Thermodynamic Strategies for CâO Bond Formation and Cleavage via

Article pubs.acs.org/accounts

Thermodynamic Strategies for C−O Bond Formation and Cleavage via Tandem Catalysis Tracy L. Lohr, Zhi Li,† and Tobin J. Marks* Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113, United States CONSPECTUS: To reduce global reliance on fossil fuels, new renewable sources of energy that can be used with the current infrastructure are required. Biomass represents a major source of renewable carbon based fuel; however, the high oxygen content (∼40%) limits its use as a conventional fuel. To utilize biomass as an energy source, not only with current infrastructure, but for maximum energy return, the oxygen content must be reduced. One method to achieve this is to develop selective catalytic methods to cleave C−O bonds commonly found in biomass (aliphatic and aromatic ethers and esters) for the eventual removal of oxygen in the form of volatile H2O or carboxylic acids. Once selective methods of C−O cleavage are understood and perfected, application to processing real biomass feedstocks such as lignin can be undertaken. This Laboratory previously reported that recyclable “green” lanthanide triflates are excellent catalysts for C−O bond-forming hydroalkoxylation reactions. Based on the virtues of microscopic reversibility, the same lanthanide triflate catalyst should catalyze the reverse C−O cleavage process, retrohydroalkoxylation, to yield an alcohol and an alkene. However, ether C−O bond-forming (retrohydroalkoxylation) to form an alcohol and alkene is endothermic. Guided by quantum chemical analysis, our strategy is to couple endothermic, in tandem, ether C−O bond cleavage with exothermic alkene hydrogenation, thereby leveraging the combined catalytic cycles thermodynamically to form an overall energetically favorable C−O cleavage reaction. This Account reviews recent developments on thermodynamically leveraged tandem catalysis for ether and more recently, ester C−O bond cleavage undertaken at Northwestern University. First, the fundamentals of lanthanide-catalyzed hydroelementation are reviewed, with particular focus on ether C−O bond formation (hydroalkoxylation). Next, the reverse C−O cleavage/ retrohydroalkoxylation processes enabled by tandem catalysis are discussed for both ether and ester C−O bond cleavage, including mechanistic and computational analysis. This is followed by recent results using this tandem catalytic strategy toward biomass relevant substrates, including work deconstructing acetylated lignin models, and the production of biodiesel from triglycerides, while bypassing the production of undesired glycerol for more valuable C3 products such as diesters (precursors to diols) in up to 47% selectivity. This Account concludes with future prospects for using this tandem catalytic system under real biomass processing conditions.

1. INTRODUCTION Biomass is one of the largest potential renewable chemical feedstocks on earth. However, due to the high oxygen content (>40%) present in typical biomass, feeds must be significantly modified prior to utilization as fuels or feedstocks in current worldwide infrastructure.1−5 Selective, efficient cleavage of C−O bonds found in biomass (ethers, esters, alcohols) remains a significant challenge, and new strategies to achieve these types of transformations are critical for both the success of the biorenewable industry, and for “greener” organic synthetic methodologies.3 To design catalytic systems for selective C−O cleavage of biomass substrates, C−O cleavage processes in model ether and ester substrates must first be thoroughly understood mechanistically and optimized. One way to study the fundamental properties of C−O cleavage is to investigate and characterize the microscopic reverse process, C−O bond formation. This Laboratory has maintained a long-standing interest in hydrofunctionalization/hydroelementation, the catalytic addition © XXXX American Chemical Society

of E−H bonds (E = heteroatom, such as B, S, N, O, Si, or P) across carbon−carbon unsaturations (e.g., eq 1), as an

atom-economical approach to creating new carbon-heteroelement bonds.6−32 Olefin hydroalkoxylation (where E−H = O−H), the transformation whereby an alcoholic RO-H bond is added across a C−C multiple bond, affords ethers in an atomeconomical process.7−11,13 Lanthanide catalysts, which are attractive due to their earth abundance33 and ionic radius tunability, are remarkable catalysts for σ-bond metathesis (eq 2) and rapidly mediate insertions of carbon−carbon unsaturations into Ln−E (E = heteroelement) bonds (eq 3).6 Received: February 8, 2016

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DOI: 10.1021/acs.accounts.6b00069 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research

Scheme 1. Cycles for Intramolecular Lanthanide-Catalyzed Hydroelementationa

The purpose of this Account is to present progress at Northwestern in experimental and computationally guided lanthanide (and Group IV) catalyzed hydroalkoxylation and retrohydroalkoxylation research relevant to biomass conversions. First, we discuss results on catalytic hydroalkoxylation/C−O bond formation processes, with focus on alkene hydroalkoxylation. After discussing scope, mechanism, and energetics of intramolecular hydroalkoxylation, we discuss our approach to C−O cleavage: coupling the microscopic reverse of C−O bond formation, retrohydroalkoxylation, with olefin hydrogenation to achieve hydrogenolysis of both ether and alcohol moieties via tandem thermodynamic leveraging. Finally, we discuss recent efforts on selective cleavage of ester C−O bonds, with focus on triglyceride substrates and lignin models.

2. LANTHANIDE CATALYZED C−O BOND FORMATION (HYDROALKOXYLATION) 2.1. Homoleptic Lanthanide Amides

Lanthanide (“Ln”) and actinide organometallic complexes have been shown to catalyze hydroelementation with heteroatoms as diverse as amines (C−N formation, hydroamination),20−26,28−32 phosphines (C−P formation, hydrophosphination),34−37 alcohols (C−O formation, hydroalkoxylation),7−11,13,16 thiols (C−S formation, hydrothiolation),6,17−19 and silanes (C−Si formation, hydrosilylation).38−41 The H−E bond polarity dictates the regiochemistry of hydroelementation. In cases where the bond polarity is modest (E = B and Si), the first step in the catalytic cycle is formation of a lanthanide hydride (Scheme 1A). This is followed by insertion of the unsaturation into the Ln−H bond, and σ-bond metathesis to release the final product. In contrast, when the E−H bond polarity is large (E = O, N, P, and S), the first step is exothermic protonolysis to generate a Ln−E species which can undergo subsequent initial C−C insertion and σ-bond metathesis/protonolysis with incoming substrate (Scheme 1B). Experimental bond enthalpy data indicate that the initial C−C insertion into Ln−O bonds is only reasonable energetically favorable for alkynes and allenes (Scheme 1).6 Unfortunately, this strategy does not apply to hydroalkoxylation of alkenes because the enthalpic impediment to olefin insertion into the Ln-O bond is predicted to be endothermic by ∼22 kcal/mol. (Scheme 1B).42 Intramolecular alkyne hydroalkoxylation catalyzed by lanthanide amide catalysts was first reported by this Laboratory in 2007.13 The reader is directed to a previously published perspective for a more in-depth analysis of alkyne hydroalkoxylation than is provided here.6

