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.     

Predicting H2S Oxidative Dehydrogenation over Graphene Oxides from First Principles

Spin-polarized periodic density functional theory was performed to characterize H₂S adsorption and dissociation on graphene oxides (GO) surface. The comprehensive reaction network of H₂S oxidation with epoxy and hydroxyl groups of GO was discussed. It is shown that the reduction reaction is mainly governed by epoxide ring opening and hydroxyl hydrogenation which is initiated by H transfer from H₂S or its derivatives. Furthermore, the presence of another OH group at the opposite side relative to the adsorbed H₂S activates the oxygen group to facilitate epoxide ring opening and hydroxyl hydrogenation. For H₂S interaction with -O and -OH groups adsorption on each side of graphene, the pathway is a favorable reaction path by the introduction of intermediate states, the predicted energy barriers are 3.2 and 10.4 kcal/mol, respectively, the second H transfer is the rate-determining step in the whole reaction process. In addition, our calculations suggest that both epoxy and hydroxyl groups can enhance the binding of S to the C-C bonds and the effect of hydroxyl group is more local than that of the epoxy.     

Structural requirements in chiral diphosphine-rhodium complexes—XI: Asymmetric homogeneous hydrogenation of z-α-acylaminocinnamic acids and esters with (1s, 2s)-trans-1,2-bis(diphenylphosphinomethyl) cyclohexane/rhodium(i) complexes

Z-α-acylaminocinnamate esters were hydrogenated with neutral rhodium(I) complexes containing (1S, 2S)-trans-1,2-bis(diphenylphosphinomethyl)cyclohexane. Increasing the steric bulk of the alcohol moiety of the ester function results in increased enantioface differentiation in favor of the re-si prochiral face to yield an excess of the S-amino acid derivatives. In the series of N-acetylphenylalanine ester products (resulting from hydrogenation of Z-α-acetamidocinnamate esters) the optical purity increased from 1% ee-(R) [Me]; 20% ee-(S) [Et]; 47% ee-(S) [i-Pr]; to 58% ee-(S) [t-Bu]. Increasing the steric bulk of the acyl function (NHCOR, where R is an alkyl moiety) favors the reduction of the si-re prochiral face [in the methyl ester substrates] to yield an excess of the R-amino acid derivatives. In the series of N-acylphenylalanine methyl ester products (resulting from hydrogenation of Z-methyl α-acylaminocinnamates) the optical purity increased from 1% ee-(R) [Me]; 13% ee-(R) [i-Pr]; to 15% ee-(R) [t-Bu and 1-adamantyl]. The α-formamido and α-benzamido substrates gave hydrogenation products having 22% ee-(R) [H] and 35% ee-(R) [Ph]. In the corresponding free acids, increasing the steric bulk of the acyl function (NHCOR, where R is an alkyl moiety) results in almost no change in the optical purity of the reduction products. In the series of N-acylphenyl-alanine products (resulting from hydrogenation of Z-α-acylaminocinnamic acids) the optical purity was 35% ee-(S) [Me]; 31% ee-(S) [i-Pr]; 33% ee-(S) [t-Bu]; and 35% ee-(S) [1-adamantyl]. The α-benzamido substrate gave a hydrogenation product having 8% ee-(S).     

Influence of the nature of chiral auxiliaries on the diastereoselective hydrogenation of ortho-substituted benzoic acid derivatives

The diastereoselective hydrogenation of o-toluic acid or o-methoxy benzoic acid covalently bound to different chiral auxiliaries was performed on Rh and Ru supported catalysts. The cis-isomers were formed predominantly, with a diastereoselectivity largely influenced by the structure of the chiral inductor and the steric hindrance brought for the preferential adsorption of one face of the aromatic substrate. The effect of the functional group on the proline auxiliary (alcohol or ester groups susceptible to modify the anchoring of the aromatic substrate) was weak. Hydrogenolysis occurred rather extensively with the methoxy benzoic acid and constituted the most important hydrogenation pathway on Rh/C. The presence of the CO group in the pyroglutamic acid methyl ester is a determining factor for obtaining good diastereoselectivity.     

