Concerning periselectivity. A method for predicting the major route in thermal intermolecular two component cycloaddition reactions having manysymmetry-favoured pathways

The application of the Woodward-Hoffmann rules to thermal, intermolecular two-component cycloaddition reactions generates a set of 'symmetry favoured' pericyclic reactions. The problem of choosing, within this set, the most favoured reaction (i.e. periselectivity) is discussed. It has been found that a second order perturbational molecular orbital (PMO) treatment, employing Huckel eigenvectors and eigenvalues, and which is restricted to include only those terms containing highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbitals, gives results which are in satisfactory agreement with the available experimental data. The treatment has been extended to cover the effect of substituents on the preferred mode of cycloaddition.