Hybrid organic-inorganic solids for heterogeneous asymmetric catalysis

This paper provides a short review of recent progress in the design and synthesis of chiral organic–inorganic hybrid solids and their applications in heterogeneous asymmetric catalysis. Homochiral hybrid solids containing catalytic sites can be readily prepared by linking appropriately designed chiral bridging ligands with metal ion or metal cluster nodes. Heterogeneous asymmetric catalysts with enantioselectivity as high as 99.2 and turnover number (TON) as high as 20,000 have been obtained based on these hybrid organic–inorganic solids. The modular nature of the present approach promises to lead to a variety of practically useful heterogeneous asymmetric catalysts for many organic transformations.     

联萘酚纳米孔道配位聚合物催化剂的研究进展

纳米多孔材料可用作催化剂、气体储存材料和光电子器件,是目前新型多孔材料的研究热点之一。手性联萘酚配体所修饰的催化剂是一种很优异的C2轴对称手性诱导源,可以催化各种α,β-不饱和羰基化合物,具有良好的催化活性和对映选择性。本文对由手性联萘酚类配体所修饰的小分子催化剂、聚合物负载的催化剂和自负载催化剂,在不饱和羰基化合物催化不对称环氧化反应中的应用进行了综述。本文介绍了对手性联萘酚纳米多孔配位聚合物的研究。     

手性树枝状大分子催化剂的合成及其在不对称催化中的应用进展

根据手性催化剂活性位点在树枝状大分子（Dendrimer）中位置的不同,综述了催化活性位点分别位于Dendrimer核心和外围以及聚合物固载化的手性Dendrimer催化剂的合成及其在不对称氢化、不对称氢转移、不对称Michael加成、硼烷对酮的不对称还原、不对称醇醛缩合反应、不对称Diels-Alder反应等不对称催化反应中的应用,重点论述了树枝状结构及代数对手性催化剂的活性、对映选择性及循环利用的影响,并对开发活性、选择性和稳定性更高的可回收手性Dendrimer催化剂前景进行展望。     

Reduced graphene oxide supported chiral Ni particles as magnetically reusable and enantioselective catalyst for asymmetric hydrogenation

A reduced graphene oxide (rGO) supported chiral-modified Ni catalyst was synthesized, characterized and employed for asymmetric hydrogenation. The prepared hybrid catalyst could produce each enantiomer with d- or l-tartaric acid as chiral modifier and exhibited a high TOF (20160 h⁻¹) and enantioselectivity (enantiomeric excess, 98.5%) for asymmetric hydrogenation of methyl acetoacetate. The high catalytic activity and enantioselectivity were mainly attributed to the unique properties of the support rGO, as it had a large specific surface area to sustain and stabilize Ni particles and its high charge carrier mobility could enable the readily transfer of electrons in the reaction process. Besides, the catalyst could also gain an enhanced reactant sorption with the support of rGO, thus achieved a greatly catalysis enhancement. The ferromagnetism of Ni made the catalyst easier for separation and reuse. The catalytic and recycling performance of the prepared chiral Ni catalyst demonstrated that rGO was indeed a promising support to improve activity, enantioselectivity and durability of catalysts, and the prepared catalysts were promising reusable heterogeneous catalysts for asymmetric hydrogenation.     

Asymmetric Catalysis with Chiral Porous Metal–Organic Frameworks

This paper provides a brief overview of recent progress in the design and synthesis of chiral metal–organic frameworks (CMOFs) and their applications in heterogeneous asymmetric catalysis. Catalytically active CMOFs can be synthesized using two distinct strategies, either by post-synthetic activation of pre-synthesized porous MOFs or by directly linking well-designed catalytically competent bridging ligands with metal ion or metal cluster nodes. Heterogeneous asymmetric catalysis with very high activity and enantioselectivity has been achieved with CMOFs. The intrinsic tunability of CMOFs promises to lead to the design and synthesis of a variety of practically useful heterogeneous asymmetric catalysts for many organic transformations.     

Chiral salen Mn(III) immobilized on sulfoalkyl modified ZSP-IPPA as an effective catalyst for asymmetric epoxidation of unfunctionalized olefins

Sulfoalkyl modified zirconium poly (styrene-isopropenyl phosphonate)-phosphate (ZPS-IPPA), with special structures of new type of organic–inorganic hybrid material were designed and synthesized for immobilization of the chiral salen Mn(III) Jacobsen’s homogenous catalyst by axial coordination. All the heterogeneous chiral salen Mn(III) catalysts with different linkage lengths obtained exhibited great catalytic activity and enantioselectivity in the asymmetric epoxidation of unfunctionalized olefins. The influence of the linkage lengths on the catalytic performance was investigated. What’s more, the catalysts were easily separated from the reaction systems and could be reused for several times without significant loss of catalytic activity.     

