Transfer Hydrogenation with Glycerol as H-Donor: Catalyst Activation

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Transfer Hydrogenation with Glycerol as H-Donor: Catalyst Activation, Deactivation and Homogeneity. Robert H. Crabtree ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00228 • Publication Date (Web): 29 Aug 2019 Downloaded from pubs.acs.org on September 2, 2019

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ACS Sustainable Chemistry & Engineering

Transfer Hydrogenation with Glycerol as H-Donor: Catalyst Activation, Deactivation and Homogeneity. Robert H. Crabtree Chemistry Department, Yale University, 225 Prospect Street, New Haven, Connecticut 06520-8107, United States & Energy Sciences Institute, Yale University, 810 West Campus Drive, West Haven, Connecticut 06516, United States

email: [email protected]

KEYWORDS: Acceptorless dehydrogenation, hydrogen evolution, biomass, catalyst homogeneity, catalyst deactivation. ________________

INTRODUCTION Hydrogen transfer catalysis began in the 20th century as a little regarded variant of the thenstandard hydrogenation with H2. The first transfer cases apparently come from Knoevenagel1 (1903) and from Wieland2 (1912) who both saw dihydroarene disproportionation to give arene and tetrahydroarene with a heterogeneous Pd catalyst at elevated temperature (>200°C). The first homogeneous case is hard to identify since early conditions were sometimes harsh and no consideration was given to the possibility of decomposition of the homogeneous catalyst precursor to give active heterogenous catalysts which, with our current much improved understanding,3 now seems more likely. In an early case under mild conditions from Henbest4 (1967), a IrHCl2(dmso)3 catalyst precursor gave 300 turnovers of chalcone reduction with iPrOH as H-donor. The first review5 of the field comes from the same year although homogeneous catalysts receive no more than a brief mention. The field remained a minor

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speciality for many years until base was recognized as a good catalyst promoter. The mechanistic idea that formation of the alkoxide from the hydrogen donor alcohol was the key that led Bäckvall6 to introduce the idea that addition of base should be beneficial; indeed it speeded the reaction by 103 - 104 times. The base effect was later shown to be the result of conversion of the initial [RuCl2(PPh3)3] catalyst to the much more active hydride, [RuH2(PPh3)3]. The solvent, iPrOH in this case, can bind to the metal and undergo deprotonation. Subsequent beta-elimination of the alkoxide to give acetone gives the metal hydride that subsequently inserts into a substrate ketone, such as the PhCOMe depicted in Fig. 1. Beyond glycerol, both homogeneous7 and heterogeneous8 catalysts have seen wide use in transfer hydrogenation in general and biomass upgrading in particular. --------------------------------------------------------------------------------------------------------

Fig. 1 The inner sphere monohydride mechanism for transfer hydrogenation catalysis. (LnM = metal and ligands) -------------------------------------------------------------------------------------------------------Since the venerable (1925) Meerwein–Ponndorf–Verley (MPV) reaction9 brings about much the same overall transformation as in Fig. 1, although by a different mechanism (Fig. 2) and with a cheap Al-based catalyst, the transition metal variant still had limited impact. Catalytic


