Electron demand in the transition state of the cyclopropylidene to allene ring opening
作者:Philip Warner、Robert Sutherland
DOI:10.1021/jo00049a042
日期:1992.11
The electronic structure of the transition state for the cyclopropylidene to allene conversion has been probed. The methodology involved the relative rates of ring opening vs trapping by MeOH for a series of variously substituted 2,3-diarylcyclopropylidenes. With the assumption that the rate of trapping was unaffected by substituents, a Hammett correlation was constructed. The negative value (-0.72) for rho indicated that the carbenic center attracts electron density in the ring-opening transition state, much like the cyclopropyl cation to allyl cation transition state. Temperature-dependent studies showed that the observed preference for ring opening was driven by entropy factors. Also, using reasonable estimates for the close to diffusion-controlled trapping activation enthalpies, the derived enthalpies for ring opening were in close agreement with the best theoretical values.