Methylenecyclopropane Rearrangement as a Probe for Free Radical Substituent Effects. Effect of Sulfur in Variouis Oxidation States

Article abstract of DOI:10.1021/jo00357a032

A series of 3-aryl-2,2-dimethylmethylenecyclopropanes 4 with the sulfur-containing substitutents SCH3, SOCH3, SO2CH3, and PS(OEt)2 in the para and meta positions have been prepared.These substrates undergo thermal rearrangement at 80 deg C in C6D6 to give the corresponding 2-arylisopropylidenecyclopropanes 5 at rates which vary as a function of substituent.The para-substituted substrates all rearrange at faster rates than the unsubstituted system.Comparison with the meta isomers suggests that the rate-enhancing effect is conjugative in nature.The singlettrimethylenemethane biradical intermediate is suggested to be stabilized by p-SCH3, p-SOCH3, and p-SO2CH3 substituents, with p-SCH3 providing the greatest stabilization.The p-PS(OEt)2 substituents is also quite effective at increasing rate.These rate effects are considered in terms of interaction of the nonbonding electron pair of SCH3 with the developing benzylic radical center.A further stabilizing interaction involving vacant d orbitals is considered to account for the greater stabilizing effect of SCH3 relative to OCH3, as well as the relatively large stabilizing effects of SOCH3, SO2CH3, and PS(OEt)2 on the transition state for this methylenecyclopropane rearrangement.