Denitrogenation of Bicyclic Azoalkanes through Photosensitized Electron Transfer: Generation and Intramolecular Trapping of Radical Cations

Article abstract of DOI:10.1021/jo00072a009

To elucidate the intermediates that intervene in the denitrogenation of bicyclic azoalkanes by photosensitized electron transfer (PET), the 2,3-diazabicyclo<2.2.1>hept-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.