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.     

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.     

Ti-containing mesoporous organosilica as a photocatalyst for selective olefin epoxidation

We previously found that Ti-containing mesoporous silica (T-S) with isolated and tetrahedrally coordinated Ti-oxide species, when photoactivated in acetonitrile with molecular oxygen (O(2)), catalyzes highly selective epoxidation of olefins (Chem. Commun. 2005, 5977). The system showed the highest epoxide selectivity among the photocatalytic systems proposed so far, but showed insufficient olefin conversion. In the present work, we have employed Ti-containing mesoporous organosilicas (T-OS), synthesized by a surfactant-templating method with an organosilane precursor, as the photocatalyst and have studied the effects on the olefin conversion and the epoxide selectivity. The T-OS catalysts demonstrate the same high epoxide selectivity as does T-S, but scarcely improve the olefin conversion. Photoluminescence measurement reveals that the T-OS catalysts with high surface hydrophobicity enhance the access of hydrophobic olefins to the photoexcited Ti-oxide species as expected, but destabilize the excited species themselves. ESR analysis demonstrates that the T-OS catalysts also destabilize the active oxygen radical (O(3)(*-)), a crucial oxidant for olefin epoxidation, formed on the excited Ti-oxide species. These destabilizations counteract the enhanced olefin access to the excited species, resulting in almost no improvement in olefin conversion. Through detailed analyses, we have summarized the changes in photocatalytic properties of the Ti-oxide species, associated with the organic modification of the catalyst.     

Asymmetric epoxidation of cis-β-methylstyrenes catalyzed by N-aryl substituted oxazolidinone-containing ketones. A beneficial substituent effect

Asymmetric epoxidation of substituted cis-β-methylstyrenes using N-aryl substituted oxazolidinone-containing ketones as catalysts shows that substituents on the phenyl group of the olefin have significant positive effects on the enantioselectivity of the epoxidation, indicating a beneficial interaction between the phenyl group of the olefin and the phenyl group of the ketone catalyst.     

Optically detected magnetic resonance in the photoexcited triplet states of Ti(IV) and Zr(IV) cylopentadienyl complexes

The class of compounds (RCp)2MX2, where M is a group IV metal, Cp is cyclopentadienyl, R is an alkyl, and X is a halide, has been of continuing interest as precursors for olefin coordination polymerization catalysts. In this communication, we demonstrate that the technique of optically detected magnetic resonance (ODMR) reveals subtle differences in the composition of the frontier molecular orbitals associated with the nature of the alkyl substituents on the Cp rings.     

[N,N]-二齿镍配合物催化烯烃聚合

烯烃聚合催化剂的设计是烯烃配位聚合领域的一个核心科学问题,通过设计合成精确结构的催化剂可以有效地调控催化聚合性能以及聚合产物的结构.后过渡金属催化剂由于其易调变性、对聚合产物支化结构的可控性及对极性单体的容忍性,在烯烃聚合领域引起了广泛的关注.本文介绍了近年来本课题组在[N,N]-二齿镍烯烃聚合催化剂设计方面的研究进展,包括四元环的中性脒基镍催化剂、五元环的-二亚胺镍催化剂、2-胺基吡啶和-胺基亚胺系列镍催化剂,以及六元环的-二亚胺和苯胺基亚胺镍催化剂在烯烃聚合的应用.通过优化后过渡金属镍催化剂结构,可成功实施烯烃活性聚合.     

Silica-Supported Tantalum Catalysts for Asymmetric Epoxidations of Allyl Alcohols

Tantalum good, titanium bad: This appears to be the case for silica-supported catalysts for the asymmetric epoxidation of allyl alcohols. Complexes such as [SiO-Ta(OEt)(4)] were prepared from silica and [Ta(=CHCMe(3))(CH(2)CMe(3))(3)], and in the presence of a tartrate and an alkyl hydroperoxide, these surface tantalum compounds lead to efficient and convenient catalysts for the asymmetric epoxidation of 2-propen-1-ol (R=H) and trans-2-hexen-1-ol (R=nPr; see reaction).     

