Formation of a stable rhodium(I) dihydride complex and its reactions with prochiral substrates of asymmetric hydrogenation

A new P-chirogenic diphosphine, (S,S)-α,α′-bis(tert-butylmethylphosphino)-o-xylene (1), was synthesized in two steps from optically active secondary phosphine-borane 4 via the corresponding diphosphine-borane 3. The Rh(I) complex (2) of 1 was characterized by X-ray study, which revealed a significantly distorted C₂-symmetry. The catalytic hydrogenation of four representative prochiral substrates gave modest enantioselection. Unlike all previously studied Rh(I) complexes of chiral diphosphines, 2 reacts with dihydrogen at -70°C, producing two diastereomers of a solvate dihydride, 5a,b, in a ratio 1:0.07. The formation of 5a,b is almost quantitative; the hydrogen is not eliminated from 5. At temperatures above 20°C, further transformation of 5 takes place, eventually yielding a bridging binuclear complex 6. The reaction of 5 with methyl Z-(α)-acetamidocinnamate (7) is slow at -80°C, and gradual accumulation of a mixture of monohydride intermediates 15a-d could be monitored by ¹H NMR spectroscopy. When 5 was reacted with dimethyl 1-benzoyloxyethenephosphonate (10), two monohydride intermediates, 19a,b, formed cleanly in a ratio 1:0.04, which corresponded to the 92% ee observed after reductive elimination and quenching the reaction mixture.