Mechanism and kinetics of homogeneous 1-methyl-carbamidopyridinyl radical reactions

Abstract

The effect of changing the substitution pattern on the mechanism and kinetics of homogeneous solution reactions of electrogenerated 1-methyl-carbamidopyridinyl radicals has been investigated. The dynamics and energetics of the reactions of 1-methyl-3-carbamidopyridinium, 1-methyl-4-carbamidopyridinium and 1-methyl-3,4-dicarbamidopyridinium perchlorate have been explored using fast scan cyclic voltammetry and double potential step chronoamperometry conducted on a microsecond timescale at ultramicroelectrodes. The reaction mechanisms have been probed by determining the dependence of the peak potential, the peak currents and current ratios on the experimental timescale, the pyridinium concentration and solution pH. 1-Methyl-3-carbamidopyridinyl radicals react via a dimerisation mechanism involving direct coupling of the electrogenerated neutral radicals at a rate of approximately 1.6±0.1×10⁷ M^-1 s^-1 in DMF containing 1.0 M tetraethylammonium perchlorate (TEAP) as supporting electrolyte. The 1-methyl-4-carbamidopyridinyl and 1-methyl-3,4-dicarbamidopyridinyl radicals react via a pH dependent ECE–DISP1 mechanism, E, C and DISP denote electron transfer, following chemical and disproportionation reactions, respectively. In this mechanism, the radical, R, generated after the first electron transfer step (E), reacts with the protonated radical, RH⁺, and RH⁺ reacts with other RH⁺ to yield pyridinium and a dihydropyridine. The rate constant for the reaction of 1-methyl-4-carbamidopyridinyl radicals in aqueous acetate buffer at pH 7 is approximately 1.6±0.1×10⁵ M^-1 s^-1. In contrast, the rate constant for the loss of 1-methyl-3,4-carbamidopyridinyl radicals is significantly smaller being approximately 1.2±0.1×10³ M^-1 s^-1. This reduced rate constant reflects both steric constraints and the electron withdrawing character of the second carbamido group which stabilises the radical species. In all three cases, temperature resolved potential step measurements reveal low activation energies (<40 kJ mol^-1) which are consistent with the rapid nature of the radical coupling reactions observed.