To Isomerize or not to Isomerize? E/Z Isomers of Cyclic Azobenzene Derivatives and Their Reactivity Upon One‐Electron Reduction

Abstract

Azo compounds are efficient electron acceptors. Upon one‐electron reduction they generally isomerize forming the thermodynamically most stable radical anion. Herein we show that the size of the central ring in 1,2‐diazocines and diazonines has a ruling influence on the configuration of the one‐electron reduced species. Markedly, diazonines, which bear a central nine membered heterocycle, show light‐induced E/Z isomerization, but retain the configuration of the diazene N=N moiety upon one‐electron reduction. Accordingly, E/Z isomerization is not induced by reduction.

Synthesis of Halogenated Dibenzo[1,2,6]triazonines and Late-Stage Functionalization of the Triazonine Ring

“…It must be noted here that triazonines 4a – g in Scheme and Table are obtained in the E form. For comparison, it may be recalled that the synthesis of the nine-membered thiadiazonine also resulted in the E isomer exclusively, while the parent diazonine was isolated as a mixture of isomers with the Z isomer as the major one . Thus, the nature of the atoms in the nine-membered ring seems to control the preferred stereochemistry, N and S heteroatoms favoring the E isomer.…”

“…Having in hand the halide-functionalized triazonines 4a – g , we have considered their use as starting materials in palladium-promoted coupling reactions, which are widely recognized for their prominent versatility and synthetic usefulness. They have enabled notably the easy functionalization of acyclic azobenzenes and diazocines. − , Thus, the 3,8-diiodotriazonine 4a , the 3,8-dibromotriazonine 4e , and the 3-iodotriazonine 4g have been subjected to Suzuki-type couplings with aryl-boronic acids in order to ascertain the compatibility of the triazonine scaffold with the quite harsh conditions of these couplings (Scheme ).…”

“…Although seven-membered cyclic azobenzene diazepines ( III ) have been known for decades, extensive investigations on their photophysical behavior have been reported only recently, showing that the E form is especially unstable with a lifetime in the millisecond range . Similarly, the photochemical isomerization of dibenzo­[ c , h ]­[1,2]­diazonine ( IV ) and dibenzo­[ c , g ]­[1,5,6]-thiadiazonine ( V ) has been disclosed starting from 2018 and cyclic azobenzenes based on [1,2,6]­triazonine scaffolds ( VI ) have been introduced in 2021 by our group . In the diazonine series, the nature of the bridging chain subtly controls the geometrical features.…”