Ketone-catalyzed asymmetric epoxidation reactions

This Account summarizes our efforts in developing organic ketone catalysts for enantioselective and diastereoselective epoxidation of olefins. We have developed an efficient and general protocol for epoxidation of olefins using dioxiranes generated in situ from activated ketones and Oxone (2KHSO5.KHSO4.K2SO4) in a homogeneous aqueous solvent system. We have also developed chiral ketone catalysts for highly enantioselective epoxidation of unfunctionalized trans-olefins and trisubstituted olefins. Excellent diastereoselectivities have been achieved for epoxidation of substituted cyclohexenes with ketone catalysts.     

环己烯催化环氧化反应的研究新进展

综述了环己烯催化氧化法合成环氧环己烷的最新研究进展。近年来,催化环己烯环氧化越来越受到关注,因为它采用了来源方便和具有环保性质的氧化剂。介绍了环己烯催化环氧化的一些新进展。反应中采用了各种氧化剂如过氧化氢(双氧水),叔丁基过氧化氢(TBHP)和分子氧(O2),以及各种含有各种配体(如席夫碱、卟啉、酞菁和杂多酸)的过渡金属催化剂。对有关反应的机理进行了讨论。     

Catalytic performance of advanced titanosilicate selective oxidation catalysts – a review

This review article aims to cover the state-of-the-art of titanosilicate catalysts for selective oxidations developed within past seven years. Many elaborated materials (e.g., layered and pillared titanosilicates, hierarchical composite materials, and others) have been prepared and thoroughly characterized; however, their catalytic properties have been usually investigated only using a single or few model substrates and compared with a benchmarking material. The main goal of this article is to summarize the novel catalysts and compare their catalytic performance with each other. The comparison is focused on epoxidation. In addition, phenol hydroxylation and sulphide oxidation are briefly covered.     

New types of catalytic oxidations in organic synthesis

Simulation of the enzymatic function of cytochrome P-450 with transition metal complex catalysts has resulted in finding biomimetic and catalytic oxidations of amines, amides, β-lactams, alcohols, phenols and hydrocarbons by using ruthenium catalyst and peroxide. Further study revealed that the catalytic formation of peracids in situ from aldehydes and molecular oxygen enables the aerobic oxidation of β-lactams, alcohols, alkanes and ketones in the presence of metal catalyst under mild conditions.     

Dioxirane oxidation of 3β-substituted Δ<Superscript>5</Superscript>-steroids

This article reviews the utility of dioxiranes in the oxidation of 3beta-substituted delta5-sterols. Dioxiranes are the smallest cyclic peroxides that contain a carbon atom. They can be generated in situ from Oxone (2KHSO5.KHSO4.K2SO4) and a ketone. Dioxiranes are versatile oxidizing agents. The most common reaction of dioxiranes is epoxidation, with nearly 1:1 ratios of alpha/beta isomer products in all cases. delta5-Steroids with different side chains were epoxidized by dioxiranes generated in situ from several commercially available ketones. Although ketones function as catalyst, they were used in about an equivalent amount or large excess to accelerate the reaction.     

Catalytic oxidation of alcohols using molecular oxygen mediated by poly(ethylene glycol)-supported nitroxyl radicals

The selective catalytic oxidation of alcohols over a mixture of copper(I) chloride and a number of linear ‘linker-less’ or ‘branched’ poly(ethylene glycol)-supported nitroxyl radicals of the 2,2,6,6-tetramethyl-piperidine-1-oxyl (TEMPO) family as a catalyst system has been investigated in the presence of molecular oxygen in a batch reactor. It is found that the activity profile of the polymer-supported nitroxyl radicals is in good agreement with that of low-molecular weight nitroxyl catalysts, for example, allylic and benzylic alcohols are oxidised faster than aliphatic alcohols. The oxidations can be tuned to be highly selective such that aldehydes are the only oxidation products observed in the oxidation of primary alcohols and the oxidations of secondary alcohols yield the corresponding ketones. A strong structural effect of the polymeric nitroxyl species on catalytic activity that is dependent upon their spatial orientation of the nitroxyl radicals is particularly noted. The new soluble macromolecular catalysts can be recovered readily from the reaction mixture by solvent precipitation and filtration. In addition, the recycled catalysts demonstrate a similar selectivity with only a small decrease in activity compared to the fresh catalyst even after five repetitive cycles.     

