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A ruthenocenyl phosphino-oxazoline-ruthenium complex (RuPHOX-Ru) was applied successfully to the asymmetric hydrogenation of β-secondary amino ketones, directly affording the corresponding chiral γ-secondary amino alcohols in up to 99% yield and with 99% ee. Reaction with β-(benzylamino)-1-phenylpropan-1-one could be performed on a gram-scale with a relatively low catalyst loading (up to 2000 S/C). The resulting hydrogenated product could be used for the synthesis of synthetically useful compounds.

The rate and, more importantly, selectivity (ketone vs aromatic ring) of the hydrogenation of 4-phenyl-2-butanone over a Pt/TiO2 catalyst have been shown to vary with solvent. In this study, a fundamental kinetic model for this multi-phase reaction has been developed incorporating statistical analysis methods to strengthen the foundations of mechanistically sound kinetic models.A 2-site model was determined to be most appropriate, describing aromatic hydrogenation (postulated to be over a platinum site) and ketone hydrogenation (postulated to be at the platinum–titania interface). Solvent choice has little impact on the ketone hydrogenation rate constant but strongly impacts aromatic hydrogenation due to solvent-catalyst interaction. Reaction selectivity is also correlated to a fitted product adsorption constant parameter. The kinetic analysis method shown has demonstrated the role of solvents in influencing reactant adsorption and reaction selectivity.

The asymmetric transfer hydrogenation of alpha,beta-propargyl ketones catalyzed by an in situ formed ruthenium-hydroxyamide complex was explored. The acetylenic alcohols were isolated in good to excellent yields with excellent ee values (typically >90%) after short reaction times at room temperature.

An efficient protocol for the asymmetric reduction of sterically hindered ketones under transfer-hydrogenation conditions was developed. The corresponding chiral alcohols were obtained in good to excellent yields with enantiomeric excess values up to 99%. The role of the cation associated with the base present in the reduction reaction was investigated. In contrast to previous studies on this catalyst system, potassium ions rather than lithium ions significantly enhanced the reaction outcome.

A convenient and general asymmetric transfer hydrogenation of a wide array of 1-aryl-3,4-dihydroisoquinoline derivatives using a [RuIICl(η6-benzene)TsDPEN] complex in combination with a 5:2 HCOOH–Et3N azeotropic mixture as a hydrogen source was developed. Under mild reaction conditions, the described catalytic transformation secured a practical synthetic access to the corresponding valuable chiral 1-aryltetrahydroisoquinoline units with high atom economy, a broad substrate scope, high isolated yields (up to 97 %) and good to excellent enantioselectivities (up to 99 % ee). It was found that the stereochemical outcome of the reaction was strongly influenced by both the structure of the catalyst and the substituents present on the substrate. The synthetic utility of the present protocol has been demonstrated through the asymmetric synthesis of several biologically important alkaloids including the antiepileptic drug agent 1c, as well as (–)-nor-cryptostyline alkaloids I and II.

Gas-phase hydrogenation of a wide range of ketones to alkanes, including hydrogenation of aliphatic ketones and acetophenone, was investigated using bifunctional metal–acid catalysis. The catalysts were comprised of a metal (Pt, Ru, Ni, and Cu) supported on acidic caesium salt of tungstophosphoric heteropoly acid Cs2.5H0.5PW12O40 (CsPW). The reaction occurred via a sequence of steps involving hydrogenation of ketone to alcohol on metal sites followed by dehydration of alcohol to alkene on acid sites and finally hydrogenation of alkene to alkane on metal sites. Catalyst activity decreased in the order: Pt > Ru >> Ni > Cu. Pt/CsPW showed the highest catalytic activity, giving almost 100% alkane yield at 100 °C and 1 bar pressure. Evidence is provided that the reaction with Pt/CsPW at 100 °C is limited by ketone-to-alcohol hydrogenation, whereas at lower temperatures (≤60 °C) by alcohol dehydration yielding alcohol as the main product. The catalyst comprised of a physical mixture of Pt/C + CsPW was found to be highly efficient as well, which indicates that the reaction is not limited by migration of intermediates between metal and acid sites in the bifunctional catalyst.

