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. Catalyst-substrate complexes containing the ligands tBu-BisP, Tangphos, Me-BPE, DCPE, DCPB, and mono- ([Rh(PP)(MeOH)2H2]BF4) or dinuclear ([{Rh(PP)H}2(μ2-H)3]BF 4) Rh-hydride species, are described (see figure). A sequence for the formation of the trinuclear Rh-hydride complexes is suggested. The presence of these complexes should be taken into account when discussing the mechanism of asymmetric hydrogenation. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.