Applied Hydroformylation - ACS Publications - American Chemical

Review pubs.acs.org/CR

Applied Hydroformylation Robert Franke,†,‡ Detlef Selent,§ and Armin Börner*,§,∥ †

Evonik Industries AG, Paul-Baumann-Str. 1, D-45772 Marl, Germany Lehrstuhl für Theoretische Chemie, Ruhr-Universität Bochum, Universitätsstraße 150, D-44780 Bochum, Germany § Leibniz-Institut für Katalyse an der Universität Rostock e.V., Albert-Einstein-Str. 29a, D-18059 Rostock, Germany ∥ Institut für Chemie der Universität Rostock, Albert-Einstein-Str. 3a, D-18059 Rostock, Germany ‡

5.2.2. Propene 5.2.3. Butenes 5.2.4. Pentenes, Hexenes, Heptenes, Octenes, Nonenes, and Decenes 5.2.5. Higher Olefins 5.3. Dienes 5.4. Functionalized Olefins 5.4.1. α-Functionalized Olefins 5.4.2. Vinyl Arenes 5.4.3. β-Functionalized Olefins 5.4.4. γ-Functionalized Olefins 5.5. Alkynes 5.6. Fatty Acid Compound 6. Stereoselective Hydroformylations 6.1. Reaction Conditions 6.2. Chiral Ligands 6.3. Diastereoselective Hydroformylations 6.4. Isoregioselective Asymmetric Hydroformylation 6.4.1. α-Functionalized Olefins 6.4.2. 1,3-Butadienes 6.4.3. Allyl Derivatives 6.5. n-Regioselective Asymmetric Hydroformylation 7. Alternatives to Syngas 8. Conclusions and Outlook Author Information Corresponding Author Notes Biographies Acknowledgments References

CONTENTS 1. Introduction 2. Classification of Mature Industrial Processes 3. Catalytic Systems 3.1. Metals 3.2. General Types of Ligands 3.3. Characterization of Ligands and Metal Complexes 3.4. Ligands Designed for Industrial Application 3.4.1. Monodentate Phosphines 3.4.2. Monodentate P−O Ligands 3.4.3. Bidentate Ligands 3.4.4. Multidentate Ligands 3.4.5. Ligands for Special Applications 3.5. Preparation of Homogeneous Precatalysts 3.6. General Reaction Conditions 4. Decomposition of Ligands and Catalysts and Measures to Combat This 4.1. General Aspects 4.2. Degradation Pathways of Ligands 4.2.1. Phosphines 4.2.2. Ligands with P−O Bonds 4.3. Degradation of Rhodium Complexes 4.4. Measures against Degradation 5. Substrates and Reactions 5.1. General Remarks 5.2. Unfunctionalized Olefins 5.2.1. Ethylene © XXXX American Chemical Society

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1. INTRODUCTION Hydroformylation is the addition of synthesis gas (“syngas”), a mixture of CO and H2, to olefins in the presence of a catalyst under the formation of aldehydes. Hydrogen (“hydro”) and a formyl group (H−CO) are added in an atom-economical manner (Scheme 1). The reaction leads, unless ethylene is used as a substrate, to a mixture of isomeric products, n-aldehydes (linear), and isoaldehydes (branched). Because double-bond isomerization of the substrate may occur prior to the hydroformylation, different branched aldehydes can be formed even when a single

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Received: May 3, 2012

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dx.doi.org/10.1021/cr3001803 | Chem. Rev. XXXX, XXX, XXX−XXX

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very recently, and general-purpose ligands, which have seen application in the asymmetric hydrogenation, are missing. Another reason for this might be that synthesis chemists working at the interface between academic and industrial research often have only limited knowledge about the potential of modern hydroformylation methodologies. By contrast, a survey about publication activities reveals that the number of academic papers dealing with special aspects of hydroformylation reactions has steadily increased over the last 25 years, whereas the number of patents has stagnated at about 80 per year (Figure 1). A detailed analysis showed that the industrial research is carried out by only a few companies. Protzmann and Wiese assumed that this trend of centralization will even continue in the next few years.5 The situation in the area of asymmetric hydroformylation, where today (2012) there exists only a small number of publications and some 20 patents, is even more problematic. We feel that there is a gap between academic hydroformylation research and its industrial relevance. One reason is that there is a lack of actual overviews showing the relevance of academic findings to applications in the lab or on an industrial scale. Thus, the only major monograph on the subject was edited by van Leeuwen and Claver over 12 years ago.6 In the same year, Chemical Reviews edited a special issue for Organometallics in Synthesis, where, among other things, some particular aspects of the hydroformylation were surveyed.7 In the years that followed, specialist books devoted only a few chapters to this reaction.8 Breit gave an outline of advances in alkene hydroformylation with an emphasis on the concept of self-assembly of catalysts.9 Special reviews were dedicated to spectroscopic aspects of catalysts and catalytic intermediates.10 In 2008, Haumann and Riisager summarized recent developments in the area of hydroformylation in room-temperature ionic liquids (RTILs).11 Likholobov and Moroz gave a résumé of hydroformylation employing solid catalysts.12 A recent book edited by Kollár mentioned some features of hydroformylation in the framework of metal-catalyzed carbonylation reactions.13 Hebrard and Kalck compiled the main aspects of the cobaltcatalyzed hydroformylation with a particular focus on mechanistic aspects.14 In 2010, Castillón, Claver, and coworkers published a special survey about the use of phosphorus donor ligands in asymmetric hydroformylation.15 In extension of this compilation, Vidal-Ferran and co-workers discussed the

Scheme 1. Principle of Hydroformylation

terminal olefin has been subjected to the reaction. Besides the rate of the reaction, the ratio of the isomers (regioselectivity) is therefore an important parameter of each hydroformylation. Independent of the catalyst used, hydroformylation proceeds in a cis-manner. With R unequal to H or CH3 (Scheme 1), the isoaldehydes become chiral; thus, one possibility of a stereoselective transformation arises. The same happens when α,α-disubstituted olefins with different substituents are employed as substrate. Hydroformylation was discovered accidently in 1938 by Otto Roelen (1897−1993), who called it “oxo process”.1,2 Already 4 years later, the first unit began its work at IG Farben Leuna/ Merseburg in Germany. Today, the transformation represents one of the largest homogeneously catalyzed reactions in industry. The year 2008 saw the production of nearly 10.4 million metric tons of oxo chemicals.3 Worldwide, almost the entire volume is manufactured in plants with an output of several hundred thousand metric tons per year. Formed aldehydes are valuable final products and intermediates in the synthesis of bulk chemicals like alcohols, esters, and amines (Scheme 2). Moreover, a reduction−elimination sequence gives access to isomerically pure olefins prolonged by an additional C1-unit. Hydroformylation should be of value in synthesis, as well as in academic laboratories and for the manufacture of fine chemicals; but unlike the homogeneous metal-catalyzed hydrogenation,4 which is related to it, we rarely find it applied there. This odd situation is probably attributable to having to work with the extremely toxic CO under pressure. Moreover, development costs are considered to be too high for a specific, small application. Asymmetric variants have become useful only Scheme 2. Products Derived from Aldehydes

B


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Figure 1. Number of articles published and patents issued over the last 60 years (source: Chemical Abstracts).

technology that was used.20 Because of their classification, the first-generation processes followed the original procedure of Roelen or used similar conditions with cobalt-based catalysts. They operated under quite smooth conditions, but production did not exceed 10 kt/year. The second generation benefitted from an unmodified Co catalyst under much more severe conditions. Differences originated from varying methods of cobalt recovery from the product (“decobalting”),21 which were implemented either by redox processes or by transformation of [HCo(CO)4] into water-soluble salts, followed by extraction.22 The feedstock basis changed from higher olefins in the first plants to mainly propene. On the basis of these processes, large units manufactured aldehydes up to 300 kt/a, which allowed worldwide production of dialkyl phthalates, the most important outlet for aldehydes, to increase to several hundred kt/year. A crucial problem of unmodified Co catalysts is the relatively low l/b-butyraldehyde ratio that is induced. This disadvantage could be overcome through modification with monodentate phosphine ligands;23 however, due to the coordination of organic phosphorus ligands, a lower level of reactivity and the formation of alcohols and alkanes became a critical issue. The new processes could be conducted under much less syngas pressure and are still used today. The reactions are usually carried out in a temperature range of 120−190 °C and a syngas pressure of 4−30 MPa in large industrial companies, such as BASF, Exxon, Sasol, and Shell. Especially the production of high-boiling aldehydes or alcohols from long-chain and branched olefins remains a domain of cobalt catalysis. There are still some patent activities in this area from Shell, which indicates a renewed interest.24 Efforts are directed toward improving the yield of the targeted alcohol by processing in multizone reactors with different temperature areas and utilizing the beneficial effect of water.25 The cleanup of offgases containing volatile Co compounds is a further issue of current industrial interest.26 A recent approach by the Institute Francais du Petrol focused on the use of nonaqueous ionic liquids as solvents with pyridines as ligands for cobalt.27 The third generation started in the 1970s and involves processes operating with P-ligand-modified Rh catalysts at low syngas pressure (1.8−6.0 MPa) and medium temperatures (85−130 °C).20 These “low-pressure oxo processes” (LPOs)

impact of chiral phosphine−phosphinites and phosphine− phosphites as ligands.16 In contrast to these reviews, herein we will concentrate on pivotal achievements and crucial problems of the hydroformylation for those chemists who are interested in simply using this versatile methodology in the academic laboratory or at an industrial scale. Therefore, we do not attempt to give a complete overview of all research activities in this area. On the basis of our own personal experiences, collected during several years of applied hydroformylation, we will show where this reaction has been already successfully implemented. Moreover, we will quote those developments of academic and industry laboratories, where we assume a potential for the manufacture of fine or bulk chemicals in the near future. After a brief overview of the classification of bulk processes that have been run in the industry for several decades, we will focus on the choice of the metal and the synthesis of easily available ligands. One section will be dedicated to the decomposition of ligands and catalysts under hydroformylation conditions and measures against it. A survey of substrates that have already been used or that are of potential value for the manufacture of bulk and fine chemicals will be of particular importance. Also those olefinic substrates will be considered that are derived from natural sources such as biomass. The hydroformylation of formaldehyde to give glycol aldehyde17 and the ring-opening reaction of epoxides with syngas will not be considered herein.18 The same applies to the mechanism of particular consecutive reactions (tandem reactions) with hydroformylation as an intermediate step.19 Thus with the exception of isomerizing hydroformylation, other tandem reactions such as hydroaminomethylation (see, e.g., Scheme 39), hydroformylation in combination with double cyclization (see, e.g., Scheme 48), or dehydration (see, e.g., Scheme 51), etc. will only be discussed with reference to examples, when natural products or compounds with some value for fine chemical synthesis yield. The review will close with some suggestions for less toxic alternatives for syngas.

2. CLASSIFICATION OF MATURE INDUSTRIAL PROCESSES Bohnen and Cornils classified important industrial hydroformylation processes up until 2000 in relation to the C


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Figure 2. Development of the Rh price over the last 12 years. Data downloaded from http://heraeus-edelmetallhandel.de.

An example of aqueous two-phase hydroformylation went on-stream at Ruhrchemie AG in 1984 (fourth generation) on their site in Oberhausen/Germany with an annual capacity of 100 kt/a.20 The current capacity is 500 kt/a. The Rh catalyst is immobilized in the aqueous phase. A sulfonated phosphine ligand (TPPTS) produces the high metal solubility in water. The catalyst is removed into the aqueous phase before distillation of the product, which avoids thermal stress. The rhodium losses are in the range of ppbs.

are still state of the art and are carried out at Dow Chemical, BASF, and Mitsubishi. Preferentially, short unfunctionalized olefins are used as substrates. About 70% of the total hydroformylation capacity, which concerns the transformation of ethylene, propene, and butenes, is based on LPOs. In general, a high excess of P-ligand is required to stabilize the Rh complex and to prevent formation of the less-desired branched aldehydes. In some recent approaches, monodentate phosphines have been replaced by bidentate diphosphites (see section 3.4.3). One of the main differences between all of these large-scale rhodium-catalyzed hydroformylations is the technology used to separate the product and the catalyst with the aim of reusing the metal. Because of cobalt’s relatively low price compared to rhodium, it does not require complete recycling; however, deposition products of Co clusters and metallic cobalt cause fouling on reactor surfaces and serious blockage of valves that can result in a shutdown of the plant. Moreover, the formation of Co carbides (CoxC, x = 2, 3), which are responsible for the formation of methane from syngas in dry spots of the reactor, must be prevented (e.g., by injection of NaHS).28 Wiese and Obst have estimated the annual financial loss in a 400 kt plant when just 1 ppm Rh/kg product is lost at several million euros;8b therefore, efficient catalyst recycling is indispensable. It may be achieved by stripping off the lowboiling product with an excess of syngas (“gas recycling”). The technology is limited to the hydroformylation of alkenes up to pentene. An alternative, more recently developed separation process is based on the distillative removal of the products (“liquid recycling”). The catalyst remains in the residue, consisting of high-boiling condensation products, and is used for the next run. This technology can also be used in the workup procedure in the hydroformylation of alkenes with chain lengths greater than C6. The lifetime of a catalyst charge may exceed 1 year under the condition that sufficient purity of the feed and careful process control is guaranteed.

3. CATALYTIC SYSTEMS 3.1. Metals

The typical catalysts for hydroformylation of olefins are homogeneous complexes of the type [HM(CO)xLy], where L can be further CO or an organic ligand. A generally accepted series of the activities of the unmodified metal is as follows:29 Rh ≫ Co > Ir, Ru > Os > Pt > Pd ≫ Fe > Ni

To date, only rhodium and cobalt are used in industrial practice. One of the main advantages of cobalt catalysts is their robustness toward poisons in the feedstock. In particular, mixtures of long-chain internal and branched alkenes can be converted, which are derived from the Shell Higher Olefin Process (SHOP) or from Fischer−Tropsch synthesis on a large scale. Because of the high hydrogenation activity of Co catalysts, product aldehydes can be immediately reduced to the corresponding alcohols, which may be desired. The main disadvantages of Co systems are the severe reaction conditions, which require high investment costs. Therefore, a renaissance of new oxo processes based on cobalt is not in sight, although there has been some research over the last few years.10b Homogeneous unmodified or ligand-modified rhodium catalysts are predominantly used for the transformation of olefins with a chain length ≤C10. Such Rh catalysts can be up to 1000 times more reactive than Co complexes. The major D


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activity in hydroformylation was found with corresponding Rh catalysts:53,54

advantages of rhodium catalysis are documented in the reduced syngas pressure and reaction temperature. These features have also been recognized by the chemical industry. Thus, in 1980 less than 10% of hydroformylation was conducted with rhodium; by 1995 this had increased to ∼80%.30 In some cases, a combination of Co and Rh can be advantageous. ICI claimed the beneficial addition of Co salts, which are soluble in organic solvents, to Rh phosphine catalysts of low concentration, which prevents deactivation of the catalyst by feed impurities, such as sulfur compounds or butadiene.31 The main problem of rhodium is its high, very volatile price over the years (Figure 2). The price on the world market is dictated by the automotive industry, which uses ∼80% of the metal in catalytic converters for vehicles. In July 2008, rhodium broke the $10,000 per ounce barrier for the first time. Because of the global financial crisis, which began in the last quarter of 2008, the rhodium price fell from this level to about $1,000 per ounce within a few months. Despite having presumptive economical advantages over rhodium, other metal catalysts have remained undeveloped. In some academic studies, the suitability of homogeneous Ru complexes was investigated.32 A comparison of Rh and Ru catalysts in the hydroformylation of linear butenes provided evidence that the latter are significantly less active.33,34 On the other hand, Ru-based catalysts are more active than osmium complexes,35 but only a narrow substrate range has been tested.36 Garland, Li, and co-workers showed that the addition of Re,37 Mo,38 or W39 carbonyls to unmodified rhodium produces synergies, which may contribute to more efficient exploitation of the precious metal. The use of Fe is almost unexplored to date,40 although a promoting effect has been attributed to this metal in hydroformylation with Rh catalysts.41 The assumed low activity and chemoselectivity of Ir catalysts has been the reason that only a few substrates (e.g., vinylsilanes) have been investigated to date,42 although it has been proven that these problems can be overcome by adding inorganic salts.43 Recently, Beller and co-workers discovered that a PPh3-modified Ir catalyst was no more than 8 times slower than its Rh counterpart in the hydroformylation of a variety of olefins.44 The same research group as well as Drent and coworkers advocated the use of homogeneous Pd catalysts, which produced excellent regioselectivities in isomerizing hydroformylation in the presence of acids.45 Modified Pd complexes were also successfully used in the hydroformylation of internal alkynes, where a remarkable synergy effect of added [Co2CO8] was observed.46 Heterobimetallic Pt/Pd complexes bridged by thiolato ligands were investigated by the group of Masdeu-Bultó.47 Homogeneous Ni and Cu complexes were tested in the carbonylation of acetylene.48 A large field of interesting academic investigations is covered by Pt49 or modified Pt/Sn catalysts,50 which have been studied intensively since the 1970s. In the asymmetric version of hydroformylation, in many cases superior enantioselectivities can be achieved in comparison to chiral Rh catalysts. 51 In general, these investigations have been limited to laboratory scale to date.

Ph3P > >Ph3N > Ph3As, Ph3Sb > Ph3Bi

Besides the lower activity in comparison to phosphines, amines as ligands cause lower chemoselectivity; alkanols as well as alkanes are formed.55 In a few instances, bridging thiolate ligands have also been used in dinuclear Rh complexes with the hope of generating cooperative effects between both metal centers,56 but with high probability the S-ligands do not remain coordinated in the active catalysts.57 Phosphine ligands (also called phosphanes) are usually characterized by three carbon atoms surrounding the central phosphorus atom (Figure 3). Exceptions are unsaturated Pheterocycles and primary or secondary phosphines; the latter play a minor role as ligands.

Figure 3. Classification of trivalent P-ligands based on the nature of the α-atom next to the phosphorus.

