PERSPECTIVE PUBLISHED ONLINE: 20 MAY 2015 | DOI: 10.1038/NCHEM.2262
Orthogonal tandem catalysis Tracy L. Lohr and Tobin J. Marks* Tandem catalysis is a growing field that is beginning to yield important scientific and technological advances toward new and more efficient catalytic processes. ‘One-pot’ tandem reactions, where multiple catalysts and reagents, combined in a single reaction vessel undergo a sequence of precisely staged catalytic steps, are highly attractive from the standpoint of reducing both waste and time. Orthogonal tandem catalysis is a subset of one-pot reactions in which more than one catalyst is used to promote two or more mechanistically distinct reaction steps. This Perspective summarizes and analyses some of the recent developments and successes in orthogonal tandem catalysis, with particular focus on recent strategies to address catalyst incompatibility. We also highlight the concept of thermodynamic leveraging by coupling multiple catalyst cycles to effect challenging transformations not observed in single-step processes, and to encourage application of this technique to energetically unfavourable or demanding reactions.
T
he quest for new methodologies to assemble complex organic molecules continues to be a great impetus to research efforts to discover or to optimize new catalytic transformations. In this connection, performing reactions in a ‘one-pot’ (single reactor), atom-efficient scenario is an attractive target. Greater economy of time and energy maximize resources and overall process simplicity, as well as decreasing materials loss from multiple iterations of reaction, workup and purification1–6. Performing multiple tandem reactions in a one-pot mode also permits thermodynamic leveraging by coupling multiple catalytic cycles7–9. One-pot reaction types include, but are not limited to, cascade (domino) processes1,10, tandem catalysis (catalysts performing sequential transformations)7,8,11–19, multifunctional catalysts having more than one active site1,2,20–22, dual catalyst systems where one catalyst enhances or alters the properties of the other catalytic cycle23, and one-pot reactions involving isolated catalytic cycles, for example where a second catalyst is added after the first has completed its transformation, or where the first catalyst is selectively deactivated later by addition of a second reagent 11. Although there have been reviews encompassing diverse one-pot catalytic reactions as a whole, as well as tandem catalytic approaches to polymerization3,24, this Perspective focuses on the specific subset of orthogonal tandem catalysis, an area that has attracted much recent attention and is being applied to increasingly challenging synthetic problems. Mechanistically distinct from cascade or domino catalysis, in which a single catalytic transformation occurs sequentially 1,12, orthogonal tandem catalysis12,25 is a one-pot reaction in which sequential catalytic processes occur through two or more functionally distinct, and preferably non-interfering, catalytic cycles (see Box 1 for a nomenclature flowchart of sequential one-pot processes). Tandem catalysis has also been subcategorized into auto- and assisted-tandem catalytic cycles (Box 1)12,25. Auto-tandem catalysis uses a single precatalyst to effect two or more mechanistically distinct catalytic cycles, typically by cooperative interaction between the various species in the system. In contrast, assisted tandem catalysis requires deliberate intervention in the system to switch between one catalytic cycle and another. The purpose of this Perspective is to survey and analyse recent (post-2011) developments in orthogonal tandem catalysis. We highlight the challenges and some recent solutions in realizing tandem catalytic systems, such as controlling and optimizing selectivity, and overcoming detrimental interactions between multiple catalysts and their ligands. We also review recent successes in thermodynamically
leveraging coupled catalytic cycles to afford products energetically unfavourable in single-catalyst systems. The objective is to reveal the power of this approach in designing catalytic transformations and to encourage its use to address current challenges.
Hydrocarbon upgrading
Orthogonal tandem catalysis first scored an important success in the area of hydrocarbon upgrading (Fig. 1a,b). Combining olefin hydrogenation/dehydrogenation with olefin metathesis leads to the overall metathesis of medium-weight (C3–C8) alkanes to a mixture of heavier and lighter n-alkanes, the former being desirable for diesel and jet fuels and the latter constituting a large portion of natural gas liquids19,26. The pioneering work on alkane metathesis using heterogeneous catalysts by Burnett and Hughes at Chevron, and work with complementary homogeneous systems by Goldman and Brookhart, were both reviewed in 2012, and readers are directed to those two articles and references therein for more detailed discussions19,26. Computational analyses of both alkane metathesis steps, alkane dehydrogenation and olefin metathesis, have also been discussed27,28 for typical (PCP)Ir catalysts (PCP = C6H3-2,6-(CH2PtBu2)2). The heterogeneous hydrocarbon upgrading system invented by Burnett and Hughes combines a solid hydrogenation catalyst (typically Pt/Al2O3) with a solid olefin metathesis catalyst (for example W/SiO2) in the same reactor to accomplish what is termed the ‘molecular redistribution and molecular averaging’ (MR/MA) of alkanes (Fig. 1a)26,29. This physical mixture of catalysts effects three key steps: (i) alkane dehydrogenation to form olefins, (ii) metathesis of the newly formed olefins, and (iii) hydrogenation of the unsaturated metathesis products. The process operates at relatively high temperatures (>343 °C) and produces a quasi-Gaussian distribution of higher and lower n-alkanes from a given alkane feedstock. The homogeneous alkane metathesis system developed by Brookhart and Goldman uses Ir pincer catalysts for hydrogenation/ dehydrogenation, and Schrock-type Mo and W catalysts for olefin metathesis (Fig. 1b)19. The homogeneous system operates at relatively low temperatures (125 °C) and for a variety of Ir pincer catalysts generally affords Gaussian distributions of alkanes (centred around the starting alkane), with moderate molecular weight selectivity (specifically converting Cn n-alkanes to C2n−2 n-alkanes + ethane) observed using a (tBu4PCP)Ir-based catalyst in tandem with W- or Mo-alkylidenes19. To enable catalyst recycling and recoverability, both the (PCP)Ir and W-alkylidene catalysts were supported on
Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, USA. *e-mail: [email protected] NATURE CHEMISTRY | VOL 7 | JUNE 2015 | www.nature.com/naturechemistry
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PERSPECTIVE
NATURE CHEMISTRY DOI: 10.1038/NCHEM.2262
Box 1 | One-pot catalyst schemes.
This Perspective surveys orthogonal tandem catalytic processes where multiple catalysts, operating under different mechanisms, are combined in a single reaction pot. Not all multi-step, one-pot processes fit this definition, and so it is distinguished here from the related domino processes, where sequential transformations occur by a single catalytic pathway, and cascade catalytic processes, where typically three or more domino processes occur. Such processes are not the subject of this Perspective. a
Are all catalysts present at time zero? No
Yes
Is there more than one mechanism?
Isolated catalytic events
No
Yes
Cascade or domino
Tandem catalysis Multiple catalysts
Single precatalyst Intervention
Orthogonal tandem catalysis Auto-tandem catalysis
Assisted tandem catalysis
Compartmentalized
b Reactant
Cat A
Intermediate
Cat B
Product
Figure B1 | Types of multi-step catalysis. a, A flowchart guide to nomenclature of multi-step one-pot catalytic processes. b, The classical picture of orthogonal tandem catalysis.
