Reaction prospecting by 31P NMR: enantioselective rhodium-DuPhos catalysed addition of ZnMe2 to diphenylphosphinoylimines

Chiral shift 31P NMR spectroscopy allows the identification of ligand leads in asymmetric catalyst systems for ZnMe2 addition to ArCH NP(O)Ph2. Subsequent GC-based optimisation shows [RhCl(CH2 CH2)2]2 and (R,R)-MeDuPhos to be the optimal pre-catalyst combination (product in 78–93% ee). Transmetallation of [(MeDuPhos)Rh{N(P(O)Ph2–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.