Co-solvent effects on the indirect reduction of alkyl and benzyl halides: experimental evidence of a link between electron transfer and SN1-like processes

Abstract

The influence of using water as co-solvent in N,N-dimethylformamide on the electron transfer process between electrochemically generated electron donors and alkyl and benzyl halides has been investigated. While the solvent effect in general is modest for primary alkyl halides and substituted benzyl chlorides, the reaction rate is enhanced substantially in the case of 2-bromo-2-methylpropane, 1-iodoadamantane, 4-methylbenzyl bromide and 4-methoxybenzyl bromide. For these compounds the results are interpreted as if the transition state possesses substantial carbonium ion character and that a shift from an electron transfer toward an S_N1-like pathway takes place as the water content is increased. The fact that the carbonium ion structure becomes even more pronounced when poor electron donors are employed may provide an explanation as to why a linear rather than the predicted quadratic free energy relationship is observed experimentally for the homogeneous reduction of 2-bromo-2-methylpropane.