Heterogeneous hydrogenation of the aromatic ketones using Ru(II) catalyst
1. Introduction
Optically active alcohols are important building blocks in the synthesis of fine chemicals, pharmaceuticals, agrochemicals, flavors and fragrances as well as functional materials (Arai & Ohkuma, 2011; Klingler, 2007). Furthermore, molecular hydrogen is without doubt the cleanest reducing agent, with complete atom efficiency. Therefore, the catalytic, asymmetric hydrogenation (AH) of prochiral ketones is the most practical and simplest method to access enantiomerically enriched secondary alcohols, on both the laboratory and industrial scales. Asymmetric transfer hydrogenation (ATH), on the other hand, represents an attractive alternative or complement to hydrogenation because it is easy to execute and a number of cheap chemicals can be used as hydrogen donors. For practical use and to address environmental issues a high catalyst activity (low loadings) and selectivity is preferable, as well as the employment of ‘’greener’’ solvents, mild operating conditions and recyclable catalyst systems. High turnover numbers (TONs) and turnover frequencies (TOFs), and satisfactory stereo- and chemoselectivities are attainable only with a combination of well-defined metal catalysts and suitable reaction conditions. The reactivity and selectivity can be finely tuned by changing the bulkiness, chirality and electronic properties of the auxiliaries on the metal center of the catalyst.
2. Homogenous, asymmetric hydrogenation and transfer hydrogenation
Since the application of very efficient, chiral BINAP-derived ruthenium complexes in the AH of functionalized ketones (β-keto esters) at a high enantioselectivity level in the homogenous phase (Noyori et al., 1987), the development of more robust and reactive molecular catalysts is still highly desirable. Furthermore, because of the structural and functional diversity of organic substrates, no universal catalysts exist. Ruthenium complexes bearing chiral ligands are among the most commonly used catalysts for AH and ATH, following by rhodium and iridium, although in recent times other transition metals, like Fe, Cu, or Os have rapidly penetrated this field.
2.1. Ru-, Rh- and Ir-catalyzed hydrogenation and transfer hydrogenation
A major breakthrough in the wide-scope AHs of ketones was the discovery by Noyori and co-workers of the conceptually new and extremely efficient ruthenium bifunctional catalysts. They found that simple ketones like 1-5, which lack anchoring heteroatoms capable of interacting with a metal center, can be reduced enatioselectively with H2 (1-8 atm) in
The XylBINAP-complex C2 proved to be very effective for the stereoselective hydrogenation of heteroaromatic ketones (2-furyl, 2- and 3-thienyl, 2-thiazolyl, 2-pyrrolyl, 2-, 3- and 4-pyridinyl) as well as aromatic-heteroaromatic and bis-heteroaromatic ketones (phenyl-thiazolyl, phenyl-imidazolyl, phenyl-oxazolyl, phenyl-pyridinyl, pyridinyl-thiazolyl) thus providing a plethora of structurally interesting heterocyclic alcohols (C. Chen et al., 2003; Ohkuma et al., 2000). In fact, the complex C2 has been established as one of the most efficient and selective pre-catalysts for the AH of a variety of ketones (Ohkuma et al., 1998) until the discovery of novel ruthenabicyclic complexes (Matsumura et al., 2011). The hydrogenation of acetophenone catalyzed by the ruthenabicyclic complex C3 with a substrate-to-catalyst molar ratio (S/C) 10000 under 50 atm of H2 in a
Since the Noyori’s standard Ru(II) complexes of the type C1 require the presence of a strong base as a co-catalyst to
The extremely high reactivity and enantio-selectivity of [TunesPhos-Ru(II)-(1,2-diamine)] complexes combined with
The discovery of new classes of hydrogenation catalysts that deviate from the Noyori-type C1 may represent a good opportunity to reduce every type of ketone substrate with high reactivity and selectivity. Indeed, while the conventional [BINAP-Ru-(1,2-diamine)] catalysts have shown poor reactivity and enantio-selectivity in the hydrogenation of sterically congested
Interestingly, a combined amine-benzimidazole ligand in the complex C7 influenced the reverse enantioselection from that typically observed in the AH of ring-substituted acetophenones and allowed the reduction to proceed in nonprotic solvents (toluene/
AH using non-phosphine-based catalysts is attractive due to the toxicity of the catalyst precursors and the product contamination when Noyori-type catalysts are used. However, the efficiency of the π-allyl Ru precursor in combination with the phosphorous-free pyridyl-containing ligand L1 did not exceed that of the original [BINAP-Ru-diamine] complexes (Fig. 4) (Huang et al., 2006). Interestingly, this new catalyst system catalyzes the hydrogenation of 1-indanone only in the absence of a base.
