AZO BRIDGES FROM AZINES .23. 1,5-LATICYCLIC CONJUGATION BETWEEN PARALLEL AZO AND O-PHENYLENE BRIDGES - STRUCTURE DEPENDENCE OF [6-PHOTOCYCLOADDITIONS(2])

Examples were synthesized of the four systems 1, 3, 5, and 7, in which rigid parallelo o-phenylene and azo bridges are connected to five- an d/or six-membered carbocyclic moities. The o-phenylene bridge was intr oduced by two routes: (A) starting from precursors already containing that bridge (24, 29) and assembling the azo bridge in consecutive step s (--> 3a, 3b, 5c, 5d, 5e, 5f, 5g); (B) starting from the systems with parallel C=C/N=N bridges (9a, 11a, 13a, 42) and completing the dihydr o-o-phenylene ring by tetrachlorothiophene dioxide. Dyotropic hydrogen transfer of the azo bridge enhances the dehydrogenation of the interm ediate dihydro-o-phenylene derivatives (22, 3cH(2), 25). This mechanis m was proved by the domino hydrogen transfer 44 --> 45 --> 5h. Via rou te B, systems 1a, 1b, 3c, 3d, 5a, 5b, 5h, and 43 were obtained. In sha rp contrast to the smooth [2 + 2] photocycloaddition of systems 9, 11, 13, and 15 (C=C/N=N bridges), [6 + 2] photocycloaddition occurs only with systems 1 and (5(C)/5(N)) and 3 (6(C)/5(N)) but not with systems 5 (5(C)/6(N)) and 7 (6(C)/6(N)). These differences are not caused by s lightly varying distances of the two bridges (X-ray data) but by the h igher n_ ionization energy of the azo group incorporated into a 2,3-di azabicyclo[2.2.1]hept-2-ene (DBH) instead of a 2,3-diazabicyclo[2.2.2] oct-2-ene (DBO) moiety, the hypsochromicity of the corresponding DBH n -pi state and the higher ground-state energy of DBH compared to DBO.