Al2O3 by installation of a basic functionality in the para position of the central pincer ring 30 and on the HIPTO ligand, respectively (Fig. 1c). Coupling supported-(PCP)Ir catalysts and solid Re2O7/ γ-Al2O3 metathesis catalysts also produces substantially higher turnover numbers (TONs) than the homogeneous analogues, and produces turnover frequencies (TOFs) as high as 2,400 h−1 at 175 °C over a period of 14 days19. More recently, Goldman, Schrock and coworkers extended the substrate scope of the homogeneous tandem system to include the cross coupling of n-alkyl arenes31. As previously noted, alkane metathesis affords a near-Gaussian product distribution of alkanes with very little product selectivity. To overcome this, Bercaw, Labinger and co-workers ingeniously approached the issue of light hydrocarbon upgrading through a homogeneous process of tandem transfer hydrogenation and alkene dimerization15,17. The idealized tandem approach for upgrading mixtures of alkanes and alkenes is shown in Fig. 1d. One catalyst mediates dimerization of the alkene component (Cn) to the corresponding C2n alkene while the second catalyst mediates the transfer hydrogenation between alkane and the C2n alkene, thereby hydrogenating the C2n alkene component to the upgraded alkane and regenerating alkene for subsequent dimerization17. The overall cycle 478
requires one equivalent each of alkane and alkene to selectively produce a higher alkane. The dual homogeneous system investigated for transfer hydrogenation uses the Ta catalyst Cp*TaCl2(alkene) developed by Schrock and co-workers32–34 for alkene dimerization and a variety of (PCP)Ir catalysts19,35,36 (Fig. 1d). Mechanistic analysis of the above tandem catalytic system reveals that the two catalysts operate independently in solution with no detrimental ‘cross-talk’. The main limitation, however, is that the two types of catalysts operate most efficiently in vastly different kinetic regimes, making it challenging to realize optimal conditions for a tandem process15. Thus, the Ta alkene dimerization catalyst operates most efficiently at high [alkene], with an approach to kinetic saturation at ~4M alkene, and at lower temperatures, with the rate being relatively insensitive to temperature at high [alkene] ~4M. In contrast, the (PCP)Ir catalyst is most active at higher temperatures (125 °C) with a rate law that is inverse order in [alkene], opposite to conditions for favourable alkene dimerization15. Therefore, the initial optimized conditions for tandem catalysis were found to be at moderate temperatures (100 °C) and [alkene] ~0.25M, to allow realistic turnover rates for both catalytic cycles. Notably, increasing the temperature and [alkene] produces a complementary effect, and optimum yields of alkane/alkene coupled products are obtained at 150 °C with [alkene] ~1M. So far, the full tandem cycle in Fig. 2d has not been fully realized, as only higher alkene products are formed over alkanes. Control experiments show that the highly branched alkenes formed using the Ta dimerization catalyst are both kinetically and thermodynamically unsuitable for transfer hydrogenation under these conditions15. Alkene dimerization catalysts that are selective for linear alkenes will be required in future attempts to incorporate this tandem system into large-scale conversion of mixed alkane/alkene feedstocks. Both Schrock and Gooβen have reported recent progress in the area of one-pot olefin isomerization/metathesis, which is mechanistically similar to the multi-step Shell higher olefin process (SHOP)37–39. Thus, Gooβen and co-workers successfully combined Pd isomerization and Ru metathesis catalysts to convert single olefins to regular distributions of olefinic products37. The latest report 38 from Schrock and Grotjahn’s groups involves one-pot tandem olefin isomerization/metathesis-coupling (ISOMET) using a Ru-based alkene ‘zipper’-type catalyst that produces equilibrium mixtures of trans- and terminal olefins combined with a W-based olefin metathesis catalyst which is selective for producing Z homo-coupled products from terminal olefins (Fig. 1e). Importantly, the Ru-based isomerization catalyst is selective towards trans-olefins, and does not react appreciably with cis-olefin products produced through the Z-selective homo-coupling of terminal olefins38. This is the first report of a tandem isomerization/metathesis system to selectively form a single internal olefinic product.
Addressing catalyst incompatibility
Although the previous section highlighted recent results in tandem catalysis with no obvious issues of catalyst incompatibility, this is often not the case. In 2011, Huff and Sanford reported a compartmentalized orthogonal tandem homogeneous system for CO2 hydrogenation, in which three different catalysts operate in sequence to afford methanol (Fig. 2a)40. The first step is hydrogenation of CO2 to formic acid using a Ru hydrogenation catalyst (Cat A), followed by product esterification mediated by the Lewis acid Sc(OTf)3 (Cat B) to yield a formate ester, and finally hydrogenation of the formate ester by the (PNN)Ru catalyst (Cat C, Fig. 2a) to produce methanol. In practice, an optimized CO2:H2 feed ratio of 10:30 bar was determined for catalysts A and C separately. Next a preliminary tandem conversion of CO2 to methanol was investigated for catalysts A and C separately, and then an initial one-pot tandem CO2 to methanol conversion was achieved using all three catalysts with TON of 2.5 over 16 hours at 135 °C. Further experiments revealed that the (PNN) NATURE CHEMISTRY | VOL 7 | JUNE 2015 | www.nature.com/naturechemistry
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PERSPECTIVE
NATURE CHEMISTRY DOI: 10.1038/NCHEM.2262 a
W
W
b
W 2 H2
SiO2 R
R
H2 C
CH2
R
R
R
R Pt
Pt
Pt
R R H3C
X
CH3
N
X = CH2, O R = t Bu, i Pr
PR2
N
Cat MH2
Mo
X' R
R
Mo
= Cat M
Ar
Statistical mixture of alkanes
Cat B
H C2
R
= Cat B R
X
R
X, X' = -OSiPh3, -OC(CF3)2CH3, N
CH2
Trip X = HIPTO =
N
X
X
PR2 Ir
Ar
PR2
Ir
R2P
CH3
Cat M
R
c
X
R
H3C
R
Al2O3
2 H2
R
Overall
R
R
O
Trip
R
O O Al
Trip
O
d
Trip
Idealized tandem scheme: R R
O O N
R 2
e
Cat 1
R
R
Cat 2 R
R
Olefin isomerization/metathesis + C2H4
Ru cat
Overall reaction:
W cat
R
+
R
Cat 1 and 2 R
R
DIPP N BArF4–
+ Ru
H3CCN t
Bu
N P
N N
PR2
W Cat 1:
O Trip
Cl
Ta
O
IrH4
Cat 2:
Cl
PR2
Trip R=
PtBu2 Ir
O
PtBu2
t Bu, i Pr
Figure 1 | Examples of orthogonal tandem catalytic systems. a, Molecular distribution/molecular averaging of alkanes over solid catalysts. b, Alkane metathesis with homogeneous (PCP)Ir and Mo-alkylidene catalysts. X, X’ = pyrrole, OSiPh3, OC(CF3)2CH3, HIPTO = O-(2,6-Trip)-C6H3, Trip = 2,4,6-tri-iPr-benzene. c, Supporting homogeneous Ir and Mo catalysts on Al2O3 via the para position of the central pincer ring and HIPTO ligand. d, Idealized scheme and catalysts for alkene dimerization/transfer hydrogenation, followed by hydrogenation to produce a new alkane mixture (Fig. 2a). e, One-pot olefin isomerization/metathesis selective for producing Z homo-coupled products. DIPP: 2,6-di-iPr-benzene; BArF4− = [B(3,5-(CF3)2-C6H3]4−.
Ru ester hydrogenation activity is severely inhibited by the Sc(OTf)3 esterification catalyst (Cat B). To suppress unwanted interactions, Huff and Sanford introduced a physical barrier in the Parr reactor between the Ru + Sc(OTf)3 catalyst mixture and the (PNN)Ru catalyst 40. Reasonably assuming that the formate ester (bp = 32 °C at STP) would transfer/condense over the partition to reach the (PNN) Ru catalyst, this system achieved a greater TON, up to 21. A recent elegant approach to address catalyst incompatibility issues was also demonstrated by the groups of Scott and Zhao41. There is great interest in converting cellulosic biomass, particularly that derived from simple carbohydrates such as glucose, into the important chemical intermediate 5-hydroxylmethylfurfural (HMF)42,43 (for a computational analysis of Brønsted acid-catalysed glucose to HMF conversion, see ref. 44). Although fructose can be directly converted to HMF by acid catalysis20,45, the enzymatic conversion of glucose to fructose, followed by conversion to HMF by a one-pot process, would be highly desirable46. Until early 2014, this strategy required two-step processes because of the incompatibility of the isomerase enzyme with typical organic solvents and aqueous acids43,45,46. To realize a single-pot process, Scott and Zhao enabled a compartmentalized orthogonal tandem process by immobilizing the enzyme within a protective mesoporous silica and using a solid, rather than aqueous,
acid catalyst to minimize interactions with the enzyme (Fig. 2b)41. A variety of enzyme/silica materials with varying pore sizes and silica surface functionalization were investigated to discover the most effective and stable material: the thermophilic Thermotoga neapolitana enzyme immobilized on NH2-FMS30 silica41. Combining this catalyst in the same reactor with an SBA-15-type solid acid in 4:1 THF:H2O produced HMF from glucose. Reaction at 90 °C for 1 hour (optimal enzymatic activity), then ramping it to 130 °C for 24 hours, produced an overall yield of 30% HMF and 61% fructose from glucose. The limitation of this one-pot tandem catalytic approach is that the enzyme catalyst cannot be recycled, because of a short (15.8-min) lifetime and the great disparity in relative rates of the two catalytic cycles41. In a similar vein, Bäckvall and co-workers have reported seminal work on tandem metal catalyst/enzyme catalyst systems for dynamic kinetic resolution of alcohols, bringing the field of tandem catalysts using enzymes to synthetic maturity 47,48. To overcome limitations involving catalyst compatibility and lifetimes, Bäckvall and co-workers were able to find conditions that enabled dynamic kinetic resolution at almost the same rate as the individual steps (alcohol racemization and enzymatic esterification) through catalyst selection and by allowing the pre-catalysts to react before substrate addition (see refs 47, 48 for more information than can be discussed here).