Replacement of one C-substituent in phosphines by an oxy group produces esters of phosphinous acid (phosphinites). Further substitution with alcohol gives esters of phosphonous acid (phosphonites) and esters of phosphorus acid (phosphites), respectively. Another possibility of modification is the stepwise incorporation of N-substituents, which produces aminophosphines, diaminophosphines, or triaminophosphines. Further variations can be produced by combining different heteroatoms. A frequently used ligand motif employed in asymmetric hydroformylation is the structure of phosphoroamidite. In some cases phosphorus halogenides (fluorophosphites) have also been used as ligands, under the condition that they are stable under the reaction conditions.58 But their main importance lies in their use as a coupling reagent (e.g., chlorophosphine, phosphorochloridite). The organic backbone can be acyclic, but the incorporation of the phosphorus in aromatic or nonaromatic heterocycles of different ring sizes is also possible.59 Carbene ligands have also been investigated for

3.2. General Types of Ligands

For industrial applications, independent of the metal used (Co, Rh), only trivalent phosphorus compounds are used as ancillary ligands. Other potential ligating compounds based on elements of the fifth row of the periodic table have not played a role to date, although trivalent compounds of As, Sb, and Bi are occasionally claimed in patents.52 The following order of E


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several years, but so far they have been limited to academic studies.60 If cobalt complexes are modified with ancillary phosphorus ligands, in most cases there is reduced activity in comparison to the parent catalyst. Among trivalent phosphorus compounds only phosphines can be used as a ligand, because, with Co catalysis, alcohols are frequently formed from the product aldehydes through the high reductive potential of the catalyst. Consequently, ligands with a P−O or P−N bond can undergo transesterification with an excess of product alcohol. In contrast, P-ligand-modified rhodium catalysts are more active than their unmodified metal analogues. In particular, the easily accessible phosphitesbeing better π-acceptors than phosphinesforce CO dissociation during the catalytic cycle and therefore result in higher reaction rates. This correlates with earlier studies, which showed that less-basic phosphines produce higher reaction rates and higher linear-to-branched ratios.61 Rh catalysts based on trialkylphosphines predominantly produce alcohols without the intermediate formation of aldehydes.62

Figure 4. Commonly used models for steric characterization of phosphorus ligands.

calculated for asymmetric ligands, where the cone angle is taken as two-thirds of the sum of the individual cone half angles. This concept can be used for all types of monodentate phosphine ligands and is, within some limits, also suitable for diphosphines. Moreover, σ-donor and π-acceptor properties of ligands can be assessed by recording the IR spectra of corresponding metal−CO complexes, because they show very specific and intensive bands, usually in the range from 1900 to 2100 cm−1.64b On the basis of this procedure, the Tolman electronic parameter (TOP) can be extracted. Good π-acceptor properties are indicated by a shift of the A1 νCO toward higher wavenumbers. In the meantime, Tolman ligand maps have become a guiding principle for the evaluation of ligand effects. Another useful correlation for estimating the donor−acceptor properties of diphosphines was found by measuring the carbonyl stretching frequencies of complexes of the type [RhCl(P−P)CO].70 The concept of the natural bite angle, α, has been especially developed for hydroformylation in order to get some insights into the relationship between activity and regioselectivity of homogeneous rhodium catalysts (Figure 4b).71 The parameter, which is derived from molecular mechanics calculations, has been designed for bidentate ligands and describes the preferred angle created by two phosphorus atoms and a “dummy” metal atom. In general, a differentiation can be made between a steric and an electronic bite angle effect. The former relates to all repulsive interactions between ligands at the metal center, including a coordinated substrate. The electronic effect characterizes the electronic changes that occur when the bite angle is altered. The effect is dependent on the hybridization of metal orbitals and can therefore change throughout the catalytic cycle. Usually, a flexibility range is considered, which accounts for the different conformations of the ligand with energies slightly above the strain energy of the natural bite angle.

3.3. Characterization of Ligands and Metal Complexes

To optimize known processes and develop new catalytic systems, knowledge of some crucial ligand parameters can be of value. There are numerous methods to test the steric and electronic properties of phosphorus ligands.63 This experimental data is now supported by computational methods.64 It should be noted, however, that due to changing geometries and oxidation states of the metal within the catalytic cycle, this data must be treated with caution to forecast catalytic properties. Simple correlations between a set of limited analytical data and catalytic results or stabilities exist rarely.65 31 P NMR spectroscopy is the method of choice for characterization of phosphorus ligands and corresponding metal complexes. The shifts of trivalent phosphorus compounds relevant for catalytic applications cover a range between ∼200 and −60 ppm.66 Measurement of the 31P−77Se coupling constant is a simple and fast method to estimate the σ-donor capacity of a P-ligand by NMR spectroscopy.67 The required phosphorus selenides can be prepared in an NMR tube by treating the trivalent phosphorus compound with an excess of elemental selenium in a deuterated solvent. Sometimes brief heating is required. The coupling constant JPSe can be in the range of several hundred hertz and thus even allows an accurate differentiation between closely related ligands. In general, the higher the magnitude of the coupling constant, the lower is the basicity of the parent phosphorus compound. Electron-withdrawing groups increase the coupling constant, whereas electron-donating groups decrease it. The presence of bulky substituents at the phosphorus atom likewise results in a decrease in the scharacter of the electron lone pair as a result of widening of the relevant bond angles. Numerous data points for the characterization of P-ligands have been accumulated after incorporation into corresponding metal carbonyl complexes. A frequently used model compound is the rapidly formed complex of the type [Ni(CO)3(phosphine)]. In this connection the Tolman cone angle, θ, characterizes the size of the organic ligand.68 The cone angle is defined as the solid angle formed with the metal at the vertex (centered at a distance of 2.28 A from the donor atom) and the van der Waals radii of the outermost atoms at the perimeter of the cone (Figure 4a).69 Cone angles can also be

3.4. Ligands Designed for Industrial Application

The price and the long-term stability of the phosphorus ligand are pivotal issues for calculating the overall costs of an industrial process. In academic laboratories, literally several hundred ligands have been prepared to explore structure−activity/ selectivity relationships, but only very few of them have any chance of finding broader application. Ideal are short and highyield synthetic routes based on cheap starting materials. The modular preparation of related structures can be a further advantage for final optimization in terms of activity, selectivity, and stability. As is usual in such systems, small changes in the ligand structure may cause large effects. F


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3.4.1. Monodentate Phosphines. Historically, in the first ligand-modified hydroformylation process with Co, and subsequently with Rh, mostly PPh3 has been used as an accessible, inexpensive, and quite air-stable ligand. The use of rather volatile trialkyl phosphines (PEt3, PBu3) has also been reported in literature.72 With some success, triphenylphosphine oxide (TPPO) has also been used as a slightly coordinating ligand for Co73 and Rh;74 however, more severe conditions have to be applied and the final addition of PPh3 is necessary to stabilize the rhodium catalyst before the distillation step for catalyst recovery.75 PPh3 is produced in a scale of ∼5 kt through the reaction of p-chlorobenzene with sodium and PCl3, for example, by BASF, Atochem, and Hoko (Scheme 3).

quick preparation of a large series of modular ligands displaying considerable π-acceptor properties. Turnover frequencies of >45 000 h−1 could be achieved with the assistance of these ligands in the Rh-catalyzed hydroformylation of terminal olefins. 3.4.2. Monodentate P−O Ligands. Organophosphites are less sensitive toward oxidation than phosphines. Because of the P−O bonds, they are weak σ-donors but strong π-acceptors. This issue facilitates the dissociation of CO from the metal center. Therefore, in Rh-catalyzed hydroformylation, the replacement of phosphines by phosphites contributes to a substantial increase in the reaction rate. The n-regiodirecting properties of the catalyst can be enhanced by incorporation of sterically demanding substituents in the organic backbone. Bulky alkyl groups in the neighborhood of the P−O bond fulfill this requirement and also contribute to an enhanced activity and hydrolysis stability of the ligand (see section 4.2). These properties were combined for the first time by van Leeuwen's group in the structure of phosphites 1 bearing a tert-butyl group in the ortho-position of the aromatic ring (Figure 5).83 Typically, such phosphites are produced by condensation of PCl3 with substituted phenols. Some of these phenols and phosphites are available in a wide variety and on a large scale because they are used as antioxidants for the stabilization of polymers.84 The first phosphites of type 1 were claimed as ligands for hydroformylation by Shell85 and UCC (since 2001, Dow Chemical)86 and are still in use in the industry. Tris(2,4di-tert-butylphenyl)phosphite 1c is widely used, also on a laboratory scale.87 More conformational stability has been realized by the incorporation of the phosphorus atom in heterocyclic rings such as 2a.88 Interestingly, related fluorophosphites like 2b89 have also been used as ligands.90 The P−F bond is remarkably thermally and hydrolytically stable under hydroformylation conditions.91 Examples of rare monophosphonites 3 were developed by Selent and Börner in cooperation with Oxeno.92 3.4.3. Bidentate Ligands. Sterically hindered, chelating Pligands predominantly induce the formation of n-aldehydes. Because of their short and easy synthetic access, diphosphites are always the first choice when the aim is n-regioselective hydroformylation of large quantities of olefins. The first ligands for this purpose were described in patents from UCC.93 Among several chelating and nonchelating polyphosphites, diphenylphosphites of the BIPHEPHOS-type became the prototype of all subsequently developed phosphite ligands (Scheme 6).94 A particular feature of some of these biphenol-based ligands is their ability to form stereoisomers due to temperaturedependent atropisomerism. Formation of diastereomers influences the catalytic properties and may complicate mechanistic studies.95,96 In general, these phosphorous acid triesters can be easily prepared in one step starting from substituted biphenols97 through reaction with phosphorochloridites, usually in the presence of a base like triethylamine in toluene or tetrahydrofuran (THF) as a solvent.98 Alternatively, to avoid side reactions or severe reaction conditions, it was suggested that the required phosphorochloridite be prepared by melting 2,2-dihydroxydiphenyl with PCl3 at 110−130 °C in the absence of any base under reduced pressure.99 HCl or its salts have to be completely removed to avoid acid-related decomposition during hydroformylation. Attention should be given to the second esterification step, because transesterification at the stage of the monoester may

Scheme 3. Technical Scale Preparation of PPh3

Shell was the first company to utilize trialkylphosphines for optimization of Co catalysts. As monophosphines, regioisomeric phosphabicyclononanes are applied (Scheme 4).76 Scheme 4. Typical Synthesis and Examples of Monophosphines Used as Ligands in Cobalt-Catalyzed Hydroformylation

Currently, these ligands are commercially available as a mixture under the collective chemical name eicosyl phobane (EP). Recently, bidentate phobanes also have been synthesized; however, their catalytic performance was quite similar to that observed with monodentate ligands.77 Sasol claimed similar bicyclic monophosphines.78 The so-called LIM ligands (e.g., LIM-10 and LIM-18) differ in the length of the monoalkyl chain on the phosphorus. The two-step synthesis of LIM-18, which produces a mixture of diastereomers, starts from easily available cyclic olefins, like limonene. Radical mediated addition of PH3 yields a secondary phosphine,79 which is reacted with a long-chain n-alkene under the same conditions or in the presence of a strong base.80 One problem of this short synthesis may be the use of the highly flammable, foul-smelling, and toxic PH3. As illustrated in Scheme 5, a similarly short synthesis for phosphabenzenes (phosphinines) has been suggested by Breit’s group in cooperation with BASF.81,82 The protocol allows G


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Scheme 5. Typical 2-Step Synthesis of a Phosphinine Ligand and Some Examples

The BASF group of Röper and Paciello correlated product linearity in the hydroformylation of 1-octene with structural data of strongly related diphosphite ligands 10a−d (Figure 7).109 A key finding was that ligands that stabilize a P−Rh−P angle of 120° lead to superior n-regioselectivity as realized perfectly with ligand 10a. Alternatively, functionalized calixarenes have been used as a sterically hindered backbone.110 Replacement of phenol substituents by pyrrole units produces corresponding phosphoroamidites as shown with ligands 11,111 12,112 and 13113 (Figure 8). Trzeciak, Ziółkowski, and co-workers showed that, in comparison to phosphites, phosphoroamidites are stronger πacceptor ligands.114 Figure 9 indicates the order of π-acceptor and σ-donor properties that were found. Another striking property of the ligands concerned is their facile synthesis by a simple reaction of PCl3 with amines. Usually the formation of P−C bonds requires the use of metal organic reagents and the application of low temperatures, and it is frequently based on multistep sequences. Therefore, safety issues and high costs may hamper the production of these ligands on a large scale. Exceptions can be justified either by extremely high activities of the corresponding catalysts or their use for the manufacture of expensive aldehydes. Phosphinites and phosphonites such as 14,115 15,116 16,117 and 17118 (Figure 10) were tested predominantly for isomerizing hydroformylation of alkenes. Xantphos is the generic name of a class of diphosphine ligands consisting of numerous individuals discovered independently by Haenel’s119 and van Leeuwen’s groups (Scheme 9).120 As evidenced by van Leeuwen, Kamer and co-workers, ligands are of particular value for the explanation of structure− activity/selectivity relationships and for the manufacture of expensive aldehydes.7a,121 Several of them are commercially available even on a larger scale. By varying the heterocycle, different P−Rh−P bite angles can be adjusted in the final Rh catalyst. A typical synthesis covers the functionalization of 2,7dimethylphenoxathiin with PCl2 and the subsequent reaction with ArMgBr, as shown by the example of Thixantphos.122,123 The use of very low temperatures and highly flammable Li-

Figure 5. Selection of industrially relevant bulky P−O ligands.

occur, which lowers the yield of the desired ligand.100 Recently, it was demonstrated that this side-reaction, which probably occurs through the formation of a bicyclic hydrophosphorane, can be advantageously used for the construction of a nonsymmetric diphosphite (Scheme 7).101 A similar transesterification process was observed in the reaction of biphenolphosphorochloridite with o-phthalic acid, where only the corresponding pyrophosphite was produced (Scheme 8).102 Synthesis of the targeted diphosphite could be achieved only with m-phthalic acid.103 Because of the simple and cheap construction principle shown above, various diphosphites were synthesized by the groups of Mitsubishi Kasei (4)104 and Oxeno in-house or in cooperation with research groups of academia (5,105 6,100 7,106 8,107 and 9;108 Figure 6). H


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Scheme 6. Synthesis of BIPHEPHOS

Scheme 7. Transesterification as an Approach for the Synthesis of an Unsymmetric Diphosphite

11 were observed. Obviously, electron-withdrawing groups force the formation of the linear aldehyde. As expected, a simultaneous increase in the turnover number frequency (TOF) was observed. BISBI was patented by Eastman Kodak126 for Rh-catalyzed hydroformylation and subsequently used by Casey and co-

organyls explains the current high price of Xantphos ligands. A cheaper alternative is the conformationally more flexible ligand DPEphos.124 Steric and electronic fine-tuning can be achieved by varying the P−Ar groups.125 With a series of Thixantphos ligands in the hydroformylation of styrene, the l/b ratios indicated in Figure I


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Scheme 8. Transesterification of Mixed Anhydrides

Figure 6. Selection of important diphosphite ligands with a biaryl scaffold.

groups in the latter by electron-withdrawing fluoro-substituted aryl groups produces IPHOS-type ligands, which have been synthesized by Beller and co-workers in cooperation with Oxeno.129 Diphosphine 18 was patented by BASF.123 van

workers for mechanistic studies (Figure 12).127 The related bidentate ligand NAPHOS is based on a binaphthyl backbone and was first tested for other metal-catalyzed reactions by Kumada and co-workers.128 Replacement of the P-phenyl J


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relation to the nature of Ar in the following order, indicating a remarkable electronic effect: p‐MeC6H4 < Ph ≈ m‐CF3C6H4 ≈ 3,5‐(CF3)2 C6H3 < 3,5‐F2C6H3 < p‐CF3C6H4

3.4.5. Ligands for Special Applications. A crucial point in the workup of a hydroformylation reaction is the separation of product and catalyst. In best-case scenarios, the latter can be recycled and used for subsequent runs. A breakthrough was the use of the sulfonated phosphine ligand TPPTS (trisodium salt of 3,3′,3″-phosphinidyne tris(benzenesulfonic acid) (Figure 13)140 in the Ruhrchemie/Rhône-Poulenc process for hydroformylation of propene, which, to the best of our knowledge, is the only aqueous two-phase hydroformylation used in industry to date. TPPTS exhibits excellent solubility in water (∼1.1 kg/ L) and is in general insoluble in most organic solvents used for two-phase catalytic reactions.141 Some attempts to incorporate sulfonate groups in more sophisticated diphosphine ligands were published, e.g., BISBIS142 or BINAS.143 Sulfonation of aryl phosphines is usually carried out in fuming sulfuric acid (oleum) at room temperature. The sequence of sulfonation can be quite complex, and it usually produces a mixture of regioisomeric products with varying degrees of sulfonation at the aryl rings. The progress of the reaction can be followed by 31P NMR spectroscopy.144 Strict exclusion of oxygen is required; otherwise, the product will be contaminated by phosphine oxides. Purification of corresponding metal complexes has been achieved with gel permeation chromatography.145 As shown by Herrmann et al., the use of boric acid can facilitate the synthesis.146 Unfortunately, to date only TPPTS is available in quantities useful for scale-up purposes, which may be explained by the rather unpleasant synthetic protocol required. Probably for this reason, only a few academic studies using BINAS and related sulfonated diphosphines have been reported to date.147,148 There are numerous attempts to link phosphines or corresponding homogeneous catalysts to solid supports, but industrial applications are not in sight due to the multistep synthesis required. Therefore, only some recent examples will be discussed here. DuPont incorporated a diphosphite unit in the framework of a polyamide (22) to provide for better

Figure 7. Selection of slightly varied diphosphites.