Selective catalytic reductions of nitriles are presented using the commercially available Ru-Macho-BH complex. A variety of aliphatic, aromatic and (hetero)cyclic nitriles including industrially important adipodinitrile are hydrogenated to the corresponding primary amines. Modelling suggests the reaction follows an outer sphere hydrogenation mechanism.

There is widespread interest in the hydrogenation of CO2 to energy-rich products such as formate. However, first-row transition metal catalysts for the hydrogenation of CO2 to formate remain rare. Copper complexes are widely used in the reduction of organic substrates, but their use in the catalytic hydrogenation of CO2 has been limited. Here, we demonstrate that the copper(I) complex LCu(MeCN)PF6 is an active catalyst for CO2 hydrogenation in the presence of a suitable base. Screening of bases and studies of catalytic reactions by in operando spectroscopy revealed important and unusual roles for the base in promoting H2 activation and turnover.

In this review, we consider the main processes for the asymmetric transfer hydrogenation of ketones from 2008 up today. The most effective organometallic compounds (derived from Ru, Rh, Ir, Fe, Os, Ni, Co, and Re) and chiral ligands (derived from amino alcohols, diamines, sulfur- and phosphorus-containing compounds, as well as heterocyclic systems) will be shown paying special attention to functionalized substrates, tandem reactions, processes under non-conventional conditions, supported catalysts, dynamic kinetic resolutions, the use of water as a green solvent, theoretical and experimental studies on reaction mechanisms, enzymatic processes, and finally applications to the total synthesis of biologically active organic molecules. We thank the continuous financial support from our Ministerio de Ciencia e Innovación (MICINN) (projects CTQ2007-62771/BQU, CTQ2010-20387, CONSOLIDER INGENIO 2010-CDS2007-00006, CTQ2011-24151, CTQ2011-24165), the Ministerio de Economía y Competitividad (MINECO) (projects CTQ2013-43446-P, CTQ2014-51912-REDC, CTQ2014-53695-P), FEDER, the Generalitat Valenciana (PROMETEO 2009/039, PROMETEOII/2014/017), and the University of Alicante.

A family of chiral (cyclopentadienone)iron complexes, featuring an (R)-BINOL-derived backbone, and their application in the asymmetric hydrogenation of ketones are described. The complexes differ from each other in the substituents at the 3,3-positions of the binaphthyl residue (H, OH, OR, OCOR, OSO2R) or at the 2,5-positions of the cyclopentadienone ring [trimethylsilyl (TMS) or Ph]. Remarkably, eight precatalysts with different 3,3-binaphthyl substitution [(R)-1c-1j] were synthesized from a common parent complex [(R)-1b] through direct functional group interconversion reactions of the complexes. The 3,3-(bis)methoxy-substituted precatalyst (R)-1b gave the best catalytic performance, and its application scope was assessed in the hydrogenation of several ketones. The observed ee values (up to 77%) are much higher than those previously reported for other chiral (cyclopentadienone)iron complexes.

Different methods for transforming N-heteroarenes into more reactive derivatives for catalytic asymmetric hydrogenation are highlighted. The first strategy consists of facilitating hydrogenation by the formation of positively charged derivatives of the heteroarene. Catalyst deactivation processes arising upon binding of the substrate to the metal center can thus be prevented and, additionally, hydrogenation of positively charged heteroarenes may also be more favored than that of their neutral analogues. The second strategy is based on introducing a ligating group onto the substrate to assist its coordination to the metal center and facilitate hydrogenation by chelation assistance. The last strategy involves breaking the aromaticity of the heteroarene by inducing a double-bond migration process. This microreview summarizes advances made in the above strategies, which have allowed the development of highly enantioselective catalytic hydrogenation of N-heteroarenes for the production of fully or partially saturated chiral heterocycles.

Pincer complexes are becoming increasingly important for organometallic chemistry and organic synthesis. Since numerous applications for such catalysts have been developed in recent decades, this Minireview covers progress in their use as catalysts for (de)hydrogenation and transfer (de)hydrogenation reactions during the last four years. Aside from noble-metal-based pincer complexes, the corresponding base metal complexes are also highlighted and their applications summarized.

