Photoinduced electron transfer double fragmentation: an oxygen-mediated radical chain process in the co-fragmentation of substituted pinacol donors with carbon tetrachloride

Abstract

Investigations of photoinduced electron transfer processes have led to the observation of several ion radical fragmentation reactions in which strong covalent bonds in the neutral (or starting) molecules rapidly cleave in the one-electron redox products. Donors that undergo rapid bond breaking reactions on one-electron oxidation include 1,2-diarylethanes, pinacols, diamines, and aminoalcohols. One-electron reduction of acceptors, such as ethers, esters and organic halides, can also result in bond cleavage. The efficiency of these reactions is determined by the competition between back electron transfer (k−et), fragmentation (kr) and separation of the ion radical pair (ksep). The quantum yields are generally low since this competition is dominated by back electron transfer (k−et ≈ 109−1011 s−1). One strategy to increase the fragmentation efficiency is to utilize very rapid co-fragmentations of both donor and acceptor thus allowing return electron transfer to be minimized. Cleavage reactions of organic halides are potentially useful in this regard because electrochemical studies and thermochemical calculations suggest that, for certain reduced halides, the electron transfer is dissociative. In this paper, we report the excited state reactivity of several amino-and methoxy-substituted pinacols with the halogenated acceptor carbon tetrachloride. Low to moderate quantum efficiencies (approximately 0.04–0.6) are observed for these reactions when irradiations are carried out under degassed conditions. However, for some pinacols, irradiation in the presence of O2 results in significantly larger quantum yields (approximately 1–10) suggesting a chain mechanism. Data from nuclear magnetic resonance (NMR), electron spin resonance (ESR) and time-resolved absorption spectroscopy suggest that the primary halide radical formed from dissociation is captured by O2 to give peroxyl radical which then propagates the chain reaction.