WO2012176930A1

WO2012176930A1 - Process for producing carboxylic acid ester

Info

Publication number: WO2012176930A1
Application number: PCT/JP2012/066312
Authority: WO
Inventors: Koji Hagiya
Original assignee: Sumitomo Chemical Co Ltd
Current assignee: Sumitomo Chemical Co Ltd
Priority date: 2011-06-20
Filing date: 2012-06-20
Publication date: 2012-12-27
Anticipated expiration: 2013-12-20
Also published as: JP2013056879A

Abstract

An object of the present invention is to provide a new process which can produce a carboxylic acid ester from an aldehyde. The object is achieved by a process for producing a carboxylic acid ester, including the step of mixing a compound represented by the formula (2-1): (wherein R² represents an alkyl group optionally having a substituent, or the like; R³ and R⁴ each represent independently an alkyl group optionally having a substituent, or the like, or R³ and R⁴ are taken together to form a divalent hydrocarbon group optionally having a substituent, or the like; Y represents a group represented by -S- or a group represented by -N(R⁵)-; R⁵ represents an alkyl group optionally having a substituent, or the like, or R⁵ is taken together with R4 to form a divalent hydrocarbon group optionally having a substituent; and X- represents an anion), a base, an alcohol, oxygen and an aldehyde to oxidize the aldehyde.

Description

DESCRIPTION

Title of Invention

PROCESS FOR PRODUCING. CARBOXYLIC ACID ESTER

Technical. Field.

[0001]

^■ The present invention relates to a process for

producing a carboxylic acid ester.

Background Art

[0002]

A carboxylic acid ester is a compound important as various chemical products. Among the carboxylic acid esters, a carboxylic acid ester such as a ketocarboxylic acid ester is known to be useful as an intermediate for producing an amino acid and the like.

[0003]

As a process for producing a carboxylic acid ester, a process for producing a carboxylic acid ester from an aldehyde has been developed. For example, Non-Patent

Literature 1 (see, for example, Table 1) describes a' process for producing methyl 4-nitrobenzoate, by reacting 4-nitrobenzaldehyde which is an aldehyde, an alcohol, iodine and sodium nitrite with one another. Non-Patent Literature 2 (for example, Supporting information, page 2) describes a process for producing a cinnamic acid ester, by reacting a cinnamic aldehyde, with quinone, in the presence of a catalyst prepared from a triazolium salt and 1,8- diazabicyclo [5.4.0] undec-7-ene .

Citation- List

Non-Patent Literatures

[0004]

Non-Patent Literature 1: Synthesis, pp. 276-28-2 (2010) Non-Patent Literature 2: Journal of the American

Chemical Society, vol. 132, pp. 1190-1191 (2010)

Summary of the Invention

Technical Problem

[0005]

An object of the present invention is to provide a new process which can produce a carboxylic acid ester from an aldehyde.

Solution to Problem

[0006]

Under such circumstances, the present inventor

intensively studied, leading to the present invention.-

[0007]

That is, the present invention provides as follows:

[1] A process for producing a carboxylic acid ester, including the step of: mixing a compound represented by the formula (2-1) :

[0008]

[Chemical formula 1]

(wherein R² represents an alkyl group optionally having a substituent or an aryl group optionally having a substituent; R³ and R⁴ each represent independently an alkyl group optionally having a substituent or an aryl group optionally having a substituent,. or R³ and R⁴ are taken together to form a divalent hydrocarbon group

optionally having a substituent or a group represented by - CH=N- optionally having a substituent; Y represents a group represented by -S- or a group represented by -N(R⁵)-; R⁵ represents an alkyl group optionally having a substituent or an aryl group optionally having a substituent, or R⁵ is taken together with R⁴ to form a divalent hydrocarbon group optionally having a substituent; and X^" represents an anion) , a base, an alcohol, oxygen and an aldehyde to oxidize the aldehyde.

[2] The process according to [1], wherein the step of oxidizing the aldehyde is performed in the presence of carbon dioxide.

[3] The process according to [1] or [2]., wherein the aldehyde is a compound represented by the formula (1) :

[0009] [Chemical formula 2]

(wherein R¹ represents a hydrocarbon group optionally having a substituent or a heteroaryl group optionally having a substituent; and n represents 0 or 1),

the alcohol is a compound represented by the formula

(4):

[0010]

[Chemical formula, 3]

R⁹ OH (4)

(wherein R⁹ represents an alkyl group optionally having a substituent) , and

the carboxylic acid ester is a compound represented by the formula (3) :

[0011]

[Chemical formula 4]

(wherein R¹, R⁹ and n are each as defined above) .

[4] The process according to any one of [1] to [3], wherein the compound represented by the formula (2-1) is a compound represented by the formula (2-2) : [00121-

[Chemical formula 5]

R²

(wherein R², Y and X^" are each as defined above; R⁶ and R⁷ -each represent independently a hydrogen atom, an alkyl group optionally having a substituent or an aryl group optionally having a substituent, or R⁶ and R⁷ are taken together with a carbon atom to which they are bound to form a ring, or R⁶ represents a hydrogen atom, an alkyl group optionally having a substituent or an aryl group optionally having a substituent, and R⁷ is taken together with R⁵ to form a divalent hydrocarbon group optionally having a substituent; and represents a single bond or a double bond) , or

a compound represented by the formula (2-3) :

[0013]

[Chemical formula 6]

(wherein R², Y and X^~ -are each as defined above; and R⁷ represents a hydrogen atom, an alkyl group optionally having a substituent or an aryl group optionally having a substituent, or R⁷ is taken together with R⁵ to form a divalent hydrocarbon group optionally having a substituent) .

[5] The process according to any one of [1] to [4], wherein the base is at least one kind ..selected from the -group consisting of an organic base and an alkali metal alkoxide.

[6] A process for producing a carboxylic acid ester,

including the step of:

oxidizing an aldehyde, in the presence of a compound obtained by contacting a compound represented by the

formula ( 2-1 ) :

[0014]

[Chemical formula 7]

(wherein R² represents an alkyl group optionally having a substituent or an aryl group optionally having a substituent; R³ and R⁴ each represent independently an alkyl group optionally having a substituent or an aryl group optionally having a substituent, or R³ and R⁴ are taken together to form a divalent hydrocarbon group

optionally having a substituent or a group represented by - CH=N- optionally having a substituent; Y represents a group represented by -S- or a group represented by -N(R⁵)-; R⁵ represents an alkyl group optionally having a substituent or an aryl group optionally having a substituent, or R⁵ is taken together with R⁴ to form a divalent hydrocarbon group optionally having a substituent; and X^" represents an anion) with a base, an alcohol, and oxygen.

[7] The process according to [6], wherein the step of oxidizing the aldehyde is performed in the presence of carbon dioxide.

[8] The process according to [6] or [7], wherein the aldehyde is a compound represented by the formula (1) :

[0015]

[Chemical formula 8]

(wherein R represents a hydrocarbon group optionally having a substituent or a heteroaryl group optionally having a substituent; and n represents 0 or 1),

the alcohol is a compound represented by the formula

(4):

[0016]

[Chemical formula 9]

R⁹ OH (4)

(wherein R⁹ represents an alkyl group optionally having a substituent), and the carboxylic acid ester is a compound represented by the formula ( 3 ) :

[0017]

[Chemical formula 10]

(wherein R¹, R⁹ and n are each as defined above) .

[9] The process according to any one of [6] to [8], wherein the compound represented by the formula (2-1) is a compound represented by the formula (2-2) :

[0018]

[Chemical formula 11]

(wherein R², Y and X^~ each are as defined above; R⁶ and R⁷ each represent independently a hydrogen atom, an alkyl group optionally having a substituent or . an aryl group optionally having a substituent, or R⁶ and R⁷ are taken together with a carbon atom to which they are bound to form a ring, or R⁶ represents a hydrogen atom, an alkyl group optionally having a substituent or an aryl group optionally having a substituent, and R⁷ is taken together with R⁵ to form a divalent hydrocarbon group optionally having a substituent; and : represents a single bond or a double bond) , or

a compound represented by the formula (2-3) :

[0019]

[Chemical formula 12]

(wherein ^'R², Y and X^" are each as defined above; and R' represents a hydrogen atom, an alkyl group optionally having a substituent or an aryl group optionally having a substituent, or R⁷ is taken together with R⁵ to form a divalent hydrocarbon group optionally having a substituent) [10] The process according to any one of [6] to [9], wherein the base is at least one kind selected from the group consisting of an organic base and- an alkali metal alkoxide.

Advantageous Effects of Invention

[0020]

According to the present invention, a new process which can produce a carboxylic acid ester from an aldehyde can be provided. Description of Embodiments

[0021]

The process for producing a carboxylic acid ester in the present invention is a process for producing a

corresponding carboxylic acid ester from an aldehyde, and includes the step of mixing a compound represented by the formula (2-1) (hereinafter, referred to as compound (2-1) in some cases), a base, an alcohol, oxygen and an aldehyde to oxidize ^'the aldehyde. In addition, the process for producing a carboxylic acid ester in the present invention is a process for producing a corresponding carboxylic acid ester from an aldehyde, and includes the step of oxidizing an aldehyde, in the presence of a compound obtained by contacting the compound (2-1) with a base, an alcohol and oxygen .

[0022]

As the aldehyde, a compound having a group represented by the -CHO can be used without any limitation. As the aldehyde, a compound represented by the formula (1)

(hereinafter, referred to as compound (1) in some cases) is preferably used.

[0023]

In the formula (1), examples of the hydrocarbon group optionally having a substituent represented by R¹ include an alkyl group optionally having a substituent, an alkenyl group optionally having a substituent and an aryl group optionally having a substituent.

[0024]

In R¹, examples of the alkyl group, in the alkyl group optionally having a substituent, include linear or branched C1-C12 alkyl groups such as a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, a pentyl group and a decyl group, and cyclic C3-C12 alkyl groups such as a cyclopropyl group, a 2,2- dimethylcyclopropyl group, a cyclopentyl group, a

cyclohexyl group and a menthyl group.

Examples of the substituent which the alkyl group optionally has include groups selected from the following group Gl.

[0025] _.

a C1.-C.10 alkoxy group optionally having a fluorine atom, a C7-C20 aralkyloxy group optionally having a C1-C10 alkoxy group,

a C7-C20 aralkyloxy group having a C6-C10 aryloxy group, a Ce-Cio aryloxy group optionally having a C1-C10 alkoxy group,

a C6-C10 aryloxy group having a C6-C10 aryloxy group, a C2-C10 acyl group optionally having a C1-C10 alkoxy group, a Ci-Cio alkylthio group,

a C2-C10 alkoxycarbonyl group,

a C6-C20 aryl group,

a C5-C20 heteroaryl group, and

a halogen atom.

