Catalytic asymmetric hydrogenation of prochiral enamides by rhodium(I) complexes containing the enantiomers of (R*,R*)-(±)-1,2-phenylenebis(methylphenylphosphine) and its arsenic isosteres

Article abstract of DOI:10.1021/om00136a029

Soluble (bicyclo[2.2.1]hepta-2,5-diene)rhodium(I) complexes containing the enantiomers of (R*,R*)-(±)-1,2-phenylenebis(methylphenylphosphine) or their arsenic isosteres have been shown to be highly efficient catalysts for the asymmetric hydrogenation of a variety of prochiral Z-substituted enamide acids and esters, producing α-amino acid derivatives with optical yields as high as 94%. The enantioselectivity of the reaction, however, is remarkably dependent upon the nature of the β-substituent on the enamide-olefin bond. The catalyst containing the bis(tertiary arsine) out performed the corresponding phosphorus compound in several instances. Both ligands form rigid dissymmetric five-membered chelate rings in which the chirality is due solely to a pair of equivalent asymmetric tertiary phosphorus or arsenic donor groups. Hydrogenation of the catalyst precursor bis(tertiary phosphine) complexes in dichloromethane produces crystalline catalytic dimers of the type [Rh₂(diphos)₂](PF₆)₂ that have been assigned structures involving arene bridging on the basis of ³¹P NMR spectroscopy. A ¹H NMR investigation of an isolated enamide complex of the bis(tertiary phosphine) has shown that hydrogenation of the minor diastereomer leads to the major amino acid product, thus supporting the view that it is the relative stabilities of intermediate product diastereomers that determines stereoselectivity in these systems. An unusual dynamic NMR behavior was observed for one of the diastereomers at temperatures below -50°C, which has been rationalized in terms of a restricted rotation of one of the phosphorus-phenyl rings by the carbomethoxy group of the coordinated enamide.