Selective C-H bond activation of arenes catalyzed by methylrhenium trioxide

Arenes, in glacial acetic acid, are oxidized to para-benzoquinones by hydrogen peroxide when methylrhenium trioxide (CH3ReO3 or MTO) is used as a catalyst. In some cases an intermediate hydroquinone was also obtained in lower yield. Oxidation of the methyl side chains of various methylbenzenes did not occur. The active catalyst species are the previously-characterized eta(2)-peroxorhenium complexes, CH3Re(O)(2)(eta(2)-O-2) and CH3Re(O)(eta(2)-O-2)(2)(H2O). Separate tests showed that hydroquinones and phenols are oxidized by H2O2-MTO more rapidly than the simple arenes; in the proposed mechanism they are intermediate products. Higher conversions were found for the more highly-substituted arenes, consistent with their being the most reactive species toward the electrophillically-active peroxide bound to rhenium. High conversions of the less substituted members of the series were not achieved, reflecting concurrent deactivation of MTO-peroxide, a process of greater import for the more slowly-reacting substrates. (C) 1998 Elsevier Science S.A.