[0026]

In the group Gl, examples of the C1-C10 alkoxy group optionally having a fluorine atom include a methoxy group, an ethoxy group, a propoxy group, an isopropoxy group, a butoxy group, an isobutoxy group, a sec-butoxy group, a tert-butoxy group and a trifluoromethyloxy group,

examples of the C₇-C₂o aralkyloxy group optionally having a C1-C10 alkoxy group include a benzyloxy group, a 4- methylbenzyloxy group and a 4-methoxybenzyloxy group,

examples of the C7-C20 aralkyloxy group having a C₆-Cio aryloxy group include a 3-phenoxybenzyloxy group,

examples of the C₃-Ci₀ aryloxy group optionally having a C1-C10 alkoxy group include a phenoxy group, a 2-- methylphenoxy group, a 4-methylphenoxy group and a 4- methoxyphenoxy group,

examples of the C₆-Cio aryloxy group having a C₆-Ci₀ aryloxy group include a 3-phenoxyphenoxy group,

examples of the C₂-Ci₀ acyl group optionally having a C1-C10 alkoxy group include an acetyl group, a propionyl group, a benzylcarbonyl _. group, a 4-methylbenzylcarbonyl group, a 4-methoxybenzylcarbonyl group, a benzoyl, group, a 2-methylbenzoyl group, a 4-methylbenzoyl group and a 4- methoxybenzoyl group,

examples of the Ci-Cio alkylthio group include a methylthio group, an ethylthio group and an isopropylthio group,

examples of the C₂-Ci₀ alkoxycarbonyl group include a methoxycarbonyl group and an ethoxycarbonyl group,

examples of the C6-C20 aryl group include a phenyl group, a 1-naphthyl group and a 2-naphthyl group,

examples of the C5-C20' heteroaryl group include a 2- pyridyl group, a 3-pyridyl group, a 4-pyridyl group, a 2- quinolyl group, a 3-quinolyl group and a 4-quinolyl group, and

examples of the halogen atom include a fluorine atom, a chlorine atom, and a bromine atom. ^■

[0027]

Examples of the alkyl group having a group selected from the group Gl include a chloromethyl group, a

fluoromethyl group, a trifluoromethyl group, a

methoxymethyl group, an ethoxymethyl group, a 1- methoxyethyl group, a 2-methoxyethyl group, a

methoxycarbonylmethyl group, a phenylmethyl group, a 2- pyridylmethyl group, a 3-pyridylmethyl group, a 1- ethoxycarbonyl-2 , 2-dimethyl-3-cyclopropyl group and a 2- methylthioethyl group.

[0028] In R¹, examples of the alkenyl group, in the alkenyl group optionally having a substituent, include linear, branched or cyclic C2-C12 alkenyl groups such as a vinyl group, a 1-propenyl group, a 1-butenyl group, a 2-methyl-l- propenyl group, and a 1-cyclohexenyl group.

Examples of a substituent which the alkenyl group optionally has include groups selected from the group Gl .

[0029]

Examples of the alkenyl group having a group selected from the group Gl include a 2-chlorovinyl group and a 2- trifluoromethylvinyl group.

[0030]

In R¹, examples of the aryl group, in the aryl group optionally having a substituent, include C6-C20 aryl groups such as a phenyl group, a 2-methylphenyl group, a 4- methylphenyl group, a 1-naphthyl group, a 2-naphthyl group and a styryl group.

Examples of the substituent which the aryl group optionally has include groups selected from the following group G2.

[0031]

a C1-C10 alkoxy group optionally having a fluorine atom or a C^'l-Cio alkoxy group,

a C6-C10 arylox.y group optionally having a C1-C10 alkoxy group, a C₆-Cio aryioxy group having a C_e-Ci₀ aryioxy group, a C₂-Cio acyl group optionally having a Ci-Cio alkoxy group,

a Ci-C₆ alkylenedioxy group,

a nitro group, and

a halogen atom.

[0032]

In the group G2, examples of the Ci-Cio alkoxy group optionally having a fluorine atom or a Ci-Cio alkoxy group include a methoxy group, an ethoxy group, a propoxy group, an isopropoxy group, a butoxy group, an isobutoxy group, a sec-butoxy group, a tert-butoxy group, a pentyloxy group, a cyclopentyloxy group, a fluoromethoxy group, a

trifluoromethoxy group, a methoxymethoxy group, an

ethoxymethoxy group and a methoxyethoxy group,

examples of the^'C₆-Cio aryioxy group optionally having a Ci-Cio alkoxy group include a phenoxy group, a 2- methylphenoxy group, a 4-methylphenoxy group and a 4- methoxyphenoxy group,

examples of the CQ-C_IO aryioxy group having a C₆-Cio aryioxy group include a 3-phenoxyphenoxy group,

examples of the C₂-Ci₀ acyl group optionally having a Ci-Cio alkoxy group include an acetyl group, a propionyl group, a benzylcarbonyl group, a 4-methylbenzylcarbonyl group and a 4-methoxybenzylcarbonyl group,

examples of the i-C^ alkylenedioxy group include a methylenedioxy group and an ethylenedioxy group, and

examples of the halogen atom include a fluorine atom and a chlorine atom.

Examples of the aryl group having a group selected from the group G2 include a 4-chlorophenyl group, a 4- methoxyphenyl group and a 3-phenoxyphenyl group.

[0033] ^'

Examples^' of the heteroaryl group, in the heteroaryl group optionally having a substituent represented by R¹, include C4-C10 heteroaryl groups having at. least one

heteroatom such as a nitrogen atom, - an oxygen atom or a sulfur atom. Specific examples thereof include a 2-pyridyl group, a 3-pyridyl group, a 4-pyridyl group, a 2-furyl group, a 3-furyl group, a 5-methyl-2-furyl group and a 2- chloro-3-pyridinyl group.

[0034]

When n in the formula (1) represents 0, examples of the compound (1) include benzaldehyde, 4-chlorobenzaldehyde, 2-methylbenzaldehyde, 4-fluorobenzaldehyde, 2- methoxybenzaldehyde , 2 , 4-dichlorobenzaldehyde , 2- nitrobenzaldehyde, 2-naphthylaldehyde, 2-pyridylaldehyde, acetaldehyde, n-propylaldehyde , isopropylaldehyde, n- butylaldehyde, n-pentylaldehyde, n-hexylaldehyde, n- heptylaldehyde , n-octylaldehyde, cyclohexylaldehyde, 4- (methylthio) -1-butanal, acrolein and 3-phenyl-2- propenaldehyde . When n in the formula (1) represents 0, the compound (1) may be a commercially available product, or may be produced according to any known method.

[0035]

When n in the formula (1) represents 1, examples of the compound (1) include phenylglyoxal, 4- chlorophenylglyoxal, 2-methylphenylglyoxal, 4- fluorophenylglyoxal, 2-methoxyphenylglyoxal , 2,4- dichlorophenylglyoxal , 2-nitrophenylglyoxal , 2- naphthylglyoxal , 2-pyridineglyoxylaldehyde, methylglyoxal , ethylglyoxal, . n-propylglyoxal , isopropylglyoxal ,

cyclohexylglyoxal , 4- (methylthio) -2-oxo-l-butanal,

vinylglyoxal and styrylglyoxal .

When n in the formula (1) represents 1, the compound (1) may be a commercially available product, or may be produced according to any known method such as a method of oxygen-oxidizing a ketoalcohol in the presence of a metal catalyst (see, for example, JP-A No. 2000-336055) .

[0036]

The alcohol can be used without any limitation. As the alcohol, a compound represented by the formula (4) (hereinafter, referred to as compound (4) in some cases) is preferably used.

[0037]

In the formula (4), examples of the alkyl group, in the alkyl group optionally having a substituent represented by R⁹, include linear or branched C1-C12 alkyl groups such . as a methyl group, an ethyl group, a propyl group, an

isopropyl group, a butyl group, an isobutyl group, a sec- butyl group, a tert-butyl group, a pentyl group and a decyl group, and cyclic C3-C12 alkyl groups such as a cyclopropyl group, a 2 , 2-dimethyl.cyclopropyl group, a cyclopentyl group, ■ a cyclohexyl group and a menthyl group.

Examples of the substituent which the alkyl group optionally has include groups selected from the following group G3.

[0038]

a C1-C10 alkoxy group optionally having a fluorine atom, a C7-C20 aralkyloxy group optionally having a C1-C10 alkoxy group,

a C7-C20 aralkyloxy group having a C₆-Cio aryloxy group, a C6-C10 aryloxy group optionally having a C1-C10 alkoxy group,

a C₆-Cio aryloxy group having a C₆-Ci₀ aryloxy group, a C₂-Cio acyl group optionally having a C1-C10 alkoxy group,

a C1-C10 alkylthio group,

a C2-C10 alkoxycarbonyl group,

a C6-C20 aryl group,

a C5-C20 heteroaryl group, and

a halogen atom. [0039]

In the group G3, examples of the Ci-Cio alkoxy group optionally having a fluorine atom include a methoxy group, an ethoxy group, a propoxy group, an isopropoxy group, a butoxy group, an isobutoxy group, a sec-butoxy group, a tert-butoxy group and a trifluoromethyloxy group,

examples of the C₇-C₂o aralkyloxy group optionally having a Ci-Ci₀ alkoxy group include a benzyloxy group, a 4- methylbenzyloxy group and a 4-methoxybenzyloxy group,

examples of the C₇-C₂₀ aralkyloxy group having a C₆-Cio aryloxy group include a 3-phenoxybenzyloxy group,

examples of the C₆-Ci₀ aryloxy group optionally having a Ci-Cio alkoxy group include a phenoxy group, a 2- methylphenoxy group, a 4-methylphenoxy group and a 4- methoxyphenoxy group,

examples of the C₆-Ci₀ aryloxy group having a C₆-C₁₀ aryloxy group include a 3-phenoxyphenoxy group,

examples of the C₂-C₁₀ acyl group optionally having a Ci-Cio alkoxy group include an acetyl group, a propionyl group, a benzylcarbonyl group, a 4-methylbenzylcarbonyl group, a 4-methoxybenzylcarbonyl group, a benzoyl group, a 2-methylbenzoyl group, a 4 -methylbenzoyl group and a 4- methoxybenzoyl group,

examples of the Ci-Cio alkylthio group include a _.

methylthio group, an ethylthio group and an isopropylthio group, examples of the C₂-Ci₀ alkoxycarbonyl group include a methoxycarbonyl group and an ethoxycarbonyl group,

examples of the C₆-C2o aryl group include a phenyl group, a 1-naphthyl group and a 2-naphthyl group,

examples of the C5-C20 heteroaryl group include a 2- pyridyl group, a 3-pyridyl group, a 4-pyridyl group, a 2- quinolyl group, a 3-quinolyl group and a. 4-quinolyl group, and

examples of the halogen atom include a fluorine atom, a chlorine atom and a bromine atom.