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PERSPECTIVE
NATURE CHEMISTRY DOI: 10.1038/NCHEM.2262
a
2H2
ROH O
3 H2 + CO2
Cat A
Cat B
OH
CH3OH + H2O
O Cat C
OR
Overall reaction: 3 H2 + CO2
75–135 oC
CH3OH + H2O
H PtBu2 PMe3 Me3P PMe3 Ru OAc Me3P Cl
N Sc(OTf)3
Cat A
b
Ru N
Cat B
C
Cat C
HO
OH
HO
Physical barrier
CO
OH O
A+B
NH22
OH
NH2
O OH
HO
Isomerase enzyme
OH
O
H22N
HO
OH
HO SO3H
SO3H
O
SO3H
H22N SiO2
SiO2
SiO2
c Cl
[Rh(cod)OH]2 BINAP
Cl
NHMs
Pd(OAc)2 XPhos
Catalyst inactivation:
Ms N
BINAP + [Rh(L)n ]
[Rh(BINAP)OH]2
inactive Ar-B(OH)2
Ar
Ar Pd(XPhos)Ln
NHMs
Pd(BINAP)Ln + XPhos inactive
Figure 2 | Physical separation of catalysts can be essential. a, In the homogeneous hydrogenation of CO2 to methanol, separating catalysts A + B from C by a physical barrier in the reactor increases the TON. b, Conversion of glucose to hydroxymethylfurfural using a supported glucose isomerase enzyme and acid catalysts, respectively. c, Tandem arylation/C–N coupling in one pot; Ms = methanesulfonyl; BINAP = racemic 2,2’-bis(diphenyl-phosphino)-1,1’-binaphthyl; cod = 1,5-cyclooctadiene; XPhos = 2-dicyclohexyl phosphino-2’,4’,6’-triisopropylbiphenyl.
The Lautens group reported a slightly different strategy to address catalyst–ligand incompatibility.49 They showed that tandem Rh-catalysed alkyne arylation + Pd-catalysed C–X (X = N, O) coupling produces new pharmacologically relevant heterocycles (Fig. 2c)49,50. This catalytic system uses two different metal/ligand combinations to selectively form a single product despite the possibility of different, multiple reaction pathways. The optimal catalyst for the alkyne arylation step (Step 1) is found to be [Rh(cod)OH]2 + BINAP, and Pd(OAc)2 + XPhos for C–N coupling (step 2)49. Control experiments showed that Rh/XPhos and Pd/BINAP exhibit very little activity for arylation (~5% yield) and no activity for C–N coupling. The authors find that Pd binds BINAP and XPhos competitively, and in a multistep catalytic sequence in the absence of XPhos, Pd competes with Rh for BINAP, depressing the yield from the first catalytic cycle from 70% to 5%. They also report that excess BINAP inhibits the C–N coupling process by competitive complexation of Pd, thereby deactivating it for this transformation49. The authors conclude that while the two catalytic cycles turn over independently with minimal mutual interference, complexities arise with the two specific ligands involved. By premixing the precatalysts with their respective ligands and determining the optimal catalyst:ligand ratio for maximum activity (5mM Rh, 1.05 eq. BINAP + 8mM Pd with 2 eq. XPhos), they achieve an overall yield for the two catalytic steps of 78%. Performing the reaction with a two-step heating sequence (60 °C to 90 °C) provides a modest increase in yield, to 81%. Lautens et al. have recently extended this tandem approach by incorporating chiral ligands to produce chiral aza-dihydrodibenzoxepines50. 480
Thermodynamic leveraging by tandem catalysis
The Marks group has recently pursued the strategy of using coupled tandem catalytic cycles to achieve etheric C–O hydrogenolysis. Capitalizing on the accrued mechanistic/thermodynamic understanding of alkenol hydroalkoxylation/cyclization catalysed by Lewis acidic lanthanide triflates51, the microscopic reverse of dehydro alkoxylation — endothermic C–O scission (Fig. 3; ΔH ≈ +14 kcal mol−1) — can be driven along the same reaction coordinate by coupling this process with exothermic alkene hydrogenation (Fig. 3; ΔH ≈ −25 kcal mol−1) in a single pot. This affords rapid, regiospecific catalysis of endothermic C–O scission to yield alkenols7, which are rapidly captured by a supported hydrogenation catalyst to close the catalytic cycle and produce saturated alkanols, leading to an overall exothermic/exoergic process (Fig. 3). Exploratory and mechanistic studies performed in the Marks group initially demonstrated that commercially available, recyclable lanthanide triflates in ionic liquids (IL, EMIM+OTf−, EMIM+ = 1-ethyl3-methylimidazolium) are efficient for C–O forming alkene hydroalkoxylation52,53. Whereas commercially available Pd-based olefin hydrogenation catalysts undergo deactivation through significant agglomeration in EMIM+OTf−, using sinter-resistant Pd nanoparticles deposited on Al2O3 by atomic layer deposition (Pd@ALD)51 affords efficient hydrogenation of the alkenol intermediate to the alkanol product7. More recently, this tandem approach was extended to other solvents and solvent-free systems, as well as to other metal triflates and hydrogenation catalysts, in combined experimental and DFT computational investigations8,18. Computations reveal that NATURE CHEMISTRY | VOL 7 | JUNE 2015 | www.nature.com/naturechemistry
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PERSPECTIVE
NATURE CHEMISTRY DOI: 10.1038/NCHEM.2262 ‡
a
(TfO)3Ln H 40
δ− O
b O
n
M(OTf)n
δ+
R1 R2
O
i M(OTf)n
Enthalpy (kcal mol–1)
30
ii i
20
O n
H 10
HO
+H2
O H
R1
n
M(OTf)n
OH
R2
ii
R2
R1
H2 Pd OH
0
HO
R1
n
R2
M(OTf)n M = Hf, n = 4
iii
H2 Pd
+ H2O
Reaction coordinate
Figure 3 | Scenario for tandem catalytic ether hydrogenolysis via retro-hydroalkoxylation. a, Approximate reaction coordinate; b, catalytic cycle showing (i) conversion of a cyclic ether into an intermediate alkenol followed by (ii) hydrogenation of the alkenol to alkanol, and (iii) subsequent conversion to saturated hydrocarbon and water, mediated by Hf(OTf)4.
metal triflates with higher effective charge densities/smaller ionic radii should exhibit greater catalytic activity. In concert, it was found experimentally that Hf(OTf)4 exhibits far greater C–O hydrogenolysis activity than the lanthanide triflates, cleaving both etheric and alcoholic C–O bonds (Fig. 3) with turnover frequencies for neat substrates as high as 4,000 h−1 at 110 °C/1 atm H2 pressure8. Note that performing these reactions in either neat substrate or saturated alkanes also permits use of Pd/C as the hydrogenation catalyst. This methodology is effective for cleaving a broad spectrum of etheric and alcoholic C–O bonds, giving high alkane yields without skeletal rearrangement, and using commercially available hydrogenation catalysts with low catalyst loadings under solvent-free reaction conditions. Gordon and coworkers have also reported a variant of this reaction type using either a Lewis acid or aqueous H+ with Pd/C/H2 to hydrogenate ketones in a single pot, followed by water elimination to form alkanes54.