The effects of ligand variation on enantioselective hydrogenation catalysed by RuH2(diphosphine)(diamine) complexes

Density functional theory calculations have been used to investigate the hydrogenation of acetophenone (ACP) catalysed by the RuH(2)(diphosphine)(diamine) complexes with emphasis on the effect of the structure of the diphosphine and diamine ligands on the enantioselectivity. The computed reaction coordinate diagrams of RuH(2)(diphosphine)[(S,S)-DPEN] catalysed reactions with different (S)-diphosphine ligands (XylBINAP, TolBINAP, and BINAP) show that the presence of two methyl groups in the meta position is critical to obtaining a high difference in activation energy for the reaction pathways associated with the (R)- and (S)-alcohols, and consequently high enantioselectivity. The effect of the diamine structure while keeping the TolBINAP and XylBINAP fixed has also been analysed. To enhance the enantioselectivity of the TolBINAP system, the addition of two methyl groups and the removal of a phenyl group of the diamine (DMAPEN) offer the necessary steric interactions. We conclude by reporting a correlation between the enantiomeric excess and the difference in the computed activation energies of the two most favourable (S) and (R) reaction pathways, which shows that the computational procedure adopted could be used to predict the enantiomeric excess of ketone hydrogenation reactions catalysed by the Noyori-type catalysts, and assist in the choice of ligand when optimising the enantiomeric excess.     

Effect of chain density and conformation on protein adsorption at PEG-grafted polyurethane surfaces

Polyurethanes were modified using monobenzyloxy polyethylene glycol (BPEG) which possesses a bulky hydrophobic benzyloxy group at one end and a hydroxyl group at the other end as a preconstructed BPEG layer, and poly(ethylene glycol) (PEG) and monomethoxyl poly(ethylene glycol) (MPEG) with various chain lengths as fillers. Our objective was to investigate the effect of PEG graft density and conformation on protein adsorption at PEGlated surface. The graft density was estimated by a chemical titration method. The combination of ATR-FTIR, AFM and titration results provide evidences that the graft density can be increased by backfilling PEG or MPEG to a BPEG layer. However, fibrinogen and albumin adsorption significantly increased on all surfaces after PEG or MPEG backfilling. We conclude that the conformation of hydrophobic benzyloxy end groups of the BPEG layer plays a key role. The benzyloxy end groups of preconstructed PEG chains stretch to the surface after PEG backfilling, which possibly accounts for the observed increase in protein adsorption. The BPEG conformation change after backfilling with PEG or MPEG was also suggested by contact angles. Additionally, protein adsorption was slightly influenced by the length of filler, suggesting a change in surface morphology.     

噻吩在γ-Mo_2N(100)表面上加氢脱硫反应的密度泛函理论研究

利用密度泛函理论研究了γ-Mo2N(100)表面上的噻吩加氢脱硫(HDS)过程.噻吩在γ-Mo2N(100)表面上不同作用形式的结构优化结果显示,η5-Mo2N吸附构型最稳定,具有最大的吸附能(-0.56 eV),此时噻吩通过S原子与Mo2原子相连平行表面吸附在四重空位(hcp位).H原子和噻吩在hcp位发生稳定共吸附,hcp位是噻吩HDS的活性位点.噻吩在γ-Mo2N(100)表面进行直接脱硫反应,HDS过程分为S原子脱除和C4产物加氢饱和两部分.过渡态搜索确定了HDS最可能的反应机理及中间产物,首个H原子的反应需要最大的活化能(1.69 eV),是噻吩加氢脱硫的控速步骤.伴随H原子的不断加入,噻吩在γ-Mo2N(100)表面上优先生成―SH和丁二烯,随后―SH加氢生成H2S,丁二烯加氢饱和生成2-丁烯和丁烷.由于较弱的吸附,H2S、2-丁烯和丁烷很容易在γ-Mo2N(100)表面脱附成为产物.     

Selectivity Control on Hydrogenation of Substituted Nitroarenes through End‐On Adsorption of Reactants in Zeolite‐Encapsulated Platinum Nanoparticles

Platinum nanoparticles encapsulated into zeolite Y (Pt@Y catalyst) exhibit excellent catalytic selectivity in the hydrogenation of substituted nitroarenes to form the corresponding aromatic amines, even after complete conversion. With the hydrogenation of p‐chloronitrobenzene as a model, the role of zeolite encapsulation toward perfect selectivity can be attributed to constraint of the substrate adsorbed on the platinum surface in an end‐on conformation. This conformation results in the activation of only one adsorbed group, with little influence on the other one in the molecule. Owing to a much lower apparent activation energy of Pt@Y for the hydrogenation of a separately adsorbed nitro group than that of the adsorbed chloro group, the Pt@Y catalyst can prevent hydrodechlorination of p‐chloronitrobenzene under mild conditions. Moreover, such a conformation results in a reduced adsorption energy of target p‐chloroaniline on the platinum surface; thus suppressing the reactivity of hydrodechlorination of p‐chloroaniline to circumvent further C−Cl bond breakage.     

