Multiphase heterogeneous catalytic enantioselective hydrogenation of acetophenone over cinchona-modified Pt/C

The multiphase heterogeneous enantioselective hydrogenation of acetophenone in the presence of cinchona-modified Pt/C was investigated. The system demonstrated the feasibility of this reaction on non-activated ketones. The reaction proceeded selectively, at room temperature and atmospheric pressure, towards the formation of 1-phenylethanol, with up to 20% ee (enantiomeric excess) of either enantiomer depending on the modifier used. A mode of action of the modifier is proposed to account for the mechanism. A comparison with other systems indicates that the investigated system likely acts by a different mechanism, and that it is quite specific for acetophenone.     

Enantioselective sensing of chiral carboxylic acids

A chiral 1,8-diacridylnaphthalene-derived fluorosensor has been prepared and used for enantioselective sensing of a broad variety of chiral carboxylic acids including amino acids, aliphatic acids, arylalkanoic acids, and halogenated carboxylic acids. Fluorescence titration experiments in acetonitrile gave linear Stern-Volmer plots for 1:1 and 1:2 complexes and enantioselectivities up to 4.5.     

Direct hydrogenation of nitroaromatics and one-pot amidation with carboxylic acids over platinum nanowires

A novel ultrathin platinum nanowire with uniform length and a diameter of 1.5 nm was synthesized by acidic etching of FePt nanowire in methanol. This nanowire was characterized by high‐resolution transmission electron microscopy (HRTEM). X‐ray diffraction (XRD) data indicated that the main plane is (111). The ability of this nanowire to catalyze the heterogeneous hydrogenation of nitroaromatics to give the corresponding amines has been investigated. The catalyst showed satisfactory activity in various solvents under mild conditions and showed excellent stability. The catalytic performance was also evaluated in the one‐pot reduction of nitroaromatics and amidation with carboxylic acids under a hydrogen atmosphere at 100 °C. These methods for the hydrogenation of nitroaromatics and the direct amidation of nitroaromatics with carboxylic acids are simple, economical, and environmentally benign, and have practical advantages for the synthesis of amines and amides without the production of toxic byproducts.     

Asymmetric hydrogenation of indene carboxylic acids: stereochemistry of hydrogen addition

The stereochemistry of hydrogen addition to ,β-unsaturated carboxylic acids was studied by means of hydrogenation of indene carboxylic acids and their derivatives. Experiments were carried out over Pd/Al₂O₃ in the presence and absence of cinchonidine as a chiral modifier. In all cases hydrogenation occurred via bottom side syn addition of two hydrogen atoms to the chemisorbed substrate. Formation of trans isomers, up to 72%, was observed due to C=C bond isomerization in the substrate, and to adsorption and hydrogenation of the unsaturated molecule in a sterically unfavourable position. Adsorption in ‘upside down’ position was promoted by N-bases. Hydrogenation of 3-methylindene-2-carboxylic acid provided up to 45% ee. Due to the high activity of Pd in C=C bond migration, a good ee can be achieved only when isomerization is negligible.     

Enantioselective hydrogenation of α,β-unsaturated carboxylic acids over cinchonidine-modified Pd catalysts: effect of substrate structure on the adsorption mode

Systematic variation of the substrate structure in the enantioselective hydrogenation of α,β-unsaturated carboxylic acids over cinchonidine-modified Pd catalysts indicated that aryl substituents in β position changed the adsorption mode of the substrates on the metal surface and eventually the configuration of the products formed in excess as compared to aliphatic substrates. The configuration of the products formed in excess by hydrogenation of eight α,β-unsaturated carboxylic acids, of which two have not been described yet, indicated that the substrates bearing aromatic ring in β position were adsorbed on the opposite face of the CC group compared to acids having aliphatic group in β position.     

Iridium-catalyzed enantioselective hydrogenation of alpha,beta-unsaturated carboxylic acids

A highly efficient iridium-catalyzed hydrogenation of alpha,beta-unsaturated carboxylic acids has been developed by using chiral spiro-phosphino-oxazoline ligands, affording alpha-substituted chiral carboxylic acids in exceptionally high enantioselectivities and reactivities.     

