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Cite This: ACS Sustainable Chem. Eng. 2017, 5, 9818-9825

Mechanistic Investigation of the Catalyzed Cleavage for the Lignin β‑O‑4 Linkage: Implications for Vanillin and Vanillic Acid Formation Torsten Rinesch,†,§ Jakob Mottweiler,†,‡,§ Marta Puche,‡ Patricia Concepción,‡ Avelino Corma,*,‡ and Carsten Bolm*,† †

Institute of Organic Chemistry, RWTH Aachen University, Landoltweg 1, 52074 Aachen, Germany Instituto de Tecnología Química (UPV-CSIC), Av. de los Naranjos s/n, 46022 Valencia, Spain

‡

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S Supporting Information *

ABSTRACT: The depolymerization of kraft lignin with a copper−vanadium hydrotalcite-like catalyst (HTc-Cu-V) and V(acac)3/Cu(NO3)2·3H2O mixtures to monomeric aromatic aldehydes and aromatic acids such as vanillin and vanillic acid using molecular oxygen as oxidant is reported. The obtained products correlate to model-based studies with erythro-1-(3,4dimethoxyphenyl)-2-(2-methoxyphenoxy)-1,3-propanediol (1a). Kinetic investigations with 1a demonstrate that there is a combined effect of V(acac)3 together with Cu(NO3)2·3H2O that enhances the catalytic activity and increases the selectivity and yield for the cleavage products veratric acid and veratraldehyde. Veratric acid is formed through reaction pathways involving either the transient oxidation products 1-(3,4dimethoxyphenyl)-3-hydroxy-2-(2-methoxyphenoxy)propan-1-one (2) and 1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)prop-2-en-1-one (5) or through overoxidation of veratraldehyde. The formation of veratraldehyde likely proceeds through C−C bond cleavage involving either a retro-aldol or a single electron transfer mechanism. KEYWORDS: Oxidation, Lignin, Vanadium, Copper, Kinetics

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INTRODUCTION Nature synthesizes 170 billion metric tons of biomass annually.1 The largest constituent among the different types of biomass is lignocellulose.2 One of the main components of lignocellulose is lignin.3 Lignin is a complex aromatic polymer and represents an attractive renewable feedstock for the production of biofuels and chemicals. Despite its great natural abundance and enticing application potential, only a few industrial processes have been established utilizing lignin as a feedstock.4−7 This can be attributed to its recalcitrant nature. However, in recent years an increasing amount of research has been devoted to meeting the challenges of lignin valorization using catalytic and stoichiometric reaction systems in cleavage reactions for lignin model compounds and/or extracted lignin.8−53 In the majority of these studies the cleavage of the lignin βO-4 linkage was investigated as it represents the most prominent interconnecting bond in native lignin.4−7,54,55 Along those lines we have recently reported on the oxidative cleavage of lignin using copper−vanadium double-layered hydrotalcites or combined V(acac)3 and Cu(NO3)2·3H2O as catalysts.56 Using both Organosolv and kraft lignin as lignin sources we observed depolymerization to dimeric and trimeric products as confirmed by GPC and MALDI measurements. Intrigued by these results we wanted to gain a deeper understanding on how the cleavage of the β-O-4 linkage actually proceeded. There are few studies in the literature where the reaction mechanism for the formation of the cleavage © 2017 American Chemical Society

products has been established in greater detail, this being the main objective of our study.57,58

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RESULTS AND DISCUSSION Due to the plethora of products that are obtained when using extracted lignin sources, model compounds were chosen as substrates for these studies. However, it needs to be considered that making assessments about the reaction behavior for extracted lignin sources by employing those model compounds is sometimes not possible because the reactivity and product selectivity observed for model-based systems do not translate to extracted lignin sources.5,7 Therefore, we first had to establish whether the same types of products are formed in both the cleavage of extracted lignin and the employed 1,3-dilignol model compound 1a (Scheme 1).56,59 GC−MS was chosen to identify the products that formed during the cleavage of Sigma−Aldrich kraft lignin 370959 with either a copper−vanadium hydrotalcite-like catalyst (HTc-CuV) or V(acac)3/Cu(NO3)2·3H2O. The amount of β-O-4 as well as other prominent aliphatic ether linkages within SigmaAldrich kraft lignin 370959 was assessed by QQ-HSQC measurements.60−62 Additionally, the H/G/S-ratio was determined in order to elucidate which types of monomeric products Received: June 2, 2017 Revised: August 14, 2017 Published: September 7, 2017 9818

DOI: 10.1021/acssuschemeng.7b01725 ACS Sustainable Chem. Eng. 2017, 5, 9818−9825

Research Article

ACS Sustainable Chemistry & Engineering Scheme 1. Isolated Products for the Cleavage of 1,3-Dilignol 1a with a Copper−Vanadium Hydrotalcite-like Catalyst (HTc-Cu-V) as Catalysta 56

a

1.0 mmol scale; yields after column chromatography.

