GB2265622A - Process for making dialkyl ethylidene dicarbamate - Google Patents

Abstract

Dimethyl ethylidene dicarbamate is formed in high yield by acid catalyzed reaction of acetaldehyde and methyl carbamate. Dialkyl ethylidene dicarbamates can be produced in nearly quantitative yield from alkyl carbamates reacted with acetaldehyde in the liquid phase by precipitating a reaction product from the reactant mixture and rousing the mother liquor from this separation in a subsequent reaction of the same nature or in a continuous process recycling the mother liquor to the reaction zone. The dialkyl ethylidene dicarbamates are useful in the formation of vinyl isocyanates or N-vinyl-O-methyl carbamate which can be polymerized to form a polymer hydrolyzable to an amine-functional polymer.

Description

PROCESS FOR MAKING DIALKYL ETHYLIDENE DICARBAMATE 2265622 This invention

relates to a process for making dialkyl ethylidene dicarbamate in high yield from acetaldehyde and alkyl carbamates. In another aspect it relates to the formation of dimethyl ethylidene dicarbamate using an acid 10 catalyst.

Investigators have known for over fifty years that certain hydrocarbyl carbanates could be condensed with aldehydes to form carbamate derivatives. J. Org. Chem. 6 is pages 87887 (1941) describes the condensation of benzyl carbamate with aliphatic and aromatic aldehydes in which one mole of aldehyde reacts with two molds of benzyl carbamate. Use of benzyl aldehyde in this reaction produced dicarbobenzoxy benzylidene diamine in 68% yield.

An extensive study by Kraft and Herbst published in J. Org. Chem 10 pages 483-497 (1945), describes the condensation of ethyl and higher alkyl carbamates with various aldehydes to form, for example, diethyl ethylidene dicarbamate in modest yields. This compound was made by interacting acetaldehyde with ethyl carbamate. The study included a series of saturated aliphatic carbamates, from ethyl carbamate to higher aliphatic esters, interacted with a group of aliphatic carbonyl derivatives. Condensation 30 was found to take place in the ratio of two moles of carbamate to one mole of carbonyl compound to form dialiphatic bis-carbamates. A few drops of concentrated hydrochloric acid served as the catalyst to initiate the exothermic reaction and the product was recovered by 35 crystallization. No mention is made in this article of methyl carbamate or its derivatives.

A different result appears to have been obtained by Merger, et al., U.S. 4,459,236 (1984) who describe making 1-alkenyl isocyanates by reacting an aldehyde and a carbamate to form N-(l-alkenyl)-carbamate which is then 5 pyrolyzed to the isocyanate. Formation of dialkyl ethylidene dicarbamate is not disclosed. Methyl carbamate is listed as a suitable starting material as is acetaldehyde among many other aldehydes, although there is no indication that acetaldehyde was actually used in the process described. Pyrolysis is carried out in the vapor phase or in a liquid solvent such as an oil boiling between 300 and 40WC.

The work of Kraft and Herbst was recognized in U.S.

4,870,203, Mullins (1989) which describes a,B-saturated geminal dicarbamates prepared by reacting a carbamate with a-B- unsaturated ether, optionally in the presence of an acid catalyst. Example 2 of this patent discloses synthesis of 2,2-N,N-bis(methyl carbamato) propane from methyl carbamate and isopropenyl methyl ether in methylene chloride solvent and p-toluene sulfonic acid as a catalyst. The yield was given as 94%.

Utility for such bis-carbamates in the manufacture of a,B-unsaturated monoisocyanates is disclosed in U.S. 4,572,804, Mullins (1986). The isocyanates are made by pyrolysis of the bis-carbamates with pyrolysis conditions including temperatures from 100 to 600C for 0.5 to 5 seconds. The geminal bis-carbamates are preferably made by reaction between a carbamate and an a-B-unsaturated ether. In Example 2, vinyl isocyanate is made by pyrolyzing dimethyl ethylidene dicarbamate and distilling off vinyl isocyanate. The undistilled material is said to be a mixture of carbamates including N-vinyl-O-methyl carbamate.

U.S. 4,701,549, Mullins (1987) discloses a similar process to the 1804 patent except in Example 2 a byproduct is recovered which is a mixture of N-vinyl-O-methyl carbamate and methyl[(ethenylamino) carbonyl] carbamate. This mixture was copolymerized to form a bonding resin.

