Biaryl Anion Radical\nFormation by Potassium Metal\nReduction of Aryl Isocyanates and Triaryl Isocyanurates

The potassium metal reduction of aryl isocyanates (aryl\n= phenyl, <i>p</i>-tolyl, 3,5-dimethylphenyl, 4-biphenylyl,\nand 1-naphthyl)\nin THF with 18-crown-6 or in HMPA results in the formation of the\ncorresponding triaryl isocyanurate anion radicals. Continued exposure\nto potassium results in loss of the isocyanurate anion radical and\nthe eventual formation of the respective biaryl anion radical. The\n1,1′-binaphthyl anion radical is found to undergo a cyclodehydrogenation\nreaction, which leads to formation of the perylene anion radical.\nWhen authentic triaryl isocyanurates are reduced with metal, the heterocyclic\nring undergoes fragmentation with elimination of carbon monoxide to\nproduce a triarylbiuret dianion. This ring opening reaction is initiated\nby the two-electron reduction of the neutral isocyanurate species.\nThe biaryl anion radical is formed when the biuret dianion is reduced\nfurther with metal. A possible mechanism for biaryl formation involves\na heterolytic cleavage of an aryl C–N bond and release of an\naryl radical once the triarylbiuret dianion is further reduced. A\nsubsequent intermolecular reaction between two aryl radicals forms\nthe corresponding biaryl, which can then be reduced to the anion radical.\nNotably, when a mixture of two different triaryl isocyanurate compounds\nis reduced in solution, the corresponding mixed biaryl anion radical\nis generated.