[0040]

Examples of the alkyl group having a group selected from the group G3 include a chloromethyl group, a

fluoromethyl group, a trifluoromethyl group, a

[0041]

Examples of the compound (4) include methanol, ethanol, 1-propanol, isopropanol, 1-butanol, isobutanol, 1-pentanol, 1-hexanol, cyclohexanol, benzyl alcohol, 2-phenylethanol^" and 2-pyridinemethanol .

[0042] The alcohol such as the compound (4) may be a

commercially available product, or may be produced

according to any known method.

[0043]

The amount of the alcohol to be used is preferably 1 mol or more based on 1 mol of the ^'aldehyde, and an upper limit thereof is not limited, but is preferably 100 mol or less from the viewpoint of economy.

[0044]

In the formula (2-1) , examples of the alkyl group, in the alkyl group optionally having a substituent represented by R³ and the alkyl group optionally having a substituent represented by R⁴, include linear or branched C₁-Ci₂ alkyl groups such as a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, a pentyl group and a decyl group, and cyclic C3-C12 alkyl groups such as a

cyclopropyl group, a 2 , 2-dimethylcyclopropyl group, a cyclopentyl group, a cyclohexyl group, and a menthyl group.

[0045]

Examples of the substituent which the alkyl group in R³ and R⁴ optionally has include groups selected from the ^' following group G3.

[0046]

a C₆-Cio aryl group optionally having a Ci-Cio alkoxy group,

a Ci-Cio alkoxy group optionally having a fluorine atom, a benzyloxy group optionally having at least one kind of a group selected from the group consisting of a Ci-Cio alkoxy group, a Ci-Cio alkyl group and a C6-C10 aryloxy group, a C₆-Cio aryloxy . group optionally having a Ci-C₁₀ alkoxy group,

a C₆-Cio aryloxy group having a C₆-Ci₀ aryloxy group, a C₂-Cio acyl group optionally having a Ci-Cio alkoxy group,

a carboxy group, and

a fluorine atom.

[0047]

In the group G3, examples of the C₆-Ci₀ aryl group optionally having a Ci-Cio alkoxy group include a phenyl group, a naphthyl group, a 4-methylphenyl group and a 4- methoxyphenyl group,

examples of the Ci-Cio alkoxy group optionally having a fluorine atom include a methoxy group, an ethoxy group, a propoxy group, an isopropoxy group, a butoxy group, an isobutoxy group, a sec-butoxy group, a tert-butoxy group and a trifluoromethoxy group,

examples of the benzyloxy group optionally having at least one kind of a group selected from the group

consisting of a Ci-Cio alkoxy group, a Ci-Cio alkyl group and a C₆-Cio aryloxy group include a benzyloxy group, a 4- methylbenzyloxy group, a 4-methoxybenzyloxy group and a 3- phenoxybenzyloxy group,

examples of the C₆-Ci₀ aryloxy group optionally having a Ci-Cio alkoxy group include a phenoxy group, a 2- methylphenoxy group, a 4 -methylphenoxy group and a 4- methoxyphenoxy group,

examples of the C₆-Ci₀ aryloxy group having a C₆-Ci₀ aryloxy group include a 3-phenoxyphenoxy group, and

examples of the C₂-Ci₀ acyl group optionally having a Ci-Cio alkoxy group include an acetyl group, a propionyl group, a benzylcarbonyl group, a 4-methylbenzylcarbonyl group, a 4-methoxybenzylcarbonyl group, a benzoyl group, a 2-methylbenzoyl group, a 4-methylbenzoyl group and a 4- methoxybenzoyl group.

[0048]

Examples of the alkyl group having a group selected from the group G3 include a fluoromethyl group, a

trifluoromethyl group, a methoxymethyl group, an

ethoxymethyl group, a methoxyethyl group, a benzyl group, a 4-fluorobenzyl group, a 4-methylbenzyl group, a

phenoxymethyl group, a 2-oxopropyl group, a 2-oxobutyl group, a phenacyl group and a 2-carboxyethyl - group .

[0049]

In the . formula (2-1), examples of the aryl group, in the aryl group optionally having a substituent represented by R³ and the aryl group optionally having a substituent represented by R⁴, include Cg-Cio aryl groups such as a phenyl group, a 2-methylphenyl group, a 4-methylphenyl group, and a naphthyl group.

Examples of the substituent which the aryl group optionally has include groups selected from the group G2.

[0050]

Examples of the aryl group having a group selected from the group G2 include a 4-chlorophenyl group and a 4- methoxyphenyl group.

[0051]

In the formula (2-1), examples of the divalent

hydrocarbon group optionally having a substituent, which is formed by R³ and R⁴ together, include an ethylene group, a vinylene group, a propane-1 , 2-diyl group, a propene-1,2- diyl group, a butane-1, 2-diyl group, a 2-butene-l , 2-diyl group, a cyclopentane-1 , 2-diyl group, a cyclohexane-1 , 2- diyl group, an o-phenylene group, a 1 , 2-diphenylethylene group and a 1 , 2-diphenylvinylene group. Examples: of the substituent which the divalent hydrocarbon group optionally has include groups selected from the group G2. In the formula (2-1) , examples of the substituent which the group represented by -CH=N- optionally has, which is formed by R³ and R⁴ together, include an alkyl group optionally having a group selected from the group G2 and an aryl group

optionally having a group selected from the group G2.

Examples of the alkyl group, in the alkyl group optionally having a group selected from the group G2, include linear or branched C1 -.C12 alkyl groups such as a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, a pentyl group and a decyl group, and cyclic C3-C12 alkyl groups such as a cyclopropyl group, a 2,2- dimethylcyclopropyl group, a cyclopentyl group, a

cyclohexyl group, and a menthyl group. Examples of the- aryl group, in the aryl group optionally having a group selected from the group G2, include C₆-Cio aryl groups such as a phenyl group, a 2-methylphenyl group, a 4-methylphenyl group, and a naphthyl group.

[0052]

R³ and R⁴ are preferably taken together to form a divalent hydrocarbon group optionally having a substituent.

[0053]

In the formula (-2-1) , examples of the alkyl group, in the alkyl group optionally having a substituent represented by R² and the alkyl group optionally having a substituent represented by R⁵, include linear or branched C1-C12 alkyl groups such as a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, an isobutyl group, a sec-butyl . group, a tert-butyl group, a pentyl group, a tert-pentyl group and a decyl group, and cyclic C3-C12 alkyl groups such a cyclopropyl group, a 2 , 2-dimethylcyclopropyl group, a cyclopentyl group, a cyclohexyl group, a menthyl group and an adamantyl group.

Examples of the substituent which the alkyl group optionally has include groups selected from the following group G4.

[0054]

a C6-C10 aryl group optionally having a C1_.-C10 alkoxy group,

a C7-C20 aralkyloxy group having a C6-C10 aryloxy group, a C₆-Cio aryloxy group optionally having a C1-C10 alkoxy group,

a C6-C10 aryloxy group having a C₆-Cio aryloxy group, and ^

a C₂-Cio acyl group optionally having a C1-C10 alkoxy group .

[0055]

In the group G4, examples of the C₆-Cio aryl group optionally having a C1-C10 alkoxy group include a phenyl group, a naphthyl group, a 4-methylphenyl group, and a 4- methoxyphenyl group,

examples of the C1-C10 alkoxy group optionally having a fluorine atom include a methoxy group, an ethoxy group, a propoxy group, an isopropoxy group, a butoxy group, an isobutoxy group, a sec-butoxy group, a. tert-butoxy group and a trifluoromethoxy group,

examples of the C7-C20 aralkyloxy group optionally having a C1-C10 alkoxy group include a benzyloxy group, a 4- methylbenzyloxy group and a 4-methoxybenzyloxy group,

examples of the C7-C20 aralkyloxy group having a C6-C10 aryloxy group include a 3-phenoxybenzyloxy group,

examples of the C6-C10 aryloxy group optionally having a C1-C10 alkoxy group include a phenoxy group, a 2- methylphenoxy group, a 4-methylphenoxy group and a 4- methoxyphenoxy group,

examples of the^" C₆-Cio aryloxy group having a C6-C10 aryloxy group include a 3-phenoxyphenoxy group, and

examples of the C₂-Cio acyl group optionally having a C1-C10 alkoxy group include an acetyl group, a propionyl group, a benzylcarbonyl group, a 4-methylbenzylcarbonyl group, a 4-methoxybenzylcarbonyl group, a benzoyl group, a 2-methylbenzoyl group, a 4-methylbenzoyl group and a 4- methoxybenzoyl group.

[0056]

Examples of the alkyl group having a group selected from the group G4 include a methoxymethyl group, an

ethoxymethyl group, a methoxyethyl group, a benzyl group, a 4 -fluorobenzyl group-, a 4-methylbenzyl group, a

phenoxymethyl group, a 2-oxopropyl group, a 2-oxobutyl group and a phenacyl group. [0057]

In the formula (2-1) , examples of the aryl group, in the aryl group optionally having a substituent represented by R² and the aryl group optionally having a substituent represented by R⁵, include C6-C20 aryl groups such as a phenyl group, a naphthyl group, a 2-methylphenyl group, a · 4-methylphenyl group, a 2 , 6-dimethylphenyl group, a 2,4,6- trimethylphenyl group, and a 2 , 6-diisopropylphenyl group.

Examples of the substituent which the aryl group optionally has include groups selected from the following group G5.

[0058]

<Group G5> ^■

a C1 -C10 alkoxy group optionally having a fluorine atom or a C1 -C 10 alkoxy group, and

a halogen atom.

[0059]

In the group G5, examples of the C1 -C10 alkoxy group optionally having a fluorine atom or a C1 -C10 alkoxy group include a methoxy group, an ethoxy group, a propoxy group, an isopropoxy group, a butoxy group, an isobutoxy group, a sec-butoxy group, a tert-butoxy group, a pentyloxy group, a cyclopentyloxy group, a fluoromethoxy group, a

trifluoromethoxy group, a methoxymethoxy group, an

ethoxymethoxy group and a methoxyethoxy group, and examples of the halogen atom include a fluorine atom and a chlorine atom.

[0060]

Examples of the aryl group having a group selected from the group G5 include a 4-chlorophenyl group, a 4- methoxyphenyl group and a 2 , 6-dichlorophenyl group.

