Dehalogenation of vicinal dibromides by electrogenerated polysulfide ions in dimethylacetamide

Abstract

Dehalogenation reactions of vic-dibromides by electrogenerated S₆^2–(⇄S₃˙^–), S₄^2– or S₃^2– polysulfide ions have been investigated in dimethylacetamide by use of spectroelectrochemistry on a series of dibromides: methyl erythro-2,3-dibromo-3-phenylpropanoate (1), diethyl meso-2,3-dibromosuccinate (2), meso-1,2-dibromo-1,2-diphenylethane (3), DL-1,2-diphenylethane (4), 1,2-dibromobutane (5), 1,2-dibromophenylethane (6), DL-2,3-dibromobutane (7), erythro-3,4-dibromoheptane (8), threo-3,4-dibromoheptane (9), meso-5,6-dibromodecane (10), trans-1,2-dibromocyclohexane (11). Substitution and elimination are competing mechanisms with 5–7 while the quantitative formation of alkenes is observed with the other substituted dibromides. In dilute solutions, indirect electrochemical reductions of vic-dibromides have been performed in the presence of small amounts of sulfur as catalyst at potentials leading to S(–⅓) ions with 1–3, or at more cathodic potentials (formation of S₄^2–/S₃^2– ions) for 8–11. On a preparative scale, stereospecific anti debrominations afford only E alkenes from chemical reaction with 3 or mediated electrolysis of 1, 2 or 10. Kinetic studies of the reactions between S(–⅓) ions and compounds 4, 8–10 imply that the dianions S₆^2– are the eliminating agents rather than S₃˙^– radical ions for the concerted anti dehalogenations.