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 C 2 -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 1 H 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.