ELECTROCYCLIC AROMATIC-SUBSTITUTION BY NITRILE YLIDES TO GIVE 3H-2-BENZAZEPINES - SUBSTITUENT EFFECTS AND MECHANISM

Benzonitrile 3,3-diarylallyl nitrile ylides (16), generated by the base-induced dehydrochlorination of imidoyl chlorides, cyclised by 1,7-ring closure to give 3H-2-benzazepines e.g. (19), in contrast to analogous diazo-compounds (1) which prefer 1,5-electrocyclisation. Asymmetrically placed substituents [R in (16b-e)] favour substitution at the ortho (2') position irrespective of their polar electronic effects. Deuterium labelling studies have shown that the cyclisation step is irreversible for these nitrite ylides in contrast to the analogous diazo-compounds (3) for which it is reversible.