Nucleophile-assisted racemizations of halosilanes: thermodynamic studies

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Abstract Thermodynamic studies have been carried out for the nucleophile-induced racemisation of PhCHMeSiMe2X (2) (X = Br or Cl). The values of ΔH ‡ (≈ 45 kJ mol −1 ) and (≈ −15–−50 J K −1 mol −1 ) for the racemization of 2 (X = Br or Cl) in the presence of N-methylimidazole and 2 (X = Br) in the presence of hexamethylphospharamide are compatible with a mechanism for racemization in which the nucleophile displaces halide ion from the silane in the first step and the rate-determining step is attack of halide ion on halosilane. For 2 (X = Cl) in the presence of hexamethylphosphoramide a curved Eyring plot shows that different racemization mechanisms are competing. At low temperatures the halide-halosilane mechanism dominates where at high temperatures a double-displacement mechanism dominates, in which the rate-determining step is the attack of nucleophile on PhCHMeSiMe2HMPA+. The approximate thermodynamic parameters for this latter process are ΔH ‡ , ≈ 0 kJ mol −1 and Δ S ‡ ≈ −180 J K −1 mol −1 . Analysis of the thermodynamics of these reactions does not allow a definitive distinction to be made between nucleophilic activation involving attack at extracoordinated silicon and that at teracoordinated silicon.