Outlook
This Perspective overviews several recent examples illustrating the breadth and attractive characteristics of orthogonal tandem catalytic processes. Although there have been successful examples in which the catalytic cycles appear not to interfere, in reality some systems are complex. For example, although two component catalysts might not interact directly, the reaction conditions for the efficient turnover of each catalyst may be vastly different 15,17,19. Ultimately, different catalysts must accommodate the same reaction conditions and catalyst lifetimes over the relevant reaction times. Substantial recent progress has been made in dealing with cases in which the different catalysts and ligands are mutually incompatible40,41,45,55. Placing a physical barrier between catalysts while allowing full access to catalytic reactants and intermediates can be achieved by means of equipment design40 or by supporting one (or both) catalysts on a protective solid phase41. But as shown by Scott and Zhao41, this tactic does not necessarily solve the problem of maintaining catalyst activity or lifetimes in these new environments or dealing with mass transfer limitations. The
Lautens group has also demonstrated that by premixing precatalyst and ligands before catalytic runs, while optimizing catalyst:ligand ratios, ligand poisoning effects can be significantly suppressed49,50. In view of the recent progress being made in orthogonal tandem catalysis, we wish also to highlight the thermodynamic leveraging approach of coupling the endothermic/endergonic reverse of a well-characterized catalytic process with a second exothermic/ exergonic process, to render the combined process thermodynamically favourable7,8. Coupling the energy of an exothermic/exergonic reaction in the same pot as a partner endothermic/endergonic reaction can drive the formation of products via reaction coordinates otherwise inaccessible under standard single-reaction conditions. This underused simple strategy has great potential. Indeed, there are multiple combinations of reactions and catalysts that are ripe for experimental and mechanistic study; for example, could this methodology be applied to other hydroelementation reactions and their reverse? The determination of new combinations of reactions that meet the prerequisite thermodynamic considerations for this type of tandem catalysis is an area with a bright future. Received 27 January 2015; accepted 13 April 2015; published online 20 May 2015
References
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7. Atesin, A. C., Ray, N. A., Stair, P. C. & Marks, T. J. Etheric C–O bond hydrogenolysis using a tandem lanthanide triflate/supported palladium nanoparticle catalyst system. J. Am. Chem. Soc. 134, 14682–14685 (2012). 8. Li, Z., Assary, R. S., Atesin, A. C., Curtiss, L. A. & Marks, T. J. Rapid ether and alcohol C–O bond hydrogenolysis catalyzed by tandem high-valent metal triflate plus supported Pd catalysts. J. Am. Chem. Soc. 136, 104–107 (2014). 9. Wang, X. & Rinaldi, R. A route for lignin and bio-oil conversion: dehydroxylation of phenols into arenes by catalytic tandem reactions. Angew. Chem. Int. Ed. 52, 11499–11503 (2013). 10. Barber, D. M., Ďuriš, A., Thompson, A. L., Sanganee, H. J. & Dixon, D. J. Onepot asymmetric nitro-mannich/hydroamination cascades for the synthesis of pyrrolidine derivatives: combining organocatalysis and gold catalysis. ACS Catal. 4, 634–638 (2014). 11. Dydio, P., Ploeger, M. & Reek, J. N. H. Selective isomerization– hydroformylation sequence: a strategy to valuable α-methyl-branched aldehydes from terminal olefins. ACS Catal. 3, 2939–2942 (2013). 12. Fogg, D. E. & dos Santos, E. N. Tandem catalysis: a taxonomy and illustrative review. Coord. Chem. Rev. 248, 2365–2379 (2004). 13. Braddock, D. C. & Matsuno, A. In situ tandem allylic acetate isomerisation-ring closing metathesis: 1, 3-dimesityl-4, 5-dihydroimidazol-2-ylidene ruthenium benzylidenes and palladium(0)-phosphine combinations. Tetrahed. Lett. 43, 3305–3308 (2002). 14. Rueping, M., Dufour, J. & Bui, L. Convergent catalysis: asymmetric synthesis of dihydroquinolines using a combined metal catalysis and organocatalysis approach. ACS Catal. 4, 1021–1025 (2014). 15. Leitch, D. C., Labinger, J. A. & Bercaw, J. E. Scope and mechanism of homogeneous tantalum/iridium tandem catalytic alkane/alkene upgrading using sacrificial hydrogen acceptors. Organometallics 33, 3353–3365 (2014). 16. Yuki, Y., Takahashi, K., Tanaka, Y. & Nozaki, K. Tandem isomerization/ hydroformylation/hydrogenation of internal alkenes to n-alcohols using Rh/Ru dual- or ternary-catalyst systems. J. Am. Chem. Soc. 135, 17393–17400 (2013). 17. Leitch, D. C., Lam, Y. C., Labinger, J. A. & Bercaw, J. E. Upgrading light hydrocarbons via tandem catalysis: a dual homogeneous Ta/Ir system for alkane/alkene coupling. J. Am. Chem. Soc. 135, 10302–10305 (2013). 18. Assary, R. S., Atesin, A. C., Li, Z., Curtiss, L. A. & Marks, T. J. Reaction pathways and energetics of etheric C-O bond cleavage catalyzed by lanthanide triflates. ACS Catal. 3, 1908–1914 (2013). 19. Haibach, M. C., Kundu, S., Brookhart, M. & Goldman, A. S. Alkane metathesis by tandem alkane‑dehydrogenation‑olefin-metathesis catalysis and related chemistry. Acc. Chem. Res. 45, 947–958 (2012). 20. Peng, W‑H., Lee, Y‑Y., Wu, C. & Wu, K. C. W. Acid–base bi-functionalized, large-pored mesoporous silica nanoparticles for cooperative catalysis of onepot cellulose‑to‑HMF conversion. J. Mater. Chem. 22, 23181–23185 (2012). 21. McInnis, J. P., Delferro, M. & Marks, T. J. Multinuclear group 4 catalysis: olefin polymerization pathways modified by strong metal–metal cooperative effects. Acc. Chem. Res. 47, 2545–2557 (2014). 22. Delferro, M. & Marks, T. J. Multinuclear olefin polymerization catalysts. Chem. Rev. 111, 2450–2485 (2011). 23. Al-Amin, M., Roth, K. E. & Blum, S. A. Mechanistic studies of gold and palladium cooperative dual-catalytic cross-coupling systems. ACS Catal. 4, 622–629 (2013). 24. Popoff, N., Mazoyer, E., Pelletier, J., Gauvin, R. M. & Taoufik, M. Expanding the scope of metathesis: a survey of polyfunctional, single-site supported tungsten systems for hydrocarbon valorization. Chem. Soc. Rev. 42, 9035–9054 (2013). 25. Wasilke, J. C., Obrey, S. J., Baker, R. T. & Bazan, G. C. Concurrent tandem catalysis. Chem. Rev. 105, 1001–1020 (2005). 26. Chen, C. Y., O’Rear, D. J. & Leung, P. Molecular redistribution and molecular averaging: disproportionation of paraffins via bifunctional catalysis. Top. Catal. 55, 1344–1361 (2012). 27. Biswas, S. et al. Olefin isomerization by iridium pincer catalysts. experimental evidence for an eta(3)-allyl pathway and an unconventional mechanism predicted by DFT calculations. J. Am. Chem. Soc. 134, 13276–13295 (2012). 28. Krogh-Jespersen, K. et al. On the mechanism of (PCP)Ir-catalyzed acceptorless dehydrogenation of alkanes: A combined computational and experimental study. J. Am. Chem. Soc. 124, 11404–11416 (2002). 29. Basset, J. M., Coperet, C., Soulivong, D., Taoufik, M. & Cazat, J. T. Metathesis of alkanes and related reactions. Acc. Chem. Res. 43, 323–334 (2010). 30. Huang, Z. et al. Highly active and recyclable heterogeneous iridium pincer catalysts for transfer dehydrogenation of alkanes. Adv. Synth. Catal. 351, 188–206 (2009). 31. Dobereiner, G. E., Yuan, J., Schrock, R. R., Goldman, A. S. & Hackenberg, J. D. Catalytic synthesis of n-alkyl arenes through alkyl group cross-metathesis. J. Am. Chem. Soc. 135, 12572–12575 (2013). 32. Mclain, S. J., Sancho, J. & Schrock, R. R. Selective dimerization of monosubstituted alpha-olefins by tantalacyclopentane catalysts. J. Am. Chem. Soc. 102, 5610–5618 (1980). 482
