The 1,7-8.rc-electron electrocyclisation of o-alkenylaryldiazoalkanes (1 ) to give 1 H-2,3-benzodiazepines (3) takes place readily for substrates with a cis-hydrogen atom at the cyclisation site, but is blocked b y 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) b y a n e w 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 t o the two possible primary processes of 1,7-electrocyclisation and 1 ,I -cycloaddition in the reactions of the isoelectronic systems (43) and (47) containing nitrilium and diazonium betaines. R H R' H R' (3) X (13; R = Me) gave the carbene 'dimer' (15; R = Me) (40%) and the azine (16; R = Me) (38%), formed by reaction of the Me H \ + -C=N=N * The conformational assignments of the proximate products (29) and (31) [and also (33) and (%)I are based on a transition state for ring closure of the type shown in (42).

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