[0061]

In the formula (2-1), R⁵ may be taken together with R⁴ to form a divalent hydrocarbon group optionally having , a substituent. Examples of the divalent hydrocarbon group include polymethylene groups such as an ethylene group, a trimethylene group and a tetramethylene group, a vinylene group, a propane-1, 2-diyl group, a propene-1 , 2-diyl group, a butane-1, 2-diyl group, a 2-butene-l, 2-diyl group, a cyclopentane-1, 2-diyl group, a cyclohexane-1, 2-diyl group and an o-phenylene group.- Examples of the substituent which the divalent hydrocarbon group optionally has include groups selected from the group G2.

[0062]

In the formula (2-1), examples of the anion

represented by X^~ include halide ions ^' such as a chloride ion, a bromide ion and an iodide ion; alkanesulfonate ions optionally having a fluorine atom such as methanesulfonate and trifluoromethanesulfonate; acetate ions optionally having a halogen atom such as . trifluoroacetate and

trichloroacetate ions; nitrate ions; perchlorate ions;^' tetrahaloborate ions such as tetrafluoroborate and tetrachloroborate ; hexahalophosphate ions such as

hexafluorophosphate ; hexahaloantimonate ions such as hexafluoroantimonate and hexachloroantimonate ;

pentahalostannate ions such as pentafluorosutannate -and pentachlorosutannate; as well as tetraarylborates

optionally having a substituen^'t such as tetraphenylborate, tetrakis (pentafluorophenyl ) borate and tetrakis [ 3 , 5- bis (trifluoromethyl ) phenyl] borate .

[0063]

The compound (2-1) is preferably a compound

represented by the formula (2-2) (hereinafter, referred to as "compound (2-2)" in some cases) or a compound

represented by the formula (2-3) (hereinafter, referred to as "compound 2-3" in some cases), more preferably a

compound (2-2) .

That is, the oxidizing of an aldehyde is preferably performed by mixing the compound (2-2) or the compound (2- 3), a base, an alcohol, oxygen and an aldehyde, and is more preferably performed by mixing the compound (2-2), a base, an alcohol, oxygen and an aldehyde. The oxidizing of an aldehyde is preferably performed in the presence of a compound obtained by contacting the compound (2-2) or the compound (2-3) with a base, an alcohol, and oxygen, and is more preferably performed in the presence of a compound obtained by contacting the compound (2-2) with a base, an. alcohol, and oxygen. [0064]

In the formula (2-2) and the formula (2-3), R² has the same meaning as that of R² in the formula (2-1) , and Y has the same meaning as that of Y. in the formula (2-1) . When Y in the formula (2-2) and the formula (2-3) is a group represented by -N(R⁵)-, R⁵ has the same meaning as that of R⁵ in the formula (2-1) . In the formula (2-2) and the formula (2-3) , X^~ has the same meaning as that of X^" in the formula (2-1) .

[0065]

In the formula (2-2), R² is preferably a bulky group. When Y is a group represented by -N(R⁵)-, it is preferable that either of R² and R⁵ is a bulky group, and it is more preferable that both of R² and R³ are a bulky group. R² and R⁵ may be the same group, or may be different groups.

[0066]

Examples of the bulky group in R² and R⁵ include C4-C12 tertiary alkyl groups such as a tert-butyl group and a tert-pentyl group; C3-C10 cycloalkyl groups such as a cyclopropyl group, a 2 , 2-dimethylcyclopropyl group, a cyclopentyl group, a cyclohexyl group, a menthyl group and an adamantyl group; phenyl groups having a substituent at least on a 2-position and a 6-position (2 , 6-disubstituted phenyl groups) such as a 2 , 6-dimethylphenyl group, a 2,6- dichlorophenyl group, a 2 , 6-dibromophenyl group, a 2,4,6- trimethylphenyl group and a 2 , β-diisopropylphenyl group; as well as a naphthyl group having a Ci-Cio alkyl group on a 2- position such as a 2-methylnaphthyl group. Examples of the substituent in the 2 , 6-disubstituted phenyl group include a ^■ C1-C12 alkyl group and a halogen atom.

As the bulky group in R² and R⁵, a tert-butyl group, a tert-pentyl group, a cyclohexyl group, an adamantyl group or a 2 , 6-disubstituted phenyl group is preferable, a 2,6- disubstituted phenyl group is more preferable, and a 2,6- dibromophenyi group or a 2 , 6-diisopropylphenyl group is further preferable.

[0067]

Examples of the alkyl group, in the alkyl group optionally having a substituent represented by R⁶ in the formula (2-2) and the alkyl group optionally having a substituent represented by R⁷ in the formula (2-2) and .the formula (2-3) , include linear, branched or cyclic C1-C10 alkyl groups such as a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, an

isobutyl group, a sec-butyl group, a tert-butyl group, a pentyl group, a decyl group, a cyclopropyl group, a 2,2- dimethylcyclopropyl group, a cyclopentyl group, a

cyclohexyl group, and a menthyl group.

Examples of the substituent which the alkyl group optionally has include groups selected from the group G3. Examples of the alkyl group having a group selected from the group G3 include a fluoromethyl group, a trifluoromethyl group, a, methoxymethyl group, an

phenoxymethyl group, a 2-oxopropyl group, a 2-oxobu^'tyl group, a phenacyl group and a 2-carboxyethyl group.

[0068]

Examples of the aryl group, in the aryl group

optionally having a substituent represented by R⁶ in the formula (2-2) and the aryl group optionally having a substituent represented by R⁷ in the formula (2-2) and the formula (2-3), include C6-C10 aryl groups such as a phenyl group, a 2-methylphenyl group, a 4-methylphenyl group, and a naphthyl group.

[0069]

Examples of the aryl group having a group selected from the group G2 include a 4 -chlorophenyl group and a 4- methoxyphenyl group.

[0070]

In the formula (2-2) , R⁶ and R⁷ may be taken together with a carbon atom to which they are bound to form a ring. Examples of such a ring include a cyclopentane ring, a cyclohexane ring and a benzene ring.

[0071]

In the formula (2-2), it is preferable that R⁶ and R⁷ each are independently a hydrogen atom or an alkyl group optionally having a substituent, and it is more preferable that R⁶ and R⁷ are both a hydrogen atom.

[0072]

In the formula (2-3) , it is preferable that R² is a linear, branched or cyclic Ci-Cio alkyl group or a C6-C10 aryl group. When Y in the formula (2-3) represents -N(R⁵)-, it is preferable that R⁵ is a linear, branched or cyclic C1 -C10 alkyl group or a C6-C10 aryl group. In the formula (2- 3) , it is preferable that . R⁷ is a linear, branched or cyclic C1-C10 alkyl group or Cg-Cio aryl group.

[0073]

In the formula (2-2) and the formula (2-3) , when Y is a group represented by -N(R⁵)-, R⁵ and R⁷ may be taken together to form a divalent hydrocarbon group optionally having a substituent. Examples of the divalent hydrocarbon group include polymethylene groups such as an ethylene group, a trimethylene group and a tetramethylene group, a vinylene group, a propane-1, 2-diyl group, a propene-1,2- ^' diyl group, a butane-1, 2-diyl group, a 2-butene-l, 2-diyl group, a cyclopentane-1 , 2-diyl group, a cyclohexane-1, 2- diyl group as well as an o-phenylene group. Examples of the substituent which the divalent hydrocarbon group optionally has include groups selected from the group G2.

[0074]

In the formula (2-2), when Y is a group represented by -N(R⁵)-, z^z^i is preferably a single bbnd, and when Y is a group represented by -S-, — is preferably a double bond.

[0075]

The compound (2-2) include

the compound (2-2) wherein Y is -N (R⁵) -, and R² and R⁵ are independently a C₄-Ci₂ tertiary alkyl group, a cyclic C₃-C₁₀ alkyl group, a phenyl group having a substituent at least on a 2-position and a 6-position or a naphthyl group having a Ci-Cio alkyl group on a 2-position;

the compound (2-2) wherein Y is -N(R⁵)-, R² and R⁵ are independently a C4-C12 tertiary alkyl group, a cyclic C₃-Ci₀ alkyl group, a phenyl group having a substituent at least on a 2-position and a 6-position or a naphthyl group having a Ci-C₁₀ alkyl group on a 2-position, and is a single bond;

the compound (2-2) wherein Y is -N(R⁵)-, and R² and R⁵. are independently a C₄-Ci₂ tertiary alkyl group, a cyclic C3-C10 alkyl group, a phenyl group having a Ci-C₁₀ alkyl group or a halogen atom at least on a 2-position- and a 6- position, or a naphthyl group having a C1 -C10 alkyl group on a 2-position;

the compound (2-2) wherein Y is -N(R⁵)-, R² and R⁵ are independently a C₄-Ci₂ tertiary alkyl group, a cyclic C3-C10 alkyl group, a phenyl group having a C1 -C10 alkyl group or a halogen atom at least on a 2-position and a β-position, or a naphthyl group having a Ci-Ci₀ alkyl group on a 2-position, and is a single bond;

the compound (2-2) wherein Y is -N(R⁵)-, and R² and R⁵ are independently a tert-butyl group, a tert-pentyl group, a cyclohexyl group, an adamantyl group, a 2,6- dimethylphenyl group, ^' a 2 , 6-dichlorophenyl group, a 2,6- dibromophenyl group, a 2 , 4 , 6-trimethylphenyl group or a 2 , 6-diisopropylphenyl group;

the compound (2-2) wherein Y is -N(R⁵)-, R² and R⁵ are independently a tert-butyl group, a tert-pentyl group, a cyclohexyl group, an adamantyl group, a 2 , 6-dimethylphenyl group, a 2 , 6-dichlorophenyl group, a 2 , 6-dibromophenyl group, a 2 , 4 , 6-trimethylphenyl group or a 2,6- diisopropylphenyl group, and is a single bond;

the compound (2-2) wherein Y is -N(R⁵)-, R² and R⁵ are independently a C₄-Ci₂ tertiary alkyl group, a cyclic C₃-Ci₀ alkyl group, a phenyl group having a Ci—Cio alkyl group or a halogen atom at least on a 2-position and a 6-position or a naphthyl group having a C_x-C₁₀ alkyl group on a 2-position,

■ is a single bond, and R⁵ and R⁷ are independently a hydrogen atom or a linear, branched or cyclic Ci-Ci₀ alkyl group optionally having a group selected from the group G3; the compound (2-2) wherein Y is -N(R⁵)-, R² and R⁵ are independently a C₄-C₁₂ tertiary alkyl group, a cyclic C₃-Ci₀ alkyl group, a phenyl group having, a Ci-Ci_0. alkyl group or a halogen atom at least on a 2-position and a 6-position or a naphthyl group having a C_!-C₁₀ alkyl group on a 2-position, ^_{s a} single bond, and R⁶ and R⁷ are hydrogen atoms; the compound (2-2) wherein Y is -N(R⁵)-, R² and R⁵ are independently a tert-butyl group, a tert-pentyl group, a

