Formation and Nature of Microheterogeneous Catalysts Based on Palladium Complexes

The mechanism of the formation and the nature of microheterogeneous catalysts for hydrogenation are discussed using acetylacetonate complexes of palladium with phosphine ligands as an example. Polynuclear palladium complexes with phosphide and phosphinidene ligands are obtained, and their role in the formation of hydrogenation catalysts is found. As distinct from phosphide complexes, amide complexes of palladium are unstable in a hydrogen atmosphere and undergo reduction to form highly dispersed palladium black.     

Chiral phosphine-phosphite ligands in the highly enantioselective rhodium-catalyzed asymmetric hydrogenation.

We have investigated a series of enantiopure phosphine-phosphite ligands (P(1)-P(2) = ligands 1-4) in the rhodium-catalyzed asymmetric hydrogenation reaction. Intermediate [Rh(P(1)-P(2))(cod)]BF(4) and [Rh(P(1)-P(2))(5)]BF(4) complexes (cod = 1,5-cyclooctadiene; 5 = methyl acetamidoacrylate ester) were observed by (31)P[(1)H] NMR. The [Rh(P(1)-P(2))(cod)]BF(4) complexes were precursors to active catalysts of the asymmetric hydrogenation reaction of several prochiral dehydroamino acid derivatives under mild reaction conditions (1 bar of hydrogen and 20 degrees C). The enantiomeric excess reached up to 99%.     

Ligand and substrate effects on the mechanism of rhodium-catalyzed hydrogenation of enamides

The rhodium-catalyzed hydrogenation reaction of enamides is studied computationally using the B3LYP/LACVP** level of theory for a range of ligands and substrates. Two model bidentate phosphine ligands, 1,2-bis(dimethylphosphino)ethane (DMPE) and (Z)-1,2-bis(dimethylphosphino) ethene (ZDMP), and two chiral bidentate phosphine ligands, (R,R)-MeDuPHOS and (R,R)-tetramethylbisoxaphospinane (TMBOP), are investigated in the hydrogenation of alpha-formamidoacrylonitrile as a model substrate. The ZDMP ligand is then studied for three additional substrates: N-(2-propenyl)formamide, (Z)-3-formamido-2-butenenitrile, and (E)-3-formamido-2-butenenitrile. The potential-energy surfaces calculated for the four ligands and alpha-formamidoacrylonitrile are in general agreement with previous computational studies using QM/MM (ONIOM) methods but show consistently higher relative barriers rather than lower. The calculated potential-energy surfaces of hydrogenations of various substrates with a common ligand indicate a mechanistic change based on substrate. The sequence of hydrogen transfer to the two olefinic carbons is calculated to change based on substrate electronics. This has a significant impact on the origins of enantioselectivity for such varied substrates as the first hydride transfer to the substrate is calculated to be irreversible for all substrates, independent of whether it occurs at the alpha or beta carbon of the olefin.     

Asymmetric Ir-catalyzed hydrogenation of 1,3-dihydro-2<Emphasis Type="Italic">H</Emphasis>-1,5-benzodiazepin-2-ones using phosphoramidites

A series of phosphoramidite ligands was tested in the asymmetric hydrogenation of 4-arylsubstituted 1,3-dihydro-2H-benzodiazepine-2-ones and up to 52% ee was achieved. The effects of various factors (solvents, hydrogen pressure, and addition of phosphine ligands) on the hydrogenation were studied.     

新的双胺双膦钌配合物的合成、表征和催化性能

Polydentate ligands, N,N'-bis[o-(diphenylphosphino)benzylidene]-1,2-propane-diamine [P2N2Me for short] and N,N'-bis[o-(diphenylphosphino)benzy1]-1,2-propanediamine [P2N2 H4Me for short] have been synthesized. The interaction of RuCl2(DMSO)4 with one equivalent of P2N2Me or P2N2H4Me in refluxing toluene gave trans-RuCl2(P2N2Me) and trans-RuCl2(P2N4H4Me) in good yield, respectively. The ligands and the complexes have been fully characterized by elemental analysis and spectroscopic methods. The complexes act as an excellent catalyst precursor in hydrogen transfer hydrogenation of acetophenone in catalyst: acetophenone :iso-PrOK of 1: 100: 15, leading to 2-phenylethanol of 89-96% yield.     

