Importance of charge-transfer effects in [4+2] cycloaddition reaction

Abstract

The self-consistent charge and configuration method for subsystems is used to study the [4+2] cycloaddition of 1- and 2-monosubstituted butadienes (C 4H 5X, X=CN, Cl, F, CH 3, OH, NH 2) with monosubstituted ethylenes (C 2H 3Y, Y=Cl, F, CH 3, OH). All calculations are performed for reactive systems (diene/dienophile) in the transition-state geometries. The system ‘electronic’ activation energies are decomposed into deformation, Heitler-London (the sum of electrostatic and exchange-repulsion), charge-transfer (CT) and polarisation components. It is shown that the CT energy, which is often considered as the starting point for constructing reactivity indicators such as: the frontier orbitals, local softnesses, and Fukui functions, is not responsible for regioselectivity. Such behaviour of CT energy suggests that these criteria have not energetic origin. Nevertheless, the CT energy is the only term, which is very sensitive to the type of substituent in diene and dienophile and depends on donor/acceptor character of the attached substituents. The results obtained confirm the ortho rule of regioselectivity for the normal, inverse, and neutral electron demand Diels-Alder reactions (1-substituted butadienes/ethylenes). The para rule is valid only for normal electron demand Diels-Alder reactions (2-substituted butadienes/ethylenes).