iV,iV'-Dimethyl-p-hydrazotoIuene (1), A7,A''-dimethyl-p-hydrazoanisole (2), and A7,A7'-dimethyl-p-hydrazobiphenyl (3) rearranged to the corresponding o-semidines (compounds 5, 12, and 23) when irradiated in cyclohexane solution. Rearrangement of 1 was relatively slow but clean. That is, after 10 hr 19% of 1 had rearranged, while 74% was recovered. In contrast, after 3 hr 63% of 2 had rearranged, 11% had undergone scission to A7-methylanisidine, and 17% was recovered. After 17 hr of irradiation of 3, 34% was recovered, 24% had rearranged, and 35% had undergone scission to A7-methyl-4-aminobiphenyl. Some 4-aminobiphenyl was also formed. Irradiation of A7,A7'-dimethylhydrazomesitylene (4) caused scission to A'-methylmesidine and some demethylation-scission to mesidine. The photochemical rearrangements of A7,A7'-dimethylhydrazobenzenes are the first photobenzidine rearrangements known to occur in more than trace amounts.Acid-catalyzed rearrangement of 1 gave the o-benzidine type product 24 and 2,7-dimethylphenazine (25) in 9 and 5% yields, respectively. Three other unidentified compounds were also obtained. Acid-catalyzed rearrangement of 2 gave 46% of the o-semidine ( 12) and 42% of A7-methylanisidine. Mechanistic details of the photorearrangement have not been explored, but it is evident that the rearrangement pathways are different from those of the well-known acid-catalyzed rearrangements.Although acid-catalyzed and thermally induced benzidine rearrangements are well known,4 almost nothing about photochemical rearrangements has appeared in the literature. Hashimoto and coworkers5 have reported the chromatographic detection of o-and p-semidine in 0.25% yields from the irradiation of hydrazobenzene. As far as we know, no other example of rearrangement of simple hydrazo aromatics has been reported. This is not surprising, since it is now known that hydrazobenzenes, which do not have substituents on the hydrazo-group nitrogen atoms, undergo dehydrogenation and, in small part, N,N bond scission when irradiated.6,7 The dehydrogenation reaction is almost quantitative at 290 nm (whereas some scission to anilines occurs at 250 nm) and involves the solvent, since the use of A,A'-dideuteriohydrazobenzene in cyclohexane gave large amounts of HD.7 Hydrogen-atom formation from singlet hydrazobenzene followed by disproportionation of hydrazyl radicals has been proposed by Shizuka6 to explain the behavior of hydrazobenzene. In contrast with the behavior of simple hydrazobenzenes, we have found that , '-dimethyihydrazobenzenes undergo rearrangement. The case of N,N '-dimethy lhy drazobenzene was reported briefly recently.2 We have been exploring the scope of the rearrangement of A,A'-dimethylhydrazobenzenes and now report on four of them.A,A'-Dimethyl-p-hydrazotoluene (1), A,A'-dimethyl-p-hydrazoanisole (2), and A, A '-dimethyi-p-hydrazobiphenyl (3) rearrange to the o-semidine (eq 1 in Table I). Authentic o-semidines 5 and 12 were synthesized, but synthesis of authentic 23 was not attempted.Another product of rearrangement, the o...
