WO1995021808A1 - Process for preparing dihydrocinnamic aldehydes - Google Patents

Abstract

Dihydrocinnamic aldehydes may be prepared in high yields and with a good odour by catalytic dehydrogenation of dihydrocinnamic alcohols at moderate temperatures.

Description

 "Process for the preparation of di-hydrozi-mtaldehyden"

Field of the Invention

The invention relates to a process for the preparation of dihydrocinnamaldehydes.

State of the art

Various routes have been described in the literature for the preparation of dihydrocinnamaldehydes (phenylpropanals). For example, US Pat. No. 2,976,321 describes a process in which the hydrochloride aldehydes are prepared by aldol condensation of benzaldehydes with acetaldehyde and other aldehydes together with subsequent partial hydrogenation of the C = C double bond. Vilsmeier syntheses of arylalkyl ketones to cinnamaldehydes are known from DE-A-28 51 024, which can be converted to the corresponding dihydrocinnamaldehydes by subsequent selective hydrogenation.

The processes known from the prior art for the production of dihydrocinnamic aldehydes, however, have considerable process engineering and / or chemical defects in some cases. For example, those who have Benzaldehydes are used as starting materials, the disadvantage that these raw materials are very difficult to access technically. The same applies to the raw materials of Vilsmeier synthesis. In addition, the cinnamaldehydes obtained in the first process step have to be selectively hydrogenated, which can generally only be achieved in moderate yields because of the high reactivity of the aldehyde group.

The starting point of the present invention is therefore the fact that there is a need in the art for alternative processes for the preparation of dihydrocinnamaldehydes.

The present invention was based on the idea of producing dihydrocinnamaldehydes by dehydration of the corresponding dihydrocinnamic alcohols.

The catalytic dehydrogenation of alcohols to carbonyl compounds has long been known to the person skilled in the art. It is also known that this reaction requires relatively high temperatures and is usually carried out above 300 ° C. For example, US 4,891,446 describes the catalytic dehydrogenation of aliphatic C9_i5 alcohols in a fixed bed reactor at temperatures in the range from 375 to 550 ° C. over a Cu / Zn catalyst to give the corresponding aldehydes.

L. Cerveny describes in Perfumer & Flavorist 1991, Ü. _/ 41-43 the production of 3-cyclohexylpropanal. The starting point of the 3-step synthesis sequence is the Prins reaction of styrene with formaldehyde to the corresponding acetal, which is then used in the presence of a copper chromite contact to speaking dihydrocinnamic alcohol (3-phenylpropanol) is opened. The aryl radical of this alcohol is then completely hydrogenated to the cyclohexyl radical with the aid of a nickel catalyst, 3-cyclohexylpropanol being obtained. The latter aliphatic alcohol is finally dehydrated at 250 ° C in the presence of a CuO-ZnO-Al2θ3 catalyst. The yield is given as 58%, the proportion of unreacted 3-cyclohexylpropanol as 41%.

Description of the invention

Due to their olfactory properties, dihydrocinnamaldehydes are of particular interest for the fragrance industry. It is very general that the purity plays a special role when using fragrances. For the production of compounds which could be used as fragrances, it is therefore necessary to contain as little as possible of by-products. This is the only way to ensure that the special olfactory properties are sufficiently effective and that the desired odor profile is affected as little as possible by impurities.

The object of the present invention was to develop a method in which byproduct-poor dihydrocinnamaldehydes are accessible in the most economical manner. The process should also be characterized in that the dihydrocinnamaldehydes are accessible in satisfactory yields. In particular, the new process should be suitable for the production of p-tert-butylpropanal. In view of the known prior art, it was to be expected that catalytic dehydrogenation of dihydrocinnam alcohols to give dihydrocinnamaldehydes would require reaction temperatures above 300 ° C., but at least above 250 ° C. However, it has been observed that under these conditions the formation of by-products takes place to an extent that is unacceptable.

Surprisingly, however, it was found that the reaction takes place at lower temperatures, namely in the range from 180 to 240 ° C., and in particular from 200 to 220 ° C., with the desired high selectivity.