Nanostructured biomimetic catalysts for asymmetric hydrogenation of enamides using molecular imprinting technology

A new class of heterogeneous catalysts for asymmetric hydrogenation of enamides was synthesized using molecular imprinting technology. These new catalysts are molecularly imprinted polymers (MIPs) made from rhodium (I) and copper (II) complexes with the bis(oxazoline) chiral ligands. One of the Rh-MIPs showed 87% ee toward L-enantiomeric product while the Cu-MIP showed 82% ee toward D-enantiomeric product. Both MIPs are easy to separate and reusable.     

硅胶固载Ru(Ⅱ)Cl_2(PPh_3)_3催化苯乙酮不对称加氢

通过共价键联的方法将RuCl_2(PPh_3)_3配合物固载于手性二胺修饰的硅胶表面,制备了多相不对称加氢催化剂。采用等离子体发射光谱(ICP)、红外光谱(FTIR)和紫外光谱(UV—Vis)等手段对制备的催化剂进行了表征。结果表明,RuCl_2(PPh_3)_3配合物固载于硅胶表面保持了原有的结构性能。在苯乙酮不对称加氢反应中,不同手性二胺对催化反应具有明显的影响,(1S,2S)-1,2-二苯基乙二胺的手性诱导能力强于其他二胺,优化条件下,苯乙酮转化率达100%,(R)-苯乙醇的对映体过量值达50%,固体催化剂具有良好的稳定性和重复使用性。     

Diastereoselective Heterogeneous Catalytic Hydrogenation of Aromatic or Heteraromatic Compounds

The diastereoselective hydrogenation catalyzed by heterogeneous metallic catalysts uses a covalently bound chiral auxiliary to induce the chirality. It remains an active synthetic methodology in the asymmetric synthesis of chiral products and may proceed with high diastereoselectivity. This review describes recent examples using this method, such as hydrogenation of C=C, C=O, and C=N bonds. The use of a chiral auxiliary group has also been successfully applied to the hydrogenation of aromatic and heteroaromatic rings. The choice of the chiral auxiliary was found to play a key role in the asymmetric hydrogenation. The results could be explained in terms of steric effect, with a preferred conformation of the adduct substrate and the addition occurring from the less bulky side. The catalytic metal, the support and the presence of additives were also found to have a significant influence.     

Chiral N,N'-dioxides: new ligands and organocatalysts for catalytic asymmetric reactions

Homochiral catalysts that can effect asymmetric transformations are invaluable in the production of optically active molecules. Researchers are actively pursuing the design of new ligands and organocatalysts by exploiting concepts derived from the application of bifunctional and C(2)-symmetric catalysts. Many homochiral catalysts containing amines, ethers, alcohols, and phosphines as electron-pair donors have been successfully developed. Amine N-oxides are highly polar substances. Despite their pronounced capacity as electron-pair donors, N-oxides have been underutilized in asymmetric reactions; they have only made a visible impact on the field in the preceding decade. Systematic studies have instead largely focused on pyridine- or quinoline-based scaffolds in organosilicon and coordination chemistry. The application of chiral tertiary amine N-oxides has not been widely pursued because of the difficulty of controlling the chirality at the tetrahedral nitrogen of the N-oxide moiety. In this Account, we outline the design of a new family of C(2)-symmetric N,N'-dioxides from readily available chiral amino acids. We then discuss the application of these chiral amine N-oxides as useful metal ligands and organocatalysts for asymmetric reactions. The high nucleophilicity of the oxygen in N-oxides is ideal for organocatalytic reactions that rely on nucleophilic activation of organosilicon reagents. These catalysts have been successfully applied in the asymmetric addition of trimethylsilylcyanide to aldehydes, ketones, aldimines, and ketimines, with good yields and excellent enantioselectivities. Asymmetric organocatalytic chlorination of β-ketoesters with N-chlorosuccinimide has also been achieved through hydrogen bond activation. The molecular framework of these N,N'-dioxides, with their multiple O-donors, also serves as a new tetradentate ligand that can coordinate a range of metal ions, including Cu(I), Cu(II), Ni(II), Mg(II), Fe(II), Co(II), In(III), Sc(III), La(III), Y(III), Nd(III), and others. These versatile metal complexes are efficient catalysts for a variety of asymmetric reactions. Asymmetric cycloadditions have been achieved with these chiral Lewis acid catalysts. We have also found success with asymmetric nucleophilic additions to C═O or C═N bonds; substrates include 3-substituted 2-oxindoles, alkenes, enamides, enecarbamates, diazoacetate esters, nitroalkanes, glycine Schiff bases, and phosphate. Notably, the first catalytic asymmetric Roskamp reaction was realized, which was successful because of the high efficiency of the catalyst. Asymmetric conjugate additions between α,β-unsaturated compounds and nucleophiles such as nitroalkane, malonate, thioglycolate, and indoles have been accomplished. The first asymmetric haloamination of chalcones was discovered, and the reaction proceeded with high regio- and enantioselectivity. In some cases, we were able to reduce the catalyst loading to just 0.01-0.05 mol % while maintaining excellent outcomes. Some particularly interesting phenomena were observed over the course of the research. These include a remarkable amplification of the asymmetry in a sulfa-Michael reaction, as well as the reversal of enantioselectivity after alteration of the central metal or the subunits of the ligand in two other reactions. These unusual results have facilitated a deeper understanding of the catalytic mechanism.     

