Recent Advances in Facile Liquid Phase Epoxidation of Light Olefins over Heterogeneous Molybdenum Catalysts

Molybdenum complexes are versatile and efficient for liquid phase olefin epoxidation reactions. Rational design of catalysts is critical to achieve high atom efficiency during epoxidation processes. Although liquid phase epoxidation has been a popular topic for decades, three key issues, (a) rational control of morphology of molybdenum nanoparticles, (b) manipulating metal‐support interaction and (c) altering electronic configuration at molybdenum center remains unsolved in this area. Therefore, in this paper, we have critically revised recent research progress on heterogeneous molybdenum catalysts for facile liquid phase olefin epoxidation in terms of catalyst synthesis, surface characterization, catalytic performance and structure‐function relationship. Furthermore, plausible reaction mechanisms will be systematically discussed with the aim to provide insights into fundamental understanding on novel epoxidation chemistry.     

钼、钨系过氧化物在Sharpless烯烃环氧化反应中的应用*

钼、钨系过氧化物是非常重要的Sharpless烯烃环氧化催化剂,有着非常广泛的工业发展前景。本文综述了钼、钨系过氧化物的种类,结构,合成方法及催化活性,分析了各种钼、钨系过氧化物的特点以及在催化环氧化领域的发展现状。同时,本文还讨论了钼、钨系过氧化物催化烯烃环氧化反应的机理。评述了在催化环氧化领域中两种主要理论——Mimoun机理与Sharpless机理的争论焦点及发展现状,以及计算化学对两种理论中间过渡态的研究进展,并且重点分析了Sharpless机理的过渡态结构。此外,本文还综述了影响钼、钨系过氧化物催化剂反应活性的各种因素以及质子所产生的副反应。最后,本文对应用于催化环氧化领域的钼、钨系过氧化物未来的研究方向提出了建议。     

Design, Synthesis of Chiral Ketone and Application in Asymmetric Epoxidation

A class of chiral ketone was synthesized for asymmetric epoxidation. High ee values have been obtained for a number of cis olefin and trans olefin. The epoxidation was stereospecific with no isomerizatiom observed in the epoxidation of acyclic system. Encourageingly high ee value has also been obtained for a number of terminal olefins. Mechanistic studies show that electronic interactions play an important role in the stereodifferentiation. O O O O O O NBoc O O t-BuO2CH2CN O O O …     

Organocatalytic asymmetric epoxidation reactions in water-alcohol solutions

The diastereo- and enantioselective organocatalytic epoxidation of alpha,beta-unsaturated aldehydes in aqueous solutions is presented. By the screening of the reaction conditions for the epoxidation of cinnamic aldehyde applying hydrogen peroxide as the oxidant and 2-[bis-(3,5-bis-trifluoromethyl-phenyl)-trimethylsilanyloxy-methyl]-pyrrolidine as the catalyst, a highly stereoselective reaction has been developed. The scope of the diastereo- and enantioselective organocatalytic epoxidation in aqueous solutions is documented by the asymmetric epoxidation of alpha,beta-unsaturated aldehydes with enantioselectivities up to 96% ee.     

温和条件下离子液体／水两相体系中α，β-不饱和羰基化合物的环氧化反应研究

在室温下，以双氧水为氧化剂，研究了六氟磷酸1—甲基—3—烷基咪唑离子液 体与水构成的两相体系内α，β-不饱和羰基化合物的环氧化反应，详细考察了反 应时间、NaOH浓度、氧化剂用量和不同底物对反应结果的影响，并对其反应机理进 行了探讨。此催化体系同传统的相转移催化体系相比，实验操作简单，反应条件更 加温和，并且有效地抑制了反应过程中环氧化合物的开环，在较为适宜的反应条件 下，甲基戊烯酮转化率达到100％，环氧化选择性可达98。     

Designing new chiral ketone catalysts. Asymmetric epoxidation of cis-olefins and terminal olefins

This paper describes a new class of chiral oxazolidinone ketone catalyst for asymmetric epoxidation. High ee values have been obtained for a number of cyclic and acyclic cis-olefins. The epoxidation was stereospecific with no isomerization observed in the epoxidation of acyclic systems. Encouragingly high ee values have also been obtained for a number of terminal olefins. Mechanistic studies show that electronic interactions play an important role in stereodifferentiation.     