33. Mclain, S. J., Schrock, R. R., Sharp, P. R., Churchill, M. R. & Youngs, W. J. Synthesis of monomeric niobium-benzyne and tantalum-benzyne benzyne complexes and the molecular-structure of Ta(Eta‑5‑C5Me5)(C6H4)Me2. J. Am. Chem. Soc. 101, 263–265 (1979). 34. Mclain, S. J., Sancho, J. & Schrock, R. R. Metallacyclopentane to metallacyclobutane ring contraction. J. Am. Chem. Soc. 101, 5451–5453 (1979). 35. Goldman, A. S. et al. Catalytic alkane metathesis by tandem alkane dehydrogenation olefin metathesis. Science 312, 257–261 (2006). 36. Choi, J., MacArthur, A. H., Brookhart, M. & Goldman, A. S. Dehydrogenation and related reactions catalyzed by iridium pincer complexes. Chem. Rev. 111, 1761–1779 (2011). 37. Ohlmann, D. M. et al. Isomerizing olefin metathesis as a strategy to access defined distributions of unsaturated compounds from fatty acids. J. Am. Chem. Soc. 134, 13716–13729 (2012). 38. Dobereiner, G. E., Erdogan, G., Larsen, C. R., Grotjahn, D. B. & Schrock, R. R. A one-pot tandem olefin isomerization/metathesis-coupling (ISOMET) reaction. ACS Catal. 3069–3076 (2014). 39. Keim, W. Oligomerization of ethylene to alpha-olefins: discovery and development of the shell higher olefin process (SHOP). Angew. Chem. Int. Ed. 52, 12492–12496 (2013). 40. Huff, C. A. & Sanford, M. S. Cascade catalysis for the homogeneous hydrogenation of CO2 to Methanol. J. Am. Chem. Soc. 133, 18122–18125 (2011). 41. Huang, H. et al. Tandem catalytic conversion of glucose to 5-hydroxymethylfurfural with an immobilized enzyme and a solid acid. ACS Catal. 4, 2165–2168 (2014). 42. Mascal, M. & Nikitin, E. B. Direct, high-yield conversion of cellulose into biofuel. Angew. Chem. Int. Ed. 47, 7924–7926 (2008). 43. Huang, R. L., Qi, W., Su, R. X. & He, Z. M. Integrating enzymatic and acid catalysis to convert glucose into 5-hydroxymethylfurfural. Chem. Commun. 46, 1115–1117 (2010). 44. Yang, G., Pidko, E. A. & Hensen, E. J. M. Mechanism of Bronsted acidcatalyzed conversion of carbohydrates. J. Catal. 295, 122–132 (2012). 45. Simeonov, S. P., Coelho, J. A. S. & Afonso, C. A. M. Integrated chemoenzymatic production of 5-hydroxymethylfurfural from glucose. ChemSusChem 6, 997–1000 (2013). 46. Grande, P. M., Bergs, C. & de Maria, P. D. Chemo-enzymatic conversion of glucose into 5-hydroxymethylfurfural in seawater. ChemSusChem 5, 1203–1206 (2012). 47. Martin-Matute, B., Edin, M., Bogar, K., Kaynak, F. B. & Bäckvall, J. E. Combined ruthenium(ii) and lipase catalysis for efficient dynamic kinetic resolution of secondary alcohols. Insight into the racemization mechanism. J. Am. Chem. Soc. 127, 8817–8825 (2005). 48. Martin-Matute, B., Edin, M., Bogar, K. & Bäckvall, J. E. Highly compatible metal and enzyme catalysts for efficient dynamic kinetic resolution of alcohols at ambient temperature. Angew. Chem. Int. Ed. 43, 6535–6539 (2004). 49. Panteleev, J., Zhang, L. & Lautens, M. Domino rhodium-catalyzed alkyne arylation/palladium-catalyzed N arylation: a mechanistic investigation. Angew. Chem. Int. Ed. 50, 9089–9092 (2011). 50. Friedman, A. A., Panteleev, J., Tsoung, J., Huynh, V. & Lautens, M. Rh/Pd catalysis with chiral and achiral ligands: domino synthesis of aza-dihydrodibenzoxepines. Angew. Chem. Int. Ed. 52, 9755–9758 (2013). 51. Seo, S. Y., Yu, X. H. & Marks, T. J. Intramolecular hydroalkoxylation/cyclization of alkynyl alcohols mediated by lanthanide catalysts. scope and reaction mechanism. J. Am. Chem. Soc. 131, 263–276 (2009). 52. Dzudza, A. & Marks, T. J. Efficient intramolecular hydroalkoxylation of unactivated alkenols mediated by recyclable lanthanide triflate ionic liquids: scope and mechanism. Chem. Eur. J. 16, 3403–3422 (2010). 53. Dzudza, A. & Marks, T. J. Efficient intramolecular hydroalkoxylation/ cyclization of unactivated alkenols mediated by lanthanide triflate ionic liquids. Org. Lett. 11, 1523–1526 (2009). 54. Sutton, A. D. et al. The hydrodeoxygenation of bioderived furans into alkanes. Nature Chem. 5, 428–432 (2013). 55. Schwartz, T. J. et al. Integration of chemical and biological catalysis: production of furylglycolic acid from glucose via cortalcerone. ACS Catal. 3, 2689–2693 (2013).
Acknowledgements
This work was supported by the US Department of Energy under contract DE-AC0206CH11357 to the EFRC Institute for Atom-Efficient Chemical Transformations, and by NSF grant CHE-1213,235 on basic f-element chemistry which supported T.L.L. We thank M. Delferro and R. S. Assary for comments and suggestions.
Additional information
Reprints and permissions information is available online at www.nature.com/reprints. Correspondence should be addressed to T.J.M.
Competing financial interests
The authors declare no competing financial interests. NATURE CHEMISTRY | VOL 7 | JUNE 2015 | www.nature.com/naturechemistry
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Orthogonal tandem catalysis Tracy L. Lohr and Tobin J. Marks* Tandem catalysis is a growing field that is beginning to yield important scientific and technological advances toward new and more efficient catalytic processes. ‘One-pot’ tandem reactions, where multiple catalysts and reagents, combined in a single reaction vessel undergo a sequence of precisely staged catalytic steps, are highly attractive from the standpoint of reducing both waste and time. Orthogonal tandem catalysis is a subset of one-pot reactions in which more than one catalyst is used to promote two or more mechanistically distinct reaction steps. This Perspective summarizes and analyses some of the recent developments and successes in orthogonal tandem catalysis, with particular focus on recent strategies to address catalyst incompatibility. We also highlight the concept of thermodynamic leveraging by coupling multiple catalyst cycles to effect challenging transformations not observed in single-step processes, and to encourage application of this technique to energetically unfavourable or demanding reactions.
T
he quest for new methodologies to assemble complex organic molecules continues to be a great impetus to research efforts to discover or to optimize new catalytic transformations. In this connection, performing reactions in a ‘one-pot’ (single reactor), atom-efficient scenario is an attractive target. Greater economy of time and energy maximize resources and overall process simplicity, as well as decreasing materials loss from multiple iterations of reaction, workup and purification1–6. Performing multiple tandem reactions in a one-pot mode also permits thermodynamic leveraging by coupling multiple catalytic cycles7–9. One-pot reaction types include, but are not limited to, cascade (domino) processes1,10, tandem catalysis (catalysts performing sequential transformations)7,8,11–19, multifunctional catalysts having more than one active site1,2,20–22, dual catalyst systems where one catalyst enhances or alters the properties of the other catalytic cycle23, and one-pot reactions involving isolated catalytic cycles, for example where a second catalyst is added after the first has completed its transformation, or where the first catalyst is selectively deactivated later by addition of a second reagent 11. Although there have been reviews encompassing diverse one-pot catalytic reactions as a whole, as well as tandem catalytic approaches to polymerization3,24, this Perspective focuses on the specific subset of orthogonal tandem catalysis, an area that has attracted much recent attention and is being applied to increasingly challenging synthetic problems. Mechanistically distinct from cascade or domino catalysis, in which a single catalytic transformation occurs sequentially 1,12, orthogonal tandem catalysis12,25 is a one-pot reaction in which sequential catalytic processes occur through two or more functionally distinct, and preferably non-interfering, catalytic cycles (see Box 1 for a nomenclature flowchart of sequential one-pot processes). Tandem catalysis has also been subcategorized into auto- and assisted-tandem catalytic cycles (Box 1)12,25. Auto-tandem catalysis uses a single precatalyst to effect two or more mechanistically distinct catalytic cycles, typically by cooperative interaction between the various species in the system. In contrast, assisted tandem catalysis requires deliberate intervention in the system to switch between one catalytic cycle and another. The purpose of this Perspective is to survey and analyse recent (post-2011) developments in orthogonal tandem catalysis. We highlight the challenges and some recent solutions in realizing tandem catalytic systems, such as controlling and optimizing selectivity, and overcoming detrimental interactions between multiple catalysts and their ligands. We also review recent successes in thermodynamically
leveraging coupled catalytic cycles to afford products energetically unfavourable in single-catalyst systems. The objective is to reveal the power of this approach in designing catalytic transformations and to encourage its use to address current challenges.