\ cyclohexyl group, an adamantyl group, a 2 , 6-dimethylphenyl group, a 2 , 6-dichlorophenyl group, a 2 , 6-dibromophenyl group, a 2 , 4 , 6-trimethylphenyl group _. or a 2,6- diisopropylphenyl group, is a single bond, and R⁶ and

R⁷ are independently a hydrogen atom or a linear, branched or cyclic Cx-Cio alkyl group optionally having a group selected from the group G3;

the compound (2-2) wherein Y is -N(R⁵)-, R² and R⁵ are independently a tert-butyl group, a tert-pentyl group, a cyclohexyl group, an adamantyl group, a 2 , 6-dimethylphenyl group, a 2 , 6-dichlorophenyl group, a 2 , 6-dibromophenyl group, a 2 , 4 , 6-trimethylphenyl group or a 2,6- diisopropylphenyl group, ^■ is a single bond, and R⁶ and

R⁷ are hydrogen atoms;

the compound (2-2) wherein Y is -S-, and R² is a C₄-Ci₂ tertiary alkyl group, a cyclic C₃-C₁₀ alkyl group, a phenyl group having a substituent at least on a 2-position and a 6-position or a naphthyl group, having a Ci-Ci₀ alkyl group on a 2-position;

the compound (2-2.) wherein Y is -S-, R² is a C₄-C₁₂ tertiary alkyl group, a cyclic C₃-C₁₀ alkyl group, a phenyl group having a substituent at least on a 2-position and a β-position or a naphthyl group having a Ci-Cio alkyl group on a 2-position, and is a double bond;

the compound (2-2) wherein Y is -S-, and R² is a C₄-Ci₂ tertiary alkyl group, a cyclic C₃-Ci₀ alkyl group, a phenyl group having a Ci-C₁₀ alkyl group or a halogen atom at least on. a 2-position and a 6-position, or a naphthyl group having a Ci-Ci₀ alkyl group on a 2-position;

the compound (2-2) wherein Y is -S-, R² is a C₄-Ci₂ tertiary alkyl group, a cyclic C₃-C₁₀ alkyl group, a phenyl group having a Ci-Ci₀ alkyl group or a halogen atom at least on a 2-position and a 6-position, or a naphthyl group having a Ci-Ci₀ alkyl group on a 2-position, and is a double bond;

the compound (2-2) wherein Y is -S-, and R² is a tert- butyl- group, a tert-pentyl group, a cyclohexyl group, an. adamantyl^' group, a 2 , 6-dimethylphenyl group, a 2,6- dichlorophenyl group, a 2 , 6-dibromophenyl group, a 2,4,6- trimethylphenyl group or a 2 , 6-diisopropylphenyl group; the_. compound (2-2) wherein Y is -S-, R² is a tert- butyl group, a tert-pentyl group, a cyclohexyl group, an adamantyl group, a 2 , 6-dimethylphenyl group, a 2,6- dichlorophenyl group, a 2 , 6-dibromophenyl group, a 2,4,6- trimethylphenyl group, or a 2 , 6-diisopropylphenyl group, and is a double bond;

the compound (2-2) wherein Y is -S-, R² is a C₄-C₁₂ tertiary alkyl group, a cyclic C₃-C₁₀ alkyl group, a phenyl group having a C₁-C₁₀ alkyl group or a halogen atom at least on a 2-position and a 6-position or a naphthyl group having a Ci-Cio alkyl group on a 2-position, is a double bond, and R⁶ and R⁷ are -independently a hydrogen atom or a linear, branched or cyclic Ci-Gio alkyl group optionally having a group selected from the group G3;

the compound (2-2) wherein Y is -S-, R² is a C₄ -Ci₂ tertiary alkyl group, a cyclic C₃-G₁₀ alkyl group, a phenyl group having a Ci-Cio alkyl group or a halogen atom at least on a 2-position and a 6-position or a naphthyl group having a Ci-Cio alkyl group on a 2-position, is a double bond, and R⁶ and R⁷ are hydrogen atoms;

the compound (2-2) wherein Y is -N(R⁵)-, R² and R⁵ are independently a tert-butyl group, a tert-pentyl group, a cyclohexyl group, an adamantyl group, a 2, 6-dimethylphenyl group, a 2 , 6-dichlorophenyl group, a 2 , 6-dibromophenyl group, a 2 , , 6-trimethylphenyl group or a 2,6- diisopropylphenyl group, and R⁶ and R⁷ are independently a hydrogen atom, a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, a pentyl group, a decyl group, a cyclopropyl group, a 2 , 2-dimethylcyclopropyl - group, a cyclopentyl group, a cyclohexyl group or a menthyl group; and

the compound (2-2) wherein Y is -N(R⁵)-, R² and R⁵ are independently a tert-butyl group, a tert-pentyl group, a cyclohexyl group, an adamantyl group, a 2 , 6-dimethylphenyl group, a 2 , 6-dichlorophenyl group, a 2 , 6-dibromophenyl^' group, a 2 , 4 , 6-trimethylphenyl group or a 2,6- diisopropylphenyl group, is a single bond, and R⁶ and

R⁷ are independently a hydrogen atom, a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, a pentyl group, a decyl group, a cyclopropyl group, - a 2 , 2-dimethylcyclopropyl group, a cyclopentyl group, a cyclohexyl group or a menthyl group.

[0076]

Examples of the compound (2-2) include 1,3-di-tert- butylimidazolium chloride, 1 , 3-di-tert-butylimidazolinium chloride, 1 , 3-dicyclohexylimidazolium chloride, 1,3- dicyclohexylimidazolinium chloride, 1,3- diadamantylimidazolium chloride, 1,3- diadamantylimidazolinium chloride, 1 , 3-diphenylimidazolium chloride, 1, 3-diphenylimidazolinium chloride, 1, 3-bis

[ (2 , 6-diisopropyl ) phenyl ] imidazolium chloride, 1,3- bis [ (2, 6-diisopropyl) phenyl] imidazolinium chloride, 1,3- bis [ (2 , 4 , 6-trimethyl) phenyl] imidazolium chloride, 1,3- bis [ (2 , 4 , 6-trimethyl) phenyl] imidazolinium chloride, 1,3- bis [ (2, 6-dibromo) phenyl] imidazolinium chloride, 1,3- bis [ (2 , 4 , 6-tribromo) phenyl] imidazolinium chloride, 4,5- dimethyl-1, 3-bis [ (2,4, 6-trimethyl) phenyl] imidazolium chloride, 4, 5-dimethhyl-l, 3-bis [ (2, 4, 6- trimethyl) henyl] imidazolinium chloride, 4 , 5-dimethyl-l , 3- bis [ (2 , 6-diisopropyl) phenyl] imidazolium chloride, 4,5- dimethyl-1, 3-bis [ (2, 6-diisopropyl) phenyl] imidazolinium . chloride, 4 , 5-dichloro-l, 3-bis [ (2 , 6- diisopropyl) phenyl] imidazolium chloride, 4 , 5-dichloro-l , 3- bis [ (2, 6-diisopropyl.) phenyl] imidazolinium chloride, 4,5- diphenyl-1, 3-bis [ (2,4, 6-trimethyl) phenyl] imidazolium chloride, 4 , 5-diphenyl-l , 3-bis [ ( 2 , 4 , 6- trimethyl ) phenyl ] imidazolinium chloride, 4 , 5-difluoro-1 , 3- bis [ (2, 6-diisopropyl) phenyl] imidazolium chloride, 4,5- difluoro-1, 3-bis [ (2, 6-diisopropyl ) phenyl ] imidazolinium chloride, 4-methyl-l, 3-bis [ (2,4,6- trimethyl) phenyl] imidazolium chloride, 4-methyl-l , 3- bis [ (2 , , 6-trimethyl ) phenyl ] imidazolinium chloride, 1,3- bis [ (2, 6-dichloro) phenyl] imidazolium chloride, 1,3- bis [ (2, 6-dichloro). phenyl] imidazolinium chloride, 1-tert- butyl-3-phenylimidazolium chloride, l-tert-butyl-3- phenylimidazolinium chloride, l-cyclohexyl-3- [ ( 2 , 6- diisopropyl) phenyl] imidazolium chloride, l-cyclohexyl-3- [ (2, 6-diisopropyl) phenyl] imidazolinium chloride, 1-phenyl- 3- [ (2 , 4 , 6-trimethyl) phenyl] imidazolium chloride, 1-phenyl- 3- [( 2 , 4 , 6-trimethyl ) phenyl ] imidazolinium chloride, 1-tert- butyl-3- [ (2, 6-diisopropyl) phenyl] imidazolium chloride, 1- tert-butyl-3- [ ( 2 , 6-diisopropyl ) phenyl ] imidazolinium chloride, l-tert-butyl-3- [ (2 , 4 , 6- trimethyl) phenyl] imidazolium chloride, l-tert-butyl-3- [ (2, 4, 6-trimethyl) phenyl] imidazolinium chloride, 3- ethylbenzothiazolium bromide, 3-butylbenzothiazolium chloride, 3- (2, 6-diisopropyl) phenyl-4 , 5-dimethylthiazolium chloride, 3-phenyl-4 , 5-dimethylthiazolium chloride, 3- benzylthiazolium chloride, 3-benzyl-4-methylthiazolium chloride, 3-n-butyl-4 -methylthiazolium chloride,^' 3-n-hexyl- 4-methylthiazolium chloride, 3-cyclohexyl-4 - methylthiazolium chloride, 3-n-octyl-4:-methylthiazolium chloride and 3- (2, 4, 6-trimethyl) phenyl-4, 5- dimethylthiazolium chloride.

[0077]

The compound (2-3) include

the compound (2-3) wherein Y is -N (R⁵) -, and R² and R⁵ are independently a linear or branched Ci-Ci₂ alkyl group or a C₅-C₂o aryl group;

the compound (2-3) wherein Y is -N(R⁵)-, R². and R⁵ are independently a linear or branched C_x-C₁₂ alkyl group or a C₆-C₂o aryl group, and R⁷ is a linear or branched Ci-Cio alkyl group or a C₆-Ci₀ aryl group;

the compound (2-3) wherein Y is -S-, R² is a linear or branched Ci-Ci₂ alkyl group or a C₆-C₂o aryl group; and

the compound (2-3) wherein Y is -S-, R² is a linear or branched Ci-C₁₂ alkyl group or a C₆-C₂₀ aryl group, and R⁷ is a linear or branched Οχ-Οχο alkyl group or a C₆-C₁₀ aryl group.

