The mechanism of thermal eliminations. Part 27. Steric acceleration in pyrolysis of 3,3,3-tris(trimethylsilyl)propyl acetate

Abstract

3,3,3-Tris(trimethylsilyl)propyl acetate has been prepared and its rate of unimolecular first-order decomposition measured between 633.1 and 687.8 K. The linear Arrhenius plot obtained from the rate data yields log A= 12.485 s^–1 and E_act= 189.60 kJ mol^–1. At 650 K this ester undergoes elimination 14.0 times faster per β-hydrogen than does ethyl acetate, measured under the same conditions. This effect of the β-C(SiMe₃)₃ substituent is attributed to steric acceleration and is the largest such effect observed in a gas-phase elimination. The results confirm that the much larger effect (125-fold per β-hydrogen) previously found to be produced by the smaller SiMe₃ substituent is due principally to C–Si hyperconjugative stabilisation of the partial carbocation produced at the α-position in the transition state of this electrocyclic reaction. Hyperconjugation involving the electrons of the C–C(SiMe₃)₃ bond is an alternative but less probable explanation of the present results.