There has been considerable interest in recent years in amine-functional polymers which have a variety of uses ranging from dye manufacture to flocculation aids and as strength additives for paper. An attractive route to such amine-functional polymers is the homopolymerization and copolymerization of N-vinylformamide. For example, U.S.

4,490,557, Dawson et al. (1984) describes the reaction of formamide and acetaldehyde in the presence of an acidic catalyst to form ethylidene bis-formamide which can be pyrolyzed to N-vinylformamide.

It would be highly desirable to develop industrially convenient routes to other monomer systems which, like N-vinylformamide, can be homopolymerized or copolymerized with other monomers, such as vinyl acetate, to make polymers which, on hydrolysis, contain amine functionality.

Promising candidates are N-vinyl-o-alkyl-carbamates, particularly N-vinyl-0-methyl-carbamate. This monomer can be obtained selectively by pyrolysis of dimethyl ethylidene dicarbamate according to a process described in a copending patent application of even priority and filing dates. In this process, dimethyl ethylidene dicarbamate is heated to a temperature of 155 to 3500C and at a pressure which permits N-vinyl-O-methyl carbamate to vaporize as it is formed; the vaporized mixture of N-vinyl-O-methyl carbamate and methyl carbamate emanating from the liquid phase is cooled and condensed; and the N-vinyl-O-methyl carbamate recovered. This route to amine-functional polymers has the advantage of using as a starting material methyl carbamate which is readily obtainable by reacting urea and methanol.

Esters of carbamic acid, including ethyl carbamate and methyl carbamate, are described in Chem. Rev. 65 page 567570 (1965). It is stated that these carbamates can form eutectic mixtures with other alkyl carbamates. Reaction of urea with alcohols is said to be the preferred commercial route to a methyl or ethyl carbamate.

According to one aspect of the present invention a process is provided for making dimethyl ethylidene dicarbamate (bis-carbamate) by reacting methyl carbamate with acetaldehyde in solution in a liquid solvent and in the presence of an acid catalyst. This bis-carbamate is useful in that it can be thermally cracked to N-vinyl-0methyl carbamate which in turn can be polymerized and copolymerized. This polymer on hydrolysis contains amine functionality. The invention thereby provides a monomeric alternative to N- vinylformamide. This alternative is particularly attractive industrially because it is based upon the reaction of methyl carbamate which, as pointed out above, can readily be prepared from urea and methanol simply by the application of heat.

In another aspect of the present invention the condensation of acetaldehyde with alkyl carbamates generally is improved significantly in yield by using, as at least a portion of the liquid solvent medium, the filtrate from a prior reaction between an alkyl carbamate and acetaldehyde from which the dialkyl ethylidene dicarbamate has been precipitated and separated. In this process a reaction mixture is formed with an alkyl carbamate in which the alkyl group has from 1 to 10 carbon atoms and acetaldehyde using a liquid solvent. The mixture is subjected to condensation conditions and after a suitable reaction period the mixture is cooled to precipitate a product of dialkyl ethylidene dicarbamate._ The precipitated solid is separated from the liquid solvent medium which contains dissolved alkyl carbamate and this solvent medium is used as at least a portion of the liquid solvent for a further condensation reaction between additional alkyl carbamate and acetaldehyde. This process can be carried out in sequential batch operations or in a continuous process in which the separated liquid solvent medium is recycled to the main reaction zone.

The process of this invention provides dialkyl ethylidene dicarbamate (bis-carbamate) which is useful as a precursor to vinyl isocyanate or, according to the invention described in the copending Application cross-referenced above, to N-vinyl-O-methyl carbamate.

In general, the bis-carbamate is prepared by reacting an alkyl carbamate dissolved in a nondeleterious solvent with acetaldehyde in a reaction zone in the presence of an acidic catalyst and at a temperature above room temperature. The resulting cloudy solution is cooled to precipitate the bis-carbamate which is separated as a solid from the mother liquor. This mother liquor is used as at least part of the solvent in another reaction of the same nature or, in a continuous process, is recycled to the main reaction zone.