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hydrogenation with H2 also became a standard reduction method. The key accelerants were the rise of green chemistry10 and the development of numerous catalysts for highly efficient asymmetric transfer hydrogenation. The green aspect is the replacement of H2 as reductant by a hydrogen-donor solvent such as i-PrOH or, in particular, glycerol that is safer and easier to handle. The best known examples of asymmetric transfer hydrogenation catalysts come from Noyori.11 These and related catalysts proved particularly well adapted to the reduction of organic carbonyl groups.12 In what is the best current review of the whole field, Wang and Astruc13 even go so far as to consider the present day a 'golden age' of transfer hydrogenation'. The recent rise to prominence of transfer hydrogenation in biomass upgrading parallels the rising promise of biorefineries14 as a future source of fuels and chemicals. Hydrogen donor solvents can also come directly from biomass. Ethanol, produced in large quantities from fermentation of corn biomass, is a competent H donor solvent in transfer hydrogenation but its dehydrogenation product, CH3CHO, as an aldehyde, has a tendency to undergo decarbonylation with CO transfer to the metal, potentially leading to catalyst deactivation. Such is the case, for example with the catalyst precursor, [Ru2(µ-Cl)3L6]Cl, which reacts with refluxing ethanol solvent under basic conditions to give [RuH2(CO)L3] while i-PrOH instead gives catalytically active [RuH2L4] (L = PMePh2) because the dehydrogenation product, Me2CO, as a ketone has no tendency to transfer CO.15 In current practice, i-PrOH and HCOOH or Et3N/HCOOH are the most common hydrogen donors, neither having any significant tendency for decarbonylative deactivation of typical catalysts. MECHANISM Transfer hydrogenation most commonly relies on the transfer of 2H or more commonly (H+ + H-) from the H-donor first to the metal and then on to the substrate, but the details are remarkably complex with many variants possible. Although an MPV mechanism involving direct transfer of a beta-hydrogen of a coordinated alkoxide to an adjacent coordinated aldehyde or ketone (Fig. 2) may apply in certain cases, the favored mechanisms for transition metal catalyzed reactions involve metal hydrides formed by beta-elimination of a coordinated alkoxide as in Fig. 1. In the standard review, Backvall and Andersson16 distinguish between dihydride and monohydride mechanisms. In the dihydride case, the CH and OH hydrogens are both transferred


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to the metal and lose their identity. The test they propose to detect this situation is the racemization of S-phenylethanol that is deuteriated at the alpha carbon: PhCD(OH)Me. If the CD and OH lose their identity during the racemization reaction the H and D will be scrambled between the two sites in the racemized phenylethanol in a near 1:1 H:D ratio at each site. In the monohydride mechanism, in contrast, only the CH hydrogen is transferred to the metal and thus ends up at the alpha C site of the racemized compound. The OH proton remains protonic throughout, for example, by first being abstracted by the base and then finally returning to the O atom of the final product. They also distinguish between inner sphere and outer sphere mechanisms. In the outer sphere case, the carbonyl compound does not coordinate to the metal but instead receives a proton and a hydride from the catalyst, as in the Noyori catalysts. Fig. 3 shows the original interpretation of the key step in the Noyori mechanism. More recently, an alternative mechanism has been proposed involving formation of a dihydrogen complex which donates a proton to the substrate, leaving a metal hydride that subsequently donates the hydride.17 An case of exactly this dihydrogen mechanism is depicted in Fig. 4. The mechanism is unambiguous because there is now no ligand-based basic site to act as proton donor to the substrate as in the Noyori catalyst.18 Dihydrogen, when bound as an undissociated molecule in this way, can be greatly acidified versus the free molecule as a consequence of the much stronger binding of the M—H that results from proton transfer over M—(H2) binding in the starting complex.19 The hydride that remains is also readily transferred to the substrate because there is a high trans effect ligand, in this case another hydride, in the trans site. --------------------------------------------------------------------------------------------------------

Fig. 2 The MPV mechanism for transfer hydrogenation catalysis.


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Fig. 3 The outer sphere Noyori proton/hydride transfer mechanism for transfer hydrogenation catalysis. ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Fig. 4 An outer sphere sequential proton/hydride transfer mechanism via a dihydrogen complex. -------------------------------------------------------------------------------------------------------One other mechanistic variant has been proposed by Voutchkova-Kostal and coworkers. This involves oxidative addition of RO—H to the metal, leading to the alkoxide intermediate that subsequently beta eliminates as before.20