Here we report the first example of a tandem metathesis-asymmetric hydrogenation protocol where the prochiral olefin generated by metathesis is hydrogenated with high enantioselectivity by an in situ formed chiral ruthenium catalyst. We show that either the ruthenium metathesis catalysts or the ruthenium species formed during the metathesis reaction can be converted into an efficient asymmetric hydrogenation catalyst upon addition of a chiral ligand and an alcohol. The performance in asymmetric hydrogenation appears to be very dependent on the solvent, the chiral ligand, and the prochiral substrate.

Cyclophosphazene was reacted with biphenyl-2,2’- diol to afford the N3P3Cl4(O2C12H8) (1). Then, pyridyloxy cyclophosphazene, spiro N3P3(O2C12H8)(O-C5H4N-3)4(2) was synthesized from the reaction of 3-hydroxy pyridine and compound 1. The reaction of 2 with [RuCl2(p-cymene)]2 afforded Ru(II) complex (3). The structures of the ligand and complex are characterized by elemental analysis, FT-IR, 1H- and 31P-NMR spectroscopy. Ru(II) complex has been employed as catalysts for the transfer hydrogenation of acetophenone in the presence of KOH using 2-propanol as a hydrogen source. The complex has demonstrated good catalytic activity in transfer hydrogenation of ketones.

A sulfonated chiral diamine ligand anion-based functionalized ionic liquid has been synthesized. Then the new ionic liquid-regulating ruthenium complexes were prepared successfully and employed as efficient catalysts for the asymmetric transfer hydrogenation of various ketones. The catalysts were highly efficient for the hydrogenation of a wide range of substrates bearing different functional groups and could be separated easily from the reaction mixture by thermoregulated phase separation, which can be efficiently recycled five times without significant changes in catalytic activity and enantioselectivity.

A trans-chelating chiral bisphosphine, PhTRAP, allows the ruthenium-catalyzed hydrogenation of various azoles to proceed with high enantioselectivity. The PhTRAP–ruthenium catalyst transformed indo...

In this article we highlight some of the recent developments in the aqueous-phase hydrogenation of alkenes and arenes. The traditional types of water-soluble catalysts applied in aqueous media are outlined together with recent trends towards the development of transition metal nanoparticle and nanocomposite catalysts. In addition, we describe some palladium nanoparticle catalysts that are stabilized by ionic polymers, which demonstrate superior activities in the aqueous-phase catalytic hydrogenation of styrene, used as a model substrate, compared to related palladium nanoparticles.

Carbohydrate ligands have been studied in transition-metal-catalyzed hydrogenations in recent decades. Herein, we report the diastereoselective synthesis of trans-dichlorido(bisphosphane)ruthenium(II) complexes with four different methyl 2,3-diamino-4,6-O-benzylidene-2,3-dideoxy-α-D-hexopyranosides and their application as catalyst precursors in asymmetric hydrogenation reactions. Depending on the hexopyranose, an enantiomeric excess of up to 78 % was obtained.

An asymmetric homogeneous hydrogenation of 2(1H)-pyridones has been developed, using a ruthenium complex bearing two chiral N-heterocyclic carbene (NHC) ligands. To the best of our knowledge, the presented reaction is the first example of a homogeneous asymmetric conversion of 2-pyridones into the corresponding enantioenriched 2-piperidones.