Suitable alkyl carbamate contain an alkyl group having from 1 to 10 carbon atoms, such as ethyl carbamate, npropyl carbamate, isopropyl carbamate, n-butyl carbamate, isobutyl carbamate, sec-butyl carbamate, n- amyl carbamate, or isoamyl carbamate, but the preferred carbamate is methyl carbamate which can be prepared by reacting methanol with urea.

Methyl carbamate is also available commercially and the commercial product should be dried under nitrogen to a constant weight at about SO'C. In general, commercial methyl carbamate contains 6 to 20% of a volatile component, and this methyl carbamate has a purity between 82 to 94%. Methyl carbamate, either as received or predried, is added to a solvent which can be any nondeleterious liquid, preferably hydrocarbyl in nature, such as toluene, xylene, cyclohexane and mixtures of such aromatic and aliphatic hydrocarbons. The solvent must be a material in which both acetaldehyde and the carbamate are soluble.

The reaction can be conducted at any combination of temperature and pressure which maintains the reaction mixture in a liquid phase without decomposition of the reactants or product. Typically the temperature is above room temperature but does not exceed 140'C. Pref erred temperatures lie in the range of 40 to 80C. Although the reaction is exothermic, some warming of the mixture, for example to about 50C, is generally needed in order to dissolve the reactants. Atmospheric pressure is suitable but higher pressures can be used if necessary to maintain the reaction in the liquid phase.

An acidic catalyst is used and examples of typical catalysts include organic acids such as benzene sulfonic acid or p-toluene sulfonic acid, mineral acids such as sulfuric acid or hydrochloric acid or polymeric acids such as sulfonated polystyrene resins or perfluorinated ion exchange resins. The preferred catalysts for this process are dilute HC1 and the sulfonated resins.

Acids which are difficult to remove from the bis carbamate, such as soluble organic acids or high boiling inorganic acids like sulfuric acid, may cause the white bis-carbamate product to darken in color when it is heat dried. The acidic resins such as sulfonated polystyrene resin and perfluorinated ion exchange resins offer the advantage that they can be filtered from the reaction 7- product mixture before precipitation of the bis-carbamate.

Dimethyl ethylidene dicarbamate will precipitate from the solution at about SO'C and this product can be produced in high yield, for example about 93%, based on the methyl carbamate. After the bis-carbamate has been precipitated it is separated from the mother liquor either by filtration or centrifugation and the mother liquor is preferably used as at least part of the liquid solvent in a repetition of the same reaction either by sequential batch operation or recycle in a continuous process to the main reaction zone.

The reaction commences on addition of the acid catalyst and reaction time is generally between 5 to 120 minutes. Preferably the reaction time is held to between 5 and 20 minutes before cooling the reaction mixture in order to precipitate the solid product. During the reaction, a slight cloudiness occurs because of the formation of water in the reaction. Precipitation of the.bis-carbamate starts at approximately SO'C and after precipitation is complete the slurry of solid product in the mother liquor is separated, preferably by filtration, and the product is washed with solvent and dried.

Dimethyl ethylidene dicarbamate is a white solid which melts within a 21C range between 128 to 134'C. One characteristic of an incomplete reaction is a narrow melting point falling somewhere within the range of 74 to 124'C. A narrow melting point observed over a 2 to 3 'C range could be misleading, since methyl carbarnate can form eutectic mixtures with other carbamates.

The stoichiometric reaction between acetaldehyde and alkyl carbamate is one mole of acetaldehyde to two moles of carbamate. In the reaction mixture the proportions of reactants should be at least stoichiometric, but can have as much as two or three moles or more of the carbamate to the acetaldehyde. Excess amounts of acetaldehyde can react with the acid catalyst to give colored products. Preferably the proportions are between stoichiometric up to a 20% excess of the acetaldehyde. The proportions of reactants in the reaction mixture can vary broadly, for example 1 to 2% of carbamate up to the solubility limits of the carbamate and acetaldehyde. A catalytically effective amount of the acid catalyst is used and this normally would be in the range of 0.1 to 0.001 equivalents of acid per mole of acetaldehyde.

The following examples are presented to illustrate the invention and should not be construed to limit the is invention. The examples present batch preparations, but the process can be operated continuously with Example 13 illustrating the advantage of recycling or reusing toluene filtrate in the original or subsequent reaction processes. By following this procedure the yield of bis-carbamate based upon the methyl carbamate starting material was almost quantitative.