A Library Approach to the Development of BenzaPhos: Highly Efficient Chiral Supramolecular Ligands for Asymmetric Hydrogenation

A library of chiral supramolecular ligands, named BenzaPhos, of straightforward preparation (two steps from commercially or readily available starting materials) and modular structure, was designed and synthesized. The ligands were screened in the search for new rhodium catalysts for the enantioselective hydrogenation of several benchmark and industrially relevant substrates. Once a series of hits were identified, structural modifications were introduced on three of the best ligands and a small second-generation library was created. Members of the latter library showed outstanding levels of activity and enantioselectivity in the hydrogenation of challenging olefins, such as enamide S4 and β-dehydroamino ester S5 (>99?%?ee: best value ever reported in both cases). A series of control experiments were undertaken to clarify the role of hydrogen bonding in determining the catalytic properties of the new ligands. The results of these experiments, together with those of computational studies carried out on four dihydride complexes involved in the catalytic hydrogenation of substrate S4, strongly suggest that a substrate orientation takes place in the catalytic cycle by formation of a hydrogen bond between the ligand amide oxygen atom and the substrate amide NH atom.     

Chiral induction effects in ruthenium(II) amino alcohol catalysed asymmetric transfer hydrogenation of ketones: an experimental and theoretical approach

The enantioselective outcome of transfer hydrogenation reactions that are catalysed by ruthenium(II) amino alcohol complexes was studied by means of a systematically varied series of ligands. It was found that both the substituent at the 1-position in the 2-amino-1-alcohol ligand and the substituent at the amine functionality influence the enantioselectivity of the reaction to a large extent: enantioselectivities (ee values) of up to 95% were obtained for the reduction of acetophenone. The catalytic cycle of ruthenium(II) amino alcohol catalysed transfer hydrogenation was examined at the density functional theory level. The formation of a hydrogen bond between the carbonyl functionality of the substrate and the amine proton of the ligand, as well as the formation of an intramolecular H...H bond and a planar H-Ru-N-H moiety are crucially important for the reaction mechanism. The enantioselective outcome of the reaction can be illustrated with the aid of molecular modelling by the visualisation of the steric interactions between the ketone and the ligand backbone in the ruthenium(II) catalysts.     

Employing the structural diversity of nature: development of modular dipeptide-analogue ligands for ruthenium-catalyzed enantioselective transfer hydrogenation of ketones

A library of novel dipeptide-analogue ligands based on the combination of tert-butoxycarbonyl(N-Boc)-protected alpha-amino acids and chiral vicinal amino alcohols were prepared. These highly modular ligands were combined with [[RuCl(2)(p-cymene)](2)] and the resulting metal complexes were screened as catalysts for the enantioselective reduction of acetophenone under transfer hydrogenation conditions using 2-propanol as the hydrogen donor. Excellent enantioselectivity of 1-phenylethanol (up to 98 % ee) was achieved with several of the novel catalysts. Although most of the ligands contained two stereocenters, it was demonstrated that the absolute configuration of the product alcohol was determined by the configuration of the amino acid part of the ligand. Employing ligands based on L-amino acids generated S-configured products, and catalysts based on D-amino acids favored the formation of the R-configured alcohol. The combination N-Boc-L-alanine and (R)-phenylglycinol (Boc-L-Ab) or its enantiomer (N-Boc-D-alanine and (S)-phenylglycinol, Boc-D-Aa) proved to be the best ligands for the reduction process. Transfer hydrogenation of a number of aryl alkyl ketones were evaluated and excellent enantioselectivity, up to 96 % ee, was obtained.     