Enantioselective synthesis of beta2-amino acids using rhodium-catalyzed hydrogenation

A series of protected beta2-dehydroamino acids has been prepared in three steps from commercially available starting materials in good yields. These were used as substrates in rhodium-catalyzed asymmetric hydrogenation applying a mixed ligand system of monodentate phosphoramidites and phosphines. Optimization of the catalyst structure was achieved by high throughput experimentation. High enantioselectivities were obtained (up to 91%) with full conversion for a number of beta-amino acids.     

Enantioselective hydrogenation of ethyl pyruvate over diop modified Pt nanoclusters. Determination of geometry of the ligand adsorption mode via DRIFTS

Immobilization of chiral ligands on the surface of metal nanoparticles is one concept for heterogenation of enantioselective catalysts. Diop modified Pt nanoclusters were found to be able to induce enantioselectivity and enhancement of the reaction rate in hydrogenation of ethyl pyruvate. The model of geometry of the diop adsorption has been proposed based on DRIFTS and molecular modelling investigations. The ability of diop to induce enantioselectivity and enhanced reaction rate was explained through the adsorption geometry and comparison with known models of enantioselectivity established for the cinchonidine on Pt system.     

Effect of cyclodextrins on the rhodium-catalysed hydrogenation of unsaturated carboxylic acids in water

Addition of cyclodextrin to the well-known RhCl₃/TPPTS (trisulfonated triphenylphosphine) hydrogenation catalytic system leads to high selectivities in the hydrogenation of unsaturated carboxylic acids in water. The presence of both cyclodextrin and sulfonate ions is required to obtain high selectivities.     

Enantioselective hydrogenation of alpha-aryloxy alpha,beta-unsaturated acids. Asymmetric synthesis of alpha-aryloxycarboxylic acids

[reaction: see text] A facile preparation of chiral alpha-aryloxy carboxylic acids via asymmetric hydrogenation of the corresponding unsaturated acids has been discovered. A number of catalysts have been identified that give high product enantioselectivity, and the scope of the reaction has been examined with respect to substitution on the aromatic ring and olefin.     

The effect of the substrate structure on the stereoselectivity in an asymmetric hydrogenation of unsaturated carboxylic acids over cinchonidine-modified palladium catalyst

An enantiomeric ratio of 10:1 (82% ee) was achieved for the first time in the hydrogenation of α,β-unsaturated acids with heterogeneous catalysts by designing the substrate structure to enhance the interaction with the catalyst, and to increase the intrinsic stereoselectivity.     

高效钴氧化物催化羧酸可控加氢制醇（英文）

羧酸选择加氢是合成醇的重要方法,廉价高效的催化体系仍然在探索中.我们利用地球上储量丰富的钴氧化物作为催化剂,通过控制催化反应过程,进而实现高选择性地催化羧酸加氢制备醇.一系列含有不同官能团的羧酸可以被选择加氢至相应的醇类化合物,反应选择性可以满足工业生产要求.通过一系列的谱学表征以及理论计算,我们证实了钴氧化物在羧酸选择加氢反应中的优选活性位点位为氧化亚钴,从而建立了催化剂与反应活性之间的构效关系,为催化剂的理性设计提供指导.首先,我们选取硬脂酸加氢反应作为模型反应,通过对地球上储量丰富的氧化镍、四氧化三铁和四氧化三钴的催化活性对比发现,四氧化三钴催化剂活性最高,在473 K,2 MPa氢气条件下,反应速率可以达到1.2 mmol/(h·g).对四氧化三钴催化剂进行不同温度的预还原处理,我们发现催化剂的活性得到显著提高,其中573 K还原的样品活性最高,反应速率可以达到7.3 mmol/(h·g),要远远高于贵金属催化剂Pd/C(0.6 mmol/(h·g))和Pt/C(1.8 mmol/(h·g)).XRD结果表明,随着还原处理温度的不断升高,催化剂由四氧化三钴变为氧化亚钴,最终变为金属态的钴.当还原温度为573 K时,催化剂的组成为单一相氧化亚钴.XPS测试结果表明,当还原温度为573 K时,样品中只含有Co~(2+)的信号峰,并且Co/O的比例为1/1,进一步证明样品是纯态的氧化亚钴.从TEM照片中可以发现,在原始的四氧化三钴样品中观察到晶面间距为0.467和0.244 nm,分别对应四氧化三钴的(111)和(311)晶面.而对于573 K还原的样品只观察到一种晶面间距(0.246 nm),对应氧化亚钴的(111)晶面.结合表征手段和硬脂酸催化加氢活性结果,我们得出氧化亚钴是573 K还原样品催化羧酸加氢反应的活性位点.理论计算结果进一步证实了这个实验结论.理论计算结果表明,在氧化亚钴(111)晶面,硬脂酸加氢转换为十八醇是非常快速和高效的,然而,对于氢解C-C键和C-O键,需要耗费更高的能量,能垒约为1.2 e V.因为硬脂酸的吸附远远强于十八醇的吸附,硬脂酸的存在会抑制十八醇氢解形成烯烃的反应,只有当硬脂酸酸完全转化为十八醇,才会发生随后的氢解反应.通过控制催化反应过程,可以实现在氧化亚钴(111)晶面高选择性催化酸加氢至醇,也就是反应控制催化过程.基于氧化亚钴在硬脂酸加氢制备十八醇上的优异催化性能,我们进一步研究了一系列含有不同官能团的羧酸化合物的催化加氢,发现氧化亚钴表现出良好的官能团容忍度,可以实现高效、广谱的酸选择加氢至醇反应.     

