In the photochemical denitrogenation of 1,4-diaryl-2,3-diazabicyclo[2.2.1]heptane (AZ6) bearing sterically hindered substituents, a curious new absorption band at about 450 nm was observed under low-temperature matrix conditions, together with the previously well-characterized planar singlet diradical pl-¹DR6 with λ_max=≈580 nm. The 450 nm species was electron paramagnetic resonance (EPR)-silent. Instead of generating the planar diradical pl-¹DR6 and the precursor azoalkane AZ6 upon warming, the ring-closed bicyclo[2.1.0]pentane derivative SB6, that is, the AZ6 denitrogenation product was identified. Based on product analysis, low-temperature spectroscopic observations, high-level quantum-mechanical computations, viscosity effect, and laser-flash photolysis, the puckered singlet diradicaloid puc-¹DR6 was assigned to the new 450 nm absorption. The latter was detected experimentally at the same time as the planar singlet diradical pl-¹DR6. Sterically demanding substituents as well as viscosity impediments were essential for the detection of the experimentally hitherto unknown puckered singlet cyclopentane-1,3-diyl diradicaloid puc-¹DR6, that is, the third isomer in homolysis. The present findings should stimulate future work on the mechanistically fascinating stereoselectivity documented in the formation of bicyclo[2.1.0]pentanes during the 2,3-diazabicyclo[2.2.1]heptane denitrogenation.

High-spin organic molecules have attracted much attention as spin sources for magnetic materials. In this paper, a novel high-spin organic molecule was designed according to a logical strategy. A 2,2-dimethylcyclopentane-1,3-diyl diradical unit was utilized as a spin source and a spin-bridged fragment. In addition, nitroxides were introduced as spin sources. Quintet electron-spin resonance (ESR) signals, which were attributed to the 1,3-di(3′-nitoroxyphenyl)-2,2-dimethylcyclopentane-1,3-diyl tetraradical (TR), were observed in a 2-methyltetrahydrofuran glassy matrix at 80 K after the photodenitrogenation of a newly synthesized azoalkanes AZ. A simulation of the signal provided the zero-field splitting parameters D (|D|/hc = 0.0127 cm⁻¹⁾ and E (|E|/hc = 0.0014 cm⁻¹) for the quintet state of the tetraradical TR. The parameter D of the quintet state of TR was found similar to those of the previously reported quintet spin states of tetraradical TR1 with two cyclobutane-1,3-diyl units (0.0190 cm⁻¹) and TR2 with two 2,2-dimethylcyclopentane-1,3-diyl diradicals (0.0116 cm⁻¹). Copyright © 2014 John Wiley & Sons, Ltd.

A stereochemical deuterium-labeling study of the thermal and photochemical denitrogenation of the exo-selectively dideuterated azoalkane 7,7-diethoxy-2,3-diazabicyclo[2.2.1]hept-2-ene (AZb-d₂) was performed in order to obtain information about the denitrogenation mechanism of 2,3-diazabicyclo[2.2.1]hept-2-ene derivatives. A double-inversion product 5,5-diethoxybicyclo[2.1.0]pentane (CPb), that is, inv-CPb, was selectively formed over the retention product ret-CPb in a ratio of inv-CPb/ret-CPb = 73/27 under flash vacuum pyrolysis (FVP) conditions (365 °C/~1.0 × 10⁻² mmHg). The formation of inv-CPb was slightly higher than that of ret-CPb in the direct photolysis of AZb-d₂ at 70 °C. An approximately 1:1 mixture of the two products was observed after the direct photolysis at 7 °C. A stereoselective formation of 1,5-diethoxycyclopent-1-ene (MG-d₂) was found in the denitrogenation of AZb-d₂ under the FVP conditions. Computational studies at the broken-symmetry (BS)-(U)CCSD/6-31G(d) level of theory revealed that the stepwise C–N bond cleavage is the energetically favored process of the denitrogenation mechanism. The S_H2 trajectory of the nitrogen-extrusion process through an equatorial conformation of the intermediary diazenyl diradical DZb-eq was found for the formation of inv-CPb. An alternative route for inv-CPb was found through an axial conformation of DZb-ax, which was suggested by the trajectory calculations using a Born–Oppenheimer molecular dynamics at the UB3LYP/6-31G(d). Copyright © 2011 John Wiley & Sons, Ltd.

Quantum chemical calculations were performed to investigate the cooperative effect of the nitrogen and silicon atoms on the singlet–triplet energy spacing and the reactivity of the singlet state in 1,2-diazacyclopentane-3,5-diyls and 1,2-diaza-4-silacyclopentane-3,5-diyls. The largest singlet–triplet energy gap ( = −36.1 kcal/mol) found so far in localized 1,3-diradicals was in the C_2v symmetry of 4,4-difluoro-1,2-diaza-4-silacyclopentane-3,5-diyl at the UB3LYP/6-31G(d) level of theory. The cooperative effect was also found in the energy differences of singlet diradicals with the corresponding ring-closing compounds, bicyclo[2.1.0]pentane derivatives. The singlet state of the 1,2-diaza-4-silacyclopentane-3,5-diyls was calculated to be energetically more stable than the ring-closing compound. The notable finding on the stability of the singlet diradicals may be attributed to the resonance structures that specifically stabilize the singlet state of diradicals. The computational studies predict that the singlet 1,2-diaza-4-silacyclopentane-3,5-diyl is a persistent molecule under conditions without intermolecular-trapping reagents. Copyright © 2010 John Wiley & Sons, Ltd.

The stereo- and regioselectivity of triplet-sensitised radical reactions of furanone derivatives have been investigated. Furanones 7 a,b were excited to the ³ππ* state by triplet energy transfer from acetone. Intramolecular hydrogen abstraction then occurred such that hydrogen was transferred from the tetrahydropyran to the β position of the furanone moiety. Radical combination of the tetrahydropyranyl and the oxoallyl radicals led to the final products 8 a,b. In the intramolecular reaction, overall, a pyranyl group adds to the α position of the furanone. The effect of conformation was first investigated with compounds 9 a,b carrying an additional substituent on the tether between the furanone and pyranyl moiety. Further information on the effect of conformation and the relative configuration at the pyranyl anomeric centre and the furanone moiety was obtained from the transformations of the glucose derivatives 12, 14, 17 and 18. Radical abstraction occurred at the anomeric centre and at the 5′-position of the glucosyl moiety. Computational studies of the hydrogen-abstraction step were carried out with model structures. The activation barriers of this step for different stereoisomers and the abstraction at the anomeric centre and at the 6‘-position of the tetrahydropyranyl moiety were calculated. The results of this investigation are in accordance with experimental observations. Furthermore, they reveal that the reactivity and regioselectivity are mainly determined in the hydrogen-abstraction step. Intramolecular hydrogen abstraction (almost simultaneous electron and proton transfer) in ³ππ* excited furanones only takes place under restricted structural conditions in a limited number of conformations that are defined by the relative configuration of the substrates. It is observed that in the biradical intermediate, back-hydrogen transfer occurs leading to the starting compound. In the case of glucose derivatives, this reaction led to epimerisation at the anomeric centre.