An Ab Initio Study on the Conformational and Endo/exo Preferences of Acrylates in Diels-Alder Reactions

Abstract

Abstract The four transition structures (TS) of the reaction between butadiene and methyl acrylate have been studied at several ab initio levels considering both the non-catalyzed and the BF3-catalyzed processes. Whereas in the non-catalyzed reaction the s-cis TS are more stable than the corresponding s-trans TS, and the exo approaches are preferred over the endo, both situations are reversed in the case of the catalyzed reaction, in which the endo s-trans is the most stable TS. The results obtained show that for the uncatalyzed reaction, the endo/exo selectivity mainly depends on the conformation of the dienophile in the TS, and therefore that the energy differences between the s-cis and s-trans TS also change from endo to exo approaches. The catalyst induces a strong preferential stabilization of the endo s-trans TS, which is in agreement with the experimental observations. © 1997 Elsevier Science Ltd.