Leeuwen and Bronger developed together with Celanese the bidentate ligand 19.131 3.4.4. Multidentate Ligands. Only a very few ligands bearing three ligating phosphorus groups, such as 1,1,1tris(diphenylphosphinomethyl)ethane (Triphos), have been investigated in hydroformylation.132 Probably one P-group dissociates from the metal during the catalytic cycle (“arm-off mechanism”).133 Tetradentate phosphite ligands derived from pentaerythritol for rhodium-catalyzed hydroformylation were claimed by Mitsubishi Kasei.104 Entirely new ligand motifs are pyrrole-based tetraphosphoroamidite 20134 and tetraphosphines 21,135 which were discovered by Zhang’s group (Scheme 10).136 Such potential tetradentate ligands are said to have enhanced chelating abilities due to multiple chelating modes, which probably increases the local phosphorus concentration around the metal center.137 Because of the large flexibility of the P-ligating groups around the biphenyl axis, a high concentration of phosphorus is always ensured at the metal center, which contributes to a greater robustness of Rh-bound phosphorus ligands toward attacking CO.138 Because of this effect, usually a low ligand/rhodium ratio (e.g., 3/1) can be chosen. With tetraphosphines of type 21, >95% linear selectivity and up to 94% yield of total aldehydes starting from 2-alkenes (2pentene, 2-hexene, and 2-octene) were observed in isomerizing hydroformylation.139 The n-regioselectivity increased in

Figure 8. Phosphoroamidite ligands. K


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Figure 9. π-Acceptor and σ-donor properties of P-ligands with different P−X (X = C, O, N) bonds.

Figure 10. Bidentate diphosphinite and diphosphonite ligands.

Scheme 9. Selected Xantphos-Type Ligands and a Typical Synthetic Access

Figure 11. Regioselectivities achieved in the Rh-catalyzed hydroformylation of styrene with Thixantphos ligands with electronically different P-aryl groups (conditions: CO/H2 (1/1, 10 bar), ligand/Rh = 10, substrate/Rh = 1746, [Rh] = 0.50 mM).

recycling of the catalyst (Figure 14).149 van Leeuwen and coworkers linked long-chain alkyl groups (R1, R2) to a Xantphostype ligand 23 by means of a Friedel−Crafts acylation, followed by reduction.150 As a result of this modification, solubility in organic solvents, such as toluene, was enhanced. Differences from 2 to 500 mmol/L were observed in relation to the length of the alkyl chain. Another Xantphos-based diphosphine (24) was designed for reaction in ionic liquids.151 L


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Figure 14. Ligands with special separation properties.

can be accomplished in a separate vessel before the hydroformylation or in situ. Unmodified Co catalysts are usually produced from Co2(CO)8, an air-sensitive solid. The compound is a strong irritant and harmful to eyes, skin, and mucous membranes. Alternatively, cobalt octanoate is used as well. As precursors for rhodium catalysts, RhCl3 × (H2O)x, Rh(OAc)3, Rh(II) carboxylates, Rh(acac)(CO)2 (acac = acetylacetonate), Rh(acac)(COD) (COD = 1,5-cyclooctadiene), and Rh4(CO)12 are used. Also thiolates of rhodium(I) have been used. Methoxides are likewise good precursors. Rh dipivaloylmethane forms more stable stock solutions than acac.120 Modification of Co catalysts with phosphorus ligands can be realized by mixing Co2(CO)8 with an excess of the ligand to produce a salt, which is rapidly converted at higher temperatures into the dimeric species Co2(CO)6P2 (Scheme 11). The corresponding precatalyst is formed in the presence of H2 or syngas.

Figure 12. Chelating diphosphine ligands.

Scheme 10. Polydentate P-Ligands and Multiple Chelation Modes

Scheme 11. Preparation of a Modified Cobalt Precatalyst

3.5. Preparation of Homogeneous Precatalysts

Catalysts for hydroformylation may contain ancillary organic ligands or not. In all cases, instead of the catalyst, a corresponding more stable precatalyst is submitted to the reaction, which is converted under syngas into the catalytically active species. Modification of the metal with organic ligands

In older experimental formulations, Rh2O3 is mixed with the P-ligand before the reaction is initiated through pressure.152 Also the commercially available hydrido complex HRh(CO)-

Figure 13. Selection of sulfonated aryl phosphines used for aqueous two-phase hydroformylation. M


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(PPh3)3 can be used directly as a precatalyst; however, addition of a bidentate ligand is occasionally needed for stabilization.153 Today, for synthetic purposes mainly Rh(acac)(CO)2 is used instead, which is reacted with the P-ligand under syngas (Scheme 12). The precatalysts formed, such as [HRh(CO)3P]

Scheme 13. Preparation of a Modified Rhodium Catalyst from an Ortho-Metalated Precursor

Scheme 12. General Method for the Preparation of a PModified Rh precatalyst

(P = bulky monophosphite), usually cannot be isolated easily and are only observable by spectroscopic methods.10,154 The progress of the formation of the P-ligand-modified precatalyst can be investigated by means of UV−vis spectroscopy or, in the case of chiral ligands, with UV−vis circular dichroism (CD) spectroscopy.155 Different high-pressure NMR techniques coupled with IR provide valuable information about the structure of the precatalyst that is formed.10,154,156 It is highly recommended that hydroformylation batches be set up under an inert atmosphere. Otherwise, precursors may transport oxygen and moisture in an undesired way. In the literature, sometimes a startup routine can be found where the precatalyst, ligand, and even olefin are mixed under air. The mixture is transferred into the autoclave and then flushed with syngas. This straightforward approach will lead to contamination with olefin hydroperoxides. Furthermore, the transformation of the precursor to the catalyst takes place in parallel to hydroformylation, and few reliable results on catalyst activity and selectivity are available. The striking effect of the methodology on the catalytic performance has been demonstrated impressively for an Ir catalyst by Beller’s group.44 Sometimes mixtures of diastereomeric P-ligands have to be used, because separation would be too costly. Investigations of individual diastereoisomers showed differing coordination properties with Co, but apparently they are not relevant in hydroformylation.157 In strong contrast, diastereomeric Pligands can affect the pathways for the generation of Rh precatalysts.158 This applies to the formation of the precatalyst as well as to the catalytic results. The generation of the catalyst from the precatalyst may lead to the formation of side-compounds (Hacac, acids, and alcohols), which, in turn, may contribute to the decomposition of ligands bearing P−O bonds. This effect is avoided by the use of ortho-metalated Rh complexes, as shown in Scheme 13.159 This catalyst precursor can be easily stored and handled. Only under syngas is the precatalyst easily liberated through breakage of the Rh−C bond by hydrogenolysis. During preformation, the 1,5-cyclooctadiene ligand is hydroformylated once and the second double bond is hydrogenated to produce cyclooctane carbaldehyde. The ortho-metalation reaction of hydroformylation catalysts has been found to occur also during distillation of the reaction mixture of a continuously driven hydroformylation reaction. Especially in the presence of an excess of olefin and after stripping hydrogen with pure carbon monoxide, the formation of ortho-metalated rhodium complexes is favored.160 Thus, possible decomposition pathways of the catalyst are blocked. In continuous processes, information about the degree of degradation of ligand and catalyst is essential to maintain the quality and quantity of the product aldehydes over the whole

reaction time. Phosphorus-containing degradation products can be analyzed by means of 31P NMR spectroscopy. Unfortunately, their concentration in technical hydroformylation solutions is very small, and they may be below the detection limit. Moreover, NMR measurements are rather timeconsuming and may be affected by problems of mass transfer of reactive gases from the head space of the NMR tube into the solution.161,162 The method of choice is in situ-Fourier transform infrared (FTIR) spectroscopy with different techniques.163 This spectroscopic technique can be directly linked to the reactor and immediately provides continuous results for nearly all components of the reaction mixture, which may be present in very low concentrations. The actual status of the catalytically active metal can be investigated on the basis of characteristic CO bands. Figures 15 and 16 show a typical laboratory-scale setup, which has the potential of being connected to a largescale reactor. An independent time-resolved determination of product aldehydes is possible with subsequent sampling or direct coupling of the autoclave to a GC device. For the device depicted in Figure 16, identical product concentration-versus-time profiles were obtained from GC with sampling done directly from the reactor and from the IR spectroscopic analysis, respectively. The same first-order rate constants were derived from both profiles. No acyl rhodium complexes were detected also with a rhodium concentration > 1 mM applied, pointing to the fact that the solution did not suffer hydrogen depletion.103 These results show that data obtained with this methodology are not necessarily altered by mass transport limitation. Of course, direct measurements with an immersion probe are highly desired. Such an approach might become possible in the future when a mutual enhanced sensitivity of the attenuated total reflection (ATR) FTIR spectroscopic technique allows for a reliable quantitative timeresolved detection of catalytic intermediates present only on the submillimolar concentration level. An elegant experimental solution for following relatively fast gas-consuming reactions has been provided with the Amsterdam in situ infrared autoclave.10c,164 The implementation of a transmission IR flow cell into the autoclave does strongly reduce the influence of the cell design on the composition of the reaction solution. For an accurate analysis of single IR bands in mixtures of compounds, programs for spectra deconvolution are required. High-quality algorithms for this purpose were developed by the groups of Garland (BTEM = band-target entropy minimizaN


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Figure 15. Scheme of the experimental setup showing the reactor system connected with an automated sampling device (ASD) and an FTIR spectrometer.154

decreased with increasing pressure. With 1-octene as a substrate at low pressure, all isomeric aldehydes were formed, whereas at high pressure, only 1- and 2-nonanal were produced. Obviously, in this reaction a high syngas pressure favors the hydroformylation over the isomerization of the starting olefin. In accordance with this conclusion, at a high pressure and with 4octene used as a substrate, mainly internal aldehydes were formed. The ratio of the partial pressures of CO/H2 also affects the result of the hydroformylation. As exemplarily shown with a Rh-calix[4]arene-based diphosphite catalyst with 1-octene as a substrate, a CO/H2 ratio of 2:1 almost blocked the reaction.168 Hydroformylation occurred with equal partial pressures of CO and H2. A slightly higher H2 pressure assists in the formation of the active precursors RhH(CO)2P2, thus improving the rate of the reaction. The best results were obtained with a CO/H2 ratio of 1:2. It should be noted, however, that an excess of hydrogen may lead to the hydrogenation of olefin and product aldehyde as undesired side-reactions. The chosen pressure depends on the activity of the metal (e.g., Rh versus Co catalyst), the ligand, and the reactivity of the substrate. Usually the reaction is conducted at 80−140 °C. For selective hydroformylation often 20−40 °C temperatures are advantageous. Breit and co-workers showed that hydroformylation can even be performed in a Schlenk tube equipped with a cross-type magnetic stirring bar at room temperature and 1 bar syngas pressure, provided a particularly active catalyst is used (Figure 17).169,170 Under these conditions and at a ratio of Rh/ligand/ substrate of 1:3:150 with a self-assembling Rh catalyst, numerous olefins could be almost completely converted within 20 h. With BIPHEPHOS as a ligand similar activity was observed, whereas the reaction with PPh3 or a Xantphos-ligand proceeded much slower.

Figure 16. Detail from the setup shown in Figure 15, showing the autoclave unit (A), from which reaction solution is transferred to the IR transmission cell (C) through a capillary process with the help of a microgear pump (P) and is recirculated.103 A modified version of a Bruker Tensor 27 FTIR spectrometer (S) is placed inside the same hood.

tion)165 and Neymeyr (PCD = pure-component spectral recovery).166 3.6. General Reaction Conditions

The conversion of the olefin, the chemoselectivity, and the regioselectivity toward the formation of the desired aldehyde are strongly dependent on the reaction conditions. In the literature, numerous and varying setups have been described; therefore, only some typical tendencies are quoted here. Albers et al. investigated the hydroformylation of linear olefins at syngas pressures ranging from 7 to 550 MPa with a ratio of CO/H2 of 1:1.167 In general, yield and conversion O


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point in technical hydroformylation, been reflected to some extent in academia; before this, it was exclusively a domain of industrial research. In 2011, van Leeuwen and Chadwick provided an overview about several decomposition pathways of ligands and catalysts used in hydroformylation.174 Beside industrially used ligands and catalysts, more sophisticated catalytic systems were also considered there. Here, only pivotal and some particular aspects will be discussed together with some very recent observations. Basically, every ligand has to compete with CO for coordination sides on the metal center. As a result, metal species are formed that differ in the number of coordinated CO and P-ligands. The shift of the equilibria depends on CO pressure, temperature, excess, and coordination properties of the P-ligand. The latter are determined by electronic and steric properties (section 3.3).63e,175 The situation is shown in Scheme 14 with the example of a typical Rh catalyst. It should be borne in mind that only [RhH(CO)3P] (I, P = monodentate ligand) and [RhH(CO)2P2] (II, P2 = bidentate ligand) represent the desired precatalyst, whereas increasing displacement of the organic ligand by CO produces unmodified rhodium with its typically poor hydroformylation performance (low activity and regioselectivity). Moreover, displacing the Pligand and lowering the CO pressure led to the formation of catalytically less active Rh clusters (“dead end”). With an increasing ratio of Rh/CO, the metal might eventually “plate out”. If bidentate P,P-ligands are used, an increasing CO pressure and higher temperatures favor the monodentate coordination mode, which reduces regioselectivity. An optimal P/Rh ratio results in relation to these features for the catalytic properties of each catalytic system. In general, an excess of P-ligand in relation to the metal (usually 2/1−200/1) is used. With an increase in the P/Rh ratio, the rate of the reaction is affected due to the formation of RhHP4 complexes, which also cause a “dead end” in the reaction. In addition, the P/Rh ratio may influence regioselectivity.176 It is clear that the subtle equilibria described above are dramatically affected by the degradation of P-ligand. In general, the main enemies of the catalyst are oxygen hydroperoxides, carbon dioxide, water, enones, alkynes, and butadienes in the feed. Studies by Nifant’ev and co-workers showed that the Rh/ substrate ratio also influences the rate of decomposition of P(OPh)3.177 Chemical and technology-based measures have been developed to remove these poisons prior to or during the reaction.174 However, product aldehydes, alcohols, and acidic degradation products of phosphorus ligands also may interfere with the original ligand. It should be noted that chlorinated

Figure 17. RTAP (room temperature/ambient pressure) hydroformylation apparatus.169

The hydroformylation can be accelerated with the effect of microwaves.171 At low syngas pressure (40 psi) and a temperature of 110 °C, the reaction with a Rh-Xantphos catalyst was finished within a few minutes. Several terminal olefins were successfully converted under these conditions. The CO/H2 ratio can influence the regioselectivity in a different way. A strong dependency on the substrate has been observed. For example, in the reaction with styrene at 20 °C, by changing the partial pressures of CO and H2 from 85:25 to 25:85 atm the regioselectivity was not affected.172 Only at higher temperatures did the l/b ratio change to some extent. In contrast, in the hydroformylation of vinyl acetate with increasing syngas pressure, the conversion decreased together with the regioselectivity in favor of the branched product.173

4. DECOMPOSITION OF LIGANDS AND CATALYSTS AND MEASURES TO COMBAT THIS 4.1. General Aspects

In contrast to simple textbook theories, catalysts lose their catalytic properties due to degradation over the reaction time. This applies to batch reactions and is even more pronounced in continuous processes. As a result, a group of more or less active and selective catalytic species is responsible for the overall result. Moreover, the properties of the product change over time. Only in recent years has this feature, which is a crucial

Scheme 14. Formation of Different P−Rh Complexes in Dependence on the Concentration of CO and P-Ligand

P


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solvents (1,2-dichloromethane, trichloroethane, and CCl4) also affect the hydroformylation.178 The situation becomes even more complicated due to the fact that the catalytic metal itself is able to stabilize the ligand or to accelerate its deterioration. The mechanism and products of the degradation are dependent on the structure of the trivalent phosphorus compound and can therefore vary even within one class of ligands. Degradation products of ligands do not necessarily lose their catalytic properties, but in comparison to the original system the catalytic performance is altered. 4.2. Degradation Pathways of Ligands

4.2.1. Phosphines. The most prominent degradation reaction of phosphines is their reaction with oxygen or alkylhydroperoxides. The major products are corresponding phosphine oxides.179 Phosphinate esters, phosphonates, and phosphates are being formed in lesser amounts. Phosphine oxides are not entirely inactive as ligands. For instance, triphenylphosphine oxide (TPPO) has a slightly coordinating property and may therefore also contribute to the hydroformylation.73,74 Oxidation has also been observed in RhTPPTS catalyst through the effect of water; consequently, the two-phase reaction might be catalyzed by colloidal particles that form.180 4.2.2. Ligands with P−O Bonds. Phosphorus ligands with P−O bonds are less prone to oxidation but react easily with water to produce the corresponding pentavalent species.181 Water is continuously formed in the continuous reactor by aldol reaction of product aldehydes under hydroformylation conditions and can be removed in some cases as water vapor.182 The mechanism of the saponification is dependent on the electronic and steric properties of organic substituents. As shown in naturally occurring cyclic and acyclic organic phosphates, stereoelectronic effects (e.g., anomeric effect) also play a role,183 but to date no investigations in the field of phosphorus ligands have been conducted in this context. In general, in cyclic phosphites mainly side-chain hydrolysis is observed.184 In nonsymmetric bidentate diphosphite ligands, the acylphosphite unit is hydrolyzed first (Figure 18).185 The trend for hydrolysis increases in the described order; apparently electron-withdrawing substituents in the salicylic ester moiety weaken the oxygen linkage of the acylphosphite unit.185 After hydrolysis of the P−O bond and formation of the pentavalent phosphorus compound, the ligating properties are not completely lost. Secondary phosphine oxides (SPOs) or heteroatom-substituted secondary phosphine oxides (HASPOs) form an equilibrium consisting of a pentavalent and a trivalent species (Scheme 15). The latter is able to bind to metals. Because of hydrolysis and subsequent equilibrium in comparison to the original ligand, new ligands are formed in the catalytic system. SPOs and HASPOs can therefore be considered as “preligands”186 and show typical properties in rhodium-catalyzed hydroformylation.187 Consequently, the results expected with the original ligand are adulterated.188 Moreover, HASPOs add to aldehydes in a temperaturedependent equilibrium.187 Formed α-hydroxy phosphonates represent acids and thus accelerate the decomposition of the original ligand with water by autocatalysis.189 Probably the wellknown stabilizing effect of added epoxides can counterbalance this effect (see section 4.4). Alper and co-workers showed that such α-hydroxy phosphonates can also serve as ligands in Rhcatalyzed hydroformylation.190

Figure 18. Order of hydrolysis in unsymmetric diphosphites.

