A Modular Furanoside Thioether‐Phosphite/Phosphinite/ Phosphine Ligand Library for Asymmetric Iridium‐Catalyzed Hydrogenation of Minimally Functionalized Olefins: Scope and Limitations

A highly modular furanoside thioether‐phosphite/phosphinite/phosphine ligand library has been synthesized for the iridium‐catalyzed asymmetric hydrogenation of minimally functionalized olefins. These ligands can be prepared efficiently from easily accessible D ‐(+)‐xylose. We found that their effectiveness at transferring the chiral information in the product can be tuned by correctly choosing the ligand components. Enantioselectivities were therefore excellent (ees up to 99%) in a wide range of E‐ and Z‐trisubstituted alkenes using 5‐deoxyribofuranoside thioether‐phosphite ligands. It should be pointed out that these catalysts are also very tolerant to the presence of a neighbouring polar group. For 1,1‐disubstituted substrates, both enantiomers of the hydrogenation product can be obtained in high enantioselectivities simply by changing the configuration of the biaryl phosphite moiety. The asymmetric hydrogenation was also performed using propylene carbonate as solvent, which allowed the iridium catalysts to be reused while maintaining the excellent enantioselectivities.     

Evolutionary Catalyst Screening: Iridium‐Catalyzed Imine Hydrogenation

A high throughput catalyst screening is presented employing an evolutionary approach. The method comprises the optimization of initial leads by subjecting the catalysts to iterative rounds of optimization, including structural elaboration of the ligands by creating new focused libraries. Highly modular supramolecular ligands, robotized synthesis combined by high throughput experimentation creates a platform for fast catalyst development. An illustrative example for the asymmetric hydrogenation of cyclic 2,3,3‐trimethyl‐3H‐indole using iridium catalysts is presented. The kinetic investigation of the best catalyst yields an unusual second order in iridium, first order in hydrogen and zeroth order in substrate. Under optimized reaction conditions a TOF of 100 mol mol⁻¹ h⁻¹ with 96% ee could be obtained with the best catalyst. A full catalyst screening and kinetic study was conducted within a three‐week time‐frame.     

Asymmetric Homogeneous Hydrogenation in Flow using a Tube‐in‐Tube Reactor

In this update, the asymmetric homogeneous hydrogenation of a number of trisubstituted olefins utilizing the recently developed tube‐in‐tube gas‐liquid flow reactor is described. A number of chiral iridium‐ and rhodium‐based catalysts and other parameters such as pressure, solvent, temperature and catalyst loading were screened. The advantage of the flow set‐up for rapid screening and optimization of reaction parameters is illustrated. Furthermore, a comparative study using batch conditions aided in the optimization of the flow reaction set‐up. The set‐up was further modified to recycle the catalyst which prolonged catalytic activity.     

Recent Advances in Transition Metal‐Catalyzed Hydrogenation of Nitriles

Amines are important building blocks possessing various applications in agrochemicals, the fine chemical industry, pharmaceuticals, materials science and biotechnology. The catalytic hydrogenation of nitriles is an important reaction for the one‐step synthesis of diverse amines. However, significant amounts of side product formation during the course of the reaction is a major issue. In recent years, an enormous amount of work has been reported using both homogeneous and heterogeneous transition metal complex catalysts for the selective reduction of nitriles. Transition metal catalysts are the most crucial factor that controls the selectivity in this reaction. Therefore, transition metal catalysts are the central point of this review. We have also briefly discussed the effect of reaction parameters, selectivity to different substrate structures and reaction mechanisms. This review provides an overview of recent developments in transition metal‐catalyzed nitrile reduction along with examples which highlight its vast potential in organic transformations.

