Diphenylpyridylmethyl radicals. Part 1. Synthesis, dimerization and ENDOR spectroscopy of diphenyl(2-, 3- or 4-pyridyl)methyl radicals; bond dissociation enthalpies of their dimers

Abstract

ortho–ortho hydrogen van der Waals repulsions are the origin of the propeller shape of the triphenylmethyl radical and the main reason for the low bond dissociation enthalpy (BDH) of its dimer 1(44.8 J mol^–1). In order to reduce these steric repulsions (eliminating some aromatic hydrogens), diphenyl(2-, 3- or 4-pyridyl)methyl radicals 2–4 were prepared through reductive dehalogenation of the corresponding triarylchloromethanes 2–4a with silver in benzene. They form α,p-dimers 2–4e exclusively through the pyridine ring. ENDOR spectroscopy shows that the structure of the radicals, does not deviate substantially from that of the parent radical, Ph₃C˙. In contrast, the BDH values of the dimers (measured using EPR spectroscopy) show strengthening of the central C–C bond in 2e(88.7 kJ mol^–1) and 3e(90.0 kJ mol^–1) and a similar value for 4e(46.4 kJ mol^–1) with respect to the trityl dimer 1. This is a consequence of the ground state stabilization of the dimers 2–4e due to relief of strain (elimination of ring hydrogens), whereas in the case of 4e, this stabilization is probably compensated by the formation of a weaker C–N bond with respect to the C–C bond. The above dimers undergo easy 1,5-H-rearrangement, autocatalysed by the basic pyridyl groups themselves.