Table 1. Characterization of Sigma−Aldrich Kraft Lignin 370959 by 2D QQ-HSQC NMR60−62 linkages (per 100 C 9 units)a S, G, H (%)a

β-O-4-OHb

β-O-4-ORc

β-5

β-β

traces, 93, 7

5

0

1

1

a

Determined by comparing the aliphatic and aromatic signal intensities (Figures S14 and S15 in the Supporing Information). b Amounts of β-O-4 linkages with free γ-OH. cAmounts of β-O-4 linkages with capped γ-OH units.

were to be expected. Table 1 summarizes the obtained values. The β-O-4 content in the applied lignin source was 5%, and it predominantly consisted of G-units (93%). On the basis of these observations, mostly guaiacyl-derived products such as vanillin and vanillic acid were expected as lignin β-O-4 cleavage products. Gratifyingly, the GC−MS spectra of the reactions catalyzed with HTc-Cu-V or V(acac)3/Cu(NO3)2·3H2O showed that vanillin and vanillic acid were the main lignin cleavage products (Figure S16 in the Supporting Information). Therefore, a good correlation could be observed between the types of products that are formed during the cleavage of 1,3dilignol 1a (veratraldehyde and veratric acid) and extracted lignin (vanillin and vanillic acid). Having established that similar products were formed during the cleavage of Sigma−Aldrich kraft lignin 370959 and 1,3dilignol model compound 1a, the reaction profiles for the cleavage of 1a with HTc-Cu-V [20 wt %] and V(acac)3/ Cu(NO3)2·3H2O [5 mol %] (Figure 1) were studied. In both cases, a similar reaction profile was observed, despite the lower initial reactivity observed for the HTc-Cu-V catalyst versus the V(acac)3/Cu(NO3)2·3H2O catalyst. In detail, as dilignol 1a was converted, veratraldehyde (3) and ketone 2 were observed as primary products. After reaching a maximum, their yields slowly decreased over time. On the contrary, the yield for veratric acid (4) increased almost linearly over time resulting with both catalysts as the main product after a reaction time of 24 h. As disclosed in our previous study, guaiacol was not stable under the employed oxidative reactions conditions. Therefore, no quantification was possible for the kinetics because guaiacol was too rapidly degraded.56 Initial reaction rates of 1.9, 2.6, and 0.36 h−1 to veratraldehyde (3), ketone 2, and veratric acid (4) were found for the V(acac)3/Cu(NO3)2·3H2O catalyst, while for the HTc-Cu-V catalyst the initial reaction rates were distinctly lower with 0.08, 0.03, and 0.021 h−1, respectively. According to the reaction profile displayed in Figure 1 and the initial formation rates, both veratraldehyde (3) and ketone 2 can be stated as primary unstable products, while veratric acid (4) appears as a secondary reaction product.

Figure 1. Reaction profile of dilignol 1a with V(acac)3 and Cu(NO3)2· 3H2O as catalyst (top) or HTc-Cu-V (bottom). Reaction conditions: 1a (0.25 mmol), V(acac)3/Cu(NO3)2·3H2O (5 mol %) or HTc-Cu-V (20 wt %), O2 (0.5 MPa), pyridine (1.25 mL), 135 °C, stirring rate 600 rpm.

