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

The effect of changing the substitution pattern on the mechanism and kineti cs of homogeneous solution reactions of electrogenerated 1-methyl-carbamido pyridinyl radicals has been investigated. The dynamics and energetics of th e reactions of 1-methyl-3-carbamidopyridinium, 1-methyl-4-carbamidopyridini um and 1-methyl-3,4-dicarbamidopyridinium perchlorate have been explored us ing fast scan cyclic voltammetry and double potential step chronoamperometr y conducted on a microsecond timescale at ultramicroelectrodes. The reactio n mechanisms have been probed by determining the dependence of the peak pot ential, the peak currents and current ratios on the experimental timescale, the pyridinium concentration and solution pH. 1-Methyl-3-carbamidopyridiny l radicals react via a dimerisation mechanism involving direct coupling of the electrogenerated neutral radicals at a rate of approximately 1.6 +/- 0. 1 x 10(7) M-1 s(-1) in DMF containing 1.0 M tetraethylammonium perchlorate (TEAP) as supporting electrolyte. The 1-methyl-4-carbamidopyridinyl and 1-m ethyl-3,4-dicarbamidopyridinyl radicals react via a pH dependent ECE-DISP1 mechanism, E, C and DISP denote electron transfer, following chemical and d isproportionation 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 pyridini um and a dihydropyridine. The rate constant for the reaction of 1-methyl-4- carbamidopyridinyl radicals in aqueous acetate buffer at pH 7 is approximat ely 1.6 +/- 0.1 x 10(5) M-1 s(-1). In contrast, the rate constant for the l oss of 1-methyl-3,4-carbamidopyridinyl radicals is significantly smaller be ing approximately 1.2 +/- 0.1 x 10(3) 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 th ree cases, temperature resolved potential step measurements reveal low acti vation energies (< 40 kJ mol(-1)) which are consistent with the rapid natur e of the radical coupling reactions observed.