     

Study on liquid‐phase hydrogenation of paranitrotoluene over Ru‐based catalysts

This paper presented a study on the role of yttrium addition to Ru‐based catalysts for liquid phase paranitrotoluene hydrogenation reaction. An impregnation‐precipitation method was used for preparation of a series of yttrium doped Ru/NaY catalysts with yttrium content in the range of 0.0026–0.0052 g/g. Properties of the obtained samples were characterized and analyzed by X‐ray diffraction (XRD), H₂‐TPR, Transmission electron microscopy (TEM), ICP atomic emission spectroscopy, and Nitrogen adsorption‐desorption. The results revealed that catalytic activity of NaY supported Ru catalysts increased with the yttrium content at first, then decreased with the further increase of yttrium content. When yttrium content was 0.0033 g/g, a Ru‐Y/NaY² catalyst showed the most excellent performance of paranitrotoluene hydrogenation reaction (paranitrotoluene conversion and the selectivity toward P‐methyl‐cyclohexylamine reached 99.9 % and 82.5 %, respectively). In addition, to compare with the performance of Ru‐Y/NaY catalysts, the active carbon supported Ru catalysts were prepared using the same method in view of its higher surface area and adsorption capacity. Finally, the effect of solvent on the reaction over Ru‐Y/NaY² catalyst has been investigated, it was found that the best performance of paranitrotoluene hydrogenation reaction took place in protic solvents (isopropanol and ethanol). This was mainly ascribed to their polarity and hydrogen‐bond accepting capability.

     

Competitive Hydrogenation of Alkyl‐Substituted Arenes by Transition‐Metal Nanoparticles: Correlation with the Alkyl‐Steric Effect

The influence of substituents on rate constants of the hydrogenation of monoalkylbenzenes by transition metal nanoparticles or by classical heterogeneous catalysts can be rationalized in terms of the Taft rule. A series of the initial reaction rate constants obtained from various competitive toluene/benzene and toluene/monoalkylbenzene hydrogenation experiments catalyzed by transition‐metal nanoparticles prepared in the presence of imidazolium ionic liquids or surfactants [Ir(0), Rh(0) and Ru(0)] or by classical heterogeneous catalysts (PtO₂, Rh/C, Rh/Al₂O₃, Ru/C, Ru/Al₂O₃ and Pd/C) have been correlated with the Taft equation . Satisfactory correlation coefficients (r) (between 0.96 and 0.99) and positive slopes (ρ) between 0.38 and 0.83 have been obtained. The results clearly show that the reaction constants for the alkyl‐substituents can be expressed by steric factors and are independent of any other non‐steric factors. It is suggested that bulky alkylbenzene substituents, for both transition metal nanoparticles and classical heterogeneous hydrogenation reactions, lower the overall hydrogenation rate, implying a more disturbed transition state compared to the initial state of the hydrogenation (in terms of the Horiuti–Polanyi mechanism). This competitive method is suitable for the estimation of the constant selectivity for couples of alkylbenzenes in which the difference in hydrogenation rates are very high and experimentally difficult to measure and also useful for the design of more selective “nano” and classical catalysts for hydrogenation reactions.     

Liquid‐Phase Catalytic Hydrogenation of p‐Chlorobenzophenone to p‐Chlorobenzhydrol over a 5 % Pd/C Catalyst

The kinetics of the liquid‐phase catalytic hydrogenation of p‐chlorobenzophenone have been investigated over a 5 % Pd/C catalyst. The effects of hydrogen partial pressure (800–2200 kPa), catalyst loading (0.4–1.6 gm dm^–3), p‐chlorobenzophenone concentration (0.37–1.5 mol dm^–3), and temperature (303–313 K) were studied. A stirring speed > 20 rps has no effect on the initial rate of reaction. Effects of various catalysts (Pd/C, Pd/BaSO₄, Pd/CaCO₃, Pt/C, Raney nickel) and solvents (2‐propanol, methanol, dimethylformamide, toluene, xylene, hexane) on the hydrogenation of p‐chlorobenzophenone were also investigated. The reaction was found to be first order with respect to hydrogen partial pressure and catalyst loading, and zero order with respect to p‐chlorobenzophenone concentration. Several Langmuir‐Hinshelwood type models were considered and the experimental data fitted to a model involving reaction between adsorbed p‐chlorobenzophenone and hydrogen in the liquid phase.     

