Isolation and Characterization of a Trinuclear Cobalt Complex Containing Trigonal-Prismatic Cobalt in Secondary Alcohol Aerobic Oxidation

Oxygen-Free Csp3-H Oxidation of Pyridin-2-yl-methanes to Pyridin-2-yl-methanones with Water by Copper Catalysis

Aromatic ketones are important pharmaceutical intermediates, especially the pyridin-2-yl-methanone motifs. Thus, synthetic methods for these compounds have gained extensive attention in the last few years. Transition metals catalyze the oxidation of Csp3-H for the synthesis of aromatic ketones, which is arresting. Here, we describe an efficient copper-catalyzed synthesis of pyridin-2-yl-methanones from pyridin-2-yl-methanes through a direct Csp3-H oxidation approach with water under mild conditions. Pyridin-2-yl-methanes with aromatic rings, such as substituted benzene, thiophene, thiazole, pyridine, and triazine, undergo the reaction well to obtain the corresponding products in moderate to good yields. Several controlled experiments are operated for the mechanism exploration, indicating that water participates in the oxidation process, and it is the single oxygen source in this transformation. The current work provides new insights for water-involving oxidation reactions.

Indium(III)/2-benzoylpyridine chemistry: interesting indium(III) bromide-assisted transformations of the ligand

Reactions of 2-benzoylpyridine, (py)(ph)CO, with InX₃ (X = Cl, Br) in EtOH at room temperature have been studied. The InCl₃/(py)(ph)CO system has provided access to complex [InCl₃{(py)(ph)CO}(EtOH)]·{(py)(ph)CO} (1) and the byproduct {(pyH)(ph)CO}Cl (2). The reaction of InBr₃ with (py)(ph)CO has led to a mixture of (L)[InBr₄{(py)(ph)CO}] (3) and [In₂Br₄{(py)(ph)CH(O)}₂(EtOH)₂] (4), where L⁺ is the 9-oxo-indolo[1,2-a]pyridinium cation and (py)(ph)CH(O)^- is the anion of (pyridin-2-yl)methanol. Based on solubility and crystallisation time differences between the two components of the mixture, complex 4 was isolated in pure form, i.e. free from 3. The formations of the counterion L⁺ and the coordinated (py)(ph)CH(O)^- anion represent clearly InBr₃-promoted/assisted transformations. Reaction mechanisms have been proposed for the formation of 2, 3 and 4. Complex 4 could also be isolated by the reaction of InBr₃ and pre-formed (py)(ph)CH(OH) in EtOH. The solid-state structures of 1, 3 and 4 were determined by single-crystal X-ray crystallography, while the identity of the salt 2 was confirmed by microanalyses and a variety of spectroscopic techniques, including ESI-MS spectra. In the indium(III) complexes, the metal ions are 6-coordinate with a distorted octahedral geometry. The halogeno groups (Cl^-, Br^-) in the three complexes are terminal. The (py)(ph)CO molecule behaves as a N,O-bidentate (1.11) ligand in 1 and 3. A terminal EtOH ligand completes the coordination sphere of In^III in 1. The alkoxo oxygen atoms of the two 2.21 (py)(ph)CH(O)^- ligands doubly bridge the In^III centers in 4 creating a {InIII2(μ-OR)₂}⁴⁺ core; a nitrogen atom of one reduced organic ligand, two bromo ions and one terminal EtOH molecule complete the 6-coordination at each metal centre. Complexes 1, 3 and 4 were characterised by IR and Raman spectroscopies, and the data were discussed in terms of their known solid-state structures. Molar conductivity data and ¹H NMR spectra were used in an attempt to probe the behaviour of the complexes in DMSO. The to-date observed metal ion-assisted/promoted transformations of (py)(ph)CO are also discussed.

Experimental insights into catalytic oxidation of 1,6-hexanediol to ε-caprolactone over (p-cymene)RuCl2(L) complexes in non-polar media

The activity-pocket site dimension (θc) dependence of (p-cymene)RuCl2(L) supports associative interchange mechanism for 1,6-hexandiol oxidation to ε-caprolactone. Methyl isobutyl carbinol, a H-accepting product, reacts with Ru, causing deactivation.

Homoleptic titanium and zirconium complexes exhibiting unusual Oiminol–metal coordination: application in stereoselective ring-opening polymerization of lactide

The synthesis and characterization of novel homoleptic Ti and Zr complexes with tridentate ONO-type Schiff base ligands and their catalytic activities towards the ring-opening polymerization (ROP) of lactide are reported.