多相手性催化剂的制备及其对不对称加氢反应的催化作用

综述了不对称加氢反应中多相手性催化剂的研究进展,包括将均相手性催化剂固定到有机聚合物、磁性的Fe₃O₄纳米颗粒、功能化修饰的分子筛等载体上;直接利用有机金属配合物中的阳离子和分子筛骨架中的阴离子相互作用实现多相化;采用金鸡纳生物碱等手性小分子为修饰剂和天然高分子为手性源制备多相催化剂。同时对不同途径制备的多相手性催化剂的结构特性、催化性能和立体选择性进行了评价。     

Synthesis of polymeric salen complexes and application in the enantioselective hydrolytic kinetic resolution of epoxides as catalysts

The polymeric (salen) Co(III) catalysts containing various counter anions have been synthesized, and the asymmetric catalytic activities of these newly synthesized polymer-type salen complexes were investigated in the HKR of terminal epoxides. The polymeric chiral salen Co(III) complexes catalyze the hydrolysis of epichlorohydrine, 1,2-epoxybutane, 1,2-epoxyhexane and epoxystyrene with very high enantioselectivities under mild conditions. The catalysts could be recovered and reused several times without further treatment after reaction, showing no loss of activity and enantioselectivity.     

Layered Double Hydroxides Containing Chiral Organic Guests: Synthesis, Characterization and Application for Asymmetric C–C Bond-Forming Reactions

We designed and developed two heterogeneous enantioselective catalysts, LDH-proline and LDH-BINOL, by incorporating chiral organic molecules viz., L-proline and S-BINOL, into layered double hydroxides (LDHs). These catalysts are characterized by IR, UV/vis and TGA analysis. These organo-LDHs have been successfully employed for various asymmetric C–C bond-forming transformations, and the products are obtained with good to excellent yields albeit low enantiomeric excess.     

Enantiomer-selective activation of racemic catalysts

Asymmetric catalysts can be evolved into highly activated catalysts by association with chiral activators. This asymmetric activation process is particularly useful in racemic catalysis through selective activation of one enantiomer of the racemic catalysts. Recently, a strategy whereby a racemic catalyst is selectively deactivated by a chiral additive has been reported to yield nonracemic products. However, we have reported a strategy that is an alternative to asymmetric catalysts but is conceptually opposite, in which a chiral activator selectively activates rather than deactivates one enantiomer of a racemic chiral catalyst. The advantage of this activation strategy over the deactivation counterpart is that the activated catalyst can produce a greater enantiomeric excess (x(act)% ee) in the products than the ee attained by the enantiomerically pure catalyst on its own. Therefore, 'asymmetric activation' could provide a general and powerful strategy for the use of not only atropisomeric and, hence, racemic ligands but also chirally flexible and 'pro-atropisomeric' ligands without enantiomeric resolution!     

Mesoporous Silica Supported Unsymmetric Chiral Mn(III) Salen Complex: Synthesis,Characterization and Effect of Pore Size on Catalytic Performance

An unsymmetric chiral Mn(III) salen complex was immobilized onto a series of MCM-41 and MCM-48 mesoporous materials with different pore sizes. The as-synthesized catalysts were characterized by XRD, FT-IR, DR UV-Vis, N₂ sorption and XPS. The results indicated that chiral Mn(III) salen complex was immobilized into the nanopores of supports and the long-range mesoporous ordering of parent supports was maintained after the immobilization. The heterogeneous catalysts were evaluated in the asymmetric epoxidation of styrene, indene, and 1-phenylcyclohexene, and the effect of 3-mercaptopropyltrimethoxysilane dosage and fine-tuning of pore size on the catalytic performance was studied. It was found that the activity and enantioselectivity were closely correlated with the pore sizes of heterogeneous catalysts. The larger pore sizes of supports could lead to higher conversions and ee values.     