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GLYCEROL AS A SUSTAINABLE SOLVENT OR STARTING MATERIAL Glycerol is available from biomass as a waste product in biodiesel production. Each triglyceride molecule, on transesterification with MeOH, ideally gives three fatty acid ester molecules, which constitute the biodiesel fuel, along with one molecule of glycerol. Since annual biodiesel production already exceeds 4 x 109 L in the US alone and ~10% w/w glycerol is typically formed in the process, an estimated >4 x 108 L of glycerol waste is expected from this source. In addition, glycerol is also available from cellulose by hydrogenolysis.21 Glycerol is nontoxic, biodegradable, recyclable and relatively involatile, and it also dissolves a wide range of solutes. All these desirable characteristics make glycerol a high priority green target for future development. It may well become a standard feedstock for conversion into useful chemicals in a future biorefinery-based economy. GLYCEROL AS HYDROGEN DONOR SOLVENT Glycerol has found applications as a solvent for standard organic reactions where its green credentials are valued. As an unexpected bonus, increased selectivity or yield have been seen when certain reactions are run in glycerol.22 For transfer hydrogenation, glycerol also has advantages over ethanol, the other plausible H-donor produced in large quantities from biomass. In contrast to transfer hydrogenation with EtOH, where acetaldehyde is formed, the corresponding dehydrogenation product from glycerol is dihydroxyacetone, (Fig. 5)23 which, as a ketone, is expected to have a much lower tendency for potentially deleterious CO transfer to the metal catalyst. To be sure, the glyceraldehyde/dihydroxyacetone tautomeric equilibrium is expected to produce a small amount of glyceraldehyde that would be able to decarbonylate15 but the equilibrium thermodynamics greatly favor the ketone,24 so even if glyceraldehyde were a kinetic product in glycerol dehydrogenation, it would tautomerize to the ketone under the usual reaction conditions. Decarbonylative deactivation of the catalyst has occasionally been authenticated, as fully discussed in the deactivation section below. --------------------------------------------------------------------------------------------------------


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Fig. 5 Dehydrogenation of glycerol leads to a tautomeric equilibrium of dihydroxyacetone and glyceraldehyde. ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Fig. 6 Conversion of glycerol to lactic acid via the glyceraldehyde tautomer of the dehydrogenation product. Only the dehydrogenation step is metal-catalyzed. --------------------------------------------------------------------------------------------------------

The most straighforward application of glycerol to transfer hydrogenation is thus as a hydrogen donor solvent22,23 and several recent reports exploit this opportunity. In one example,23 acetophenone is converted to phenylethanol while the glycerol is converted to dihydroxyacetone


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with [(diene)Ir(chelate)Cl] (diene = 1,5-hexadiene or -cyclooctadiene; chelate = C-methylated bipyridine and phenanthrene derivatives) catalyst precursor at 100°C with NaOH or K2CO3 as base. The hexadiene complexes and the weaker base gave the best results. Interestingly, the amount of dihydroxyacetone detected in the product mixture was less than expected from the idea that one mole of this product would be present per mole of acetophenone converted—often only one third of the expected amount was in fact seen. A possible reason is the tendency of dihydroxyacetone to undergo base-catalyzed conversion to lactate, as has been seen both with catalysts 1-3 (Fig. 7) and with heterogeneous catalysts.25,26 Any lactate coproduct could have easily escaped detection by being involatile. In order to avoid the loss of the desired dihydroxyacetone, the Crotti group carried out the reaction in the absence of base with a more reactive Ir catalyst having P,N chelate ligands. This reaction then took place at the comparatively mild temperature of 100°C.27 Dihydroxyacetone is now formed in good yields and conversion to other products is minimized. N-heterocyclic carbene complexes have taken a prominent role in the area. In one case, complexes 4-5 (Fig. 7) were shown to be active for reduction of acetophenone with glycerol as H donor solvent with KOH as base at 80-120°C for 15-24h with yields of up to 45%.28 The reaction showed some chemoselectivity for the reduction of the C=C bond over the C=O bond in benzylidene and dibenzylidene ketones. --------------------------------------------------------------------------------------------------------


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Fig. 7 Structures of catalyst precursors 1-6. -------------------------------------------------------------------------------------------------------Peris et al.29 have also reported the reduction of aldehydes and ketones with glycerol as H-donor solvent and a number of catalyst precursors including 4 and 6 (Fig. 7). The Ir(III) complex 4 proved to be most efficient in the reduction of ketones, while the Ir(I) complex, 6, was more active for aldehydes. The effect of microwave and of particularly of ultrasound activation was reported, with ultrasound having the most dramatic effect on shortening the reaction time, for example from 900 min to 5 min. for 4. The formation of nanoparticulate metal was detected by electron microscopy but the homogeneous complex was nevertheless thought to be the true catalyst because nanoparticle formation was considered to be a catalyst deactivation product. If authentic homogeneous catalysis is the aim, ultrasound is nevertheless a risky method to apply to homogeneous catalysis in general because the hot spots formed during cavitation are likely to produce catalyst decomposition with the possibility for formation of nanoparticles that could initiate heterogeneous catalysis.30 Nor is microwave irradiation entirely risk-free, especially in the presence of polyols such as glycerol, since it is involved in a well known procedure, the microwave-polyol nanoparticle synthesis process.31 Microwave conditions have proved very


