WHAT MAKES A NEUTRAL IMINO DIENEOPHILE UNDERGO A THERMAL, NONCATALYZED, DIELS-ALDER REACTION

Ab initio calculations have been carried out at the MP2/6-31G level o f theory on the aza-Diels-Alder reaction of a range electron deficient imines (with sulfonyl and/or carbonyl substituents) with buta-1,3-die ne. In all cases but one, an early cyclic transition-state could be lo cated 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 (dien ophile) controlled and that the most electron deficient imines should be more reactive than simple alkyl or aryl imines and imines possessin g less electron withdrawing groups. Evidence is also found for exo-lon e pair effects, but these effects become obscured by other stereoelctr onic effects as electron withdrawing substituents are added to the imi ne. (C) 1998 Elsevier Science Ltd. All rights reserved.