Electron-Transfer, Halogen-Metal Exchange and Direct Processes in Formal Nucleophilic Substitutions on Alkyl Halides by Trimethyltinsodium

Formal nucleophilic substitutions have been studied by simple trapping techniques designed to separate and estimate contributions of reactions proceeding by way of free radicals, by way of anions, and by way of geminate or synchronous processes. Reactions of trimethyltinsodium with organic halides in tetrahydrofuran at 0 °C were examined, using dicyclohexylphosphine for trapping free radicals and teri-butylamine for free anionoids. Among 22 halides included in this study nine have been shown to involve two or all three of the mechanistic pathways. Primary chlorides reacted predominantly by a direct mechanism (Sn2, geminate reaction of intermediates, or multicenter process). Branching, as in isobutyl and neopentyl, led to contributions from electron-transfer (free radical) and halogen-metal exchange (anionoid) mechanisms. Secondary bromides reacted predominantly by the ET process (major) and HME (minor) while the relative contributions from these were reversed in importance with the iodides. Triethylcarbinyl chloride reacted exclusively by elimination while the bromide reacted by ET alone or in competition with elimination. 1- and 2-bromoadamantanes reacted by ET, and 1 -chloroadamantane showed no reaction after 10 days at -4 °C.