Hydrocarbon upgrading
Orthogonal tandem catalysis first scored an important success in the area of hydrocarbon upgrading (Fig. 1a,b). Combining olefin hydrogenation/dehydrogenation with olefin metathesis leads to the overall metathesis of medium-weight (C3–C8) alkanes to a mixture of heavier and lighter n-alkanes, the former being desirable for diesel and jet fuels and the latter constituting a large portion of natural gas liquids19,26. The pioneering work on alkane metathesis using heterogeneous catalysts by Burnett and Hughes at Chevron, and work with complementary homogeneous systems by Goldman and Brookhart, were both reviewed in 2012, and readers are directed to those two articles and references therein for more detailed discussions19,26. Computational analyses of both alkane metathesis steps, alkane dehydrogenation and olefin metathesis, have also been discussed27,28 for typical (PCP)Ir catalysts (PCP = C6H3-2,6-(CH2PtBu2)2). The heterogeneous hydrocarbon upgrading system invented by Burnett and Hughes combines a solid hydrogenation catalyst (typically Pt/Al2O3) with a solid olefin metathesis catalyst (for example W/SiO2) in the same reactor to accomplish what is termed the ‘molecular redistribution and molecular averaging’ (MR/MA) of alkanes (Fig. 1a)26,29. This physical mixture of catalysts effects three key steps: (i) alkane dehydrogenation to form olefins, (ii) metathesis of the newly formed olefins, and (iii) hydrogenation of the unsaturated metathesis products. The process operates at relatively high temperatures (>343 °C) and produces a quasi-Gaussian distribution of higher and lower n-alkanes from a given alkane feedstock. The homogeneous alkane metathesis system developed by Brookhart and Goldman uses Ir pincer catalysts for hydrogenation/ dehydrogenation, and Schrock-type Mo and W catalysts for olefin metathesis (Fig. 1b)19. The homogeneous system operates at relatively low temperatures (125 °C) and for a variety of Ir pincer catalysts generally affords Gaussian distributions of alkanes (centred around the starting alkane), with moderate molecular weight selectivity (specifically converting Cn n-alkanes to C2n−2 n-alkanes + ethane) observed using a (tBu4PCP)Ir-based catalyst in tandem with W- or Mo-alkylidenes19. To enable catalyst recycling and recoverability, both the (PCP)Ir and W-alkylidene catalysts were supported on
Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, USA. *e-mail: [email protected] NATURE CHEMISTRY | VOL 7 | JUNE 2015 | www.nature.com/naturechemistry
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PERSPECTIVE
NATURE CHEMISTRY DOI: 10.1038/NCHEM.2262
Box 1 | One-pot catalyst schemes.
This Perspective surveys orthogonal tandem catalytic processes where multiple catalysts, operating under different mechanisms, are combined in a single reaction pot. Not all multi-step, one-pot processes fit this definition, and so it is distinguished here from the related domino processes, where sequential transformations occur by a single catalytic pathway, and cascade catalytic processes, where typically three or more domino processes occur. Such processes are not the subject of this Perspective. a
Are all catalysts present at time zero? No
Yes
Is there more than one mechanism?
Isolated catalytic events
No
Yes
Cascade or domino
Tandem catalysis Multiple catalysts
Single precatalyst Intervention
Orthogonal tandem catalysis Auto-tandem catalysis
Assisted tandem catalysis
Compartmentalized
b Reactant
Cat A
Intermediate
Cat B
Product
Figure B1 | Types of multi-step catalysis. a, A flowchart guide to nomenclature of multi-step one-pot catalytic processes. b, The classical picture of orthogonal tandem catalysis.
Al2O3 by installation of a basic functionality in the para position of the central pincer ring 30 and on the HIPTO ligand, respectively (Fig. 1c). Coupling supported-(PCP)Ir catalysts and solid Re2O7/ γ-Al2O3 metathesis catalysts also produces substantially higher turnover numbers (TONs) than the homogeneous analogues, and produces turnover frequencies (TOFs) as high as 2,400 h−1 at 175 °C over a period of 14 days19. More recently, Goldman, Schrock and coworkers extended the substrate scope of the homogeneous tandem system to include the cross coupling of n-alkyl arenes31. As previously noted, alkane metathesis affords a near-Gaussian product distribution of alkanes with very little product selectivity. To overcome this, Bercaw, Labinger and co-workers ingeniously approached the issue of light hydrocarbon upgrading through a homogeneous process of tandem transfer hydrogenation and alkene dimerization15,17. The idealized tandem approach for upgrading mixtures of alkanes and alkenes is shown in Fig. 1d. One catalyst mediates dimerization of the alkene component (Cn) to the corresponding C2n alkene while the second catalyst mediates the transfer hydrogenation between alkane and the C2n alkene, thereby hydrogenating the C2n alkene component to the upgraded alkane and regenerating alkene for subsequent dimerization17. The overall cycle 478
requires one equivalent each of alkane and alkene to selectively produce a higher alkane. The dual homogeneous system investigated for transfer hydrogenation uses the Ta catalyst Cp*TaCl2(alkene) developed by Schrock and co-workers32–34 for alkene dimerization and a variety of (PCP)Ir catalysts19,35,36 (Fig. 1d). Mechanistic analysis of the above tandem catalytic system reveals that the two catalysts operate independently in solution with no detrimental ‘cross-talk’. The main limitation, however, is that the two types of catalysts operate most efficiently in vastly different kinetic regimes, making it challenging to realize optimal conditions for a tandem process15. Thus, the Ta alkene dimerization catalyst operates most efficiently at high [alkene], with an approach to kinetic saturation at ~4M alkene, and at lower temperatures, with the rate being relatively insensitive to temperature at high [alkene] ~4M. In contrast, the (PCP)Ir catalyst is most active at higher temperatures (125 °C) with a rate law that is inverse order in [alkene], opposite to conditions for favourable alkene dimerization15. Therefore, the initial optimized conditions for tandem catalysis were found to be at moderate temperatures (100 °C) and [alkene] ~0.25M, to allow realistic turnover rates for both catalytic cycles. Notably, increasing the temperature and [alkene] produces a complementary effect, and optimum yields of alkane/alkene coupled products are obtained at 150 °C with [alkene] ~1M. So far, the full tandem cycle in Fig. 2d has not been fully realized, as only higher alkene products are formed over alkanes. Control experiments show that the highly branched alkenes formed using the Ta dimerization catalyst are both kinetically and thermodynamically unsuitable for transfer hydrogenation under these conditions15. Alkene dimerization catalysts that are selective for linear alkenes will be required in future attempts to incorporate this tandem system into large-scale conversion of mixed alkane/alkene feedstocks. Both Schrock and Gooβen have reported recent progress in the area of one-pot olefin isomerization/metathesis, which is mechanistically similar to the multi-step Shell higher olefin process (SHOP)37–39. Thus, Gooβen and co-workers successfully combined Pd isomerization and Ru metathesis catalysts to convert single olefins to regular distributions of olefinic products37. The latest report 38 from Schrock and Grotjahn’s groups involves one-pot tandem olefin isomerization/metathesis-coupling (ISOMET) using a Ru-based alkene ‘zipper’-type catalyst that produces equilibrium mixtures of trans- and terminal olefins combined with a W-based olefin metathesis catalyst which is selective for producing Z homo-coupled products from terminal olefins (Fig. 1e). Importantly, the Ru-based isomerization catalyst is selective towards trans-olefins, and does not react appreciably with cis-olefin products produced through the Z-selective homo-coupling of terminal olefins38. This is the first report of a tandem isomerization/metathesis system to selectively form a single internal olefinic product.
