Quantitative Electromerism of a Well-defined Mononuclear Copper Complex Through Reversible Intramolecular Metal-Ligand Electron Transfer.

Copper amine oxidases are enzymes that exhibit in their active site a mononuclear copper complex and a 2,4,5-trihydroxyphenylalanine quinone (TPQ) cofactor; in the oxidative half of the catalytic cycle, the enzymes regulate their activity by a temperature-dependent electron transfer equilibrium between the CuII complex with the reduced, aminoquinol form of the cofactor and the reactive CuI complex with the corresponding oxidized, semiquinone form of the cofactor. Here, we report the first mononuclear copper complex with redox-active ligands showing quantitative, reversible electromerism between a CuII eletromer with reduced, neutral ligand and a CuI electromer with an oxidized, radical monocationic ligand. The CuII form, being exclusively present at low temperature, exhibits a lower enthalpy (like the enzymes), but the CuI complex exhibits a higher entropy and is exclusively present at room temperature in CH2 Cl2 solution. Further analysis, based on six different copper complexes, discloses a large solvent effect on electromerism.

Kinetics and Mechanism of Pyrrolidine Buffer-Catalyzed Fulvene Formation.

Rapid synthesis of fulvenes is achieved using pyrrolidinium/pyrrolidine buffers in anhydrous acetonitrile. Time-dependent UV-vis absorption and NMR spectroscopy reveal that the rate and yield of fulvene formation depend strongly on both the presence of acid in the medium and the choice of solvent, and they are negatively affected by water. Kinetic data have been collected for various substrates, and the synthetic benefits of the adjusted reaction conditions are showcased. Enhancements of reaction rates are found in comparison to literature procedures. α-Unsaturated fulvenes that were previously difficult to access can now be obtained in good yields.