Catalytic oxidation of alcohols using molecular oxygen mediated by poly(ethylene glycol)-supported nitroxyl radicals

The selective catalytic oxidation of alcohols over a mixture of copper(I) chloride and a number of linear ‘linker-less’ or ‘branched’ poly(ethylene glycol)-supported nitroxyl radicals of the 2,2,6,6-tetramethyl-piperidine-1-oxyl (TEMPO) family as a catalyst system has been investigated in the presence of molecular oxygen in a batch reactor. It is found that the activity profile of the polymer-supported nitroxyl radicals is in good agreement with that of low-molecular weight nitroxyl catalysts, for example, allylic and benzylic alcohols are oxidised faster than aliphatic alcohols. The oxidations can be tuned to be highly selective such that aldehydes are the only oxidation products observed in the oxidation of primary alcohols and the oxidations of secondary alcohols yield the corresponding ketones. A strong structural effect of the polymeric nitroxyl species on catalytic activity that is dependent upon their spatial orientation of the nitroxyl radicals is particularly noted. The new soluble macromolecular catalysts can be recovered readily from the reaction mixture by solvent precipitation and filtration. In addition, the recycled catalysts demonstrate a similar selectivity with only a small decrease in activity compared to the fresh catalyst even after five repetitive cycles.     

醇类的分子氧催化氧化反应研究进展

目前,国外广泛地使用分子氧(或空气)催化氧化醇类以合成羰基化合物。分子氧是化工生产中很受欢迎的一种“绿色”氧化剂而且便宜易得。此处介绍的是目前国外已发现的多种这类反应的以铂系金属为主的催化剂。它们大多数具有高的催化转化率和很好的选择性。     

Environmentally friendly liquid phase oxidation: enhanced selectivity in the aerial oxidation of alkyl aromatics,epoxidations and the Baeyer–Villiger oxidation using novel silica supported transition metal ions

Active transition metal species (Co, Cu, Cr, Ni or Mn) supported on a chemically modified silica gel are used as heterogeneous catalysts in a range of liquid phase oxidation reactions: alkyl aromatic side chain oxidations, epoxidations of alkenes and Baeyer–Villiger oxidations of linear ketones to esters and cyclic ketones to lactones. The catalyst employs metal centres bound to the silica surface via a hydrophobic spacer chain and is thus chemically robust and has a relatively high loading for a supported reagent (c 0.4 mmol g⁻¹). The Cr version of the catalyst promotes the oxidation of ethylbenzene to acetophenone in a solvent‐free system at a rate of 5.5% h⁻¹ (>370 turnover h⁻¹). It is also active for the oxidation of p‐chlorotoluene and p‐xylene to p‐chlorobenzoic acid and p‐toluic acid respectively. Cyclohexene is converted to its oxide at room temperature at a rate of c 28% h⁻¹ (c 12 turnover h⁻¹) using either the Ni or Cu versions of the catalyst. The room temperature Baeyer–Villiger oxidation of cyclohexanone is achieved at a rate of 44% h⁻¹ (49 turnover h⁻¹) using the Ni‐containing catalyst. The same material also promotes the Baeyer–Villiger oxidation of linear aliphatic ketones and aromatic side chains. All the above systems use either air or molecular oxygen as the oxidant rather than peroxides or peracids. © 1999 Society of Chemical Industry     

Base Catalysis in the Synthesis of Fine Chemicals

The catalytic properties of hydrotalcites are investigated for reactions of aldolisation, Meerwein–Ponndorf–Verley hydrogenations, epoxidations and oxidations, Michael and Wadsworth–Emmons additions. Excellent yields have been reported using mild conditions and catalysts which can be easily separated from the reaction medium and could be regenerated. The catalytic properties are controlled by the activation of the solid, and traces of chlorine appear to be deleterious. In many cases rehydration increases activity dramatically. Solid bases give a very specific catalysis for instance with Michael or Wadsworth–Emmons reactions of aldolisable compounds due to the exclusion of carbonyls from the surface by the other reactant.     