Highly enantio- and diastereoselective one-pot synthesis of acyclic epoxy alcohols with three contiguous stereocenters

Two highly enantio- and diastereoselective one-pot procedures for the synthesis of epoxy alcohols with up to three contiguous stereocenters are reported. Route one involves asymmetric addition of an alkylzinc reagent to an enal followed by diastereoselective epoxidation. Route two entails asymmetric vinylation of an aldehyde with divinylzinc reagents and subsequent diastereoselective epoxidation. The oxidant for the epoxidation is generated by exposure of the allylic alkoxide intermediate and the remaining organozinc reagent to dioxygen. Upon addition of catalytic titanium tetraisopropoxide, the directed epoxidation yields the epoxy alcohols with good to excellent yields.     

Epoxidation of syndiotactic 1,2-polybutadiene with peracids

The epoxidation of syndiotactic 1,2-polybutadiene (84 and 16% 1,2 and 1,4 units, respectively) with carboxylic peracids prepared in situ and m-chloroperbenzoic acid was studied. In the course of epoxidation in the presence of carboxylic peracids, oxirane groups are formed only through epoxidation of double bonds in the macromolecular backbone, whereas m-chloroperbenzoic acid is responsible for the chemical modification of 1,2 and 1,4 units of polybutadiene. The basic kinetic parameters of 1,2-polybutadiene epoxidation with peracids of various chemical structures were determined.     

Olefin epoxidation by alkyl hydroperoxide with a novel cross-bridged cyclam manganese complex: demonstration of oxygenation by two distinct reactive intermediates

Olefin epoxidation provides an operative protocol to investigate the oxygen transfer process in nature. A novel manganese complex with a cross-bridged cyclam ligand, MnIV(Me2EBC)(OH)2(2+) (Me2EBC = 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane), was used to study the epoxidation mechanism with biologically important oxidants, alkyl hydroperoxides. Results from direct reaction of the freshly synthesized manganese(IV) complex, [Mn(Me2EBC)(OH)2](PF6)2, with various olefins in neutral or basic solution, and from catalytic epoxidation with oxygen-labeled solvent, H2 18O, eliminate the manganese oxo moiety, Mn(IV)=O, as the reactive intermediate and obviate an oxygen rebound mechanism. Epoxidations of norbornylene under different conditions indicate multiple mechanisms for epoxidation, and cis-stilbene epoxidation under atmospheric 18O2 reveals a product distribution indicating at least two distinctive intermediates serving as the reactive species for epoxidation. In addition to alkyl peroxide radicals as dominant intermediates, an alkyl hydroperoxide adduct of high oxidation state manganese(IV) is suggested as the third kind of active intermediate responsible for epoxidation. This third intermediate functions by the Lewis acid pathway, a process best known for hydrogen peroxide adducts. Furthermore, the tert-butyl peroxide adduct of this manganese(IV) complex was detected by mass spectroscopy under catalytic oxidation conditions.     

Energy-gaining formation and catalytic behavior of active structures in a SiO(2)-supported unsaturated Ru complex catalyst for alkene epoxidation by DFT calculations

The formation and catalytic behavior of active structures in a SiO(2)-supported unsaturated Ru complex catalyst for alkene epoxidation were studied by means of hybrid density functional theory (DFT) calculations. An energy-gaining route for the catalyst activation was found to allow the formation of active unsaturated Ru complexes and the remarkable alkene epoxidation via an exothermic reaction path between isobutyraldehyde and oxygen. In the proposed Bartlett mechanism, Ru promotes the formation of peracid intermediate and facilitates the intermolecular hydrogen transfer in the peracid intermediate, while alkene molecules do not directly coordinate to the Ru site. It was found that stilbene epoxidation is easier to achieve than ethene epoxidation thanks to the electron donating phenyl groups.     

