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A large-scale synthesis of l-isopropoxy carbonyl-1,2-diazepine (17) was performed using a thin film ultraviolet reactor. 1,5-Disubstituted diazepines 10 and 12 and 1,3,7-trisubstituted 1,2-diazepine 14 have been prepared.Iron tricarbonyl complexes could be obtained only with diazepines which are unsubstituted on the butadiene moiety. Photosensitization of a 1-iminopyridinium ylide of type 1, using triplet sensitizers, dramatically increased the photolytic N-N bond cleavage, a process which is in competition with the photoisomerization to 1.2diazepines. A singlet excited state is postulated for the formation of 1,2-diazepines of type 3. Base-induced ring opening of diazepine 17 led to the cis-cis dienaminonitrile 18 in good yield, whereas its pyrolysis led only to trace amounts of the isomeric dienaminonitriles 19 and 20. Thermal rearrangement of 17 in o-xylene and in acetic acid solution gave 2-isopropoxycarbonylaminopyridine ( 21) and l-isopropoxycarbonyliminopyridinium ylide (15), respectively, and suggests the existence of an equilibrium between diazepine 17 and its valence tautomer 16.