Halogen Transfer Research Articles

Alkenes from halides and epoxides by reductive eliminations with Cr II complexes

A variety of β-substituted alkyl halides and epoxides are reductively eliminated in high yields to alkenes by Cr IIen at room temperature. The reducing agent is easily prepared in situ from chromous salts and ethylenediamine. Reductive elimination and protolytic reduction of β-substituted alkyl halides by chromous reagent proceed by similar mechanisms, and involve free radicals and alkylchromium species as intermediates. In the former process, β-substituents such as hydroxy, phenoxy, acetoxy, tosyloxy, bromo, chloro, amino and trimethylammonium groups are readily eliminated from the β-substituted alkylchromium intermediate to generate alkene. Protolytic reduction of the alkylchromium species is a significant competing reaction only with such β-substituents as cyano, benzamido and phthalimido groups. Factors which determine the effect of groups on the relative rates of reductive elimination and protolytic reduction of the alkylchromium species are discussed. The formation of a free radical by an initial ligand transfer of halogen from the alkyl halide to chromous ion is enhanced by β-bromo and trimethylammonium groups. Neighboring group participation by these groups in homolytic processes to form bromine or trimethylammonium-bridged alkyl radicals as intermediates is postulated. Cr IIen is a more powerful reductant than Cr II and the scale of relative reactivity of various halides is compressed in the former relative to the latter. o-Diiodobenzene is reduced by Cr IIen stepwise to benzene, and benzyne does not appear to be an intermediate in the reaction.

Isotopic Exchange Reactions. IV. Evidence for the Tetrachloroborate Anion from Kinetic Studies1

The halogen exchange between BC3 and a number of chloride salts has been studied in both the liquid and gas phase using Cl36 as a radiotracer. The heavy alkali metal salts do not undergo exchange at 0 in several hours. LiCl shows a small amount of halogen transfer in liquid BCl3 . Both (CH3)4/NCl and (CH3CH/2)4/NCl solute exchange chlorine rapidly with liquid BCl3 and the exchange with the former has been exploited to prepare radiochlorine labelled BC3. Silicon tetrachloride and trimethylchlorosilane do not exchange halogens with BCl3 in the gas phase, although the latter should form a 1:1 addition compound with boron trichloride. These results are interpreted on the basis of an ionic mechanism which involves the formation of the tetrachloroborate anion in BCl3 solution.

The Faworskii rearrangement in the formation of 17-methyl steroids : The configuration of C-17 bromides

The configuration of 17-bromo-20-keto pregnanes and the 17-methyletianic esters have been determined. The Faworskii rearrangement of the former to give the latter has been observed to be abnormal. It is suggested that C-17 → C-21 halogen transfer intervenes.