Electron transfer. 143. Reductions of benzopyrazine derivatives with indium(I)

Solutions of In(I)(aq) in 0.03-0.20 M H+ reduce quinoxaline (Q), phenazine (P), and the N-methylphenazonium cation (MP+) in kinetically separable step s, yielding first a colored radical, then a colorless dihydro compound. The initial stage of these reductions yields 2 equiv. of the radical, and the much slower second stage likewise entails loss of nearly 2 mol of radical p er mol of In(I). Individual kinetic profiles exhibit no irregularities attr ibutable to the intervention of In(II). Reactions of phenazine and its N-me thylated cation are exceptionally photosensitive. Reductions of phenazine a nd quinoxaline to their radicals are favored by N-protonation, but reductio n of the N-methyl cation is independent of [H+], as are the further reducti ons of both phenazonium radicals. Reduction of the quinoxalinium radical ut ilizes paths at two protonation levels (QH(.) and QH(2)(.+)). Although oxid ants and reductant alike have access to 2 e(-) paths, observations in all t hree systems are consistent with reactions via successive 1 e(-) steps. (C) 2000 Elsevier Science S.A. All rights reserved.