Addressing catalyst incompatibility
Although the previous section highlighted recent results in tandem catalysis with no obvious issues of catalyst incompatibility, this is often not the case. In 2011, Huff and Sanford reported a compartmentalized orthogonal tandem homogeneous system for CO2 hydrogenation, in which three different catalysts operate in sequence to afford methanol (Fig. 2a)40. The first step is hydrogenation of CO2 to formic acid using a Ru hydrogenation catalyst (Cat A), followed by product esterification mediated by the Lewis acid Sc(OTf)3 (Cat B) to yield a formate ester, and finally hydrogenation of the formate ester by the (PNN)Ru catalyst (Cat C, Fig. 2a) to produce methanol. In practice, an optimized CO2:H2 feed ratio of 10:30 bar was determined for catalysts A and C separately. Next a preliminary tandem conversion of CO2 to methanol was investigated for catalysts A and C separately, and then an initial one-pot tandem CO2 to methanol conversion was achieved using all three catalysts with TON of 2.5 over 16 hours at 135 °C. Further experiments revealed that the (PNN) NATURE CHEMISTRY | VOL 7 | JUNE 2015 | www.nature.com/naturechemistry
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PERSPECTIVE
NATURE CHEMISTRY DOI: 10.1038/NCHEM.2262 a
W
W
b
W 2 H2
SiO2 R
R
H2 C
CH2
R
R
R
R Pt
Pt
Pt
R R H3C
X
CH3
N
X = CH2, O R = t Bu, i Pr
PR2
N
Cat MH2
Mo
X' R
R
Mo
= Cat M
Ar
Statistical mixture of alkanes
Cat B
H C2
R
= Cat B R
X
R
X, X' = -OSiPh3, -OC(CF3)2CH3, N
CH2
Trip X = HIPTO =
N
X
X
PR2 Ir
Ar
PR2
Ir
R2P
CH3
Cat M
R
c
X
R
H3C
R
Al2O3
2 H2
R
Overall
R
R
O
Trip
R
O O Al
Trip
O
d
Trip
Idealized tandem scheme: R R
O O N
R 2
e
Cat 1
R
R
Cat 2 R
R
Olefin isomerization/metathesis + C2H4
Ru cat
Overall reaction:
W cat
R
+
R
Cat 1 and 2 R
R
DIPP N BArF4–
+ Ru
H3CCN t
Bu
N P
N N
PR2
W Cat 1:
O Trip
Cl
Ta
O
IrH4
Cat 2:
Cl
PR2
Trip R=
PtBu2 Ir
O
PtBu2
t Bu, i Pr
Figure 1 | Examples of orthogonal tandem catalytic systems. a, Molecular distribution/molecular averaging of alkanes over solid catalysts. b, Alkane metathesis with homogeneous (PCP)Ir and Mo-alkylidene catalysts. X, X’ = pyrrole, OSiPh3, OC(CF3)2CH3, HIPTO = O-(2,6-Trip)-C6H3, Trip = 2,4,6-tri-iPr-benzene. c, Supporting homogeneous Ir and Mo catalysts on Al2O3 via the para position of the central pincer ring and HIPTO ligand. d, Idealized scheme and catalysts for alkene dimerization/transfer hydrogenation, followed by hydrogenation to produce a new alkane mixture (Fig. 2a). e, One-pot olefin isomerization/metathesis selective for producing Z homo-coupled products. DIPP: 2,6-di-iPr-benzene; BArF4− = [B(3,5-(CF3)2-C6H3]4−.
Ru ester hydrogenation activity is severely inhibited by the Sc(OTf)3 esterification catalyst (Cat B). To suppress unwanted interactions, Huff and Sanford introduced a physical barrier in the Parr reactor between the Ru + Sc(OTf)3 catalyst mixture and the (PNN)Ru catalyst 40. Reasonably assuming that the formate ester (bp = 32 °C at STP) would transfer/condense over the partition to reach the (PNN) Ru catalyst, this system achieved a greater TON, up to 21. A recent elegant approach to address catalyst incompatibility issues was also demonstrated by the groups of Scott and Zhao41. There is great interest in converting cellulosic biomass, particularly that derived from simple carbohydrates such as glucose, into the important chemical intermediate 5-hydroxylmethylfurfural (HMF)42,43 (for a computational analysis of Brønsted acid-catalysed glucose to HMF conversion, see ref. 44). Although fructose can be directly converted to HMF by acid catalysis20,45, the enzymatic conversion of glucose to fructose, followed by conversion to HMF by a one-pot process, would be highly desirable46. Until early 2014, this strategy required two-step processes because of the incompatibility of the isomerase enzyme with typical organic solvents and aqueous acids43,45,46. To realize a single-pot process, Scott and Zhao enabled a compartmentalized orthogonal tandem process by immobilizing the enzyme within a protective mesoporous silica and using a solid, rather than aqueous,
acid catalyst to minimize interactions with the enzyme (Fig. 2b)41. A variety of enzyme/silica materials with varying pore sizes and silica surface functionalization were investigated to discover the most effective and stable material: the thermophilic Thermotoga neapolitana enzyme immobilized on NH2-FMS30 silica41. Combining this catalyst in the same reactor with an SBA-15-type solid acid in 4:1 THF:H2O produced HMF from glucose. Reaction at 90 °C for 1 hour (optimal enzymatic activity), then ramping it to 130 °C for 24 hours, produced an overall yield of 30% HMF and 61% fructose from glucose. The limitation of this one-pot tandem catalytic approach is that the enzyme catalyst cannot be recycled, because of a short (15.8-min) lifetime and the great disparity in relative rates of the two catalytic cycles41. In a similar vein, Bäckvall and co-workers have reported seminal work on tandem metal catalyst/enzyme catalyst systems for dynamic kinetic resolution of alcohols, bringing the field of tandem catalysts using enzymes to synthetic maturity 47,48. To overcome limitations involving catalyst compatibility and lifetimes, Bäckvall and co-workers were able to find conditions that enabled dynamic kinetic resolution at almost the same rate as the individual steps (alcohol racemization and enzymatic esterification) through catalyst selection and by allowing the pre-catalysts to react before substrate addition (see refs 47, 48 for more information than can be discussed here).
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479
PERSPECTIVE
NATURE CHEMISTRY DOI: 10.1038/NCHEM.2262
a
2H2
ROH O
3 H2 + CO2
Cat A
Cat B
OH
CH3OH + H2O
O Cat C
OR
Overall reaction: 3 H2 + CO2
75–135 oC
CH3OH + H2O
H PtBu2 PMe3 Me3P PMe3 Ru OAc Me3P Cl
N Sc(OTf)3
Cat A
b
Ru N
Cat B
C
Cat C
HO
OH
HO
Physical barrier
CO
OH O
A+B
NH22
OH
NH2
O OH
HO
Isomerase enzyme
OH
O
H22N
HO
OH
HO SO3H
SO3H
O
SO3H
H22N SiO2
SiO2
SiO2
c Cl
[Rh(cod)OH]2 BINAP
Cl
NHMs
Pd(OAc)2 XPhos
Catalyst inactivation:
Ms N
BINAP + [Rh(L)n ]
[Rh(BINAP)OH]2
inactive Ar-B(OH)2
Ar
Ar Pd(XPhos)Ln
NHMs
Pd(BINAP)Ln + XPhos inactive
Figure 2 | Physical separation of catalysts can be essential. a, In the homogeneous hydrogenation of CO2 to methanol, separating catalysts A + B from C by a physical barrier in the reactor increases the TON. b, Conversion of glucose to hydroxymethylfurfural using a supported glucose isomerase enzyme and acid catalysts, respectively. c, Tandem arylation/C–N coupling in one pot; Ms = methanesulfonyl; BINAP = racemic 2,2’-bis(diphenyl-phosphino)-1,1’-binaphthyl; cod = 1,5-cyclooctadiene; XPhos = 2-dicyclohexyl phosphino-2’,4’,6’-triisopropylbiphenyl.