Synthesis of Some New Long-chain Salen Catalysts

In the research of enantioselective epoxidation of unfunctionalized alkenes using Salen compounds, Katasuki1 and Jacobsen2 pointed out that 3 and 3′ groups of Salen were very important to increase the e.e of the catalytic product. Recently, in order to explore useful information concerning molecular design of metal catalysts for enantioselective epoxidation of trans-disubstituted alkenes, which remains an unresolved problem in the field of metal-catalyzed asymmetric epoxidation of unfunctionalized alkenes,some new Salen compounds containing long-chain in 3 and 3′ were designed and synthesized. We think these 3 and 3′ long-chain groups not only do as bulky group, but also act as second introduced chiral source. The following is the route:     

Olefin cis-dihydroxylation versus epoxidation by non-heme iron catalysts: two faces of an Fe(III)-OOH coin

The oxygenation of carbon-carbon double bonds by iron enzymes generally results in the formation of epoxides, except in the case of the Rieske dioxygenases, where cis-diols are produced. Herein we report a systematic study of olefin oxidations with H(2)O(2) catalyzed by a group of non-heme iron complexes, i.e., [Fe(II)(BPMEN)(CH(3)CN)(2)](2+) (1, BPMEN = N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)-1,2-diaminoethane) and [Fe(II)(TPA)(CH(3)CN)(2)](2+) (4, TPA = tris(2-pyridylmethyl)amine) and their 6- and 5-methyl-substituted derivatives. We demonstrate that olefin epoxidation and cis-dihydroxylation are different facets of the reactivity of a common Fe(III)-OOH intermediate, whose spin state can be modulated by the electronic and steric properties of the ligand environment. Highly stereoselective epoxidation is favored by catalysts with no more than one 6-methyl substituent, which give rise to low-spin Fe(III)-OOH species (category A). On the other hand, cis-dihydroxylation is favored by catalysts with more than one 6-methyl substituent, which afford high-spin Fe(III)-OOH species (category B). For catalysts in category A, both the epoxide and the cis-diol product incorporate (18)O from H(2)(18)O, results that implicate a cis-H(18)O-Fe(V)=O species derived from O-O bond heterolysis of a cis-H(2)(18)O-Fe(III)-OOH intermediate. In contrast, catalysts in category B incorporate both oxygen atoms from H(2)(18)O(2) into the dominant cis-diol product, via a putative Fe(III)-eta(2)-OOH species. Thus, a key feature of the catalysts in this family is the availability of two cis labile sites, required for peroxide activation. The olefin epoxidation and cis-dihydroxylation studies described here not only corroborate the mechanistic scheme derived from our earlier studies on alkane hydroxylation by this same family of catalysts (Chen, K.; Que, L, Jr. J. Am. Chem. Soc. 2001, 123, 6327) but also further enhance its credibility. Taken together, these reactions demonstrate the catalytic versatility of these complexes and provide a rationale for Nature's choice of ligand environments in biocatalysts that carry out olefin oxidations.     

A structure-activity study of Ni-catalyzed alkyl-alkyl Kumada coupling. Improved catalysts for coupling of secondary alkyl halides

A structure-activity study was carried out for Ni catalyzed alkyl-alkyl Kumada-type cross coupling reactions. A series of new nickel(II) complexes including those with tridentate pincer bis(amino)amide ligands ((R)N(2)N) and those with bidentate mixed amino-amide ligands ((R)NN) were synthesized and structurally characterized. The coordination geometries of these complexes range from square planar, tetrahedral, to square pyramidal. The complexes had been examined as precatalysts for cross coupling of nonactivated alkyl halides, particularly secondary alkyl iodides, with alkyl Grignard reagents. Comparison was made to the results obtained with the previously reported Ni pincer complex [((Me)N(2)N)NiCl]. A transmetalation site in the precatalysts is necessary for the catalysis. The coordination geometries and spin-states of the precatalysts have a small or no influence. The work led to the discovery of several well-defined Ni catalysts that are significantly more active and efficient than the pincer complex [((Me)N(2)N)NiCl] for the coupling of secondary alkyl halides. The best two catalysts are [((H)NN)Ni(PPh(3))Cl] and [((H)NN)Ni(2,4-lutidine)Cl]. The improved activity and efficiency was attributed to the fact that phosphine and lutidine ligands in these complexes can dissociate from the Ni center during catalysis. The activation of alkyl halides was shown to proceed via a radical mechanism.     

Transition metal catalyzed hydroarylation of olefins using unactivated substrates: Recent developments and challenges

The addition of aromatic C-H bonds across olefin CC bonds (olefin hydroarylation) is an important synthetic methodology for the preparation of alkylated arenes. Traditional methods utilizing Friedel-Crafts catalysts result in a significant waste stream and substantial polyalkyated products. In order to achieve high overall yield of monoalkylated compounds, Friedel-Crafts catalysis must be combined with a second transalkylation catalytic step. Reactions that incorporate substituted olefins are selective for branched products. In addition, the regioselective synthesis of disubstituted alkyl arenes is difficult to achieve. Recent development of transition metal based catalysts provides non-Friedel-Crafts pathways for the production of alkyl arenes. Systems based on Ru(II), Ir(III), and Pt(II) have been developed, and mechanistic studies have begun to provide insight into the details of these catalysts. Herein, we review recently published results in the area of transition metal catalyzed olefin hydroarylation with a comparison of known catalysts and discussion of challenges yet to be overcome.     

