Photochemical generation and methanol trapping of localized 1,3 and 1,4 singlet diradicals derived from a spiroepoxy-substituted cyclopentane-1,3-diyl

The direct and benzophenone-sensitized photodenitrogenation of the azoalkane 2,3-diazabicyclo-[2.2.1]hept-2-ene-7,3'-spiro-2',2'-diphenyloxirane 1 at ambient temperature (ca. 15 OC) in benzene afforded exclusively the 6-oxabicyclo[3.2.0]hept-1-ene 3. The labile oxetane 3 is proposed to be formed by the selective cleavage of the C-O bond in the epoxy ring of the initially photogenerated, spiroepoxy-substituted, localized cyclopentane-1,3-diyl diradical 1,3-DR to the 1,4-DR diradical and subsequent cyclization of the latter. Even at -100 degrees C, the highly strained (strain energy estimated to be ca. 94 kcal/mol) spiroepoxy housane 2 could not be observed by H-1 NMR (600 MHz) spectroscopy. In MeOH, instead of the oxetane 3, the two regioisomeric MeOH adducts 6 (trapping product of the diradicals 1,3-DR and 1,4-DR) and 6' (methanolysis product of 3) were obtained in high yields. For the first time methanol trapping of a cyclopentane-1,3-diyl singlet diradical was achieved; the required dipolar character is presumably induced in this symmetric species by the polar methanol medium. Computational work (UB3LYP/6-31G*) reveals that the S-1,3-DR singlet diradical possesses a singlet ground state (Delta E-ST ca. 1 kcal/mol), for which ring closure (E-A ca. 2.6 kcal/mol) to the spiroepoxy housane 2 requires more activation than epoxide-ring opening (E-A Ca. 0.7 kcal/mol); the T-1,3-DR triplet species ring-opens spontaneously (E-A Ca. 0 kcal/mol) to the corresponding T-1,4-DR. Stereoelectronic effects dictate the appropriate conformational alignment of the C-O bond with the radical p orbital in the 1,3-DR for the selective cleavage of the C-O rather than C-C bond in the epoxy ring.