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useful for catalytic biomass transformations in general,32 but care is needed for any mechanistic interpretations. --------------------------------------------------------------------------------------------------------

Fig. 8 Structures of catalyst precursors 7-12. --------------------------------------------------------------------------------------------------------

Voutchkova-Kostal et al.33 have also reported on some recyclable catalysts, including a sulfonated Ru(bis-NHC) compound (not shown), and to a lesser extent, 7 and 8 (Fig. 8), 8 being an unusual example of a bis-abnormal carbene complex with the metal bound at C4 rather than the usual C2. These are active for reduction of imines, aldehydes, olefins and ketones with glycerol as H-donor solvent at 120°C for 2h or with microwave irradiation at 150° for 2h. Reductive amination proved possible with 8; for example benzylamine and benzaldehyde gave


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the corresponding secondary amine in 99% NMR yield. This system was thought to proceed through a monohydride mechanism and was more active for transfer hydrogenation of imines than of aldehydes, ketones, and olefins. A dihydride mechanism was thought more likely for a different catalyst, 5, which was more active for carbonyls than imines and olefins. Singh et al. report an unusual areneruthenium(II) S,Se-donor chelate complex, 9 (Fig. 8. Ar = 2,6diisopropylphenyl), as catalyst precursor for hydrogen transfer from glycerol in aqueous solution at 110°C with 89% isolated yield of phenylethanol from acetophenone as substrate.34,35 The same group36 find that Ir and Rh catalysts 10 and 11 are effective for transfer hydrogenation from glycerol to a variety of ketones at 120°C with KOH, 11 being the first case of a successful Rh precatalyst. They reported similar results with the precatalyst 12 that is catalytic for reduction of a series of aldehydes and ketones at 80°C for 3h with glycerol as H-donor and KOH as base. The group also tested alternate H-donors and found 2-propanol to be the best, closely followed by glycerol.37 Allyl alcohols have been converted to the saturated forms by precatalyst [(6-C6Me6)Ru(µ-Cl)Cl]2 with CsCO3 as base at 82°C first involving an isomerization to the aldehyde and then on to the alcohol by hydrogen transfer from the solvent, initially i-PrOH but subsequently glycerol, either pharmaceutical or technical grade. In the glycerol case, the precatalyst was the closely related [(6-p-Cymene)Ru(µ-Cl)Cl]2 and the reaction conditions were 60°C for 3h but ultrasound was required.22,38-40 A patent41 reports the reduction of carboxylic acids, RCOOH, to the corresponding alcohols, RCH2OH (R = Ph or n-alkyl groups from C3 to C8) in 90-95% yield, by transfer hydrogenation from glycerol at 140-150°C with CoCl2 as catalyst and KOH as base (Fig. 9). In spite of the homogeneous precursor, a heterogeneous active catalyst seems more likely in this case. Whatever the mechanism this may prove a useful, greener alternative to the usual harsh reagents such as LiAlH4. --------------------------------------------------------------------------------------------------------


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Fig. 9 Conversion of carboxylic acids to alchols by transfer hydrogenation from glycerol. -------------------------------------------------------------------------------------------------------Carbon dioxide reduction from combustion effluent has attracted much increased recent interest in connection with upgrading what would otherwise be a waste material. The two most common reduction products, CO or HCOOH, tend to be formed by inner and outer sphere pathways, respectively (Fig 10). While CO often comes from direct metal attack at the CO2 carbon (inner sphere), HCOOH usually arises by metal hydride attack on an outer sphere CO2. With aqueous basic glycerol as the H-transfer solvent and [RuCl2(PPh3)3] as precatalyst, along with the formate coming from CO2, dihydroxyacetone is initially also formed from the glycerol but then is transformed into glycolic acid by decarbonylation.42 Relatively few turnovers were seen, however, compared with similar work43 in which i-PrOH was the donor. A similar transfer hydrogenation from glycerol leading to reduction of carbon dioxide to HCOOH or of bicarbonate to formate was carried out under aqueous conditions at 150-225°C with catalyst 13, bearing a potentially tricoordinate 'pincer' ligand. In this case, the thermodynamics of the process was compared for different H-donors, H2, i-PrOH and glycerol, with the latter being preferred, particularly with HCO3- rather than CO2 as substrate.44 With HCO3- reduction to formate an inner sphere pathway seems likely and was proposed,44 contrary to the generalization of Fig. 10 in which inner sphere pathways are invoked for cases where CO is the product rather than formate. Choudhury and coworkers report45 an unusual [Cp*Ir(aNHC)I] precatalyst (aNHC = abnormal46 NHC) for the conversion of ambient pressure CO2 to formate at 110°-150°, although the integrity of the full Cp*Ir(aNHC) unit may not be fully maintained under the relatively harsh conditions since Cp*is readily lost from similar precatalysts under similar conditions.47 --------------------------------------------------------------------------------------------------------


