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It has been experimentally demonstrated that viscosity differentiates the extent to which inverted and retained housanes are formed in the liquid-phase denitrogenation of a substituted diazabicyclo[2,2,1]hept-2-enes (DBH)-type azoalkane, namely syn -2,3-diaza-4,7-dimethyl-1-ethyl bicyclo[2,2,1]heptene. The fact that the viscosity profiles coincide for the inversion/retention product ratio, assessed for a set of diverse solvents at constant temperature and through the temperature variation in a single solvent ( n -butanol), shows that the temperature dependence of the stereoselectivity is controlled by bulk viscosity effects. The simple free-volume model rationalizes the viscosity behavior of the inversion/retention ratio. Since the viscous medium differentiates between the inversion and retention reaction channels, we conclude that an unsymmetrical, nitrogen-containing intermediate, presumably the singlet diazenyl diradical, intervenes in the liquid-phase deazetation process.

Radical anions of 2,3-diazabicyclo[2.2.1]hept-2-ene 1 and its 18 substituted and tricyclic derivatives (2a–11c) have been characterized by their hyperfine data with the use of EPR and ENDOR spectroscopy. The structural modifications of 1, yielding 2a–11c, are methyl and/or phenyl substitutions in the 1,4-positions, introduction of methyl groups or a spirocyclopentane ring in the 7,7-positions and annelation by a cyclopropane, cyclobutane, cyclobutene, cyclopentane or cyclopentene ring in the 5,6-endo-positions. Although these modifications do not markedly alter the π-spin and charge distribution in 1˙-, they have a strong effect on the long-range coupling constant of the γ-protons in the 5,6-exo-positions: the pertinent aH(γ) value varies from +0.26 to +0.64 mT. The effect is particularly impressive for the 5,6-endo-annelation by a cycloalkane or a cycloalkene ring and it goes along with only slight changes in the geometry of the carbon framework in 1˙-. Thus, owing to the through-bond mechanism of π,σ-spin transfer (homohyperconjugation), the coupling constant, aH(γ), of the 5,6-exo-protons responds sensitively to such changes in the geometry.

Photochemical electron transfer (PET) and chemical electron transfer (CET) studies have been conducted in solution and within zeolite cavities for the bicyclo[2.1.0]pentanes (2a−j), prepared by direct photolysis of the corresponding azoalkanes 1. The advantage of the CET oxidations is that they proceed catalytically in a clean manner to afford the rearranged cyclopentenes 3 in excellent yields. A complete reversal in the regioselectivity of the 1,2 migration has been observed for the unsymmetrical derivatives of bicyclo[2.1.0]pentane, namely 2b (methyl substitution) versus 2c,i (phenyl substitution). Both in solution and in the zeolite cavities, the less substituted cyclopentene 3b‘ is obtained for the methyl derivative 2b and the more substituted cyclopentenes 3c,i for the phenyl cases 2c,i. This unexpected fact is rationalized in terms of delocalization of the positive charge into the aromatic ring for the phenyl-substituted radical cation, as corroborated by AM1 calculations. Furthermore, the electron ...

The chemical behavior of the radical cations derived from 2,3-diazabicyclo[2.2.1]hept-2-enes 1 and bicyclo[2.1.0]pentanes 2 by oxidation with tris (aryl)aminium hexachloroantimonates has been examined.

Cyclopentane-1,3-diyl radical cations were generated from the 1-methyl- and 1,4-dimethyl substituted bicyclo-[2.1.0]pentanes 1b,c through photoinduced electron transfer (PET) and radiolytic oxidation. The unsymmetrical bridgehead-substituted bicyclopentane 1b rearranged spontaneously and exclusively to the 3-methylcyclopentene 3b under PET conditions. ESR studies showed similarly that 3b[sup [sm bullet]+] was the only final oxidation product of 1b; the initial radical cation 1b[sup [sm bullet]+] was not detected because it rearranges rapidly and stereoselectively by a 1,2-hydrogen shift to 3b[sup [sm bullet]+], even at 80 K, and no trace of the more stable 1-methylcyclopentene radical cation 3a[sup [sm bullet]+] was observed. This contra-thermodynamic regioselectivity is rationalized in terms of essential localization of positive charge at the tertiary center as the reaction proceeds in the 1,3-diyl radical cation 1b[sup [sm bullet]+]. The symmetrical dimethyl derivative 1c rearranged much more reluctantly than 1b despite its lower oxidation potential, and this is attributed to the greater persistence of radical cation 1c[sup [sm bullet]+] through its reluctance to undergo a 1,2-H shift. The oxidation of these azoalkanes generates highly reactive transients, presumably diazenyl radical cations, which readily denitrogenate and undergo 1,2-H shifts in either a consecutive or concerted manner to form olefin radical cations. 25 refs., 6 figs., 2more » tabs.« less