Iron(II)‐Catalyzed Biomimetic Aerobic Oxidation of Alcohols

We report the first Fe^II‐catalyzed biomimetic aerobic oxidation of alcohols. The principle of this oxidation, which involves several electron‐transfer steps, is reminiscent of biological oxidation in the respiratory chain. The electron transfer from the alcohol to molecular oxygen occurs with the aid of three coupled catalytic redox systems, leading to a low‐energy pathway. An iron transfer‐hydrogenation complex was utilized as a substrate‐selective dehydrogenation catalyst, along with an electron‐rich quinone and an oxygen‐activating Co(salen)‐type complex as electron‐transfer mediators. Various primary and secondary alcohols were oxidized in air to the corresponding aldehydes or ketones with this method in good to excellent yields.     

Mechanism of Pd(OAc)2/Pyridine Catalyst Reoxidation by O2: Influence of Labile Monodentate Ligands and Identification of a Biomimetic Mechanism for O2 Activation

Aerobic oxidation : Mechanisms of aerobic oxidation of the Pd^II(OAc)₂/pyridine catalyst system were evaluated by using density functional theory methods. The results reveal that labile monodentate ligands, such as pyridine, favor a catalyst reoxidation pathway that proceeds via Pd⁰, rather than direct reaction of O₂ with a Pd^II–hydride intermediate (see scheme).

     

Binary Pd–Polyoxometalates and Isolation of a Ternary Pd–V–Polyoxomolybdate Active Species for Selective Aerobic Oxidation of Alcohols

Binary Pd–polyoxometalates [Pd(dpa)₂]₃[PW₁₂O₄₀]₂ ? 12 DMSO ( 2 ), [Pd(dpa)₂]₃[PMo₁₂O₄₀]₂ ? 12 DMSO ? 2 H₂O ( 3 ), and [Pd(dpa)(DMSO)₂]₂[HPMo₁₀V₂O₄₀] ? 4 DMSO ( 4 ) were synthesized by reaction of [Pd(dpa)(OAc)₂] ? 2 H₂O ( 1 ; dpa=2,2′‐dipyridylamine) with three Keggin‐type polyoxometalates and fully characterized by single‐crystal and powder XRD analyses, IR spectroscopy, and elemental analyses. The synthesis is facile and straightforward, and the complicated ligand‐modification procedure often used in the traditional charge‐transfer method can be omitted. In 2 – 4 , Pd complexes and polyoxometalate anions are coupled through electrostatic interaction. Compound 4 is more active than the other three compounds in the selective aerobic oxidation of alcohols at ambient pressure. Interestingly, during catalytic recycling of compound 4 , unprecedented ternary Pd–V–polyoxometalate [Pd(dpa)₂{VO(DMSO)₅}₂][PMo₁₂O₄₀]₂ ? 4 DMSO ( 5 ), which was captured and characterized by single‐crystal XRD, proved to be the true active species and showed high catalytic activity for the selective aerobic oxidation of aromatic alcohols (98.1–99.8 % conversion, 91.5–99.1 % selectivity). Moreover, on the basis of control experiments and EPR and UV/Vis spectra, a plausible reaction mechanism for the oxidation of alcohols catalyzed by 5 was proposed.     

Aerobic Oxidative N-Heterocyclic Carbene Catalysis

The ease with which simple starting materials can be transformed into highly functionalized products has made oxidative N-heterocyclic carbene (NHC) catalysis an area of significant interest. However, the use of stoichiometric amounts of high molecular weight oxidants in most reactions generates an undesired equivalent amount of waste. To address this issue, the use of oxygen as the terminal oxidant in NHC catalysis has been developed. Oxygen is attractive due to its low cost, low molecular weight, and ability to generate water as the sole by-product. However, molecular oxygen is challenging to use as a reagent in organic synthesis due to its unreactive ground state, which often requires reactions to be run at high temperatures and results in the formation of kinetic side-products. This review covers the development of aerobic oxidative carbene catalysis, including NHC-catalyzed reactions with oxygen, strategies for oxygen activation, and selectivity issues under aerobic conditions.     

