Journal of the American Chemical Society, Vol.125, No.21, 6434-6446, 2003
Metal-ligand charge-transfer-promoted photoelectronic Bergman cyclization of copper metalloenediynes: photochemical DNA cleavage via C-4' H-atom abstraction
Metal-to-ligand charge-transfer (MLCT) photolyses (lambda greater than or equal to 395 nm) of copper complexes of cis-1,8-bis(pyridin-3-oxy)oct-4-ene-2,6-diyne (bpod, 1), [Cu(bpod)(2)]PF6 (2), and [Cu(bpod)(2)](NO3)(2) (3) yield Bergman cyclization of the bound ligands. In contrast, the uncomplexed ligand 1 and Zn(bpod)(2)(CH3COO)(2) compound (4) are photochemically inert under the same conditions. In the case of 4, sensitized photochemical generation of the lowest energy (3)pi-pi* state, which is localized on the enediyne unit, leads to production of the trans-bpod ligand bound to the Zn(II) cation by photoisomerization. Electrochemical studies show that 1, both the uncomplexed and complexed, exhibits two irreversible waves between E-p values of -1.75 and -1.93 V (vs SCE), corresponding to reductions of the alkyne units. Irreversible, ligand-based one-electron oxidation waves are also observed at +1.94 and +2.15 V (vs SCE) for 1 and 3. Copper-centered oxidation of 2 and reduction of 3 occur at E-1/2 = +0.15 and +0.38 V, respectively. Combined with the observed Cu(I)-to-pyridine(pi*) MLCT and pyridine(pi*)-to-Cu(II) ligand-to-metal charge transfer (LMCT) absorption centered near similar to315 nm, the results suggest a mechanism for photo-Bergman cyclization that is derived from energy transfer to the enediyne unit upon charge-transfer excitation. The intermediates produced upon photolysis degrade both pUC19 bacterial plasmid DNA, as well as a 25-base-pair, double-stranded oligonucleotide. Detailed analyses of the cleavage reactions reveal 5'-phosphate and 3'-phosphoglycolate termini that are derived from H-atom abstraction from the 4'-position of the deoxyribose ring rather than redox-induced base oxidation.