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Fig. 10 Key steps often invoked in the conversion of CO2 to CO (inner sphere, upper) or to HCOOH (outer sphere, lower). -------------------------------------------------------------------------------------------------------A more exotic transformation was reported by Sanz and coworkers48 who found that glycerol as H-donor reduced sulfoxides to the corresponding thioethers at 170°-230°C under thermal or microwave conditions. The catalyst precursor, MoO2Cl2(dmf)2, was certainly homogeneous although the nature of the active catalyst was unclear. The fate of the glycerol was probed by C-13 NMR with the result that formate was the only significant product in addition to CO2. Consequently the glycerol was able to reduce up to 6 equivalents of the substrate, not simply one as in the cases described above where each glycerol was a donor of 2H. Whatever the mechanism, this seems a greener means of sulfoxide reduction considering that the standard reductants are phosphorus(III) compounds or silanes. Aqueous glycerol also permits the catalytic transformation of aliphatic or benzylic amines into alcohols by a hydrogen borrowing pathway. Jang and coworkers49 report a bis-NHC iridium catalyst that operates by dehydrogenative activation of the amine to give the imine. This undergoes hydrolysis to give the aldehyde that is subsequently reduced to the alcohol using the hydrogen abstracted from the starting amine. Yields of 60-70% were typical for benzylic amines and mechanistic studies with deuterated amines showed that the α-hydrogens of the aminederived alcohols come from the α-hydrogens of glycerol. --------------------------------------------------------------------------------------------------------


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Fig. 11 Catalysts 13-17. -------------------------------------------------------------------------------------------------------GLYCEROL AS PRECURSOR TO USEFUL PRODUCTS Many useful compounds can be obtained from glycerol50 but perhaps the simplest glycerol conversion is catalytic dehydrogenation, a reaction closely allied mechanistically to transfer hydrogenation. The catalyst dehydrogenates the H-donor as in classical transfer hydrogenation but instead of subsequent transfer of 2H to acetophenone or some other substrate molecule, H2 is instead released. This is termed an acceptorless dehydrogenation and results in an oxidation without the need for an oxidant and therefore without the waste formation that could otherwise result, a green outcome. In the study of H-acceptor-driven glycerol dehydrogenation to dihydroxyacetone that was mentioned above27 the acceptorless version with release of H2 also proved possible with the P,N catalyst, 14 (Fig. 11). Lactic acid is considered as a platform chemical with applications in food, cosmetics, pharmaceuticals, fine chemicals, and in the synthesis of polylactic acid, a biodegradable polymer.51 Sharninghausen et al. were able to convert glycerol to lactate in at 115°C with Ir N-