EXAMPLE 1

The reagent methyl carbamate was 81.3% pure based on vacuum oven drying. In a 100 ml round-bottom flask equipped with a magnetic stirrer and thermometer, methyl carbamate (4.93 g, 65.7 mmol) and toluene (60 ml) were heated to approximately SO'C. A prepared solution of acetaldehyde (1.6 g, 36.4 mmol) in about 10 ml toluene was added to the methyl carbamate/toluene solution.

Concentrated HCl (3.3 mmol) was added and the source of heat immediately removed. The solution was stirred for 60 minutes as it cooled to room temperature. The resulting solids were filtered, washed with toluene and dried in a vacuum oven for 2 hours at 5WC. The white solid weighed 9_ 5.4 g and had a melting point of 128-130'C. The yield was 93% based on charge methyl carbamate. The filtrate was stirred with 0.5 Reillex 425 resin to remove any acid, the resin filtered and volatiles removed to yield 0.4 g solids that melted 90-111'C.

Reillex 425 is a cross-linked poly-4-vinylpyridine marketed by Reilly Tar & Chemical Corp.

EXAMPLE 2-4

The same procedure was followed as in Example 1 except for changes in amount of added HCl. In all cases, additional solids were isolated from the mother liquor, and their melting points ranged from approximately 70 to 11WC indicating a mixture of the bis-carbamate and methyl carbamate. The carbamate mixture was verified by 'H NMR analysis. Yields are given in Table 1.

TABLE 1

Ex. HC1 (mmol) Reaction Time (hr) Durincr Coolincr Yield 2 0.85 1 93 3 0.43 1 93 4 0. 04 1 87 0.4 g of a stock solution: 10-4 mol HCl per gram solution.

EXAMPLE 5-7

The same procedure was followed as in Example 1 except the reaction solution was kept at approximately 5WC for the indicated time and then rapidly cooled to room temperature by an external wet ice bath. Results are shown in Table 2.

TABLE 2

Ex. HC1 (mmol) is 8 Reaction Time (min) % Yield 6 7 0. 06 0.1 0.1 2:.

5 10 87 78 93 0.6 g of a stock solution: 10-4 mol HCl per gram solution 0.1 g of a stock solution: 10-3 mol HCl per gram solution EXAMPLE 8-9

The same procedure was followed as in Example 1 except for changes in the type of acid catalyst. The dried solid was brownish. Results are given in Table 3.

TABLE 3

Ex. Acid (mmol) H,S04 (0 - 1) - 9 p-toluenesulfonic acidH,0 (0. 1) Reaction Time (min) 0.1 g of a stock solution: solution EXAMPLE 10-12 % Yield m.p. range 90-120'C indicating starting material and product m.p. range 100-105,c indicating starting material and product 10-3 mol H,S04 per gram The same procedure was followed as in Example 1 except ion-exchange resins were used as acid catalysts. Results are in Table 4.

TABLE 4

Ex. Acid (mMol) Reaction Time % Yield 10 Amberlyst 15 150 90 (0.94) 11 Amberlyst 15 90 95 (1.9) 12 NR50 (0.88) 30 68 Polystyrene sulfonic acid resin of Rohm and Haas Company.

Perfluorinated ion exchange resin (NAFION by DuPont).

EXAMPLE 13

The reagent methyl carbamate was 91% pure based on vacuum oven drying. In a 250 ml round-bottom flask equipped with a magnetic stirrer and thermometer, methyl carbamate (13.6 g, 181.8 mmol) and toluene (180 ml) were heated to approximately 50'C. A prepared solution of acetaldehyde (4.97 g, 112.9 mmol) in about 30 ml toluene was added to the methyl carbamate/toluene solution.

At approximately 6TC, 0.3 mmol HCl was added at once.

Within 15 minutes. the temperature increased to 671C. After cooling with external wet ice, the solids were filtered and dried. The solids weighed 14.65 g and had a m.p. of 132 134C. The toluene filtrate was used as the solvent in the next run.

