Recent developments in the homogeneous hydrogenation of carboxylic acid esters

Abstract

A concise review highlighting the more successful catalyst designs used in homogeneous hydrogenation of carboxylic acid esters is presented. An interesting range of multidentate and heterobidentate ligands have been employed, generally as complexes of ruthenium, but the use of osmium and iridium catalysts is also noted. An estimate of the relative reactivity of the catalysts is made based on average turnover frequencies for comparable substrates. Issues relating to substrate scope are also discussed.

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Background

The most widely used methods for reducing esters to alcohols involve the use of LiAlH₄ and BH₃·THF. However, these reagents are not ideal from the perspective of cost, safety and atom economy or simplicity of work-up procedures. Catalytic hydrogenation can therefore provide a more efficient method for this reduction. Heterogeneous hydrogenation is a relatively mature technology suitable for large scale reductions. This heterogeneous catalytic method is likely to be less useful for the production of functionalised fine chemicals and pharmaceuticals intermediates, or for organic syntheses. This is because heterogeneous hydrogenation catalysts generally also reduce, or cause hydrogenolysis of many other functional groups such as nitro, nitrile, ketone, alkene, halides, heterocycles, imines, benzyl ethers (OBn), and Bn or Cbz protected amines. Indeed, in the heterogeneous hydrogenation of dimethyl benzene carboxylate, even the aromatic ring is hydrogenated before the ester function is also reduced.¹ There is therefore a clear demand for more functional group tolerant ester hydrogenation technology, and while new classes of chemoselective heterogeneous catalysts would also be valuable, this is most obviously achieved using homogeneous catalysts. The last 10 years have seen some important developments in this area, to the point that this reaction can be seriously considered as a step in a target synthesis (and in fact has been used at 2200 Kg scale). This mini-review does not attempt to provide a comprehensive coverage of this field, but will hopefully provide the reader with an indication of some of the structural features present in successful catalysts and the applications that can be realised at this point in time. It is hoped some people reading this will be tempted to use this reaction in a synthesis, or to investigate new ligands and invent new applications.

Possibly the first report of homogeneous ester hydrogenation was made by Grey et al. in 1979.² In this paper and in several subsequent papers using different catalysts, it became apparent that esters containing strongly electron-withdrawing groups were the only substrates to give any reduction product.³ Dimethyl oxalate is one such activated substrate, and while it could be hydrogenated to methyl glycolate at 180 °C, a double hydrogenation to ethylene glycol was challenging, even at 220 °C. With this background in mind, the results of Elsevier and co-workers were quite a step forward.⁴ A catalyst derived from [Rh(acac)₃] and the facially co-ordinating TRIPHOS ligand gives high conversion to ethylene glycol at 120 °C and 80 bar pressure (TON = 160). Other ligands like PPh₃, dppe and even the meridinally co-ordinating PhP(C₂H₄PPh₂)₂ did not produce any of the double hydrogenation product. Using the Ru/TRIPHOS system, but with 24% HBF₄ promoter, dimethylphthalate could be hydrogenated all the way to 1,2-dihydroxymethylbenzene and less activated benzyl benzoate was also hydrogenated. Some recent observations on Ru/TRIPHOS catalysed hydrogenation of dimethyloxalate show that the reaction is zero order in dimethyl oxalate substrate, with the kinetic data pointing towards the second hydrogenation of methyl glycolate being of similar difficulty to the first using Ru/TRIPHOS.⁵ In any case, while these catalysts are quite modest by the very latest standards, they were clearly influential to other scientists starting out in this area and multidentate ligands are most frequently employed in the hydrogenation of esters.

Catalysts derived from tridentate and tetradentate ligands

In 2004, we began a project investigating chiral tridentate P,N,X (X = NR₂, OH, SR, PR₂) ligands (e.g.1, Fig. 1) in a range of challenging hydrogenations⁶ and ester hydrogenation was one of the first applications we investigated. Catalyst 2 was found to be moderately effective in the hydrogenation of activated esters such as dimethylphthalate or perfluorobutanoic acid–methyl ester at around 140 °C.^6a However simple aromatic esters initially gave poor conversions. An oversight in this initial work was the assumption that a base/catalyst ratio of 2 : 1 would be sufficient in this catalysis. This assumption was made since this was found to be a good ratio in the enantioselective hydrogenation of ketones using this catalyst. However, subsequent publications that will be described later always make use of quite a high base/catalyst ratio (around 50 : 1). When the system was revisited using base/catalyst ratios of 30–50, massively increased reactivity was seen.^6e

Fig. 1 Some of the ester hydrogenation catalysts that are derived from tri- and tetra-dentate ligands.

