Synthesis and redox properties of crowded triarylphosphines carrying a nitroxide radical and related compounds

Crowded triarylphosphines carrying a nitroxide radical were synthesized to investigate intramolecular interaction between the crowded triarylphosphine redox centers and a neutral radical. A crowded triarylphosphine carrying a bromoaryl group was developed as a key synthetic intermediate. Lithiation of the (bromoaryl)phosphine, followed by reaction with 2-methyl-2-nitrosopropane, afforded a crowded triarylphosphine carrying a nitroxide after aerobic oxidation. Similar procedure using isoamyl nitrite (isoamyl = 3-methylbutyl) gave nitroxide and azobenzene derivatives with two crowded triarylphosphine moieties. The azobenzene derivative was alternatively prepared by photolysis of the corresponding (azidoaryl)phosphine. EPR spectra of the nitroxide radicals showed hyperfine couplings with ³¹P nuclei, which were larger than previously reported phosphinoaryl nitroxide radicals. Cyclic voltammograms of the newly synthesized triarylphosphines showed reversible redox waves corresponding to the phosphorus redox centers. The two triarylphosphine moieties connected by a nitroxide linkage showed a reversible two-step oxidation reflecting considerable interaction between the phosphorus redox sites across the nitroxide linkage. Oxidation of the crowded triarylphosphines carrying a nitroxide and photolysis of the chemically oxidized (azidoaryl)phosphme was studied by EPR spectroscopy.