4.3. Degradation of Rhodium Complexes

Besides the degradation of the noncoordinated ligand, its ligation to the metal may lead to additional deterioration. For example, Abatjoglou and colleagues showed that, in the continuous hydroformylation of propene with PPh3 besides benzene and benzaldehyde, a considerable amount of npropyldiphenylphosphine can be detected as a ligand, which arises from an Rh-mediated replacement of one P-aryl by an alkyl group.191 The conversion is dependent on the partial pressure of the propene. It is assumed that a trinuclear metal cluster plays a central role in this transformation.192 A similar decomposition process was observed with TPPTS.193 Because the alkyldiarylphosphine formed coordinates more strongly to the metal due to its higher basicity, the catalytic properties of the original catalyst are changed. Deactivation is especially rapid when the P−Ar substituents are strongly electron-donating, such as p-methoxy and pdimethylamino.194 Cleavage of the P−Ar bond was also observed during the hydroformylation with phosphinemodified cobalt catalysts, where the formation of orthometalated Co species as intermediates were assumed.195 Alcohols present in the reaction mixtures can modify the original ligand containing P−O bonds. Rhodium-catalyzed intramolecular transesterification with a phosphonite ligand bearing a phenolic group was observed by Selent, Börner, and co-workers (Scheme 16).196 The reaction where a P−C bond is cleaved can be considered as the reverse synthesis of the ligand and does not take place in the absence of rhodium. 4.4. Measures against Degradation

Different measures against deterioration of ligands are suggested, mainly in the patent literature. With regard to phosphite ligands, observations made in the stabilization of polymeric materials, which are widely used as antioxidants, provide additional information.197 Two general methodologies can be differentiated to increase the long-term stability of ligands and catalysts: (i) appropriate steric and electronic Q


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Scheme 15. SPOs and HASPOs as Preligands for the Formation of Rh Complexes

Remarkably stable toward oxidation are dialkylbiaryl phosphines.199 Recently, Williams and co-workers showed that airstable phosphine−boranes can be directly used for the generation of Rh phosphine catalyst; this may help to stabilize the ligand at least until it is released into the reactor.200 Cleavage of the P−BH3 adducts occurs in situ through the excess of CO. The rate of hydrolysis of tri-n-alkyl phosphites was found to decrease in the following order, which clearly accounts for a decelerating steric effect:201

Scheme 16. Rhodium-Mediated Transesterification of a Hydroxy Phosphonite

P(OMe)3 > P(OEt)3 > P(OPh)3 > P(OPr)3 > P(OBu)3

Triethyl and higher phosphites are stable in pure water even at 100 °C. The rate of hydrolysis of triphenyl phosphite is between those of triethyl and tripropyl phosphite.202 Branched alkyl groups enhance the stability toward water as is subsequently illustrated.201,203 P(OPh)3 < (iOctyl)P(OPh)2 < (iOctyl)2 P(OPh) < (iOctyl)3 P

design (premeasurement) and (ii) external stabilization in the presence of additives (postmeasurement). Phosphines with an alkyl group are more prone to oxidation than their aryl counterparts, which should be taken into consideration in the ligand design. The rate of Rh-catalyzed oxidation increases in the following order:198

In polymer chemistry, where phosphites are used as flame retardants, a typical “rule of thumb” is that the cost of highperformance phosphites is directly proportional to their hydrolytic stability. In other words, efforts for their synthesis can be correlated with their resistance toward water, and ultimately, a compromise has to be found for commercial use.

PPh3 < PBuPh 2 < PEt3

Figure 19. Rate of hydrolysis of P−O bond-bearing compounds in water at 60 °C. R


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a life in water of only ∼5 h. However, coordination to Rh enhanced its stability toward hydrolysis to 4 months. Consequently, it could be successfully used even in aqueous two-phase hydroformylation. Several patents claim enhancement of the hydrolytic stability of phosphites by inorganic bases or amines. K2CO3 is useful for stopping the deterioration of phosphoroamidites in water.211 The life of the hydroformylation catalyst RhH(CO)(PPh3)3/ P(OPh)3 could be prolonged from 4 to 10 h by the addition of tri-n-octylamine.212 Back in 1961, Imaev found that the addition of organic and inorganic bases retards the hydrolysis of trialkyl phosphites.213 In this respect, triethylamine is superior in comparison to pyridine. The author assumes that through the formation of a salt the base collects the secondary phosphite that had initially been formed and thus delays further hydrolysis. The different rates of hydrolysis of acyclic and cyclic phosphites have been taken into account by the stabilization of the latter with tertiary amines.214 It should be noted that many amines also catalyze the aldol condensation of product aldehydes, which leads to the formation of high-boiling byproduct.215 Long-chain amines have been used in blends with phosphites to improve their hydrolytic stability in polymers.216 For the same purpose, organic phosphonites and phosphites containing sterically hindered amines (hindered amine light stabilizers = HALS) were developed by Habicher and co-workers.217 In the reaction with water, the amine unit neutralizes the formed acid to form a betaine (Scheme 17). This structure is extremely stable against hydrolysis. Neither H3PO3 nor water at 70 °C for 90 h could cause hydrolysis, which was ascribed to a decrease in the enhanced electron density at the phosphorus atom. This principle was adopted on the Rh-catalyzed hydroformylation with phosphite ligands, where external HALS with long-chain aliphatic bridges has been proposed as intermolecularly acting stabilizers (Figure 22).218 Incorporation of long alkyl chains into the P-ligand enhances its hydrophobicity; therefore, hydrolysis is restricted to the phase boundary. When a mixture of ortho- and para-nnonylphenolphosphites was used instead of the pure orthosubstituted phosphite, a significant increase of the hydrolysis stability was observed.219 A group at UCC suggested the addition of cyclohexene oxide to mitigate depletion of BIPHEPHOS-type ligands (Scheme 18).220 The epoxide reacts with phosphoric acids, which result

The hydrolysis rates of bulky phosphites have been measured in water at 60 °C.204 Bis(2,4-di-tert-butylphenyl) pentaerythritol diphosphite (30),205 which has been used as a ligand in the hydroformylation of allylbenzene derivatives,206 was fully hydrolyzed within 7 h; phosphite 31 completely disappeared after 18 h (Figure 19). The phosphonite 32 was hydrolyzed within ∼90 h. The monophosphite 1c, one of the most preferred monodentate ligands in hydroformylation, did not show any sign of hydrolysis until 400 h of exposure. The rate of hydrolysis of phosphinites is dependent on the CPC angle (Figure 20).207 The reaction is accelerated with an

Figure 20. Rate of hydrolysis in relation to the CPC angle.

increase in the CPC angle. Thus, the phosphinite 33 is stable even after 17 days of treatment with water and has been successfully tested by Pringle and colleagues as a long-term stable ligand for Rh-catalyzed hydroformylation.208,209 A remarkable stabilizing effect of rhodium on the hydrolysis rate of the monophosphite 34 derived from sulfonated calix[4]arene was observed (Figure 21).210 The phosphite has

Figure 21. Sulfonated phosphite and its hydrolysis-stable Rh complex.

Scheme 17. Stabilization Effect of HALS on the Hydrolysis of Phosphites

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configurated C-atoms is unfavorable (Keulemans’ rule).226 Meanwhile there are exceptions to this rule, but they are mostly restricted to olefins with neighboring activating groups, e.g., hydroxyl227 or esters groups,228 which allow the chelation of the substrate to the metal center (“substrate-directed hydroformylation”).229 Moreover, functional groups may alter the “normal” regioselectivity of the hydroformylation of the parent olefin due to an electronic effect. A typical example is styrene, where usually the branched aldehyde is formed. Prior isomerization of the double bond may change the structure of the original substrate. Gladfelter and co-workers observed that, in the presence of a small amount of a Rh diphosphite catalyst under syngas, 1-octene is immediately converted into cis- and trans-2-octene before hydroformylation begins.161 The isomerization is a reversible reaction and dependent on the temperature. Behr et al. investigated the thermodynamic equilibrium of trans-4-octene under similar conditions.230 Within 45 min equilibrium was reached consisting of 2% 1-octene, 11% 2-octenes, and 28% 3-octenes. Only 59% of the starting 4-octene remained. In most industrial bulk processes, single olefins are not available at an economically feasible price; therefore, mixtures of isomers are used as feedstock. Examples are the hydroformylation of “tributene” (trimerized 2-butene)231 or di- and tricyclopentadiene.232 In mixtures of acyclic olefins, besides terminal compounds also internal and branched compounds are present. Because in several cases the production of terminal aldehydes is desired, hydroformylation with prior isomerization is targeted. In the optimal case this tandem reaction is achieved by a single Rh catalyst based on sterically demanding bidentate ligands.233 There have also been some attempts using multicomponent or multifunctional catalysts.234 For example, the high isomerization tendency of homogeneous Ru complexes (e.g., Ru3(CO)12) has been proven in combination with a rhodium hydroformylation catalyst.235,236 High-branched selectivity is predominantly achieved by using an internal olefin together with a catalyst of low isomerization activity.

Figure 22. HALS proposed for stabilization of phosphite ligands.

from phosphite degradation. In this way the autocatalytic cascade effect is stopped. Scheme 18. Epoxides Used to Decelerate Autocatalytic Cascade-Degradation Processes

Fluorophosphites also can be stabilized by continuously adding epoxides.221 In this case, epoxides trap not only phosphoric acids but also HF.222 Alternatively, the addition of homogeneous late transition metal complexes other than rhodium, like Ru3(CO)12, Pt(acac)2, Pd(acac)2, Os3(CO)12, or Co2(CO)8 in combination with sterically hindered groups at the phosphite ligand, has been patented for the stabilization of hydroformylation catalysts.223

5.2. Unfunctionalized Olefins

Important unfunctionalized acyclic alkenes used in industry are in particular ethylene (C2) and propene (C3), isomeric butenes (C4), octenes (C8), and olefins up to a chain length of C18. In general, a distinction is made between short-chain (C3−C4), medium-chain (C5−C12), and long-chain (C13−C19) oxo products. Some linear α-olefins (LAOs), such as 1-butene, 1hexene, 1-octene, or 1-decene, can be extracted selectively from Fischer−Tropsch processes. As exemplarily conducted in Sasol’s SYNTHOL process, a range of olefins with a broad distribution of odd and even carbon numbers can be obtained.237,238 In the case of low-cost ethylene, dimerization may lead to 1-butene. Trimerization or tetramerization affords 1-hexene or 1-octene, respectively.239 In 2008, global consumption of aldehydes was 10 million metric tons, one-third of which was consumed in Western Europe.240 In this connection, the manufacture of chemical commodities is aimed at. It is worthy of note that hydroformylation also has some potential for removing unsaturated compounds from the refinery cracking process. Commonly these undesired olefins, which might produce viscous polymers or solids capable of blocking the carburetors and injectors of vehicles, are converted into harmless alkanes by hydrogenation. For this reason, several studies have been carried out into Rh-

5. SUBSTRATES AND REACTIONS 5.1. General Remarks

In bulk chemical processes mainly nonfunctionalized olefins of different chain lengths are employed, but functionalized substrates are also interesting targets. Principally terminal CC bonds react faster than internal olefins. The rate of the hydroformylation falls with increasing steric hindrance of the substrate.224 The order indicated in Figure 23 was found to be independent of the metal (Rh, Co) used.225 The reaction of branched olefins requires more severe reaction conditions or alternatively a more active catalyst. Principally, the hydroformylation of trifold substituted sp2-

Figure 23. Rate of the hydroformylation in dependence on the steric hindrance. T


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catalyzed hydroformylation of refinery naphtha cuts.241 These investigations will be not be discussed in detail here. 5.2.1. Ethylene. Hydroformylation of ethylene is usually associated with model studies in gas-phase reactions using metals on supports or metal oxides.242 A serious side-reaction under these conditions is the hydrogenation of CO that produces methanol and C2 oxygenates. There is another problem with alkyne impurities, which could probably be overcome in the future by using homogeneous catalysts. Thus, Puckette (Eastmann) found that rhodium complexes based on phosphite ligands tolerate up to 1000 ppm of acetylene in the feed stream and convert ethylene cleanly to propionaldehyde.243 Only a few homogeneous catalysts, such as the Wilkinson complex ([RhCl(PPh3)3]),244 were studied for ethylene hydroformylation to give propionaldehyde (Scheme 19).245 Subsequent aldol reaction with formaldehyde in the

condensation to produce the corresponding unsaturated C8aldehyde.249 The latter is hydrogenated on a heterogeneous catalyst to produce 2-ethylhexanol (2-EH), which is used to manufacture bis(2-ethylhexyl) phthalate (DEHP), the standard plasticizer used in today’s polyvinyl chloride (PVC) industry. Alternatively, esterification with adipic acid produces bis(2ethylhexyl) adipate (DEHA), another plasticizer alcohol, which has also been used as a hydraulic fluid and as a component of aircraft lubricants. Isobutyraldehyde, which is an isomeric byproduct from the hydroformylation of propene, can be reacted with formaldehyde to produce 2,2-dimethyl-1,3-propanediol (neopentyl glycol), as suggested by Reinius, Krause, and co-workers.250 Recycling of propene is likewise possible by pyrolysis at 250− 350 °C, which produces CO and H2.251 5.2.3. Butenes. Butenes are usually derived from Crack-C4 from naphtha steam cracking.252 After the removal of butadiene and isobutene from the crude stream, the so-called Raffinate II contains 1-butene, cis/trans-2-butene, and the isomeric butanes. The nonselective hydroformylation of linear butenes produces n-valeraldehyde and isovaleraldehyde; the former is predominantly required for further transformations (Scheme 21).

Scheme 19. Production of Methacrolein via Hydroformylation of Ethylene

Scheme 21. n-Regioselective Hydroformylation of Isomeric Butenes and a Subsequent Transformation

presence of a base transforms it to methacrolein; the process has been patented by BASF.246 Advantageously, hydroformylation and aldol condensation can be performed as a tandem reaction in an aqueous two-phase system.247 5.2.2. Propene. From a quantitative point of view, the most important oxo chemical is n-butyraldehyde with worldwide annual consumption of >50% of all aldehydes by weight, based on the total weight of all oxo aldehydes consumed. In comparison, the world production of isobutyraldehyde is just 15%. The main producers are, for example, BASF, Dow Chemical, Oxea, and Eastman. A particularly innovative method for the reaction is the aqueous two-phase hydroformylation with a Rh-TPPTS catalyst.20,248 n-Butyraldehyde is used for the manufacture of n-butanol and n-butyric acid (Scheme 20). The main part is subjected to aldol

On a technical scale, the regioselectivity problem can be resolved with a two-step process using two different Rh catalysts, as suggested by Celanese.131 In the first step, 1-butene

Scheme 20. n-Regioselective Hydroformylation of Propene and Important Subsequent Transformations

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Figure 24. Process for the n-regioselective hydroformylation of isomeric butenes (adapted from ref 253).

is converted with a l/b-selectivity of 85/15 using an inexpensive Rh monophosphine catalyst. After removal of n-pentanal, the residue, containing mainly 2-butenes, is subjected to a second hydroformylation procedure under isomerizing conditions with the use of a Rh complex based on a more expensive bidentate P-ligand. Oxeno patented a process for the n-regioselective hydroformylation of Raffinate II, which can be operated continuously.253 Figure 24 shows the general process. The products are isolated by condensation and distillation. The reactor is linked to a filter where insoluble decomposition products of the phosphorus ligand are removed at a lower pressure and temperature. A similar unit is operating at Dow Chemical as part of the UNOXOL 10 process.254 The main application for n-valeraldehyde is transformation into 2-propylheptanol (2-PH) by aldol condensation and subsequent hydrogenation of the product (Scheme 21).253,255 Like 2-EH, 2-PH is also an important plasticizer alcohol. nValeraldehyde is also used as an ingredient in flavoring mixtures. For the isoregioselective hydroformylation of 1butene, which is of minor economic importance, Dow suggested the use of calixarene-based diphosphites as ligands for rhodium.256 The hydroformylation of 2-butene under nonisomerizing conditions produces selectively 2-methylbutanal,257 which can be converted in the presence of an acid, such as boron phosphate, into isoprene (Scheme 22).258 Oxidation and subsequent esterification with ethanol opens up the access to ethyl 2-methyl butanoate, which is an important aromatic ingredient of perfumes and liqueurs. When bidentate phosphorodiamidite ligands (depicted in Figure 8) with four electron-withdrawing pyrolyl groups were used, a dramatic increase in n-regioselectivity was observed.259 5.2.4. Pentenes, Hexenes, Heptenes, Octenes, Nonenes, and Decenes. Isomeric pentenes, hexenes, and octenes and some other olefins with a defined structure have been used

Scheme 22. Formation of Isoprene or 2-Methyl Butanoate via Hydroformylation

frequently in academic research to study the properties of new catalytic systems. For example, 1-pentene and 2-pentene have been used as a model substrate for the isomerizing activity of Ru260 or Rh catalysts.139,261 1-Pentene can be produced by codimerization of ethylene with propene.262 1-Hexene is prepared by SHOP263 or via trimerization of ethylene239,264 and, on a much smaller scale, by the dehydration of corresponding alcohols.265 n-Regioselective hydroformylation of hexene produces 1-heptanal, which is converted into the short-chain fatty acid heptanoic acid by oxidation. The latter is used to manufacture polyol esters and plasticizer alcohols. When hydroformylation of hexene is followed by a reaction with glycerol in the presence of p-toluene sulfonic acid (PTSA), corresponding 5-membered and 6-membered cyclic acetals are formed, which are new fuel additives, partly generated from renewable resources (Scheme 23).266 This reaction has been carried out by the Kragl group in a 60 L pilot plant. 1-Heptene is used as a substrate in the first step of Sasol’s process (using likely Dow/UCC technology) for its homologation to 1-octene.267 The hydroformylation is presumably V


