Hydrogenation of derivatives of 2-oxo-4-(2-thienyl)butenoic acid at palladium and nickel catalysts

Abstract

The hydrogenation of sodium and ethyl 2-oxo-4-(2-thienyl)butenoate at Raney nickel and pai~dium black was investigated. With Raney nickel the reaction products were the corresponding derivatives of 2-oxo-4-(2-thienyl)butyric acid and 2-hydroxy-4-(2-thienyl)butyric acid and the products from bimolecular condensation.If palladium black is used, the reaction can be directed toward the selective formation of the corresponding derivative of 2-oxo-4-(2-thienyl) The derivatives of 4-substituted 2-oxobutyric acids are important synthons for the production of antihypertensive products [1], homoamino acids, hydroxamic acids, and other compounds [2]. In order to obtain these synthons we investigated the hydrogenation of derivatives of the 2-oxo-4-phenylbutenoic acid series at nickel and palladium catalysts [2][3][4]. During the hydrogenation of sodium 2-oxo-4-phenylbutenoate at nickel catalysts the corresponding salt of 2-hydroxy-4-phenylbutyric acid is obtained with yields of up to 94% [3]. At palladium catalysts a mixture of the salts of 2-hydroxy-4-phenylbutyric and 2-oxo-4-phenylbutyric acids is formed [4, 5]. Under strictly defined conditions the process can be directed toward the selective formation of sodium 2-oxo-4-phenylbutyrate. The 2-oxo-4-phenylbutenoic esters are hydrogenated at a higher rate but less selectively with respect to the formation of the corresponding saturated oxo compound than the corresponding sodium salt.During the hydrogenation of sodium 2-oxo-4-phenyl-(2-furyl)butenoate at Raney nickel catalyst the corresponding salts of 2-oxo-4-(2-furyl)butyric and 2-hydroxy-4-(2-furyl)butyrie acids and aliphatic compounds from the hydrogenolysis of the initial compound are formed [5]. Sodium 2-hydroxy-4-(tetrahydrofuryl)butyrate and a condensation product [a derivative of 3-(2-furylmethyl)-4-(2-furyl)-2-oxo-3,4-didehydroglutaric acid] were also detected in the products.Comparison of the hydrogenation of the derivatives of 4-substituted furyl-and phenylbutenoic acids at Raney nickel catalysts indicates that the formation of sodium 2-oxo-4-(2-furyl)butyrate (yield 66%) is highly selective compared with the hydrogenation of the corresponding phenyl derivative, with which sodium 2-hydroxy-4-phenylbutyrate is mainly formed. The increased selectivity for the formation of the salt of the c~-keto acids of the furan series is probably due to complexation between the compound and the nickel.In the present communication we present data on the hydrogenation of sodium and ethyl 2-oxo-4-(2-thienyl)butenoate at Raney nickel and palladium black. There is no information in the literature on the hydrogenation of these compounds. The hydrogenation was carried out at 17-80~ with the compound at an initial concentration of 0.18-0.21 M in water-alcohol solution and a compound-Raney nickel (wet) ratio 1:1 and a compound--palladium black (wet) ratio 1:0.86 (Table I). The hydrogenation products contained sodium 2-oxo-4-(2-thienyl)butyrate and 2-hydroxy-4-(2-thienyl)butyrate and condensation products. In the esterification products b...