GB1586805A

GB1586805A - Manufacture of alkyl a-formylpropionates

Info

Publication number: GB1586805A
Application number: GB3973777A
Authority: GB
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 1976-09-25
Filing date: 1977-09-23
Publication date: 1981-03-25
Also published as: JPS5340716A; DE2643205A1; FR2365549B1; CH629175A5; FR2365549A1; BE858725A

Abstract

Alkyl alpha -formylpropionates are prepared by hydroformylating acrylic esters. The conditions employed are 80 to 140 DEG C and 180 to 400 bar. The reaction is carried out in the presence of catalysts comprising rhodium complexes. The ligand is a phosphorus compound of the formula III. The symbols in the formula III are defined in Claim 1. <IMAGE>

Description

(54) MANUFACTURE OF ALKYL -FORMYLPROPIONATES (71) We, BASF AKTIENGESELL- SCHAFT, a German Joint Stock Company of 6700 Ludwigshafen, Federal Republic of Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to the manufacture of alkyl a-formylpropionates of the general formula I

where Alk is alkyl of 1 to 18 carbon atoms.

The manufacture of these compounds, which are valuable intermediates for methacrylic acid esters, by a Claisen condensation of formic acid esters and propionic acid esters, has been disclosed, but this procedure is uneconomical because it consumes stoichiometric amounts of an alkali metal alcoholate (Houben-Weyl, volume VII/I, 1954, pages 46 et seq.).

It has also been disclosed to manufacture the compounds I by hydroformylating acrylic acid esters II

where Alk has the above meaning, but these processes are uneconomical in several respects. If cobalt compounds are used as catalysts, the undesired p-formylpropionic acid esters are obtained in the main, and only at the relatively low temperature of about 60"C can the isomer ratio be shifted to about 4:1 in favor of the a-formyl compounds. However, these low temperatures require reaction times which are from about 10 to 15 times as long as those at the conventional temperatures of from 100 to 130"C. In addition, a disproportionate amount-up to 27%-of valueless residue is formed when these long reaction times are used (Bull. Soc. Chem.

Jap., 39 (1966), 2,430 et seq. and Chem.

Ber., 97 (1964), 863 et seq.).

It is true that rhodium catalysts preferentially give the a-formyl isomers at the conventional temperatures, but only in yields of up to 62% (Japanese Laid-Open Application 3,020/64, Netherlands Laid Open Application 65/16,193).

The present invention seeks to provide a more economical process than hitherto for obtained alkyl sg-formylpropionates I.

We have found that an alkyl - formylpropionate of the general formula I

where Alk is alkyl of 1 to 18 carbon atoms, can be obtained in an advantageous manner, generally with substantially improved space-time yields, by hydroformylating a corresponding acrylic acid ester II

at from 50 to 2000C and from 100 to 700 bars in the presence of a rhodiumcontaining catalyst, if the catalyst used is a rhodium complex which contains, as a ligand bound to rhodium, a phosphorus compound of the general formula III P[(O)nR]3 Ill where the radicals R are identical or different alkyl, aromatic aralkyl or cycloalkyl radicals and n is 1 or zero.

Further, we have found that it is particularly advisable to carry out the reaction in the presence of the compound I11 in the free form in a concentration of from 3 to 10 moles per mole of rhodium, in order thereby to counteract the decomposition of the Rh complex.

It is true that the use of phosphorus compounds III in rhodium-catalyzed hydroformylations has been disclosed, though for the purpose of thereby increasing the proportion of the straightchain hydroformylation products, which in the present case would be the a - formylpropionic acid esters (German Laid-Open Applications DOS 1,793,069 and 2,062,703).

However, it is not only for this reason that the success of the process of the invention is noteworthy, but also because it is known that acrylic acid esters and methacrylic acid esters tend to polymerize in the presence of phosphines and phosphites.

The nature of the radical Alk in the starting compounds II depends on the methacrylic acid esters which it is ultimately desired to manufacture; on the other hand, it has no detectable influence on the process according to the invention. The commonest methacrylic acid esters are the methyl ester and the ethyl ester, so that the process is mostly applied to the corresponding acrylic acid esters.

Suitable phosphorus compounds III are above all the cheapest members of this category, namely triphenylphosphine and triphenyl phospite. In general terms, phosphines and phosphites in which phenyl, chlorophenyl, alkylphenyl (with alkyl of I to 3 carbon atoms), alkoxyphenyl (with alkoxy of 1 to 3 carbon atoms), carbalkoxyphenyl, carboxylatophenyl, benzyl, cyclohexyl and alkyl of 1 to 18 carbon atoms as identical or different radicals R are suitable; the total number of carbon atoms should preferably be from 12 to 30. Compounds with a total of fewer than 12 carbon atoms admittedly also serve their purpose in chemical respects, but are somewhat more difficult to remove from the hydroformylation mixture.