简单的配体改变调控钌配合物催化还原CO2的活性：硼基配体的对位效应和Ru―H键的性质

开发高效的催化剂用于催化还原CO₂转化为甲酸和它的盐类已经成为研究的热点,是因为将CO₂转化为C1产物不仅可以解决CO₂的含量升高带来的环境问题,还可以解决化石能源燃烧日趋严重的问题。贵金属配合物催化CO₂转化为甲酸和甲酸盐类是目前这类反应最有效的方式,尤其是Ru、Ir和Rh等贵金属。我们之前的研究结果表明Ir(Ⅲ), Ru(Ⅱ)类配合物催化还原CO₂转化为甲酸盐的活性是由配合物Ru―H键的成键性质决定的。它们能高活性的催化CO₂是由于它们都含有同一种特点的Ru―H键,是由Ru的sd²杂化轨道和H的1s轨道杂化而成的,而且这一特点可以被活性氢的对位配体显著影响。鉴于硼基配体具有强的对位效应,我们基于高活性的均相催化剂Ru(PNP)(CO)H₂ (PNP = 2, 6-二(二叔丁基磷甲基)-吡啶)设计了Ru-PNP-HBcat和Ru-PNP-HBpin,并计算了二者催化还原CO₂的活性。Bcat和Bpin配体是实验上常用的硼基配体。我们的计算结果表明Ru-PNP-HBcat和Ru-PNP-HBpin有比Ru-PNP-H₂更长的Ru―H键、亲核性更强的活性氢,其Ru―H键中的Ru原子的d轨道杂化成分的贡献也比Ru-PNP-H₂的更少。相应地Ru-PNP-HBcat和Ru-PNP-HBpin活化CO₂的能垒比Ru-PNP-H₂低。而且Ru-PNP-H₂、Ru-PNP-HBcat和Ru-PNP-HBpin催化CO₂转化为甲酸盐的能垒分别为76.2、67.8、54.4 kJ∙mol^-1,表明Ru-PNP-HBpin具有最高的催化活性。因此,钌配合物催化还原CO₂的活性可由硼基配体强的对位效应和Ru―H键的成键性质来调控。     

Mechanistic investigation of imine formation in ruthenium‐catalyzed N‐alkylation of amines with alcohols

Imines are observed frequently in ruthenium‐catalyzed N‐alkylation of amines with alcohols. Herein, nitrogen–phosphine functionalized carbene ligands were developed and used in ruthenium‐catalyzed N‐alkylation to explore the mechanism of imine formation. The results showed that strongly electron‐donating ligands were beneficial for imine formation and alcohol dehydrogenation to generate acid. In addition, with an increase of electron density of nitrogen atom in substituted amines, the yield of imines in N‐alkylation was improved. At the same time, with electron‐rich imines as substrates, the transfer hydrogenation of imines became difficult. It is suggested that strongly electron‐donating ligands and substrates caused an increase of electron density on the ruthenium center, which resulted in the elimination of hydrogen atoms in active species [LRuH₂] as hydrogen gas rather than transfer onto the imine coordinated with the ruthenium center.     

Heterogeneous Hydrogenation with Hydrogen Spillover Enabled by Nitrogen Vacancies on Boron Nitride-Supported Pd Nanoparticles

Heterogeneous hydrogenation with hydrogen spillover has been demonstrated as an effective route to achieve high selectivity towards target products. More effort should be paid to understand the complicated correlation between the nature of supports and hydrogenation involving hydrogen spillover. Herein, we report the development of the hydrogenation system of hexagonal boron nitride (h-BN)-supported Pd nanoparticles for the hydrogenation of aldehydes/ketones to alcohols with hydrogen spillover. Nitrogen vacancies in h-BN determine the feasibility of hydrogen spillover from Pd to h-BN. The hydrogenation of aldehydes/ketones with hydrogen spillover from Pd proceeds on nitrogen vacancies on h-BN. The weak adsorption of alcohols to h-BN inhibits the deep hydrogenation of aldehydes/ketones, thus leading to high catalytic selectivity to alcohols. Moreover, the hydrogen spillover-based hydrogenation mechanism makes the catalyst system exhibit a high tolerance to CO poisoning.     