This is an Account of our development of iron-based catalysts for the asymmetric transfer hydrogenation (ATH) and asymmetric pressure hydrogenation (AH) of ketones and imines. These chemical processes provide enantiopure alcohols and amines for use in the pharmaceutical, agrochemical, fragrance, and other fine chemical industries. Fundamental principles of bifunctional reactivity obtained by studies of ruthenium catalysts by Noyori's group and our own with tetradentate ligands with tertiary phosphine and secondary amine donor groups were applied to improve the performance of these first iron(II) catalysts. In particular the correct positioning of a bifunctional H-Fe-NH unit in an iron hydride amine complex leads to exceptional catalyst activity because of the low energy barrier of dihydrogen transfer to the polar bond of the substrate. In addition the ligand structure with this NH group along with an asymmetric array of aryl groups orients the incoming substrate by hydrogen-bonding, and steric interactions provide the hydrogenated product in high enantioselectivity for several classes of substrates. Enantiomerically pure diamines or diphenylphosphino-amine compounds are used as the source of the asymmetry in the tetradentate ligands formed by the condensation of the amines with dialkyl- or diaryl-phosphinoaldehydes, a synthesis that is templated by Fe(II). The commercially available ortho-diphenylphosphinobenzaldehyde was used in the initial studies, but then diaryl-phosphinoacetaldehydes were found to produce much more effective ligands for iron(II). Once the mechanism of catalysis became clearer, the iron-templated synthesis of (S,S)-PAr2CH2CH2NHCHPhCHPhNH2 ligand precursors was developed to specifically introduce a secondary amine in the precatalyst structures. The reaction of a precatalyst with strong base yields a key iron-amido complex that reacts with isopropanol (in ATH) or dihydrogen (in AH) to generate an iron hydride with the Fe-H bond parallel to the secondary amine N-H. In the AH reactions, the correct acidity of the intermediate iron-dihydrogen complex and correct basicity of the amide are important factors for the heterolytic splitting of the dihydrogen to generate the H-Fe-N-H unit; the acidity of dihydrogen complexes including those found in hydrogenases can be estimated by a simple additive ligand acidity constant method. The placement of the hydridic-protonic Fe-H···HN interaction in the asymmetric catalyst structure influences the enantioinduction. The sense of enantioinduction is predictable from the structure of the H-Fe-N-H-containing catalyst interacting with the ketone in the same way as related H-Ru-N-H-containing catalysts. The modular construction of the catalysts permits large variations in order to produce alcohol or amine products with enantiomeric excess in the 90-100% range in several cases.

A new family of ruthenium complexes bearing the carbodicarbene-type ligand "cyclic bent allene" (CBA) have been synthesized from the common precursor RuHCl(CO)(PPh3)3. Complexes were evaluated for catalytic activity in the room-temperature hydrogenation of unactivated olefins and were found to be significantly more active than known ruthenium hydrido-carbonyl phosphine or NHC complexes. In particular, RuH(OSO2CF3)(CO)(SIMes)(CBA) was found to be among the most active hydrogenation catalysts, achieving comparable activity to Crabtree's catalyst in the hydrogenation of unactivated trisubstituted olefins and superior activity in the hydrogenation of styrene derivatives in side-by-side catalytic runs. RuH(OSO2CF3)(CO)(SIMes)(CBA) was also found to be highly active in olefin selective hydrogenation in the presence of a variety of unsaturated functional groups, and can achieve exceptional diastereoselectivity in functional-group-directed hydrogenations at very low catalyst loadings.

In the fine chemicals industry the preference for heterogeneous over homogeneous catalysts is well known. However, the activity and selectivity of heterogeneous metal complexes are usually better than those of their homogeneous counterparts. There has, consequently, been an increasing demand for heterogenized metal complexes which combine the advantages of both types of catalyst. In the production of fine chemicals, selectivity is probably its most important property of a catalyst. Chemoselectivity, regioselectivity, and enantioselectivity are very important in the synthesis of fine chemicals. Several examples of hydrogenation with heterogenized metal complexes have been studied, with emphasis on selectivity. Catalytic transfer hydrogenation is also discussed, as an alternative to classical hydrogenation.

Exhaustive hydrogenation of 2-butyne-1,4-diol to 1,4-butanediol on suspended palladium and Raney nickel catalysts under atmospheric pressure at 40°C was studied with the aim to determine the mechanism of 1-butanol formation. The previously unknown pathway of 1-butanol synthesis is realized under these conditions. The content of 1-butanol precursors in hydrogenation catalyzates was estimated by gas-liquid chromatography. The graphic dependence of the content of the intermediates and 1-butanol on time was found. The possibility of increasing the hydrogenation selectivity on Raney Ni catalysts with respect to the target product was revealed.

