Impact of Transition-State\nAromaticity on Selective\nRadical–Radical Coupling of Triarylimidazolyl Radicals

Radical coupling reactions are generally known to have\na low selectivity\ndue to the high reactivity of radicals. In this study, high regio\nand substrate selectivity was discovered in the dimerization of triarylimidazolyl\nradicals (<b>TAIR</b>), a versatile photochromic reaction. When\ntwo different radicals, 2-(4-cyanophenyl)-4,5-diphenyl-1<i>H</i>-imidazolyl radical (<b>CN-TAIR</b>) and 2-(4-methoxyphenyl)-4,5-diphenyl-1<i>H</i>-imidazolyl radical (<b>OMe-TAIR</b>), were simultaneously\ngenerated in situ, a hexaarylbiimidazole, formed by selective coupling\nat the nitrogen atom at position 1 of <b>CN-TAIR</b> and the\ncarbon atom at position 2 of <b>OMe-TAIR</b>, was isolated with\nhigh selectivity as the main product among 24 possible radical dimer\nhexaarylbiimidazole derivatives. This high regio and substrate selectivity\ncannot be explained solely by the stability of the product and/or\nthe electrophilicity and nucleophilicity of the radicals but originates\nfrom the aromaticity of the transition state in the radical–radical\ncoupling reaction. To date, the selectivity of radical coupling reactions\nhas been thought to be controlled by steric hindrance and radical\nspin density, but this study revealed a new factor for controlling\nradical coupling, that is, transition-state aromaticity. Aromaticity\nhas been reported to have an important effect not only in the reactivity\nand structure of ground-state molecules but also on the electronically\nexcited states and transition states in pericyclic reactions such\nas the Diels–Alder reaction and the Cope–Claisen rearrangement.\nThis study demonstrated for the first time that radical coupling reactions\ncan also be controlled by transition-state aromaticity.