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

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 lb 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.l]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.