Theoretical study of Diels-Alder reaction: Role of substituent in regioselectivity and aromaticity

A comparative study on the influence of the substituents on the Diels-Alder reaction was performed. The energy profiles for 11 sets of Diels-Alder reaction between monosubstituted derivatives of butadiene and ethylene have been studied and the structures of all transition states were located at B3LYP/6-31+G* level. Four pathways were independently investigated; the reaction between substituted ethylene and 1-substituted butadiene leading to ortho (a ₁) and meta (a ₂) adducts, and in the same manner, the reaction between substituted ethylene and 2-substituted butadiene yields para (b ₁) and meta (b ₂) adducts. Inspection of both the activation barriers and the reaction energies for 44 reactions revealed that the pathway b ₁ is both thermodynamically and kinetically more favorable in all types of Diels-Alder reactions; while the pathway a ₁ can be labeled only as kinetic pathway. The aromaticity of all 44 transition state structures was measured using para delocalization index to study the effect of aromaticity on the reaction path. The calculations suggest that in normal and neutral DA reactions there is a gain in aromatic stabilization of the transition state which reduces slightly the activation barrier of the kinetic pathway a ₁.