AU629978B2 - Process for the preparation of methyl 2-formylbutyrate - Google Patents

Abstract

The invention relates to a method for the production of methyl 2-formylbutyrate by hydroformylation of methyl crotonate in the presence of catalysts which contain rhodium and organic phosphines.

Description

COMMONWEALTH OF AUSTRAL 9 97 Form PATENTS ACT 1952-69 COMPLETE SPECIFICATION

(ORIGINAL)

Class Int. Class Application Number: Lodned: Complete Specification Lodged: Accepted: Published: Priority Related Art Name of Applicant Address of Applicant Actual Inventor Address for Service HOECHST AKTIENGESELLSCHAFT 50 Bruningstrasse, D-6230 Frankfurt/Main 80, Federal Republic of Germany JURGEN WEBER, PETER LAPPE and HELMUT SPRINGER WATERMARK PATENT TRADEMARK ATTORNEYS.

LOCKED BAG NO. 5, HAWTHORN, VICTORIA 3122, AUSTRALIA Complete Specification for the invention entitled: PROCESS FOR THE PREPARATION OF METHYL 2-FORMYLBUTYRATE The following statement is a full description of this inventfon, including the best method of performing it known to us -rr~t~uC -la- Process for the preparation of 'methyl 2-formylbutyrate The invention relates to a process for the preparation of methyl 2-formylbutyrate by reaction of methyl crotonate with hydrogen and carbon monoxide.

2-Formylbutyric acid esters are useful intermediate products in chemical syntheses. They can be converted by reduction into hydroxy compounds, which are used, for example, for the preparation of polyesters. Reaction with ammonia and hydrogen gives esters of aminomethylbutyric acids, and oxidation of the formylbutyric acid esters *leads to half-esters of dicarboxylic acids.

S. The hydroformylation of crotonic acid esters always leads to product mixtures which essentially contain 3- and 4-formylbutyric acid esters and, by hydrogenation of the crotonic acid ester, also butyric acid esters. The reaction has been investigated repeatedly.

seee Adkins et al. in J.Am.Chem.Soc. 71 (1949), page 3051 et seq. thus report on the reaction of ethyl crotonate with S 20 watergas (CO H 2 1 1) at 120 to 125 0 C under a total pressure of 200 to 300 atmospheres in the presence of cobalt as a catalyst and benzene as the reaction medium.

The reaction gives ethyl 3-formylbutyrate in a yield of 71%.

According to Piacenti et al., Ullmanns Encyklopddie der technischen Chemie (Ullmann's Encyclopedia of Industrial Chemistry) 62, Volume 13, page 65, ethyl 3- and 4formylbutyrate are nbtained in a ratio of 15 15 70 in yield from ethyl crotonate, evidently in the presence of a CO catalyst.

Falbe et al. have published the results of the hydroformylation of ethyl crotonate in the presence of 1% by weight of Rh 2 03 in a two-stage reaction in Brennstoff L I'' l :r 'lai^flfUWs' 2 Chemie 48 (1967), page 46 et seq. Thus, a reaction mixture which essentially consists of p-methyl-S-butyrolactone, ethyl butyrate, &-valerolactone and ethyl ohydroxymethylbutyrate thus results at 135 0 C under a pressure of 200 atmospheres in the first stage and at 200 C under a pressure of 300 atmospheres in the second stage.

Lai and Ucciani (Adv.Chem.Ser. 1974, (132), page 1 et seq.) hydroformylated methyl crotonate under various conditions and found that the selectivity of the reaction decreases if rhodium is used as the catalyst in comparison with the cobalt-catalyzed reaction. The selectivity becomes even lower if rhodium is used together with triphenylphosphine. The main product of the reaction .15 catalyzed by cobalt is methyl 4-formylbutyrate. Rhodium alone gives the 3-formyl compound as the main product, and rhodium with triphenylphosphine gives predominantly the 2-formyl compound, but the selectivity of the reaction is completely unsatisfactory when rhodium catalysts 20 are used.

