The mechanism of asymmetric homogeneous hydrogenation. Rhodium complexes formed by dehydroamino-acids co-complexed with trans-4,5-bisdiphenylphosphino-2,2-dimethyldioxolan and achiral models

Abstract

Enamiderhodium complexes formed by six- and seven-ring chelate-forming biphosphines have been investigated by 31P and 13C n.m.r. spectroscopy. Compared with five-ring chelate complexes discussed earlier, these generally undergo much readier dynamic exchange and possess lower binding constants. Their structure has been determined by observing chemical shifts and coupling constants in the 13C n.m.r. spectrum of suitably labelled dehydroamino-acids. Derivatives of Z-dehydroamino-acids are co-ordinated to rhodium through olefin and amide (with possible intervention of the carboxylate group at low temperatures) whereas derivatives of E-dehydroaminoacids are co-ordinated to rhodium through olefin and carboxylate groups. Two anomalous complexes have been observed. With an excess of Z-acetamidocinnamic acid complexes of 2 : 1 olefin: rhodium were observed, particularly at low temperatures with the rhodium solvate of 1,3-bisdiphenylphosphinopropane. E-Benzamidocinnamic acid in deficiency to the rhodium solvate of 1,4-bisdiphenylphosphinobutane forms a binuclear complex where the enamide may act as a bridging ligand.