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.     

Bifunctional nanocrystalline MgO for chiral epoxy ketones via Claisen-Schmidt condensation-asymmetric epoxidation reactions

Design and development of a truly nanobifunctional heterogeneous catalyst for the Claisen-Schmidt condensation (CSC) of benzaldehydes with acetophenones to yield chalcones quantitatively followed by asymmetric epoxidation (AE) to afford chiral epoxy ketones with moderate to good yields and impressive ee's is described. The nanomagnesium oxide (aerogel prepared) NAP-MgO was found to be superior over the NA-MgO and CM-MgO in terms of activity and enantioselectivity as applicable in these reactions. An elegant strategy for heterogenization of homogeneous catalysts is presented here to evolve single-site chiral catalysts for AE by a successful transfer of molecular chemistry to surface metal-organic chemistry with the retention of activity, selectivity/enantioselectivity. Br?nsted hydroxyls are established as sole contributors for the epoxidation reaction, while they add on to the CSC, which is largely driven by Lewis basic O2-sites. Strong hydrogen-bond interactions between the surface -OH on MgO and -OH groups of diethyl tartrate are found inducing enantioselectivity in the AE reaction. Thus, the nanocrystalline NAP-MgO with its defined shape, size, and accessible OH groups allows the chemisorption of TBHP, DET, and olefin on its surface to accomplish single-site chiral catalysts to provide optimum ee's in AE reactions.     

Silica-supported,single-site titanium catalysts for olefin epoxidation. A molecular precursor strategy for control of catalyst structure

A molecular precursor approach involving simple grafting procedures was used to produce site-isolated titanium-supported epoxidation catalysts of high activity and selectivity. The tris(tert-butoxy)siloxy titanium complexes Ti[OSi(O(t)Bu)(3)](4) (TiSi4), ((i)PrO)Ti[OSi(O(t)Bu)(3)](3) (TiSi3), and ((t)BuO)(3)TiOSi(O(t)Bu)(3) (TiSi) react with the hydroxyl groups of amorphous Aerosil, mesoporous MCM-41, and SBA-15 via loss of HO(t)Bu and/or HOSi(O(t)Bu)(3) and introduction of titanium species onto the silica surface. Powder X-ray diffraction, nitrogen adsorption/desorption, infrared, and diffuse reflectance ultraviolet spectroscopies were used to investigate the structures and chemical natures of the surface-bound titanium species. The titanium species exist mainly in isolated, tetrahedral coordination environments. Increasing the number of siloxide ligands in the molecular precursor decreases the amount of titanium that can be introduced this way, but also enhances the catalytic activity and selectivity for the epoxidation of cyclohexene with cumene hydroperoxide as oxidant. In addition, the high surface area mesoporous silicas (MCM-41 and SBA-15) are more effective than amorphous silica as supports for these catalysts. Supporting TiSi3 on the SBA-15 affords highly active cyclohexene epoxidation catalysts (0.25-1.77 wt % Ti loading) that provide turnover frequencies (TOFs) of 500-1500 h(-1) after 1 h (TOFs are reduced by about half after calcination). These results demonstrate that oxygen-rich siloxide complexes of titanium are useful as precursors to supported epoxidation catalysts.     

Mechanism and activity of ruthenium olefin metathesis catalysts.

This report details the effects of ligand variation on the mechanism and activity of ruthenium-based olefin metathesis catalysts. A series of ruthenium complexes of the general formula L(PR(3))(X)(2)Ru=CHR(1) have been prepared, and the influence of the substituents L, X, R, and R(1) on the rates of phosphine dissociation and initiation as well as overall activity for olefin metathesis reactions was examined. In all cases, initiation proceeds by dissociative substitution of a phosphine ligand (PR(3)) with an olefinic substrate. All of the ligands L, X, R, and R(1) have a significant impact on initiation rates and on catalyst activity. The origins of the observed substituent effects as well as the implications of these studies for the design and implementation of new olefin metathesis catalysts and substrates are discussed in detail.     

磺甲基化ZPS-IPPA轴向固载手性Salen Mn(Ⅲ)催化剂及催化烯烃环氧化反应

以聚(苯乙烯-异丙烯膦酸)-磷酸氢锆(ZPS-IPPA)为载体,对其苯环氯甲基化、磺酸化后与手性Salen Mn(Ⅲ)轴向配位,合成了一类新的固载型手性Salen Mn(Ⅲ)催化剂,采用FTIR,XPS,SEM,TEM,N2吸附等对催化剂表征.分别在次氯酸钠溶液和间氯过氧苯甲酸氧化体系中,考察了固载催化剂对苯乙烯、α-甲基苯乙烯、茚等非功能化烯烃不对称环氧化反应的催化性能,结果表明,与均相催化剂相比,对映体选择性明显提高.尤其在m-CPBA/NMO氧化体系中0℃反应12h,茚的环氧化物的转化率及ee值均达到99%以上.并且催化剂容易分离,回收使用5次仍能保持较好的催化性能.     

