A bowl-shaped phosphine as a ligand in rhodium-catalyzed hydrosilylation: Rate enhancement by a mono(phosphine) rhodium species

Bowl-shaped phosphine (BSP) ligands markedly accelerated the rhodium-catalyzed hydrosilylation of ketones and ketimines as compared with the effect of conventional phosphine ligands such as PPh ₃ and P(t-Bu) ₃. ³¹P NMR study of a mixture of [RhCl(C ₂H ₄) ₂] ₂ and phosphines at various P/Rh ratios revealed that coordination of BSP to the rhodium metal was successfully regulated, and the resultant rhodium complex bearing only one phosphine ligand (a mono(phosphine) rhodium species) was responsible for the acceleration. Structural comparison between BSP and the conventional phosphines was carried out using HF/6-31G(d)-CONFLEX/MM3-optimized structures. The mono(phosphine) rhodium complex having 1,5-cyclooctadiene as a ligand was isolated, and its X-ray molecular structure was determined.