a

(A) E = B, Si; (B) hydroalkoxylation with estimated enthalpies (Cp = cyclopentadienyl).

Table 1. Intramolecular Alkene Hydroalkoxylation

Ln

ionic radius (Å)

Nt (h‑1)

La Nd Sm Yb Lu

1.160 1.109 1.079 1.008 0.977

0.57 1.21 1.37 46.97 47.15

ionic liquids, with turnover frequencies (Nt) increasing with falling Ln(III) ionic radius (Table 1).42 A detailed kinetic/ mechanistic study indicates the rate law is first-order in both substrate and catalyst (eq 4), which is significantly different from hydroelementation processes mediated by homoleptic lanthanide amides (Scheme 1B), which are commonly zero-order in

2.2. Lanthanide Trifluoromethanesulfonate Catalysts for Alkene Hydroalkoxylation

This Laboratory reported that lanthanide trifluoromethanesulfonate complexes (triflates, Ln(OTf)n) are excellent Lewis acids for activating carbonyl groups in Friedel−Crafts reactions.43 These environmentally friendly, hard Lewis acids are air- and moisture-tolerant, and can be recycled with little loss in catalytic activity. Unlike homoleptic lanthanide amide catalysts, lanthanide triflates mediate intramolecular alkene hydroalkoxylation with high efficiency and selectivity in recyclable imidazolium

rate = k[M(OTf)n ]1 [substrate]1

(4)

28

substrate. Zero-order dependence on substrate concentration typically indicates that intramolecular C−C unsaturation insertion of the bound substrate into the Ln−E bond is turnover-limiting, arguing that Ln(OTf)n-catalyzed olefin hydroalkoxylation follows a different mechanistic pathway. Proton B


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Accounts of Chemical Research

coordination via the hydroxyl moiety, followed by intramolecular nucleophilic attack of the oxygen functionality across the olefin. This is followed by ring formation and substrate dissociation to yield the final product. Yb(OTf)3 mediates the clean, selective intramolecular hydroalkoxylation of primary, secondary, aliphatic, and aromatic alkenols, and the reaction scope is presented in Table 2. For primary alkenol cyclization, products favor 5 > 6 membered rings, consistent with a sterically controlled ring formation transition state. Since the mechanism of this Lewis acid-catalyzed hydroalkoxylation involves activation of the hydroxyl group proton, one might anticipate that a Brønsted acid would affect the same transformation. In fact, examples of Brønsted acid-catalyzed intra- and intermolecular addition of olefins have also been reported.44−49 In summary, triflate Lewis acid-catalyzed C−O bond forming reactions show great potential for the selective atom-economic syntheses of alcohols, ethers, and esters from the corresponding protonic precursors and olefins.

olefin and alcohol by coupling to a very exothermic reaction, for example, olefin hydrogenation, which compensates the unfavorable thermodynamics, providing a saturated alkane and alcohol (Figure 1). The overall reaction becomes exothermic through the thermodynamic leveraging of the two catalytic cycles in tandem.51 The key to realizing such a transformation is to devise a system in which the hydrogenation catalyst and metal triflate do not interfere with one another. After extensive experimentation, the combination of lanthanide triflates in imadizolium ionic liquids with a Pd nanoparticle hydrogenation catalyst stabilized by atomic-layer deposited (ALD) alumina50 yielded the desired stable and efficient system. This tandem catalyst cleaves the alkyl C−O bonds of a variety of cyclic ethers, affording the corresponding alcohols (Table 3). A variety of lanthanide triflates were screened, and catalytic activity increases with falling Ln(III) atomic radius, paralleling activity trends observed in C−O forming hydroalkoxylation processes. Tertiary and secondary ethers are readily hydrogenolyzed to corresponding saturated alkyl and alcohol moieties. However, primary C−O bonds do not participate in the hydrogenolysis under the conditions applied (Table 3). Note that the strong Brønsted acid trifluoromethanesulfonic acid (triflic acid, TfOH) does not provide comparable activity. Tandem ether C−O bond cleavage is first-order in both substrate and M(OTf)n, and zero-order in H2, yielding a rate law, υ = [substrate]1[M(OTf)n]1, in accord with the C−O cleavage step being turnover-limiting and mirroring the rate law for the microscopic reverse C−O formation.7−9 The tandem catalytic process shown in Figure 1 was further examined by DFT computation (B3LYP). Computations reveal the first step of the tandem sequence, retro-hydroalkoxylation, is highly endothermic for all substrates examined (Table 3, ΔH1), while hydrogenation of the corresponding alkenol is sufficiently exothermic to drive the overall reaction (Table 3, ΔH2).52 Second, the mechanistic scenario is supported by the agreement between experimental (2.7) and computed (2.4) kinetic isotope effects (KIE) for the substrate in Table 3, entry 11. Lastly, computational analysis reveals that Yb3+ has the greatest charge density and La3+ the smallest of the series, in accordance with experimental trends. Higher effective charge density corresponds to higher Lewis acidity, hence increased C−O activation activity.

