ELECTRON-TRANSFER-INDUCED REDUCTIVE DEALKOXYLATION OF ALKYL ARYL ETHERS .2. REDUCTIVE CLEAVAGE OF ISOMERIC DIMETHOXYBENZENES

The reactivity of the three isomeric dimethoxybenzenes in the reductio n 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 f ormer prevailing for the ortho isomer in THF and for both isomers in i so-octane. Furthermore, in iso-octane, particularly at high temperatur e, significant amounts of benzene, the product of double demethoxylati on, were formed. Exclusive demethylation was instead observed under al l reaction conditions for the para isomer, which showed to be the leas t 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 f ragmentation 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 r eaction medium or are reduced by single electron transfer to the corre sponding anions. As concerns demethylation, however, a mechanism invol ving fragmentation of a dianion cannot be excluded. In light of previo us results, a rationalization of the different reactivity of the three isomers is discussed.