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

Regio- and diastereoselective rearrangement of cyclopentane-1,3-diyl radical cations generated by electron transfer

“…35,36 The low energy barrier of ca. 3 kcal mol 21 for the hydrogen shift is corroborated by the EPR findings in that the 1,3-radical cation 6• + does not persist even at 77 K. 24 The alternative 1,2-shift through a puckered transition-state structure (not shown in Fig. 1) requires a prohibitive activation barrier of ca.…”

“…Indeed, the one-electron oxidation of the unsymmetrical methyl-substituted housane 6a yielded exclusively 3-methylcyclopentene (8a), whereas its phenyl analog 6b gave mainly 1-phenylcyclopentene (7b) as rearrangement product (Scheme 3). 4,24 Moreover, the distinctly different product distributions in the PET reactions of the azoalkanes 5 and the corresponding housanes 6 provide strong evidence for the involvement of diazenyl radical cations in the denitrogenation of the oxidized azoalkanes 5. This was already documented by the matrix EPR studies in the radiolytic oxidation of housanes and azoalkanes.…”

“…Besides EPR spectroscopy under matrix-isolation conditions 23,24 and pulse-radiolysis 25,26 studies, also chemical trapping 7,27 proved helpful in detecting and characterizing the transient radical cations derived from azoalkanes and housanes. Thereby valuable mechanistic insight has been gained into the chemical behavior of these short-lived reactive intermediates.…”

“…Thus, like the diazenyl diradicals, the corresponding radical cations 5• + expel N 2 with a concomitant 1,2-hydrogen shift through backside attack on the remaining C-N bond. 24,33 Recent ab initio calculations furnished a detailed mechanistic trajectory for the 1,2-shift in the methyl-substituted cyclopentanediyl radical cation 6• + (Fig. 1).…”

Radical ion cyclizations

“…Although cyclic azoalkanes are well known as biradical precursors [159] they have been used as 1,2-and 1,3-radical cation precursors only recently [160][161][162][163][164]. Apart from the rearrangement products bicyclopentane 161 and cyclopentene 163, the PET-oxidation of bicyclic azoalkane 158 yields mostly unsaturated spirocyclic products [165].…”