Benzoxepine formation by the 1,7 electrocyclisation of diene-conjugated carbonyl 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 11/12 in which carbonyl 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 13∶15 and 14∶16, has been determined for a series of 6-substituted compounds. Alkenyl groups and the thiophene ring are found to be ca. 10–20 times more reactive than phenyl. In cases where the 6-substituent is a substituted aryl group it has been found that, unlike the analogous nitrile ylides, aromatic substituents have little effect on the reactivity of the ring. The selectivity is unaffected by the nature of the substituent on the terminal atom of the carbonyl ylide. Mechanistic studies using deuteriated reactants show that the cyclisation step is irreversible.