Formation of Trinuclear Rhodium-Hydride Complexes [{Rh(PP)H} sub(3)- ( mu sub(2)-H) sub(3)( mu sub(3)-H)][anion] sub(2)-During Asymmetric Hydrogenation?

• Published in: Chemistry : a European journal Vol. 19; no. 23; pp. 7443 - 7451
• Main Authors: Kohrt, C, Baumann, W, Spannenberg, A, Drexler, H-J, Gridnev, I D, Heller, D
• Format: Journal Article
• Language: English
• Published: 01-06-2013
• Subjects: Asymmetry; Catalysts; Coordination compounds; Formations; Hydrogenation; Ligands
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201204336

Recently described and fully characterized trinuclear rhodium-hydride complexes [{Rh(PP*)H} sub(3)( mu sub(2)-H) sub(3)( mu sub(3)-H)][anion] sub(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)]BF sub(4), [Rh(Tangphos)(mac)]BF sub(4), [Rh(Me-BPE)(mac)]BF sub(4), [Rh(DCPE)(mac)]BF sub(4), [Rh(DCPB)(mac)]BF sub(4)), as well as rhodium-hydride species, both mono-([Rh(Tangphos)- H sub(2)(MeOH) sub(2)]BF sub(4), [Rh(Me-BPE)H sub(2)(MeOH) sub(2)] BF sub(4)), and dinuclear ([{Rh(DCPE)H} sub(2)( mu sub(2)-H) sub(3)]BF sub(4), [{Rh(DCPB)H} sub(2)( mu sub(2)-H) sub(3)]BF sub(4)), 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) sub(2)H sub(2)]BF sub(4)) or dinuclear ([{Rh(PP*)H} sub(2)( mu sub(2)-H) sub(3)]BF sub(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.