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Scheme 23. Manufacture of Fuel Additives through Hydroformylation/Acetalization from Renewable Resources

Scheme 24. Hydroformylation of 1-Decene and Subsequent Transformations

carried out either with a rhodium catalyst based on PPh3 or with a diphosphite as a ligand, which would eventually produce n-regioselectivities of 85−92%. 1-Heptene is formed by hydrogenation of the aldehyde followed by heat-induced elimination of water. The hydroformylation of mixtures of C8-olefins is a task with large economic importance. A typical example is “di-n-butene”, consisting of isomeric n-octenes, methylheptenes, and dimethylhexenes. This mixture is produced from Raffinate II, where isomeric butenes are dimerized (e.g., by IFP Dimersol268 or Octol process269). Hydroformylation of di-n-butene produces linear and alkyl-branched C9-aldehydes, which are converted to diisononyl phthalate (DINP), another additive for flexible PVC with industrial relevance. For this application, the use of terminal aldehydes is preferred. Nonenes, commonly manufactured by Fischer−Tropsch synthesis, are converted with low n-regioselectivity with Rh complexes of monodentate ligands into isomeric decanals.270 Hydroformylation of 1-nonene in scCO2 has been investigated with monodentate arylphosphite ligands with long alkyl chains at the aromatic rings.271 1-Decanal produced by this protocol is sufficiently pure that it crystallizes directly from the product mixture. Alternatively, mixtures of nonenes were transformed in an aqueous two-phase reaction with water-soluble Rh catalysts based on phosphines bearing sulfonium or carboxylate groups with ammonium or phosphonium anions. 272 When a calixarene-based diphosphite was used as a ligand, the terminal aldehyde was produced almost exclusively.273

Hydroformylation of 1-decene for the production of 1undecanal is an important process in perfume manufacturing (Scheme 24). The first process developed by Ruhrchemie is based on a Co catalyst.274 More recent reports from academic laboratories recommend the use of biphasic systems. Thus, Monflier, Tilloy, and co-workers showed that the presence of methylated cyclodextrines can enhance n-regioselectivity in water.275 Alternatively, the reaction in micellar systems276 or the use of ionic liquids277 or scCO2278 as a solvent has been proposed in order to overcome problems of mass transfer. Bulky P-ligands contribute significantly to the formation of the linear aldehyde.279 After hydrogenation of 1-undecanal, followed by elimination of water, pure 1-undecene can be derived.280 Alternatively, 1-undecanal can be transferred by aldol condensation with formaldehyde and subsequent hydrogenation into 2-methylundecanal (methylnonyl acetaldehyde = MNA).281 MNA is a natural product found in kumquat peel oil. It is a sought-after principal ingredient in perfumery because of its strong fragrance of oranges and incense. Its current market for the production of flavors and fragrances is estimated to be 500−600 t/a. Important producers or/and suppliers are Symrise, Givaudan, Firmenich, and Kao Corporation. Nozaki and co-workers discovered that, with the assistance of the electron-rich Me-BISBI as a ligand, one-pot hydroformylation/hydrogenation is possible to directly produce 1undecanol, which might also be useful for the production of MNA.282 In the reaction, almost double H2 partial pressure was chosen in comparison to the partial pressure of CO. Recently, W


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Scheme 25. Hydroformylation of 1,3-Butadiene and Possibilities of Subsequent Transformations

Scheme 26. Product Distribution in the Hydroformylation of 1,3-Butadiene

A two-step sequence using, in the first step, a phosphinemodified rhodium complex and, in the second step, a cobalt catalyst was claimed by BASF to produce adipaldehyde and the corresponding diol with ∼60% selectivity.290 Mitsubishi reported 37% selectivity in the formation of 1,6-hexanedial with Rh complexes containing diphosphines with a natural bite angle of 102−113°.291 Alternatively, diphosphites as ligands has been suggested by UCC.292 The reaction with conjugated dienes does not necessarily produce the desired dialdehydes. Thus, a rhodium-mediated reaction with 1,3-butadiene may produce mainly n-valeraldehyde with high regioselectivity (Scheme 26).293 The reason for this preference is the intermediate formation of the corresponding α,β-unsaturated aldehyde.294 This compound undergoes hydrogenation much faster than hydroformylation. DuPont has benefitted from this feature by claiming a process for the selective production of 3- and 4-pentenal.295 Isomeric penten-1-ols can be prepared with a selectivity of 87% by hydroformylation of butadiene with a Rh catalyst with the strong basic phosphine PEt3 as a ligand (Scheme 27). Subsequent treatment of the mixture of 3- and 4-penten-ol with a Pd(II)/Sn(II) catalyst in the presence of syngas produced εcaprolactone with a yield of ∼50%.296 Like 1,3-butadiene, other dienes like isoprene,297 cyclohexadiene, and pentadiene also predominantly form unsaturated monoaldehydes under the conditions of Rh catalysis.298 In the hydroformylation of 1,5-cyclooctadiene, the main product is formyl cyclooctane (see also Scheme 13).299 In contrast, cycloheptatriene is formylated twice. Zhang and coworkers investigated the reaction of 1,5-hexadiene with a Rh complex with a tetraphosphorus ligand and obtained pimelaldehyde with up to 98% selectivity.300,301 The hydroformylation of natural monoterpenes frequently containing multiple double bonds is an interesting area for the

Vogt’s group showed that the selectivity toward the formation of the desired alcohol can be improved further by up to 93.2% through reaction in an aqueous alcoholic solution and using Xantphos or P(n-Bu)3 as a ligand.283 5.2.5. Higher Olefins. Because of serious decomposition problems with Rh catalysts during the separation of highboiling products, most commercial plants for long-chain aldehydes (>C10) operate with Co catalysts. These approaches are based on unmodified catalysts under rather severe conditions (30 MPa, 200 °C). Besides alcohols, alkanes are also formed. Through modification of the Co catalyst with phosphines, the pressure can be lowered (90% tricyclodecanedialdehyde (TCD-dialdehyde) within a few hours with unmodified Rh at high pressure.314 The product Y


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Scheme 31. Regioselective Hydroformylation of Citronellene

Scheme 33. Synthesis of Lactic Acid starting from Vinyl Chloride

is used as a starting material for the production of TCD-diol and TCD-diamine (Scheme 32). The former is widely used as a diol component for the production of glass fiber-reinforced plastic and solvent-free fast-drying lacquers.315 The hydroformylation of allenes has not been explored to date. Only recently, Guo and Ma reported about a very rare example, the conversion of 1,2-allenyl phosphine oxides and -phosphonates under the control of [RhH(CO)PPh3], which produces exclusively the terminal aldehyde.316

added. The 2-aldehyde is obtained with excellent regioselectivity. After oxidation of the aldehyde group and replacement of the chloro substituent by a hydroxyl group in basic media, racemic lactic acid might be produced by this method.323 Vinyl ethers as substrates usually direct the attack to the position next to the oxygen atom.324 Moreover, when treated, for example, with a Rh catalyst containing P(OPh)3 as a ligand, vinyl acetate produces mainly 2-acetoxy propionaldehyde (Scheme 34).325 Also with bidentate diphosphine ligands, the branched product was produced.326 In the presence of a weak base, formyl acetates are converted into β-acetoxyketones.327 At higher temperatures and with the use of an unmodified Co catalyst, the monoacylated diol was formed.152 Through modification of the metal with a tridentate phosphorus ligand at relatively low temperature, the original regiochemistry can be reversed and MeCO2CH2CH2CH2OH is formed with >99.9% purity.328 The latter can be hydrolyzed to produce propane-1,3diol (PDO). Alternatively, the ester can be used directly without saponification for the preparation of polytrimethylene terephthalate (PTT) fiber through condensation with terephthalic acid (TPA). Acetals of acrylaldehyde react in dependence of the hydroformylation conditions to methylmalonaldehyde or succinaldehyde monoacetals (Scheme 35).329,330 Cleavage of the acetal and final reduction produces isomeric butanediols. Acrylonitrile was reacted with unmodified Co to produce βformylpropionitrile, which in turn was selectively reduced to γhydroxybutyronitrile.331 In contrast, the use of a Rh complex modified with P(OPh)3 produced mainly α-formylbutyronitrile, which is a starting material for the synthesis of the PMMAmonomer methyl methacrylate.332 In several instances, trapping of the formed cyanopropionaldehyde as acetal proved to be

5.4. Functionalized Olefins

Hydroformylation of functionalized olefins provides access to aldehydes with one or more additional functional groups. Such functionalized aldehydes can be sold as final products or are used as intermediates in the synthesis of fine chemicals, pharmaceuticals, and fragrances. Moreover, the use of unsaturated fatty compounds is an interesting alternative for a nonpetroleum based feedstock. The hydroformylation may be significantly influenced by the functional group/heteroatom (“substrate-directed hydroformylation”)317 and show distinct differences compared to the reaction with unfunctionalized alkenes. These differences are ascribed to the intermediate formation of stable metallacycles affecting the rate and the regioselectivity of the reaction.318 To cleave the metallacycle, the use of high pressure or a reaction in aqueous biphasic systems is frequently required.319 Under these reaction conditions, vinyl groups conjugated with a carbonyl moiety can be hydrogenated.320

5.4.1. α-Functionalized Olefins. Mitsui Toatsu patented the Rh-catalyzed hydroformylation of vinyl chloride to give the thermally unstable 2-chlorpropanal (Scheme 33).321 Because the reaction can take place under much milder conditions than with Co catalysts, the use of Rh is superior.322 To avoid deterioration of the catalyst by HCl, a buffer or an amine was

Scheme 32. Production of TCD-Dialdehydes by Hydroformylation and Subsequent Transformations

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Scheme 34. Hydroformylation of Vinyl Acetate in Dependence on the Catalyst Used

Scheme 35. Hydroformylation of Acrylaldehyde Acetal Using Different Reactions Conditions

Scheme 36. Hydroformylation of Methyl Methacrylate as Part of the Synthesis of a α,α-Branched β-Amino Carboxylic Acid Ester

advantageous.333 2-Trifluoromethyl acrylic acid, a monomer for copolymerization, has been prepared by the Rh-catalyzed hydroformylation of 3,3,3-trifluorpropene followed by oxidation, halogenation, and elimination.334 Acrylamides also are predominantly hydroformylated to the corresponding isoaldehydes.335 Because the amide group directs the reaction toward the attack at the α-carbon atom, ligands for Rh simple phosphites, such as P(OPh)3 or monodentate phosphines, performed better than bidentate phosphorus ligands.336,337 Likewise, the hydroformylation of acrylates usually takes place in the α-position to the functional group.338 Also α-branched acryl acid derivatives can react at the tertiary carbon atom, provided monodentate P-ligands are used, which contradicts Keulemans’ empirical rule.228b,c,339 Hydrogenation of the olefin may take place as an undesired sidereaction. The aldehydes can be transformed into α,α-branched β-amino carboxylic esters, as shown in Scheme 36. Bulky ester groups (tBu), the use of bidentate ligands at high temperature,340 or the use of Zhang’s tetraphos ligands (Scheme 10)341 force the reaction at the terminus. A group at Chirotech Technology showed that, when crotonaldehyde is protected prior to the hydroformylation as cyclic acetal, it can be converted with a Rh-BIPHEPHOS catalyst into glutaraldehyde monoethylene acetal with l/bselectivity of ∼15/1 (Scheme 37).342 The reaction was used for the large-scale synthesis of allysine ethylene acetal, which is a key intermediate in the manufacture of angiotensine-I converting enzyme (ACE) and neutral endopeptic (NEP) inhibitors, such as ilepratil and omapatrilat. Preliminary trials to treat crotononitrile with syngas in the presence of a Rh phosphite catalyst failed and produced only the hydrogenated product.343 Reduction was likewise observed in the Ir-catalyzed reaction of vinyl silanes, whereas a Rh complex produced an excellent yield of corresponding

aldehydes with a slight dominance in favor of the terminal product.42b Hydroformylation of vinyl sulfones and sulfoxides produced only the branched aldehyde, which opens up the possibility of a chiral version of the reaction.344 5.4.2. Vinyl Arenes. Besides 1-octene, styrene is the most frequently used standard substrate for testing new ligands, catalysts,345 and additives346 and is widely used for mechanistic studies.168 Recently, for this reaction, Ley and co-workers recommended a commercially available gas-flow reactor operating at a syngas pressure of 5−25 bar, at 20−100 °C, and with a flow rate of up to 1 mL/min.347 Usually vinyl aromatics and vinyl heteroaromatics348 direct the hydroformylation toward the formation of the branched aldehydes. This result has been rationalized by the formation of a stabilizing η3-allyl intermediate,349 a hypothesis which has been controversially discussed in the literature.350 p-Substituents at the phenyl ring can alter the α-selectivity, which increases in the following order:351 MeO < Me ≤ tert ‐Bu ≤ H < Cl < F

By using a tetraphosphorus ligand (see Scheme 10), it is possible to override the preferred isoregioselectivity of styrene derivatives, but also here an electronic effect of the psubstituent is visible (l/b ratios: p-F-styrene, 14.2; 4-Mestyrene, 19.4; styrene, 21.2; 4-MeO-styrene, 26.0).352 Similarly, bulky ortho-substituents on the aryl rings contribute to the formation of the linear aldehyde.352,353 With a heterogenized Rh(acac)(CO)2 catalyst, m-diisopropenylbenzene can be monohydroformylated under the conditions that the progress AA


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Scheme 37. Chirotech Technology’s Hydroformylation Access to Peptidic Drugs

of the reaction is carefully controlled.354 The product serves as an intermediate for the total synthesis of the floral odorant Florhydral manufactured by Givaudan (Scheme 38).355

active compound. Hydroformylation and reductive amination can even be carried out advantageously in a single step without isolation of intermediate aldehyde and imine (hydroaminomethylation).357 The rhodium-catalyzed hydroformylation of vinyl-1,1-diarenes has been intensively studied by Botteghi’s and Paganelli’s groups and offers the potential of replacing older cobalt-based protocols.358 It was used, for example, in combination with reductive amination for the preparation of the neuroleptic drugs fluspirilen and penfluridol (Figure 25).359 Also racemic tolterodine, a urological drug with annual consumption of several hundred kilograms, can be derived with this protocol.360 This transformation was also the method of choice for the production of fenpiprane, a drug used for functional gastrointestinal disorders.361 It is noteworthy that pheniramines exhibiting antihistamic activity could not be produced by this methodology because the pyridine ring directed the attack toward the undesired α-position in contrast to the generalization of Keulemans’ rule.362 5.4.3. β-Functionalized Olefins. As with vinyl substrates, the nature of the functional group also affects the hydroformylation of functionalized allyl compounds. Zhang’s group established the order of regioselectivity indicated in Figure 26 by testing a rhodium catalyst containing a chiral bidentate phosphine−phosphoroamidite ligand.363 The n-regioselective hydroformylation of allyl alcohol to produce 4-hydroxybutyraldehyde (HBA) is well-documented.364 It can be used to prepare 1,4-butanediol and tetrahydrofuran. Sometimes besides the branched aldehyde, hydroformylation also produces some C3-side products, such as n-propanol and propionaldehyde.365 In first attempts the reaction was carried out with unmodified rhodium.366 Later on Rh complexes based on monodentate367 or bidentate phosphorus ligands368 became the catalysts of choice. Also 1buten-3-ol reacts predominantly in the γ-position to produce αhydroxy-β-methyltetrahydrofuran.369 This transformation can be used to produce substituted dihydrofurans by elimination of water in a subsequent step (Scheme 40).370 Allyl acetate gives, depending on the ligand used, either the branched or the linear product.371 The hydroformylation of

Scheme 38. Hydroformylation of m-Diisopropenylbenzene

Amgen claimed an approach for the total synthesis of cinacalcet (Sensipar, Mimpara) prescribed as a calcimimetic (Scheme 39).356 The synthesis commences with the hydroformylation of m-trifluormethyl styrene. The formed aldehyde is converted by reductive amination into the pharmaceutically Scheme 39. Amgen’s Route to Cinacalcet

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Figure 25. Pharmaceutically important compounds targeted by hydroformylation/reductive amination of vinyl aromatics.

by BASF (Scheme 41).372 Hoffmann La Roche suggested a similar procedure, but based on the hydroformylation of the isomeric 1,4-diacetoxybut-2-ene.373 Several thousand tons of vitamin A are consumed annually. Claver and co-workers have studied effects that govern the regioselectivity of the hydroformylation of acyclic allyl ethers using Rh catalysts with monodentate phosphines and phosphites.374,375 As expected, α-branching enhances the degree of n-regioselectivity. The regiodirecting effect of oxygen in heterocycles was studied with isomeric dihydrofurans and dihydropyrans (Scheme 42).376 In general, the former require conditions

Figure 26. Regioselectivities observed in the Rh-catalyzed hydroformylation of allyl compounds in relation to the functional group with (R,S)-Yanphos as ligand (see also Figure 31).

