理論探討胺催化α、β不飽和酮的自身Diels-Alder反應

Abstract

In the course of understanding Diels-Alder (DA) reaction mechanism, the global electrophiliciy and empirical nucleophilicity defined within the density functional theory (DFT) have been adopted as reaction indices to account for the relative activation energies found experimentally. These published results are accomplished by DA reactions of a specified diene with substituted ethylene series. One general rule can be summarized by the statement that the increase of the electron-deficiency character of the dienophile, together with the increase of the electron-rich character of diene, would lead to lower activation energy. In this work we have first employed DFT calculations to investigate the electronic nature of substituents with the aim to understand how chemical substitution affects the energy levels of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). Substituents having low-LUMO can press down the LUMO of an ethylene derivative (dienophile), which in return increases the magnitude of electrophiliciy of this dienophile. This effect is corresponding to the electron-deficiency argument for the dienophile, which enables the DA reactions to proceed with low activation energies. This also explains the low activation energy found for reaction systems involving hydrogen-bonding formation between the Lewis acid and the substituent. The studies have been extended to substituents having pi-type electron lone-pair orbital(s). It is found that those substituents would elevate levels of both HOMO and LUMO of the dienophile, the latter results in un-favored DA reactions.Since the destabilization of HOMO is corresponding to enhance the nucleophilicity, the substitution conducted on a diene would favor the charge transfer from diene to dienophile. In other words, chemical substitution using those substitutents conducted on “dienes” can also make DA reactions fast.This conclusion can be verified by the reported DA reactions, in which enamine {pyrrolidinyl-substituted (E)-buta-1,3-dien-1ylbenzene} was used as diene and dienophile. The role of the pyrrolidine moiety is to elevate the HOMO and enable a favored DA reaction. Effects of chemical substitution, performed on the pyrrolidinyl carbon, on HOMO/LUMO energies have also been investigated. It is found that, as expected, the inductive effects exerted by the substituents prevail.