3. ETHER AND ALCOHOL C−O BOND CLEAVAGE

3.2. Second-Generation Ether and Alcohol C−O bond Hydrogenolysis

transfer is part of the turnover-limiting step indicated by a kinetic isotope effect of 2.48(9). The proposed mechanism is presented in Scheme 2. The first step in the catalyst cycle is substrate Scheme 2. Proposed Mechanism for Ln(OTf)n-Catalyzed Intramolecular Hydroalkoxylation of a Terminal Alkenol

When hafnium triflate (Hf4+, higher effective charge density, Figure 2) is used as an ether hydrogenolysis catalyst, neither ionic liquid solvents nor ALD-stabilized palladium catalysts are necessary. Ethers are rapidly cleaved using Hf(OTf)4 and commercially available Pd/C in neat substrate. Reaction conditions depend upon substrate substitution pattern, where reactivity generally tracks corresponding carbocation stability. Hydrogenolysis reactions do not stop at alcohols, but proceed thermodynamically downhill to saturated alkanes (Table 4).53 DFT computations (B3LYP) were performed to scrutinize the energy profile from ether to alkane utilizing 2-methyltetrahydropyran (Table 4 entry 10) as a model. The first part of the reaction sequence is similar to the Ln(OTf)3-catalyzed C−O hydrogenolysis described above. The second part of the reaction sequence is Lewis acid-catalyzed dehydration-hydrogenation affording water and alkane. The computed mechanism and KIE suggest that the Lewis acid mediates both C−O cleavage and proton transfer.

3.1. First-Generation Ether C−O Bond Cleavage

The best known application of microscopic reversibility principles is that a catalyst which efficiently catalyzes a forward reaction should also excel catalyzing the reverse reaction. Metal triflates, as discussed above, effectively catalyze olefin hydroalkoxylation with alkenols to form ethers, and thus microscopic reversibility argues, they should also catalyze the reverse retrohydroalkoxylation reaction (also termed dehydroxyalkylation, or dehydroalkoxylation, Scheme 3). Unfortunately, thermodynamics do not favor uncatalyzed retro-hydroalkoxylation, cleaving an alkyl ether to an olefin and alcohol. Using the accrued understanding of metal triflatecatalyzed hydroalkoxylations noted above, the direct reverse, metal triflate catalyzed retrohydroalkoxylation enthalpy change is predicted to be endothermic with ΔH ≈ + 10−20 kcal/mol (Scheme 3).50 In practice, such equilibria lie predominantly toward the etheric functionality. However, modifying the reaction thermodynamics could drive the equilibrium toward C


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Accounts of Chemical Research Table 2. Scope of Yb(OTf)3-Catalyzed Alkenol Hydroalkoxylation

Scheme 3. Lewis Acid-Catalyzed Hydroalkoxylation and the Microscopic Reverse, Retrohydroalkoxylation

4. ESTER C−O CLEAVAGE With the success of applying the tandem M(OTf)n + Pd/C catalyst system for ether C−O bond cleavage, strategies for cleaving other C−O functionalities were pursued. Esters are ubiquitous in nature as triglycerides, and are commonly utilized as a source of biodiesel (fatty acid methyl esters), which are produced by transesterification with methanol.54−56 Therefore, there is interest in discovering new selective methods to cleave ester C−O bonds. Thermodynamically, DFT computations predict that ester C−O bond cleavage to produce a

Figure 1. Thermodynamic leveraging for ether C−O bond cleavage. D


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Accounts of Chemical Research Table 3. Scope of Catalytic Ether C−O Hydrogenolysis

Combining Hf(OTf)4 + Pd/C promotes rapid ester hydrogenolysis. Metal triflate effects on experimental rates parallel those in ether C−O hydrogenolysis, where trilfates with increased Lewis acidity/charge density exhibit higher activity. A variety of secondary and tertiary alkyl esters are efficiently cleaved ≤125 °C in neat substrate at 1 bar H2, with tertiary esters reacting fastest (Table 5). Primary alkyl esters are least reactive, undergoing cleavage at 200 °C. To probe the electronic and steric requirements of this transformation, various carboxylate functionalities were screened for their reactivity (Table 6). Thus, more electronic-withdrawing carboxylate groupsthose whose conjugate acid has the lowest pKa significantly increase the reaction rate. Steric bulk at the carboxylate moiety depresses reaction rates, likely reflecting steric repulsion between the substrate and metal triflate catalyst. Kinetically, ester C−O bond hydrogenolysis is first-order in [M(OTf)n] catalyst and zero-order in both [substrate] and H2 pressure, yielding the rate equation υ = k[M(OTf)n]1. Unlike ether C−O cleavage, where the reaction is first-order in [substrate], ester C−O cleavage is operationally zero-order in [substrate], essentially running under saturation kinetics. DFT computations on the model substrate cyclohexyl acetate yield an

Figure 2. Catalytic activity correlates with DFT-derived effective metal charge density (ρ).

carboxylic acid and alkene is slightly exergonic (Figure 3), however, when coupled in tandem with a largely exergonic olefin hydrogenation step, the overall reaction is driven forward to completion.57 E


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Accounts of Chemical Research Table 4. Catalytic Hydrogenolysis of Alcohol and Ether C−O Bondsa

a

Conditions: 0.5 mol % Hf(OTf)4, 0.2 mol % Pd/C (10 wt %), neat solvent, 1 bar H2.

strongly/essentially irreversibly to Hf(OTf)4 through the carbonyl oxygen before undergoing C−O cleavage to yield a cyclohexyl cation and carboxylate anion, stabilized by a OTfoxygen and the Hf center, respectively. H+ transfer from the cyclohexyl cation to the coordinated carboxylate anion is ratelimiting, with a computed KIE of 6.3, in good agreement with the experimental value of 6.5 (0.5) (Figure 4). The computed mechanistic profile is also consistent with the observation that electronic-withdrawing carboxylates turnover more rapidly. More electron-withdrawing carboxylates form much less stable catalyst-substrate complexes (B), which compensates for the less favorable H+ transfer to the less “basic” carboxylate anion (E, Figure 4).