The Lautens group reported a slightly different strategy to address catalyst–ligand incompatibility.49 They showed that tandem Rh-catalysed alkyne arylation + Pd-catalysed C–X (X = N, O) coupling produces new pharmacologically relevant heterocycles (Fig. 2c)49,50. This catalytic system uses two different metal/ligand combinations to selectively form a single product despite the possibility of different, multiple reaction pathways. The optimal catalyst for the alkyne arylation step (Step 1) is found to be [Rh(cod)OH]2 + BINAP, and Pd(OAc)2 + XPhos for C–N coupling (step 2)49. Control experiments showed that Rh/XPhos and Pd/BINAP exhibit very little activity for arylation (~5% yield) and no activity for C–N coupling. The authors find that Pd binds BINAP and XPhos competitively, and in a multistep catalytic sequence in the absence of XPhos, Pd competes with Rh for BINAP, depressing the yield from the first catalytic cycle from 70% to 5%. They also report that excess BINAP inhibits the C–N coupling process by competitive complexation of Pd, thereby deactivating it for this transformation49. The authors conclude that while the two catalytic cycles turn over independently with minimal mutual interference, complexities arise with the two specific ligands involved. By premixing the precatalysts with their respective ligands and determining the optimal catalyst:ligand ratio for maximum activity (5mM Rh, 1.05 eq. BINAP + 8mM Pd with 2 eq. XPhos), they achieve an overall yield for the two catalytic steps of 78%. Performing the reaction with a two-step heating sequence (60 °C to 90 °C) provides a modest increase in yield, to 81%. Lautens et al. have recently extended this tandem approach by incorporating chiral ligands to produce chiral aza-dihydrodibenzoxepines50. 480
Thermodynamic leveraging by tandem catalysis
The Marks group has recently pursued the strategy of using coupled tandem catalytic cycles to achieve etheric C–O hydrogenolysis. Capitalizing on the accrued mechanistic/thermodynamic understanding of alkenol hydroalkoxylation/cyclization catalysed by Lewis acidic lanthanide triflates51, the microscopic reverse of dehydro alkoxylation — endothermic C–O scission (Fig. 3; ΔH ≈ +14 kcal mol−1) — can be driven along the same reaction coordinate by coupling this process with exothermic alkene hydrogenation (Fig. 3; ΔH ≈ −25 kcal mol−1) in a single pot. This affords rapid, regiospecific catalysis of endothermic C–O scission to yield alkenols7, which are rapidly captured by a supported hydrogenation catalyst to close the catalytic cycle and produce saturated alkanols, leading to an overall exothermic/exoergic process (Fig. 3). Exploratory and mechanistic studies performed in the Marks group initially demonstrated that commercially available, recyclable lanthanide triflates in ionic liquids (IL, EMIM+OTf−, EMIM+ = 1-ethyl3-methylimidazolium) are efficient for C–O forming alkene hydroalkoxylation52,53. Whereas commercially available Pd-based olefin hydrogenation catalysts undergo deactivation through significant agglomeration in EMIM+OTf−, using sinter-resistant Pd nanoparticles deposited on Al2O3 by atomic layer deposition (Pd@ALD)51 affords efficient hydrogenation of the alkenol intermediate to the alkanol product7. More recently, this tandem approach was extended to other solvents and solvent-free systems, as well as to other metal triflates and hydrogenation catalysts, in combined experimental and DFT computational investigations8,18. Computations reveal that NATURE CHEMISTRY | VOL 7 | JUNE 2015 | www.nature.com/naturechemistry
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PERSPECTIVE
NATURE CHEMISTRY DOI: 10.1038/NCHEM.2262 ‡
a
(TfO)3Ln H 40
δ− O
b O
n
M(OTf)n
δ+
R1 R2
O
i M(OTf)n
Enthalpy (kcal mol–1)
30
ii i
20
O n
H 10
HO
+H2
O H
R1
n
M(OTf)n
OH
R2
ii
R2
R1
H2 Pd OH
0
HO
R1
n
R2
M(OTf)n M = Hf, n = 4
iii
H2 Pd
+ H2O
Reaction coordinate
Figure 3 | Scenario for tandem catalytic ether hydrogenolysis via retro-hydroalkoxylation. a, Approximate reaction coordinate; b, catalytic cycle showing (i) conversion of a cyclic ether into an intermediate alkenol followed by (ii) hydrogenation of the alkenol to alkanol, and (iii) subsequent conversion to saturated hydrocarbon and water, mediated by Hf(OTf)4.
metal triflates with higher effective charge densities/smaller ionic radii should exhibit greater catalytic activity. In concert, it was found experimentally that Hf(OTf)4 exhibits far greater C–O hydrogenolysis activity than the lanthanide triflates, cleaving both etheric and alcoholic C–O bonds (Fig. 3) with turnover frequencies for neat substrates as high as 4,000 h−1 at 110 °C/1 atm H2 pressure8. Note that performing these reactions in either neat substrate or saturated alkanes also permits use of Pd/C as the hydrogenation catalyst. This methodology is effective for cleaving a broad spectrum of etheric and alcoholic C–O bonds, giving high alkane yields without skeletal rearrangement, and using commercially available hydrogenation catalysts with low catalyst loadings under solvent-free reaction conditions. Gordon and coworkers have also reported a variant of this reaction type using either a Lewis acid or aqueous H+ with Pd/C/H2 to hydrogenate ketones in a single pot, followed by water elimination to form alkanes54.
Outlook
This Perspective overviews several recent examples illustrating the breadth and attractive characteristics of orthogonal tandem catalytic processes. Although there have been successful examples in which the catalytic cycles appear not to interfere, in reality some systems are complex. For example, although two component catalysts might not interact directly, the reaction conditions for the efficient turnover of each catalyst may be vastly different 15,17,19. Ultimately, different catalysts must accommodate the same reaction conditions and catalyst lifetimes over the relevant reaction times. Substantial recent progress has been made in dealing with cases in which the different catalysts and ligands are mutually incompatible40,41,45,55. Placing a physical barrier between catalysts while allowing full access to catalytic reactants and intermediates can be achieved by means of equipment design40 or by supporting one (or both) catalysts on a protective solid phase41. But as shown by Scott and Zhao41, this tactic does not necessarily solve the problem of maintaining catalyst activity or lifetimes in these new environments or dealing with mass transfer limitations. The
Lautens group has also demonstrated that by premixing precatalyst and ligands before catalytic runs, while optimizing catalyst:ligand ratios, ligand poisoning effects can be significantly suppressed49,50. In view of the recent progress being made in orthogonal tandem catalysis, we wish also to highlight the thermodynamic leveraging approach of coupling the endothermic/endergonic reverse of a well-characterized catalytic process with a second exothermic/ exergonic process, to render the combined process thermodynamically favourable7,8. Coupling the energy of an exothermic/exergonic reaction in the same pot as a partner endothermic/endergonic reaction can drive the formation of products via reaction coordinates otherwise inaccessible under standard single-reaction conditions. This underused simple strategy has great potential. Indeed, there are multiple combinations of reactions and catalysts that are ripe for experimental and mechanistic study; for example, could this methodology be applied to other hydroelementation reactions and their reverse? The determination of new combinations of reactions that meet the prerequisite thermodynamic considerations for this type of tandem catalysis is an area with a bright future. Received 27 January 2015; accepted 13 April 2015; published online 20 May 2015
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NATURE CHEMISTRY | VOL 7 | JUNE 2015 | www.nature.com/naturechemistry
© 2015 Macmillan Publishers Limited. All rights reserved
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.2262
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Acknowledgements
This work was supported by the US Department of Energy under contract DE-AC0206CH11357 to the EFRC Institute for Atom-Efficient Chemical Transformations, and by NSF grant CHE-1213,235 on basic f-element chemistry which supported T.L.L. We thank M. Delferro and R. S. Assary for comments and suggestions.
Additional information
Reprints and permissions information is available online at www.nature.com/reprints. Correspondence should be addressed to T.J.M.
Competing financial interests
The authors declare no competing financial interests. NATURE CHEMISTRY | VOL 7 | JUNE 2015 | www.nature.com/naturechemistry
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