Regioselective rearrangement of bridgehead-methyl-substituted radical cations derived from bicyclo[2.1.0]pentanes and 2,3-diazabicyclo[2.2.1]hept-2-enes through photoinduced electron transfer and radiolytic oxidation. Product distribution and matrix ESR studies

Article abstract of DOI:10.1021/ja00085a043

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^-+ was the only final oxidation product of 1b; the initial radical cation 1b^-+ was not detected because it rearranges rapidly and stereoselectively by a 1,2-hydrogen shift to 3b^-+, even at 80 K, and no trace of the more stable 1-methylcyclopentene radical cation 3a^-+ 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^-+. 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^-+ through its reluctance to undergo a 1,2-H shift. This was confirmed by direct ESR observation, which also showed that the rearrangement of 1c^-+ is much slower than that of the parent cyclopentane- 1,3-diyl radical cation 1^-+. This difference is attributed to a larger effect of methyl stabilization on the reactant than on the product, leading to a decrease in exothermicity and an increase in the activation energy for the rearrangement of 1c^-+ relative to that of 1^-+. The 1-methyl and 1,4-dimethyl substituted 2,3-diazabicyclo [2.2.1]hept-2-enes 2b,c on PET reaction also yielded evidence for the intermediacy of 1,3-diyl radical cations; however, the product distributions suggest that denitrogenation can also be accompanied by concomitant 1,2-H shifts at the stage of the intermediate diazenyl radical cations, albeit with lower efficiency. ESR studies on the oxidation of 2c failed to detect the very stable 1c^-+ species on the pathway to the 1,3-dimethylcyclopentene radical cation 3C^-+, indicating that denitrogenation of 2C^-+ results in a rapid rearrangement to 3c^-+ even under matrix-isolation conditions at 77 K. Consequently, 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.