Moving away from the chiral system to the very simple catalyst, 3 and using substrate/catalyst ratios of 200–500 enabled rapid hydrogenation of methyl benzoate. A range of aromatic esters were then successfully hydrogenated including those that had 3-pyridyl, bromo-aryl and chloro-aryl functionalities. In one case the reaction was found to proceed, albeit slowly, at just 30 °C, and an average of around 150 turnovers per hour were realised at just 50 °C for methyl benzoate hydrogenation.

Only one example of an alkyl ester was reported and while that was hydrogenated smoothly, we also failed to hydrogenate a more bulky ester with an alpha tertiary alkyl group; the scope of this catalyst with alkyl esters is not known, and the data known to date suggests steric effects might have significant effects on substrate scope.

A phosphino-alcohol ligand that acts as a P,N,OH neutral tridentate ligand for Ru^6c (i.e. ligand 1 with the NH₂ swapped for an OH) was also found to give almost the same levels of activity as the P,N,N systems.^6e We have also found what we believe may be the first iridium catalyst for ester hydrogenation using the same ligand systems. While a single Ir catalyst formed from ligand 1 and [Ir(COD)Cl]₂ is slightly less active than Ru, it does hydrogenate methyl 4-fluorobenzoate at S/C of 200 at 100 °C (33% after 1 h, giving T.O.F. of 66, >99% after 16 h);^6f perhaps other ligand systems will lead to iridium catalysts that can outperform ruthenium.

Milstein and co-workers developed a very interesting complex that has several related applications, and reported its use in ester hydrogenation in 2006.⁷ Using ruthenium hydrido-carbonyl complexes of a P,N,N ligand, the reaction with base leads to a deprotonation of the ligand such that it operates as an anionic tridentate P,N,N ligand as in 4. A P,N,P ligand also forms similar complexes, but the P,N,P complex gave quite low activity in ester hydrogenation. Complex 4 is firstly notable that no further base is needed during the catalytic reaction providing the deprotonated complexes are used. Using a S/C of 100, methyl benzoate can be reduced at 115 °C (average T.O.F ∼ 25). Ethyl acetate could also be reduced with a T.O.F. of around 8 mol mol⁻¹ h⁻¹, but tert-butyl acetate gave low conversions. The catalyst could also hydrogenate cyclic diesters such as glycolide and (S,S)-lactide to give the diols,^7b and in the case of lactide, with no racemisation. This complex has also been used in the related hydrogenation of carbonates, carbamates and formates.^7c

A logical extension to the Milstein type systems was exchanging the ‘one phosphine–two tertiary amine’ donors in catalyst 4 for one carbene and two tertiary amines in the ligand system. A couple of these systems have been prepared.^8,9 Based on the results with ethyl acetate, it appears that the carbene system, 5 is slightly more active than the parent system since using a S/C of 100 at 105 °C and a base/catalyst ratio of 8, then an average around 50 turnovers per hour can be realised compared to the average of around 8 quoted above. However, it has not been reported what performance catalyst 4 gives with added base present. One clear contrast is the easy reduction of tert-butyl acetate using catalyst 5. Another paper from the same time period reports a slightly different design (catalyst 6); in this case around 235 turnovers per hour were realised at 110 °C with a base/catalyst ratio of 1. These results suggest a slightly higher activity for these carbene analogues relative to 4.

Most ester hydrogenation catalysts use some form of phosphine ligand. However, a tridentate S,S,S ligand has also been used with [Ru(acac)₃] for the hydrogenation of dimethyloxalate giving the single hydrogenation product, methyl glycolate.¹⁰ It is not clear from the nature of the study on one target, but it seems likely that this catalyst is not as active as any of the others discussed here.

Although PNP systems did not give good results in the study by Milstein above, Takasago have recently shown that a Ru-PNP system, named Ru-MACHO, 7 can operate at a S/C of up to 4000 and at just 30 °C giving an average T.O.F of around 150 at what is a very low temperature for ester hydrogenation.¹¹ The reduction of methyl lactate was scaled up to 2200 Kg scale, demonstrating the viability of ester hydrogenation in practical large-scale organic synthesis (Scheme 1). The relatively high base/catalyst loadings typical in ester hydrogenation (in this case, 24) were clearly acceptable even in this commercial process. This catalyst could also be used for the hydrogenation of L-menthyloxyacetic acid methyl ester at multigram scale to the cooling flavour 2-(L-menthoxy)ethanol with a S/C of 2000 at 80 °C (av. T.O.F. ∼ 400). These results, particularly combined with the large scale demonstration show how far this technology has come in the last 10 years.