A ruthenium complex modified with (1S,2S)-1,2-diphenyl-1,2-ethanediamine efficiently catalyzes asymmetric hydrogenation of 3-chloro-1-phenylpropan-1-one in the presence of potassium hydroxide solution in isopropyl alcohol, specific optical rotation of the synthesized products reaching −24.6°. Effects of the reaction time, hydrogen pressure, and the alkali concentration on the reaction course have been elucidated.

Phosphoramidite ligands based on pinene-derived chiral amines have been prepared by a straightforward procedure in good yields. The key step of the synthetic protocol is a stereoselective hydrogenation of annulated pinene–pyridine derivatives leading to (diastereoisomeric) secondary amines that were separated and treated with different chlorophosphites to yield the envisaged phosphoramidites. The absolute configurations of the ligands were assigned on the basis of NMR analyses and corroborated by X-ray diffraction analysis of a borane adduct of a typical ligand. The new ligands were employed in the asymmetric hydrogenation of imines and olefins. The iridium-catalyzed hydrogenation of imines provided up to 81 % ee, whereas in the rhodium-catalyzed hydrogenation of functionalized olefins enantioselectivities of up to 99 % ee were achieved. In this particular application, the different chiral elements of the ligand structure led to synergistic effects and the enantioselectivity is dominated by the chiral diol moiety.

Neutral and cationic ruthenium(II) complexes bearing a symmetrical 2,6-bis(imidazo[1,2-a]pyridin-2-yl)pyridine were synthesized and structurally characterized by NMR analysis and X-ray crystallographic determinations. These complexes have exhibited good catalytic activity in the transfer hydrogenation of ketones. In refluxing isopropyl alcohol, the conversion of the substrates reached up to 99%, and a TOF value of 356 400 h–1 with 0.1 mol % catalyst was achieved.

This paper describes a novel approach to CO2 hydrogenation, in which CO2 capture with aminoethanols at low pressure is coupled with hydrogenation of the captured product, oxazolidinone, directly to MeOH. In particular, (2-methylamino)ethanol or valinol captures CO2 at 1–3 bar in the presence of catalytic Cs2CO3 to give the corresponding oxazolidinones in up to 65–70 and 90–95% yields, respectively. Efficient hydrogenation of oxazolidinones was achieved using PNN pincer Ru catalysts to give the corresponding aminoethanol (up to 95–100% yield) and MeOH (up to 78–92% yield). We also have shown that both CO2 capture and oxazolidinone hydrogenation can be performed in the same reaction mixture using a simple protocol that avoids intermediate isolation or purification steps. For example, CO2 can be captured by valinol at 1 bar with Cs2CO3 catalyst followed by 4-isopropyl-2-oxazolidinone hydrogenation in the presence of a bipy-based pincer Ru catalyst to produce MeOH in 50% yield after two steps.

The selective hydrogenation of aromatic and aliphatic nitriles into amines and imines is described. Using a ruthenium pincer complex, the selectivity towards amines or imines can be controlled by simple parameter changes. The reactions are conducted under very mild conditions between 50–100 °C at 0.4 MPa H2 pressure without any additives at low catalytic loadings of 0.5–1 mol %, which results in quantitative conversions and high selectivity.

Hydrogenation of carbazole and N-ethylcarbazole over Raney-Ni catalyst were realized in the temperature range of 393–503 K. 4[H] adduct dominated the hydrogenation products and the formation of 2[H] adduct was the rate-limiting step during the period, in which the conversion of carbazole was less than 40%. The hydrogenation process followed pseudo-first-order kinetics and the hydrogenation activation energies of carbazole and N-ethylcarbazole were 90 kJ/mol and 115 kJ/mol, respectively. The reaction starting position as well as the pathway of the hydrogenation of (N-ethyl)carbazole were investigated by comparing the kinetic characteristics of hydrogen uptake of carbazole and N-ethylcarbazole. The results showed that the reaction was a stepwise hydrogenation process and the first H 2 was added to the C1 = C10 double bond in the hydrogenation.

A new Rh(III) complex containing the TsDPEN ligand and an η6-arene connected through a carbon tether is reported. The asymmetric transfer hydrogenation of a series of ketones catalyzed by this complex using the formic acid/triethylamine system provided the corresponding alcohols with complete conversions and a high level of enantioselectivity.