5. BIOMASS SUBSTRATES 5.1. Triglycerides

Ester C−O cleavage is of great interest for biomass conversion58,59 As one of the major molecular classes that Nature uses to store energy, triglycerides, feature multiple ester functionalities. Transesterification of fats to biodiesel and glycerol has been widely adopted in multiple countries, however the crude glycerol byproduct presents challenges due to the cost of purifying/ using the glycerol stream produced by the biodiesel industry.60−62

Figure 3. Thermodynamic profile for ester C−O hydrogenolysis.

activation enthalpy of 24.6 kcal/mol, in good accord with the experimental value of 25 (2) kcal/mol. Thus, the ester binds F


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Accounts of Chemical Research Table 5. Catalytic Hydrogenolysis of Ester C−O Bonds; Selected Scopea

a

Conditions: 0.5 mol % Hf(OTf)4, 0.2 mol % Pd/C (10 wt %), neat substrate, 1 bar H2.

Table 6. Carboxylate Substituent Effects on Ester C−O Cleavage

a

entry

R

conversion (%)

1 2 3 4 5a 6 7 8

−CH3 −CH2CH3 −CH(CH3)2 −C(CH3)3 −CF3 −CH2Cl −CHCl2 CCl3

89 60 42 33 83 93 61 44

10 min.

In a recent report, we proposed a “detoured” pathway to biodiesel, whereby the C3 backbone is converted to useful mono- and dioxygenates and alkanes rather than to glycerol (Scheme 4).63 Hydrogenolysis of the model triglyceride tricaprylin (1) was investigated at 150 and 200 °C, and at 1/30 bar H2 with a range of metal triflates to assess C3 selectivity to 1,3-PDO dioctanoate

Figure 4. DFT computed profile for the cleavage of cyclohexyl acetate.

(2) vs 1,2-PDO dioctanoate (3), vs n-propyl octanoate (4).63 Using the most active Hf(OTf)4 catalyst affords high conversions (86.2%) of tricaprylin over 2 h at 200 °C/1 bar H2, however, the G


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Accounts of Chemical Research Scheme 4. “Detoured” Biodiesel Production

Figure 6. Example of guaiacyl-based lignin. Key linkages denoted in blue.

ether linkages, a secondary benzylic alcohol at the α position, and a primary alcohol at the γ position (Figure 6). Recently, this Laboratory reported an alternative strategy for deconstructing acetylated lignin model compounds (5, Table 7) that employs both ether and ester C−O bond cleavage.81 Acetylation not only enhances solubility, but modifies the reactivity of the benzylic α-alcohol by converting it to an α-ester. Performing reactions in polar, noncoordinating recyclable chlorinated solvents solubilizes the lignin dimer 5 without sacrificing metal triflate catalytic activity commonly incurred when very polar, coordinating solvents are utilized. Initial attempts at breaking down 5 with the highly active Hf(OTf)4 catalyst at 70 °C under 1 psi H2 or 100 °C under 600 psi H2 did not result in the cleavage of dimer 5 to monomers (Table 7). The products instead are thick waxy solids with broad, uninformative 1H NMR spectra, and with no recovery of starting dimer 5. Analysis of the waxy solids by GC-MS revealed styrenic 6 in minor amounts, indicating that such species may be the source of the waxy solids, products of either radical or cationic polymerization. Viewing all of the cleavage reactions in a stepby-step fashion (Table 7) and using the accrued mechanistic knowledge of both ester and ether C−O bond cleavages, the first cleavage event likely occurs at the secondary α benzylic ester, yielding styrenic 6, which can be hydrogenated to compound 7. Compound 7 then undergoes etheric C−O cleavage to produce guaiacol (8) and dimethoxy phenyl monomers 9a,b, respectively. Note that using strongly Lewis acidic Hf(OTf)4 affords C−O cleavage rates to 6 which are substantially greater than the rate of 6 → 7 hydrogenation. Therefore, to minimize the steady-state concentration of 6 and minimize minimize production of oligomeric/polymeric byproducts, efforts were made to retard the C−O cleavage rate relative to olefin hydrogenation. This was achieved using a less Lewis acidic metal triflate (La(OTf)3), increasing the H2 pressure, and increasing Pd catalyst loading. Gratifyingly, employing a 1:1 La:Pd catalyst ratio affords almost quantitative recovery of monomers, 97 and 96% for 8 and 9a−c, respectively, demonstrating that kinetic manipulation of the relative rates of the two tandem reactions enhances cleavage selectivity in acetylated lignin dimers.

Figure 5. C3 selectivity for tricaprylin hydrogenolysis at 200 °C.

condensed phase (2 − 4) only contains 12% of the total C3 yield (Figure 5). Mass spectrometry reveals that propane (C3H8) comprises >96% of the gaseous phase, with minor amounts of C2H6, CO2, CH4, C2H4, and C3H6. For larger ionic radius/less Lewis acidic metal triflates, C−O cleavage is slower but more selective, and a larger fraction of the C3 backbone remains in the condensed phase. The highest selectivity to 1,3-PDO dioctanoate is currently in the 18−20% conversion range at 200 °C/1 bar H2. C3 selectivities for different M(OTf)n catalysts are summarized in Figure 5, with selectivity to the diesters 2 and 3 increasing with decreasing Lewis acidity (Ce ∼ Sc > Al > Hf), reaching 48% using Ce(OTf)3. M(OTf)n’s are potent transesterification catalysts, and addition of methanol to the reaction vessel affords quantitative conversion to the corresponding methyl ester (biodiesel) at 25 °C/4 h. 5.2. Lignin Structures

