Benzil asymmetric hydrogenation

The asymmetric hydrogenation of C—O bonds have now been achieved in optical yields up to 95%, rivalling the performance of alkenes. Here also, rhodium complexes have been used almost exclusively, but some success has been obtained with cobalt catalysts. Using [Co(HDMG)2] in presence of optically active bases, benzil could be reduced to benzoin (equation 54) in an optical yield of 78%. Quinine or quinidine were the chiral bases employed. The best optical yields were obtained with quinine (60). It was found that when benzylamine was also present, the rate of hydrogenation was greatly enhanced without any decrease in the optical yield.276... 

Diketones Asymmetric hydrogenation of a-diketones is rare. However, benzil is hydrogenated with a quinine-benzylamine-Co(dmg)2 catalyst system to give (S)-benzoin with... 

Taking advantage of these findings, a catalytic asymmetric hydrogenation of ketones has been effected [27]. It should be noted in this connection that asymmetric reduction of benzil to benzoin has been found to occur with Z>is(dimethyl-glyoximato)cobalt(II)-quinine system as a catalyst [28]. 

Only a few other cobalt complexes of the type covered in this review (and therefore excluding, for example, the cobalt carbonyls) have been reported to act as catalysts for homogeneous hydrogenation. The complex Co(DMG)2 will catalyze the hydrogenation of benzil (PhCOCOPh) to benzoin (PhCHOHCOPh). When this reaction is carried out in the presence of quinine, the product shows optical activity. The degree of optical purity varies with the nature of the solvent and reaches a maximum of 61.5% in benzene. It was concluded that asymmetric synthesis occurred via the formation of an organocobalt complex in which quinine was coordinated in the trans position (133). Both Co(DMG)2 and cobalamin-cobalt(II) in methanol will catalyze the following reductive methylations ... 

More successful asymmetric reductions have been based on amine (particularly alkaloid) complexes of bis(dimethylglyoximato) cobalt(II), also known as cobaloxime(II) and represented Co(dmg)2 (compound VII). Cobaloxime-chiral amine complexes have been used to catalyze the hydrogenation of both olefinic and ketonic substrates (Fig. 24). It has been determined that hydroxyamine modifiers, for example, alkaloids such as quinine, quinidine, and cinchonidine, are most effective. The highest optical purity obtained thus far has been 71%, observed for reduction of benzil in benzene solution at 10° using quinine as the... 

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