A Comparative Study of Isocyanates and Isothiocyanates via One Electron Reduction

Isocyanates (R-N=C=O) are a versatile class of compounds with application primarily in the formation of industrial and commercial products. Isocyanates are widely used as building blocks for more complex molecular systems, particularly for various polymerization products such as polyurethane. While quite a bit of chemistry has been reported for isocyanates, there is little known about the one electron reduction of these systems. Herein, we report on the room temperature alkali metal reduction of aryl isocyanates. The use of electron paramagnetic resonance (EPR) spectroscopy has revealed that upon reduction, the phenyl isocyanate yields the formation of an anion radical species in solution, and the identity of this species greatly depends upon the solvent used. Reduction of phenyl isocyanate in hexamethylphosphoramide (HMPA) via exposure to sodium metal yields the anion radical phenyl isocyanate, where the unpaired electron exhibits coupling to one nitrogen and five unique protons. The formation of this monomer anion radical is strongly supported by the reduction of isotopically labeled compounds and computational analysis. When phenyl isocyanate is reduced in tetrahydrofuran (THF), a solvent where ion association effects are common, the anion radical phenyl isocyanate is not observed. Rather, a cyclotrimerization occurs (initiated by the anion radical monomer), generating the triphenyl isocyanurate anion radical where the unpaired electron is predominately localized in one of the carbonyl moieties. Preliminary studies in the one electron reduction of isothiocyanates (R-N=C=S) are also discussed. Isothiocyanates are a class of organosulfur compounds with somewhat similar structure to isocyanates, but the differences in size and electronegativity in the NCS moiety greatly affect the chemistry occurring during the reduction process. We report the formation of three unique anion radical species throughout the course of a reduction of ethyl isothiocyanate in a THF solution containing 18-crown-6 ether through exposure to sodium metal. Experiments were performed using a combination of EPR and nuclear magnetic resonance (NMR) spectroscopy techniques, as well as the addition of an internal standard to observe the growth and decay of these signals.