Theoretical study of the mechanism of the cycloaddition of acetylene to α-pyrone: Asynchronism and substituent effects

Abstract

Ab-initio SCF-MO gradient methods have been applied to the cycloaddition reaction of acetylene and α-pyrone. This current study elucidates structures of the reactants and adduct (eqn. 1, 1c). The asynchronicity constant, A, is evaluated by assuming a symmetric, concerted pathway for this reaction. The structure of the transition state along this symmetric pathway indicates early bond formation at C-3 on the pyrone, and repulsion between C-4 and the approaching acetylene. The ab-initio activation energy and exothermicity (in kcal mol −1) are respectively: STO-3G: 65.5, −72.7; STO-4G: 66.3, −69.0. These values were compared to MINDO/3 and MM2 calculations. The effects of fluorine substituents on both diene and dienophile were studied in detail.