A New Family of Chelating Diphosphines with a Transition Metal Stereocenter in the Backbone: Novel Applications of “Chiral-at-Rhenium” Complexes in Rhodium-Catalyzed Enantioselective Alkene Hydrogenations

Abstract

The title compounds are accessed by sequences starting with racemic and enantiomerically pure [(η5-C5H5)Re(NO)(PPh3)(CH3)]. Reactions with chlorobenzene/HBF4, PPh2H, and tBuOK give the phosphido complex [(η5-C5H5)Re(NO)(PPh3)(PPh2)] (3). Reactions with Ph3C+BF4−, PPh2H, and tBuOK give the methylene homologue [(η5-C5H5)Re(NO)(PPh3)(CH2PPh2)] (9). Treatment of 3 or 9 with nBuLi or tBuLi and then PPh2Cl gives the diphosphido systems [(η5-C5H4PPh2)Re(NO)(PPh3)((CH2)nPPh2)] (n=0/1, 5/11). Reactions of 5 and 11 with [Rh(NBD)Cl]2/AgPF6 (NBD=norbornadiene) give the rhenium/rhodium chelate complexes [(η5-C5H4PPh2)Re(NO)(PPh3)((μ-CH2)nPPh2)Rh(NBD)]+ PF6− (n=0/1, 6+/12+ PF6−; 30–32 % overall from commercial Re2(CO)10). The crystal structures of 6+ PF6− and 12+ PF6− are compared to those of 3 and 9, and other rhodium complexes of chelating bis(diphenylphosphines). The chiral pockets defined by the PPh2 groups show unusual features. Four alkenes of the type (Z)-RCH=C(NHCOCH3)CO2R′ are treated with H2 (1 atm) and (R)-6+ PF6− or (S)-12+ PF6− (0.5 mol %) in THF at room temperature. Protected amino acids are obtained in 70–98 % yields and 93–82 % ee [(R)-6+ PF6−] or 72–60 % ee [(S)-12+ PF6−]. Pressure and temperature effects are defined, and turnover numbers of >1600 are realized.