Lignins (Figure 6) are aromatic polymers representing approximately 15 to 25% of lignocellulosic biomass (lignin, cellulose, and hemicellulose).2,64,65 There has been great interest in converting lignocellulosic biomass into useful materials as it is the largest source of nonedible biomass on earth that does not compete with food sources. However, lignin depolymerization processes require extreme/harsh conditions (>300 °C) and produce low yields of aromatic monomers.66−69 Recent progress has been achieved using oxidized lignin,70−79 where aromatic monomers were isolated in up to 52% yield from hardwood lignin.80 Evaluating new strategies for lignin deconstruction initially targeted lignin model structures with functional group moieties representative of those in the lignin. This includes β-O-4 aryl

6. SUMMARY AND PROSPECTS Using mechanistic knowledge obtained by characterizing lanthanide-mediated C−O bond-forming processes (hydroalkoxylation), new methodologies for the selective cleavage of etheric and esteric C−O bonds via thermodynamic leveraging and tandem catalysis were developed using highly Lewis acidic metal triflate catalysts. Depending upon further development of H


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Accounts of Chemical Research Table 7. Acetylated Dimer Hydrogenolysis with a Tandem M(OTf)n + Pd/C Catalysta

yields (%)

a

mass balance %

entry

M(OTf)n

Pd/C

T (°C)

H2 (psi)

t (h)

5

7

8

9a

9b

9c

G:DM

1 2 1 2 3 4 5 6b

0.5% Hf 0.5% Hf 5% La 10% La 5% La 10% La 5% La 5% La

0.2% 0.2% 1% 2% 3% 6% 5% 5%

70 100 140 140 140 140 140 140

1 600 600 600 600 600 600 600

2 2 2 2 16 16 16 16

0 0 0 0 0 0 0 0

0 0 0 8 24 23 27 28

0 0 39 13 14 43 69 56

0 0 0 2 2 9 18 6

0 0 0 1 3 9 19 19

0 0 0 2 6 17 32 11

0:0 0:0 39:0 21:13 38:35 66:58 97:96 78:64

Reaction conditions: 4 mL of Cl2C2H4, 0.1 mmol 5 ([5] = 0.25 mM). b6 mL (0.17 mM).

effective catalysts, C−O cleavage reactions should be of interest for converting highly oxidized biomass structures into fuels and chemicals.

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National Academy of Sciences, and a Fellow of the Royal Society of Chemistry. Among other recognitions, he received the U.S. National Medal of Science and the Dreyfus Prize in Chemical Sciences.

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AUTHOR INFORMATION

ACKNOWLEDGMENTS This work was supported by NSF Grant CHE-1213235 and CHE-1464488 on basic homogeneous catalysis and f-element chemistry. Z.L. was supported by the Institute of Atom-efficient Chemical Transformation (IACT), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Basic Energy Sciences.

Corresponding Author

*E-mail: [email protected]. Present Address †

Z.L.: ShanghaiTech University, 100 Haike Road, Pudong District, Shanghai 201210, China.

■

Notes

The authors declare no competing financial interest. Biographies

REFERENCES

(1) Deuss, P. J.; Barta, K.; de Vries, J. G. Homogeneous catalysis for the conversion of biomass and biomass-derived platform chemicals. Catal. Sci. Technol. 2014, 4, 1174−1196. (2) Tuck, C. O.; Perez, E.; Horvath, I. T.; Sheldon, R. A.; Poliakoff, M. Valorization of Biomass: Deriving More Value from Waste. Science 2012, 337, 695−699. (3) Sheldon, R. A. Utilisation of biomass for sustainable fuels and chemicals: Molecules, methods and metrics. Catal. Today 2011, 167, 3− 13. (4) Huber, G. W.; Corma, A. Synergies between bio- and oil refineries for the production of fuels from biomass. Angew. Chem., Int. Ed. 2007, 46, 7184−7201. (5) Corma, A.; Iborra, S.; Velty, A. Chemical routes for the transformation of biomass into chemicals. Chem. Rev. 2007, 107, 2411−2502. (6) Weiss, C. J.; Marks, T. J. Organo-f-element catalysts for efficient and highly selective hydroalkoxylation and hydrothiolation. Dalton Trans. 2010, 39, 6576−6588. (7) Seo, S.; Marks, T. J. Lanthanide-Catalyst-Mediated Tandem Double Intramolecular Hydroalkoxylation/Cyclization of Dialkynyl Dialcohols: Scope and Mechanism. Chem. - Eur. J. 2010, 16, 5148−5162. (8) Motta, A.; Fragala, I. L.; Marks, T. J. Atom-Efficient CarbonOxygen Bond Formation Processes. DFT Analysis of the Intramolecular Hydroalkoxylation/Cyclization of Alkynyl Alcohols Mediated by Lanthanide Catalysts. Organometallics 2010, 29, 2004−2012.

Tracy L. Lohr was born in Victoria B.C. Canada. She received her B.Sc. (Honors with distinction, Coop) from the University of Victoria in 2008 and completed her NSERC and Alberta Innovates funded Ph.D from the University of Calgary under Professor Warren E. Piers in 2013. After 2 years of postdoctoral research with Tobin Marks at Northwestern University she joined the Northwestern faculty as a Research Assistant Professor in 2016. Zhi Li was born in Jilin, China. He obtained a B.S. degree from University of Science and Technology of China in 2006, and a Ph.D. from the University of Chicago under Professor Hisashi Yamamoto in 2011. He conducted postdoctoral research at Northwestern University with Professor Tobin Marks before starting independent research as a tenure-track Assistant Professor at ShanghaiTech University in Shanghai, China. He is an awardee of the “1000 Youth Talents” program of China. Tobin J. Marks was born in Washington, D.C. He is currently Ipatieff Professor of Chemistry and Professor of Materials Science and Engineering at Northwestern University. He received a B.S. degree from the University of Maryland and Ph.D. from MIT in Chemistry. He is a member of the U.S. National Academy of Sciences, a Fellow of the American Academy of Arts and Sciences, a Member of the German I