[0078] Examples of the compound (2-3) include 1 , 4-dimethyl- lH-1, 2, 4-triazol-4-ium chloride, 1, 3, 4-triphenyl-lH-l , 2, 4- triazol- -ium chloride, 6, 7-dihydro-2-pentafluorophenyl-5H- pyrrolo [2,_.1-c] -1, 2, 4-triazolium chloride, , 3, 5-diphenyl- 1, 3, 4-thiadiazolium chloride, and 3-methyl-5-phenyl-l , 3, 4- thiadiazolium chloride.

[0079]

Examples of the compound (2-2) and the compound (2-3) also include the compound (2-2) and the compound (2-3) in which the "chloride" ^' is replaced with, for example,

"iodide", "bromide", "methanesulfonate" ,

"trifluoromethanesulfonate", "nitrate", "perchlorate" ,

"tetrafluoroborate" , "tetrachloroborate" ,

"hexafluorophosphate" , "hexafluoroantimonate" ,

"hexachloroantimonate", "pentafluorostannate" ,

"pentachlorostannate", "tetraphenylborate" ,

"tetrakis (pentafluorophenyl) borate", or "tetrakis [3, 5- bis ( trifluoromethyl ) phenyl] borate".

[0080]

Examples of the compound (2-1). include commercially available products or compounds produced according to the process described in, for example, J. Organometallic . Chem., 606, 49 (2000) and J. Org. Chem., 73, 2784 (2008).

The amount of the compound (2-1) to be used is

preferably in a range of 0.001 mol to 0.5 mol, more

preferably in a range of 0.01 mol to 0.3 mol based on 1 mol of the aldehyde.

[0081]

It is preferable that the base is at least one kind selected from the group consisting of an organic base and an alkali metal alkoxide.

[0082]

Examples of the organic base include tertiary amines such as triethylamine, trioctylamine, diisopropylethylamine and 4-dimethylaminopyridine; nitrogen-containing aliphatic cyclic compounds such as 1 , 8-diazabicyclo [ 5 , , 0 ] -7-undecene and 1, 5, 7-triazabicyclo [4, 4, 0] -5-decene; as well as

nitrogen-containing aromatic compounds such as pyridine and imidazole .

[0083]

Examples of the alkali metal of the alkali metal alkoxide include lithium, sodium, and potassium. In addition, examples of the alkoxide include methoxide, ethoxide, n-propoxide, isopropoxide, t-butoxide, and sec- butoxide. It is preferable that the alkoxide is at least one kind of an alkali metal alkoxide selected from the group consisting of lithium alkoxide, sodium alkoxide and potassium alkoxide.

The alkali metal alkoxide may be used alone, or may be used in an alcohol solution obtained by dissolving the alkali metal alkoxide in an alcohol solvent. In this case, it is preferable in obtaining a carboxylic acid ester at a high purity that the alcohol solvent contained in the alcohol solution is the same as the alcohol such as the compound ( 4 ) .

[0084]

The amount of the base to be used is, for example, in a range of 0.1 mol to 10 mol, preferably in a range of 0.5 mol to 3 mol based on 1 mol of the compound (2-1) .

[0085]

Oxygen may be an oxygen gas, an oxygen gas diluted with an inert gas such as nitrogen, or oxygen contained in the atmosphere. Alternatively, oxygen may be a gas

obtained by diluting oxygen contained in the atmosphere with an inert gas such as nitrogen.

The amount of oxygen to be used is preferably in a range of 1 mol to 100 mol based on 1 mol of the aldehyde.

[0086]

It is preferable that the. mixing of the compound (2-1) , a base, an alcohol, oxygen and an aldehyde is performed in the presence of carbon dioxide. Carbon dioxide may be gaseous, solid (dry ice) or in the supercritical state.

Gaseous carbon dioxide may be carbon dioxide diluted with an inert gas such as nitrogen.

The amount of carbon dioxide to be used is preferably 1 mol or more based on 1 mol of the aldehyde, and an upper limit thereof is not limited. However, it is, for example, 100 mol or less from the viewpoint of productivity. [0087]

The mixing may be further performed in the presence of a solvent.

The solvent is not limited as far as it does not inhibit the oxidizing of the aldehyde. Examples of the solvent include ether solvents such as tetrahydrofuran, methyl tert-butyl ether, cyclopentyl methyl ether, and diisopropyl ether, ester solvents such as ethyl acetate and butyl acetate, aromatic solvents such as toluene and chlorobenzene, nitrile solvents such as acetonitrile and propionitrile, and a mixed solvent of these.

The amount of the solvent to be used is not limited. It is practical that the amount to be used is, for example, 100 parts by weight or less based on 1 part by weight of the aldehyde.

[0088]

In the mixing, the order of mixing reaction reagents is not limited. Preferable embodiments thereof include a ^■ method of mixing an aldehyde, the compound (2-1), oxygen, if necessary, carbon dioxide, an alcohol, and if necessary, a solvent, and adding a base to the resulting mixture. · [0089]

The mixing is performed under any condition of under reduced pressure, under normal pressure and under pressure, and is preferably performed under normal pressure or under pressure . [0090]

The reaction temperature of the mixing differs

according to the kind, amount to be used and the like of the compound (2-1) , and the kind, amount to be used and the like of the base, and is preferably in a range of -20°C to 150°C, more preferably in a range of 0°C to 100°C. When the mixing temperature is lower than -20°C, there is a tendency that the reaction rate of the oxidizing of the aldehyde is lowered, and when the mixing temperature is higher than 150°C, there is a tendency that selectivity of the

oxidizing of the aldehyde is lowered.

[0091]

The progress of the oxidizing of the aldehyde can be confirmed by an analysis means such as gas chromatography, high performance liquid chromatography, thin layer

chromatography, nuclear magnetic resonance spectrum

analysis, or infrared absorption spectrum analysis.

[0092]

After completion of the oxidizing of the aldehyde, oxygen, carbon dioxide used as necessary, and carbon monoxide generated from carbon dioxide by the reaction can be easily removed as.a gas from the resulting reaction mixture. A carboxylic acid ester can be taken out by, for example, subjecting the resulting reaction mixture to a concentration treatment as necessary and, thereafter, performing a cooling treatment or the like. The carboxylic acid ester taken out can be purified by a purification means such as distillation, column

chromatography, or crystallization.

[0093]

The carboxylic acid ester is a compound represented by the formula (3) (hereinafter, referred to as compound (3) in some cases) is obtained, when the aldehyde is the compound (1) and the alcohol is the compound (4) .

[0094] .

When n in the formula (1) and the formula (3)

represents 0, examples of the compound (3) include methyl benzoate, methyl 4-chlorobenzoate, methyl 2-methylbenzoate, methyl 4-fluorobenzoate, methyl 4-methoxybenzoate, methyl 2-nitrobenzoate, methyl 2 , 4-dichlorobenzoate, methyl 2- naphthalenecarboxylate, methyl 2-pyridinecarb^'oxylate, methyl acetate, methyl propionate, methyl butanoate, methyl pentanoate, methyl hexanoate, methyl heptanoate, methyl octanoate, methyl cyclohexanoate; methyl 4-

(methylthio) butanoate, methyl 3-butenoate, methyl acrylate, methyl 3-phenyl-2-propenoate, ethyl benzoate, ethyl 4- chlorobenzoate, ethyl 2-methylbenzoate, ethyl^" 4- fluorobenzoate, ethyl 4-methoxybenzoate, ethyl 2- nitrobenzoate, ethyl 2, 4-dichlorobenzoate, ethyl 2- naphthalenecarboxylate, ethyl 2-pyridinecarboxylate, ethyl acetate, ethyl propionate, ethyl butanoate, ethyl

pentanoate, ethyl hexanoate, ethyl heptanoate, ethyl octanoate, ethyl cyclohexanoate, ethyl 4- (methylthio) butanoate, ethyl-3-butenoate, - ethyl acrylate, ethyl 3-phenyl-2-propenoate, benzyl benzoate, benzyl 4- chlorobenzoate, benzyl 2-methylbenzoate, benzyl 4- fluorobenzoate, benzyl 4-methoxybenzoate, benzyl 2- nitrobenzoate, benzyl 2 , 4-dichlorobenzoate, benzyl 2- naphthalenecarboxylate, benzyl 2-pyridinecarboxylate , benzyl acetate, benzyl propionate, benzyl butanoate, benzyl pentanoate, benzyl hexanoate, benzyl heptanoate, benzyl octanoate, benzyl cyclohexanoate, benzyl 4-

(methylthio) butanoate, benzyl 3-butenoate, benzyl acrylate and benzyl 3-phenyl-2-propenoate .

[0095]

When n in . the formula (1) and the formula (3)

represents 1, examples of the compound (3) include methyl benzoylformate, methyl 4 -chlorobenzoylformate , methyl 2- methylbenzoylformate, methyl 4-fluorobenzoylformate, methyl -methoxybenzoylformate , methyl 2-nitrobenzoylformate, methyl 2 , 4-dichlorobenzoylformate, methyl 2- naphthoylformate, methyl α-οχο-2-pyridineacetate, methyl pyruvate, methyl 2-oxobutanoate, methyl 2-oxopentanoate, methyl 3-methyl-2-oxobutanoate, methyl a-oxo- cycl.ohexaneacetate, methyl 4- (methylthio) -2-oxo-butanoate, methyl 2-oxo-3-butenoate, methyl 2-oxo-4-phenyl-3-butenoate, ethyl benzoylformate, ethyl 4-chlorobenzoylformate, ethyl .

2-methylbenzoylformate, ethyl 4-fluorobenzoylformate, ethyl 4 -methoxybenzoylformate, ethyl 2-nitrobenzoylformate, ethyl 2 , 4-dichlorobenzoylformate, ethyl 2-naphthoylformate, ethyl α-οχο-2-pyridineacetate, ethyl pyruvate, ethyl_.2- oxobutanoate, ethyl 2-oxopentanoate, ethyl 3-methyl-2- oxobutanoate, ethyl a-oxo-cyclohexaneacetate, ethyl 4- (methylthio) -2-oxo-butanoate, ethyl 2-oxo-3-butenoate, ethyl 2-oxo-4-phenyl-3-butenoate, benzyl benzoylformate , benzyl 4-chlorobenzoylformate, benzyl 2- methylbenzoylformate, benzyl 4 -fluorobenzoylformate , benzyl 4 -methoxybenzoylformate, benzyl 2-nitrobenzoylformate, benzyl 2 , 4 -dichlorobenzoylformate , benzyl 2- naphthoylformate , benzyl α-οχο-2-pyridineacetate , benzyl pyruvate, benzyl 2-oxobutanoate, benzyl 2-oxopentanoate, benzyl 3-methyl-2-oxobutanoate, benzyl a-oxo- cyclohexaneacetate, benzyl 4- (methylthio) -2-oxo-butanoate, benzyl 2-oxo-3-butenoate and benzyl 2-oxo-4-phenyl-3- butenoate.