An efficient and practical protocol for the flow hydrogenation of aromatic rings was developed. The hydrogenation of a variety of aromatic compounds, such as benzene, furan, and pyridine derivatives, could be completed within only 20 s during a single pass through a catalyst cartridge containing 10 % rhodium on carbon (Rh/C) or 10 % ruthenium on carbon (Ru/C) under neutral conditions. The protocol was successfully applied to a 10 mmol scale reaction. Furthermore, the 10 % Rh/C and 10 % Ru/C did not deteriorate during the entire study.

The first asymmetric hydrogenation (AH) of 2,6-disubstituted and 2,3,6-trisubstituted 1,5-naphthyridines, catalyzed by chiral cationic ruthenium diamine complexes, has been developed. A wide range of 1,5-naphthyridine derivatives were efficiently hydrogenated to give 1,2,3,4-tetrahydro-1,5-naphthyridines with up to 99 % ee and full conversions. This facile and green protocol is applicable to the scaled-up synthesis of optically pure 1,5-diaza-cis-decalins, which have been used as rigid chelating diamine ligands for asymmetric synthesis.

The catalytic cycle for the production of formic acid by CO2 hydrogenation and the reverse reaction have received renewed attention because they are viewed as offering a viable scheme for hydrogen storage and release. In this Forum Article, CO2 hydrogenation catalyzed by iridium complexes bearing sophisticated N^N-bidentate ligands is reported. We describe how a ligand containing hydroxy groups as proton-responsive substituents enhances the catalytic performance by an electronic effect of the oxyanions and a pendent-base effect through secondary coordination sphere interactions. In particular, [(Cp*IrCl)2(TH2BPM)]Cl2 (Cp* = pentamethylcyclopentadienyl; TH2BPM = 4,4',6,6'-tetrahydroxy-2,2'-bipyrimidine) enormously promotes the catalytic hydrogenation of CO2 in basic water by these synergistic effects under atmospheric pressure and at room temperature. Additionally, newly designed complexes with azole-type ligands were applied to CO2 hydrogenation. The catalytic efficiencies of the azole-type complexes were much higher than that of the unsubstituted bipyridine complex [Cp*Ir(bpy)(OH2)]SO4. Furthermore, the introduction of one or more hydroxy groups into ligands such as 2-pyrazolyl-6-hydroxypyridine, 2-pyrazolyl-4,6-dihydroxypyrimidine, and 4-pyrazolyl-2,6-dihydroxypyrimidine enhanced the catalytic activity. It is clear that the incorporation of additional electron-donating functionalities into proton-responsive azole-type ligands is effective for promoting further enhanced hydrogenation of CO2.

Catalytic reduction of aromatic ketones is an interesting reaction that leads to the obtaining of alcohols. Some of these alcohols are employed as intermediaries to produce chemical fine compounds. The bimetallic catalysts are widely used in the chemoselective hydrogenation. The Surface Organometallic Chemistry on Metals (SOMC/M) is a methodology employed to obtain bimetallic systems. In the present work the hydrogenation of ketones derived from acetophenone has been studied. The aim was observe the effect on activity and selectivity to aromatic alcohols by the presence of substituents in the ring. The chemical characteristics of substituents groups could influence the way aromatic ketones are absorbed on the metallic surface. This fact can be explained by a combination of geometric and electronic effects.

Ir nanocatalyst Ir@CN was prepared by pyrolysis of the IrCl3 complex with 1,10-phenanthroline in the activated carbon. The iridium nanocatalyst Ir@CN was highly selective for the transfer hydrogenation of aldehydes and ketones including benzaldehyde derivatives, and the hydrogenative alkylation of C=O bonds was suppressed effectively. The iridium nanocatalyst Ir@CN is a heterogeneous catalyst and can be reused several times for the transfer hydrogenation of aldehydes and ketones. Ir nanocatalyst Ir@CN was prepared and the Ir@CN was found to be highly selective for the transfer hydrogenation of ketones and aldehydes including benzaldehyde derivatives in water. The iridium nanocatalyst Ir@CN is a heterogeneous catalyst and can be reused several times.