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Accounts of Chemical Research (9) Dzudza, A.; Marks, T. J. Efficient Intramolecular Hydroalkoxylation of Unactivated Alkenols Mediated by Recyclable Lanthanide Triflate Ionic Liquids: Scope and Mechanism. Chem. Eur. J. 2010, 16, 3403−3422. (10) Seo, S.; Yu, X.; Marks, T. J. Intramolecular Hydroalkoxylation/ Cyclization of Alkynyl Alcohols Mediated by Lanthanide Catalysts. Scope and Reaction Mechanism. J. Am. Chem. Soc. 2009, 131, 263−276. (11) Dzudza, A.; Marks, T. J. Efficient Intramolecular Hydroalkoxylation/Cyclization of Unactivated Alkenols Mediated by Lanthanide Triflate Ionic Liquids. Org. Lett. 2009, 11, 1523−1526. (12) Dzudza, A.; Marks, T. J. Lanthanide triflate-catalyzed arene acylation. Relation to classical Friedel-Crafts acylation. J. Org. Chem. 2008, 73, 4004−4016. (13) Yu, X.; Seo, S.; Marks, T. J. Effective, selective hydroalkoxylation/ cyclization of alkynyl and allenyl alcohols mediated by lanthanide catalysts. J. Am. Chem. Soc. 2007, 129, 7244−7245. (14) Dudnik, A. S.; Weidner, V. L.; Motta, A.; Delferro, M.; Marks, T. J. Atom-efficient regioselective 1,2-dearomatization of functionalized pyridines by an earth-abundant organolanthanide catalyst. Nat. Chem. 2014, 6, 1100−1107. (15) Wobser, S. D.; Stephenson, C. J.; Delferro, M.; Marks, T. J. Carbostannolysis Mediated by Bis(pentamethylcyclopentadienyl)lanthanide Catalysts. Utility in Accessing Organotin Synthons. Organometallics 2013, 32, 1317−1327. (16) Wobser, S. D.; Marks, T. J. Organothorium-Catalyzed Hydroalkoxylation/Cyclization of Alkynyl Alcohols. Scope, Mechanism, and Ancillary Ligand Effects. Organometallics 2013, 32, 2517−2528. (17) Weiss, C. J.; Wobser, S. D.; Marks, T. J. Lanthanide- and ActinideMediated Terminal Alkyne Hydrothiolation for the Catalytic Synthesis of Markovnikov Vinyl Sulfides. Organometallics 2010, 29, 6308−6320. (18) Weiss, C. J.; Marks, T. J. Organozirconium Complexes as Catalysts for Markovnikov-Selective Intermolecular Hydrothiolation of Terminal Alkynes: Scope and Mechanism. J. Am. Chem. Soc. 2010, 132, 10533−10546. (19) Weiss, C. J.; Wobser, S. D.; Marks, T. J. Organoactinide-Mediated Hydrothiolation of Terminal Alkynes with Aliphatic, Aromatic, and Benzylic Thiols. J. Am. Chem. Soc. 2009, 131, 2062−2063. (20) Yu, X.; Marks, T. J. Organophosphine oxide/sulfide-substituted lanthanide binaphtholate catalysts for enantioselective hydroamination/ cyclization. Organometallics 2007, 26, 365−376. (21) Stubbert, B. D.; Marks, T. J. Constrained geometry organoactinides as versatile catalysts for the intramolecular hydroamination/ cyclization of primary and secondary amines having diverse tethered CC unsaturation. J. Am. Chem. Soc. 2007, 129, 4253−4271. (22) Stubbert, B. D.; Marks, T. J. Mechanistic investigation of intramolecular aminoalkene and aminoalkyne hydroamination/cyclization catalyzed by highly electrophilic, tetravalent constrained geometry 4d and 5f complexes. Evidence for an M-N sigma-bonded insertive pathway. J. Am. Chem. Soc. 2007, 129, 6149−6167. (23) Amin, S. B.; Marks, T. J. Organolanthanide-catalyzed synthesis of amine-capped polyethylenes. J. Am. Chem. Soc. 2007, 129, 10102− 10103. (24) Zhao, J.; Marks, T. J. Recyclable polymer-supported organolanthanide hydroamination catalysts. Immobilization and activation via dynamic transamination. Organometallics 2006, 25, 4763−4772. (25) Motta, A.; Fragala, I. L.; Marks, T. J. Organolanthanide-catalyzed hydroamination/cyclization reactions of aminoalkynes. Computational investigation of mechanism, lanthanide identity, and substituent effects for a very exothermic C-N bond-forming process. Organometallics 2006, 25, 5533−5539. (26) Ryu, J. S.; Marks, T. J.; McDonald, F. E. Organolanthanidecatalyzed intramolecular hydroamination/cyclization/bicyclization of sterically encumbered substrates. Scope, selectivity, and catalyst thermal stability for amine-tethered unactivated 1,2-disubstituted alkenes. J. Org. Chem. 2004, 69, 1038−1052. (27) Kawaoka, A. M.; Marks, T. J. Organolanthanide-catalyzed synthesis of phosphine-terminated polyethylenes. J. Am. Chem. Soc. 2004, 126, 12764−12765.

