Classical Versus Nonstatistical Behavior in the C²-C6/Ene Cyclization of Enyne-Allenes: Intramolecular Kinetic Isotope Effects and Radical Clock Reactions

Abstract

The effects of aryl groups on the mechanism of the C²-C6/ene cyclization of enyne-allenes were studied by means of radical clock openings and intramolecular kinetic isotope effects. Upon attachment of a single aryl group at either the alkyne or the allene terminus, the thermal reaction proceeds by a stepwise mechanism that shows significant nonstatistical dynamic effects. Despite this, we were able to intercept the intermediate diradical intramolecularly by using the ultrafast diphenylcyclopropyl radical clock reaction. When aryl groups were present at both the alkyne and allene termini, the intramolecular kinetic isotope effects were consistent with a classical stepwise mechanism. The present study thus demonstrates the shift in reaction mechanism from a nonstatistical stepwise mechanism to a classical stepwise behavior, depending on the substituents.