Hydrogenation of m‐dinitrobenzene to m‐phenylenediamine over La2O3‐promoted Ni/SiO2 catalysts

BACKGROUND: Liquid‐phase catalytic hydrogenation of m‐dinitrobenzene is an environmentally friendly routine for m‐phenylenediamine production. The key to increasing product yield is to develop catalysts with high catalytic performance. In this work, La₂O₃‐modified Ni/SiO₂ catalysts were prepared and applied to the hydrogenation of m‐dinitrobenzene to m‐phenylenediamine. The effect of La₂O₃ loading on the properties of Ni/SiO₂ was investigated. The reaction kinetic study was performed in ethanol over Ni/3%La₂O₃–SiO₂ catalyst, in order to clarify the reaction mechanism of m‐dinitrobenzene hydrogenation. RESULTS: It was found that the activity of the silica supported nickel catalysts is obviously influenced by La₂O₃ loading. Ni/3%La₂O₃–SiO₂ catalyst exhibits high activity owing to its well dispersed nickel species, with conversion of m‐dinitrobenzene and yield of m‐phenylenediamine up to 97.1% and 94%, respectively. The results also show that Ni/3%La₂O₃–SiO₂ catalyst can be reused at least six times without significant loss of activity. CONCLUSION: La₂O₃ shows strong promotion of the effect of Ni/SiO₂ catalyst for liquid‐phase hydrogenation of m‐dinitrobenzene. La₂O₃ loading can affect the properties of Ni/SiO₂ catalyst. Based on the study of m‐dinitrobenzene hydrogenation kinetics over Ni/3%La₂O₃–SiO₂ catalyst, a possible reaction mechanism is proposed. Copyright © 2009 Society of Chemical Industry     

Kinetics of hydrogenation of 4‐chloro‐2‐nitrophenol catalyzed by Pt/carbon catalyst

Hydrogenation of 4‐chloro‐2‐nitrophenol (CNP) was carried out at moderate hydrogen pressures, 7–28 atm, and temperatures in the range 298–313 K using Pt/carbon and Pd/γ‐Al₂O₃ as catalysts in a stirred pressure reactor. Hydrogenation of CNP under the above conditions gave 4‐chloro‐2‐aminophenol (CAP). Dechlorination to form 2‐aminophenol and 2‐nitrophenol is observed when hydrogenation of CNP is carried out above 338 K, particularly with Pd/γ‐Al₂O₃ catalyst. Among the catalysts tested, 1%Pt/C was found to be an effective catalyst for the hydrogenation of CNP to form CAP, exclusively. To confirm the absence of gas–liquid mass transfer effects on the reaction, the effect of stirring speed (200–1000 rpm) and catalyst loading (0.02–0.16 g) on the initial reaction rate at maximum temperature 310 K and substrate concentration (0.25 mole) were thoroughly studied. The kinetics of hydrogenation of CNP carried out using 1%Pt/C indicated that the initial rates of hydrogenation had first order dependence with respect to substrate, catalyst and hydrogen pressure in the range of concentrations varied. From the Arrhenius plot of ln rate vs 1000/T, an apparent activation energy of 22 kJ mol^?1 was estimated. © 2001 Society of Chemical Industry     

Viscosity Reduction of Heavy Oil during Slurry‐Phase Hydrocracking

The saturates, aromatics, and resins fractions of maltenes from upgraded oils obtained by slurry‐phase hydrocracking (SPH) under low‐severity reaction conditions using analytical‐grade and mineral catalysts were obtained by chromatographic separation. The reactions in the SPH at low severity with the catalysts used occur by free radicals, and their subsequent hydrogenation is from heavier fractions (asphaltenes and resins) to lighter fractions (aromatics, saturates, and light cuts). The degree of conversion depends on the type of catalyst used (Mo > Fe) and is also proportional to the active metal content of the catalyst. The enhanced conversion of aromatics and resins towards the saturates fraction depends on the hydrogenation capacity of the catalyst. The better flow properties of the maltenes are due to the conversion of heavier to lighter fractions and to the upgrading of the resins fraction properties.     