Solution pH-regulated interfacial adsorption of diblock phosphorylcholine copolymers

Spectroscopic ellipsometry has been used to examine the pH-responsive interfacial adsorption of a series of biocompatible diblock copolymers incorporating 2-methacryloyloxyethyl phosphorylcholine-based (MPC) residues and 2-(dialkylamino)ethyl methacrylate residues, with a specific focus on 2-(diethylamino)ethyl groups (referred to as MPCm-DEAn, where m and n refer to the mean degrees of polymerization of each block) at the hydrophilic silicon oxide/water interface. For all the copolymers studied the surface excess shows only weak concentration dependence. Increasing the length of the DEA block has little effect on the dynamic or equilibrated adsorption at pH 7, indicating that the DEA block adopts a flat conformation on the silicon oxide surface at this pH. With increasing pH, however, the surface excess shows a dramatic increase, followed by a subsequent decline. The observed maximum in surface excess represents a balance between charge over-compensation of the copolymer with the oppositely charged surface and the subsequently reduced charge density of the copolymer. Variations in the observed maxima for various MPCm-DEAn diblock copolymers indicate different surface conformations at high pH. Salt addition does not affect copolymer adsorption. This behavior is attractive for biomedical applications in which the ionic strength is variable. It was also found that the preadsorbed diblock copolymers immobilized DNA from solution to an extent that is proportional to the relative charge ratio between the anionic DNA and the cationic DEA block of the copolymer.     

噻吩在γ-Mo2N(100)表面上加氢脱硫反应的密度泛函理论研究

利用密度泛函理论研究了γ-Mo₂N(100)表面上的噻吩加氢脱硫(HDS)过程. 噻吩在γ-Mo₂N(100)表面上不同作用形式的结构优化结果显示, η5-Mo₂N吸附构型最稳定, 具有最大的吸附能(-0.56 eV), 此时噻吩通过S原子与Mo2原子相连平行表面吸附在四重空位(hcp 位). H原子和噻吩在hcp位发生稳定共吸附, hcp位是噻吩HDS的活性位点. 噻吩在γ-Mo₂N(100)表面进行直接脱硫反应, HDS过程分为S原子脱除和C₄产物加氢饱和两部分. 过渡态搜索确定了HDS最可能的反应机理及中间产物, 首个H原子的反应需要最大的活化能(1.69 eV),是噻吩加氢脱硫的控速步骤. 伴随H原子的不断加入, 噻吩在γ-Mo₂N(100)表面上优先生成―SH和丁二烯, 随后―SH加氢生成H₂S, 丁二烯加氢饱和生成2-丁烯和丁烷. 由于较弱的吸附, H₂S、2-丁烯和丁烷很容易在γ-Mo₂N(100)表面脱附成为产物.     

Enantio-differentiating hydrogenation of methyl acetoacetate over modified nickel catalysts: effects of nickel dispersion on the enantio-selectivity of catalysts

Enantio-differentiating hydrogenation of methyl acetoacetate was performed over the supported nickel catalysts modified by the solution of (R,R)-tartaric acid or (S)-malic acid and NaBr. The reduction temperature of supported nickel was the most important factor determining the enantio-selectivity of catalysts. The reduction temperature changed the nickel dispersion, by which the quantity and coverage of modifier adsorption were varied. The enantio-selectivity of modifiers both (R,R)-tartaric acid and (S) -malic acid were compared at various reduction temperatures. (R,R)-tartaric acid with two hydroxyl groups in a molecule showed an optimum coverage on the nickel surface that gave the maximum ee value. The maximum ee value was 72% at the reduction temperature of 973 K. In contrast, (S)-malic acid with one hydroxyl group in a molecule showed a monotonous decrease in ee and decreasing amounts of adsorbed modifier with increasing reduction temperatures.     