To further elaborate on these observations we looked at the reaction profile when ketone 2 was used as a starting reagent under the standard reaction conditions. As can be seen in Figure 2, independent of the catalyst employed, ketone 2 is almost exclusively converted to veratric acid (4), with an initial rate of 0.14 and 1.3 h−1 for HTc-Cu-V and V(acac)3/ Cu(NO3)2·3H2O, respectively. After a reaction time of 24 h the yield of acid 4 was 90% with V(acac)3/Cu(NO3)2·3H2O and 80% with HTc-Cu-V. The yields for veratraldehyde (3) remained below 2% over the course of the entire reaction. This result is of additional importance considering previous density function theory (DFT) calculations where the oxidation of the benzylic alcohol in dilignol 1a to ketone 2 has a strong influence on whether the C−C or C−O bond cleavage is favored.63,65 The bond dissociation enthalpy (BDE) for the C− 9819


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Figure 2. Reaction profile of ketone 2 with V(acac)3 and Cu(NO3)2· 3H2O as catalyst (top) or HTc-Cu-V (bottom). Reaction conditions: 2 (0.25 mmol), V(acac)3/Cu(NO3)2·3H2O (5 mol %) or HTc-Cu-V (20 wt %), O2 (0.5 MPa), pyridine (1.25 mL), 135 °C, stirring rate 600 rpm.

Figure 3. Reaction profile of aldehyde 3 with V(acac)3 and Cu(NO3)2· 3H2O as catalyst (top) or HTc-Cu-V (bottom). Reaction conditions: 3 (0.25 mmol), V(acac)3/Cu(NO3)2·3H2O (5 mol %) or HTc-Cu-V (20 wt %), O2 (0.5 MPa), pyridine (1.25 mL), 135 °C, stirring rate 600 rpm.

O bond within the β-O-4 linkage changes from 247.9 kJ mol−1 for dilignol 1a to 161.1 kJ mol−1 for ketone 2. At the same time the BDE for Cα−Cβ increases from 264.3 kJ mol−1 for 1a to 294.2 kJ mol−1 for 2.63 These bond dissociation enthalpies imply that the products formed from ketone 2 likely undergo C−O bond cleavage at the β-O-4 linkage, yielding veratric acid as the final product. Next, we studied the reaction behavior of veratraldehyde (3) under the standard reaction conditions to elucidate whether the overoxidation of aldehyde 3 to veratric acid (4) was a key route for the acid formation (Figure 3). For both catalytic systems, overoxidation of veratraldehyde indeed occurred though to a much lower extent than the conversion of ketone 2 or dilignol 1a. These results provide critical evidence that ketone 2 was converted directly to veratric acid and did not primarily proceed through an oxidative cleavage to veratraldehyde (3) followed by a subsequent oxidation to acid 4. To conclude our studies on the reaction pathways leading to the formation of the cleavage products, i.e., veratraldehyde (3) and veratric acid (4), enol ether enone 5 (see Scheme 2) was

Scheme 2. Cleavage of Enol Ether Enone 5 Using Either HTc-Cu-V [20 wt %] or V(acac)3/Cu(NO3)2·3H2O [5 mol %] as Catalyst

used as starting material. Enol ether enone 5 is a minor product in both the HTc-Cu-V [20 wt %]- and V(acac)3/Cu(NO3)2· 3H2O [5 mol %]-catalyzed cleavage of 1a, with yields below 5% over the course of the reaction. With HTc-Cu-V [20 wt %] 40% of veratric acid was obtained with a conversion of 83% after 17 h under the standard reaction conditions. With V(acac)3/ 9820


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ACS Sustainable Chemistry & Engineering Cu(NO3)2·3H2O [5 mol %] the conversion was 92%, and the yield for veratric acid was 25% after 17 h. Veratraldehyde was not observed as a product with either of the catalysts. This demonstrates that the intermediate enol ether enone 5 participates in the formation of veratric acid (4). However, its contribution to the final yield of veratric acid is rather minor. From all the data presented above, we can propose two major reaction pathways, in which four reactions occur in parallel leading to the formation of veratric acid (4) (Scheme 3). Scheme 3. Main Reaction Pathways for the Cleavage of Dilignol 1a Leading to the Formation of Veratraldehyde and Veratric Acid

Figure 4. Reaction profile of dilignol 1a with Cu(NO3)2·3H2O as catalyst. Reaction conditions: 1a (0.25 mmol), Cu(NO3)2·3H2O (5 mol %), O2 (0.5 MPa), pyridine (1.25 mL), 135 °C, stirring rate 600 rpm.

