Theoretical analysis of concerted and stepwise mechanisms of the hetero-Diels–Alder reaction of butadiene with formaldehyde and thioformaldehyde

Abstract

The concerted and stepwise mechanisms of the hetero-Diels–Alder reaction of butadiene with formaldehyde and thioformaldehyde were studied by a CASSCF molecular orbital method. The energy barrier of the concerted reaction of butadiene with formaldehyde is about 21 kcal/mol higher than that of butadiene with thioformaldehyde at the CAS-MP2 calculation level. For the stepwise reaction paths, the energy barrier for the first step process of the reaction of butadiene with formaldehyde is about 17 kcal/mol above that of butadiene with thioformaldehyde. The concerted pathways for both systems are more favorable by 9–12 kcal/mol than the stepwise pathways. The electronic mechanisms for the concerted reactions of both reaction systems are also discussed by a CiLC analysis.