Direct Arylations for Study of the Air-Stable P-Heterocyclic Biradical: From Wide Electronic Tuning to Characterization of the Localized Radicalic Electrons

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• 作者：Shigekazu Ito ; Yasuhiro Ueta ; Trang Thi Thu Ngo ; Makoto Kobayashi ; Daisuke Hashizume ; Jun-ichi Nishida ; Yoshiro Yamashita ; Koichi Mikami
• 刊名：Journal of the American Chemical Society
• 出版年：2013
• 出版时间：November 20, 2013
• 年：2013
• 卷：135
• 期：46
• 页码：17610-17616
• 全文大小：398K
• 年卷期：v.135,no.46(November 20, 2013)
• ISSN：1520-5126

文摘

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鈮; 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 S_NAr 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 S_NAr 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 蟺-overlap in the crystalline state. The single-electron transfer reaction involving Mes*C鈮, 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鈥揅 bonds in the four-membered ring, and no covalent transannular interaction.