Scheme 1 The large scale hydrogenation of (R)-methyl lactate.

Recently, Gusev and co-workers reported on some Ru and Os dimers of an electron rich PNN bridging ligand that give very good results in the hydrogenation of esters.^12a A T.O.F. of 1470 was realised using complex 8 in the hydrogenation of methyl benzoate at 80 °C. This paper also shows that an analogous Os complex is highly competent, albeit slightly less active than ruthenium. Some examples of other substrates are reported; while the catalyst does not touch a bromoaryl substituted ester, several alkyl esters were reduced easily and while α,β-unsaturated esters were reduced with simultaneous reduction of the C=C bond, esters with an alkene group remote from the ester group can be reduced with high chemoselectivity.

Very recently, another highly active system based on a Ru complex of a PNN ligand (Ph₂P(CH₂)₂NHCH₂(2-pyridine) has been reported by the same group, giving up to 1250 turnovers per hour in the hydrogenation of ethyl acetate to ethanol at 40 °C. While only a few substrates were tested, this catalyst looks very promising.^12b

A team working at Firmenich were the first to show the surprising chemoselective reduction of esters in the presence of a C=C double bond. Most of their studies make use of a P,N bidentate ligand discussed later, but it seems that Ru complex, 9, derived from a tetradentate P,N,N,P ligand is the catalyst of choice when hydrogenating unsaturated esters. Alkenes that are more highly substituted and remote from the ester group are essentially untouched by this catalyst and hence give access to unsaturated alcohols with high chemoselectivity (Scheme 2). This catalyst is also highly competent in the hydrogenation of alkyl esters.^13a,b

Scheme 2 Hydrogenation of unsaturated esters using catalyst 9.

Catalysts derived from bidentate ligands

Diphosphines and diamines as ligands

Several groups have investigated Noyori-type catalysts for ester hydrogenation. Saudan and co-workers patented the use of [RuCl₂(BINAP)(DPEN)] and several related complexes with different diamines for the hydrogenation of esters in 2006.^13c Good reactivity was observed, providing a relatively high base/catalyst ratio is used (as noted before with other catalysts). This is not such a huge disadvantage since relatively low catalyst loadings can be used. Bergens and co-workers published a report in 2009 that studied the Ru-dihydride species, 10 that can be made from the Noyori catalysts.¹⁴ These operate well in ester hydrogenation, but in contrast to the Milstein Ru-hydride complexes, further amounts of base are needed to sustain good reactivity. Mechanistic studies show that the actual transfer of hydride to the ester is not a difficult process and occurs below room temperature (Fig. 2).

Fig. 2 Some ester hydrogenation catalysts based on bidentate ligands.

Scientists at Takasago prepared a different type of Noyori catalyst, moving away from the BINAP ligand and using the cheaper ligand dppp.¹⁵ They also found that using a borohydride complex of type 11 enables base-free hydrogenations to take place. This included a range of optically active esters that were reduced with no loss of enantiomeric purity at a S/C of 100 at 60 °C. This study then goes on to demonstrate both the high activity of this catalyst and its functional group tolerance to Boc or Bn protected amino-acid esters as shown in Scheme 3. If the amine group is β to the ester function, then even unprotected primary and secondary amines are tolerated. This paper also notes that while methyl benzoate can be reduced in high yield with S/C of 500, the tert-butyl ester was not reduced. We had carried out some studies in St Andrews on Noyori catalysts in ester hydrogenation. The only aspect worth noting was that having found it slow-going making a library of isolated Noyori catalysts with a range of both diphosphine and diamine components, we developed an in situ synthesis where catalysts are formed from [RuCl₂(NBD)(Py)₂] after treating with diphosphine and diamine. Thus far the only potentially useful result stemming from this enhanced screening ability was the finding that very cheap [RuCl₂(dppp)(amino-methyl-pyridine)], 12 was quite a good catalyst.^6e

Scheme 3 Hydrogenation of some amino-acid ester derivatives with retention of configuration.

Ikariya and co-workers discovered that diamines can be used as ligands in Ru catalysed ester hydrogenation. At S/C of 100, and a base/catalyst ratio of 25, an average T.O.F. of around 10–17 was realised at 100 °C. This paper is also notable for reporting the first examples of an ester (lactone) hydrogenation that occurs with a Dynamic Kinetic Resolution (DKR) as shown in Scheme 4.¹⁶

Scheme 4 An ester hydrogenation that proceed with a Dynamic Kinetic Resolution.

