Azepinones. V. The mechanisms of ring contractions of some 5-substituted 6H-4,5-Dihydro-3,7-diphenyl-1,2-diazepines

Abstract

The mechanism of the rearrangement to 4-substituted-3,6-diphenylpyridazines of 6H-4,5-dihydro-3,7-diphenyl-1,2-diazepine (I; R = H) and its 5-methyl and -phenyl (I; R = CH₃; C₆H₅) derivatives when treated with N-chlorosuccinimide has been investigated by a combination of u.v., N.M.R., and mass spectrometric methods. The reaction involves a stereospecific 1,3-dehydrohalogenation to form a diazanorcaradiene (111) intermediate. In the presence of excess N-chlorosuccinimide, ring opening of the three-membered ring occurs by radical attack on the protonated intermediate. In the absence of N-chlorosuccinimide, ring opening proceeds by protonation of the cyclopropane ring of the diazanorcaradiene to yield an appropriately 4-substituted 3,6-diphenylpyridazine. The data exclude the participation of any 5H-1,2-diazepines, the valence tautomers of the norcaradiene intermediates.