Ar. Bassindale et al., NUCLEOPHILE-ASSISTED RACEMIZATIONS OF HALOSILANES - THERMODYNAMIC STUDIES, Journal of organometallic chemistry, 499(1-2), 1995, pp. 137-141
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.