Competing β-scission and hydrogen transfer to the pinanyl radical in the addition of methyl thioglycolate to β-pinene derivatives

Abstract

Despite previous reports to the contrary, addition of methyl thioglycolate to β-pinene always leads to both pinane and p-menthene adducts, the proportion of the latter increasing with the reaction temperature. This is attributable to the substantially higher activation entropy for β-scission than for hydrogen transfer, while the activation enthalpy is higher for the latter reaction. Any substituent at the 3-carbon of β-pinene, trans with respect to the gem-dimethyl bridge, increases the p-menthene yield: polar effects of the mainly electron-withdrawing substituents appear to disfavour hydrogen transfer to the tertiary pinanyl radical and favour β-scission. Analysis of the temperature dependence of the pinane∶p-menthene ratio for the substituted β-pinenes indicates that variations in the activation entropy difference for the two reactions are at least as important as those of the activation enthalpy.