Ruthenium (Ru) nanoparticles were prepared using polyoxyethylene–polyoxypropylene–polyoxyethylene triblock copolymer (P123) micelles in water as a stabilizing agent. The P123–Ru micellar catalyst was first used in the hydrogenation of α-pinene to pinane, and the selectivity for cis-pinane reached 98.9%. This result is attributed to the formation of vesicles. The isolated catalyst phase could be used seven times with no treatment, and its catalytic activity and selectivity were almost unchanged. The preparation process of the catalyst and hydrogenation reaction of α-pinene was under mild and environmentally friendly conditions. This research offers an effective method for the hydrogenation of α-pinene and provides a reference for other hydrophobic natural products in hydrogenation reactions.

A straightforward synthesis of a tetraarylphosphonium (TAP)-supported Noyori–Ikariya catalyst is described. The TAP-supported ruthenium precatalyst provided good conversions and high enantioselectivities for the asymmetric transfer hydrogenation of ketones in water. In addition the catalyst was easily recovered and used in multiple catalytic cycles.

Heterogeneous asymmetric hydrogenation of CO and CC bonds over chiral molecule-modified metal particles represents an important route for the production of chiral compounds. In this mini review, we first briefly introduced the background of heterogeneous asymmetric hydrogenation and the remaining challenges in this field. Then, we highlighted recent important progress in the understanding of the reaction mechanism in terms of the acid–base properties of supports and the effects of the size/shape of metal particles. Finally, we summarized the possible models proposed for the substrate–modifier adsorption and their interaction in asymmetric hydrogenation reactions.

The Noyori catalyst RuCl2(R-Binap)(dmf)n has been successfully encapsulated in C-FDU-12 by using the active chlorosilane Ph2Cl2Si as the silylating agent. 31P-NMR results show that there is no strong interaction between the molecular catalyst and the solid support, thus the encapsulated molecular catalyst could move freely in the nanoreactor during the catalytic process. The solid catalyst exhibits high activity and enantioselectivity for the asymmetric hydrogenation of a series of β-keto esters due to the preserved intrinsic properties of RuCl2(R-Binap)(dmf)n encapsulated in the nanoreactor. The solid catalyst could be recycled by simple filtration and be reused at least four times.

Development of the methodology for hydrogenation is of great significance in organic chemistry. Ionic hydrogenation is attractive for its chemoselectivity and unique feature of product structures which is a result of the combination of reduction, cyclization and intermolecular addition. It is mostly suitable for the reduction of carbonyl groups, unsaturated hydrocarbons, imines and reductive amination of carbonyl compounds. Recent advances in ionic hydrogenation as well as its fundamental mechanism are summarized and discussed.

A series of the supported nickel catalysts have been synthesized through impregnation, characterized by X-ray diffraction, N 2 sorption measurement, temperature programmed H 2 reduction and X-ray photoelectron spectroscopy. The morphology and particle size of catalysts are imaged by scanning electron microscope. The catalysts have efficiently catalyzed the one-step hydrogenation of 1,5-dinitronaphthalene to 1,5-diaminodecahydronaphthalene. The catalyst 10%Ni/SiO 2 -1a is the most active for the titled hydrogenation with 100 mol% conversion and 95.3% selectivity, applicable for the catalytic one-step hydrogenation of nitro-aromatic compounds. The reducibility of NiO species on different supports has been carefully studied. Some factors such as the type of carriers, the addition of modifiers, the silica source, the Ni loading, the H 2 pressure and the reaction temperature and time play important roles in controlling the hydrogenation. In addition, the negative effect of the water amount has been observed. The recycling tests reveal the recyclability and stability of the supported Ni catalyst.

The asymmetric hydrogenation of N-arylimines with the chiral phosphine-phosphoramidite ligand, (Sc,Sa)-PEAPhos 2b, has been developed. The results revealed that the presence of the substituents on the 3,3′-positions of the binaphthyl backbone significantly improved the enantioselectivity. The utility of this methodology was demonstrated in the synthesis of the chiral fungicide (R)-metalaxyl.