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heterocyclic carbene precatalysts 1-3 in basic conditions.Turnover numbers exceeded 30,000 after 90h, with 91% yield and 95% selectivity.52 Initial dehydrogenation led to release of H2, but the further conversion to lactate is not plausible from the expected dihydroxyacetone intermediate, so the glyceraldehyde tautomer was invoked. This is thought to follow the pathway depicted in Fig. 6, where only the first step is metal-catalyzed, all the others being spontaneous organic processes in basic solution. The Williams group reported on complex 15 (Fig. 11) and find that it converts neat glycerol to lactate at 145-180°C with KOH as base. By running the reaction for 32 days, an impressive turnover number of 4.56 x 106 was achieved. Beta-hydride elimination was thought to be the turnover-limiting step and since n-PrOH and i-PrOH reacted at essentially identical rates there is probably low selectivity between the primary and secondary alcohol positions in the case of glycerol. A homogeneous origin was proposed for the catalysis on the grounds of its clean, light yellow physical appearence, well-behaved kinetics and tolerance of both Hg(0) and 1,10phenanthroline. The reaction rate is even unaffected by the presence of up to 0.5 equivalents of PPh3 per Ir. The lactate obtained in the process was also further converted to rac- and mesolactide, monomers for the synthesis of the desirable green polymer, poly(lactic acid).53 Conversion of vegetable-derived unsaturated triglycerides with NaOMe to fatty acid methyl esters (FAMEs) results in biodiesel fuels. The commonly available soybean and corn oils, however, gives FAMEs that are heavily unsaturated, having a high content of the doubly and triply unsaturated C18 linoleic and linolenic esters. Since this unsaturation is undesirable in biodiesel applications, the starting oil was treated with the Ir catalyst, in an extension of the prior work, leading to H-transfer from the glycerol backbone to reduce the level of unsaturation in the resulting material, making for improved performance.54 Other catalyst systems that produce lactic acid by glycerol dehydrogenation include Voutchkova-Kostal's Ir(I) NHC precatalyst,20 a polymer-supported Ir(I) NHC precatalyst from Huang and Tu55 and an Ru(PNP) pincer precatalyst from Dixneuf, Beller et al.56 The Wayland group57 has reported on a transfer hydrogenation in which glycerol is converted to dihydroxyacetone in high conversion and selectivity with benzoquinone or even with air as oxidant with [Pd(phen*)(µ-OAc)]2 (phen* = 2,9-dimethyl-1,10-phenanthroline) as catalyst. A special feature here is the room temperature operation and the absence of base,


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largely preserving the dihydroxyacetone from conversion to other products. The first order dependence on benzoquinone suggests that reoxidation of the reduced Pd species is turnover limiting. A limitation of much work in the field is its reliance on precious metals, typically Ir and Ru, but the glycerol to lactate conversion also proved possible with the iron pincer catalyst 16 at 140°C with a TON of 770.58 This appears to be the only cheap metal case so far reported. GLYCEROL AS A PRECURSOR TO FREE H2 Other studies regard glycerol as a precursor to free H2, rather than to lactic acid. This makes it a type of hydrogen storage except that it is a release-only version, without any easy way to regenerate the glycerol, but since the latter is freely available this is not a big limitation as long as the resulting lactic acid can also find a use. In a classic early example from Cole-Hamilton,59 [RuH2(N2)(PPh3)3] catalyzed H2 release from glycerol and base at 150°C over 2h although with a modest turnover frequency of 12.4 h-1. Ethylene glycol and n-BuOH proved much more active H2 sources with turnover frequencies exceeding 500 h-1. Dixneuf, Beller et al.56 reported a ruthenium pincer catalyst, 17, that gave over 250,000 turnovers of H2 at 140°C after 24h. The lactic acid yield (67%) and selectivity (67%) were somewhat lower than some of the others discussed here. One limitation of much research work in the application of glycerol in transfer hydrogenation and related processes, however, is the exclusive use of pure glycerol rather than biodiesel waste, a brown material of unappealing aspect and only ca. 60% (w/w) glycerol content. Catalyst 17 proved effective with 'industrial glycerol' and 1-3 were effective even for biodiesel waste. CATALYST ACTIVATION, DEACTIVATION AND HOMOGENEITY This section covers topics which receive less attention than they should in the field of homogeneous catalysis. The reason that they are particularly relevant for the present discussion is that the conditions used in transfer hydrogenation from glycerol commonly involve unusually high temperatures, harsh bases and even microwave and ultrasound irradiation. These conditions are likely to bring about changes in the catalyst precursor, either minor or major, even going to the extreme of forming metal nanoparticles that could act as heterogeneous catalysts for the