Initial hydrogenations of pyridine on MoP(001): a density functional study

The initial hydrogenations of pyridine on MoP(001) with various hydrogen species are studied using self-consistent periodic density functional theory (DFT). The possible surface hydrogen species are examined by studying interaction of H(2) and H(2)S with the surface, and the results suggest that the rational hydrogen source for pyridine hydrogenations should be surface hydrogen atoms, followed by adsorbed H(2)S and SH. On MoP(001), pyridine has two types of adsorption modes, i.e., side-on and end-on; and the most stable η(5)(N,C(α),C(β),C(β),C(α)) configuration of the side-on mode facilitates the hydrogenation of pyridine. The optimal hydrogenation path of pyridine with surface hydrogen atoms in the Langmuir-Hinshelwood mechanism is the formation of 3-monohydropyridine, followed by producing 3,5-dihydropyridine, in which the two-step hydrogenations take place on the C(β) atoms. When adsorbed H(2)S is considered as the source of hydrogen, slightly higher hydrogenation barriers are always involved, while the energy barriers for hydrogenations involving adsorbed SH are much lower. However, the hydrogenation of pyridine should be suppressed by the adsorption of H(2)S, and the promotion effect of adsorbed SH is limited.     

非螯合型手性双膦/钌催化的不对称氢化反应

对RuCl₃和手性双膦(2S,5S)-2,5-双-(二苯膦)-1,4∶3,6-双脱水-2,5-双去氧-L-艾杜醇(BDPI)催化的不对称氢化反应进行了研究,反应的转化率为100%,光学收率受[双膦]/[RuCl₃]比值的影响较大.在α-乙酰胺基肉桂酸的催化氢化反应中,[双膦]/[RuCl₃]=2.0时e.e.值最大,为68%;对衣糠酸的催化氢化,[双膦]/[RuCl₃]=3时e.e.值最大,为92%.     

(1S,2S)-DPEN修饰的3%Ir/SiO2/2TPP催化苄叉丙酮的不对称加氢

在温和的条件下制备了负载型3%(w)Ir/SiO2/2TPP(三苯基膦)催化剂, 并且考察了(1S,2S)-1,2-二苯基乙二胺[(1S,2S)-DPEN]作为手性修饰剂对其催化苄叉丙酮不对称加氢反应性能的影响. 结果表明, 手性修饰剂(1S,2S)-DPEN的加入, 对苄叉丙酮不对称加氢反应活性和C=O加氢的选择性都有很好的促进作用. 经优化条件, 在40 ℃下, LiOH浓度为0.375 mol·L-1的甲醇溶液中, 氢气压力为6 MPa, 反应8 h后, 苄叉丙酮的转化率大于99.0%, 对不饱和醇的选择性大于99.0%, 不饱和醇的对映选择性(ee)值达到48.1%.     

(1S,2S)-1,2-二苯基乙二胺修饰Ir/HAP催化苯乙酮及其衍生物的不对称加氢反应

采用浸渍法在温和条件下制备了羟基磷灰石负载的铱催化剂(Ir/HAP), 并以X射线衍射(XRD), 透射电子显微镜(TEM), X射线光电子能谱(XPS), 比表面积测定(BET)以及附带能量散射X射线谱的扫描电子显微镜(SEM-EDS)等手段对载体和催化剂进行了表征. 以(1S,2S)-1,2-二苯基乙二胺((1S,2S)-DPEN)为手性修饰剂时, 该催化剂对苯乙酮及其衍生物不对称加氢反应表现出较高活性和对映选择性(ee). 在氢气压力为3.0 MPa、303 K条件下反应3 h, 苯乙酮及其衍生物的加氢转化率在94.7%以上, 其中生成2'-(三氟甲基)苯乙醇的对映选择性高达81.5%. 在不使用其它配体作稳定剂的情况下, 该结果比目前文献报道值高. 通过对比研究发现, 羟基磷灰石作为载体优于二氧化硅等其它无机载体. 催化剂通过简单离心分离可循环使用多次, 无明显的金属铱流失.     

Enhanced selectivity in acetylene hydrogenation by ligand modified Pd/TiO2 catalysts

Modification of Pd/TiO(2) catalyst by adsorption of triphenylphosphine and phenyl sulfide leads to markedly enhanced selectivity for acetylene hydrogenation in the presence of ethylene and excess hydrogen. Similar selectivities were maintained in cases where ligand decomposition took place and sulfur was retained on the catalyst surface.     

