title: Direct arylations for study of the air-stable P-heterocyclic biradical: from wide electronic tuning to characterization of the localized radicalic electrons.
abstract: We have developed methods for installing aryl substituents directly on the phosphino groups of the 1,3-diphosphacyclobutane-2,4-diyl system. The aryl substituents tuned the electronic and structural characteristics of the biradical unit both in solution and in the solid state. 1-tert-butyl-2,4-bis(2,4,6-tri-tert-butylphenyl)-1,3-diphosphacyclobuten-4-yl anion, prepared from phosphaalkyne (Mes*C≡P; Mes* = 2,4,6-tBu3C6H2) and t-butyllithium, was allowed to react with an electron-deficient N-heterocyclic reagent. The corresponding N-heteroaryl-substituted P-heterocyclic biradicals were produced via SNAr reactions. Biradicals bearing perfluorinated pyridyl substituents exhibited photoabsorption properties comparable to those of previously reported derivatives because the highest occupied and lowest unoccupied molecular orbit levels were reduced by a similar amount. In contrast, the triazine substituent reduced the band gap of the biradical unit, and the large red shift in the visible absorption and high oxidation potential were further tuned via subsequent SNAr and Negishi coupling reactions. The amino-substituted triazine structure provided a strongly electron-donating biradical chromophore, which produced unique p-type semiconducting behavior even though there was no obvious π-overlap in the crystalline state. The single-electron transfer reaction involving Mes*C≡P, phenyllithium, and iodine afforded 1,3-diphenyl-2,4-bis(2,4,6-tri-tert-butylphenyl)-1,3-diphosphacyclobutane-2,4-diyl via the intermediate P-heterocyclic monoradical. The tetraaryl-substituted symmetric biradical product was used to determine the electron density distribution from the X-ray diffraction data. The data show highly localized radicalic electrons around the skeletal carbon atoms, moderately polarized skeletal P-C bonds in the four-membered ring, and no covalent transannular interaction.
authors.affiliation: Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology , 2-12-1 Ookayama, Meguro, Tokyo 152-8552, Japan.
authors.forename: Shigekazu
authors.initials: S
authors.lastname: Ito
authors.forename: Yasuhiro
authors.initials: Y
authors.lastname: Ueta
authors.forename: Trang Thi Thu
authors.initials: TT
authors.lastname: Ngo
authors.forename: Makoto
authors.initials: M
authors.lastname: Kobayashi
authors.forename: Daisuke
authors.initials: D
authors.lastname: Hashizume
authors.forename: Jun-ichi
authors.initials: J
authors.lastname: Nishida
authors.forename: Yoshiro
authors.initials: Y
authors.lastname: Yamashita
authors.forename: Koichi
authors.initials: K
authors.lastname: Mikami
mesh_terms: D000388:Air; D004583:Electrons; D005609:Free Radicals / Q000138:chemical synthesis / Q000737:chemistry; D006571:Heterocyclic Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D008958:Models, Molecular; D015394:Molecular Structure; D009943:Organophosphorus Compounds / Q000737:chemistry*; D011789:Quantum Theory
chemical_list: D005609:Free Radicals; D006571:Heterocyclic Compounds; D009943:Organophosphorus Compounds