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

• Published in: Journal of the American Chemical Society 1994-03, Vol.116 (6), p.2576-2584
• Main Authors: Adam, Waldemar, Sahin, Coskun, Sendelbach, Juergen, Walter, Herbert, Chen, Guo-Fei, Williams, Ffrancon
• Format: Article
• Language: English
• Subjects: 400102 - Chemical & Spectral Procedures; 400201 - Chemical & Physicochemical Properties; 400500 - Photochemistry; ALKANES; ALKENES; CATIONS; CHARGED PARTICLES; CHEMICAL RADIATION EFFECTS; CHEMICAL REACTIONS; CHROMATOGRAPHY; CYCLOALKANES; CYCLOALKENES; DATA; DECOMPOSITION; ELECTRON SPIN RESONANCE; ELECTRON TRANSFER; EXPERIMENTAL DATA; GAS CHROMATOGRAPHY; HYDROCARBONS; INFORMATION; INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; IONS; MAGNETIC RESONANCE; MASS SPECTROSCOPY; NUMERICAL DATA; ORGANIC COMPOUNDS; OXIDATION; PHOTOCHEMICAL REACTIONS; RADIATION EFFECTS; RADICALS; RADIOLYSIS; RESONANCE; SEPARATION PROCESSES; SPECTROSCOPY
• ISSN: 0002-7863; 1520-5126
• 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[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., 2 tabs.