Mechanism of thermal eliminations. Part 30. Pyrolysis of 2-trimethylsilylethanol and 1-aryl derivatives

Abstract

The Arrhenius parameters (E_act= 45.57 kcal mol^–1, log A/s^–1= 12.865) and rate coefficient at 600 K (1.84 × 10^–4 s^–1) for the cisβ-thermal elimination of trimethylsilanol from 2-trimethylsilylethanol are identical (within experimental error) with those reported in the literature for elimination of methyl trimethylsilyl ether from 1-methoxy-2-(trimethylsilyl)ethane. This indicates that the driving force for the reaction, formation of the Si–O bond, is such that alteration of the nucleophilicity of oxygen has little effect on the reaction rate. A series of 1-aryl-2-trimethylsilylethanols have been prepared and their rates of elimination determined. The activating effect of the 1-aryl substituent (5.2-fold) is much less than the corresponding effect in the pyrolysis of ethyl acetates (63-fold) and 2-trimethylsilylethyl acetates (87-fold). Breaking of the α-C–O bond is thus kinetically less important than in the other reactions, and this conclusion is confirmed by a correlation of the rate data with the Yukawa–Tsuno equation which gives ρ=–0.4, r= 0.3. Si–O bond formation in the reaction is thereby shown to be of over-riding kinetic importance. For the 4-methylphenyl- and 4-methoxyphenyl compounds a minor competing reaction was the elimination of water to give the corresponding 1-aryl-2-trimethylsilylethene, and this has a higher activation energy than for the elimination of trimethylsilanol. NMR spectra for the 1-aryl-2-trimethylethanols show that interaction between the OH and SiMe₃ groups inhibits free rotation about the C(1)–C(2) bond. Preparation of 1-aryl-2-trimethylsilylethanols gave 1,3-diaryl-4-trimethylsilylbutyl trimethylsilyl ethers as byproducts arising from elimination of water from two molecules of the alcohols in a process believed to be two-step.