Carbanionic Rearrangements of (Halomethylene)cycloalkanes

Article abstract of DOI:10.1021/ja00186a043

The mechanism for the unusual base-induced ring-enlargement reaction of (halomethylene)cyclobutanes to 1-halocyclopentenes was examined by C-13 labeling studies with (bromomethylene)cyclobutane (1), cis- and trans-1-(bromomethylene)-3-ethoxy-2,2-dimethylcyclobutane (15 and 16), and 1-(bromomethylene)-2,2,4,4-tetramethylcyclobutane (22).Two competing processes were found to lead from the vinyl anion to rearranged products: (1) rehybridization of the vinyl anion to a 1,2-carbene-anion, which subsequently undergoes rearrangement, and (2) a Beckmann-like simultaneousmigration of bromide and ring carbon.Both processes subtly bypass the "forbidden" alkyl-to-carbanion shift.