Theoretical Investigation of the Mechanisms and Stereoselectivities of Reductions of Acyclic Phosphine Oxides and Sulfides by Chlorosilanes

Computational studies were performed to explain the highly varied stereoselectivities obtained in the reductions of acyclic phosphine oxides and sulfides by different chlorosilanes. The reductions of phosphine oxides by HSiCl3, HSiCl3/Et3N, and Si2Cl6 and the reductions of phosphine sulfides by Si2Cl6 (all in benzene) were explored by means of B3LYP, B3LYP-D, and SCS-MP2 calculations. For the reductions of phosphine oxides by HSiCl3, the calculations support the mechanism proposed by Homer in which a hydride is transferred from silicon to phosphorus through a four-centered, frontside transition state. This mechanism leads to retention of stereochemistry at phosphorus. For the other three reductions, two classes of mechanisms were explored. Phosphorane-based mechanisms that were previously proposed by Mislow and involve SiCl3- were compared with novel alternative mechanisms that involve nonionic rearrangement processes. In one of these, donor-stabilized SiCl2 is formed as an intermediate. The calculations support a phosphorane-based mechanism for the reductions of phosphine oxides by HSiCl3/Et3N and Si2Cl6 (which proceed with inversion) but favor the rearrangement pathways for the reductions of phosphine sulfides by Si2Cl6 (which proceed with retention).