According to Tanaka et al., Bull.Chem.Soc.Jap. 50 (1977), 2351 et seq., the product distribution on hydroformylation of methyl crotonate in the presence of rhodium depends very greatly on the nature of the ligands used.

25 If triphenylphosphine is used as the ligand, 3-formylbutyric acid ester is formed as the main product in a moderate yield, and the 2-formyl compound is formed only in a minor amount. Using (H 5

C

6 2

P(CH

2 )4P(C 6

H

5 2 as the ligand, ethyl 2-formylbutyrate is preferably obtained.

Finally, Okano et al., Bull.Chem.Soc.Jap. 54 (1981), 3799 et seq. found that mainly butyric acid and in addition small amounts of methyl 2- and 3-formylbutyrate are formed in the reaction of methyl crotonate with water in the presence of rhodium-phosphine complex compounds as catalyste on the basis of the watergas equilibrium which is established. This reaction route also does not offer 3 the possibility of an economically satisfactory preparation of methyl 2-formylbutyrate on an industrial scale.

There was therefore the object of developing a process which allows hydroformylation of methyl crotonate to give methyl 2-formylbutyrate in a high yield with a high selectivity using readily accessible catalysts.

This object is achieved by a process for the preparation of methyl 2-formylbutyrate by hydroformylation of methyl crotonate in the presence of catalysts containing rhodium 10 and organic phosphines. It comprises carrying out the ;reaction at temperatures of 80 to 120°C, under pressures of 20 to 30 MPa, with a rhodium concentration of 10 to 500 ppm, based on the methyl crotonate employed, and in an organic solvent as the reaction medium.

h thyl crotonate is employed as the starting compound for the preparation of methyl 2-formylbutyrate. It can be used in the pure form, i.e. in the distilled form.

However, it has been found that crude products which also contain other constituents in addition to methyl crotonate can also be employed without a disadvantage to the yield and selectivity. Thus, for example, mixtures which are obtained on esterification of crotonic acid with methyl alcohol and contain 40 to 60% by weight of methyl crotonate, 30 to 40% by weight of methanol and 5 to 25 by weight of water are successfully used directly as the starting substances.

Carbon monoxide and hydrogen are in general employed in the form of synthesis gas, which is obtained.by partial oxidation of carbonaceous material in the presence of water. It contains carbon monoxide and water in a ratio of about 1:1. However, mixtures in which one of the components is present in excess can also successfully be used as reaction partners. In practice, CO/H 2 mixtures which contain 0.8 to 1.2 mol of CO per mol of H 2 have proved to be suitable. It goes without saying that 4 impurities which may lead to poisoning of the catalyst must be removed from the gas. Thus, for example, sulfur can be tolerated only up to a maximum concentration of about 2 ppm.

The starting substances are reacted in the presence of a catalyst system comprising rhodium and an organic phosphine. The concentration of the rhodium, based on the methyl crotonate originally employed, is 10 to 500 ppm, preferably 50 to 200 ppm and in particular 80 to 120 ppm.

The rhodium is employed as the metal, advantageously in finely divided form, or as a compound. In practice, Rh 2ethylhexanoate has proved to be a particularly suitable starting substance for the catalyst. The second constituent of the catalyst system is an organic phosphine. These 15 are understood as being alkyl- and arylphosphines. Trin-butylphosphine and triphenylphosphine have proved to be particularly suitable. 1 to 50, in particular 2 to 20 mol of phosphine are used per g-atom of rhodium. The catalyst system can be added to the reaction mixture in finished 20 form. In this case it is obtained in its own process step independent of the actual reaction. However, the catalyst S can also be prepared in situ, i.e. in the reaction mixture under hydroformylation conditions, with an equally good result.

25 It is an essential feature of the invention that certain pressure and temperature ranges are observed during the hydroformylation, i.e. 80 to 120 0 C and 20 to 30 MPa. It Sis advantageous to use temperatures of 90 to 110°C, and in particular 90 to 100oC, and pressures of 200 to 300 bar. Temperatures higher than those mentioned above lead to increased hydrogenation, and lower temperatures lead to a reduction in the conversion.

