Synthesis and isolation of stable cation radicals of triarylphosphines, and development to functional molecules

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Crowded triarylphosphines of advanced properties were developed. One of the most crowded triarylphosphines, tris(2,4,6-tricyclohexylphenyl)phosphine was synthesized and the corresponding cation radical was prepared by oxidation with nitrosonium salts and isolated as purple crystals for the first time as a cation radical of triarylphosphine. X-ray crystallography revealed planar structure of phosphorus and anisotropic hfc constants were obtained from angular dependence of EPR spectra of a single crystal. Several crowded triarylphosphines carrying various aryl groups such as 2,6-dialkylphenyl, 2,6-diarylphenyl, and 2-alkyl-6-arylphenyl groups were synthesized and detailed structure, dynamic stereochemistry, especially inversion barrier of the phosphine and interconversion of isomers arising from helicity of propeller composed of three aromatic rings attached to the phosphorus atom. Intramolecular interaction between the crowded triarylphosphine moiety and the other functional site was also investigated. Several triarylphosphines carrying a donar or an acceptor at the 4-position to the phosphorus were synthesized and the substituent effects on electronic spectra were studied. Electron withdrawing groups such as a formyl group were found to cause red shift of the longest absorption wavelength to give orange to red colored phosphines. Crowded triarylphosphines carrying quinones were synthesized by Suzuki coupling and relation between redox properties and intramolecular charge transfer was systematically clarified. (4-Bromo-2,6-dicyclohexylphenyl)bis(2,4,6-tricyclohexylphenyl)phosphine was developed as a key synthetic intermediate for crowded triarylphosphines which can be oxidized to isolable cation radicals as well as possess functional sites. Diphosphines carrying the two crowded triarylphosphine moieties connected by biphenyl, azo, and nitroxyl linkage were synthesized.