Reaction prospecting by 31P NMR: enantioselective rhodium-DuPhos catalysed addition of ZnMe2 to diphenylphosphinoylimines
摘要:
Chiral shift P-31 NMR spectroscopy allows the identification of ligand leads in asymmetric catalyst systems for ZnMe2 addition to ArCH=NP(C)Ph-2. Subsequent GC-based optimisation shows [RhC](CH2=CH2)(2)](2) and (RR)-MeDuPhos to be the optimal pre-catalyst combination (product in 78-93% ee). Transmetallation of [(MeDuPhos)Rh{N(P(O)Ph-2-CHMeAr}] with ZnMe2 appears to be the rate limiting step of the catalytic cycle as competing coordination by the imine starting material leads to Ph2P(O)NHCH2Ar via MVP hydrogen-transfer. This limitation can largely be overcome by the slow addition of the imine. (C) 2009 Elsevier Ltd. All rights reserved.