Non-symmetric bidentate ligands

One of the leading catalysts for ester hydrogenation are the Ru complexes of P,N ligands, 13, developed by researchers at Firmenich almost 10 years ago. The hydrogenation of benzyl benzoate at S/C of 2000 with base/catalyst ratio of 100 gives an average T.O.F. of 2000 mol mol⁻¹ h⁻¹ at 100 °C using catalyst 13. Lactones were readily reduced to diols and cyclohexane carboxylic acid methyl ester, bearing a secondary alkyl group alpha to the ester, and hence quite deactivated, also reduces well under these conditions.^13a This catalyst system operates under a range of process conditions (many solvents) and has been applied in a range of functionalised molecules including valuable perfumery ingredients.^13d Mecking and co-workers used these conditions for the hydrogenation of dimethyl-1,19, nonadecandioate to give nonadecane-1,19-diol.¹⁷ This substrate can be made in two steps from olive oil or rapeseed oil.

In 2010, Morris and co-workers reported a carbene-primary amine and its Ru(II) complex, 14.¹⁸ While this complex was only examined for one ester substrate, the results are promising since it was active at 25 °C and at 50 °C, 838 turnovers per hour could be realised at a base to catalyst ratio of 8 in the hydrogenation of methyl benzoate. Another very recent paper reports the use of simple P,N phosphine imidazole ligands in ester hydrogenation. While the turnover frequencies of around 20 at 80 °C are lower than some of the catalysts discussed, quite a range of esters were examined. A bromoaryl substituent was problematic again, but chloride, trifluoromethyl, methoxy and unprotected alcohols did not interfere with the ester hydrogenation when they were present on the substrate.¹⁹

Mechanism of ester hydrogenation

The only paper devoted to the mechanism of ester hydrogenation is the report by Bergens and coworkers showing that esters and Ru-hydrides react below room temperature with catalyst 10. In the case of γ-butyrolactone, a Ru-hemiacetaloxide complex, 15 is generated that reacts further with hydrogen (by an unknown mechanism) to give a Ru-alkoxide complex of 1,4-butane-diol, 16. The alkoxide can remove a proton from the amino ligand, DPEN, and dissociate to give 1,4-butane-diol and a Ru-amido complex, 17 in an equilibrium process. It is quite reasonably proposed that this equilibrium is the origin of product inhibition observed in the system, since the amido complex has been proven to react with a range of alcohols to generate alkoxide complexes such as 16. It is also proposed that the presence of additional base might help push this equilibrium towards the diol product and Ru-amido complex; the latter being the catalytic species that can react with hydrogen and continue the catalytic cycle. However, whether this is the main role that excess base has in promoting the reaction has not been established. This paper and points out that it is perfectly feasible that while the well-known bifunctional mechanism may be in operation for the reaction of the lactone with the Ru-dihydride, some form of direct hydride transfer from Ru to ester with gradual formation of a Ru-alkoxide bond could be the dominant reaction mechanism, or occur in parallel. In any case, the alkoxide that is forming does in this case end up bound to Ru so the reaction cannot be considered true outer-sphere catalysis. We have carried out a kinetic study on the hydrogenation of ketones using catalyst 3, relative to a fully N-methylated variant, but also compared [RuCl₂(BINAP)(DPEN)] with the tertiary amine [RuCl₂(BINAP)(Py)₂]. It was noted that the catalysts with NH functions show first order kinetics in ketone substrate (implying hydride transfer is slower that the activation of hydrogen). Meanwhile, the catalysts that lack the NH function needed to utilise the bifunctional mechanism are slower catalysts and are zero order in ketone. This latter observation implies that knocking out of the NH function mainly has a detrimental effect on the ability of the complex to activate hydrogen. While it is perfectly likely that the ketone can form a weak hydrogen bond to the NH function in the NH-functionalised catalysts giving a well defined transition state, the fact that NH-ligands utilise highly reactive Ru-amido complexes during the catalytic cycle is more significant in explaining their remarkable reactivity in our view. While several papers point out that the poor performance of N-methylated catalysts lends support for the outer-sphere bifunctional mechanism, they perhaps do not point out that these methylated catalysts are more bulky than the primary amine ligands and could hinder the association of the ketone/ester or molecular hydrogen to Ru. Such ligands might also have lower association constants and therefore give catalysts more prone to decomposition. Mechanistic information is at its most useful when it can inform the design of other catalysts and improve catalyst performance, and regardless of any subsequent further details that may appear on the exact reaction path for a specific catalyst–substrate combination, the literature suggests that while there may be some advantage to the use of NH-functionalised ligands, this might, in part, be due to their very streamlined steric properties. The variety of catalysts used successfully in ester hydrogenation and the mechanistic data described above suggests that providing the complex can find a path to activate hydrogen that does not come at the expense of any other steps, and is relatively sterically unhindered, then there is some good possibility for the addition of Ru–H across the double bond of an ester. Hence, quite a lot of ligands that do not possess a primary or secondary amine function may lead to very competent ester hydrogenation catalysts and further research in this context would be useful.