Examples

[0096]

The present invention will be explained in more detail below by way of examples .

[0097]

Synthesis of 1, 3-bis (2, 4, 6-tribromophenyl) imidazolinium chloride A 300 mL flask replaced with nitrogen was charged with 25 g of 2, 4, 6-tribromoaniline, 200 g of chloroform and 9.2 g of triethylamine . To the resulting mixture was added dropwise 11.5 g of oxalyl -chloride at 0°C over 30 minutes. After the resulting mixture was stirred at 0°C for 2 hours, the_. mixture was further stirred at room temperature for 18 hours. To the- resulting reaction mixture was added 100 g of water, thereby, a crystal was precipitated. Then, after the precipitated crystal was recovered by a filtration procedure, the recovered crystal was washed with 10 g of water and 20 g of diethyl ether and further dried to obtain 20.4 g of a white crystal. It was confirmed from the result of GC-MS analysis that the resulting white crystal- is N, N' -bis (2 , 4 , 6-tribromophenyl ) ethanediamide . Yield: 76%. MS (m/z) : 713 (M+)

[0098]

After a 200 mL stainless autoclave was charged with 10.1 g of N, N' -bis ( 2 , 4 , 6-tribromophenyl ) ethanediamide obtained above and 85 mL of a 1 M solution of

BH₃-tetrahydrofuran, the mixture was heated with stirring at 75°C for 16 hours. After cooled to room temperature, the reaction .liquid was added to a mixed liquid of 170 g of methanol and 8.5 g of 35% hydrochloric acid in portions, and the mixture was stirred. The low boiling substances were distilled off from the resulting mixture, 150 g of methanol was further added to the residue, and the low boiling .substances were distilled off again to obtain 9.1 g of a white crystal. It was confirmed from the result of GC-MS analysis that the resulting crystal is Ν,Ν'- bis (2, 4, 6-tribromophenyl ) -1, 2-ethanediamine hydrochloride. Yield: 89%

MS (m/z) : 685 (M+, free amine)

[0099]

A 200 mL flask replaced with nitrogen was charged with 9 g of N, N' -bis ( 2 , 4 , 6-tribromophenyl.) -1 , 2-ethanediamine hydrochloride obtained above and 100 g of triethyl

orthoformate, and the resulting mixture was refluxed for 1 hour, and cooled to room temperature to precipitate a crystal. Then, after the precipitated crystal was

recovered by a filtration procedure, the recovered crystal was washed with 10 g of tetrahydrofuran and dried to obtain 3.1 g of a white crystal. It was confirmed from the result of ¹H-NMR analysis that the resulting crystal is 1,3- bis [ (2, 4, 6-tribromo) phenyl] imidazolinium chloride. Yield: 32 "6 ·

1H-NMR (δ/ppm, DMSO-d₆, tetramethylsilane standard): 4.66 (s,4H), 8.3 (s,4H), 9.70 (s,lH)

[0100]

A 100 mL stainless pressure-resistant reaction tube equipped with a magnetic rotor was charged with 150 mg of methylglyoxal monohydrate, 35 mg of 3- (2, 6- diisopropyl ) phenyl-4 , 5-dimethylthiazolium chloride, 500 mg of methanol and 3 g of tetrahydrofurari, and the resulting mixture was cooled in a dry ice bath of -70°C while a nitrogen gas was blown into the mixture. After 2 g of dry ice and 6 mg of sodium methylate were added to the cooled mixture, the pressure-resistant reaction tube was sealed.

The resulting mixture was stirred to react at 60°C for 4 hours .

After completion of the reaction, carbon dioxide, and carbon monoxide which had been generated as a byproduct were removed as a gas from the reaction mixture. The resulting reaction mixture was analyzed by a gas

chromatography internal standard method to obtain a yield of methyl pyruvate, and the yield was found to be 10%.

Unreacted methylglyoxal remained in an amount of 50% in the reaction mixture after completion of the reaction.

[0101]

A 100 mL stainless pressure-resistant reaction tube equipped with a magnetic rotor was charged with 100 mg of 4- (methylthio) -2-oxo-l-butanal, 20 mg of 3- (2, 6- diisopropyl) phenyl-4 , 5-dimethylthiazolium chloride, 500 mg of methanol and 3 g of tetrahydrofuran, and the resulting mixture was cooled in a dry ice bath of -70°C while a nitrogen gas was brown into the mixture. After 2 g of dry ice and β mg of sodium methylate were added to the cooled mixture, the pressure-resistant reaction tube was sealed. The resulting mixture was stirred to react at 60°C for 4 hours .

chromatography internal standard method to obtain a yield of methyl 4- (methylthio) -2-oxo-butanoate , and the yield was found to be 50%. Unreacted 4- (methylthio) -2-oxo-l-butanal remained in an amount of 10% in the reaction mixture after completion of the reaction.

[0102]

A 100 mL stainless pressure-resistant reaction tube equipped with a magnetic rotor was charged with 130 mg of phenylglyoxal monohydrate, 36 mg of 1 , 3-bis [ ( 2 , 4 , 6- tribromo) phenyl ] imidazolinium chloride, 300 mg of methanol and 3 g of tetrahydrofuran, and the resulting mixture was cooled in a dry ice bath of -70°C while a nitrogen gas was brown into the mixture. After 2 g of dry ice and 10 mg of a 28% solution of sodium methylate in methanol were added to the cooled mixture, the pressure-resistant reaction tube was sealed.. After the resulting mixture was pressurized to 1 MPa with the air, the mixture was stirred to react at 60°C for 3 hours. After completion of the reaction, the reaction was cooled to room temperature, and thereafter, the pressure was allowed to be reduced back to the normal pressure. The resulting reaction mixture was analyzed by a gas

chromatography internal standard method to obtain a yield of methyl benzoylformate, and the yield was found to be 66%. Unreacted phenyl glyoxal remained in an amount of 6% in the reaction mixture after completion of the reaction.

[0103]

According to the same manner as that of Example 3 except that 18 mg of 3- ( 2 , 6-diisopropyl ) phenyl-4 , 5- dimethylthiazolium chloride was used in place of 36 mg of 1 , 3-bis [( 2 , 4 , 6-tribromo) phenyl ] imidazolinium chloride in Example 3, the experiment was carried out. When a yield of methyl benzoylformate was obtained, the yield was found to be 60%. Unreacted phenylglyoxal remained in an amount of 15%. in the reaction mixture after completion of the

reaction.

[0104]

According to the same manner as that of Example 3 except that 7 mg of 1 , 8-diazabicyclo [ 5 , 4 , 0 ] -7-undecene was used in place of _.10 mg of a 28% solution of sodium

methylate in methanol and 3 g of toluene was used in place of 3 g of tetrahydrofuran in Example 3, the experiment was carried out. When a yield of methyl benzoylformate was obtained, the yield was found to -be.44%. Unreacted

phenylglyoxal remained in an amount of 50% in the reaction mixture after completion of the^" reaction.

[0105]

<Example 6> ^' _r .

A 100 mL Schlenk tube equipped with a magnetic rotor was charged with 130 mg of phenylglyoxal monohydrate, 36 mg of 1, 3-bis [ (2, 4, 6-tribromo) phenyl] imidazolinium chloride, 300 mg of methanol and 3 g of tetrahydrofuran, 10 mg of a 28% solution of sodium methylate in methanol was added to the resulting mixture, and the mixture was stirred to react at 60°C for 3 hours in the air atmosphere.

After completion of the reaction, the reaction was cooled to room temperature, thereafter, the resulting reaction mixture was analyzed by a gas chromatography internal standard method to obtain a yield of methyl benzoylformate, and the yield was found to be 48·%..

Unreacted phenylglyoxal remained in an amount of 34% in the reaction mixture after completion of the reaction.

[0106]

A 100 mL stainless pressure-resistant reaction tube equipped with a magnetic rotor was charged with 100 mg of 4- (methylthio) -2-oxo-l-butanal, 18 mg of 3- (2, 6- diisopropyl) phenyl-4 , 5-dimethylthiazolium chloride, 300 mg of methanol and 3 g of tetrahydrofuran, and the resulting mixture was cooled in a dry ice bath of -70°C while a nitrogen gas was blown into the mixture. After 2 g of dry ice and 10 mg of a 28% solution of sodium methylate in methanol were added to the cooled mixture, the pressure- resistant reaction tube was sealed. The resulting mixture was pressurized to 1 Pa with the air, and the mixture was stirred to react at 60°C for 3 hours.

After completion of the reaction, the reaction was cooled to room temperature, and thereafter, the pressure was allowed to be reduced back to the normal pressure. The resulting reaction mixture was analyzed by a gas

chromatography internal standard method to obtain a yield of methyl 4- (methylthio) -2-oxo-butanoate , and the yield was found to be 20%. Unreacted 4- (methylthio) -2-oxo-l-butanal remained in an amount of 20% in the reaction mixture after completion of the reaction.

[0107]

According to the same manner as that of Example 7 except that 36 mg of 1, 3-bis [ (2, 4, 6- tribromo) phenyl] imidazolinium chloride was used in place of 18 mg of 3- (2, 6-diisopropyl) phenyl-4, 5-dimethylthiazolium chloride in Example 7, the experiment was carried out.

When a yield of methyl 4- (methylthio) -2-oxo-butanoate was obtained, the yield was found to be 57%. Unreacted 4- (methylthio) -2-oxo-l-butanal remained in an amount of 6% in the reaction mixture after completion of the reaction.

[0108]

A 100 mL Schlenk tube equipped with a magnetic rotor was charged with 100 mg of 4- (methylthio) -2-oxo-l-butanal, 36 mg of 1 , 3-bis [ ( 2 , 4 , 6-tribromo ) phenyl ] imidazolinium chloride, 300 mg of methanol and 3 g of tetrahydrofuran, 10 mg of a 28% solution of sodium methylate in methanol was added to the resulting mixture, and the mixture was stirred to react at 60°C for 3 hours under the air atmosphere.

After completion of the reaction, the reaction was cooled to room temperature, thereafter, the resulting reaction mixture was analyzed by a gas chromatography internal standard method to obtain a yield of methyl 4-

(methylthio) -2-oxo-butanoa.te, and the yield was found to be 65%. Unreacted 4- (methylthio) -2-oxo-l-butanal remained in an amount of 4% in the reaction mixture after completion of the reaction.