Hydrogen bonding makes a difference in the rhodium-catalyzed enantioselective hydrogenation using monodentate phosphoramidites

A new generation of monodentate phosphoramidite ligands bearing a primary amine moiety was found to display comparable or better efficiency than bisphosphines in the Rh-catalyzed asymmetric hydrogenation of challenging substrates, such as (Z)-methyl alpha-acetoxyacrylate or (E)-beta-aryl itaconate derivatives, affording the corresponding hydrogenation products with excellent enantioselectivities (up to >99% ee). The presence of intermolecular hydrogen bonding (HB) between two monodentate ligands in the catalyst was found to be critical for excellent catalyst performance. This finding provides a basis for design and development of further catalyst systems using this type of monodentate phosphoramidite ligands.     

Chiral Proline‐Based P,O and P,N Ligands for Iridium‐Catalyzed Asymmetric Hydrogenation

Two new classes of proline‐based P,O and P,N ligands were prepared and applied in the iridium‐catalyzed asymmetric hydrogenation of alkenes. Both types of ligands induced high enantioselectivities in the hydrogenation of trisubstituted C?C bonds. Iridium complexes derived from P,O ligands bearing sterically demanding amide or urea groups at the pyrrolidine N‐atom proved to be especially efficient catalysts for the conjugate reduction of α,β‐unsaturated esters and ketones, whereas analogous P,N ligands led to better results with dialkyl‐phenyl‐substituted alkenes and an allylic alcohol as substrates.     

手性磷酸酯配体的设计合成及其在不对称加氢中的应用研究新进展

合成对映体纯的药品、农用化学品及风味调料,对化学家来说是个巨大的挑战．目前采用多种不同的方法可以合成这些光学纯化合物,从工业化生产的角度来看,不对称催化反应作为获得光学纯化合物的一种手段,在众多方法中最具经济效益,同时也最具挑战性．该领域的大量出版物,     

Changes in coordination of sterically demanding hybrid imidazolylphosphine ligands on Pd(0) and Pd(II)

Low-coordinate organometallic complexes are important in structure and catalysis, and hemilability or secondary interactions such as hydrogen bonding enabled by hybrid ligands are receiving increasing attention. To study the factors controlling these phenomena, three new imidazol-2-ylphosphine ligands, L, were made. In these ligands, the bulk around P and the hindrance at the basic and potentially coordinating imidazole N-3 were varied. Remarkably, L(2)Pd(0) complexes 3a-c were shown to be two-coordinate, 12-electron species, despite the availability of imidazole N-3 to enter into eta(2)-P,N chelation. In oxidative additions of C-X bonds to the Pd(0) complexes, reaction rates and products could be controlled by the nature of the C and X groups and the R groups on the phosphine. Most significantly, whereas 4c-PhI and 4c-MeOTf from 3c are normal trans-bis(phosphine)Pd(R)(X) species, 5a-PhI, 5a-PhBr, and 5b-PhI from 3a and 3b were shown by X-ray diffraction to be a monomeric species with a single eta(2)-P,N-chelating phosphine. From 3a and methyl triflate, an ionic complex [6a-Me](+)[OTf](-) with one chelating and one nonchelating phosphine was formed, with temperature-dependent windshield-wiper exchange of the two, showing hemilability. Thus, large phosphine substituents (R = tert-butyl rather than isopropyl) favor chelation. The chelate Pd-imidazole N-3 bond is longer when the heterocyclic nitrogen is hindered by an adjacent tert-butyl group at C-4 (comparing 5a-PhI and 5b-PhI). Finally, whereas in [8b-Ph](+)[OTf](-) from 5b-PhI and isopropylamine, the amine coordinates without chelate opening or hydrogen bonding, in [10c-Me](+)[OTf](-) made from 4c-MeOTf and isopropylamine, the amine is not only coordinated at N but also donates a hydrogen bond to each phosphine imidazol-2-yl substituent.     

