β-Styrylcobaloximes: mechanism of formation from β-styryl halides and mechanism of cleavage by electrophiles

Abstract

The ease of reaction of cis-β-halogenostyrenes (PhCH:CHX) with the nucleophillic bis(dimethylglyoximato)-pyridinecobaltate(I) ion decreases in the order (X =) I Br Cl > F. This order, together with the absence of isomerism of either reagent halogenostyrene of product cis-styrylcobaloxime during the reaction indicates that the displacement of halide ion and the attack of the nucleophile at the β-carbon are synchronous process occurring with retention of configuration. The absence of incorporation of deuterium in the vinylcobaloxime formed from the corresponding reaction between vinyl chloride and the same cobaltate(I) ion in deuteriated methanol is in accord with this mechanism. Reactions of both cis- and trans-β-styrylcobaloxime with bromine, chlorine, and iodine and with mercury(II) acetate in pure acetic acid are also stereospecific substitions; since cis-β-styrylcobaloxime can be formed in good yield (60%) from phenylacetylene, this reaction provides a useful route to pure cis-β-bromo-, -chloro-, and -iodo-styrene and to cis-β-styrylmercury(II) derivatives, but the method is of limited utility for the formation of other cis-vinyl derivatives.