After having established the main reaction pathways for the formation of veratric acid (4) during the cleavage of 1,3-dilignol model compound 1a, we investigated the individual effect of each transition metal in the homogeneous V(acac) 3 / Cu(NO3)2·3H2O reaction system. In our previous study, we reported that Cu(NO3)2·3H2O was not very active as a catalyst for the conversion of 1a, independent of whether the catalyst loading was 5 or 10 mol %.56 After 24 h the conversion of dilignol 1a was only 31%, and the yield for each cleavage product was below 5% (Figure 4). The initial rates for the formation of veratraldehyde (initial rate r3) and ketone 2 (initial rate r2) are close to zero. Interestingly, when ketone 2 was used as the starting reactant, the initial rate for formation of veratric acid (initial rate r42) is 1.1 h−1. After 17 h, ketone 2 was almost completely converted (96%), and veratric acid (4) was formed in 60% yield with a selectivity of 63%. (Note: Selectivity is defined here as the percentage of converted starting material that has reacted to the cleavage products veratric acid and veratraldehyde.) These results thus indicate a higher impact of the copper catalyst for the cleavage of the intermediately formed ketone 2 which is in line with reports in the literature on the copper-catalyzed cleavage of 2.63,64 As disclosed in our previous results, V(acac)3 by itself is much more active than Cu(NO3)2·3H2O for the conversion of 1a. Figure 5 shows the reaction profile with a catalyst loading of 5 mol %. The initial rate constant for r3 (formation of aldehyde 3) is 2.0 h−1, and that for r2 (formation of ketone 2) is 6.4 h−1. When V(acac)3 was used alone for the cleavage of ketone 2, the initial rate for the formation of veratric acid (r42) is 1.4 h−1. After 17 h, 76% of ketone 2 is converted yielding 43% veratric acid (4) with a selectivity of 57%. At this point, after taking into consideration that Cu(NO3)2· 3H2O is not active for the conversion of 1a, a combined effect of vanadium and copper species for the V(acac)3/Cu(NO3)2· 3H2O [5 mol %] catalyst can be clearly established where V(acac)3/Cu(NO3)2·3H2O [5 mol %] is more active than V(acac)3 [5 mol %] for the conversion of dilignol 1a to

Figure 5. Reaction profile of dilignol 1a with V(acac)3 as catalyst. Reaction conditions: 1a (0.25 mmol), V(acac)3 (5 mol %), O2 (0.5 MPa), pyridine (1.25 mL), 135 °C, stirring rate 600 rpm.

veratraldehyde (3) (initial r3 rates relative to only vanadium active species are 3.8 and 2.0 h−1, respectively). In addition to the activity increase, the selectivity is also improved. In fact, 9821


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ACS Sustainable Chemistry & Engineering when V(acac)3 [5 mol %] is employed as catalyst for the cleavage of dilignol 1a, the selectivity for 3 and 4 after 24 h and complete conversion is 53%, compared to 87% with V(acac)3/ Cu(NO3)2·3H2O [5 mol %]. Furthermore, for the cleavage of ketone 2 both V(acac)3 [5 mol %] and Cu(NO3)2·3H2O [5 mol %] are similarly active (initial r42 rates of 1.4 and 1.1 h−1; respectively), but the selectivity is clearly improved when using V(acac)3/Cu(NO3) 2·3H2O [5 mol %]. Thus, at equal conversion (76%) V(acac)3 [5 mol %] affords 57% selectivity toward veratric acid and V(acac)3/Cu(NO3)2·3H2O [5 mol %] 75% selectivity. Accordingly, Cu(NO3)2·3H2O [5 mol %] furnishes 63% selectivity and V(acac)3/Cu(NO3)2·3H2O [5 mol %] 83% selectivity at a conversion of 96%. In conclusion, a clear combined effect can be evidenced when using the V(acac)3/Cu(NO3)2·3H2O [5 mol %] catalyst. After proving a combined effect of copper and vanadium and going deeper into the reaction mechanism, we have studied the reactivity of various derivatives of 1a in which the hydrogen atoms in different positions have been exchanged by methyl groups (Table 2). We have previously employed these model compounds in mechanistic studies for base mediated lignin cleavage in the ball mill and for ruthenium-catalyzed β-O-4 cleavage.14,28 In contrast to veratric acid, we believe that the formation of veratraldehyde likely proceeds through Cα−Cβ bond cleavage of 1a; however, details about the reaction mechanism have not been addressed yet. Baker and co-workers propose that there are potentially two competing reaction pathways for the copper-catalyzed cleavage of 1a to veratraldehyde with their reaction system.48 The first one is a retro-aldol mechanism. Stahl and co-workers described in their study on the metal free oxidation of the benzylic alcohol within the β-O-4 linkage that the oxidation of the γ-hydroxyl group enables a retro-aldol mechanism leading to the formation of veratraldehyde.47 Recently, we reported an efficient organocatalytic one-pot two-step degradation strategy using a TEMPO/DAIB system for the selective oxidation of the β-O4 γ-hydroxyl group followed by a proline-catalyzed retro-aldol reaction.66 The second proposed mechanism is a single electron transfer mechanism in which an aryl cation is generated that subsequently undergoes Cα−Cβ bond cleavage. This mechanism is well-established in the literature especially for enzymatic oxidations.67−74 Entries 2 and 4 in Table 2 show that when the benzylic alcohol or both the benzylic and the primary alcohol of model compound 1a are methylated, the conversion is negligible, and neither product 3 or 4 is formed. A similar detrimental effect upon the yields of veratric acid and veratraldehyde is observed when the hydrogen at the benzylic carbon is exchanged (entry 5, 1e), but instead of veratraldehyde, 3,4-dimethoxy acetophenone is formed in 22% and 19% yield with V(acac)3/ Cu(NO3)2·3H2O and HTc-Cu-V, respectively. The conversion of 1e for both catalysts is higher than 60%. The results obtained with substrate 1e (entry 5) are both in accordance with a single electron transfer mechanism as well as a retro-aldol mechanism. However, when only the primary alcohol is methylated (entry 3), 72% conversion is achieved with V(acac)3/Cu(NO3)2·3H2O and 67% with HTc-Cu-V, while the yields of veratraldehyde decrease significantly to below 5% independent of the catalyst. The effect on the yield of veratric acid is less drastic as it is formed in 27% and 26%, respectively. The significance of the primary alcohol group for potential cleavage pathways that lead to veratraldehyde as product is further exemplified in entry 6. In substrate 1f the CH2 group of the primary alcohol is changed to

