Phenyl isocyanate anion radicals and their cyclotrimerization to triphenyl isocyanurate anion radicals.

Room-temperature sodium metal reduction of phenyl isocyanate (PhNCO) in hexamethylphosphoramide yields the anion radical (PhNCO(•-)) where the unpaired electron exhibits coupling to one nitrogen and five unique protons. The extent of coupling to the carbon in the NCO group was obtained via the reduction of (13)C-labeled PhN(13)CO. Remarkably, this coupling is over 2 orders of magnitude smaller than that found for the alkyl-substituted analogue. This large attenuation indicates that the electron is not localized within the isocyanate group (as in the alkyl analogues) but is distributed throughout the entire π system including the phenyl ring. The consequence of this delocalization is that the isocyanate is expected to remain linear upon reduction of PhNCO. The anion radicals of p-tolyl- and p-methoxyphenyl isocyanate have also been generated. We find that these electron-donating substituents on the phenyl ring have little effect on the nitrogen coupling. Hence, the NCO group has the same geometry as the PhNCO(•-). When PhNCO is reduced in tetrahydrofuran, a solvent where ion association effects are common, PhNCO(•-) is not observed. Here, a cyclotrimerization occurs (initiated by PhNCO(•-)), generating the triphenyl isocyanurate anion radical where the unpaired electron is predominately localized in one of the carbonyl moieties.