The stereochemistry of phosphine-induced debromination reactions

Abstract

The stereochemistry of dehalogenation reactions with triphenylphosphine (a two-electron dehalogenating reagent) has been investigated. Both eythro-(or meso-) and threo-[or (±)-] dibromides give entirely trans-olefin, except for (±)-stilbene dibromide which gave a 36 : 65 cis–trans-mixture (the olefins formed were identified by n.m.r. spectroscopy). Authentic samples of cis-olefin in each case were shown not to isomerise to the trans-form under the reaction conditions. The mechanism of dehalogenation is briefly discussed in terms of initial attack of phosphine on halogen to give an intermediate ion pair. Evidence for this is available from the reaction of 1,2-dibromo-2-nitrostyrene with triphenylphosphine in methanol, which gave 2-nitro-1-phenylethyltriphenyl-phosphonium bromide. With erythro-2,3-dibromobutyric acid, dehydrohalogenation, rather than debromination, occurred to give a mixture of α-bromocrotonic and α-bromoisocrotonic acids. Attempts to extend the dehalogenation reaction to the formation of three- and six-membered rings were unsuccessful.