Recent developments in the application of Baeyer-Villiger monooxygenases as biocatalysts

Baeyer-Villiger monooxygenases (BVMOs) represent a specific class of monooxygenases that are capable of catalyzing a variety of oxidation reactions, including Baeyer-Villiger oxidations. The recently elucidated BVMO crystal structures have provided a more detailed insight into the complex mechanism of these flavin-containing enzymes. Biocatalytic studies on a number of newly discovered BVMOs have shown that they are very potent oxidative biocatalysts. In addition to catalyzing the regio- and enantioselective Baeyer-Villiger oxidations of a wide range of carbonylic compounds, epoxidations, and enantioselective sulfoxidations have also been shown to be part of their catalytic repertoire. This review provides an overview on the recent developments in BVMO-mediated biocatalytic processes, identification of the catalytic role of these enzymes in metabolic routes and prodrug activation, as well as the efforts in developing effective biocatalytic methodologies to apply BVMOs for the synthesis of high added value compounds.     

丙烯环氧化合成环氧丙烷新技术的研究进展

评述了以分子氧和双氧水为氧源的丙烯环氧化催化剂体系的最新研究进展。其中包括金属Ag催化剂。金属熔盐混合物，钛硅沸石TS-1和过渡金属络合物催化剂体系，根据氧转移机理。金属Ag催化剂和熔盐体系是以分子氧为直接氧源，而TS-1和最近报道的反应相转移含钨催化剂本质上是以过氧化氢为直接氧源。TS-1和反应相转移含钨催化剂和优点是反应条件较温和，生成环氧丙烷的选择性和产率较高。如一种负载成“蛋壳醇作为溶剂），环氧丙烷选择性和产率（以双氧水计）分别可达92%和90%以上。催化剂运转1000h以上性能稳定，今后的一个重要发展方向是开发以分子氧为起始氧源，简单高效的原位双氧水工艺。     

Oxidovanadium(V) amine bisphenolates as epoxidation,sulfoxidation and catechol oxidation catalysts

Air-stable oxidovanadium(V) complexes with tetradentate amine bisphenolate ligands were made by the reaction of VOSO₄·xH₂O and ligand precursors in MeOH solutions. Isolated compounds were studied as catechol oxidase models as well as catalysts for epoxidation and sulfoxidation reactions. All compounds can catalyse such oxidation reactions without notable structure-activity correlations. The ⁵¹V NMR studies indicate that the complexes turn to the number of different species during the catalytic experiments.     

过渡金属多相催化剂催化的分子氧选择性氧化邻二醇研究进展

邻二醇选择性氧化断键生成醛、酮和羧酸是重要和基本的有机反应,氧化剂通常采用化学计量的高碘酸盐或四乙酸铅,无论从经济还是环保的角度考虑,这些传统方法都有违绿色化工的发展理念。理论上以分子氧为氧化剂选择性氧化邻二醇断键的副产物只有水,而且分子氧廉价易得,因而具有环境友好和经济可行的双重优势。经过多年探索,人们已发展了多个基于不同过渡金属为活性组分的多相催化剂或催化体系用于分子氧选择性氧化邻二醇断键反应,如钌、金、锰、钴,为未来广泛应用奠定了基础。本文将按贵金属和非贵金属分类综述此类催化剂及相关反应工艺的研究进展,并指出该领域的发展方向     

使用分子氧催化氧化法合成肉桂醛的研究

分子氧(或空气)不但便宜易得,而且也是在化工生产中很受欢迎的“绿色”氧化剂。近几年来,在国外它被广泛地用于催化氧化醇类以合成羰基化合物的研究。目前已发现了多种这类反应的催化剂。这里用Ru／Al2O3载体催化剂,由分子氧氧化肉桂醇制取肉桂醛。此催化剂具有高的催化转化率和很好的选择性。     

