2390-40-1Relevant academic research and scientific papers
A minimalist approach to C-H activation by copper
Kuebel-Pollak, Anita,Ruettimann, Stephane,Dunn, Nichola,Melich, Xavier,Williams, Alan F.,Bernardinelli, Gerald
, p. 841 - 853 (2007/10/03)
The complex [Cu2(1)2]2+ (1=1,3-bis(1-methyl-1H-benzimidazol-2-yl)benzene) undergoes slow oxidation by dioxygen in DMF solution to give the hydroxylated product [Cu 2(2-H)2]2+ (2=2,6-bis(1-methyl-1H-benzimidazol- 2-yl)phenol) characterized by an X-ray crystal-structure analysis. The oxidation occurs much faster when CuII is mixed with 1 in the presence of H2O2, with 80% hydroxylation observed within a few minutes. The mononuclear complex formed with 1-methyl-2-phenyl-1H-benzimidazole (3) shows no hydroxylation under these conditions. It is concluded that the hydroxylation requires the presence of a ligand capable of stabilizing a binuclear species, but no special coordinative activation of the copper is required.
The photocyclization of o-alkoxy phenyl ketones
Wagner, Peter J.,Meador, Michael A.,Park, Bong-Ser
, p. 5199 - 5211 (2007/10/02)
Several o-alkoxybenzophenones and o-(benzyloxy)benzophenones and -acetophenones photocyclize to 3-hydroxy-2,3-dihydrobenzofurans. Quantum yields generally are quite high, except for o-(benzyloxy)acetophenone. The o-ethoxy and o-benzyloxy ketones form two diastereomeric products, the Z isomer being highly preferred in hydrocarbon solvents, the E isomer being formed in comparable yield in methanol or with added pyridine. The reaction involves δ-hydrogen abstraction by the ketone triplets followed by cyclization of the 1,5-biradical intermediates. The biradicals have such short lifetimes that they usually cannot be detected by flash spectroscopy or trapped by thiols. Triplet state lifetimes, determined both by steady-state quenching studies and by flash kinetics, reveal that hydrogen abstraction rate constants are quite low. Arrhenius plots for triplet decay indicate activation energies of 3-5 kcal/mol and A values of 109 for the δ-hydrogen abstraction. MMX calculations and spectroscopic data all indicate that the ketones exist primarily in conformations with the carbon α to the ether oxygen twisted well away from the carbonyl. The low observed rate constants are ascribed to even lower equilibrium populations of conformers in the geometry required for reaction in the triplet state than in the ground state. 2,6-Diacylphenyl ethers show ten times the triplet reactivity of their monoacyl equivalents. In these cases, the ether function is twisted 90° such that the target C-H bond is much closer to a carbonyl. The large solvent effects on the stereochemistry of cyclization despite short biradical lifetimes suggest that bond rotations may induce intersystem crossing of the triplet biradicals. The low cyclization quantum yield from o-(benzyloxy)acetophenone and the formation of o-benzoylacetophenone as a major side product suggest that the 1,5-biradicals partially cyclize into the benzene ring to generate spiroenol intermediates. Rate constants for quenching of the triplet ketones by 2,5-dimethyl-2,4-hexadiene were measured. The kq values are ≥5 × 109 M-1 s-1 for the o-methoxy ketones but only 1-3 × 109 for the o-benzyloxy ketones. This rare steric effect on triplet energy transfer is attributed to twisting of the benzoyl chromophores caused by steric congestion.
