Mechanism of the Diels-Alder Reaction: Reactions of Butadiene with Ethylene and Cyanoethylenes

摘要

PMO theory suggests that the effects of substituents on the rate of the Diels-Alder reaction between ethylene and 1,3-butadiene cannot be explained on the basis of a synchronous mechanism. Calculations are reported, using the RHFl, UHF, and CI versions of MNDO and AM1, for the Diels-Alder reactions of 1,3-butadiene with ethylene, acrylonitrile, maleonitrile, fumaronitrile, and 1,1-dicyanoethylene. The results confirm the PMO conclusions, indicating unambiguously that the reactions involving the cyanoethylenes cannot by synchronous. The evidence suggests that Diels-Alder reactions can be predicted on this basis, more simply and more reliably than they can in terms of frontier orbital theory. The mechanism of the simplest example, i.e., the reaction of ethylene with butadiene, remains uncertain.