Homolytic displacements at carbon centres. Part 1. Reaction of allyl- and allenyl-cobaloximes with polyhalogenomethanes

Abstract

Allyl- and allenyl-cobaloximes react with bromotrichloromethane to give 4,4,4-trichlorobutene and 4,4,4-trichlorobutynes, respectively, and bromocobaloxime(III). The reactions of the allylcobaloximes, carried out at room temperature, are nearly quantitative, but those of the allenylcobaloximes, performed at higher temperatures, are dependent upon the nature of the allenyl group and of the other axial ligand; the yield of trichlorobutyne is appreciably higher with imidazole than with pyridine as the axial ligand, and decreases with increasing substitution on the allenyl ligand. The reactions are believed to involve novel chain processes in which the chain-carrying trichloromethyl radical, formed by the reaction of cobaloxime(II) with the bromotrichloromethane, attacks regiospecifically at the γ-carbon of the allyl or allenyl ligand to displace the other chain propagating species, cobaloxime(II), and give the observed organic product. Since cobaloxime(II) neither dimerises nor disproportionates under these conditions, two important chain terminating steps are eliminated. The main side reaction with allenylcobaloximes involves the formation of the corresponding acetylenic hydrocarbon, the additional hydrogen atom being derived from the dimethylglyoximato hydroxy group, probably as a consequence of attack of the trichloromethyl radical on the metal. The application of the reaction to other polyhalogenomethanes and related radical precursors is also demonstrated.