62184-82-1Relevant articles and documents
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
Adam,Sahin,Sendelbach,Walter,Chen,Williams
, p. 2576 - 2584 (2007/10/02)
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.
Denitrogenation of Bicyclic Azoalkanes through Photosensitized Electron Transfer: Generation and Intramolecular Trapping of Radical Cations
Adam, Waldemar,Sendelbach, Juergen
, p. 5316 - 5322 (2007/10/02)
To elucidate the intermediates that intervene in the denitrogenation of bicyclic azoalkanes by photosensitized electron transfer (PET), the 2,3-diazabicyclohept-2-ene (BDH) derivative 1b was synthesized, and its PET reactions were examined.With triphenylpyrylium tetrafluoroborate (TPT) or 9,10-dicyanoanthracene (DCA) as sensitizers and biphenyl as cosensitizer, the azoalkane 1b gave through intramolecular cyclization the spiro ethers 5 and 6 as trapping products, in addition to the bicyclopentane 2b and the cyclopentenes 3b,b', the latter as rearrangement products.Comparison with 1,4-dimethyl-DBH (1c) revealed that trapping of 1 1,3-diyl radical cation as the major pathway is unlikely.PET experiments with the regioisomeric cyclopentenes 3b,b', which both led to the spiro ether 5, imply the involvement of the allyl cation 3b(-H)(+) as the decisive intermediate in the nucleophilic trapping reactions.Comparison of the PET chemistry of the azoalkane 1b and the corresponding bicyclopentane 2b gave further insight into the mechanism of denitrogenation of azoalkanes through single electron transfer.The latter results lend additional support for the involvement of the diazenyl radical cation 1(.+), which to date has escaped direct detection.