芳香酮类化合物的合成研究进展

介绍了由芳香族化合物的Friedel—Crafts酰基化反应和芳香醇的氧化脱氢反应等合成芳香酮类化合物的研究进展,比较了各种方法的优缺点,指出以固体酸为催化剂的Friedel—Crafts酰基化反应和以双氧水或氧为氧化剂的芳香醇的氧化反应是今后芳香酮类化合物合成的发展方向。     

Rapid evaluation of oxidation catalysis by gas sensor system: total oxidation, oxidative dehydrogenation, and selective oxidation over metal oxide catalysts

The rapid evaluation of catalysis is an indispensable technology for the success of combinatorial chemistry. A small-sized, less expensive, easily operating screening is desirable for parallel settings which dramatically shortens the evaluation time. Recent advances in gas sensors have enabled us to use them for the rapid evaluation of oxidation catalysis. Three typical catalytic oxidations over metal oxide catalysts were evaluated by gas sensor systems optimized for each catalytic system. The first one is the total oxidation of carbon monoxide in air. Five catalytic combustion-type gas sensors were used in a parallel reactor system to shorten the evaluation time. The second one is the oxidative dehydrogenation (ODH) of ethane over the mixed oxide of nickel and iron. The evaluation of the ODH catalysis was performed by a selective olefin sensor which determines the concentration of C₂H₄ in C₂H₆. The third one is the selective oxidation catalysis of propane over alkali modified Fe/SiO₂. The effluents including CO, CO₂, aldehydes and ketones in propane were analyzed by the CO, CO₂ and semiconductor-type gas sensors selective toward aldehydes and ketones. These evaluation results indicated that gas sensors have a good potential for the rapid evaluation of oxidation catalysts.     

丙烯环氧化反应的研究新进展

综述了不同氧源与丙烯环氧化反应的研究新进展。主要包括:氢氧原位制备过氧化氢丙烯环氧化,直接以分子氧为氧化剂的光诱导环氧化反应以及以臭氧和一氧化二氮为氧化剂的研究。指出开发活性高、选择性好的催化剂是解决问题的关键,并对其应用前景进行了讨论。     

Catalytic activity of ethylene oxidation over Au,Ag and Au–Ag catalysts: Support effect

Au, Ag and Au–Ag catalysts on different supports of alumina, titania and ceria were studied for their catalytic activity of ethylene oxidation reactions. An addition of an appropriate amount of Au on Ag/Al₂O₃ catalyst was found to enhance the catalytic activity of the ethylene epoxidation reaction because Au acts as a diluting agent on the Ag surface creating new single silver sites which favor molecular oxygen adsorption. The Ag catalysts on both titania and ceria supports exhibited very poor catalytic activity toward the epoxidation reaction of ethylene, so pure Au catalysts on these two supports were investigated. The Au/TiO₂ catalysts provided the highest selectivity of ethylene oxide with relatively low ethylene conversion whereas, the Au/CeO₂ catalysts was shown to favor the total oxidation reaction over the epoxidation reaction at very low temperatures. In comparisons among the studied catalysts, the bimetallic Au–Ag/Al₂O₃ catalyst is the best candidate for the ethylene epoxidation. The catalytic activity of the gold catalysts was found to depend on the support material and catalyst preparation method which govern the Au particle size and the interaction between the Au particles and the support.     

Epoxidation of propylene in the gas phase

The gas-phase epoxidation of propylene using N₂O, air and air-ammonia mixture as an oxidants was studied. Propylene can be epoxidized by nitrous oxide with a yield as high as 13.3% over silica supported iron oxide catalysts modified by amines. The iron oxide dispersion, the acidity of the support and the nitrogen-containing modifiers are the key factors determining the catalytic performance. We suggest a reaction pathway involving two concurrent mechanisms: the radical oxidation of propylene to acroleine, hexanediene, etc., and a non-radical oxidation leading to epoxide. Propylene is epoxidized with air over silica-supported iron oxide catalysts at a conversion of about 0.2%. Using air as an oxidizing agent, the presence of gaseous ammonia improves the propylene conversion by 10-fold preserving the considerable selectivity (up to 60%). This observation suggests a reaction mechanism involving the oxidation of ammonia to nitrous oxide in the first step, which subsequently produces active oxygen species, which selectively oxidize propylene to propylene oxide (PO).