Asymmetric organocatalytic epoxidation of alpha,beta-unsaturated aldehydes with hydrogen peroxide

The first asymmetric organocatalytic epoxidation of alpha,beta-unsaturated aldehydes is presented. A chiral bisaryl-silyl-protected pyrrolidine acts as a very selective epoxidation organocatalyst using simple oxidation agents, such as hydrogen peroxide and tert-butyl hydroperoxide. The asymmetric epoxidation reactions proceed under environmental friendly reaction condition in, for example, water mixtures of alcohols, and the scope of the reaction is demonstrated by the formation of optically active alpha,beta-epoxy aldehydes in high yields and enantioselectivities >94% ee. Furthermore, the direct synthesis of the sex pheromone from an acaric mite by asymmetric epoxidation of citral is presented.     

手性酮催化的烯烃不对称环氧化反应

手性酮是催化非官能化烯烃不对称环氧化的一类重要催化剂 ,它与过氧硫酸氢钾可原位产生对贫电子和富电子烯烃均很有效的氧化剂———手性二氧杂环丙烷 .综述了各种结构的手性酮在反式烯烃、三取代烯烃和顺式烯烃等的不对称环氧化反应中的应用研究进展 ,总结了手性酮结构及反应条件对其催化活性和不对称诱导作用的影响     

多金属氧酸盐催化烯烃环氧化研究进展

综述了以过氧化氢为氧源,多金属氧酸盐催化烯烃环氧化的研究进展,尤其是针对基于多金属氧酸盐的反应控制相转移催化体系近年来的研究进展进行了详细阐述.     

Why copper is intrinsically more selective than silver in alkene epoxidation: ethylene oxidation on Cu(111) versus Ag(111)

The heterogeneously catalyzed epoxidation of alkenes is experimentally challenging, theoretically interesting, and technologically important. Although large-scale ethylene epoxidation is universally carried out with Ag catalysts, recent laboratory studies on single crystal surfaces show that Cu is intrinsically much more selective than Ag in the epoxidation of a variety of terminal alkenes. The reasons for this striking difference between Ag and Cu have been investigated by means of density functional theory. It is found that the fundamental cause is the inversion in the ordering of activation barriers for the competing pathways to epoxide formation versus acetaldehyde formation (the latter being the first step on the route to combustion). On Cu, epoxide formation is less activated than aldehyde formation; the opposite is true on Ag. This behavior is associated with a late transition state to epoxidation on Cu (i.e., product-like) compared to an early (reactant-like) transition state to epoxidation on Ag.     

Transition-metal-free catalysts for the sustainable epoxidation of alkenes: from discovery to optimisation by means of high throughput experimentation

Transition-metal-free oxides were studied as heterogeneous catalysts for the sustainable epoxidation of alkenes with aqueous H?O? by means of high throughput experimentation (HTE) techniques. A full-factorial HTE approach was applied in the various stages of the development of the catalysts: the synthesis of the materials, their screening as heterogeneous catalysts in liquid-phase epoxidation and the optimisation of the reaction conditions. Initially, the chemical composition of transition-metal-free oxides was screened, leading to the discovery of gallium oxide as a novel, active and selective epoxidation catalyst. On the basis of these results, the research line was continued with the study of structured porous aluminosilicates, gallosilicates and silica-gallia composites. In general, the gallium-based materials showed the best catalytic performances. This family of materials represents a promising class of heterogeneous catalysts for the sustainable epoxidation of alkenes and offers a valid alternative to the transition-metal heterogeneous catalysts commonly used in epoxidation. High throughput experimentation played an important role in promoting the development of these catalytic systems.     

