Silk fibroin -palladium catalysts

Heterogeneous catalysts modified by the addition of chiral substances have been used to hydrogenate olefins asymmetrically, but only a few effective chiral heterogeneous catalyst systems have been found. Palladium deposited on silk fibroin was used to hydrogenate 4-benzylidene-2-methyl-5-oxazolone asymmetrically to give, after hydrolysis, optically active phenylalanine (Fig. lc). The optical purity1 of the product was found to be dependent on the origin of the fibroin and its chemical pretreatment (4-6). 

Raney nickel has been modified with amino acids and other chiral reagents to give catalysts that have been used to effect asymmetric reductions (7). However, these catalysts suffer from some of the same kinds of vagaries that have been observed for the palladium on silk fibroin catalysts. For example, the optical purities of the products were found to be very dependent on pH and the method of catalyst preparation. 

Enantioselective heterogeneous catalytic hydrogenation using a chiral catalyst was pioneered by Aka-bori and Izumi, who prepared a palladium catalyst supported on silk fibroin. The oxime acetates of diethyl a-ketoglutarate or of ethyl phenylpyruvate were hydrogenated to form glutamic acid (7-15% ee) and phenylalanine (30% Similarly, a palladium-poly-L-leucine catalyst was used for the asym-... 

The active site of the catalyst has been considered to be metallic palladium deposited in the -structure of silk fibroin as an a mmetric chelate. However, as a result of the examination of the reproducibility of these atymmetric hydrogenation reactions, the asymmetric modification Cpommii ) of palladium by S-a-amino acid was suspected, the latter being concomitantly formed by the l rdrolysis of silk protein with hydrochloric acid from palladium(II) chloride in the course of the catalyst preparation (4). 

Alternatively, the surface of a metal can be modified with an enantiomerically pure additive. For example, in 1956 palladium metal/silk fibroin was used for the hydrogenation of alkenes with moderate enantioselectivity. However, most success with modified metal siufaces has been achieved in the hydrogenation of ketones. The reduction of 3-keto esters with a Raney nickel/tartaric acid/sodium bromide catalyst provides good enantioselectivities. For example, P-keto ester (3.108) affords the P-hydroxy ester (3.109). Platinum metal modified with cinchona alkaloids has been successfully used with a-keto esters. The reaction yields product with up to 90% ee in the reduction of a-keto ester (3.110). ... 

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