膦化聚2,6—二甲基1,4—苯醚负载钯催化剂的表征及其催化性能

报道了四种不同P/Pd摩尔比的膦化聚2,6-二甲基1,4-苯醚负载把催化剂的加氢和异构化性能;通过XPS、电镜和远红外对催化剂进行了表征;并考察了溶剂和温度对催化剂活性的影响.     

Recent topics of transfer hydrogenation

A number of transition metal catalysts have been developed for transfer hydrogenation of organic molecules. This method provides a useful process for the reduction of unsaturated molecules without the need for explosive hydrogen gas. An important development in this area is the design of new ligands that improve activity and selectivity under mild reaction conditions. Polydentate ligands are good candidates for producing high performance metal catalysts. This digest describes recent developments in transfer hydrogenation as well as asymmetric reactions using metal catalysts containing polydentate ligand systems.     

Aminosulf(ox)ides as ligands for Iridium(I)-catalyzed asymmetric transfer hydrogenation

A new class of efficient catalysts was developed for the asymmetric transfer hydrogenation of unsymmetrical ketones. A series of chiral N,S-chelates (6-22) was synthesized to serve as ligands in the iridium(I)-catalyzed reduction of ketones. Both formic acid and 2-propanol proved to be suitable as hydrogen donors. Sulfoxidation of an (R)-cysteine-based aminosulfide provided a diastereomeric ligand family containing a chiral sulfur atom. The two chiral centers of these ligands showed a clear effect of chiral cooperativity. In addition, aminosulfides containing two asymmetric carbon atoms in the backbone were synthesized. Both the sulfoxide-containing beta-amino alcohols and the aminosulfides derived from 1,2-disubstituted amino alcohols gave rise to high reaction rates and moderate to excellent enantioselectivities in the reduction of various ketones. The enantioselective outcome of the reaction was favorably affected by selecting the most appropriate hydrogen donor. Enantioselectivities of up to 97% were reached in the reduction of aryl-alkyl ketones.     

Highly enantioselective Rh-catalyzed hydrogenation based on phosphine-phosphite ligands derived from carbohydrates.

A new class of efficient catalysts was developed for the asymmetric hydrogenation of alpha,beta-unsaturated carboxylic acid derivatives by synthesizing a series of novel phosphine-phosphite ligands (4a-d) derived from readily available D-(+)-xylose. Excellent enantioselectivities (> 99%) were achieved under very mild reaction conditions (1 bar H(2) and 20 degrees C). Varying the biphenyl substituents in the phosphite moiety greatly affected the enantioselectivity in the hydrogenation reactions. The results also indicate that the sense of enantioselectivity is mainly controlled by the configuration of the phosphite moiety. (31)P[(1)H] NMR and kinetic studies on intermediates of the catalytic cycle show that the [Rh(P(1)-P(2))(enamide)]BF(4) species is the resting state and that the rate dependence is first order in rhodium and hydrogen pressure and zero order in enamide concentration.     

Iridium‐Catalyzed Asymmetric Hydrogenation of Imines

We have evaluated a wide range of iridium complexes derived from chiral oxazoline‐based N,P ligands for the asymmetric hydrogenation of imines and identified three efficient catalysts. These catalysts are readily synthesized by straightforward convenient routes and are air and moisture stable. In the reduction of acetophenone N‐arylimines and related acyclic substrates, excellent enantioselectivities (up to 96 % ee) were obtained by using 0.1–0.5 mol % of catalyst at ?20 °C and 5–50 bar hydrogen pressure.