Kinetics and Regioselectivity of Ring Opening of Substituted Cyclopropylmethyl Radicals

Article abstract of DOI:10.1021/jo00272a043

Accurate analysis of the mixture of hydroxylamines 8 formed when suitable peroxides 4 undergo homolysis in the presence of the nitroxyl radical 1,1,3,3-tetramethylisoindolin-2-yloxy (T) has afforded rate constants for ring opening of the cyclopropylmethyl radicals 2a, 2c, 2d^cis, 2d^trans, and 2e and cyclization of the but-3-enyl radicals 1a, 1c, and 1d.The presence of methyl substituents enhances the rates both of cyclization and of ring opening to give primary radicals.In the case of the trans-2-methylcyclopropyl radical, 2d^trans, this effect leads to preferential formation of the less thermodynamically stable possible product, 1d, below about 60 deg C.In general, the effects of substituents support the view that the transition structure for the cyclopropylmethyl-butenyl radical interconversion is dipolar.Combination of data from four sets of workers using three different kinetic techniques affords the following recommended Arrhenius equation for the cyclopropylmethyl radical clock reaction: log k₁(2a) = 13.31 - 7.6/2.3RT.