Ti-MWW/H2O2体系催化官能化烯烃环氧化

研究了Ti-MWW/H2O2催化体系对多种官能化烯烃液相环氧化的催化性能.结果表明,与钛硅分子筛TS-1相比,Ti-MWW具有更高的催化活性和环氧化产物选择性.溶剂对Ti-MWW催化环氧化反应的活性影响较大,其中水是催化丙烯酸乙酯和乙酸烯丙酯的最佳溶剂,随着C=C双键相邻官能团吸电子能力的增强,环氧化反应的催化活性下降.     

Olefin epoxidation with hydrogen peroxide catalyzed by lacunary polyoxometalate [gamma-SiW10O34H2O2]4-

The tetra-n-butylammonium (TBA) salt of the divacant Keggin-type polyoxometalate [TBA](4)[gamma-SiW(10)O(34)(H(2)O)(2)] (I) catalyzes the oxygen-transfer reactions of olefins, allylic alcohols, and sulfides with 30 % aqueous hydrogen peroxide. The negative Hammett rho(+) (-0.99) for the competitive oxidation of p-substituted styrenes and the low value of (nucleophilic oxidation)/(total oxidation), X(SO)=0.04, for I-catalyzed oxidation of thianthrene 5-oxide (SSO) reveals that a strongly electrophilic oxidant species is formed on I. The preferential formation of trans-epoxide during epoxidation of 3-methyl-1-cyclohexene demonstrates the steric constraints of the active site of I. The I-catalyzed epoxidation proceeds with an induction period that disappears upon treatment of I with hydrogen peroxide. (29)Si and (183)W NMR spectroscopy and CSI mass spectrometry show that reaction of I with excess hydrogen peroxide leads to fast formation of a diperoxo species, [TBA](4)[gamma-SiW(10)O(32)(O(2))(2)] (II), with retention of a gamma-Keggin type structure. Whereas the isolated compound II is inactive for stoichiometric epoxidation of cyclooctene, epoxidation with II does proceed in the presence of hydrogen peroxide. The reaction of II with hydrogen peroxide would form a reactive species (III), and this step corresponds to the induction period observed in the catalytic epoxidation. The steric and electronic characters of III are the same as those for the catalytic epoxidation by I. Kinetic, spectroscopic, and mechanistic investigations show that the present epoxidation proceeds via III.     

An Overview of Heterogeneous Transition Metal-Based Catalysts for Cyclohexene Epoxidation Reaction

Cyclohexane epoxide, which contains highly active epoxy groups, plays a crucial role as an intermediate in the preparation of fine chemicals. However, controlling the epoxidation pathway of cyclohexene is challenging due to issues such as the allylic oxidation of cyclohexene and the ring opening of cyclohexane epoxide during the cyclohexene epoxidation process to form cyclohexane oxide. This review focuses on the structure-activity relationships and synthesis processes of various heterogeneous transition metal-based catalysts used in cyclohexene epoxidation reactions, including molybdenum(Mo)-based, tungsten(W)-based, vanadium(V)-based, titanium(Ti)-based, cobalt(Co)-based, and other catalysts. Initially, the mechanism of cyclohexene epoxidation by transition metal-based catalysts is examined from the perspective of catalytic active centers. Subsequently, the current research of cyclohexene epoxidation catalysts is summarized based on the perspective of catalyst support. Additionally, the differences between alkyl hydroperoxide, hydrogen peroxide (H₂O₂), and oxygen (O₂) as oxidants are analyzed. Finally, the main factors influencing catalytic performance are summarized, and reasonable suggestions for catalyst design are proposed. This work provides scientific support for the advancement of the olefin epoxidation industry.     

1,4-Cyclohexadienes as mechanistic probes for the Jacobsen epoxidation: evidence for radical pathways

1,4-Cyclohexadienes allow a direct comparison of epoxidation and C-H oxidation within the same molecule and give evidence for radical pathways during the Jacobsen epoxidation.     

Co2+-exchanged faujasite zeolites as efficient heterogeneous catalysts for epoxidation of styrene with molecular oxygen

Co2+-exchanged faujasite zeolites can efficiently catalyze the epoxidation of styrene with molecular oxygen, and the Co2+ ions located in supercages are suggested to account for the activation of O2 for the epoxidation of styrene.