HYDROGEN-ATOM TRANSFERS FROM THIOPHENOLS TO TRIARYLMETHYL RADICALS - RATES, SUBSTITUENT EFFECTS, AND TUNNEL EFFECTS

Rates of reaction between tris-p-tert-butylphenylmethyl radical and some substituted thiophenols are reported. The rate-determining step is Ar3Cη + HSArʹ → Ar3CH + ηSArʹ; it is followed by the fast step Ar3C-+ -SArʹ → Ar3CSArʹ. Rates are also reported for Ar = Ph, Arʹ = Ph and 2, 4, 6-trimethylphenyl, where the trityl dimer dissociation rates must be considered. Substituent effects in Arʹ are not large, covering a range of less than a factor of 20 at 0 °C. The rate constants do not correlate with ordinary substituent constants, although a modestly good correlation with the Taft σR suggests that polar contributions are small, and the rate differences arise from differences in the product ArʹSη. Large variations are unexpectedly found in the preexponential factor (from about 1 X 107 to 3 X 109). They correlate with activation energy and are shown to be attributable to substantial variation in the tunnel correction. A rough model calculation shows that the effects are not unreasonable. The tunnel correction varies considerably because there is a substituent effect on the endothermicity of the rate-determining step. © 1979, American Chemical Society. All rights reserved.