Table 2. Selectivity Studies When Various Positions Are Blocked in the Model Compound Dilignol 1ac

a

Conversions and yields determined by HPLC. bYield of 3,4dimethoxy acetophenone determined by HPLC. cReaction conditions: substrate (0.25 mmol) 1.25 mL pyridine, 0.5 MPa O2, 600 rpm, 135 °C, 17 h reaction time.

the corresponding dimethyl analogue. The conversion is similar to 1a, and the yields for veratric acid are even higher than that for substrate 1a with 65% and 58%, respectively. However, the yield for 3 is only 11% for both catalysts. The results for model compounds 1c and 1f indicate that the reaction pathway starting with the oxidation of the primary alcohol, enabling the cleavage of the Cα−Cβ bond through a retro-aldol reaction, is likely substantially involved in the veratraldehyde formation. The fact that veratraldehyde (3) is still being formed in low quantities from substrates 1c and 1f despite the unavailability of the retro-aldol pathway also suggests the involvement of the previously mentioned single electron transfer mechanism. Accordingly, we can state that the formation of veratraldehyde through Cα−Cβ bond cleavage likely involves both a retroaldol and a single electron transfer mechanism (Scheme 4). 9822


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into a 25 mL glass autoclave equipped with a magnetic stirrer. Pyridine (1.25 mL) was added, subsequently. The autoclave was pressurized with 0.5 MPa of oxygen and stirred with 600 rpm in a preheated oil bath at 135 °C. After reaching the desired reaction time the autoclave was taken out of the oil bath and cooled to room temperature, followed by the release of the remaining oxygen pressure. To the reaction solution was added 1.000 mL of a standard solution (3,4dimethoxybenzyl alcohol in methanol, c = 0.2 mol/L) with an Eppendorf pipet. The reaction mixture was directly filtered into approximately 50 mL of a 1 M HCl solution. The aqueous phase was then extracted with dichloromethane (3 × 20 mL). The combined organic phases were washed with 1 M HCl and brine, dried over MgSO4, and concentrated under reduced pressure. For HPLC measurements, 2.0−3.0 mg of the reaction product was weighed into a vial. Then, 0.5 mL of ethyl acetate and 0.5 mL of acetonitrile were added to the vial, and after the complete dissolution, the solution was filtered into an HPLC vial.