Phosphines and phosphites of a total of more than 30 carbon atoms are in most cases more difficult to obtain and hardly offer advantages over triphenyl-phosphine.

The amount of the phosphorus compound III, based on rhodium as the central atom of the active complex, that is present for best results is at least the stoichiometric amount which corresponds to the preferred rhodium complexes HRhCO(PPh3)3 and ClRhCO(PPh3)2 (Ph=phenyl).

Either the finished complex compounds, or the rhodium salts, e.g. RhCl3 or RhCl3. 3H2O, and the phosphorus compounds separately, may be supplied to the hydro-formylation. In the latter case, the active catalysts are formed in situ under the hydroformylation conditions.

In accordance with the conventional hydroformylation technology, the amount of the catalyst is advantageously such that from 0.01 to 0.1 mole of rhodium is present during the reaction per mole of acrylic acid ester II.

An excess of III over the stoichiometric amount for complex formation, i.e. over the amount actually complexed, has an advantageous effect on the yield of I, which can thereby be increased by from 10 to 15%.

This excess, which represents III present in free form, is advantageously from 3 to 10, preferably from 3 to 7 and especially from 3 to 5, moles per mole of rhodium. Smaller excesses produce no significant effect whilst larger excesses can cause side reactions such as polymerization or Michael addition of I to II.

In order to counteract the polymerization of the acrylic acid esters II, it is furthermore advisable to have present a polymerization inhibitor, e.g. hydroquinone, in particular in amounts of from 1 to 10 mmoles per mole of II.

In other respects, the hydroformylation may be carried out batchwise or continuously in accordance with the conventional techniques, i.e. at from 50 to 2000C, preferably from 80 or 85 to 1400C, under a pressure of from 100 to 700 bars, advantageously from 180 to 400 bars, preferably from 200 to 300 bars, using a carbon monoxide/hydrogen mixture, preferably in equimolar or about equimolar proportions of CO:H2.

The presence of an inert solvent, e.g.

cyclohexane, benzene or tetrahydrofuran, is frequently advantageous from the point of view of facilitating the working up of the reaction mixture. Advantageously, from 0.5 to 1.5 liters of the solvent are employed per liter of II.

The working up of the reaction mixture may also be carried out in the conventional manner, preferably by distillation. The catalyst-containing distillation residue thus obtained can be repeatedly recycled to the hydroformylation stage. Since the amount of residue in this way increases in the course of time, a catalyst regeneration, e.g. by oxidatively destroying the residue with nitric acid and isolating the rhodium nitrate from the resulting aqueous solution, is necessary from time to time.

The products I are usually obtained in yields of from 78 to 86%. In addition, from 0.3 to 0.8% of the p-formyl isomers are formed. The cr-formylpropionic acid esters I are intermediates for numerous organic syntheses, above all for the manufacture of methacrylic acid esters, which are obtained by hydrogenating I and dehydrating the product.

EXAMPLE I (a) 320 g (3.7 moles) of methyl acrylate were hydroformylated with an equimolar CO/H2 mixture at 1050C and 280 bars in the presence of 400 g of cyclohexane, 508 mg of triphenylphosphine and 1 g of hydroquinone, with the aid of 704 mg of the complex HRhCO(PPh3)3 (Ph=phenyl).

The conventional working up of the reaction mixture by distillation gave the pure methyl cr-formylpropionate in a yield of 69%. In addition, 0.3% of the p-isomer was produced.

It proved possible to reuse the catalystcontaining residue several times, with virtually the same success.

(b) Using the same conditions, but the equivalent amount of dicarbonyl-rhodium acetylacetonate as the catalyst in place of the phosphine-containing complex, the yield of a-isomer was 46% and the yield of p- isomer 2%.

EXAMPLE 2 A continuously operating experimental unit under a pressure of 280 bars of an equimolar CO/H2 mixture was fed hourly, at from 105 to 1080C, with a solution of 780 g of cyclohexane, 420 g of methyl acrylate, 1.1 g of the rhodium complex mentioned in Example 1 and 100 mg of triphenylphosphine.

The reactor output was first freed from solvent in a column operating under atmospheric pressure, after which the hydroformylated acrylic acid esters were removed in a second column under 50 mbars. The distillation residue was returned to the reactor, as was the cyclo-hexane, after fresh acrylic acid ester had been added to it.