Highly Enantioselective Hydrogenation of Quinoline and Pyridine Derivatives with Iridium‐(P‐Phos) Catalyst

The use of a chiral iridium catalyst generated in situ from the (cyclooctadiene)iridium chloride dimer, [Ir(COD)Cl]₂, the P‐Phos ligand [4,4′‐bis(diphenylphosphino)‐2,2′,6,6′‐tetramethoxy‐3,3′‐bipyridine] and iodine (I₂) for the asymmetric hydrogenation of 2,6‐substituted quinolines and trisubstituted pyridines [2‐substituted 7,8‐dihydroquinolin‐5(6H)‐one derivatives] is reported. The catalyst worked efficiently to hydrogenate a series of quinoline derivatives to provide chiral 1,2,3,4‐tetrahydroquinolines in high yields and up to 96% ee. The hydrogenation was carried out at high S/C (substrate to catalyst) ratios of 2000–50000, reaching up to 4000 h⁻¹ TOF (turnover frequency) and up to 43000 TON (turnover number). The catalytic activity is found to be additive‐controlled. At low catalyst loadings, decreasing the amount of additive I₂ was necessary to maintain the good conversion. The same catalyst system could also enantioselectively hydrogenate trisubstituted pyridines, affording the chiral hexahydroquinolinone derivatives in nearly quantitative yields and up to 99% ee. Interestingly, increasing the amount of I₂ favored high reactivity and enantioselectivity in this case. The high efficacy and enantioselectivity enable the present catalyst system of high practical potential.     

Ir/C催化剂氯代硝基苯选择性催化加氢反应性能

采用浸渍法制备Ir/C和Pd/C催化剂,并用于氯代硝基苯选择性催化加氢反应,采用ICP-MS、XRD、XPS和TEM等手段对催化剂进行表征。结果表明,在相同负载量条件下,Ir在活性炭表面的分散与Pd有显著差异,Ir颗粒粒径更小且更均匀。在氯代硝基苯液相催化加氢反应中,Ir催化剂的催化活性较低,对氢解脱氯副反应有较好的抑制作用,这些性能与Ir的特殊性能有关。     

Some catalytic properties of silica‐supported base and metal porphyrins for hydrocarbon cracking and hydrogenation

A base porphyrin, etioporphyrin (EPI), has been synthesised and a number of metal–etioporphyrin compounds have been derived from EPI by metal insertion, these being nickel, vanadyl, palladium and platinum. The metal–etioporphyrins were supported on silica gel with loadings of 0.5–5.0% (w/w) to be employed as catalysts for hydrocarbon cracking and to a minor extent for hydrogenation. The porphyrins themselves were characterised using temperature programmed decomposition (TPD), temperature programmed reduction (TPR), mass spectroscopy (MS) and infra‐red (IR) spectroscopy. TPD studies up to 550 °C indicated complete stability and TPR studies (20–500 °C) showed interaction with hydrogen, nickel–EPI and Pd–EPI especially showing strong interaction. MS studies showed that metal insertion had occurred for VO–EPI and Ni–EPI and Pd insertion was demonstrated to have occurred using an analytical method. IR spectroscopy carried out on VO–EPI and Ni–EPI showed an absence of ? NH linkages, again confirming metal insertion. The behaviour of the catalysts for hydrocarbon cracking was studied using 2,2‐dimethylbutane (2,2‐DMB) as the model reactant in the temperature range 440–550 °C and thermally in the temperature range 440–600 °C and at 1 at, m (101.3 kPa) pressure. All porphyrins, even the base porphyrin, exhibited cracking activity and the catalysed reaction had an energy of activation, depending on the porphyrin, in the range 78–113 kJ/mol^?1, compared with a value of 210 kJ mol^?1 for the thermal reaction. The product distribution was dominated by C₁ and C₂ hydrocarbons and is typical of a free radical reaction, the thermal reaction giving a similar product distribution, so that the porphyrin catalyst acts as a free radical initiator. Hydrogenation studies using hex‐1‐ene at 150 °C and at 1 atm. pressure showed that Pd–EPI/SiO₂ was an active and possibly stable hydrogenation catalyst, whereas Ni–EPI/SiO₂ while of only slightly lower activity initially, lost that activity so that the Pd–EPI catalyst was over 16 times more active at the end of a 2 h period. © 2001 Society of Chemical Industry     