Trinuclear nickel and cobalt complexes containing unsymmetrical tripodal tetradentate ligands: syntheses, structural, magnetic, and catalytic properties

The coordination chemistries of the tetradentate N2O2-type ligands N-(2-pyridylmethyl)iminodiethanol (H2pmide) and N-(2-pyridylmethyl)iminodiisopropanol (H2pmidip) have been investigated with nickel(ii) and cobalt(ii/iii) ions. Three novel complexes prepared and characterized are [(Hpmide)2Ni3(CH3COO)4] (1), [(Hpmide)2Co3(CH3COO)4] (2), and [(pmidip)2Co3(CH3COO)4] (3). In 1 and 2, two terminal nickel(ii)/cobalt(ii) units are coordinated to one Hpmide(-) and two CH3CO2(-). The terminal units are each connected to a central nickel(ii)/cobalt(ii) cation through one oxygen atom of Hpmide(-) and two oxygen atoms of acetate ions, giving rise to nickel(ii) and cobalt(ii) trinuclear complexes, respectively. Trinuclear complexes 1 and 2 are isomorphous. In 3, two terminal cobalt(iii) units are coordinated to pmidip(2-) and two CH3CO2(-). The terminal units are each linked to a central cobalt(ii) cation through two oxygen atoms of pmidip(2-) and one oxygen atom of a bidentate acetate ion, resulting in a linear trinuclear mixed-valence cobalt complex. 1 shows a weak ferromagnetic interaction with the ethoxo and acetato groups between the nickel(ii) ions (g = 2.24, J = 2.35 cm(-1)). However, 2 indicates a weak antiferromagnetic coupling with the ethoxo and acetato groups between the cobalt(ii) ions (g = 2.37, J = -0.5 cm(-1)). Additionally, 3 behaves as a paramagnetic cobalt(ii) monomer, due to the diamagnetic cobalt(iii) ions in the terminal units (g = 2.53, |D| = 36.0 cm(-1)). No catalytic activity was observed in 1. However, 2 and 3 showed significant catalytic activities toward various olefins with modest to good yields. 3 was slightly less efficient toward olefin epoxidation reaction than 2. Also 2 was used for terminal olefin oxidation reaction and was oxidised to the corresponding epoxides in moderate yields (34-75%) with conversions ranging from 47-100%. The cobalt complexes 2 and 3 promoted the O-O bond cleavage to ∼75% heterolysis and ∼25% homolysis.

Pd(OAc)2-catalysed regioselective alkoxylation of aryl (β-carbolin-1-yl)methanones via β-carboline directed ortho-C(sp(2))-H activation of an aryl ring

Synthesis of (2-alkoxyphenyl)(9H-pyrido[3,4-b]indol-1-yl)methanone via Pd(OAc)2-catalyzed regioselective alkoxylation of aryl (β-carbolin-1-yl) methanones employing β-carboline directed ortho-C(sp(2))-H activation of an aryl ring under oxidative conditions is described.

Mechanism of the CuII-catalyzed benzylic oxygenation of (aryl)(heteroaryl)methanes with oxygen

A mechanistic study of the copper-catalyzed oxidation of the methylene group of aryl(di)azinylmethanes was performed. Initial reaction rates were measured making use of in situ IR reaction monitoring and a kinetic analysis of the reaction was executed. The reaction proved to be first order in oxygen concentration. For substrate and acid concentration, saturation kinetics due to O2 mass transfer limitation were observed. The occurrence of mass transfer limitation was further confirmed by examining the effect of the stirring rate on the initial reaction rate. Interestingly, the effect of the concentration of the catalyst on the rate shows that higher loadings result in a maximal initial rate, followed initially by a steady decrease and subsequently a rate plateau when the concentration is increased further. Mass transfer limitation and increased concentration of dinuclear catalytically active species rationalizes this hitherto unprecedented rate behavior. Continuous-wave and pulsed electron paramagnetic resonance methods were used to characterize the catalytic species present in the solution during the reaction and confirmed the presence of both mono- and dinuclear copper species. Analysis of a diverse substrate scope points towards imine-enamine tautomerization as a crucial process in the oxidation reaction. DFT calculations of these equilibrium constants (pKeq) provided us with a qualitative tool to predict whether or not a substrate is viable for oxidation under the reaction conditions developed.

Iron(iii) and cobalt(iii) complexes with both tautomeric (keto and enol) forms of aroylhydrazone ligands: catalysts for the microwave assisted oxidation of alcohols

Aroylhydrazone Fe(iii) and Co(iii) complexes of both keto and enol tautomeric forms of the ligand are reported and shown to catalyze the microwave-assisted solvent-free peroxidative oxidation (by TBHP) of primary and secondary alcohols.

Aerobic oxidation at benzylic positions catalyzed by a simple Pd(OAc)2/bis-triazole system

An extremely active palladium catalyst system for the aerobic oxidation of benzyl alcohols and benzylic C–H oxidation is described.