The most efficient AH catalysts tend to mimic that of Noyori as its excellent enantioselectivity is proposed to be a result of the synergistic effect of chiral phosphane and chiral amine ligands. Nevertheless, commercially available achiral diphosphanes (DPPF, DPEphos) in conjunction with rigid chiral biisoindoline-based diamines have been applied in the Ru-catalyzed AH of (hetero)aromatic ketones, affording excellent enantioselectivities (up to 99%
Since ketones coordinate more weakly to metals than olefins, many Rh-phosphane complexes show no activity for hydrogenation of simple ketones. However, the highly enantioselective direct hydrogenation of simple ketones 19−24 using an
The complex prepared from [Rh(COD)OCOCF3)]2 and the amide-phosphine-phosphinite ligand L3 catalyzed the AH of trifluoromethyl ketones 25 giving almost quantitative yields of the corresponding alcohols in 83-97%
The hydrogenation of ketones catalyzed by chiral iridium complexes has been well studied and developed because iridium is less expensive than rhodium (Malacea et al., 2010). Generally, Ir(I) or Ir(III) complexes with chiral diamines, diphosphines or a combination of both, very similar to those in Ru-catalyzed hydrogenation, have been successfully employed in the AH of various aromatic ketones and β-keto esters. On the other hand, chiral Ir(I) complexes bearing N-heterocyclic carbenes as ligands proved to be far less efficient (Diez & Nagel, 2009). Although complexes of [Ir(COD)Cl]2 and planar-chiral ferrocenyl phosphine-thioethers (
With its origin in Meerwein-Pondorf-Verley reduction, and later developed in its asymmetric version, the transfer hydrogenation of ketones has emerged as an operationally simpler and significantly safer alternative to catalytic H2-hydrogenation as there is no need for special vessels and high pressures (Ikariya & Blacker, 2007; Palmer & Wills 1999). Moreover, chemo-, regio- and stereoselectivity can often be different from that of AH. In the ATH process, the transition-metal catalyst is able to abstract a hydride and a proton from the hydrogen donor and deliver them to the carbonyl moiety of the ketone. Suitable catalysts for ATH are typically complexes of homochiral ligands with Ru, Rh or Ir, whilst
In parallel with the discovery of efficient ruthenium catalysts for AH, Noyori and co-workers found a prototype of chiral (arene)Ru(II) catalysts of type C8 bearing
The stereochemically rigid β-amino alcohols L7 or L8 work very well as ligands for Ru-catalyzed ATH in basic
An
It was first disclosed by Noyori, that a N-H moiety is necessary for an efficient transfer of hydrogen from the metal hydride. However, the Ru complex with the oxazolyl-pyridyl-benzimidazole-based NNN ligand L10 featuring no N-H functionality exhibited a high catalytic activity in the ATH of different acetophenones (Fig. 7) (Ye et al., 2011).
