A New Dimeric Copper(II) Complex of Hexyl Bis(pyrazolyl)acetate Ligand as an Efficient Catalyst for Allylic Oxidations

A new dimeric copper(II) bromide complex, [Cu(LOHex)Br(μ-Br)]2 (1), was prepared by a reaction of CuBr2 with the hexyl bis(pyrazol-1-yl)acetate ligand (LOHex) in acetonitrile solution and fully characterized in the solid state and in solution. The crystal structure of 1 was also determined: the complex is interlinked by two bridging bromide ligands and possesses terminal bromide ligands on each copper atom. The two pyrazolyl ligands in 1 coordinate with the nitrogen atoms to complete the Cu coordination sphere, resulting in a five-coordinated geometry—away from idealized trigonal bipyramidal and square pyramidal geometries—which can better be described as distorted square pyramidal, as measured by the τ and χ structural parameters. The pendant hexyloxy chain is disordered over two arrangements, with final site occupancies refined to 0.705 and 0.295. The newly synthesized complex was evaluated as a catalyst in copper-catalyzed C–H oxidation for allylic functionalization through a Kharasch–Sosnovsky reaction without any external reducing agent. Using 0.5 mol% of this catalyst, and tert-butyl peroxybenzoate (Luperox) as an oxidant, allylic benzoates were obtained with up to 90% yield. The general reaction time was only slightly decreased to 24 h but a very significant decrease in the alkene:Luperox ratio to 3:1 was achieved. These factors show relevant improvements with respect to classical Kharasch–Sosnovsky reactions in terms of rate and amount of reagents. The present study highlights the potential of copper(II) complexes containing functionalized bis(pyrazol-1-yl)acetate ligands as efficient catalysts for allylic oxidations.

Theoretical Study on Magnetic Interaction in Pyrazole-Bridged Dinuclear Metal Complex: Possibility of Intramolecular Ferromagnetic Interaction by Orbital Counter-Complementarity

A possibility of the intramolecular ferromagnetic (FM) interaction in pyrazole-bridged dinuclear Mn(II), Fe(II), Co(II), and Ni(II) complexes is examined by density functional theory (DFT) calculations. When azide is used for additional bridging ligand, the complexes indicate the strong antiferromagnetic (AFM) interaction, while the AFM interaction becomes very weak when acetate ligand is used. In the acetate-bridged complexes, an energy split of the frontier orbitals suggests the orbital counter-complementarity effect between the dxy orbital pair, which contributes to the FM interaction; however, a significant overlap of other d-orbital pairs also suggests an existence of the AFM interaction. From those results, the orbital counter-complementarity effect is considered to be canceled out by the overlap of other d-orbital pairs.

The Mechanism of Rhodium Catalyzed Allylic C–H Amination

The mechanism of catalytic allylic C–H amination reactions promoted by Cp*Rh complexes is reported. Reaction kinetics experiments, stoichiometric studies, and DFT calculations demonstrate that allylic C–H activation to generate a Cp*Rh(π-allyl) complex is viable under mild reaction conditions. The role of external oxidant in the catalytic cycle is elucidated. Quantum mechanical calculations, stoichiometric reactions, and cyclic voltammetry<b></b>experiments support an oxidatively induced reductive elimination process of the allyl fragment with an acetate ligand. Lastly, evidences supporting the amination of an allylic acetate intermediate is presented. Both nucleophilic substitution catalyzed by Ag⁺that behaves as a Lewis acid catalyst and an inner-sphere amination catalyzed by Cp*Rh are shown to be viable for the last step of the allylic amination reaction.

The Mechanism of Rhodium Catalyzed Allylic C–H Amination

The mechanism of catalytic allylic C–H amination reactions promoted by Cp*Rh complexes is reported. Reaction kinetics experiments, stoichiometric studies, and DFT calculations demonstrate that allylic C–H activation to generate a Cp*Rh(π-allyl) complex is viable under mild reaction conditions. The role of external oxidant in the catalytic cycle is elucidated. Quantum mechanical calculations, stoichiometric reactions, and cyclic voltammetry<b></b>experiments support an oxidatively induced reductive elimination process of the allyl fragment with an acetate ligand. Lastly, evidences supporting the amination of an allylic acetate intermediate is presented. Both nucleophilic substitution catalyzed by Ag⁺that behaves as a Lewis acid catalyst and an inner-sphere amination catalyzed by Cp*Rh are shown to be viable for the last step of the allylic amination reaction.