Scheme 40. Hydroformylation of Allyl Alcohols and Formation of Dihydrofurans

Scheme 42. Isomerizing Hydroformylation of Dihydrofurans

milder than those used for dihydropyrans, which has been rationalized by the more planar structure of the 5-membered ring, hence facilitating the formation of substrate metal complexes. 2H,5H-Dihydrofurans isomerize easily to form the

isomeric butenyl diacetates with unmodified Rh catalysts is a main step in the manufacture of vitamin A, originally developed Scheme 41. BASF’s Approach to Produce Vitamin A

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Scheme 43. Hydroformylation Followed by Reaction with an Amine and Subsequent Hydrogenation (Hydroaminomethylation) for the Production of Antiarrhythmic Agents

corresponding 2H,3H-isomer, which is in turn subjected to reaction with syngas. With the bulky monophosphite 1a as a ligand, predominantly 2-formyl-tetrahydrofuran was formed, whereas the use of PPh3 forced the production of the 3carbaldehyde. A similar feature was noted for dihydropyran. The first protocol may serve as a model for the incorporation of a C1-unit at the anomeric center in carbohydrates.377 A high l/b ratio was observed by Whiteker and co-workers in the hydroformylation of branched allyl silylethers and allyl alcohols with the use of a Rh-BIPHEPHOS catalyst. The reaction was studied within the framework of the total synthesis of the antiarrhythmic agent ibutilide378 and of the methyl ester of the nondrowsy antihistamine fexofenadine (Scheme 43).379 Sheldon and co-workers investigated the n-regioselective hydroformylation of the water-soluble N-acetyl allylamide in an aqueous biphasic system (Scheme 44).380 Only mediocre

Scheme 45. Hydroformylation Pathway to Functionalized Lactams

Scheme 44. Synthesis of Melatonin de Vries and co-workers showed that a Rh complex based on monophosphite 1c hydroformylates allyl cyanide with a l/b selectivity of 77/23.343 At the other hand, the use of chiral bidentate diphosphites preferably formed the branched aldehyde with high enantioselectivity (see section 6.4.3).384 Lazzaroni and Settambolo realized the synthesis of chiral indolizidines with high enantioselectivity starting from optically pure α-amino acids (Scheme 46).385 In the first steps, chiral N-allyl pyrroles serving as substrates were prepared. Subsequent n-regioselective hydroformylation regioselectivities but high activities were noted with a Rh-PPh3 catalyst. In contrast, the use of Xantphos as a bidentate ligand produced a l/b ratio of up to 20/1, but with a much slower reaction rate. The best results were obtained with a watersoluble Rh-TPPTS catalyst. 4-Acetamidobutanal in turn was converted into N-acetyl-5-methoxytryptamine (melatonin), a natural compound that regulates the sleep−wake cycle. Functionalized allyl amides were hydroformylated with a Rh complex containing monophosphite 1c or Xantphos as a ligand (Scheme 45).381,382 The reaction proceeds with excellent n-regioselectivity and provides intermediates for the manufacture of β-lactams frequently prescribed as antibiotics. Subsequent incorporation of an enamido moiety produces compounds with potential anticancer activity. Similarly, N-allyl-phthalimide has been used as a substrate, where with a Rh-BIPHEPHOS catalyst 95% of the desired aldehyde was produced with a l/b ratio of 18/1.383

Scheme 46. Synthesis of Chiral Indolizidines

AD


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Scheme 47. Synthesis of Benzofused Nitrogen Heterocycles by Rhodium-Catalyzed Cyclohydrocarbonylation in Acidic Media

Scheme 48. Synthesis of an Alkaloid via Domino Hydroformylation/Double Cyclization

Scheme 49. Synthesis of Chiral C5 Synthons

bicyclization starts with the hydroformylation of a Nsubstituted allyl amide, producing the linear aldehyde as the main product. It undergoes spontaneous intramolecular cyclization. The final product of this domino reaction is formed by the reaction with the solvent (AcOH). Subsequent oxidation of the acylic keto group to the corresponding ester and reduction with LiAlH4 produced the targeted racemic natural compound with a 33% overall yield over 4 steps. Meek and co-workers synthesized trifunctionalized chiral C5 synthons through hydroformylation of (R)-N-phthalimidovinylglycinol (Scheme 49).388 The linear aldehyde was produced with the assistance of an Rh complex based on the specially designed biphenolphosphite 35. BIPHEPHOS as a ligand produced high conversion but only poor regioselectivity. The formed aldehyde cyclized to the corresponding 6-membered

with an unmodified Rh catalyst produced the aldehydes, which were immediately dehydrated to give the desired fused heterocycles. Ojima and co-workers showed that the hydroformylation of allyl amides catalyzed by Rh-BIPHEPHOS in acetic acid can be advantageously used as the initial step of a cyclization reaction (Scheme 47).386 Under these conditions, the intermediate aldehyde undergoes ring-closure with the participation of the aromatic ring. Final reduction of the amide moiety with LiAlH4 produces crispine A, a potent antitumor agent. Harmicine and analogue compounds were synthesized in a similar fashion; the former is a compound with strong anti-Leishmania activity. Chiou et al. used a similar protocol for the synthesis of the indolizidine alkaloide tashiromine isolated from an Asian deciduous shrub Maackia tashiroi (Scheme 48).387 The AE


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The influence of remote functional groups in unsaturated steroids on the diastereoselective hydroformylation has been examined by Bayon’s group.395 Mann and co-workers used the hydroformylation of a homoallylamine side-chain in the total synthesis of a steroid analogue (Scheme 51).396 n-Regioselective hydroformylation was realized by means of an Rh-BIPHEPHOS catalyst. With the effect of p-toluene sulfonic acid (PTSA), the desired cyclic 6-membered ring amide produced in this cyclohydrocarbonylation was the only product. The isomeric compound was not observed. Final hydrogenation produced desmethylveramiline, a micromolar hedgehog inhibitor. Enantiopure 2-amino-2-phenylethanol served as stereodirecting auxiliary for the preparation of (R)-4-tosylamido-hept-1-ene (Scheme 52).397 The chiral olefin was used as a starting material for the synthesis of pseudoconhydrine, one of the alkaloids from hemlock (Conium maculatum). Three steps were required for the synthesis of the olefinic substrate. Subsequent hydroformylation with a Rh(BIPHEPHOS) catalyst followed by a tandem cyclodehydration mechanism produced exclusively the desired 6-membered ene−sulfonamide. With P(OPh)3 as a ligand, the undesired five-membered ring was also formed together with traces of a hemiaminal. Dihydroxylation of the enamide, acid-mediated formation of the ketopiperidine, diastereoselective reduction of the keto group, and final removal of the tosyl group produced the alkaloid with an overall yield of 37%. de Vries and co-workers hydroformylated cinchona alkaloids with high n-regioselectivity in the presence of a Rh catalyst modified with Mitsubishi Kasei’s tetraphosphite ligand 36104 (Scheme 53).398 The reactions were performed on a 100 g scale and, with the exception of cinchonine, produced good yields of the n-aldehydes. Remarkably, TPPTS as a ligand for Rh failed entirely. The synthetic aim was to link these important chiral auxiliaries for asymmetric catalysis to a heterogeneous surface. Reductive amination or hydrogenation produced the desired building blocks. Breit and co-workers reacted mono-399 and bisallyl azides with syngas in the presence of a Rh-BIPHEPHOS to produce the terminal aldehydes (Scheme 54).400 After hydrogenation of the azido group, ring-closure took place under the formation of piperidines. With appropriately substituted enantiopure azides, this transformation enables total synthesis of the quinolizidine alkaloids lupinine and epiquinamide. Kuraray claimed the hydroformylation of 3-methylbut-3-en1-ol for the production of 2-hydroxy-4-methyltetrahydropyran (MHP), which is a starting material for the production of 3methyl-1,5-pentanediol and β-methyl-δ-valerolactone, both useful for the manufacture of polyesters and polyurethanes (Scheme 55).401 A clear difference was found by comparing the hydroformylation of (1-vinyloxy-ethyl)benzene and (1-methyl-but-3enyl)benzene with an unmodified Rh catalyst. The vinyl ether reacted exclusively in the vicinity of the oxygen atom, whereas the latter was converted into a mixture of both regioisomers.402 The importance of choosing the right ligand to achieve high n-regioselectivity was demonstrated by Cuny and Buchwald in the reaction with 5-hexene-2-one (Scheme 56).383 By using monodentate ligands, the isomeric products were produced in l/b ratios of ∼3/1, whereas with BIPHEPHOS the only observable product was 5-oxoheptanal. Ding and co-workers investigated n-regioselective hydroformylation of the allyl side-chain of enantiopure ethyl

ring hemiacetal, which could be converted into several useful chiral building blocks. With 1c as a ligand of the hydroformylation, the formation of the relevant five-membered ring was favored. The highly isoregioselective hydroformylation of allyl arenes is of considerable interest,389 because the reaction with the monoterpenes eugenol, safrol, estragol, and their double-bond isomers produces aldehydes with many applications in the flavor, perfume, and pharmaceutical industries (Figure 27). For

Figure 27. Starting materials and products for perfume production.

example, 2-selective hydroformylation of commercial safrol produces Helional, a valuable odorous substance with floral and green notes.390 Only poor selectivities were reported with nanodispersed rhodium.391 High n-regioselectivity was achieved with a NAPHOS-based catalyst, whereas the use of bis(diphenylphosphino)propane (dppp) as a ligand predominantly produced the 2-aldehyde with ∼70% selectivity, as shown by Gusevskaya and dos Santos.392 With monodentate phosphites in the reaction with eugenol, up to 84% n-aldehyde was achieved.206 A strong temperature effect was found in the hydroformylation of eugenol and isoeugenol.393 5.4.4. γ-Functionalized Olefins. Hydroformylation of allyl amines can be used to construct N-heterocycles. Thus, Mann and co-workers reacted substituted N-Cbz allylamines with syngas (Scheme 50).394 Scheme 50. Hydroformylation of Allylamides for the Construction of Alkaloids

In methanol the corresponding saturated cyclic N,O-acetal was formed, whereas in THF in the presence of pyridinium ptoluenesulfonate (PPTS) dehydration occurred immediately after the reaction and the corresponding tetrahydropyridines were produced. With appropriate substituents (R), this methodology gives access to the racemic alkaloids coniine, dihydropinidine, anabasine, and some enantiopure tetraponerines. AF


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Scheme 51. Tandem Hydroformylation/Cyclodehydration for the Construction of an Aza Steroid Analogue of Veramiline

Scheme 52. Tandem Hydroformylation/Cyclodehydration for the Total Synthesis of Pseudoconhydrine

Scheme 53. Hydroformylation of Cinchona Alkaloids

cyclopentanone-2-carboxylate (Scheme 57).403 The substrate, which is available in a gram scale, was treated with syngas in the

presence of a specially designed diphosphoroamidite ligand404 to produce the linear aldehyde. A subsequent reaction AG


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reacted several monoalkynes with Rh-BIPHEPHOS at 1 atm syngas pressure and room temperature (Scheme 58).407

Scheme 54. Synthesis of Functionalized Piperidines via Hydroformylation

Scheme 58. Hydroformylation of Alkynes

Scheme 55. Hydroformylation of a Branched Homoallyl Alcohol

Symmetrically substituted dialkyl alkynes produced the expected enaldehydes. Such enaldehydes are inhibitors of the hydroformylation.408 Indeed after some time, the reaction was accompanied by hydrogenation of the product and saturated aldehydes were increasingly formed. Especially with aryl alkynes, hydrogenation of the triple bond becomes a serious competitive reaction. Thus, besides the unsaturated aldehyde, diphenylacetylene also produced cisstilbene. Unsymmetrically substituted dialkyl alkynes exhibited no regiodifferentiation. Scampi et al. reacted β-amino alkynes with syngas in presence of a Rh catalyst and obtained pyrrole derivatives (Scheme 59).409 The reaction is believed to proceed via an unsaturated aldehyde.

Scheme 56. Hydroformylation of 5-Hexene-2-one

Scheme 59. Hydroformylation of β-Amino Alkynes

Scheme 57. Synthesis of an Enantioenriched Spiro[4,4]nonanone

5.6. Fatty Acid Compound

The hydroformylation of unsaturated fatty acids was first investigated in the late 1960s and early 1970s. Thereafter, both university and industrial research became more interested in olefins derived from petrochemical sources. However, a real renaissance of the hydroformylation of these substrates has been observed for the past decade.410,411 The global market for fats and oils from renewable resources amounts to ∼130 million tons, with soybean, palm, rapeseed, peanut, linseed, and sunflower oil being the most important.412 These unsaturated compounds provide an interesting and environmentally friendly alternative to the use of alkenes derived from mineral oil. However, while the petrochemical alkenes are mainly short and sometimes branched hydro-

sequence, including oxidation, esterification, ring-closure of the intermediate 1,4-dicarboxylate, and, finally, decarbonylation, produced spiro[4,4]nonanone with a high optical purity. 5.5. Alkynes

There are only a few studies on the hydroformylation of alkynes.405 First investigations were carried out with conjugated dialkynes, which, in the presence of Rh catalysts, produced formylbutadienes with low yields.406 Buchwald and co-workers AH


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Figure 28. Typical fatty acid compounds.

carbons without substituents, unsaturated fatty acid compounds are linear and predominantly have a chain length of C18 or longer. They contain a carboxylic or ester group (esters of glycerol or methanol) and often more than one double bond (e.g., ω3- and ω6-fatty acids). Ricinolic acid also contains a hydroxyl function. These functional groups may interact with transition metal catalysts, causing their deactivation. Moreover, multiple double bonds are frequently hydrogenated and/or isomerization takes place prior to the reaction with syngas. Therefore, the experimental knowledge accumulated in the hydroformylation of unfunctionalized petrochemicals cannot simply be transferred to the transformation of fatty compounds. The aldehydes produced can be incorporated, for instance, after hydrogenation of ester and/or formyl groups as an alcohol component in plasticizers for PVC413 or in novel polyurethanes.414 The properties of the polyurethanes are dependent on the metal used for the hydroformylation.415 At high conversion rates with a rhodium catalyst, a rigid polyurethane is formed, whereas under the conditions of Co catalysis and low conversion, a hard rubber with lower mechanical strength is produced. Acetalization with glycerol or methanol forms materials with plasticizing properties for polyvinyl chloride (PVC).416 In the past, methyl oleate has frequently been used as a model substrate, but in some cases linoleates and linolenates have also been submitted to the reaction. Technical feedstock usually contains a mixture of unsaturated and saturated fatty acids as shown in Figure 28. With methyl oleate as a substrate, hydroformylation can produce two isomeric formyl stearic methyl esters, but, due to double bond isomerization, other regioisomers with the formyl group between C10 and C18 may also be formed as side products. Pioneering attempts to hydroformylate fatty compounds, in particular oleic acid derivatives, were reported by Lai, Naudet, and Ucciani with Co2CO8 as a catalyst.417 Yields of up to 80% isomeric formyl carboxylic acids could be achieved. To overcome the undesired formation of alcohols under the severe conditions of Co catalysis (250−300 bar syngas pressure, 180 °C),418 Frankel carried out the reaction in methanol as a solvent, from which the aldehydes were trapped as dimethyl acetal (Scheme 60).419 Replacement of the homogeneous cobalt complex with rhodium layered on 5% alumina (activated rhodium catalyst = ARC)420 with PPh3 or P(OPh)3 as modifiers allowed smoother conditions (syngas pressure = 850−900 psi, 95−120 °C).421 It is assumed that the trivalent phosphorus compounds extract the metal from the solid and form the catalytically active species. Hydroformylation of methyl oleate under these conditions

Scheme 60. Hydroformylation of Methyl Oleate with a Co Catalyst and Trapping of the Aldehyde as Acetal

takes place predominantly at the original position of the double bond and gives 9- and 10-formyloctadecanoic acid methylester in more or less equal amounts with an overall yield of >80%. Related to this work, Friedrich and co-workers constructed a batch process including recovery of the Rh catalyst (Figure 29).422 The latter is recycled in a rotary roaster at a high temperature. A preliminary estimate for a hypothetical plant with 1000 t-scale production was made on the basis of this arrangement. da Rosa and co-workers investigated the hydroformylation of a technical-grade soybean oil with Rh-PPh3 catalyst.423 The reaction at 40 bar syngas pressure and 100 °C was complete within 4 h. By comparison, RhCl3 did not catalyze the hydroformylation, but some double-bond isomerization was observed. Conjugation of the double bonds did not affect the rate or the selectivity. Besides esters, oleic acid itself has also been hydroformylated with an ARC modified with PPh3 in an ∼1-kg scale (Scheme 61).424 A subsequent synthetic sequence involving acidcatalyzed esterification, acetalization, and thermal cracking produced methyl 9(10)-methoxymethylenestearate. Natural oils and fats containing, as well as oleates, esters of linolenates and linoleates are hydroformylated with a Co catalyst layered on SiO2 at typically high reaction temperatures of >160 °C to produce mainly the monoformylated products (89−99%).425 This result can be rationalized by the fast isomerization of polyolefins to conjugated double-bond systems.426 The latter are immediately hydrogenated to produce the corresponding mono-olefins, which finally react with syngas. A remarkable feature is the reported high percentage of formyl groups situated at the end of the alkyl chain, which is caused by the high isomerization activity of Co. With an ARC at ∼140 bar syngas pressure and 110 °C, isomerization is almost suppressed and the formation of diformylated products is favored.427 Unsaturated and saturated monoformyl esters were likewise detected as well as triformyl esters derived from methyl linolenate. The formation of 1,4AI


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Figure 30. Predominance of the formation of 1,4-formylated products.

inantly produce monoformyl esters, whereas the corresponding diolefin with isolated double bonds produces mainly diformyl esters. Subsequent transformations of the formylated products were reviewed by Pryde covering acetalization, oxidation with air to polycarboxylic acid, and catalytic hydrogenation to hydroxymethyl compounds.428 These reactions were examined on the basis of mono-, di-, and triformyl fatty esters. Other reactions concerned the reductive amination and the condensation with formaldehyde of the monoformyl esters. Several methods have been tested to separate the PPh3modified ARCs from the products. Thus, Dufek and List suggested extraction with a mixture of aqueous triethylamine and HCN.429 A more practical method is based on vacuum distillation of the crude hydroformylation product, after which the still bottoms containing the decomposed solubilized catalysts are burned off in the presence of spent rhodium on alumina catalyst.430 The rhodium is thereby resupported on the alumina and recycled. The use of a homogeneous rhodium catalyst with a bulky monophosphite allows the conversion of methyl oleate in a yield of 85−90% at a syngas pressure of 20−80 bar and 80−100 °C.431 Mainly 9- and 10-formyl octadecanoic acid methyl esters were produced. The use of a technical feedstock decreased conversion and yield. Detailed investigations showed that cisdouble bonds isomerize rapidly to trans-configurated olefins; the latter undergo hydroformylation only slowly. The kinetics of the hydroformylation of triglycerides with oleic and linoleic functionalities as a model for soybean oil have been investigated by Kandanarachchi et al. in the presence of a homogeneous Rh catalyst modified with PPh3 and P(OPh)3.432 In contrast to methyl esters, the formation of π-allyl Rh complexes does not cause a lowered reaction rate. A particular challenge with rhodium is the isomerizing hydroformylation of fatty acid esters under the formation of ωformyl esters. Behr and co-workers reacted methyl oleate with syngas in the presence of a Rh-BIPHEPHOS catalyst at relatively low syngas pressure (Scheme 62).433 The desired 18formyl stearic acid methyl ester was produced with a yield of 26%. Some higher yields were observed with the methyl ester of linoleic acid. Under the applied conditions besides isomeric aldehydes, also a considerable amount of the hydrogenation product (methyl stearate) was formed, which was attributed to the strong steering effect of the ester group. Proof for this hypothesis came from the reaction with α,β-unsaturated esters, such as ethyl sorbate, which under the same conditions produced exclusively the corresponding hydrogenation products (Scheme 63). Vogl investigated the hydroformylation of individual fatty acid esters and mixtures of fatty acids in sunflower oil with subsequent isolation of the Rh-TPPTS catalyst on ionic liquids.