Another type of ligands lacking a basic NH group like L11 are based on a combination of
The combination of [RuCl2(
There is a continuing search for stable catalysts that would not degrade easily during the hydrogenation process, thus making it possible to execute as many as possible catalytic cycles. In this respect, the covalent linkage from the diamine to the
It has been shown that the Rh complex with the ‘’achiral’’ but tropos benzophenone-derived ligand L14 and a chiral diamine activator (
The unique phenomenon of an enhancement of the enantioselectivity by using the chiral bulky alcohol (
2.2. Hydrogenation and transfer hydrogenation employing other transition metals
Although Ru(II) complexes have enzyme-like properties reaching high TONs and TOFs, many times near to room temperature, and deliver the secondary alcohols in near-quantitative
The first hydrogenation of ketones catalyzed by a well-defined iron catalyst was effected with an iron hydride Shvo-type complex C14 (Casey & Guan, 2007), while later on Morris and co-workers succeeded in the ATH of simple ketones catalyzed by iron complexes containing chiral PNNP tetradentate ligands, attaining
An asymmetric Shvo-type iron complex C17 was found to be a very poor catalyst for the transfer hydrogenation of acetophenone with FA/TEA, since after 48 hours only a 40% conversion and a 25%
Enantioselective, copper-catalyzed homogenous H2-hydrogenation was introduced by Shimizu and co-workers, who used a catalyst system based on [Cu(NO3){P(3,5-Xylyl)3}2], (
Owing to the stronger bonding of Os compared to Ru, robust and thermally stable complexes can be obtained, which is important for achieving highly productive catalysts. Os(II) CNN pincer complexes C18 exhibited a high catalytic activity and productivity in both the AH (5 atm H2/
2.3. Hydrogenation and transfer hydrogenation in water and ionic liquids
As a consequence of the increasing demand for ‘’greener’’ laboratory and industrial applications, the development of water-operating catalytic systems for the asymmetric hydrogenation of ketones has been of great interest (Wu & Xiao, 2007). The main disadvantage, however, is the low solubility of the homogenous metal catalysts and most of the organic substrates when going from organic to aqueous media, which may be reflected in a reduced activity and selectivity. To circumvent this, either hydrophilic, often charged, functionalities can be introduced to ligands to render the catalysts water-soluble, or different surfactants can be added in order to solvate the reaction partners, although in some cases water-insoluble catalysts can deliver a superior activity and selectivity.
Water-soluble Ru, Ir or Rh catalysts were prepared
Surfactants are often added as co-solvents to obtain a sufficient solubility of the reactants, products and metal catalysts, thus retaining the activity and selectivity of the hydrogenation process. The ATH of ketones, particularly -bromomethyl aromatic ketones, was successfully performed with HCO2Na by employing the unmodified and hydrophobic Ru-, Rh- and Ir-TsDPEN complexes C22 and C23 in the presence of single-tailed, cationic and anionic surfactants and to form micelles and vesicles (Fig. 11) (Wang et al., 2005). It is notable that catalysts embedded in these micro-reactors can be separated from the organic phase and reused for at least six times without any loss of activity and enantioselectivity.
In recent years ionic liquids (ILs) have attracted an increasing interest because of their non-volatility, non-flammability and low toxicity. Additionally, ILs are capable of immobilizing homogenous catalysts and facilitating the recycling of catalysts. Ideally, organic products can be easily separated by extraction with a less polar solvent and the IL phase containing catalyst can be reused. Such an immobilization of catalysts also promises to prevent the leaching of toxic metals into the organic products, which is especially desirable in the production of pharmaceutical intermediates.
Various aromatic ketones were reduced with FA/TEA in an ionic liquid L25 at 40 °C, catalyzed by an
While for the AH of β-alkyl β-ketoesters high enantioselectivities can be attained by using the Ru-BINAP system, for the analogous β-aryl ketoesters much more inferior
2.4. Mechanistic considerations
Homogenous hydrogenation and transfer hydrogenation may be mechanistically closely related because both reactions involve a metal hydride species under catalytic conditions, thus sharing a multistep pathway of hydride transfer to the ketone,
Noyori and co-workers proposed metal-ligand bifunctional catalysis for their Ru catalysts containing chiral phosphine-amine ligands and for (arene)Ru-diamine catalysts, which consequently resulted in a widely accepted mechanism to be responsible for the highly enantio-selective hydrogenation and transfer hydrogenation of prochiral ketones (Noyori et al., 2001, 2005). The actual catalysts, Ru-hydrides 31 or 34, are usually created in a basic alcoholic solution (under H2 or not) at the beginning of the catalytic reaction from the Ru precursors 30 or 33. Note that only the
Depending on transition-metal catalysts, an ionic mechanism has also been proposed where the proton and hydride transfer occur in separate steps (Bullock, 2004).
