STEREOCHEMISTRY OF REDUCTIVE ELIMINATION BY CHROMIUM(2) COMPLEXES

The reductive elimination of erythro- and threo-2-bromo-3-butyl derivatives of hydroxide, acetate, p-tosylate, chloride, and bromide with CrII was carried out quantitatively under a variety of conditions. With the exception of vicinal dibromides, all of these derivatives gave the same mixture of cis- and trans-butene-2 from each epimeric pair. In the initial and rate-determining step, an equilibrated free radical is postulated as a common intermediate from each erythro- and threo-3-substituted butyl bromide by reaction with CrII. Stereospecificity in reductive elimination of vicinal dibromides is attributed to a bromine-bridged radical formed by neighboring group participation in the homolytic removal (ligand transfer) of bromine by CrII. This is in accord with previous measurements of enhanced rates of reduction of 1,2-dibromides by CrII. Stereospecific trans elimination of vicinal bromines by CrII can be induced by dimethyl sulfoxide or pyridine as solvent or by the use of excess ethylenediamine as ligand. The bromine-bridged radical is opened stereospecifically by a second CrII to yield a β-bromoalkylchromium intermediate. Factors which influence trans elimination from the latter are discussed. Epoxides also yield alkenes quantitatively with CrII but the reduction is not stereospecific. Episulfides showed partial stereoselectivity; a more labile sulfur-bridged radical is postulated as an intermediate. © 1968, American Chemical Society. All rights reserved.