Figure 29. Batch process for the hydroformylation of methyl oleate with an activated Rh catalyst (ARC) and trapping of the aldehyde as acetal.

Scheme 61. Sequential Reactions of Oleic Acid Involving Hydroformylation, Acetalization, and Elimination

formyl esters in the hydroformylation of methyl linoleate has been explained by the thermodynamic stability of the Rh-acyl complex A over the smaller ring size chelate B (Figure 30). Moreover, it was found that, under these conditions, besides some 1,4-diformyl products, conjugated linoleates predomAJ


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Scheme 62. Isomerizing Hydroformylation of Methyl Oleate

Scheme 65. Hydroformylation of Oleyl Alcohol with a Recyclable Catalyst

Scheme 63. Attempt to Hydroformylate Ethyl Sorbate

ethylhexyl)amine allowed the catalyst to be reused. The formyl alcohols in turn were converted into the corresponding nonadecandiols by hydrogenation on a heterogeneous Ni catalyst. A typical run was performed in a pilot plant with 330 g of olefin.

(Scheme 64).434 Superior results were achieved by using AMMOENG 102. The catalyst could be recycled 9 times;

6. STEREOSELECTIVE HYDROFORMYLATIONS Isoregioselective hydroformylation of olefins, with the exception of propene, produces chiral aldehydes.437 When the reaction is carried out with stereoface control, products with considerable potential for the production of chiral fine chemicals are formed. Stereoface differentiation can be achieved by stereodirecting chiral groups in the substrate or by using a chiral catalyst.438 To enhance the stereoselectivity, sometimes catalysts with chiral ligands are applied to chiral substrates in order to achieve a matched pair effect.439

Scheme 64. Hydroformylation of Methyl Oleate with an Ionic Liquid

6.1. Reaction Conditions

Syngas pressure as well as the ratio of the individual partial gas pressures may influence not only the rate and the regioselectivity (see section 3.6) but also the stereoselectivity. It seems that the effects are strongly dependent on the particular catalytic system, and general conclusions are hard to draw. With styrene and vinyl acetate, the enantiomeric excess (ee) remained constant over a syngas pressure from 3.5 to 28 bar.173 With styrene as a substrate, the enantioselectivity could be improved with increasing CO partial pressure under the condition that the H2 partial pressure was kept constant.440 In contrast, at a constant CO partial pressure, the ee values were not affected because of an increase of the H2 partial pressure. The enantioselectivity of the asymmetric hydroformylation of the same substrates was not affected by increasing the temperature from 40 to 100 °C.173

however, a maximum was already observed after 4 cycles. Conversions of 96% and yields of up to 89% were noted in the best runs. Besides acids and esters, corresponding unsaturated longchain alcohols have also been subjected to the addition of syngas.435 For example, Ruhrchemie claimed the hydroformylation of oleyl alcohol using a sulfonated triphenylphosphine as a ligand under neat conditions (Scheme 65).436 The rhodium catalyst has been modified with phosphine ligands bearing dialkylammonium sulfonate groups. After completion of the reaction, the catalyst was separated from the product by extraction with an aqueous NaOH solution. Acidification of the formed tris(sulfonylphenyl)phosphine sodium (TPPTS) and subsequent neutralization with bis(2-

6.2. Chiral Ligands

An appropriate chiral ligand has to provide not only for high isoselectivity (for exceptions, see section 6.5) but also for high enantioselectivity. In first studies chiral trivalent phosphorus ligands, which have been successfully applied in other asymmetric transformation, were tested, but in most cases they did not meet the high expectations. Therefore, new structures were designed, frequently mimicking motifs of wellestablished ligands of nonasymmetric hydroformylation. Their synthesis is always based on multistep syntheses, which explains their high prices on the market. In a study with chiral bisphospholanes as ligands, maximal enantioselectivity was AK


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Scheme 66. Synthesis of (R,S)-BINAPHOS

Scheme 67. Synthesis of (S,S,S)-Bisdiazophos

mainly by the absolute configuration of the phosphine group.444 Moreover, the relative configuration of the phosphite unit plays a crucial rule in the degree of enantioselectivity. Usually (R,R)BINAPHOS induces inferior ee values.445 Significantly enhanced efficiency for the hydroformylation of a wide range of substrates was achieved with a series of bisphospholane ligands discovered by Dowpharma and Dow Chemical446 in cooperation with Landis’s group.447 In particular (S,S,S)-Bisdiazophos and its enantiomer, belonging to a family of ligands called BDP, is now one of the most frequently used chiral ligands in asymmetric hydroformylation. A typical synthesis is shown in Scheme 67. In the first step in a one-pot reaction, an appropriately substituted azine is reacted with 1,2-diphosphinobenzene in the presence of succinyl chloride to produce a bisphospholane with a yield of ∼30%. After the reaction with an optically pure amine with the assistance of the peptide coupling reagent PyBOP, the corresponding diastereomeric amide is formed, which is finally separated through liquid chromatography.

observed with ligands displaying a P−Rh−P bite angle of almost 85°.173 Another investigation with strongly related diphosphites with bite angles of 59.8−80.0° revealed that, with decreasing bite angle, there is an increase in regio- and enantioselectivity.441 One of the first ligands, (R,S)-BINAPHOS, with potential for the manufacture of chiral fine chemicals, was developed by Takaya, Nozaki, and co-workers 20 years ago.442 A typical synthesis patented by Mitsubishi Gas Chemicals starts with commercial (R)-binaphthol (BINOL), which is esterified with trifluoromethane sulfonic anhydride (Scheme 66).443 A Pdcatalyzed reaction with diphenylphosphine oxide produces mainly the monophosphorylated product. Reduction of the phosphine oxide with silane followed by hydrolysis of the ester group produces a hydroxy phosphine. A subsequent reaction with (S)-binaphthophosphorochloridite gives (R,S)-BINAPHOS. It is important to remember that in asymmetric hydroformylation with BINAPHOS enantioface selection is governed AL


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Figure 31. Representative examples of other chiral ligands developed for asymmetric hydroformylation reactions.

particularly promising results in the asymmetric hydroformylation of alkyl alkenes.455

By replacing the exocyclic oxygen atom in (R,S)BINAPHOS, Zhang and co-workers’s (R,S)-NEt-Yanphos is produced, which significantly improves the enantioselectivities over the parent ligand in the asymmetric hydroformylation of vinyl styrenes (Figure 31).448 Another breakthrough in chiral ligand design came with the disclosure of Chiraphite, originally developed by UCC for asymmetric hydroformylation of vinyl arenes.449 Kelliphite with a biphenol backbone is a ligand developed by Dowpharma.384 A rough estimation of the efficiency of (R,S)-BINAPHOS, Chiraphite, Kelliphite, and some analogues in the hydroformylation of styrene, allyl cyanide, and vinyl acetate can be derived from a parallel screening study where mixtures of olefins were used.450 Claver, Castillón, and co-workers synthesized numerous sugar-based diphosphites such as 37 by varying the type and stereochemistry of sugar backbones.15,451 ESPHOS and related ligands were developed by Wills and co-workers and successfully tested for asymmetric hydroformylation of vinyl acetate.452 With the well-known modularity of Xantphos-type ligands in mind, Reek and co-workers developed chiral phosphine phosphonites such as 38 based on this scaffold.453 In rare cases, also chiral ligands that had been developed for other metal-catalyzed enantioselective reactions (e.g., Rhcatalyzed asymmetric hydrogenations) were successfully tested for the related hydroformylation. Combinations of such approved fragments produces the efficient ligand type BIPHEN, as recently demonstrated by Dr. Reddy’s Laboratories.454 A diastereomeric ligand of the latter, called (Sax,S,S)bobphos (“the best of both phosphorus ligands”), gives

6.3. Diastereoselective Hydroformylations

Many stereoselective hydroformylations benefit from the chirality in the substrate. Chirality can be part of the original substrate, but intermediary incorporation of stereodirecting groups is also possible (“scaffolding”).456 These groups (e.g., acyl, acetal, or ortho-ester groups containing trivalent phosphorus as a ligating atom), which also have an impact on the regioselectivity, are linked to the substrate before or during the reaction. This very recent methodology will not be discussed here in detail. Rhodium-catalyzed reaction of α-(−)-pinene, which can be derived in large quantities from the cluster pine (known as maritime pine in Europe), reacts with syngas in a diastereoselective manner to produce (+)-3-formylpinane (Scheme 68).457 In contrast, the use of an unmodified Co catalyst led to the formation of (−)-2-formylbornane. The Wagner−Meerwein rearrangement is probably mediated by the acidic Co hydrido catalyst. In both cases no chiral ligand was necessary to achieve the observed diastereoselectivity. Prior isomerization of α-pinene to the exocyclic olefin (β-pinene) can be avoided by applying a high CO partial pressure and low temperature. The hydroformylation of enantiopure hydroxyl-functionalized cyclopentenes produces carbohydrate analogue compounds. For example, the reaction of a Bn-protected tetrol with syngas in the presence of the Wilkinson catalyst produced a mixture of three aldehydes that could not be separated AM


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alternatively carried out with a Rh-(R,R)-Kelliphite catalyst (Scheme 72).460 The diastereomerically almost pure product (diastereomeric ratio (dr) = 25/1) was obtained after a single crystallization process. The reaction has been up-scaled and used for the production of multifunctionalized cyclopentane derivatives by hydrogenation and saponification. Use of (S,S)Kelliphite as a ligand produced the opposite enantiomer. The formyl group in the cyclopentanecarbaldehyde has been subjected to Bayer−Villiger oxidation to produce the corresponding alcohol, which takes place while retaining the configuration. Compounds featuring this typically functionalized cyclopentane unit are the platelet aggregation inhibitor compound (39) reported by AstraZeneca, the HCV NS3 NS4A protease inhibitor (40) claimed by Tibotec for the treatment of hepatitis C, and a Neuraminidase (Sialidase) inhibitor (41) discovered by Biocryst for the treatment of influenza (Figure 32). Likewise the widely used nucleoside analogue reverse transcriptase inhibitor Abacavir (42), used to treat AIDS and produced by GlaxoSmithKline, and Medivir’s beta-secretase 1 (BACE 1) inhibitor (43) for the treatment of Alzheimer’s dementia can be derived from this hydroformylation protocol. In 2008, Sherill and Rubin reported on the stereoselective hydroformylation of cyclopropenes (Scheme 73).461 When Tunephos was used as a chiral ligand, which was originally developed for rhodium-catalyzed asymmetric hydrogenation, high diastereoselectivities and good ee values were achieved.

Scheme 68. Diastereoselective Hydroformylation of (−)-αPinene in Relation to the Metal Used

(Scheme 69).458 Fortunately, subsequent hydrogenation of the carbaldehyde group produced alcohols, from which only one isomer was resistant to the conditions of the acid-mediated elimination of benzyl alcohol. Removal of the protective groups from the former produced the desired carba-D-fructofuranose. Liu and Jacobsen used the diastereoselective hydroformylation of an exocyclic cis-butadiene moiety in the total synthesis of the antifungal agent (+)-ambruticin (Scheme 70).459 Reaction with a chiral Rh-(R,S)-BINAPHOS catalyst did not affect the branched double bonds and produced mainly the 2formyl compound in a diastereomeric ratio of 96/4. Diastereoselective hydroformylation of vinyl-β-lactam precursors, which subsequently can be transformed into antibiotics, was realized with a Rh complex based on the chiral phosphine phosphinite ligand (R)-BIPNITE (Scheme 71).382a,c It was assumed that BIPNITE, which was particularly developed for this transformation, is particularly suitable for industrial use because of its high crystallinity. Apparently, the electron-withdrawing nature of the β-lactam plays an important role in favoring the branched regioselectivity. A total yield of aldehyde of 95% was observed. Regioselectivity could be further improved by protecting the secondary amide with Boc.382b As patented by Dr. Reddy’s Laboratories, the diastereoselective hydroformylation of an enantiopure bicyclic lactam, which is produced on a multiton scale by Chirotech, can be

6.4. Isoregioselective Asymmetric Hydroformylation

Usually, asymmetric hydroformylation (AHF) is associated with the isoregioselective reaction of olefins. With the exception of ethylene and propene, chiral aldehydes are produced in this reaction (Scheme 74). One would expect that this type of regioselectivity is supported by sterically less-hindered ligands. However, to achieve high enantioselectivity, an effective steric interaction between the substrate and the catalyst is necessary, which also accounts for bulky ligands. It seems that high isoregioselectivity may contradict in some cases high enantioselectivity and vice versa. Probably this is a reason why enantioselective hydroformylation is less advanced to date compared with other metal-catalyzed asymmetric reactions, such as hydrogenation. Only a small number of highly selective

Scheme 69. Diastereoselective Hydroformylation of a Chiral Cyclopentene

AN


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Scheme 70. Synthesis of (+)-Ambruticin Based by Diastereoselective Hydroformylation

Scheme 71. Diastereoselective Hydroformylation of a Vinyl-β-lactam Precursor

Scheme 72. Diastereoselective Hydroformylation of a Bicyclic Lactam and Subsequent Transformations

the stereoface-discriminating abilities of new chiral ligands.464 In spite of these more academic efforts, enantioselective synthesis of pharmaceutical aryl propionic acids on an industrial scale by hydroformylation remains a challenge. The high manufacturing costs of the vinyl substrates and the hitherto achieved low catalytic activities in the hydroformylation have to compete with other methods of preparation (mainly resolution of racemates). Moreover, administration of the racemic mixture can be advantageous, because in the human body the less-active enantiomer (distomer) is converted into the more active one (eutomer) (a typical example concerns ibuprofen) or the eudismic ratio is too small and, therefore, the stereoeselective synthesis and medical application of the single enantiomer are not meaningful.465 Vinyl heteroatomatic compounds could also be successfully converted into the branched aldehydes. Such aldehydes may be

catalytic systems are known, and the range of substrates has been extended only over the past few years. The problem of high isoregioselectivity is less pronounced in functionalized prochiral substrates, like vinyl arenes or vinyl ethers, which direct the reaction in favor of the branched product (see sections 5.4.1 and 5.4.2). 6.4.1. α-Functionalized Olefins. To date, most efforts have focused on asymmetric hydroformylation of vinyl arenes to get access to enantiomerically pure 2-aryl propionic acids (Scheme 75). Some of these compounds constitute a class of nonsteroidal inflammatory drugs, such as (S)-naproxen, (S)ibuprofen, (R)-flurbiprofen, and (S)-ketoprofen.462 For this reason, asymmetric hydroformylation of styrene and related vinyl arenes derivatives has been addressed frequently in the literature.463 Styrene usually induces high isoselectivity (see section 5.4.2) and is therefore particularly suitable for testing AO


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Figure 32. Pharmaceutically useful products, which may be derived from the highly stereoselective hydroformylation of a bicyclic lactam.

hydroformylation was followed by oxidation of the formed aldehyde with sodium chlorite to produce the desired acid in 84% ee. 1,2-Substituted alkenes, like cis- or trans-1-propenylbenzene, stilbene, and indene, have been converted with a Rh-(S,S,S)Bisdiazophos catalyst into the corresponding aldehydes with turnover numbers (TONs) of 65−500 and in up to 96% ee (Figure 33).467 With p-substituted styrenes, the isoregioselectivity increased with more electron-withdrawing substituents. Interestingly, no correlation with the enantioselectivity was observed. It is worth mentioning that the partial pressures of CO/H2 were varied from 120/40 psi (α-olefins) to 35/35 psi (1,2-disubstituted acylic olefins) and 75/75 psi (cyclic olefins). The highly enantioselective hydroformylation of heteroatomsubstituted olefins, like vinyl carboxylates, can be achieved advantageously with a Rh-(R,S)-BINAPHOS catalyst at 100 bar syngas pressure and 40−80 °C.444a Regioselectivites in favor of the branched aldehyde of 84/16 to 96/4 and ee values up to 98% have been observed. The catalyst is likewise suitable for enantioselective conversion of fluoro-substituted olefins. Finetuning of the parent ligand (R,S)-BINAPHOS can be achieved by modifying the P-aryl groups, which is particularly beneficial for the hydroformylation of styrene, 1-hexene, (Z)-2-butene, and indene.468 Stahl, Landis, and co-workers reacted vinyl amides (N-acetyl, N-Boc, N-Cbo, N-CF3CO, and phthaloyl) with syngas in the presence of the (S,S,S)-Bisdiazophos-based Rh catalyst and achieved excellent b/l ratios and excellent ee values.469 Tertiary amides or substitution at the olefin produced lower levels of enantioselectivity. With this method β3-aminoaldehydes, which are valuable precursors for the synthesis of nonproteinogenic amino acids, can be produced (Scheme 77). Under the applied conditions the undesired isomerization of the terminal into the nonreactive internal enamides could be strongly suppressed.