The active species in catalytic cycles, Ru-hydride (31 or 34) and Ru-amido complexes (32 or 35), have not only been detected but also isolated in some cases (Abdur-Rashid et al., 2001, 2002; Haack et al 1997).
The absolute configuration of the alcohol product in AH is determined in the six-membered transition state resulting from the reaction of a chiral diphosphine-diamine-RuH2 complex with a prochiral ketone (Noyori et al., 2005). Because the enantiofaces of the ketone are differentiated on the molecular surface of the saturated RuH2 complex, a suitable combination of the catalyst and substrate is necessary for high efficiency. The prochiral ketone (
The stereoselectivity in the hydrogenation of prochiral aryl ketones catalyzed by (arene)Ru(II) complexes (mostly in ATH) has been ascribed not only to the chiral environment originating from the amine ligand, but also to the contribution of the arene ligand to the stabilization of the transition state through the CH/π interaction (Fig 14 (b)) (Yamakawa et al., 2001). This interaction as well as the NH/π interaction occurring in the transition states with diphosphine-Ru-(1,2-diamine) systems may explain why aryl ketones usually give better
Depending on the ligands attached to the metal center (M = transition metal) the inner-sphere mechanisms, in which monohydride or dihydride species are involved, can operate in H2-hydrogenation and transfer hydrogenation (Clapham et al., 2004, Samec et al., 2006; Wylie et al., 2011). In contrast to the outer-sphere mechanism, here the ketone and alcohol interact with the metal center.
3. Heterogeneous hydrogenation
For the heterogeneous, asymmetric, catalytic reduction of the C=O functionality, there are two types of heterogeneous catalysts. One is chirally modified supported metals, and the other is the immobilized homogeneous catalyst on a variety of organic and inorganic polymeric materials. There are also two major reasons for preparing and studying heterogeneous catalysts: firstly, and most importantly, the better and advanced separation and handling properties, and, secondly, the potential to create catalytic positions with an improved catalytic performance. The ultimate heterogeneous catalyst can easily be renewed, reused without of loss of activity and selectivity, which are at least as good or even better than those of the homogeneous analogue.
3.1. Immobilized chiral complexes
The immobilization of a homogeneous metal coordination complex is a useful strategy in the preparation of new hydrogenation catalysts. Much effort has been devoted to the preparation of such heterogenized complexes over the past decade due to their ease of separation from the reaction mixture and the desired minimal product contamination caused by metal leaching, as well as to their efficient recyclability without any significant loss of activity. Preferably, Rh, Ir, and Ru complexes have been employed in the hydrogenations of carbonyl functionality (Corma et al., 2006). Chemically different supports have been used for the immobilization of various homogeneous complexes, including polymeric organic and inorganic supports (Saluzzo et al., 2002; Bergbreiter, 2002; Fan et al., 2002). Due to their chemical nature, organic polymeric supports have some drawbacks concerning reduced stability that affects the reusability of the catalysts, mainly due to their swelling and deformation (Bräse et al., 2003; Dickerson et al., 2002). Supports of an inorganic nature are more suitable owing to their physical properties, chemical inertness and stability (with respect to swelling and deformation) in organic solvents. The above-mentioned properties of the inorganic supports will facilitate the applications of the materials in reactions carried at higher temperatures and their use in continuous-flow reactions. In the past decade a lot of research effort has been devoted to the development of adequate procedures to attach homogenous catalysts onto inorganic supports (Merckle & Blümel, 2005; Crosman et al., 2005; Corma et al., 2005; Jones et al., 2005; Melero et al., 2007). Immobilization via covalent bonds is undoubtedly the most convenient, but on the other hand, it is the most challenging method for immobilization to perform on such supports (Jones et al., 2005; Steiner et al., 2004; Pugin et al., 2002; Sandree et al., 2001). For example, micelle templated silicas (MTS) featuring a unique porous distribution and high thermal and mechanical stabilities can be easily functionalized by the direct grafting of the functional organo-silane groups on their surfaces (McMorn & Hutchings, 2004; Heckel & Seebach, 2002; Bigi et al., 2002, Clark & Macquarrie, 1998; Tada & Iwasawa, 2006). On the other hand, polar solvents such as water or alcohols and high temperatures during the catalytic procedure can promote the hydrolysis of the grafted moieties.