To the recycled toluene filtrate., methyl carbamate (13.69 g, 182.5 mmol) was added and the solution temperature increased to approximately 70'C. Acetaldehyde (5.12 g, 116.4 mmol) in 30 ml toluene was added to the solution followed by 0.3 mmol of HCl. After 15 minutes, the solution was cooled to room temperature to yield 17.32 g of solids after drying. The m.p. was 132-13CC.

The combined yield of the bis-carbamate was 99.8%.

In separate UV experiments, it was shown that acetaldehyde stoichiometrically reacts with two moles of methyl carbamate. Therefore, the acetaldehyde selectivity is 100%. The bis-carbamate product had the following spectroscopic characteristics.

'H NMR (6, CDC13): 5.93 (br d, 2 H), 5.20 (sextet, 1H), 3.66 (s, 6 H), 1.46 (d, 3 H).

H NMR (6, D,O/CD3OD): sextet collapses to quartet.

UV (dry methano 1) Xm,x (lg e) [ nm]: 2 0 5 (1. 9) IR (KBr) [cm-']: 3340, 2990, 2950, 1700, 1550, 1240, 1060.

The near quantitative yield of desired product was quite surprising, particularly in view of the lower yield obtained in the first run. In other words, the results of the two runs combined, where the second run used filtrate from the first to make up the liquid phase, were more than additive.

other advantages and features of the invention will be apparent to those skilled in the art from the foregoing disclosure without departing from the scope of the invention.

Claims (19)

CLAIMS:

1. A process for making dimethyl ethylidene dicarbamate which comprises reacting methyl carbamate and acetaldehyde in solution in a liquid solvent in the presence of an acid catalyst.

2. A process as claimed in Claim 1, wherein said catalyst is HCl.

3. A process as claimed in Claim 1, wherein said catalyst is polystyrene sulfonic acid resin.

4. A process as claimed in any one of the preceding is claims, wherein said solvent is hydrocarbon.

5. A process as claimed in any one of the preceding claims, wherein said reaction is conducted at 40 to 800C.

6. A process as claimed in any one of the preceding claims, wherein methyl carbamate and acetaldehyde are present in the range stoichiometric to 200 molar % excess of methyl carbamate.

7. A process as claimed in any one of the preceding claims, further comprising cooling said reaction mixture after 5 to 120 minutes to precipitate a solid product, and separating said solid from remaining liquid reaction mixture.

8. A process as claimed in Claim 7, wherein said remaining liquid reaction mixture is introduced into a reaction zone as at least part of the liquid solvent for methyl carbamate and acetaldehyde in a reaction to form 35 dimethyl ethylidene dicarbamate.

9. A process for making dialkyl ethylidene dicarbamate which comprises is (a) contacting in a reaction mixture an alkyl carbamate in which the alkyl group has from 1 to 10 carbon atoms with acetaldehyde under condensation conditions in a liquid solvent medium, (b) cooling reaction mixture from step (a) to precipitate dialkyl ethylidene dicarbamate product solids, (c) separating said solids from liquid solvent medium containing dissolved alkyl carbamate, and (d) using at least a portion of said liquid solvent medium from step (c) to serve as solvent in further condensation reaction between additional alkyl carbamate as used in step (a) and additional acetaldehyde.

10. A process as claimed in claim 9, wherein said steps (a) and (d) are each batch reactions.

11. A process as claimed in Claim 9, wherein step (a) is a continuous reaction to which said liquid solvent medium of step (d) is recycled.

12. A process as claimed in any one of Claims 9 to 11, wherein said alkyl group is methyl.

13. A process as claimed in any one of Claims 9 to 11, wherein said condensation reactions are conducted in the presence of an acid catalyst.

-is-

14. A process as claimed in claim 13, wherein said catalyst is HCl.

15. A process as claimed in claim 13, wherein said catalyst is polystyrene sulfonic acid resin.

16. A process as claimed in Claim 1 and substantially as hereinbefore described in any one of the Examples.

17. A process as claimed in Claim 9 and substantially as hereinbefore described in Example 13.

18. Dimethyl ethylidene dicarbamate whenever prepared by a process as claimed in any one of the preceding claims.

19. Di(CI-Clo alkyl) ethylidene dicarbamate whenever prepared by a process as claimed in any one of Claims 9 to 15 and 17.