Scheme 4. Competing Reaction Mechanisms Involved in the Cleavage of 1a to Veratraldehyde

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CONCLUSIONS We have shown that a copper−vanadium hydrotalcite (HTcCu-V) and V(acac)3/Cu(NO3)2·3H2O catalyze the depolymerization of a kraft lignin source to monomeric aromatic aldehydes and aromatic acids such as vanillin and vanillic acid. These products correlate with the ones that are obtained when using 1,3-dilignol 1a as reaction model compound. Kinetic investigations for the cleavage of 1a show that there is a combined effect of V(acac)3 and Cu(NO3)2·3H2O that enhances the catalytic activity for the cleavage of 1a to veratraldehyde and increases the selectivity and yield for the cleavage products veratric acid and veratraldehyde. Both cleavage products are obtained through different reaction pathways. Veratric acid is formed through the transient oxidation products ketone 2 and enol ether enone 5 and by oxidation of vetraldehyde. The corresponding bond dissociation enthalpies imply that the formation of veratric acid through ketone 2 proceeds by C−O bond cleavage. Studies with derivatives of 1a in which the hydrogen atoms in different positions had been exchanged by methyl groups suggest that veratraldehyde is formed by two reaction mechanisms involving C−C bond cleavage: one being a retro-aldol pathway that involves the oxidation of the primary alcohol in the β-O-4 linkage and the other being a single electron transfer mechanism, in which an aryl cation is generated that subsequently undergoes Cα−Cβ bond cleavage.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01725. Materials and methods, experimental details, product preparation and characterization, spectroscopic data, kinetic experiments, and GC−MS investigations (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Avelino Corma: 0000-0002-2232-3527 Carsten Bolm: 0000-0001-9415-9917 Author Contributions §

T.R. and J.M. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Cluster of Excellence “Tailor Made Fuels from Biomass” (TMFB) funded by the Excellence Initiative of the German federal and state governments and by Prometeo II/203/011 and SEV2012-0267 funded by the Spanish Ministry of Science and Innovation. J.M. is grateful to the NRW Graduate School BrenaRo and the German Academic Exchange Service (DAAD) for predoctoral stipends. We thank Dr. M. Palomino-Schätzlein (Centro de Investigación Prı ́ncipe Felipe) and Prof. Dr. N. Loening (Lewis & Clark College) for the helpful discussions during the NMR investigations and Dr. M. Palomino-Schätzlein for performing the QQ-HSQC measurements. Prof. Dr. R. Palkovits and A. Willms (both RWTH Aachen University) are acknowledged for calcination of HTc-Cu-V. Thanks also go to F. Krauskopf, S. Knippertz, and Dr. C. A. Dannenberg for synthetic contributions (all RWTH Aachen University) and S. Dabral and Dr. C. A. Dannenberg for proofreading the manuscript.

EXPERIMENTAL SECTION

Homogeneous System with V(acac)3 and Cu(NO3)2·3H2O. The respective model compound (0.25 mmol), V(acac)3 (2.5, 5, 10, or 20 mol %), and Cu(NO3)2·3H2O (2.5, 5, 10, or 20 mol %) were weighed into a 25 mL glass-autoclave equipped with a magnetic stirrer. Pyridine (1.25 mL) was added, subsequently. The autoclave was pressurized with 0.5 MPa of oxygen and stirred with 600 rpm in a preheated oil bath at 135 °C. After reaching the desired reaction time, the autoclave was taken out of the oil bath and cooled to room temperature, followed by the release of the remaining oxygen pressure. To the reaction solution was added 1.000 mL of a standard solution (3,4-dimethoxybenzyl alcohol in methanol, c = 0.2 mol/L) with an Eppendorf pipet. The reaction mixture was directly filtered into approximately 50 mL of a 1 M HCl solution. The aqueous phase was then extracted with dichloromethane (3 × 20 mL). The combined organic phases were washed with 1 M HCl and brine, dried over MgSO4, and concentrated under reduced pressure. For HPLC measurements, 2.0−3.0 mg of the reaction product was weighed into a vial. Then, 0.5 mL of ethyl acetate and 0.5 mL of acetonitrile were added to the vial, and after the complete dissolution, the solution was filtered into an HPLC vial. Heterogeneous System with HTc-Cu-V. The respective model compound (0.250 mmol) and HTc-Cu-V (20 wt %) were weighed

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REFERENCES

(1) Corma, A.; Iborra, S.; Velty, A. Chemical Routes for the Transformation of Biomass into Chemicals. Chem. Rev. 2007, 107, 2411−2502.

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