Finally, the novel process comprises the use of organic solvents as the reaction medium. Aliphatic hydrocarbons have proved to be particular]- suitable, and cyclohexane is preferably employed. The proportion of the reaction L

ILLI~

medium in the reaction mixture can vary within wide ranges and can be from 20 to 80% by weight. About 50% by weight are preferred.

The process according to the invention can be carried out discontinuously or continuously. In the case of a discontinuous procedure, the ester, the reaction medium and the catalyst, which is formed beforehand or employed in the form of its components, are introduced into the reactor and the desired pressure is established by forcing in carbon monoxide and hydrogen, while heating at the same time. It is advisable to stir the reaction mixture if adequate mixing is not already achieved by the introduc- 1 tion of carbon monoxide and hydrogen. Synthesis gas is fed to the reactor at the rate at which it is consumed by 15 the reaction. The reaction has ended as soon as no further uptake of gas occurs.

The reaction is preferably carried out continuously. In this case, synthesis gas, methyl crotonate and reaction medium are introduced continuously into the reactor containing the catalyst system. The reaction product and some of the reaction medium are removed continuously.

A

S•stationary state is maintained in the reactor by appropriate adjustment of the feed rate of the reaction partners and the removal rate of the products. Interruption 25 of the reaction only becomes necessary when the catalyst stability has decreased significantly. This point in time can be delayed by replacing some of the spent catalyst by fresh catalyst from time to time.

The methyl 2-formylbutyrate prepared by the process claimed is purified in a known manner by distillation.

The yields are more than 85%. In addition to methyl 2formylbutyrate, the crude product of the hydroformylation contains about 2 to 5% of methyl 3-formylbutyrate and to 10% of methylbutyrate.

The novel process is described in more detail in the i e i 6 examples which follow.

Example 1 500 g of methyl crotonate, 500 g of cyclohexane, 0.6 g of triphenylphosphine and 100 ppm of Rh (as Rh 2-ethylhexanoate) are initially introduced into a 2 1 autoclave with a piston stirrer. The mixture is allowed to react at under a CO/H 2 pressure of 27 MPa for about 8 hours, the contents of the autoclave are then cooled and the reaction mixture is analyzed by gas chromatography.

i 10 It has the following composition (in by weight, without taking into account cyclohexane): Smethyl butyrate 4.23 methyl crotonate 0.59 methyl 2-formylbutyrate 91.67 methyl 3-formylbutyrate 2.05 other substances 1.46 "I Example 2 4* 4 250 g of methyl crotonate, 250 g of cyclohexane, 0.83 g of triphenylphosphine and 100 ppm of Rh (as Rh 2-ethylhexanoate) are initially introduced into a 1 1 autoclave with a piston stirrer. The mixture is allowed to react at 90 0 C under a CO/H 2 pressure of 27 MPa for about 7 hours, the contents of the autoclave are then cooled and the f 0 reaction mixture is analyzed by gas chromatography.

It has the following composition (in by weight, without taking into account the cyclohexane): methyl butyrate 5.23 methyl crotonate 0.11 methyl 2-formylbutyrate 88.82 methyl 3-formylbutyrate 4.21 other substances 1.63 L 7 Example 3 250 g of methyl crotonate, 250 g of cyclohexane, 0.83 g of triphenylphosphine and 100 ppm of Rh (as Rh 2-ethylhexanoate) are initially introduced into a 1 1 autoclave with a piston stirrer. The mixture is allowed to react at 100°C under a CO/H 2 pressure of 28 MPa for about 7 hours, the contents of the autoclave are then cooled and the reaction mixture is analyzed by gas chromatography.

It has the following composition (in by weight, without 10 taking into account the cyclohexane): ,methyl butyrate 7.11 methyl crotonate 0.08 methyl 2-formylbutyrate 87.55 methyl 3-formylbutyrate 4.01 other substances 1.25 oo% 0.00o0 r,