Synthesis and characterization of a new chiral polyurea-based catalyst

A series of chiral poly(urea)s was synthesized by solution polyaddition. The polymers are characterized by NMR, FTIR, DSC, viscosimetry, and microanalysis. After deposition of the rhodium, the insoluble chiral polymers were used as catalysts in the reduction of acetophenone by hydrogen transfer. The influence of the structure of the polymers on the catalytic activity and selectivity was investigated. The enantiomeric excesses (e.e.) obtained were up to 60%, and the polymeric catalysts were reused without loss of activity and selectivity. © 1996 John Wiley & Sons, Inc.     

Ring-opening polymerizations of heterocycles with organometallic catalysts

Recent developments in the field of ring-opening polymerizations using organometallic initiators are discussed, particularly in the case of epoxides, episulfides, lactones and cyclosiloxanes. For classical anionic polymerizations, generally initiated by compounds of alkali metals, the complexity of the kinetics is linked to the presence of aggregates of active centres. But kinetics could be largely simplified by a suppression of these aggregates through the addition of complexing agents of the counter ions. This permits the measurements of absolute rate constants on various ionic species. This addition generally leads to large increases in the observed rates, and may modify the relative rate of formation of cyclic oligomers. When the monomer contains at least one chiral centre, stereospecific polymerizations may be realized both in homogeneous and heterogeneous phase, using derivatives of bivalent metals such as Zn or Cd. If asymmetric catalysts are used, the resolution of monomer racemic mixtures may be performed, giving optically active polymers and permitting in some cases the preparation of pure enantiomers.     

Functional monomers and polymers, 98. Asymmetric induction by chiral polymers containing bornyl groups

In order to determine the catalytic ability of the polymers containing chiral bornyl groups to the asymmetric induction, the asymmetric addition reaction of dodecanethiol to isopropenyl methyl ketone was studied using synthetic chiral polymers having pendant bornyl groups, in comparison with the case of using a monomeric model compound, i.e. d-bornyl acetate, and also poly(methyl methacrylate). It was found that the polymers having chiral borynl groups were effective for the asymmetric induction, and the optical yield depended on the bornyl group content in the polymers.     

Organofunctionalized catalyst surfaces highly active and selective for carbon–carbon bond-forming reactions

This article illustrates two types of organofunctionalized heterogeneous catalysts for variety of organic carbon–carbon bond-forming reactions, summarizing our previous reports and also presenting new data. Organic amines with an alkoxysilane moiety were immobilized on inorganic silica-alumina surfaces (SA-NR₂) by simple silane-coupling reactions between the silica-alumina surface (SA) and the alkoxysilane. This SA-NR₂ acted as acid–base bifunctional heterogeneous catalysts for carbon–carbon bond-forming reactions, such as cyano-ethoxycarbonylation, Michael reaction of nitriles, and nitro-aldol reaction. These reactions did not occur with either SA or homogeneous amine compounds. In addition, the mixture of SA and homogeneous amine showed low catalytic activity due to undesirable acid–base neutralization reaction. Achiral organic silane-coupling reagents with a variety of functional groups were also immobilized on a SiO₂ surface that had been immobilized with chiral bis(oxazoline) (BOX), to which Cu ions were coordinated to make chiral Cu–BOX complexes on the SiO₂ surface. The SiO₂-supported Cu–BOX complex catalyst functionalized with achiral 3-methacryloxypropyltrimethoxysilane dramatically increased enantioselectivity in the asymmetric Diels–Alder reaction of cyclopentadiene and 3-acryloyl-2-oxazolidinone. The organofunctionalized catalysts showed much better performances for the C–C bond-forming reactions compared to the corresponding homogeneous systems. The heterogeneous catalysts thus obtained were characterized by solid-state ¹³C and ²⁹Si MAS NMR, FT-IR, UV/vis, XAFS, ESR, XRF, and elemental analysis.     

不对称催化合成手性泛解酸内酯的研究进展

泛解酸内酯是合成D-泛酸钙和D-泛醇的重要中间体,国内主要采用生物拆分法,虽取得一定进展,但存在底物浓度偏低、反应条件苛刻、光学纯度不高和催化剂活性不强等问题。化学不对称合成法成为近年来制备手性泛内酯的研究热点。根据手性源的不同,介绍过渡金属配合物催化不对称还原酮基泛内酯,过渡金属配合物催化羟醛缩合反应,有机小分子及其衍生物不对称催化羟醛缩合反应并经还原内酯化合成泛内酯以及光学活性化合物作为反应底物或非手性底物中加入手性助剂的合成手性泛内酯工艺。其中,有机小分子催化乙醛酸酯与醛的反应表现出良好的催化效果,且催化剂易得,反应条件温和,操作简单。缩合产物的收率和对映选择性均不高,设计具有高活性和高选择性的有机小分子手性催化剂是今后研究的重点。