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reaction. If product formation is the sole goal, no great harm may be done, but any mechanistic understanding is greatly complicated by a misunderstanding of the true nature of the active catalyst. Such understanding is often used to guide catalyst improvement, so even for the limited goal of product formation, such misunderstanding could still be problematic. Considering catalyst activation first, it is often the case that the catalyst precursor initially present needs to undergo some change before becoming active. For example, in a series of closely related transfer hydrogenation catalyst precursors, 1-3 (Fig. 7), the Cp* (1), cyclooctadiene (2) or CO groups are apparently lost under transfer hydrogenation conditions because the presumed active catalyst, [IrH2(IMe)2(solvent)2] + (IMe = N,N'-dimethylimidazol-2ylidene), detected in mass spectral data from the active solutions, as well as the catalyst activity is the same for all three. The most surprising case was loss of Cp* from 1 because this is often considered as a reliable spectator ligand.47 Activation can be a problem if DFT calculations are performed on the precursor and not on the real system. Another difficulty is that a ligand might be modified, such as the Cp*, in the hope of modifying the activity or introducing asymmetry, only to have that ligand lost in the activation step. It is not at all easy to determine the active form, specially if it is only a minor component of a broad distribution of metal species. Understanding catalyst deactivation can lead to the development of a better catalyst system. For example, since bis-carbene catalyst 3 decomposes in part to give inactive triscarbene forms, such as [Ir(IMe)3H2CO]+, during the catalysis, Williams60 thought that a monocarbene must be formed in the disproportionation and that this could be the true active species. This led to their development of the much better chelating monocarbene catalyst 15, one of the very best available to date. Deactivation products are rarely identified but in principle these may give a clue on how to minimize deactivation. In the case of a glycerol dehydrogenation by the homogeneous, Nheterocyclic carbene Ir catalyst, 1, a number of different carbonyl cluster polyhydrides were detected as deactivation products, for example [Ir6(IMe)8(CO)2(H)14]2+ and [Ir4(IMe)8(H)10]2+ (18 and 19. Fig. 12). Cluster 19 is held together only by bridging hydrides, while 18 also has one direct M—M bond as well. Both are so inert that they do not lose hydrogen even at 140°C and were likely formed from the small fraction of glyceraldehyde in the tautomeric equilibrium from


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catalytic glycerol dehydrogenation.61,62 In earlier work, Aresta et al.42 suggest that this decarbonylative deactivation reaction impeded their ability to achieve more than a few turnovers for transfer hydrogenation of CO2 from glycerol with their catalyst system. Similarly, Voutchkova-Kostal et al.33 find that a chelating Ru bis-NHC complex only retains full activity with iPrOH as H-donor solvent but not with glycerol. As might be expected, the importance of decarbonylative deactivation seems to be very catalyst-dependent, for example, in contrast with the cases mentioned above, catalyst 13 gives high TONs for CO2 reduction even with glycerol as H-donor.44

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Fig. 12 Deactivation products 18-19. (L = IMe) -------------------------------------------------------------------------------------------------------In another case of solvent intervention, an unexpected ring opening of N-methyl-2pyrrolidinone in the basic medium was reported in the study on catalyst 16. In this case, however, the transformation proved not to affect, and to be unaffected by, the metal catalyst. The ring opening of the solvent to give sodium-4-N-methylaminobutanoate is therefore purely dependent on the basic conditions and relatively high temperature. Although the ring opening did not significantly affect the catalytic reaction in this case, it might still be a problem in other cases.


