What makes a neutral imino dieneophile undergo a thermal, non-catalysed, Diels-Alder reaction?

Abstract

Ab initio calculations have been carried out at the MP2/6-31G ∗ level of theory on the aza-Diels-Alder reaction of a range electron deficient imines (with sulfonyl and/or carbonyl substituents) with buta-1,3-diene. In all cases but one, an early cyclic transition-state could be located in which most often, the forming NC bond was more fully formed than the forming CC bond. However, examples were found in which this order of bond formation was reversed. Examination of HOMO-LUMO energy levels show that aza-Diels-Alder reactions are HOMO (diene)-LUMO (dienophile) controlled and that the most electron deficient imines should be more reactive than simple alkyl or aryl imines and imines possessing less electron withdrawing groups. Evidence is also found for exo-lone pair effects, but these effects become obscured by other stereoelctronic effects as electron withdrawing substituents are added to the imine.