NUCLEOPHILE-ASSISTED RACEMISATIONS OF HALOSILANES - KINETIC-STUDIES

Kinetic studies have been carried out on the nucleophile-induced racem isation of PhMeCHSiMe(2)X, 2, X = triflate, Br or Cl. Thirteen nucleop hiles were studied. The results are interpreted in terms of two compet ing mechanisms for racemisation: (a) nucleophile attack on a silane-nu cleophile complex formed by displacement of the halide by the nucleoph ile, and (b) halide-halosilane exchange, with inversion of configurati on. Solvent effects were examined, and kinetic orders in the nucleophi le and in one case for the halosilane were determined. The order in ad ded nucleophile varied between one and two, with strong nucleophiles i n polar media. Anomalously high orders in nucleophile were observed in non-polar media and are ascribed to aggregation of the nucleophile. A kinetic analysis of the competing mechanisms was attempted, and was c onsistent with the experimental findings. In this particular series of reactions involving compounds with good leaving groups and relatively powerful nucleophiles there was no evidence for intermediates involvi ng extracoordinate silicon.