Rearrangements accompanying the formation of alkynyl- and allyl-cobaloximes and their reactions with sulphur dioxide

Abstract

The bis(dimethylglyoximinato)pyridinecobaltate(I) ion reacts with allyl and alkynyl halides by both S_N2 and S_N2′ mechanism. In most cases only one of these reactions is apparent because of the major influence of steric effects. In the case of the relatively unhindered prop-2-ynyl bromide, the initial attack of the metal nucleophile is believed to be an S_N2 reaction, but this is followed by an S_N2′ displacement of the first introduced cobalt by a second cobalt nucleophile. The products obtained therefore depend upon the concentration of the nucleophile, the reaction time, and the temperature. The absence of rearrangement of the organic group in the reaction of the metal nucleophile with 6-bromohex-1-ene also supports the nucleophilic displacement mechanism at saturated carbon. The alkynyl- and alkynyl-cobaloximes formed in the reactions react with sulphur dioxide to give insertion products, either with or without rearrangement of the organic group. The mechanism of these reactions is discussed, and the low yields of normal insertion products from the hex-5-enyl- and but-3-enyl-cobaloximes are interpreted in terms of an initial homolysis of the carbon–cobalt bond.