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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 (DBH) derivative lb 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 lb 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 (lc) revealed that trapping of a 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 lb 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.

On direct irradiation, the azoalkanes anti-hexahydro-1,4:5,8-dimethanophthalazines (Sa, 1,4-H; Sb, 1,4-Me;SC, 1,4-Ph) do not only give the expected housanes 6 through a C-N bond cleavage and subsequent denitrogenation but also yield, in the order Sa > Sb > 5c, the rearranged aziranes 7 through /3 C-C bond cleavage. The increased formation of the heterocycles 7 on triplet sensitization with benzophenone and the suppression of these rearrangement products by the triplet quenchers trans-piperylene and 1,3-cyclohexadiene show that the aziranes 7 are derived from the triplet state of the azoalkanes 35* by /3 C-C bond cleavage. The higher amounts of housanes 6 at higher temperatures suggest that intersystem crossing (ISC) to 5* competes with denitrogenation in the singlet-excited azoalkanes 15*.Since ISC should be relatively temperature independent, the temperature effect is rationalized in terms of an activation barrier for a C-N bond cleavage in the singlet-excited azoalkane IS*, which is more readily overcome at higher temperatures to form increased amounts of housane 6. From the temperature dependence of the direct and sensitized photoreactions of azoalkane Sa, the relative activation energies were estimated to be 13.3 and 210.5 kcal/mol for CY C-N scission from the singlet and triplet states and 17.9 kcal/mol for the p C-C bond cleavage from the triplet.

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The azoalkanes la-c extrude nitrogen upon photosensitized electron transfer (PET) to yield 1,3radical cation intermediates 2,+, which undergo interesting transformations. Electron back transfer (BET) affords the unrearranged housanes 2, but significant rearrangement to dicyclopentadiene derivatives 3 occurs prior to BET. In some cases, fragmentation to cyclopentadienes 4 is observed, a cycloreversion that occurs at the 1,3-radial cation stage rather than through the corresponding 1,3-biradical intermediates. The diphenyl-substituted azoalkane la affords high yields of the symmetric cyclopentadiene 4a, while the alkyl-substituted lb produces only small amounts of a rearranged cyclopentadiene 4'b. The housanes 2a,b are also oxidized by PET, but are more reluctant to rearrange. Strong electron acceptors such as triphenylpyrylium tetrafluoroborate (TPT) or cosensitization with biphenyl (PI12) must be utilized to induce rearrangement to 3a,b and fragmentation to 4 and 4'.

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