The thermal and photochemical rearrangement of 1,2(4H)-diazepine derivatives

Thermolysis of 3,5,7-triphenyl-1,2(4H)-diazepine (1) gives, besides the known 2,4,6-triphenyl-pyridine (2), 3,5,6-triphenyl-1,2(4H)-diazepine (3) and 3,5,6-triphenyl-1,2(1H)-diazepine (4) whose structures are deduced on the basis of spectroscopic and chemical evidence. Neat thermolysis of 1 and the related 5-(p-methoxyphenyl)-3,7-diphenyl-1,2(4H)-diazepine (24a) and 5-(p-chlorophenyl)-3,7-diphenyl-1,2(4H)-diazepine (24c) gives only the corresponding pyridine derivatives 2, 26a, and 26c in low yields. Photolysis of 1 in ethanol affords the somewhat unstable 3,5,7-triphenyl-1,2-diazabicyclo[3.2.0]hepta-2,6-diene (21) which upon thermolysis undergoes cycloreversion to 1 and upon hydrolysis provides 3,5-diphenylpyrazole (23) and aceto-phenone. In contrast, photolysis of 1 as well as of 24a and 24c in acetone results in the formation of the pyridine (26a, 26c) and pyrazole (25a, 25c) derivatives respectively in low yields. Irradiation of 5-(P-dimethylaminophenyl)-3,7-diphenyl-1,2(4H)-diazepine (24d) under these conditions gives the photo-oxygenation product 24e. The observed thermolysis and photolysis results of 1 are compared and contrasted with those reported for the closely related 4,4-dimethyl-3,7-diphenyl-1,2(4H)-diazepine (16).