Dimerization Mechanism of the 1-Alkyl-4-phenylpyridinyl Radicals Generated from the Photosensitive Dimer as Studied by Kinetic ESR Spectroscopy

The dimerization mechanism of 1-methyl- (1), 1-ethyl- (2), and 1-isopropyl-4-phenylpyridinyl (3) radicals is discussed on the basis of the kinetic and thermodynamic studies. Absolute rate constants for dimerization are determined from the ESR decay curves of radicals generated by pulse photolysis of the dimer. Intermediate formation is implied by a two-step decay of the radical. The activation energy (Ea = 6.7 kcal/mol) for intermediate formation is independent of the 1-alkyl group, while that for the second slow step is dependent on the alkyl group, showing Ea's of 11.3, 12.2, and 13.3 kcal/mol for 1, 2, and 3, respectively. The equilibrium between monomeric radical and the dimer is measured and the heat of the bond formation is estimated for each radical. NMR measurement for 1 at low temperature revealed the dimer to be a 2,2′-dimer. It is proposed that the intermediate is a 4,4′-dimer which produces a 2,2,-dimer through [3,3] sigmatropic intramolecular rearrangement.