The present invention accordingly relates to a process for the preparation of dihydrocinnamaldehydes of the general formula (I)

R ¹ -C6H3-CH2-CH ₂ -CHO (I)

wherein the radicals R ¹ and R-2 independently of one another hydrogen or linear or branched alkyl radicals having 1 to 6 carbon atoms, which are each arranged in the ortho, meta or para position with respect to the group -CH2-CH2-CHO can, where the corresponding dihydrocinnamic alcohols are catalytically dehydrogenated at a temperature in the range from 180 to 240 ° C.

In a preferred embodiment of the invention, the reaction temperature is set in the range from 200 to 220 ° C. The process according to the invention can be carried out in apparatus made of almost any material. The process can be carried out batchwise or continuously. Because of the possibility of working at normal pressure or reduced pressure, glass apparatus is technically advantageous. The continuous mode of operation in a fixed bed reactor, as is usually used for heterogeneously catalyzed reactions, is particularly preferred.

The continuous implementation of the process according to the invention in a fixed bed reactor ensures to a very special degree that the proportion of by-products is low. The reason for this is to be seen on the one hand in the fact that, in this mode of operation, the volume flow of the dihydrocinnamic alcohols, ie. H. the flow rate of the educt is easily possible to keep the reaction time low; on the other hand, the heterogeneous catalysis used here guarantees that the reaction product is not contaminated with significant amounts of the catalyst.

The catalyst which is used in the reaction zone of the fixed bed reactor is in the form of solid particles (heterogeneous catalyst). The particles can have a wide variety of sizes and shapes, e.g. B. tablets, lumps, cylinders, rods, rings. The size of the particles is not critical in itself. Usually, however, it is adapted to the respective reactor dimensions in such a way that the liquid phase and carrier gas can pass through the reaction zone unhindered and no undesirable pressure drop occurs in this area. Typical suitable particle sizes range from an average diameter of approx. 1 millimeter to approx. 10 millimeters, although larger or smaller particle sizes are not excluded.

A typical laboratory apparatus for carrying out the method according to the invention comprises a fixed bed reactor made of a double-walled glass tube. The inner tube contains the heterogeneous dehydrogenation catalyst and serves as a reaction zone; the space between the shells is used for heating with a liquid medium. The educt can be fed continuously by means of a controllable piston diaphragm pump via heatable feed lines. The hydrogen formed in the course of the reaction can, for. B. remove that the reactor in cocurrent with an inert carrier gas, for. B. nitrogen flushes. The reaction products formed can be left after leaving the reactor z. B. freeze out in a downstream cold trap.

The typical technical equipment for carrying out the method according to the invention, which is described using the example of a laboratory apparatus, can easily be transferred to correspondingly larger technical or production equipment. In principle, all technically customary tube or tube bundle reactors can be used for this.

The cinnamon alcohols to be dehydrogenated can be contained in an inert solvent in the course of the process according to the invention, for example B. a linear or cyclic Kohlenwas¬ serstoff. In a preferred embodiment of the invention, however, the catalytic dehydrogenation is carried out in the absence of a solvent. The choice of the dehydrogenation catalyst per se is not subject to any particular restrictions. From a practical point of view, it is therefore advisable to use commercially available dehydration catalysts. Examples of such commercially available dehydrogenation catalysts are those which contain copper, chromium, nickel, zinc, palladium, platinum, ruthenium alone or in combination with one another. However, it may also be desirable to use dehydrogenation catalysts specially developed for this. The catalysts are usually in lumpy form, e.g. B. as tablets.

In order to avoid further dehydration of the dihydrocinnamaldehydes (I) as effectively as possible, it has proven to be advantageous for the continuous mode of operation in a fixed bed reactor to adjust the volume flow of the educt to LHSV values in the range from 0.3 to 3.0 h " ¹ , in particular 0.5 to 1.2 h ^_1 . At LHSV values significantly below 0.5 h ^{-1 it} has been shown that the content of the reaction mixture in such undesirable by-products which affect the olfactory quality of the compounds (I LHSV value ("liquid hourly space velocity") is the volume flow of the liquid - based on the volume of the solid catalyst - as is usual in the literature.

The volume flow of the inert carrier gas, which as already mentioned is used to Was¬ formed in the course of the reaction serstoffs from the fixed bed reactor to be removed, is in Rah¬ this invention men usually at GHSV values in the range of 200-1000 h ^_1, preferably 250 to 500 h ^-1 set. Below GHSV value ("gaseous hourly space velocity") is included as usual in the literature, the volume flow of the carrier gas - based on the volume of the solid catalyst - should be understood.