(28) Hong, S.; Marks, T. J. Organolanthanide-catalyzed hydroamination. Acc. Chem. Res. 2004, 37, 673−686. (29) Ryu, J. S.; Li, G. Y.; Marks, T. J. Organolathanide-catalyzed regioselective intermolecular hydroamination of alkenes, alkynes, vinylarenes, Di- and trivinylarenes, and methylenecyclopropanes. Scope and mechanistic comparison to intramolecular cyclohydroaminations. J. Am. Chem. Soc. 2003, 125, 12584−12605. (30) Hong, S. W.; Tian, S.; Metz, M. V.; Marks, T. J. C-2-symmetric bis(oxazolinato)lanthanide catalysts for enantioselective intramolecular hydroamination/cyclization. J. Am. Chem. Soc. 2003, 125, 14768− 14783. (31) Hong, S.; Kawaoka, A. M.; Marks, T. J. Intramolecular hydroamination/cyclization of conjugated aminodienes catalyzed by organolanthanide complexes. Scope, diastereo- and enantioselectivity, and reaction mechanism. J. Am. Chem. Soc. 2003, 125, 15878−15892. (32) Hong, S. W.; Marks, T. J. Highly stereoselective intramolecular hydroamination/cyclization of conjugated aminodienes catalyzed by organolanthanides. J. Am. Chem. Soc. 2002, 124, 7886−7887. (33) Cotton, S. A. Scandium, yttrium, the lanthanides. Annu. Rep. Prog. Chem., Sect. A: Inorg. Chem. 2013, 109, 208−220. (34) Motta, A.; Fragalà, I. L.; Marks, T. J. Energetics and mechanism of organolanthanide-mediated phosphinoalkene hydrophosphination/ cyclization. A density functional theory analysis. Organometallics 2005, 24, 4995−5003. (35) Douglass, M. R.; Marks, T. J. Organolanthanide-catalyzed intramolecular hydrophosphination/cyclization of phosphinoalkenes and phosphinoalkynes. J. Am. Chem. Soc. 2000, 122, 1824−1825. (36) Douglass, M. R.; Stern, C. L.; Marks, T. J. Intramolecular Hydrophosphination/Cyclization of Phosphinoalkenes and Phosphinoalkynes Catalyzed by Organolanthanides: Scope, Selectivity, and Mechanism. J. Am. Chem. Soc. 2001, 123, 10221−10238. (37) Kawaoka, A. M.; Douglass, M. R.; Marks, T. J. Homoleptic lanthanide alkyl and amide precatalysts efficiently mediate intramolecular hydrophosphination/cyclization. Observations on scope and mechanism. Organometallics 2003, 22, 4630−4632. (38) Fu, P.-F.; Marks, T. J. Silanes as chain transfer agents in metallocene-mediated olefin polymerization. Facile in situ catalytic synthesis of silyl-terminated polyolefins. J. Am. Chem. Soc. 1995, 117, 10747−10748. (39) Koo, K.; Marks, T. J. Silanolytic chain transfer in ziegler-natta catalysis. Organotitanium-mediated formation of new silapolyolefins and polyolefin architectures. J. Am. Chem. Soc. 1998, 120, 4019−4020. (40) Fu, P.-F.; Brard, L.; Li, Y.; Marks, T. J. Regioselection and enantioselection in organolanthanide-catalyzed olefin hydrosilylation. A kinetic and mechanistic study. J. Am. Chem. Soc. 1995, 117, 7157−7168. (41) Koo, K.; Fu, P.-F.; Marks, T. J. Organolanthanide-mediated silanolytic chain transfer processes. Scope and mechanism of single reactor catalytic routes to silapolyolefins. Macromolecules 1999, 32, 981−988. (42) Dzudza, A.; Marks, T. J. Efficient Intramolecular Hydroalkoxylation of Unactivated Alkenols Mediated by Recyclable Lanthanide Triflate Ionic Liquids: Scope and Mechanism. Chem. Eur. J. 2010, 16, 3403−3422. (43) Dzudza, A.; Marks, T. J. Lanthanide Triflate-Catalyzed Arene Acylation. Relation to Classical Friedel−Crafts Acylation. J. Org. Chem. 2008, 73, 4004−4016. (44) Li, Z.; Zhang, J.; Brouwer, C.; Yang, C.-G.; Reich, N. W.; He, C. Brønsted Acid Catalyzed Addition of Phenols, Carboxylic Acids, and Tosylamides to Simple Olefins. Org. Lett. 2006, 8, 4175−4178. (45) Yanagisawa, A.; Nezu, T.; Mohri, S.-i. Brønsted Acid-Promoted Hydrocyanation of Arylalkenes. Org. Lett. 2009, 11, 5286−5289. (46) Dang, T. T.; Boeck, F.; Hintermann, L. Hidden Brønsted Acid Catalysis: Pathways of Accidental or Deliberate Generation of Triflic Acid from Metal Triflates. J. Org. Chem. 2011, 76, 9353−9361. (47) Shapiro, N. D.; Rauniyar, V.; Hamilton, G. L.; Wu, J.; Toste, F. D. Asymmetric additions to dienes catalysed by a dithiophosphoric acid. Nature 2011, 470, 245−249. (48) McKinney Brooner, R. E.; Widenhoefer, R. A. Stereochemistry and Mechanism of the Brønsted Acid Catalyzed Intramolecular J


