Azo Bridges from Azines, XXIII. 1,5‐Laticyclic Conjugation Between Parallel Azo and o‐Phenylene Bridges. Structure Dependence of [6 + 2] Photocycloadditions

Abstract

AbstractExamples 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.