Partial oxidation of 4-tert-butyltoluene catalyzed by homogeneous cobalt and cerium acetate catalysts in the Br-/H2O2/acetic acid system: insights into selectivity and mechanism

The partial oxidation of 4-tert-butyltoluene to 4-tert-butylbenzaldehyde by hydrogen peroxide in glacial acetic acid, catalyzed by bromide ions in combination with cobalt(II) acetate or cerium(III) acetate, has been studied in detail. Based on the observed differences in reaction rates and product distributions for the different catalysts, a reaction mechanism involving two independent pathways is proposed. After the initial formation of a benzylic radical species, either oxidation of this intermediate by the metal catalyst or reaction with bromine generated in situ occurs, depending on which catalyst is used. The first pathway leads to the exclusive formation of 4-tert-butylbenzaldehyde, whereas reaction of the radical intermediate with bromine leads to formation of the observed side products 4-tert-butylbenzyl bromide and its hydrolysis and solvolysis products 4-tert-butylbenzyl alcohol and 4-tert-butylbenzyl acetate, respectively. The cobalt(II) catalysts Co(OAc)(2) and Co(acac)(2) are able to quickly oxidize the radical intermediate, thereby largely preventing the bromination reaction (i.e., side-product formation) from occurring, and yield the aldehyde product with 75-80 % selectivity. In contrast, the cerium catalyst studied here exhibits an aldehyde selectivity of around 50 % due to the competing bromination reaction. Addition of extra hydrogen peroxide leads to an increased product yield of 72 % (cerium(III) acetate) or 58 % (cobalt(II) acetate). Product inhibition and the presence of increasing amounts of water in the reaction mixture do not play a role in the observed low incremental yields.     

Mechanism of Alkyne Alkoxycarbonylation at a Pd Catalyst with P,N Hemilabile Ligands: A Density Functional Study

A detailed mechanism for alkyne alkoxycarbonylation mediated by a palladium catalyst has been characterised at the B3PW91‐D3/PCM level of density functional theory (including bulk solvation and dispersion corrections). This transformation, investigated via the methoxycarbonylation of propyne, involves a uniquely dual role for the P,N hemilabile ligand acting co‐catalytically as both an in situ base and proton relay coupled with a Pd⁰ centre, allowing for surmountable barriers (highest ΔG^≠ of 22.9 kcal mol^?1 for alcoholysis). This proton‐shuffle between methanol and coordinated propyne accounts for experimental requirements (high acid concentration) and reproduces observed regioselectivities as a function of ligand structure. A simple ligand modification is proposed, which is predicted to improve catalytic turnover by three orders of magnitude.     

Catalytic Methane Oxidation Over Pd Supported on Nanocrystalline and Polycrystalline TiO2 Mn3O4, CeO2 and ZrO2

The catalytic oxidation of methane has been examined over Pd supported on nanocrystalline (n-) and polycrystalline (p-) TiO₂, Mn₃O₄, CeO₂ and ZrO₂. In all cases the Pd supported on the nanocrystalline oxides performs better on a mass basis than Pd supported on the polycrystalline oxides. Conversion vs temperature curves indicate that n-ZrO₂ is more active than p-ZrO₂ and that calcining both n-ZrO₂ and p-ZrO₂ at 500°C produces better catalysts than calcining at 280°C. n-CeO₂ is a very good catalysts for methane oxidation, while p-CeO₂ is not, and Pd supported on n-CeO₂ performs much better than bare n-CeO₂ and somewhat better than Pd supported on p-CeO₂; Pd supported on n-Mn₃O₄ or p-Mn₃O₄ does not perform as well as CeO₂-supported Pd catalysts. The 5 wt.% Pd/n-ZrO₂ catalyst calcined at 500°C performs very well, achieving 100% conversion at 320°C for the reactor conditions used, while 5 wt.% Pd/n-CeO₂ exhibits initial activity at the lowest temperature of about 100°C. The best catalyst tested in this study is 30 wt.% Pd/n-TiO₂, which achieves 100% conversion at 300°C.     

可见光催化需氧氧化在有机合成中的应用

可见光催化作为一种新兴且强大的有机合成手段,具有洁净节能、优秀的官能团兼容性和良好的化学选择性等特征。氧气作为一种廉价、无污染的氧化剂,与可见光催化相结合,极大地促进了绿色化学的发展。本文主要对我们课题组近年来在可见光需氧氧化领域取得的成果做了简明的综述,并对其发展进行了展望。