2012 年 70 巻 7 号 p. 731-741
Whereas biradicals are generally observed only under cryogenic inert conditions, highly stabilized 1,3-diphosphacyclobutane-2,4-diyls, phosphorus congeners of cyclobutane-1,3-diyl, can be stored at room temperature and even in air. We succeeded in establishing a promising synthetic protocol for the extremely stable biradicals by utilizing a bulky phosphaalkyne Mes*C≡P(Mes*=2,4,6-tri-t-butylphenyl). The singlet ground state biradicals were characterized by spectroscopies including NMR, and the X-ray structural parameters indicated effects of the phosphorus atoms and sterically encumbered aryl substituents. Conversion into the cyclic phosphorane and thermal ring-expansion were facilitated by the presence of amino- and acyl substituents, respectively. Dual hydrogen loadings on the biradical skeleton were accomplished to afford cyclobutadiene-like P-heterocycles. Most of the air-stable P-heterocyclic biradicals showed high electron-donating properties, and catenation of two or three biradical units induced unique intramolecular through-space interactions between the biradical chromophores. Stable P-heterocyclic neutral monoradical was successfully obtained by utilizing the biradical synthetic procedures.