Captodative Rate Enhancements in the Methylenecyclopropane Rearrangement

Article abstract of DOI:10.1021/jo00364a009

A series of 2-aryl-2-carbethoxy-3,3-dimethylmethylenecyclopropanes, 7, with substitution in the para position of the aromatic ring, have been prepared.These substrates rearrange thermally to 2-aryl-2-carbethoxyisopropylidenecyclopropanes, 8, by fragmentation of the cyclopropane bond which gives a biradical intermediate.Rates of this rearrangement are substituent-dependent.Electron-donor substituents such as p-OCH3 and p-SCH3 enhance the rearrangement rate to a greater extent than that predicted by their rate-enhancing effect on the "parent" system, 3-aryl-2,2-dimethylmethylenecyclopropane (4).This enhanced rearrangement rate is attributed to the captodative effect in which the acceptor carbethoxy group and the donor group on the aromatic ring exert a synergistic stabilizing effect on the benzylic portion of the biradical intermediate. p-Fluoro and p-methyl substituents are also capable of acting as true donor groups in captodative systems.Rearrangement rates of 7 containing electron-donor groups correlate with ?⁺ values, which are a measure of the donor abilities of various groups.The captodative effect has been used as a probe for the mechanism of stabilization of free radicals by the sulfinyl group, SOCH3.No captodative rate enhancement is observed when 7-p-SOCH3 is rearranged.This implies that stabilization of free radicals by the sulfinyl group does not involve utilization of the sulfur nonbonding electron pair in a donor fashion, but probably involves this group acting as an acceptor group.