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Accounts of Chemical Research Hydrofunctionalization of an Unactivated Cyclic Alkene. Chem. - Eur. J. 2011, 17, 6170−6178. (49) Akiyama, T. Stronger Brønsted Acids. Chem. Rev. 2007, 107, 5744−5758. (50) 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. 2012, 134, 14682−14685. (51) Lohr, T. L.; Marks, T. J. Orthogonal tandem catalysis. Nat. Chem. 2015, 7, 477−482. (52) 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. 2013, 3, 1908−1914. (53) 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. 2014, 136, 104−107. (54) Borugadda, V. B.; Goud, V. V. Biodiesel production from renewable feedstocks: Status and opportunities. Renewable Sustainable Energy Rev. 2012, 16, 4763−4784. (55) Pinzi, S.; Leiva, D.; López-García, I.; Redel-Macías, M. D.; Dorado, M. P. Latest trends in feedstocks for biodiesel production. Biofuels, Bioprod. Biorefin. 2014, 8, 126−143. (56) Shahid, E. M.; Jamal, Y. Production of biodiesel: A technical review. Renewable Sustainable Energy Rev. 2011, 15, 4732−4745. (57) Lohr, T. L.; Li, Z.; Assary, R. S.; Curtiss, L. A.; Marks, T. J. Thermodynamically Leveraged Tandem Catalysis for Ester RC(O)O-R ′ Bond Hydrogenolysis. Scope and Mechanism. ACS Catal. 2015, 5, 3675−3679. (58) Besson, M.; Gallezot, P.; Pinel, C. Conversion of biomass into chemicals over metal catalysts. Chem. Rev. 2014, 114, 1827−1870. (59) Haveren, J. v.; Scott, E. L.; Sanders, J. Bulk chemicals from biomass. Biofuels, Bioprod. Biorefin. 2008, 2, 41−57. (60) Ayoub, M.; Abdullah, A. Z. Critical review on the current scenario and significance of crude glycerol resulting from biodiesel industry towards more sustainable renewable energy industry. Renewable Sustainable Energy Rev. 2012, 16, 2671−2686. (61) Solecki, M.; Scodel, A.; Epstein, B. Advanced Biofuel Market Report 2013 - Capacity through 2016; Environmental Entrepreneurs: San Francisco, 2013. (62) Zhou, C. H.; Zhao, H.; Tong, D. S.; Wu, L. M.; Yu, W. H. Recent Advances in Catalytic Conversion of Glycerol. Catal. Rev.: Sci. Eng. 2013, 55, 369−453. (63) Lohr, T. L.; Li, Z.; Assary, R. S.; Curtiss, L. A.; Marks, T. J. Monoand tri-ester hydrogenolysis using tandem catalysis. Scope and mechanism. Energy Environ. Sci. 2016, 9, 550−564. (64) Ragauskas, A. J.; Beckham, G. T.; Biddy, M. J.; Chandra, R.; Chen, F.; Davis, M. F.; Davison, B. H.; Dixon, R. A.; Gilna, P.; Keller, M.; Langan, P.; Naskar, A. K.; Saddler, J. N.; Tschaplinski, T. J.; Tuskan, G. A.; Wyman, C. E. Lignin Valorization: Improving Lignin Processing in the Biorefinery. Science 2014, 344, 709−719. (65) Azadi, P.; Inderwildi, O. R.; Farnood, R.; King, D. A. Liquid fuels, hydrogen and chemicals from lignin: A critical review. Renewable Sustainable Energy Rev. 2013, 21, 506−523. (66) Zakzeski, J.; Bruijnincx, P. C. A.; Jongerius, A. L.; Weckhuysen, B. M. The Catalytic Valorization of Lignin for the Production of Renewable Chemicals. Chem. Rev. 2010, 110, 3552−3599. (67) Xu, C. P.; Arancon, R. A. D.; Labidi, J.; Luque, R. Lignin depolymerisation strategies: towards valuable chemicals and fuels. Chem. Soc. Rev. 2014, 43, 7485−7500. (68) Strassberger, Z.; Tanase, S.; Rothenberg, G. The pros and cons of lignin valorisation in an integrated biorefinery. RSC Adv. 2014, 4, 25310−25318. (69) Sheldon, R. A. Green and sustainable manufacture of chemicals from biomass: state of the art. Green Chem. 2014, 16, 950−963. (70) Rahimi, A.; Azarpira, A.; Kim, H.; Ralph, J.; Stahl, S. S. Chemoselective Metal-Free Aerobic Alcohol Oxidation in Lignin. J. Am. Chem. Soc. 2013, 135, 6415−6418.

(71) Lancefield, C. S.; Ojo, O. S.; Tran, F.; Westwood, N. J. Isolation of Functionalized Phenolic Monomers through Selective Oxidation and CO Bond Cleavage of the beta-O-4 Linkages in Lignin. Angew. Chem., Int. Ed. 2015, 54, 258−262. (72) Bohlin, C.; Andersson, P. A.; Lundquist, K.; Jonsson, L. J. Differences in stereo-preference in the oxidative degradation of diastereomers of the lignin model compound 1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)-1,3-propanediol with enzymic and nonenzymic oxidants. J. Mol. Catal. B: Enzym. 2007, 45, 21−26. (73) Partenheimer, W. The Aerobic Oxidative Cleavage of Lignin to Produce Hydroxyaromatic Benzaldehydes and Carboxylic Acids via Metal/Bromide Catalysts in Acetic Acid/Water Mixtures. Adv. Synth. Catal. 2009, 351, 456−466. (74) Werhan, H.; Mir, J. M.; Voitl, T.; von Rohr, P. R. Acidic oxidation of kraft lignin into aromatic monomers catalyzed by transition metal salts. Holzforschung 2011, 65, 703−709. (75) Tran, F.; Lancefield, C. S.; Kamer, P. C. J.; Lebl, T.; Westwood, N. J. Selective modification of the beta-beta linkage in DDQ-treated Kraft lignin analysed by 2D NMR spectroscopy. Green Chem. 2015, 17, 244− 249. (76) Hanson, S. K.; Baker, R. T. Knocking on Wood: Base Metal Complexes as Catalysts for Selective Oxidation of Lignin Models and Extracts. Acc. Chem. Res. 2015, 48, 2037−2048. (77) Sedai, B.; Diaz-Urrutia, C.; Baker, R. T.; Wu, R.; Silks, L. A. P.; Hanson, S. K. Aerobic Oxidation of beta-1 Lignin Model Compounds with Copper and Oxovanadium Catalysts. ACS Catal. 2013, 3, 3111− 3122. (78) Sedai, B.; Diaz-Urrutia, C.; Baker, R. T.; Wu, R.; Silks, L. A. P.; Hanson, S. K. Comparison of Copper and Vanadium Homogeneous Catalysts for Aerobic Oxidation of Lignin Models. ACS Catal. 2011, 1, 794−804. (79) Hanson, S. K.; Baker, R. T.; Gordon, J. C.; Scott, B. L.; Thorn, D. L. Aerobic Oxidation of Lignin Models Using a Base Metal Vanadium Catalyst. Inorg. Chem. 2010, 49, 5611−5618. (80) Rahimi, A.; Ulbrich, A.; Coon, J. J.; Stahl, S. S. Formic-acidinduced depolymerization of oxidized lignin to aromatics. Nature 2014, 515, 249−252. (81) Lohr, T. L.; Li, Z.; Marks, T. J. Selective Ether/Ester C−O Cleavage of an Acetylated Lignin Model via Tandem Catalysis. ACS Catal. 2015, 5, 7004−7007.

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