DIHETEROARYLMETHANES .4. OXIDATIONS AND METAL-PROMOTED COUPLING REACTIONS OF BIS(BENZOHETEROARYL)METHANES

Bis(2-benzothiazolyl)methane, 1, bis(2-benzoxazolyl)methane, 2, and bi s(N-methyl-2-benzimidazolyl)methane, 3, undergo easy oxidation by comm on oxidizing agents (air, KMnO4, K2Cr2O7, CrO3, H2O2) and give rise to a variety of products showing different levels of oxidation. They ran ge from hydroxylation of the bridge methylene, e.g. in 25, to formatio n of the corresponding ketone, e.g. in 19, to dimerization to ethane a nd/or ethene derivatives as 23-24 and 20-22, respectively. The output of the oxidations is strongly dependent upon the nature of the second heteroatom in the azole ring (sulfur, oxygen or nitrogen), the oxidant , and the reaction conditions. Also the treatment of compounds 1 and 2 with the acetates of poorly azaphilic transition-metal ions [e.g. Mn( II), Fe(II), Pb(II)] promotes the dimerization couplings to the ethane or ethene products instead of leading to the formation of neutral che lates MetI(2). Hydroxylation al the bridge methine in derivatives of 1 substituted at this position with a functionalized benzylic residue m ay result particularly easy, being simply accomplished by exposure to air. Although no mechanistic studies have been carried out on these re actions, it appears likely that a carbanionic or an enaminic intermedi ate is responsible for the air- and metal-promoted oxidations and oxid ative couplings.