Recent progress in catalytic oxygenation of aromatic C–H groups with the environmentally benign oxidants H2O2 and O2

Developing efficient approaches for the selective oxygenation of aromatic C–H groups has been a challenging goal, interesting from both fundamental and practical perspectives. Designing catalyst systems for the direct production of phenol from benzene holds significant practical promise; on the contrary, novel strategies for the late-stage selective introduction of oxygen into aromatic rings of complex organic molecules could be useful for the synthesis of fine chemicals, including biologically active compounds. Another aspect of the overall picture is toughening environmental constraints, resulting in a steady increase in the number of catalyst systems relying on environmentally benign oxidants such as H₂O₂ and O₂. Overall, today, the realm of selective catalytic aromatic C–H oxidations is developing rapidly. This review summarizes the progress made in homo- and heterogeneous catalyst systems of aromatic C–H oxygenations with hydrogen peroxide and molecular oxygen, achieved in the last decade.     

Titanium Salan Catalysts for the Asymmetric Epoxidation of Alkenes: Steric and Electronic Factors Governing the Activity and Enantioselectivity

A new insight into the highly enantioselective (up to >99.5 % ee) epoxidation of olefins in the presence of chiral titanium(IV) salan complexes is reported. A series of 14 chiral ligands with varying steric and electronic properties have been designed, and it was found that electronic effects modulate the catalytic activity (without affecting the enantioselectivity), whereas the steric properties account for the enantioselectivity of the epoxidation. Competitive oxidations of p‐substituted styrenes reveal the electrophilic nature of the oxygen‐transferring active species, with a Hammett ρ value of ?0.51; the enantioselectivity is unaffected by the electron‐donating (or withdrawing) ability of the p‐substituents. Mechanistic studies provide evidence in favor of a stepwise reaction mechanism: in the first (rate‐determining) stage, olefin most probably coordinates to the active species, followed by intramolecular enantioselective oxygen transfer. The enantioselectivity increases with decreasing temperature. The modified Eyring plots for the epoxidation of styrene and (Z)‐β‐methylstyrene are linear, indicating a single, enthalpy‐controlled mechanism of stereoselectivity, with ΔΔH^≠=?6.6 kJ mol^?1 and ?5.4 kJ mol^?1, respectively.     

State of components in PdCu/Al2O3 catalysts obtained via anchored metal complexes

IRS and XPS data have shown that the reduction of pdCu/Al₂O₃ catalysts is followed by the formation of 30–40 Å homogeneous bimetallic particles with a surface enriched in copper atoms.     

Formation of Pd superoxo complexes by interacting potassium superoxide with palladium acetate in organic solvents

Two types of palladium superoxo complexes resulting from the interaction of potassium superoxide with palladium acetate in organic solvents have been detected by ESR.

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Low-Spin and High-Spin Perferryl Intermediates in Non-Heme Iron Catalyzed Oxidations of Aliphatic C−H Groups

The selectivity patterns of iron catalysts of the Fe(PDP) family in aliphatic C−H oxidation with H₂O₂ have been studied (PDP=N,N′-bis(pyridine-2-ylmethyl)-2,2′-bipyrrolidine). Cyclohexane, adamantane, 1-bromo-3,7-dimethyloctane, 3,7-dimethyloctyl acetate, (−)-acetoxy-p-menthane, and cis-1,2-dimethylcyclohexane were used as substrates. The studied catalyst systems generate low-spin (S=1/2) oxoiron(V) intermediates or high-spin (S=3/2) oxoiron(V) intermediates, depending on the electron-donating ability of remote substituents at the pyridine rings. The low-spin perferryl intermediates demonstrate lower stability and higher reactivity toward aliphatic C−H groups of cyclohexane than their high-spin congeners, according to the measured self-decay and second-order rate constants k₁ and k₂. Unexpectedly, there appears to be no uniform correlation between the spin state of the oxoiron(V) intermediates, and the chemo- and regioselectivity of the corresponding catalyst systems in the oxidation of the considered substrates. This contrasts with the asymmetric epoxidations by the same catalyst systems, in which case the epoxidation enantioselectivity increases when passing from the systems featuring the more reactive low-spin perferryl intermediates to those with their less reactive high-spin congeners.     

Investigation of pyramidal inversion of nitrogen atom in carbamate and thiocarbamate ions formed at the reaction of СО<Subscript>2</Subscript>, СOS,CS<Subscript>2</Subscript> with 2-aminoethanol

Dynamic NMR was applied to measuring the value ΔG^≠ characterizing the height of the barrier to the pyramidal inversion of the nitrogen atom in carbamate and thiocarbamate anions formed at the reaction of 2-aminoethanol with CO₂ and COS. The refinement was introduced in formerly suggested cyclic structures of anions containing an intramolecular hydrogen bond NH···O(S), which contradicted the found large values of the barrier of inversion (ΔG ^≠ ～ 70 kJ mol^–1). The hydrogen bond in the cyclic anions of carbamates and thiocarbamates is two-electron and three-center. Analogous cyclic structure with a multicenter hydrogen bond does not form in the case of dithiocarbamate anion that is the product of 2-aminoethanol reaction with CS₂.     

13C and1H NMR spectroscopic characterization of titanium(IV) alkylperoxo complexes Ti(OOtBu)n(OiPr)4?n with n=1, 2, 3, 4

Using¹³C and¹H NMR spectroscopy, titanium(IV) alkylperoxo complexes Ti(OOtBu)_n(OiPr)_4−n with n=1, 2, 3 and 4 were characterized in the reaction of Ti(OiPr)₄ withtBuOOH in CH₂Cl₂ and CDCl₃.     

ESR study of the kinetics and mechanism of ligand substitution reactions in Cu(II) bis-salicylaldehydate

The kinetics and mechanism of successive substitution of chelate ligands in Cu(II) bissalicyladehydate have been studied by ESR method.

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Intermediates of Asymmetric Oxidation Processes Catalyzed by Vanadium(V) Complexes

The [VO(acac)₂]/Schiff base [R-2-(N-3,5-di-tert-butylsalicylidene)amino-2-phenyl-1-ethanol, S-2-(N-3,5-di-tert-butylsalicylidene)amino-3,3-dimethyl-1-butanol, S-2-(N-3,5-di-tert-butylsalicylidene)amino-3-methyl-1-butanol, or R-2-(N-3,5-di-tert-butylsalicylidene)amino-3-phenyl-1-propanol]/H₂O₂ catalytic systems for the asymmetric oxidation of sulfides and the [VO(acac)₂]/(3bR,4aR)-2-(3,4,4-trimethyl-3b,4,4a,5-tetrahydrocyclopropa[3,4]cyclopenta[1,2-c]pyrazol-1-yl)ethanol/tert-butyl hydroperoxide/TBHP and VO(OAlkyl)₃/[2,2]paracyclophane-4-carboxylic acid N-(1,1-dimethylethyl)-N-hydroxamide/TBHP catalytic systems for the asymmetric epoxidation of allylic alcohols were studied using ¹³C, ⁵¹V, and ¹⁷O NMR spectroscopy. The key intermediates of these systems (peroxo and alkylperoxo complexes of vanadium(V)) were detected, their structures in solution were studied, and the reactivity was evaluated.