In addition, it has been shown that the yield of the dihydrocinnamic aldehydes (I) increases significantly when the catalytic dehydrogenation is carried out under reduced pressure. Pressures in the range from 0.1 to 100 mbar, in particular 1 to 50 mbar, are particularly preferred.

The following examples serve to illustrate the invention and are not to be understood as restrictive.

B e i s p i e l e

1. General

1.1. Attempts at the state of the art

Table 1 (Experiments VI to V6) uses the example of the catalytic dehydrogenation of octanol-1 to octanal to show the knowledge of the person skilled in the art that the dehydrogenation requires relatively high temperatures in order to lead to somewhat acceptable yields . The yield specifications relate to the area percentages determined by gas chromatography. Furthermore mean:

1) LHSV = volume flow of liquid - based on the volume of the catalyst - per hour

2) GHSV = volume flow of the carrier gas - based on the volume of the catalyst - per hour.

The following catalysts were used:

A: Cu chromite catalyst (Engelhard; type "CU-0203T") B: Cu chromite catalyst (Engelhard; type "CU-1808T")

Experiments VI to V6 were carried out in a fixed bed reactor made of a double-walled glass tube. The inner tube had a length of 30 cm and a diameter of approx. 28 mm and a volume of approx. 180 mml. The heterogeneous catalyst A or B was filled in the form of tablets of 4x4 mm in a quantity of 160 g into the inner tube of the reactor, which served as the reaction zone. The space between the shells of the reactor was heated for the purpose of heating flows through the respective desired temperature with a heat transfer oil. The educt (octanol-1) was metered into the reactor in a quantity of 136 g via heatable feed lines by means of an adjustable piston diaphragm pump. The hydrogen formed in the course of the reaction was removed by flushing the reactor with nitrogen as the inert carrier gas. The reaction mixture of octanal-1 (= dehydrogenation product) and octanol-1 (= unreacted starting material) was frozen out in a downstream cold trap after leaving the reactor and then analyzed by gas chromatography. It was found that temperatures around 300 ° C. were necessary in order to achieve satisfactory yields in the range of 75-80%, the yield increasing with temperature.

Table 1: Dehydrogenation of octanol-l ^a )

VI V2 V3 V4 V5 V6

Catalyst AAAABB temperature (° C) 200 270 295 315 270 295 pressure (mbar) 1000 1000 1000 1000 1000 1000 LHSV (h " ¹ ) 1.0 1.0 1.0 1.0 1.0 1.0 GHSV (h " ¹ ) 500 500 500 500 500 500

Octanol-1 69.0 25.7 5.1 11.2 29.3 18.0 Octanal-1 27.0 55.7 74.0 74.8 68.4 80.0

^a ) The information on the product composition in the lower block of the table was determined by gas chromatography (% by area) 1.2. Preparation of the precursors (cinnamon alcohols)

a) 4- (4-t-Butylphenyl) -1,3-dioxane

4- (4-tert-Butylphenyl) -l, 3-dioxane was prepared as follows by Prins reaction of 4-tert-butylstyrene with formaldehyde:

In a 1 1 three-neck reactor, 300 ml of formaldehyde solution (37% in water, stabilized with 10% methanol) and 20 ml of conc. Sulfuric acid (96%) mixed carefully with cooling. 80 g (0.46 mol) of 4-tert-butyl-1-vinyl-benzene (“4-tert-butylstyrene”, from Lancaster, 92% strength) were added to the stirred mixture in one portion under intergas added. The two-phase system was stirred under nitrogen at an internal temperature of 70 ° C. for 24 hours. The conversion achieved afterwards was determined by gas chromatography to be 79%.

Workup: The organic phase was separated and the aqueous phase extracted three times with 150 g of ether. The combined organic phases were washed neutral with sodium bicarbonate solution and then with water, dried over magnesium sulfate and concentrated on a Rotavapor under slightly reduced pressure (700 mbar). The acetal was suctioned off through a glass suction filter and dried in vacuo (0.8 mbar) at 25 ° C. to constant mass. The yield was 50.5 g (47% of theory) of odorless 4- (4-tert-butylphenyl) -1, 3-dioxane (bp 125 ° C / 0.003 mbar; GC purity 94.7% ) Analysis: The IR spectrum (melt between NaCl, Perkin-Elmer 983) showed absorption bands 2958, 2844, 1474, 1463, 1403, 1363, 1246, 1205, 1193, 1173, 1114, 1085, 1031, 985, 901, 838 and 820 cm " ¹ .