Summary

In summary, there are now a reasonable range of ester hydrogenation catalysts available. However, the number of active catalysts and the extent of their applications is very small compared to the level of research carried out on many other catalytic reactions (e.g. alkene hydrogenation). Thus, there is a demand for ligand synthesis specialists and organometallic chemists to investigate new transition metal complexes (including different metals) in ester hydrogenation. It appears from the published literature that relatively little work has been done on process chemistry aspects of catalytic ester hydrogenation. Thus, it is likely that many studies are carried out under sub-optimal conditions with poorly stirred reactors, solvent effects not fully evaluated and ester substrates used as received. The relatively high pressures used may alleviate gas-mixing issues, and relatively high catalyst or co-catalyst loadings may, in part, be possible to be reduced under optimised conditions. These aspects will generally be evaluated as catalytic procedures are scaled up for commercial development. While some companies and end-users can easily deal with any pressures below around 100 bar, others may have a preference for working near atmospheric pressure, giving another possible angle for improvements. Other end-users such as medicinal chemists may wish to use supported catalysts and co-catalysts in order to enable direct access to pure product solutions with no purifications steps, and this would also be a welcome development. At larger scale, most companies prefer to use extremely low catalyst loadings such that products with <10 ppm metal contaminants can be obtained with minimum effort, and only a few of the catalysts noted are getting close to these levels. An alternative is working in a flow system using a recyclable catalyst; this might be viable option as such equipment becomes more widespread and also requires a proof of concept demonstration. While some very good results have been obtained, there is still relatively little known about the substrate scope of the catalysts; it is likely that further research will reveal a number of further limitations that it is hoped will then be solved by the invention of new catalysts. Finally, something outside the scope of this review, but worth noting is that ester hydrogenation catalysts may well be good candidates for even less developed hydrogenations like the hydrogenation of carboxylic acids, amides and related compounds.²⁰ In summary, while great advances have been made, there is certainly room for further research in this area and new processes and catalysts are eagerly awaited.

Notes and references

1. (a) S. Nishimura, Handbook of Heterogeneous Catalytic Hydrogenation for Organic Synthesis, John Wiley and sons, New York, 2001, p. 392; (b) A. B. Hungria, R. Raja, R. D. Adams, B. Captain, J. Meurig Thomas, P. A. Midgley, V. Golovko and B. F. G. Johnson, Angew. Chem., Int. Ed., 2006, 45, 4782 and refs therein.
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4. (a) H. T. Teunissen and C. J. Elsevier, Chem. Commun., 1997, 667; (b) H. T. Teunissen and C. J. Elsevier, Chem. Commun., 1998, 1367.
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13. (a) L. A. Saudan, C. M. Saudan, C. Debieux and P. Wyss, Angew. Chem., Int. Ed., 2007, 46, 7473; (b) (Tetradentate ligands) L. Saudan, P. DuPau, J. J. Riedhauser, P. Wyss, WO2006106483(A1) (Firmenich); (c) (diphosphine/diamine complexes) L. Saudan, C. Saudan, WO2008/065588 (Firmenich); (d) Dr Saudan has kindly informed us of a comprehensive account including some previously unseen results appearing in a forthcoming book covering literature up to early 2011: see L. Saudan, in Applied Homogeneous Catalysis with Organometallic Compounds: a Comprehensive Handbook in three volumes, ed. B. Cornils, W. A. Herrmann, M. Beller and R. Paciello, Wiley CVH, Weinheim, in press.
14. S. Takebayashi and S. H. Bergens, Organometallics, 2009, 28, 2349.
15. W. Kuriyama, Y. Ino, O. Ogata, N. Sayo and T. Saito, Adv. Synth. Catal., 2010, 352, 92.
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