PHOTOSENSITIZED ELECTRON-TRANSFER FROM AZOALKANES - GENERATION, FRAGMENTATION, AND REARRANGEMENT OF RADICAL CATIONS STRUCTURALLY RELATED TO DICYCLOPENTADIENE

The azoalkanes 1a-c extrude nitrogen upon photosensitized electron transfer (PET) to yield 1,3-radical 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 (Ph2) must be utilized to induce rearrangement to 3a,b and fragmentation to 4 and 4'.