Competition at chiral metal surfaces: fundamental aspects of the inversion of enantioselectivity in hydrogenations on platinum

O-Phenylcinchonidine (PhOCD) is known to efficiently induce inversion of enantioselectivity with respect to cinchonidine (CD) in the enantioselective hydrogenation of various activated ketones on Pt/Al(2)O(3). To understand the origin of the switch of enantioselective properties of the catalyst, the adsorption of PhOCD has been studied by in situ ATR-IR spectroscopy, in the presence of organic solvent and dissolved hydrogen, i.e., under conditions used for catalytic hydrogenation. The adsorption structures and energies of the anchoring group of CD and PhOCD were calculated on a Pt 38 cluster, using relativistically corrected density functional theory (DFT). Both approaches indicate that both modifiers are adsorbed via the quinoline ring and that the spatial arrangement of the quinuclidine skeleton is critical for the chiral recognition. New molecular level information on the conformation of CD relative to PhOCD adsorbed on a surface is extracted from the ATR spectra and supported by DFT calculations. The result is a clearer picture of the role played by the phenyl group in defining the chiral space created by the modifiers on Pt. Moreover, when CD was added to a pre-equilibrated adsorbed layer of PhOCD, a chiral adsorbed layer was formed with CD as the dominant modifier, indicating that CD adsorbs more strongly than PhOCD. Conversely, when PhOCD was added to preadsorbed CD, no significant substitution occurred. The process leading to nonlinear effects in heterogeneous asymmetric catalysis has been characterized by in situ spectroscopy, and new insight into a heterogeneous catalytic R-S switch system is provided.     

A physico-chemical investigation of poly(ethylene oxide)-block-poly(L-lysine) copolymer adsorption onto silica nanoparticles

The adsorption behavior of poly(ethylene oxide)-b-poly(L-lysine) (PEO(113)-b-PLL(10)) copolymer onto silica nanoparticles was investigated in phosphate buffer at pH 7.4 by means of dynamic light scattering, zeta potential, adsorption isotherms and microcalorimetry measurements. Both blocks have an affinity for the silica surface through hydrogen bonding (PEO and PLL) or electrostatic interactions (PLL). Competitive adsorption experiments from a mixture of PEO and PLL homopolymers evidenced greater interactions of PLL with silica while displacement experiments even revealed that free PLL chains could desorb PEO chains from the particle surface. This allowed us to better understand the adsorption mechanism of PEO-b-PLL copolymer at the silica surface. At low surface coverage, both blocks adsorbed in flat conformation leading to the flocculation of the particles as neither steric nor electrostatic forces could take place at the silica surface. The addition of a large excess of copolymer favoured the dispersion of flocs according to a presumed mechanism where PLL blocks of incoming copolymer chains preferentially adsorbed to the surface by displacing already adsorbed PEO blocks. The gradual addition of silica particles to an excess of PEO-b-PLL copolymer solution was the preferred method for particle coating as it favoured equilibrium conditions where the copolymer formed an anchor-buoy (PLL-PEO) structure with stabilizing properties at the silica-water interface.     

The crystal structures of a chiral aminoalkoxide cluster and its adduct with benzyllithium

The crystal structures of the chiral aminoalkoxide cluster (S)-2 and its unexpected adduct with benzyllithium (S)-3 have been determined. For compound (S)-2, a hexameric cyclic ring-ladder is observed in the solid state, which leads to an unshielded coordination site in a chiral pocket that is only accessible from one direction. The presence of (S)-2 leads to the deprotonation of toluene by n-butyllithium, giving benzyllithium. In contrast to earlier studies on lithium alkoxides, the resulting adduct (S)-3 between benzyllithium and (S)-2 is not formed by the exchange of alkoxy groups for alkanide units, as has been observed for a multitude of examples. Compound (S)-3 therefore represents a thus far unconsidered type of structure: the unshielded "top side" of the aminoalkoxide cluster (S)-2 capped by the lithium center of benzyllithium via three Li-O interactions.     

An amino alcohol ligand for highly enantioselective addition of organozinc reagents to aldehydes: serendipity rules

[reaction: see text] Amino alcohol 4 (or its enantiomer) is prepared in two simple steps. Commercial (1R,2S)-2-amino-1,2-diphenylethanol is dialkylated with bis(2-bromoethyl) ether. Subsequent hydrogenation over 5% Rh on alumina in the presence of morpholine unexpectedly stops at the hexahydro derivative 4. Amino alcohol 4 promotes the enantioselective addition of diethylzinc to aldehydes at room temperature in up to 99% enantiomeric excess.