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

The title compounds are accessed by sequences starting with racemic and ena ntiomerically pure [(eta (C5H5)-C-5)Re(NO)(PPh3)(CH3)]. Reactions with chlo robenzene/HBF4, PPh2H, and tBuOK give the phosphido complex [(eta (5)-C5H5) Re(No) (PPh3)(PPh2)] (3). Reactions with Ph3C+BF4-, PPh2H, and tBuOK give t he methylene homologue [(eta (5)-C5H5)Re(No)(PPh3)(CH2PPh2)] (9). Treatment of 3 or 9 with nBuLi or tBuLi and then PPh2Cl gives the diphosphido system s [(eta (5)-C5H4PPh2)Re(NO)(PPh3)((CH2)(n)PPh2)] (n = 0/1, 5/11), Reactions of 5 and 11 with [Rh(NBD)Cl](2)/AgPF6 (NBD = norbornadiene) give the rheni um/rhodium chelate complexes [(eta (5)-C5H4PPh2)Re(No)(PPh3)((mu -CH2)(n)PP h2)Rh- (NBD)]+PF6- (n=0/1, 6(+)/12(+) PF6-; 30-32% overall from commercial Re,(CO),,). The crystal structures of 6(+) PF6- and 12(+) PF6- are compared to those of 3 and 9, and other rhodium complexes of chelating bis(diphenyl phosphines). The chiral pockets defined by the PPh2 groups show unusual fea tures. Four alkenes of the type (Z)RCH=C(NHCOCH3)CO2R ' are treated with H- 2 (1 atm) and (R)-6(+) PF6- or (S)-12(+) PF6- (0.5 mol%) in THF at room tem perature. Protected amino acids are obtained in 70-98% yields and 93-82% ee [(R)-6(+)PF(6)(-)] or 72-60% ee [(S)-12(+) PF6-]. Pressure and temperature effects are defined, and turnover numbers of > 1600 are realized.