Diels–Alder reactions between cyclic five-membered dienes and acetylene

Abstract

Computational studies at hybrid density functional theory (B3LYP), CCSD(T) and semiempirical levels were performed on the Diels–Alder reactions of a variety of cyclic five-membered dienes, (CH) 4X (X=CH 2, SiH 2, O, NH, PH, and S) with acetylene as dienophile. The results were compared with those obtained with ethylene as the dienophile. The semiempirical AM1 and PM3 methods, which showed good agreement in modeling the cycloaddition reactions with ethylene as the dienophile, were found to be grossly inadequate for both quantitative and qualitative prediction. In contrast, the HF level although quantitatively no better than the semiempirical methods has shown good qualitative agreement with higher levels of theory. Activation and reaction energies are very similar for ethylene and acetylene for the given set of dienes. B3LYP level is in very good agreement with the CCSD(T) and this furthers our confidence in using B3LYP methodology in modeling the Diels–Alder reactions.