b) 3- (4-t-Butylphenyl) propan-1-ol

3- (4-tert-Butylphenyl) propan-l-ol was prepared from 4- (4-tert-butyl) -l, 3-dioxane by reductive cleavage as follows:

50 g (0.22 mol) of 4- (4-tert-butylphenyl) -1, 3-dioxane (prepared as described under 1.2.a), 100 g of anhydrous ethanol and 2.5 g were placed in a 500 ml steel autoclave insert Ni 55/5 TS (commercially available nickel hydrogenation catalyst from Ruhrchemie / Hoechst) mixed. After purging with nitrogen, 50 bar of hydrogen were initially added at room temperature, and after a heating phase of one hour at 150 ° C., hydrogen was added to 150 bar. After 2 hours the mixture no longer took up hydrogen.

Work-up: After cooling and releasing the pressure, the contents of the autoclave were filtered and the solvent was distilled off. The remaining 50 g residue was distilled in a high vacuum over a 12 cm Vigreux column. The main amount of 45 g went over at head temperatures of 80-88 ° C./0.07 mbar and was fractionated on a rotating belt column. 40.8 g main fraction with a nice, but not very intense, floral smell passed at head temperatures of 88 - 102 ° C / 0.1 mbar; the GC purity was 75.6% (the yield corresponds to approximately 73% of theory). Analysis: The IR spectrum (film between NaCl, Perkin-Elmer 983) showed absorption bands at 3343, 2962, 2866, 1510, 1474, 1403, 1392, 1363, 1268, 1172, 1109, 1041 and 833 cm ~ l.

2. Embodiments

example 1

34.9 g (0.137 mol) of 3- (4-tert-butylphenyl) propan-1-ol (prepared as described under 1.2.b.) Were prepared in a nitrogen stream (GHSV = 500 h ^-1 ) on a laboratory continuous system ))) on a commercially available copper chromite contact (type "G-99B-0", Südchemie AG) at flow rates LHSV = 0.5 h ^-1 and at a temperature of 220 ° C to 3- (4-tert. - Butylphenyl) propanal dehydrated. The dehydrogenation resulted in a conversion of 78% after 24 hours. The crude product of 32.5 g was distilled at a top temperature between 93-97 ° C / 0.03 mbar on a 40 cm Vigreux column. This gave 19.2 g of an odor-acceptable product (GC purity: 97%).

Claims

Patent claims 1 . Process for the preparation of dihydrocinnamaldehydes of the general formula (I) R ¹ -C6H3-CH2-CH2-CHO (I) I R2 wherein the radicals R and R-2 independently of one another hydrogen or linear or branched alkyl radicals having 1 to 6 carbon atoms, which are each arranged in the ortho, meta or para position with respect to the group -CH2-CH2-CHO be, characterized in that the corresponding dihydrocinnamic alcohol is catalytically dehydrogenated at a temperature in the range from 180 to 240 ° C. 2. The method according to claim 1, wherein the dehydrogenation is carried out at a temperature in the range from 200 to 220 ° C. 3. The method according to claim 1 or 2, wherein one carries out the dehydration in a continuous process in a fixed bed reactor. 4. The method according to any one of claims 1 to 3, wherein one carries out the dehydrogenation in the absence of a solvent. 5. The method according to any one of claims 1 to 4, wherein adjusting the volume flow of the carrier gas - based on the volume of the dehydrogenation catalyst - in the range from 200 to 1,000 h ^_1 (GHSV value). 6. The method according to any one of claims 1 to 5, wherein in the general formula (I) R ^{1 is} tert-butyl and R2 is hydrogen, with the proviso that the radical R ¹ in relation to the group -CH2 -CH2-CHO is arranged in the para position. 7. The method according to any one of claims 1 to 6, wherein the corresponding dihydrocinnamic alcohols to be used for the preparation of the dihydrocinnamaldehydes (I) are prepared in that first i) the appropriately substituted styrene is converted into the acetal by Prins reaction with formaldehyde and this ii) opens to the dihydrocinnamic alcohol in the presence of a catalyst. 8. Use of the dihydrocinnamaldehydes (I) prepared by the process according to one of claims 1 to 7 as fragrances.