[0109]

A 100 mL stainless pressure-resistant reaction tube equipped with a magnetic rotor was charged with 150 mg of n-hexylaldehyde, 60 mg of 1 , 3-bis [ (2 , 4 , 6- trimethyl) phenyl] imidazolinium tetrafluoroborate , 500 mg of methanol and 3 g of tetrahydrofuran, and the resulting mixture was cooled in a dry ice bath of -70°C while a nitrogen gas was blown into the mixture. After 2 g of dry ice and 16 mg of sodium methylate were added to the cooled mixture, the pressure-resistant reaction tube was sealed. The resulting mixture was stirred to react at 60°C for 4 hours .

chromatography internal standard method to obtain a yield of methyl hexanoate, and the yield was found to be 2%.

Unreacted n-hexylaldehyde remained in an amount of 95% in the reaction mixture after completion of the reaction.

[0110]

A 100 mL stainless pressure-resistant reaction tube equipped with a magnetic rotor was charged with 150 mg of n-hexylaldehyde, 64 mg of 1 , 3-bis [ ( 2 , 6- diisopropyl ) phenyl ] imidazolium chloride, 500 mg of methanol and 3 g of tetrahydrofuran, and the resulting mixture was cooled in a dry ice bath of -70°C while a nitrogen gas was blown into the mixture. After 2 g of dry ice and 16 mg of sodium methylate were added to the cooled mixture, the pressure-resistant reaction tube was sealed-. The resulting mixture was stirred to react at 60°C for 4 hours. After completion of the reaction, carbon dioxide, and carbon monoxide which had been generated as a byproduct were removed as a gas from the reaction mixture. The resulting reaction mixture was analyzed by a gas

chromatography internal standard method to obtain a yield of methyl hexanoate, and the yield. was found to be 0.5%. Unreacted n-hexylaldehyde remained in an amount of 98% in the reaction mixture after completion of the reaction.

[0111]

A 100 mL stainless pressure-resistant reaction tube equipped with a magnetic rotor was charged with 150 mg of n-hexylaldehyde, 34 mg of 1, 4-dimethyl-lH-l, 2, 4-triazol-4- ium chloride, 500 mg of methanol and 3 g of tetrahydrofuran, and the resulting mixture was cooled in a dry ice bath of - 70°C while a nitrogen gas was blown into the mixture.

After 2 g of dry ice and 16 mg of sodium methylate were added to the cooled mixture, the pressure-resistant

reaction tube was sealed. The resulting mixture was

stirred to react at 60°C for 4 hours.

After completion of the reaction, carbon dioxide, and carbon monoxide which had been generated as a byproduct were removed as a gas from the reaction mixture. The resulting reaction mixture, was analyzed by a gas

chromatography internal standard method to obtain a yield of methyl hexanoate, and the yield was found to be 1.0%. Unreacted n-hexylaldehyde remained in an amount of 98% in the reaction mixture after completion of the reaction.

[0112]

A 100 mL stainless pressure-resistant reaction tube equipped with a magnetic rotor was charged with 150 mg of n-hexylaldehyde, 36 mg of 1 , 3-bis [ ( 2 , 4 , 6- tribromo ) phenyl ] imidazolinium chloride, 300 mg of methanol and 3 g of tetrahydrofuran, and the resulting mixture was cooled in a dry ice bath of -70°C while a nitrogen gas was blown into the mixture. After 2 g of dry ice and 3 mg of sodium methylate were added to the cooled mixture, the pressure-resistant reaction tube was sealed. The resulting mixture was pressurized to 1 MPa with the air, and the mixture was stirred to react at 60°C for 3 hours.

chromatography internal standard method to obtain a yield of methyl hexanoate, and the yield was found to be 1.8%. Unreacted n-hexylaldehyde remained in an amount of 20% in the reaction mixture after completion of the reaction.

[0113]

<Example 11> ■

A 100 mL stainless pressure-resistant reaction tube equipped with a magnetic rotor was charged with 200 mg of n-hexylaldehyde, 35 mg of 3- (2, 6-diisopropyl) phenyl-4, 5- dimethylthiazolium chloride, 900 mg of ethanol and 3 g of toluene, and the resulting mixture was cooled in a dry ice bath of -70°C while a nitrogen gas was blown into the mixture. _ After 2 g of dry ice and 10 mg of triethylamine were added to the cooled mixture, the pressure-resistant reaction tube was sealed. The resulting mixture was pressurized to 0.5 MPa with the air, and the mixture was stirred to react at 60°C for 3 hours.

After completion of the reaction, the reaction was cooled to room temperature, and thereafter, the pressure was allowed to be reduced back to the normal, pressure. The resulting reaction mixture was analyzed by a gas

chromatography internal standard method to obtain a yield of ethyl hexanoate, and the yield was found to be 8%.

Unreacted n-hexylaldehyde remained in an. amount of 53% in the reaction mixture after completion of the reaction.

[0114]

A 100 mL stainless pressure-resistant reaction tube equipped with a magnetic rotor was charged with 180 mg of benzaldehyde, 30 mg of 3- (2 , 6-diisopropyl ) phenyl-4 , 5- dimethylthiazolium chloride, 500 mg of , methanol and 2 g of toluene, and the resulting mixture was cooled in a dry ice bath of -70°C while a nitrogen gas was blown into the mixture. After 2 g of dry ice and 13 mg of 1,8- diazabicyclo [5, 4, 0] -7-undecene were added to the cooled mixture, the pressure-resistant reaction tube was sealed. The resulting mixture was pressurized to 1 MPa with the air, and the mixture was stirred to react at 60°C for 3 hours.

chromatography internal standard method to obtain a yield of methyl benzoate, and the yield was found to be 21%.

Unreacted benzaldehyde remained in an amount of 70% in the reaction mixture after completion of the reaction.^' Industrial Applicability

[0115]

A carboxylic acid ester is a compound important as various chemical products. A carboxylic acid ester such as a ketocarboxylic acid ester is known to be useful also as an intermediate for producing amino acids and the like.

The present invention is industrially applicable as a process for producing. such a carboxylic acid ester.

Claims

CLAIMS [Claim 1] ' A process for producing a carboxylic acid ester, comprising the step of: mixing a compound represented by the formula (2-1) :

[Chemical formula 1]

(wherein R² represents an alkyl group optionally having a substituent or an aryl group optionally having a substituent; R³ and R⁴ each represent independently an alkyl group optionally having a substituent or an aryl group optionally having a substituent, or R³ and^' R⁴ are taken together to form a divalent hydrocarbon group

optionally having a substituent or a group represented by - CH=N- optionally having a substituent; Y represents a group represented by -S- or a group represented by -N(R⁵)-; R⁵ represents an alkyl group optionally having a substituent or an aryl group optionally having a substituent, or R⁵ is taken together with R⁴ to form a divalent hydrocarbon group optionally having a substituent; and X^~ represents an anion) , a base, an alcohol, oxygen and an aldehyde to oxidize the aldehyde.

[Claim 2] The process according to claim 1, wherein the step oxidizing the aldehyde is performed in the presence of carbon dioxide.

[Claim 3]

The process according to claim 1 or 2, wherein the aldehyde is a compound represented by the formula (1) :

[Chemical formula 2]

the alcohol is a compound represented by the formula

(4) :

[Chemical formula 3]

R⁹- OH (4)

(wherein R⁹ represents an alkyl group optionally having a substituent), and

the carboxylic acid ester is a compound represented by the formula (3) :

[Chemical formula 4]

(wherein R¹, R⁹ and n are each as defined above) . [Claim 4]

The process according to any one of claims 1 to 3, wherein the compound represented by the formula (2-1) is a compound represented by the formula (2-2) :

[Chemical formula 5]

(wherein R², Y and X^" are each as defined above; R⁶ and

R⁷ each represent independently a hydrogen atom, an alkyl group optionally having a substituent or an aryl group optionally having a substituent, or R⁶ and R⁷ are taken together . with a carbon atom to which^' they are bound to form a ring, or R⁶ represents a hydrogen atom, an alkyl group. optionally having a substituent or an aryl group optionally having a substituent, and R⁷ is taken together with R⁵ to form a divalent hydrocarbon group optionally having a substituent; and represents a single bond or a double bond) , or a compound represented by the formula (2-3) :

[Chemical formula 6]

(wherein R^' 2 Y and X are each as defined above; and R 7 represents a hydrogen atom,, an.alkyl group optionally having a substituent or an aryl group optionally having a substituent, or R⁷ is taken together with R⁵ to form- a divalent hydrocarbon group optionally having a substituent) .

[Claim 5]

The process according to any one of claims 1 to 4, wherein the base is at least one kind selected from the group consisting of an organic base and an alkali metal alkoxide.

[Claim 6]

A process for producing a carboxylic acid ester, comprising the step of:

formula (2-1) :

[Chemical formula 7]

optionally having a substituent or a group represented by - CH=N- optionally having a substituent; Y represents a group represented by -S- or a group represented by -N(R⁵)-; R⁵ represents an alkyl group optionally having a substituent or an aryl group optionally having a substituent, or R⁵ is taken together with R⁴ to form a divalent hydrocarbon group optionally having a substituent; and X^~ represents an anion) with a base, an alcohol, and oxygen.

[Claim 7]

The process according to claim 6, wherein the step of oxidizing the aldehyde is performed in the presence of carbon dioxide.

[Claim 8]

The process according to claim 6 or 7, wherein the aldehyde is a compound represented by the formula (1) :

[Chemical formula 8]

the alcohol is a compound represented by the formula

(4) :

[Chemical formula 9]

R⁹ OH (4)

(wherein R⁹ represents an alkyl group optionally having a substituent), and

the carboxylic acid ester is a compound represented by the formula ( 3 ) :

[Chemical formula 10]

(wherein R¹, R⁹ and n are each as defined above) .

[Claim 9]

The process according to any one of claims 6 to 8, wherein the compound represented by the formula (2-1) is a compound represented by the formula (2-2) :

[Chemical formula 11] H

R² N÷ X- (2-2)

C-^'^C

R R⁷

(wherein R², Y and X^~ each are as defined above; R⁶ and R⁷ each represent independently a hydrogen atom, an alkyl group optionally having a substituent or an aryl group optionally having a substituent, or R⁶ and R⁷ are taken together with a carbon atom to which they are bound to form a ring, or R⁵ represents a hydrogen atom, an alkyl group optionally having a substituent or an aryl group optionally having a substituent, and R⁷ is taken together with R⁵ to form a divalent hydrocarbon group optionally having a substituent; and represents a single bond or a double bond) , or

a compound represented by the formula (2-3) :

[Chemical formula 12]

(wherein R², Y and X^~ are each as defined above; and R⁷ represents a hydrogen atom, an alkyl group optionally having a substituent or an aryl group optionally having a substituent, or R⁷ is taken together with R⁵ to form a divalent hydrocarbon group optionally having a substituent) .

[Claim 10]

The process according to any one of claims 6 to 9, wherein the base is at least one kind selected from the group consisting of an organic base and an alkali metal alkoxide.