The First Structure Determination of a Diastereomeric Hydrido−Olefin Putative Intermediate in Catalytic Enantioselective Hydrogenation

The active catalyst [Ru(( R )-BINAP)(H)(MeCN) n (THF) 3 - n ](BF 4 ) ( 2; n = 0−2) is generated by hydrogenation (1 atm of H 2, 25 °C, 5 min) of the catalyst precursor [Ru(( R )-BINAP)(MeCN)(1−3:5,6- η -C 8 H 11 )](BF 4 ) ( 3 ) in THF. NMR spectra recorded at −40 °C in THF- d 8 show that the active catalyst exists as a mixture of [Ru(( R )-BINAP)(H)(MeCN)(THF- d 8 ) 2 ](BF 4 ) ( 4; ≈50%), [Ru(( R )-BINAP)(H)(MeCN) 2 (THF- d 8 )](BF 4 ) ( 5; ≈25%), and [Ru(( R )-BINAP)(H)(THF- d 8 ) 3 ](BF 4 ) ( 6; ≈25%). These complexes rapidly exchange MeCN and THF at room temperature. Reaction of the catalyst system 2 with 1 equiv of the substrate ( Z )-methyl α -acetamidocinnamate (MAC) in THF- d 8 at −40 °C over 1 h forms the diastereomeric catalyst−olefin adduct [Ru(( R )-BINAP)(H)(MAC)(MeCN)](BF 4 ) ( si - 7 ) as the major product. The structure and absolute configuration of the hydrido−olefin transient si - 7 were unambiguously determined by low-temperature NMR methods. Complex si - 7 undergoes first-order olefin−hydride insertion to directly generate [Ru(( R )-BINAP)(( S )-MACH)(MeCN)](BF 4 ) ( 1; t 1/2 ≈ 110 min at −20 °C, corresponding to k ≈ 1.0 × 10 - 4 s - 1 ). Complexes si - 7 and 1 both have the same absolute configuration as the major product enantiomer (( R )- N -acetylphenylalanine methyl ester (( R )-MACH 2 )) from the catalytic hydrogenation of MAC using 2 as catalyst.