Absolute Rate Constants for the Addition of the 1-(tert-Butoxy)carbonylethyl Radical to Alkenes in Solution

Abstract

Absolute rate constants and some of their Arrhenius parameters are reported for the addition of the 1-[(tert-butoxy)carbonyl]ethyl radical (MeC. HCO2Me3) to several mono- or 1,1-disubstituted alkenes in acetonitrile as obtained by time-resolved electron spin resonance spectroscopy. At 295 K, the rate constants range from 470 M−1 s−1 (but-1-ene) to 2.4⋅105 M−1 s−1 (1,1-diphenylethene), the experimental activation energies range from 26.8 kJ/mol (but-1-ene) to 14.7 kJ/mol (styrene), and the frequency factors obey on the average log (A/M−1 s−1)=7.9±0.5. The rate constants of the secondary 1-[(tert-butoxy)carbonyl]ethyl radical are close to the geometric mean of those of the related primary [(tert-butoxy)carbonyl]methyl and the tertiary 2-(methoxycarbonyl)propan-2-yl radicals. The activation energies for addition of these three carboxy-substituted alkyl radicals are mainly governed by the addition enthalpy but are also substantially lowered by ambiphilic polar effects. The results support a previously derived predictive analysis, and relations to rate constants of acrylate polymerizations are discussed.