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 ¹³C-labeled PhN¹³CO. 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 <i>p</i>-tolyl- and <i>p</i>-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