Scheme 73. Stereoselective Hydroformylation of 3,3Disubstituted Cylopropenes

Scheme 74. Isoregioselective Hydroformylation as a Precondition for Asymmetric Hydroformylation

reduced to the corresponding primary alcohols and serve as building blocks for the synthesis of complex biomolecules. Alternatively, oxidation of the formyl group gives access to propionic acid derivatives. Thus, Nozaki and co-workers showed that vinylfuranes and vinylthiophenes react with high regio- and enantioselectivity to the desired branched aldehydes in the presence of a Rh catalyst with a slightly modified (R,S)BINAPHOS ligand (Scheme 76).466 The protocol was used for the synthesis of (S)-tiaprofenic acid, one of the most popular nonsteroidal anti-inflammatory drugs. To achieve this aim, AP


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Scheme 75. Asymmetric Hydroformylation of Vinyl Arenes and Chiral Products with Relevance to Pharmacology

based on (S,S,S)-Bisdiazaphos and proceeds with >90% conversion and a TOF of 19 400 h−1.471 The aldehyde was obtained with 96.8% ee. Tetraglyme was used as solvent to enable easy product recovery by vacuum distillation and catalyst recycling. In turn, α-acetoxyacetaldehyde was converted into the corresponding amino alcohol, isoxazolines, or imidazol derivatives. The enantiomeric products can be manufactured by the use of (R,R,R)-Bisdiazaphos as ancillary ligand. The same catalytic system proved to be advantageous for the reaction with a more sophisticated vinyl acetate as a substrate (Scheme 81).472 Thus, the Z-configurated olefin was hydroformylated to produce exclusively the α-branched aldehyde. Without isolation, the latter was reacted with Bestmann’s reagent in a Wittig olefination process to produce a chiral macrocycle. Enzyme-catalyzed removal of the O-acetyl group yielded (+)-patulolide C with a diasteromeric ratio (dr) of 96.6:3.4, a compound exhibiting both antifungal and antibacterial activity. Dihydrofurans independent of the position of the double bond were hydroformylated with a Rh catalyst based on chiral sugar-based diphosphites451c or a phosphine phosphonite Xantphos-type ligand453 to produce, in the best cases, exclusively 3-formyltetrahydrofuran in 84−91% ee (Scheme 82). Enantioselective hydroformylation of dialkyl acrylamides was investigated by Clarke and co-workers in detail.337 It was found that these substrates undergo hydroformylation more slowly than styrene. Up to 82% ee was realized in the best trials. A serious problem is caused by the epimerization of chiral aldehydes by unmodified rhodium hydride complexes. Therefore, the reaction times should be kept short and low temperatures are recommended. Moreover, aldehydes that

Scheme 76. Synthesis of (S)-Tiaprofenic Acid via Hydroformylation

Rh-(S,S,S)-Bisdiazophos is also a highly efficient catalyst for the conversion of isomeric 4H,5H-pyrroles, where predominantly the 2-formyl compound was formed with excellent ee values (Scheme 78). By using the 4-membered ring homologue, the formyl group was mainly attached at the 3-position. The corresponding 6-membered ring heterocycle did not react. In 2012, Clemens and Burke synthesized both enantiomers of Garner’s aldehyde by asymmetric hydroformylation of N-Boc protected 2,2-dimethyl-1,3-oxazoline (Scheme 79).470 In the presence of Rh-Bisdiazophos, the formyl group was linked in the α-position to the nitrogen and the chiral C3-building block was formed with >90% ee. Yields, regioselectivities, and ee values varied slightly with the use of enantiomeric Bisdiazophos ligand. A similar catalytic system was used for asymmetric hydroformylation of vinyl acetate on a 150−180 g scale (Scheme 80). The reaction proceeded with a rhodium catalyst

Figure 33. Chiral aldehydes derived from the stereoselective hydroformylation with Rh-(S,S,S)-Bisdiazaphos. AQ


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Scheme 77. Production of a β3-Amino Acid via Hydroformylation

Scheme 78. Asymmetric Hydroformylation of N-Protected 2Pyrroline

1,3-Pentadiene was also converted with good chemoselectivity, but only very poor enantioselectivity independent of the configuration of the second double bond was observed. Recently, similar values were reported by Watkins and Landis using (S,S,S)-Bisdiazophos as a ligand; they also broadened the scope of the substrates.474 These authors also observed a remarkable dependency of the enantioelectivity on the doublebond isomers of the butadiene. 6.4.3. Allyl Derivatives. Allyl substrates, such as allyl alcohol, O-Bn and O-silyl protected allyl alcohols, and allyl acetals, were converted with a Rh catalyst based on (S,S,S)Bisdiazophos with moderate regioselectivity but excellent enantioselectivity (86−97%) into the corresponding chiral aldehydes.469 This method allows inexpensive production of the so-called “Roche aldehyde”, which is usually prepared from the “Roche ester” via a more expensive pathway (Scheme 83). The linear aldehyde was removed from the branched one by flash chromatography to obtain the chiral aldehyde with no loss of enantioselectivity. Allyl cyanide was converted with Rh-(R,S)-BINAPHOS with a b/l ratio of 72/28 and 66% ee.343 Researchers from Dowpharma were able to improve this result using (R,R)Kelliphite to a ratio of b/l of 20/1 and 80% ee (Scheme 84).384 After optimization of the reaction conditions, the hydroformylation was carried out in a 0.93-mmol scale of substrate in a 300 mL vessel. Hydrogenation of the functional groups has been carried out in two steps by using two different heterogeneous catalysts. The chiral methyl-substituted 1,4amino alcohol is useful for the synthesis of Merck’s nonpeptide gonadotropin-releasing hormone antagonist or a novel

Scheme 79. Preparation of Garner’s Aldehyde via Asymmetric Hydroformylation

form should be immediately reduced to the corresponding alcohol or converted into acetals. 6.4.2. 1,3-Butadienes. Back in 1996, Nozaki, Takaya, and co-workers found that, by using a Rh(R,S)-BINAPHOS catalyst, 1,3-butadienes can be converted into the β,γunsaturated aldehydes with high regioselectivity and enantioselectivity (Table 1).473

Scheme 80. Asymmetric Hydroformylation of Vinyl Acetate and Subsequent Transformations

AR


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Scheme 81. Synthesis of a Macrocyclic Natural Compound Using Asymmetric Hydroformylation

lyst.363 The choice of the N-protective group (Boc, Bn, pNO2PhSO2, p-MeOPhSO2, Ts, or phthaloyl) influences the ee values to some extent, but influences the b/l-regioselectivity even more. Finally, the reaction with N,N-bis(Boc)-N-allylamine, where 99% ee was achieved, was carried out in a 10mmol scale with a ratio of substrate/Rh = 10 000/1 for 24 h without affecting the regio- and stereochemistry (Scheme 85). The N-monoprotected Boc-product was converted into the corresponding β2-amino carboxylic acid by oxidation or into the corresponding 1,3-hydroxyamine by reduction. Both are valuable chiral building blocks. Alternatively, the aldehyde can be used as a starting material for the construction of the leukocyte adhesion inhibitor cyclamenol A. Prochiral 7-membered acetals have been transformed with an Rh catalyst based on (R,S)-BINAPHOS or sugar-based diphosphites into the 5-formyl compound with good enantioselectivity (Scheme 86).475 The enantio-enriched compounds can be converted in 4 steps into 3-hydroxymethyl butyrolactone, an important chiral building block. By using the same catalysts in the hydroformylation of Nprotected 2H,4H-pyrrole, almost exclusively the saturated 3formyl compounds were produced in up to 71% ee (Scheme 87).475 Under these conditions the N-Boc-protected substrate produced a lower ee value than the corresponding N-acetyl derivative. This problem can be overcome with a Rh-(S,S,S)Bisdiazophos) catalyst that produces up to 91% ee with this particular substrate.469

Scheme 82. Asymmetric Hydroformylation of Isomeric Dihydrofurans

Table 1. Representative Enantioselectivities Achieved in the Asymmetric Rh-Catalyzed Hydroformylation of Butadienes with Two Different Chiral Ligands

6.5. n-Regioselective Asymmetric Hydroformylation

a

The problem of high isoregioselectivity desired for asymmetric hydroformylation can be overcome by taking benefit from Keulemans’ rule.476 According to this rule, 1,1-substituted alkenes react predominantly at the terminus. In the presence of two different substituents at the terminal olefin, the product aldehyde becomes chiral, and thus, the tricky compromise between high regio- and/or high enantioselectivity discussed previously can be avoided (Scheme 88). The only challenge for the catalyst is to provide for an efficient stereodiscrimination of both enantiotopic faces of the prochiral substrate. Recently, Wang and Buchwald used this strategy to generate chiral ß-formyl esters from α-alkyl acrylates, which were formed in good yields and high enantioselectivities (Scheme 89).469,477 Remarkably, prominent ligands developed for asymmetric hydroformylation produced poor results. The optimal catalyst was based on the ligand (R,R)-BenzP* designed for other asymmetric metal-catalyzed reactions. To achieve full conversion, a 1:5 mixture of CO/H2 was used.

b/[l+α,β-unsaturated achiral aldehyde].

tachykinin NK1 receptor antagonist developed by Ono Pharmaceuticals. Various N-protected allyl amines were converted into the corresponding chiral aldehydes with good isoselectivity and excellent enantioselectivity using a Rh-(S,R)-Yanphos cataAS


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Scheme 83. Alternative Synthetic Access to the Roche Aldehyde by Asymmetric Hydroformylation

Scheme 84. Asymmetric Hydroformylation of Allyl Cyanide and Subsequent Steps to Pharmaceutically Interesting Compounds

Scheme 85. Asymmetric Hydroformylation of Allyl Amide and Subsequent Steps to Chiral Building Blocks or Cyclamenol A

in-house production is not efficient. The price of CO or syngas can vary considerably on the market. Special safety conditions for the transportation of the highly toxic gases may also contribute to the current high price and can be decisive as regards the economic efficiency of a process. For several years there has been increased interest in easierto-handle alternatives for syngas.478 Back in 1994, Somasunderam and Alper successfully used formic acid as a hydrogen source together with CO gas for the hydroformylation of 1decene in the presence of a heterogeneous Rh catalyst.479 The use of an aqueous solution of formaldehyde (formalin) or paraformaldehyde seems even more promising.480 Recently, Morimoto and co-workers showed that the use of two different Rh catalysts (Rh-BINAP and Rh-Xantphos) can

Chiral aldehydes can be used for the construction of biologically active compounds containing a 2-isopropyl- or a cyclohexyl-1,4-dicarbonyl moiety prepared by Pfizer or Roche.

7. ALTERNATIVES TO SYNGAS Syngas can be manufactured by partial oxidation technology or by steam reforming. In general, as a side-product of the water gas shift reaction, it can be derived from almost every carbon source. Besides low-boiling hydrocarbons, also heavier oils and byproducts from various processes, including hydroformylation, have been employed. Sometimes, besides coal also other solid materials like biomass and waste plastics are used. Most large companies engaged in hydroformylation run their own facilities for the production of syngas. For small-scale hydroformylation, AT


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Scheme 86. Asymmetric Hydroformylation of Unsaturated 7-Membered Acetals

Scheme 90. Hydroformylation with Formaldehyde

under pressure. The reaction can be accelerated with the effect of microwaves.482 Alternatively, CO can be derived in situ from the reversed water gas shift reaction (RWGS) (Scheme 91). With an excess Scheme 91. Hydroformylation Using CO from RWGS

Scheme 87. Asymmetric Hydroformylation of N-Protected 3Pyrrolins

of H2, syngas is formed. This procedure was used for hydroformylation of olefins by the groups of Tominaga483 and Haukka.484 Homogeneous Ru complexes, such as Ru3(CO)12, H4Ru4(CO)12, [Ru(bpy)(CO)2Cl]2] (bpy = bipyridine), or [PPN]Ru(CO)3Cl3 (PPN = (NPPh3)2) were used as catalysts. In general, increasing total pressure of H2 and CO2 promotes the reverse water gas shift, and the yield of the hydroformylation product is enhanced.485 Additives like LiCl, Li2CO3, or ionic liquids ([BMIM]Cl) prevent the direct hydrogenation of the alkene. Because of the high hydrogenation activity of the Ru catalysts, aldehydes that formed were immediately reduced to the corresponding alcohol. This can be avoided by an increase of the CO2 pressure.485 There is no doubt that these and other syngas-free systems will be of particular value for the manufacture of fine chemicals, provided their efficiency can be improved in the near future. An intrinsic problem that has to be overcome is faced by the CO generation in situ, which usually leads to low partial concentration of CO and, therefore in consequence, to low reaction rates of hydroformylation.

Scheme 88. Production of Chiral Aldehydes by nRegioselective Asymmetric Hydroformylation

Scheme 89. Asymmetric Hydroformylation of αAlkylacrylates and Pharmaceutically Useful Products

8. CONCLUSIONS AND OUTLOOK Hydroformylation is one of the most important homogeneously catalyzed reactions on an industrial scale. A clear dominance in the manufacture of bulk chemicals can be observed. Large-scale Co- and Rh-based processes are mature technologies that have been developed over the past 60 years. Currently, the search for more stable ligands and more active and regioselective catalytic systems is the focus of research. Interestingly, the potential of hydroformylation for the production of fine chemicals, especially for the manufacture of optically pure compounds, has not been fully explored to date. The reasons for this disparate behavior are the enormous price of the chiral ligands and the average activities and selectivities of corresponding catalysts. Moreover, safety issues in working with the toxic CO, relatively high investment costs, and the current high price of carbon monoxide hampers the broad use of this innovative technology for the production of fine chemicals. There is no doubt that mainly economic reasons

be beneficial, one for the decomposition of formaldehyde and the second for the hydroformylation of olefins (Scheme 90).481 With this system, aldehydes could be produced with a yield of 95% and excellent n-regioselectivity without the use of CO AU


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will reopen the discussion about alternative syngas sources and the best metals in industrial hydroformylation in the near future.

on catalyst development for olefin hydroformylation and the application of situ NMR and FTIR spectroscopy.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies

Armin Börner studied education and chemistry at the University of Rostock and completed a Ph.D. on the synthesis of carbohydrates in the group of Prof. Dr. H. Kristen in 1984. Between 1984 and 1992, he was a scientific co-worker in the field of complex catalysis at the Academia of Science under Prof. Dr. H. Pracejus. After finishing a postdoctoral term in the group of Prof. Dr. H. B. Kagan in Orsay/ France in 1993, he went to the Max-Planck-Group for Asymmetric Catalysis where he was awarded his professorial research degree (Habilitation) in 1995. Since 2000 he has been professor for organic chemistry at the University of Rostock and head of a research department in the Leibniz-Institute for Catalysis (LIKAT). His research focuses on applied homogeneous catalysis.

Robert Franke studied chemistry with focuses on industrial chemistry and theoretical chemistry at Bochum University in Germany. He earned his doctorate degree in 1994 in the field of relativistic quantum chemistry under Prof. Dr. W. Kutzelnigg. After working for a period as a research assistant, in 1998 he joined the process engineering department of the former Hüls AG, a predecessor company of Evonik Industries AG. He is now Director Innovation Management Hydroformylation. He was awarded his professorial research degree (Habilitation) in 2002, since when he has taught at the University of Bochum. In 2011 he was made adjunct professor. His research focuses on homogeneous catalysis, process intensification, and computational chemistry.

ACKNOWLEDGMENTS We appreciate the helpful discussions with Stephan Doerfelt (Bitterfeld), Clark R. Landis (Madison), Susan Lühr (Rostock), Joost N. H. Reek (Amsterdam), and Dieter Vogt (Eindhoven). We thank all referees for their stimulating and valuable advices. REFERENCES (1) Roelen, O. (to Chemische Verwertungsgesellschaft Oberhausen m.b.H.) German Patent DE 849548, 1938/1952; U.S. Patent 2327066, 1943; Chem. Abstr. 1944, 38, 3631. (2) In 1949 the name “oxo process”, which refers to the fact that ethylene and some hindered alkenes react with synthesis gas to aldehydes and ketones, was replaced by “hydroformylation”, but it is still used in industry. Adkins, H.; Krsek, G. J. Am. Chem. Soc. 1949, 71, 3051. (3) Naqvi, S. Oxo Alcohols. Process Economics Program Report 21E; SRI Consulting: Menlo Park, CA, 2010. (4) Handbook of Homogeneous Hydrogenation; de Vries, J. G., Elsevier, C. J., Eds.; Wiley−VCH: Weinheim, Germany, 2007; Vol. 1−3. (5) Protzmann, G.; Wiese, K.-D. Erdöl, Erdgas, Kohle 2001, 117, 235. (6) Rhodium Catalyzed Hydroformylation; van Leeuwen, P. W. N. M., Claver, C., Eds.; Kluwer Academic Publishers: Dordrecht, Netherlands, 2000. (7) (a) van Leeuwen, P. W. N. M.; Kamer, P. C. J.; Reek, J. N. H.; Dierkes, P. Chem. Rev. 2000, 100, 2741. (b) Torrent, M.; Solà, M.; Frenking, G. Chem. Rev. 2000, 100, 439. (8) (a) Bohnen, H.-W.; Cornils, B. Adv. Catal. 2002, 47, 1. (b) Wiese, K.-D.; Obst, D. In Catalytic Carbonylation Reactions; Beller, M., Ed.; Topics in Organometallic Chemistry 18; Springer: Heidelberg, Germany, 2008; pp 1−33. (9) Breit, B. Top. Curr. Chem. 2007, 297, 139. (10) (a) Dwyer, C.; Assumption, H.; Coetzee, J.; Crause, C.; Damoense, L.; Kirk, M. Coord. Chem. Rev. 2004, 248, 653. (b) Damoense, L.; Datt, M; Green, M.; Steenkamp, C. Coord. Chem. Rev. 2004, 248, 2393. (c) Kamer, P. C. J.; van Rooy, A.; Schoemaker,

Detlef Selent received his Ph.D. in 1982 for his work on ethene hydroamination in the laboratory of Prof. Dr. R. Taube at the Technical University of Leuna-Merseburg, Germany. He has been a researcher at the Academy of Sciences of the GDR and of the Center of Heterogeneous Catalysis in Berlin. From 1994 to 1996, he was a WIP fellow of the German Federal Ministry of Research and Technology at the Institute of Inorganic and Analytical Chemistry of the Technical University Berlin, studying heterometallic early late transition metal complexes as models and precursors for heterogeneous catalysts. Being a group leader at LIKAT, he now concentrates AV


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