The heterogenized catalysts can potentially combine the advantages of both homogenous and heterogeneous systems. In 2003, Hu and coworkers developed a novel chiral porous solid catalyst based on zirconium phosphonates for the practically useful enantio-selective hydrogenation of unfunctionalized aromatic ketones (Fig. 15) (Hu et al., 2003a).
With the built-in Ru-BINAP-DPEN moieties, porous solids of Ru-(
Substrate | Ru-( | Ru-( | MNP | MNP |
Ar = Ph, R = Me | 96.3 | 79.0 | 87.6 | 81.7 |
Ar = 2-naphtyl, R = Me | 97.1 | 82.1 | 87.6 | 82.0 |
Ar = 4- | 99.2 | 91.5 | 95.1 | 91.1 |
Ar = 4-MeO-Ph, R = Me | 96.0 | 79.9 | 87.6 | 77.7 |
Ar = 4-Cl-Ph, R = Me | 94.9 | 59.3 | 76.6 | 70.6 |
Ar = 4-Me-Ph, R = Me | 97.0 | 79.5 | 87.9 | 80.5 |
Ar = Ph, R = Et | 93.1 | 83.9 | 88.9 | 86.3 |
Ar = Ph, R = | 90.6 | – | – | – |
Ar = 1-naphtyl, R = Me | 99.2 | 95.8 | – | – |
Heterogeneous chiral Ru(II)-TsDPEN-derived catalysts based on Noyori’s (1
Additionally, Li and coworkers (J. Li et al., 2009) developed a Ru(II)-TsDPEN-derived catalyst that was immobilized in a magnetic siliceous mesocellular foam material. The heterogeneous catalyst showed comparable activities and enantioselectivities (
Chiral Ru and Ir, mesoporous, silica-supported catalysts were introduced by Liu and coworkers (G. Liu et al., 2008a, 2008b). The Ir-C28-SBA-(
Two magnetic chiral Ir and Rh catalysts were prepared
The mesoporous SBA-15 anchored 9-amino
The chiral RuCl2-diphosphine-diamine complex with siloxy functionality was successfully immobilized on mesoporous silica nanospheres with three-dimensional channels (Fig. 19) (Mihalcik & Lin, 2008). Upon activation with
Differently substituted Rh complexes were anchored on an Al2O3 support and applied for the enantioselective C=O hydrogenation with reasonable activity and enantioselectivities with
The immobilization of the rhodium complexes [Rh((
Furthermore, a series of polystyrene-supported TsDPEN ligands were prepared in one step and converted to the corresponding Ru(II) complexes by a treatment with [RuCl2(
A series of dendrimers and hybrid dendrimers based on Noyori-Ikariya’s TsDPEN ligand were prepared and the application of their Ru(II) complexes in the ATH of acetophenones was studied. A high catalytic activity and completely maintained enantio-selectivity (acetophenone, 93.4-98.2%
3.2. “Self-supported” and solid-supported heterogeneous catalysts
Among various approaches for homogeneous catalyst immobilization, the “self-supported” strategy exhibits some relevant characteristics, such us easy preparation, good stability, high density of catalytically active sites, and high stereocontrol performance, as well as simple recovery (Dai, 2004; Ding et al., 2007). Self-supported Noyori-type catalysts C37-C40 for the AH of ketones by the programmed assembly of bridged diphosphine and diamine ligands with Ru(II) ions were developed (Fig. 20) (Liang et al., 2005; Liang et al., 2006). The enantioselectivity of the hydrogenation of the aromatic ketones under the catalysis of the self-supported catalyst C40 was in some cases significantly higher than the
A very interesting example is the asymmetric synthesis of the chiral alcohol function that makes use of the strength of ion pairing in ionic liquids (Schulz et al., 2007). The hydrogenation of substrate 46 using H2 (60 bar) at 60 oC in the presence of the heterogeneous, achiral catalyst Ru/C in an ethanolic solution, gave the corresponding hydroxyl-functionalized ionic liquid in a quantitative yield and up to 80%
Importing chirality to a catalytic active metal surface by the adsorption of a chiral organic molecule (often referred to as a chiral modifier) seems to be one of the promising strategies to obtain new chiral heterogeneous catalytic systems. In the hydrogenation of C=O function, chirality-modified supported metal catalysts represent a promising approach with synthetic potential. Orito et al. introduced the strategy of a cinchona-alkaloid-modified platinum catalyst system in 1979 (Orito et al., 1979). Following the early work of Blaser et al. (Studer et al., 1999, 2000, 2003; Blaser et al., 2000), Baiker et al. (Heinz et al., 1995, von Arx et al., 2002), and others, the methodology developed in the sense of the substrate scope, and on the other hand, extensive efforts were carried out to get more insight into understanding the mechanistic aspects of the transformation. The method was found to have excellent performance in the hydrogenation of activated ketones (Fig. 22).
