NUCLEOPHILE-ASSISTED RACEMIZATIONS OF HALOSILANES - THERMODYNAMIC STUDIES

Thermodynamic studies have been carried out for the nucleophile-induce d racemisation of PhCHMeSiMe(2)X (2) (X=Br or Cl). The values of Delta H double dagger (approximate to 45 kJ mol(-1)) and Delta S double dag ger(approximate to -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 hexamethylphosphoramide are compatible with a mechanism fo r racemization in which the nucleophile displaces halide ion from the silane in the first step and the rate-determining step is attack of ha lide ion on halosilane. For 2 (X=Cl) in the presence of hexamethylphos phoramide a curved Eyring plot shows that different racemization mecha nisms are competing. At low temperatures the halide-halosilane mechani sm dominates where at high temperatures a double-displacement mechanis m dominates, in which the rate-determining step is the attack of nucle ophile on PhCHMeSiMe(2)HMPA(+). The approximate thermodynamic paramete rs for this latter process are Delta H double dagger, = 0 kJ mol(-1) a nd Delta S double dagger, approximate to -180 J K-1 mol(-1). Analysis of the thermodynamics of these reactions does not allow a definitive d istinction to be made between nucleophilic activation involving attack at extracoordinated silicon and that at teracoordinated silicon.