Cycloaddition of benzonitrile oxide to acetonitrile, propyne and propene - A comparative theoretical study of the reaction mechanism and regioselectivity

Quantum chemical calculations (MP2/6-31G* and B3LYP/631G*) were used to compare the reactivity, regioselectivity and orbital involvement of the reaction of benzonitrile oxide with the dipolarophiles acetonitrile, propyne and propene. All reactions are thermodynamically favoured. The product stability decreases in an order propyne > acetonitrile > propene, and reflects the degree of aromatic stabilisation of the product. The activation barriers depend strongly on the computational method used and decrease in the expected order of increasing reactivity (acetonitrile > propyne > propene) in the MP2 calculations, but are similar to each other when B3LYP is applied. The regiochemistry is correctly predicted for all reactions and the experimentally observed regioisomer is both thermodynamically and kinetically favoured. The transition state geometries indicate that, in some of the reactions, the benzonitrile oxide does not interact through its frontier orbitals, as traditionally assumed. Instead, the FMO+/-2 are involved in the reaction, indicating that the classical FMO concept should be applied with care as it might lead to wrong conclusions. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)