Electron-transfer-induced reductive dealkoxylation of alkyl aryl ethers .2. Reductive cleavage of isomeric dimethoxybenzenes

The reactivity of the three isomeric dimethoxybenzenes in the reduction with potassium metal in THF and iso-octane in the absence of proton donors has been studied. The ortho and meta isomers reacted readily to afford products of both demethoxylation and demethylation, with the former prevailing for the ortho isomer in THF and for both isomers in iso-octane. Furthermore, in iso-octane, particularly at high temperature, significant amounts of benzene, the product of double demethoxylation, were formed. Exclusive demethylation was instead observed under all reaction conditions for the para isomer, which showed to be the least reactive of the three isomers. Labelling experiments have also been carried out in order to shed light on the mechanism of demethoxylation. The results are consistent with a competition between two modes of fragmentation of a radical anion, leading to aryl radical and methoxide anion on one side (demethoxylation) and to phenoxide anion and methyl radical on the other side (demethylation); in a subsequent step, the radicals thus formed abstract a hydrogen atom from components of the reaction medium or are reduced by single electron transfer to the corresponding anions. As concerns demethylation, however, a mechanism involving fragmentation of a dianion cannot be excluded. In light of previous results, a rationalization of the different reactivity of the three isomers is discussed