Pyridines from allyl cobaloximes: a new decomposition pathway

Thermochemical and mass spectrometric studies on allyl cobaloxime indicate that it thermally decomposes in a stepwise manner. In comparison with the behaviour of benzyl cobaloximes, which are also precursors to stabilised carbon radicals, there are superficial similarities but also significant differences. Both show a final highly exothermic transition and both produce a heterocyclic ring during thermolysis. Whereas benzyl cobaloximes produce the corresponding 3,4-dimethyl-5-arylisoxazole in moderate to good yields, allyl cobaloximes provide low yields of the corresponding 2,3-dimethylpyridines. Each route involves the cyclisation of an analogous monoxime. The allyl series favours cyclisation through dehydration whereas the benzyl series follows an oxidative ring closure. It is proposed that the low yields of pyridines are due to the relatively ineffective capture of allyl radicals by the dimethylglyoxime monoanion. (C) 1999 Elsevier Science S.A. All rights reserved.