Heterogeneous asymmetric hydrogenation of heteroaromatic methyl ketones catalyzed by cinchona-modified iridium catalysts

A heterogeneous iridium catalyst was synthesized with silica particles as support for the hydrogenation of heteroaromatic methyl ketones. The catalyst and support were characterized by solid-state NMR, HTEM, SEM, XPS, and BET. A series of heteroaromatic methyl ketones were investigated at room temperature. The catalytic system was effective and more than 99% conversion and up to 83.6% enantioselectivity were obtained in the hydrogenation of heteroaromatic methyl ketones.     

Homogeneous catalytic hydrogenation of free carboxylic acids in the presence of cluster ruthenium carbonyl hydrides

Saturated monocarboxylic acids up to C₆, several bicarboxylic acids and some of the corresponding anhydrides are hydrogenated in the homogeneous phase with H₄Ru₄(CO)₈(PBu₃)₄ as catalyst to give the corresponding alcohols (present among the reaction products as esters) or lactones at 100–200°C under a pressure of 100–200 atm of hydrogen. Anhydrides react at temperatures lower than those needed for acids. Esters are not reduced. Only δ-valerolactone is hydrogenated to 1,5-pentanediol. Ruthenium carbonyl carboxylates have been recovered at the end of the reaction and appear to be catalytically active intermediates.     

Selective esterification of non-conjugated carboxylic acids in the presence of conjugated or aromatic carboxylic acids over active carbon supported methanesulfonic acid

Non-conjugated carboxylic acids are selectively esterified in good yields in the presence of conjugated or aromatic carboxylic acids by stirring over active carbon supported methanesulfonic acid in di-chloromethane at room temperature.     

Enantioselective recognition of carboxylates: a receptor derived from alpha-aminoxy acids functions as a chiral shift reagent for carboxylic acids

Enantioselective recognition of carboxylates has important implications in asymmetric synthesis and drug discovery. We have prepared a novel C2-symmetric receptor 1 from alpha-aminoxy acids in a high overall yield. A series of chiral recognition studies indicate that receptor 1 not only can bind to carboxylate ions tightly but also has a good ability to recognize enantiomers of a broad variety of carboxylic acids in the 1H NMR spectra. Thus, the receptor 1 can be used as a chiral shift reagent for the determination of enantiomeric purities of chiral carboxylic acids by 1H NMR directly and rapidly.     

Enantioselective synthesis of branched allylic esters via rhodium-catalyzed coupling of allenes with carboxylic acids

We report on the first intermolecular asymmetric catalytic regio- and enantioselective addition of carboxylic acids to terminal allenes to form valuable branched allylic esters, employing a rhodium(I)/(R,R)-DIOP catalyst system.     

Enantioselective Pd-catalyzed hydrogenation of enesulfonamides

Asymmetric hydrogenation of cyclic enesulfonamides affords chiral cyclic sulfonamides using Pd(OCOCF(3))(2)/diphosphine complexes as catalysts with up to 98% ee.