The 1,7-8π-electron electrocyclisation of o-alkenylaryldiazoalkanes (1) to give 1H-2,3-benzodiazepines (3) takes place readily for substrates with a cis-hydrogen atom at the cyclisation site, but is blocked by phenyl or methyl groups at that position. Such compounds react only via loss of nitrogen, the former, e.g. (18), to give naphtho[a]cycloheptenes (20) by a new intramolecular carbene reaction, and the latter to give carbene ‘dimers’, azines, products of solvent insertion, and in one case an indene (37). The blocking effect of cis-methyl and -phenyl groups is attributed to steric hindrance in a helical transition state (42) for the 1,7-electrocyclisation. The results are also discussed in relation to the two possible primary processes of 1,7-electrocyclisation and 1,1-cycloaddition in the reactions of the isoelectronic systems (43) and (47) containing nitrilium and diazonium betaines.