The reactions of [RuCl2(η6-C6H6)]2 with chiral aroylthiourea ligands yielded pseudo-octahedral half-sandwich “piano-stool” complexes. All the Ru(II) complexes were characterized by analytical and spectral (UV-visible, FT-IR, 1H NMR and 13C NMR) studies. The molecular structures of the ligands (L2 and L4) and the complexes (2, 4 and 5) were confirmed by single crystal XRD. All the complexes were successfully screened as catalysts for the asymmetric transfer hydrogenation (ATH) of ketones using 2-propanol as the hydrogen source in the presence of KOH. The ATH reactions proceeded with excellent yields (up to 99%) and very good enantioselectivity (up to 99% ee). The scope of the present catalytic system was extended to substituted aromatic ketones and few hetero-aromatic ketones. Density functional theory (DFT) calculations predicted non-classical, concerted transition states for the ATH reactions. The catalytic activity of Ru–benzene complexes toward asymmetric reduction of ketones was significantly higher compared to that of p-cymene complex analogues. Such enhanced efficiency and product selectivity of Ru–benzene complexes compared to those of Ru–p-cymene complexes were rationalized by the computational study.

A series of chiral conjugated microporous polymers (CMPs) based on the chiral (R)-BINAP ligand (BINAP-CMPs) were synthesized with tunable BET surface areas. These solid catalysts show high activities and enantioselectivities for the asymmetric hydrogenation of β-keto esters after coordination with ruthenium species. Moreover, CMPs can realize spatial isolation. Through preventing the formation of dimers and trimers, BINAP-CMPs show much higher activity than BINAP for the Ir-catalyzed asymmetric hydrogenation of quinaldine.

A new method was developed for in situ generation of active Fe catalysts for the hydrogenation of olefins from bench-stable Fe(II) complexes and easily accessible LiAlH4. This method makes the hydrogenation very easy to handle and enables the development of several new Fe catalysts for olefin hydrogenation through practical ligand evaluation. One of the Fe catalysts derived from a Fe complex of a phosphine-bipyridine ligand exhibited unprecedented activity for the hydrogenation of olefins, with turnover numbers up to 10 000 and turnover frequencies up to 37 740 h−1. The NMR studies of the active Fe catalyst showed that a Fe-hydride species stabilized by Al might be a real catalyst.

An efficient palladium-catalyzed asymmetric hydrogenation of fluorinated aromatic pyrazol-5-ols has been developed via capture of the active tautomers. A wide variety of 2,5-disubstituted and 2,4,5-trisubstituted pyrazolidinones have been synthesized with up to 96% and 95% ee, respectively. The hydrogenation pathway includes Bronsted acid promoted tautomerization of pyrazol-5-ols and Pd-catalyzed asymmetric hydrogenation of the active tautomer.

A diverse series of chiral bicyclic NHC/Ir complexes were prepared via a previously developed divergent synthesis of chiral imidazolium salts. Among the complexes, 8dz was found to be an excellent catalyst precursor for the Ir-catalyzed asymmetric transfer hydrogenation of ketones. The reaction of ketones with 8dz proceeded smoothly to give corresponding alcohols with high enantioselectivities (up to 98%) and productivities (TON up to 4500).

A hydrazine group-containing nitrogen-phosphine ligand and corresponding ruthenium complexes were synthesized. When these complexes were used for hydrogenation of esters, excellent performance was observed (TON up to 17, 200). A wide substrate scope was suitable for this catalytic system.

A mild catalyst system for the synthesis of chiral amino alcohols via asymmetric transfer hydrogenation (ATH) of α-phthalimide ketones has been developed by using a chiral Ru-TsDPEN complex as the catalyst in DMF/MeOH at 40 °C. The reaction exhibits high reaction activity and excellent enantioselectivity where up to 96% yield and 99% ee of the product were obtained.

Selective catalytic (10% Pd/C) hydrogenation of the double bond in the oxabicyclo[2.2.1]heptene fragment of substituted fused 1-oxo-3a,6-epoxyisoindoles is described. A continuous-flow hydrogenation device that incorporates in situ hydrogen generation by electrolysis of water was used. Changing the hydrogen source from water to deuterium oxide provides possibility to synthesize deuterated oxoepoxyisoindolones. Hydrogenation is stereoselective to give exclusively exo-cis deuterated derivatives.