Comparative study of aromatization selectivity during N‐heptane reforming on sintered Pt/Al2O3 and Pt‐Re/Al2O3 catalysts

BACKGROUND: The metal dispersed over a support can be present as small crystallites with sizes less than 5 nm. The smaller crystallites favour aromatization while larger crystallites favour cracking/hydrogenolysis. Sintering results in the agglomerization of smaller metal crystallites. Correlation of size with aromatization selectivity was investigated. RESULTS: The primary products of n‐heptane reforming on fresh Pt were methane, toluene, and benzene, while on fresh Pt‐Re, the only product was methane. Both catalysts exhibited enhanced aromatization selectivity at different oxygen sintering temperatures. The reaction products ranged from only toluene at 500 °C sintering temperature to methane at a sintering temperature of 650 °C with no reaction at 800 °C for the Pt/Al₂O₃ catalyst. On Pt‐Re/Al₂O₃ catalyst, methane was the sole product at a sintering temperature of 500 °C while only toluene was produced at a sintering temperature of 800 °C. CONCLUSION: This is the first time that sintering has been used to facilitate aromatization of supported Pt and Pt‐Re catalysts. A superior selectivity behaviour associated with bi‐metallic Pt catalysts is established. It was found that no reaction occurred on Pt catalyst after sintering at 800 °C whereas sintering Pt‐Re at 800 °C promoted aromatization solely to toluene. Copyright © 2008 Society of Chemical Industry     

Homogeneous Hydrogenation of Tri‐ and Tetrasubstituted Olefins: Comparison of Iridium‐Phospinooxazoline [Ir‐PHOX] Complexes and Crabtree Catalysts with Hexafluorophosphate (PF6) and Tetrakis[3,5‐bis(trifluoromethyl)phenyl]borate (BArF) as Counterions

Four iridium complexes with achiral phosphino‐oxazoline (PHOX) ligands were readily prepared in two steps starting from commercially available phenyloxazolines. The air‐stable complexes with tetrakis[3,5‐bis(trifluoromethyl)phenyl]borate (BAr_F) as counterion showed high reactivity in the hydrogenation of a range of tri‐ and tetrasubstituted olefins. The best results were obtained with an iridium complex ( 11 ) derived from a dicyclohexylphosphino‐oxazoline ligand bearing no additional substituents in the oxazoline ring. With several substrates, which gave only low conversion with the Crabtree catalyst, [Ir(Py)(PCy₃)(COD)]PF₆, full conversion was observed. The productivity of the Crabtree catalyst could be strongly increased by replacing the hexafluorophosphate anion with tetrakis[3,5‐bis(trifluoromethyl)phenyl]borate. In one case, in the hydrogenation of a tetraalkyl‐substituted CC bond, [Ir(Py)(PCy₃)(COD)]BAr_F gave higher conversion than catalyst 11 . However, with several other substrates complex 11 proved to be superior.     

Homogeneous catalytic hydrogenation of hydroxyl‐terminated liquid nitrile rubber

It was difficult to obtain high degree of hydrogenation of hydroxyl‐terminated liquid nitrile rubber (HTBN) by using homogeneous noble metal catalyst because the hydroxyl (? OH) in HTBN was likely to cause catalyst poisoning. In this study, with hexamethyl disilylamine protecting ? OH, a good yields of hydrogenated HTBN was synthesized through the use of homogeneous metal catalyst. The effects of catalyst concentration, reaction time, hydrogen pressure, and temperature on the hydrogenation of HTBN were investigated and obtained the following optimum process parameter values: catalyst mass fraction of 0.8%, reaction time of 8 h, pressure of 1.6 MPa, and temperature of 100°C. Fourier transform infrared spectroscopy and nuclear magnetic resonance spectroscopy were used to characterize the hydrogenation product of the protected HTBN, indicating that under certain conditions a high degree of hydrogenation of HTBN can be achieved. Only the carbon–carbon double bonds (C?C), not the ? CN bonds, are subject to hydrogenation. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Structured Raney Nickel Catalysts for Liquid‐Phase Hydrogenation

