Formation of trinuclear rhodium-hydride complexes [{Rh(PP*)H}3-(µ2-H)3(µ3-H)][anion]2--during asymmetric hydrogenation?

ABSTRACT

Recently described and fully characterized trinuclear rhodium-hydride complexes [{Rh(PP*)H}3(µ2-H)3(µ3-H)][anion]2 have been investigated with respect to their formation and role under the conditions of asymmetric hydrogenation. Catalyst-substrate complexes with mac (methyl (Z)-N-acetylaminocinnamate) ([Rh(tBu-BisP*)(mac)]BF4, [Rh(Tangphos)(mac)]BF4, [Rh(Me-BPE)(mac)]BF4, [Rh(DCPE)(mac)]BF4, [Rh(DCPB)(mac)]BF4), as well as rhodium-hydride species, both mono-([Rh(Tangphos)-H2(MeOH)2]BF4, [Rh(Me-BPE)H2(MeOH)2]BF4), and dinuclear ([{Rh(DCPE)H}2(µ2-H)3]BF4, [{Rh(DCPB)H}2(µ2-H)3]BF4), are described. A plausible reaction sequence for the formation of the trinuclear rhodium-hydride complexes is discussed. Evidence is provided that the presence of multinuclear rhodium-hydride complexes should be taken into account when discussing the mechanism of rhodium-promoted asymmetric hydrogenation.