1-Alkyl-2-(carbomethoxy)pyridinyl Radicals: Monomers and Dimers Defined through Chemical and Photochemical Properties and Electron Paramagnetic Resonance

Article abstract of DOI:10.1021/ja00406a021

1-Methyl-2-(carbomethoxy)pyridinium ion (2⁺) is reduced to the radical dimer (2-2) which is in equilibrium with a small amount of 1-methyl-2-(carbomethoxy)pyridinyl radical (2<*>).Distillation of the dimer leads to thin films (at 77 K) of the radical 2<*>, which dimerizes in solution.Chemical properties and EPR spectra of dimer solutions reflect the presence of the monomeric radical in equilibrium with the dimer; 2⁺ polarography also indicates a monomer-dimer equilibrium.Laser pulse dissociation of the dimer in solution gives the monomer reversibly, with k_assoc of ca. 10⁶ M^-1 s^-1.The dimerization constant, K_a (titration method), is 4*10⁵ M^-1; electrochemical techniques lead to a K_a value at least 7*10⁴ M_-1, a k_assoc value of ca. 1-4*10⁴ M^-1 s^-1, and k_dissoc of 1 s^-1.The activation energy of dimer dissociation (from EPR spectral broadening) is 7.7 kcal/mol (CH3CN) or 9.4 kcal/mol (2-MTHF).The 1-methyl-2-acetylpyridinyl radical <(CH3CO)2<*>> is easily prepared and distilled and has a stability similar to that of the 1-methyl-4-(carbomethoxy)pyridinyl radical (4<*>) EPR spectra of the 2-acetyl and 2-(carbomethoxy)pyridinyls are presented and analyted.A convenient electrochemical cell for the generation and analysis of air-sensitive one-electron reduced species (such as the pyridinyl radicals) or unstable pyridinyl radical dimers) is described.