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Homogeneous catalysts are relatively delicate compared to heterogeneous catalysts in the sense that they often fail to survive harsh conditions as de Vries7 has noted in a recent review: "The Achilles heel of homogeneous catalysts is not their problematic separation from the products, as many people think; but rather their instability." If homogeneous catalysis is to play a bigger role in practical biomass conversion, robustness must be given greater future attention and this means that catalyst deactivation pathways63 need to be identified so that they can be more effectively countered. When homogeneous catalyst precursors are employed in glycerol as solvent, a number of special hazards arise that do not apply to a conventional low-boiling H-donor such as i-PrOH. Glycerol is a high boiling material (290°C) so, unlike the case of i-PrOH (b.pt. 82.5°C), maintaining reflux does little to protect the catalyst from thermal decomposition and big thermal excursions could occur if ultrasound or even microwave radiation is involved. This is a problem because some homogeneous catalysts can decompose at high temperatures, typically beyond 150°C. If the catalyst merely loses activity as a result, no great harm is done other than loss of the active material, but if catalytically active metal nanoparticles are formed, the catalytic activity may be wrongly assumed to be homogeneous in origin. This error is easy to make because nanoparticle suspensions can have the outward appearance of normal solutions, for example even concentrated suspensions of Ir nanoparticles have a very light straw color. They can also be very active heterogeneous catalysts, and in such a case, the resulting catalysis risks being interpreted in error as homogeneous. Formation of catalytically active nanoparticles is not very difficult. In a recent study, [(cod)IrCl(NHC)] (NHC = 1,3-bis((2,6-diisopropyl)phenyl)imidazol-2-ylidene) was hydrogenated (10 atm., 100°C) to generate NHCstabilized Ir nanoparticles that proved active for catalytic conversion of basic aqueous glycerol.64 Tests for nanoparticulate catalysis have only been carried out in a few of the studies described here and so the possible heterogeneous origin of the catalysis has always to be kept in mind. To be sure, the rise of pincer ligands65 has raised the thermal stability of homogeneous catalysts. Although pincers are rarely seen in the existing reports on glycerol conversion, they are likely to become much more common in future. Another problem that can arise from using glycerol as an H-donor solvent. Nanoparticles were detected in the previously described study on catalysts 7


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and 8, but selective poisoning tests with 1,10-phenanthroline and Hg(0) as well as comparisons with authentic Ir nanoparticles suggested the catalysis was predominantly homogeneous. Likewise, the authors of the study on precatalysts 10-11 looked into the possibility of a heterogenous origin for the catalysis by applying the Hg(0) test, which was negative, implying homogeneity. The identity of the H-donor solvent also plays a role. In the previously mentioned study on the recyclable catalysts, 7-8, the presence of glycerol was identified as somewhat problematic, because replacement of this H-donor with i-PrOH gave much greater recyclability. Carbonyl transfer from the glyceraldehyde intermediate seems likely as a catalyst deactivation mechanism in this case and this points to a weakness of glycerol as a suitable H-donor solvent where recyclability of the catalyst is important. Identification of deactivation resistant catalysts will be needed if glycerol is to be a practical H-donor for homogeneous catalysis. CONCLUSIONS Glycerol has several advantages as an H-donor solvent in reduction reactions since it is easily available, nontoxic, biodegradable, recyclable and relatively involatile. A disadvantage, though, is the potential for CO transfer from a glyceraldehyde dehydrogenation product to the catalyst, potentially altering its properties or even deactivating it. In transfer hydrogenation, the strongly basic and generally high temperature conditions are harsh enough to produce metalcontaining nanoparticles in some cases. Even though appropriate tests were consistent with the predominant homogeneous character of the catalysis in the relatively few cases where such tests have been made, nanoparticle formation is still a problem. Not only would this lead to the loss of some of the active metal from solution, but a potentially misleading heterogeneous component may contribute to, or even entirely dominate, product formation. More account needs to be taken of the possibility of the formation of nanoparticles in these reactions. This field could become a good test bed for devising better tests for homogeneity more stable homogeneous catalysts, for example ones with pincer ligands. This search for greater stability is a key to successful practical applications of homogeneous catalysis and one which has received less attention than it deserves if the promise of homogeneous catalysis for green chemistry goals is to be fully realized.


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ACKNOWLEDGMENTS I thank a referee for very helpful comments.

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ABSTRACT: The title topic has risen to prominence with the increasing availability of the biomass-derived polyol, glycerol. Homogeneous precatalysts, particularly with the precious metals, have taken a role in these developments with promising initial results but with much yet to be done, notably in broadening the range of accessible valorization products and in moving to cheaper catalysts. The topics of catalyst activation, deactivation and homogeneity are particularly relevant here because of the relatively harsh conditions commonly employed in transfer hydrogenation from glycerol. This may therefore be a good field in which to address the problem of catalyst instability which has been considered the "Achilles heel" of homogeneous catalysis. ________________ Table of Contents graphic

________________ SYNOPSIS Glycerine, a biodiesel byproduct, formerly merely waste, now has numerous uses thanks to catalysis, such as conversion to lactic acid.


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Transfer Hydrogenation with Glycerol as H-Donor: Catalyst Activation

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