Benzazepine formation by the 1.7 electrocyclisations of diene-conjugated nitrile ylides: studies on relative rates of cyclisation via intramolecular competition reactions

Abstract

A series of reactions has been carried out using reactants of the type 19 in which nitrile ylide cyclisation on to the substituent at the 6 position is in competition with cyclisation on to the unsubstituted phenyl group at the 2 position. The relative reactivity of the two groups, determined by measuring the product ratio 20 : 21, was determined for a series of 6-substituents as shown in Table 8. This is the first collection of such data for the electrocyclisation of 1,3-dipolar intermediates. Alkenyl groups and the thiophene ring were found to be > 100 × more reactive than phenyl. In cases where the 6-substituent was a substituted aryl group it was found that all aromatic substituents at the 3′ and 4′ positions, irrespective of their electronic nature, increased the reactivity of the ring relative to that of the unsubstituted phenyl group. In contrast, a methyl group at the 2′ position produced strong deactivation. The results are discussed in terms of the steric and electronic effects of the substituents.