In the course of 10 days, 100 kg of the acrylic acid ester were reacted in this way without having to regenerate the catalyst.

The yield of methyl a-formylpropionate was 83% whilst the proportion of the p-isomer was negligible. Without the addition of excess phosphine, the yield fell in the course of time due to the decomposition of the Rh complex.

- WHAT WE CLAIM IS: 1. A process for the manufacture of an alkyl a-formylpropionate of the general formula I

where Alk is alkyl of 1 to 18 carbon atoms, which process comprises hydroformylating a corresponding acrylic acid ester of the general formula II

the hydroformylation being carried out at a temperature of from 50 to 2000C and a pressure of from 100 to 700 bars and in the presence as catalyst of a rhodium complex which contains, as a ligand bound to rhodium, a phosphorus compound of the general formula III

where R1, R2 and R3 are, each independently, an alkyl, aromatic, aralkyl or cycloalkyl radical and n is 1 or zero.

2. A process as claimed in claim 1, in which the reaction is carried out in the presence of from 3 to 10 moles of a compound of the general formula III in the free form per mole of rhodium.

3. A process as claimed in claim 1 or claim 2 wherein the compound of formula III is triphenylphosphine or triphenylphosphite.

4. A process as claimed in any one of claims I to 3 wherein a mixture of the reactants with a compound of formula III and a rhodium salt reactive with said compound to form the catalyst complex is subjected to hydroformylation, the salt and compound of formula III reacting thereby so as to effect in situ formation of the catalyst complex.

5. A process as claimed in claim 4 wherein the compound of formula III is present in said mixture in an amount of from 3 to 7 moles (per mole of rhodium) in excess of the amount participating in complex formation.

6. A process as claimed in any one of claims I to 5 wherein from 0.01 to 0.1 mole of rhodium is present in the form of the rhodium complex per mole of the acrylic acid ester of formula II used.

7. A process as claimed in any preceding claim wherein the hydroformylation is effected in the presence of a polymerization inhibitor effective to inhibit polymerization of the acrylic acid ester of formula II.

8. A process as claimed in claim 7 wherein the polymerization inhibitor is hydroquinone and is present in an amount of from I to 10 mmoles per mole of acrylic acid ester.

9. A process as claimed in any preceding claim carried out at a temperature of from

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. EXAMPLE I (a) 320 g (3.7 moles) of methyl acrylate were hydroformylated with an equimolar CO/H2 mixture at 1050C and 280 bars in the presence of 400 g of cyclohexane, 508 mg of triphenylphosphine and 1 g of hydroquinone, with the aid of 704 mg of the complex HRhCO(PPh3)3 (Ph=phenyl). The conventional working up of the reaction mixture by distillation gave the pure methyl cr-formylpropionate in a yield of 69%. In addition, 0.3% of the p-isomer was produced. It proved possible to reuse the catalystcontaining residue several times, with virtually the same success. (b) Using the same conditions, but the equivalent amount of dicarbonyl-rhodium acetylacetonate as the catalyst in place of the phosphine-containing complex, the yield of a-isomer was 46% and the yield of p- isomer 2%. EXAMPLE 2 A continuously operating experimental unit under a pressure of 280 bars of an equimolar CO/H2 mixture was fed hourly, at from 105 to 1080C, with a solution of 780 g of cyclohexane, 420 g of methyl acrylate, 1.1 g of the rhodium complex mentioned in Example 1 and 100 mg of triphenylphosphine. The reactor output was first freed from solvent in a column operating under atmospheric pressure, after which the hydroformylated acrylic acid esters were removed in a second column under 50 mbars. The distillation residue was returned to the reactor, as was the cyclo-hexane, after fresh acrylic acid ester had been added to it. In the course of 10 days, 100 kg of the acrylic acid ester were reacted in this way without having to regenerate the catalyst. The yield of methyl a-formylpropionate was 83% whilst the proportion of the p-isomer was negligible. Without the addition of excess phosphine, the yield fell in the course of time due to the decomposition of the Rh complex. - WHAT WE CLAIM IS:

1. A process for the manufacture of an alkyl a-formylpropionate of the general formula I

80 to 1400C and a pressure of from 180 to 400 bars.

10. A process as claimed in any of claims I to 8 carried out at a temperature of from 85 to 1400C and a pressure of from 200 to 300 bars.

11. A process for the manufacture of an alkyl a-formylpropionate carried out substantially as described in either of the foregoing Examples la and 2.

12. An alkyl a-formylpropionate whenever obtained by a process as claimed in any preceding claim.

13. An alkyl methacrylate when obtained from an alkyl a-formylpropionate claimed in claim 12 by hydrogenation and dehydration.