Mechanistic Evidence regarding the Magnesium Halide Transformation of Cyclopropylmethanols into Homoallylic Halides

Article abstract of DOI:10.1021/jo00336a016

Cyclopropylmethanols are converted into homoallylic halides in high yield by treatment with magnesium bromide or iodide in refluxing, anhydrous diethyl ether.For uncovering of the details of the reaction mechanism, (cyclopropylphenylmethoxy)magnesium bromide (3a) was prepared by treatment of cyclopropylphenylmethanol (1) with hydridomagnesium bromide.Alkoyoxymagnesium bromide 3a was stable in refluxing diethyl ether and was not changed when treated with tetrabutylammonium bromide but was transformed into 4-bromo-1-phenyl-1-butene by treatment with hydrogen bromide or magnesium bromide.These results, together with first-order kinetics for the reaction of magnesium halide with 1, suggest a mechanism involving rapid formation of an intermediate ion pair (4), a magnesium oxonium bromide, which undergoes rate-determining ring opening to give homoallylic halide.A Hammett study of the reaction of substituted cyclopropylphenylmethanols with magnesium iodide provides a p value of -1.82, revealing substantial positive charge development on the carbinol carbon in the latter step.This investigation provides one of only a very few reported examples of Hammett studies used to probe positive charge development for a reaction carried out in anhydrous diethyl ether.