Bridges, parallel o-phenylene and azo
Dyotropic hydrogen transfer
[6 + 2] Photocycloaddition
Wiley InterScience Backfile Collection 1832-2000
Chemistry and Pharmacology
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- and/or six-membered carbocyclic moities. The o-phenylene bridge was introduced by two routes: (A) starting from precursors already containing that bridge (24, 29) and assembling the azo bridge in consecutive steps (→ 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 dihydro-o-phenylene ring by tetrachlorothiopene dioxide. Dyotropic hydrogen transfer of the azo bridge enhances the dehydrogenation of the intermediate dihydro-o-phenylene derivatives (22, 3cH2, 25). This mechanism was proved by the domino hydrogen transfer 44 → 45 → 5h. Via route B, systems 1a, 1b, 3c, 3d, 5a, 5b, 5h, and 43 were obtained. In sharp 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 (5C/5N) and 3 (6C/5N) but not with systems 5 (5C/6N) and 7 (6C/6N). These differences are not caused by slightly varying distances of the two bridges (X-ray data) but by the higher n_ ionization energy of the azo group incorporated into a 2,3-diazabicyclo[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-π* state and the higher ground-state energy of DBH compared to DBO.
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