The modifiers derived from CD and quinine (QN) lead to an excess of (
A systematic structure-selectivity study of the hydrogenation of activated ketones catalyzed by a modified Pt-catalyst revealed a high substrate specificity of the catalytic system. Relatively small structural changes in the substrate or modifier can strongly affect the enantio-selectivity and often in the opposite manner, especially when comparing reactions in toluene and AcOH (Exner et al., 2003). Fluorinated β-diketones can be enantioselectively hydrogenated on cinchona-alkaloids-modified Pt/Al2O3 catalysts. Methyl, ethyl, and isopropyl 4,4,4-trifluoroacetoacetates were hydrogenated in the presence of MeOCD-modified Pt/Al2O3 catalysts, producing the corresponding alcohols in 93-96%
Synthetically obtained (
A supported (SiO2) iridium catalyst, which is stabilized by PPh3 and modified by a chiral diamine, derived from cinchona alkaloids, exhibits a high activity and high enantioselectivity for the hydrogenation for the simple aromatic ketones (Fig. 23). The addition of different bases (
A series of silica (SiO2) supported iridium catalysts stabilized by cinchona alkaloids were also prepared and applied in the heterogeneous asymmetric hydrogenation of acetophenone. Cinchona alkaloids display a substantial capability to stabilize and disperse the Ir particles. A synergistic effect between the (
Besides improving the cinchonidine-platinum catalyst system, extensive efforts have been made in developing a reliable mechanistic interpretation. To understand the adsorption behavior of the modifier and reactant, their conformation, and their intra-molecular interactions at solid-liquid interface, an
An inversion of the enantioselectivity occurs in the asymmetric hydrogenation of the activated ketones by changing the solvent composition, including water and acid additives (von Arx et al., 2001b; Bartók et al., 2002). Hydrogenation of the ethyl pyruvate over Pt/Al2O3 (Huck et al., 2003a) and 4-methoxy-6-methyl-2-pyrone over Pd/TiO2 (Huck et al., 2003b), an equimolar mixture of cinchona alkaloids CD and QD resulted in
4. Conclusions
This chapter discusses the transition-metal-catalyzed, asymmetric, homogenous and heterogeneous hydrogenation of prochiral ketones, not so much focusing on the reactions providing valuable chiral alcohols, but rather it gives prominent and interesting examples of the ketone substrates and catalyst systems that are found in the recent literature. Despite the tremendous effort being made in the catalytic, asymmetric hydrogenation of prochiral ketones, approaching the enzymatic performance in some cases, there is still much potential for the continued development of these reactions. Concerning the environmental and economic issues, the introduction of non-toxic, cheap, and at the same time efficient and universal catalyst systems, being able to operate under mild conditions in a highly selective manner and for a broad range of substrates, remains a challenge for future research. Additionally, more rational catalyst designs are possible with better mechanistic understandings of the catalytic cycles in catalytic AH and ATH reactions.
Acknowledgement
The Ministry of Higher Education, Science and Technology of the Republic of Slovenia, the Slovenian Research Agency (P1-0230-0103), EN→FIST Centre of Excellence, and Krka, Pharmaceutical company, d.d. are gratefully acknowledged for their financial support.
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