Asymmetric addition of alkylzinc reagents to cyclic alpha,beta-unsaturated ketones and a tandem enantioselective addition/diastereoselective epoxidation with dioxygen

Although over 100 catalysts have been reported to catalyze the asymmetric addition of alkyl groups to aldehydes, these catalysts fail to promote additions to ketones with >90% enantioselectivity. This paper describes the asymmetric 1,2-addition of alkyl groups to conjugated cyclic enones to give allylic alcohols with chiral quaternary centers. The resultant allylic alcohols are converted into epoxy alcohols with excellent diastereoselectivities. Treatment of the epoxy alcohols with BF3.OEt2 induces a semipinacol rearrangement to provide alpha,alpha-dialkyl-beta-hydroxy ketones with all-carbon chiral quaternary centers. We also report a one-pot procedure for the asymmetric addition/diastereoselective epoxidation reaction. Simply exposing the reaction mixture to dioxygen after the asymmetric addition reaction is complete results in epoxidation of the allylic alcohol with high diastereoselectivity.     

有机金属烯烃配位聚合催化剂

有机金属烯烃配位聚合催化剂具有高催化活性和良好的分子剪裁性,通过调节催化剂的微结构,如配体的取代基?配位原子以及配位中心的电子与立体环境等,可以在分子层次上实现烯烃聚合物的分子设计与组装;实现聚合物物理性质的调控,从而得到各种具有新型功能和立体异构的聚合物。本文综述了金属烯烃聚合催化剂研究进展,并展望该领域的发展趋势。     

Synthesis and Application of Novel Chiral Poly-nuclear-Mn(Ⅲ) Catalysts on Asymmetric Epoxidation of Alkenes

The interest of the coordination chemistry of manganese has been driver by the important roles of metalloenzymes and highly valuable catalysts in olefin expoxidation.1 Jacobsen Salen-Mn complexes with a simple structure have been commercially utilized to catalyze asymmetric epoxidation of unfunctionalized cis-alkenes2, but the catalytic enantioselectivity for trans-alkenes, unfortunately, are lower and this kinds of complexes are unstable and difficult to be recovered for reuse.3 In order to improve the catalytic activity and recyclability, many new catalysts including the supported catalysts,heterogeneous catalysts and else modified catalysts have been studied, however their comprehensive effects are unsatisfying.4Recently, some studies in interrelated realm showed that the catalytic performance of bi- or poly-nuclear complexes was superior to that of monomer.5 Meanwhile, our previous studies also showed that properly increasing the molecular weight of catalysts as well as the extent of conjugation of active center would not only result in high activity or reactivity but also its stability and recyclablity, aiding product isolation and catalyst recovery.6For these reasons we designed and synthesized the chiral poly-Mn(Ⅲ) complexes in which active sites were conjugated in certain distance side by side though central nucleus of 4, 6-dihydroxy- isophthalaldehyde (see the scheme). These novel Mn(Ⅲ) complexes have been investigated for the first time as catalysts (lmol%)for the asymmetric epoxidation of alkenes by using pure urea-H2O2 as oxidant and NH4OAc as additive in CH2Cl2/MeOH, showing high activity and good enantioselectivity. All reactions were finished in 1.5h. Rather surprisingly, a marginal increase in ee was observed when the concentration of the substrate was increased from 0.01 to 0.5M. The poly-nuclear complex formation enhanced the catalyst's reactivity and stability. It, unlike mononuclear, could be easily recovered and reused several cycles with a decline of yield but a keep of ee's.A low concentration requirement of catalyst, high ee and chemical activity, mild reaction conditions and catalytic recycles render the kinds of complexes attractive.     

Selective oxidation of styrene on an oxygen-covered Au(111)

For the first time, we demonstrate olefin epoxidation promoted by an extended Au surface. The oxidation of styrene to styrene epoxide, benzoic acid, and benzeneacetic acid is promoted on Au(111) covered with 0.2 ML of oxygen atoms. The estimated selectivity for styrene epoxide formation is approximately 53%. Total combustion to CO2 accounts for approximately 20% of the styrene reaction. We propose that styrene epoxide, benzoic acid, and benzeneacetic acid are produced via two possible oxametallacycle intermediates. Our work demonstrates that extended Au is an effective material for olefin oxidation, which has implications for understanding the activity of nanoscale Au catalysts.