Intramolecular and intermolecular reactivity of localized singlet diradicals: The exceedingly long-lived 2,2-diethoxy-1,3-diphenylcyclopentane-1,3-diyl

The direct and benzophenone-sensitized photodenitrogenation of the diethoxy-substituted diazene 3 in nonprotic solvents, e.g. n-hexane, benzene, and acetonitrile, afforded exclusively the housane 4 through the intermediary 2,2-diethoxy-1,3-diphenylcyclopentane 1,3-diyl diradical 2b. Alternatively, in the presence of methanol, the formation of the adduct 6 competed through trapping of the allylic cation 5. The intervention of the intermediates was corroborated by laser-flash photolysis experiments of the diazene 3. The diradical intermediate was characterized by its strong transient absorption at 545 nm, which decayed with a first-order lifetime in the microsecond range. The diradical lifetimes increased in the order n-hexane (0.52 mu s), benzene (0.88 mu s), acetonitrile (1.01 mu s), and chloroform (3.73 mu s). The singlet ground state of the 2,2-diethoxy-substituted 1,3-diradical 2b is suggested by the absence of EPR signals at,low temperature, the lack of quenching by molecular oxygen, and the calculated (UB3LYP/6-31G*) singlet preference of ca. 4 kcal mol(-1) for 2,2-dihydroxy- 1,3-diphenyl-l,3-cyclopentanediyl as a model. Trapping of the 1,3-diradical 2b by external additives, e.g., olefins, dienes, and TEMPO, was not observed. In protic solvents, a decrease of the diradical lifetimes with increasing acidity was noted, e.g., the lifetime in acetic acid dropped to 70 ns. Moreover, a concomitant growth of an additional transient at 470 nm was found in acidic solvents, which was assigned to the allylic cation 5. The factors governing the intramolecular and intermolecular chemical reactivity of the localized singlet diradical 2b are discussed.