Structured catalysts consisting of metal sheets on which Raney nickel was deposited by the thermal spraying method were tested for the liquid‐phase hydrogenation of glucose to sorbitol and 2‐nitrotoluene to 2‐methylaniline, used as model reactions. Catalytic tests performed in a bench‐scale (1 L) reactor showed that the catalytic activity of Raney Ni sheets is significantly higher than the one of the pellets used for fixed‐bed applications, but lower than the activity of the powder catalyst used in slurry mode. The activity could be significantly improved when applying a two‐phase co‐current flow through a monolith. In this case, the activity was superior to the one obtained with the slurry catalyst. These results confirm the potential of Raney Ni monoliths as structured catalysts.     

Efficient Heterogeneous Dual Catalyst Systems for Alkane Metathesis

A fully heterogeneous and highly efficient dual catalyst system for alkane metathesis (AM) has been developed. The system is comprised of an alumina‐supported iridium pincer catalyst for alkane dehydrogenation/olefin hydrogenation and a second heterogeneous olefin metathesis catalyst. The iridium catalysts bear basic functional groups on the aromatic backbone of the pincer ligand and are strongly adsorbed on Lewis acid sites on alumina. The heterogeneous systems exhibit higher lifetimes and productivities relative to the corresponding homogeneous systems as catalyst/catalyst interactions and bimolecular decomposition reactions are inhibited. Additionally, using a “two‐pot” device, the supported Ir catalysts and metathesis catalysts can be physically separated and run at different temperatures. This system with isolated catalysts shows very high turnover numbers and is selective for the formation of high molecular weight alkanes.     

Enantioselective Hydrogenation of N‐Acetyldehydroamino Acids over Supported Palladium Catalysts

The enantioselective hydrogenation of two N‐acetyldehydroamino acids over Cinchona alkaloid‐modified, supported palladium catalysts has been studied. Moderate enantioselectivities, up to 36 %, were obtained in the hydrogenation of 2‐acetamidocinnamic acid over cinchonidine‐modified Pd/TiO₂ under low hydrogen pressure. Increase in the pressure or use of benzylamine as additive led to a gradual decrease in the enantiomeric excess and eventually inversion of the sense of the enantioselectivity. On the contrary, the optical purity of the product resulting from the hydrogenation of 2‐acetamidoacrylic acid was significantly increased by addition of benzylamine to the reaction mixture. Enantiomeric excess values up to 58 % and 60 % were obtained over Pd/Al₂O₃ modified by cinchonidine and cinchonine, respectively. These optical purities are the best obtained in the hydrogenation of dehydroamino acid derivatives over chirally modified heterogeneous metal catalysts.     

Continuous Chirality Measure in Reaction Pathways of Ruthenium‐Catalyzed Transfer Hydrogenation of Ketones

Here we report theoretical studies on the ruthenium‐catalyzed reduction of acetophenone (and 2‐hexanone) with the intent of understanding the relative roles of catalyst and substrate along the reaction path. Overall ten reaction pathways are examined. The first eight are for acetophenone: they arise from the presence of two catalysts, with the more enantioselective one labeled 1 , and the poorer one labeled 2 , multiplied by the two configurations that the metal center of the catalysts can assume, multiplied by the two approaches, Re‐ and Si‐side, of the substrate to the catalyst. Two pathways are examined for 2‐hexanone and entail the two approaches to the ketone of the more effective catalyst. Density functional theory calculations provide structures of the minima and transition states, which subsequently have been assessed with the “continuous chirality measure” model developed by Avnir and collaborators. The picture that emerges is that the asymmetric induction is due to the interplay between the organometallic system and the organic substrate. This is effective only for catalyst 1 , which can interact effectively with acetophenone along only one in four of the reaction pathways, but not for 2 for which two out of four pathways are opened. For the hydrogenation of 2‐hexanone, the same catalyst 1 cannot produce enantiomeric excesses because the conformation of the substrate in the transition state induced by the catalyst has a relative low chirality.