- Rh(III)-photosensitized interconversion of norbornadiene and quadricyclane
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The utility of two Rh(III) diimine complexes, Rh(phen)33+ and Rh(phi)2(phen)3+ (phen = 1,10-phenanthroline, phi = 9,10-phenanthrenequinone diimine), as sensitizers for the interconversion of norbornadiene (N) and quadricyclane (Q) has been investigated using steady-state photochemical and laser flash photolysis (LFP) techniques. Irradiation of acetonitrile solutions of Rh(phen)33+ and N causes slow conversion to Q. The reaction is reversible; irradiation of Rh(phen)33+ in the presence of Q leads to N. Irradiation of acetonitrile solutions of Rh(phi)2(phen)3+ and Q yields N. However, this reaction is irreversible; irradiation of the Rh-(III) complex in the presence of N fails to afford Q. Irradiation of methanol solutions of either Rh(III) complex in the presence of N or Q affords minor amounts of two methanol-C7 adducts but fails to quench the N-Q interconversion reaction. The results are consistent with N-Q interconversion via an exciplex intermediate. The Rh(III)-sensitized deazatization of two cyclic azoalkane derivatives (Azo-N, Azo-Q) of N and Q was also investigated. Deazatization was achieved by Rh(phen)33+ but not Rh(phi)2(phen)3+ sensitization. The results are consistent with a mechanism involving triplet energy transfer, but the involvement of exciplex intermediates cannot be ruled out. Bimolecular rate constants for quenching of the Rh(III) excited states by N, Q, Azo-N, and Azo-Q were determined by LFP.
- Sluggett, Gregory W.,Turro, Nicholas J.,Roth, Heinz D.
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p. 8834 - 8838
(2007/10/03)
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- Electron Transfer Photochemistry of Norbornadiene and Quadricyclane. Nucleophilic Capture of Radical Cations, Free-Radical Rearrangements, and Hydrogen Abstraction
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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-methoxybicyclohept-5-en-2-yl (N(.+) -> CH3O-B(.)) and anti-5-methoxytricyclo2,6>heptan-3-yl (Q(.+) -> CH3O-C(.)) undergo rapid molecular rearrangements to CH3O-C(.) and syn-7-methoxybicyclohept-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, -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.
- Weng, Hengxin,Roth, Heinz D.
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p. 4136 - 4145
(2007/10/02)
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