Cinnamic acid catalysts, rhodium complexes

One last remark concerning the two catalysts we have discussed in more detail, cationic rhodium catalysts and the neutral chloride catalyst of Wilkinson. The difference of the catalytic system discussed above from that of the Wilkinson catalyst lies in the sequence of the oxidative addition and the alkene complexation. The hydrogenation of the cinnamic acid derivative involves a cationic catalyst that first forms the alkene complex the intermediate alkene (enamide) complex can be observed spectroscopically. 

Heterogeneisation of chiral rhodium complex of 1,2-diphosphines already known as very efficient catalysts for enantioselective hydrogenation32 was achieved through amine functionality borne by pyrrolidine molecule. The supported Rh complex revealed as its homogeneous counterpart very high enantioselectivity (<90%) in hydrogenation of a-(acetylamino)cinnamic acid and its methyl ester. 

Asymmetric Hydrogenation.—The asymmetric hydrogenation of a-acylamino-acrylates and cinnamates using chiral rhodium(i) diphosphine complexes as catalysts is now established as one of the best methods for obtaining optically pure a-amino-acids (see previous reviews in this series). In the past year, some new chiral diphosphines have been added to the already considerable number of such ligands. A bis(diphenylphosphino)-derivative of pyrrolidine in conjunction with Rh can be used to hydrogenate a-acetamidocinnamates and itaconic acid with chiral inductions of 90%, whereas an Rh -diphos complex derived from natural tartaric acid effects the reduction of some a-acylaminoacrylic acids to natural (5)-a-acylamino-acids with optical yields of between 80 and 100%. ... 

Hydrogenation of alkynes. This cationic complex of rhodium has been found to be superior to other catalysts, including the Lindlar catalyst, for hydrogenation of alkynes to c/y-olefins. Thus CaH5CsCCOOC2Ha is hydrogenated in acetone with this catalyst to ethyl c/i-cinnamate in essentially quantitative yield with no trace of the franj-isomer or of the completely reduced acid. In this particular case, use of Lindlar catalyst results primarily in complete reduction. 

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