Electron Transfer Photochemistry of Norbornadiene and Quadricyclane. Nucleophilic Capture of Radical Cations, Free-Radical Rearrangements, and Hydrogen Abstraction

Article abstract of DOI:10.1021/jo00118a034

The photoinduced electron donor-acceptor reactions between norbornadiene (N) or quadricyclane (Q) and acceptor/sensitizers generate products of several structure types, depending on the nature of sensitizers and solvents.Irradiation of 1,4-dicyanobenzene (DCB) in acetonitrile/methanol leads to methanol adducts 2 and 3, NOCAS products 4-7, and two acetonitrile adducts 8 and 9, which are formed only from N.The products are rationalized via stereospecific nucleophilic attack by methanol on the radical cations, N(.+) and Q(.+), from the exo-face.The resulting free radicals, exo-3-methoxybicyclo<2.2.1>hept-5-en-2-yl (N(.+) -> CH3O-B(.)) and anti-5-methoxytricyclo<2.2.1.0^2,6>heptan-3-yl (Q(.+) -> CH3O-C(.)) undergo rapid molecular rearrangements to CH3O-C(.) and syn-7-methoxybicyclo<2.2.1>hept-5-en-2-yl, (CH3O-E(.)), respectively, before forming products 2-7.The methanol adducts 2 and 3 are ascribed to hydrogen abstraction by CH3O-C(.) and CH3O-E(.), most likely due to the insufficient reducing ability of DCB(.-).The abstraction reaction is supported by isotropic labeling studies and by acetonitrile adducts 8 and 9 formed by attack of (.)CH2CN on N.The reduced singlet energy of 1-cyanonaphthalene (CNN) causes the electron transfer from N to be less favorable, whereas the reducing ability of CNN(.-) is increased.The reaction leads to methanol adducts 1-3, <2 + 2>-cycloadducts, and several 1:1:1 adducts of CNN, N, and methanol.The formation of methanol adducts is initiated by nucleophilic capture of the radical cations; isotopic labeling studies suggest a competition between hydrogen abstraction and reduction/protonation mechanisms.