Alkynyl<i>N</i>-Nosylhydrazones: Easy Decomposition to Alknynl Diazomethanes and Application in Allene Synthesis
作者:Yang Yang、Zhaohong Liu、Alessio Porta、Giuseppe Zanoni、Xihe Bi
DOI:10.1002/chem.201701462
日期:2017.7.6
reaction with boronic acids under metal-free conditions is reported, giving rise to a wide array of di- and trisubstituted allenes. Preliminary mechanistic investigations demonstrated that γ-protodeboration of propargyl boric acid was responsible for the initial allene formation. This methodology based on the nosyl group allows for novel transformations that involve an alkynylcarbene transient species
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.