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Aerobic Oxidation of 2-Phenoxyethanol Lignin Model Compounds Using Vanadium and Copper Catalysts Christian Díaz-Urrutia, Baburam Sedai, Kyle C Leckett, R. Tom Baker, and Susan Kloek Hanson ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b02420 • Publication Date (Web): 17 Oct 2016 Downloaded from http://pubs.acs.org on October 22, 2016

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ACS Sustainable Chemistry & Engineering

Aerobic Oxidation of 2-Phenoxyethanol Lignin Model Compounds Using Vanadium and Copper Catalysts Christian Díaz-Urrutia,† Baburam Sedai,† Kyle C. Leckett,† R. Tom Baker,†,* and Susan K. Hanson‡,* †

Department of Chemistry and Biomolecular Sciences and Centre for Catalysis Research and

Innovation, University of Ottawa, 30 Marie Curie, Ottawa, ON K1N 6N5 Canada ‡

Chemistry Division, Los Alamos National Laboratory, Los Alamos, NM 87545 USA

*corresponding authors: [email protected]; [email protected]

KEYWORDS: Biomass, lignin models, vanadium, copper, C-C bond cleavage, aerobic oxidation.

ABSTRACT: Lignin is the most abundant renewable aromatic-containing macromolecule in Nature. Intensive research efforts are underway to obtain additional value from lignin beyond current low-value heating. Aerobic oxidation has emerged as one promising alternative for the selective depolymerization of lignin and a variety of models for the most abundant β-O-4 linkage have been employed. In this work, aerobic oxidation of the simple β-O-4 lignin models 2phenoxyethanol (2) and 1-phenyl-2-phenoxyethanol (3) were investigated using the oxovanadium complex (HQ)2VV(O)(OiPr) (HQ = 8-oxyquinolinate) and CuCl/TEMPO/2,6lutidine as catalysts in several different solvents at 100 °C (TEMPO = 2,2,6,6tetramethylpiperidine-1-oxyl). Using the vanadium catalyst, reactions proceed more readily in pyridine (vs. dimethylsulfoxide) presumably via an initial base-assisted alcohol dehydrogenation followed by oxidative C-C and C-O bond cleavage to afford phenol, formic acid and CO2. In contrast, the copper-catalyzed reactions suffer from extensive formylation of the substrate and radical coupling to give TEMPO-functionalized products. These results suggest that use of more

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complex β-O-4 lignin models is required for accurate comparison of selective oxidation catalysts.

INTRODUCTION As global demand for fossil fuels and petroleum-derived chemicals increases, there is continued interest in the development of alternative carbon sources. Attention has been focused on non-food biomass (lignocellulose) as an important renewable feedstock for the production of fuels and chemicals.1-3 One of the main components of lignocellulose is lignin, a complex macromolecule composed of relatively electron-rich methoxyphenyl propanoid subunits.4 While lignin constitutes up to 30 wt % of lignocellulosic biomass, developing efficient processes to obtain valuable aromatic chemicals from lignin has been a major challenge due to its irregular composition. One promising approach involves the use of homogeneous catalysts for relatively low temperature (< 150 °C) oxidative cleavage of the β-O-4 linkage (Figure 1),5,6,7,8 the most prevalent feature (ca. 55%) in the lignin structure.9

Figure 1. Lignin and lignin models containing the most abundant β-O-4 linkage (highlighted in blue).

Lignin model compounds provide an opportunity to understand and compare the reactivity and selectivity of different catalysts. Arylglycerol β-aryl ether compounds such as 1 (Figure 1)

Figure 2. Oxovanadium and copper catalysts.

have been extensively employed as models of the β-O-4 linkage.5 Using the oxovanadium complex 6a with a salen-type ligand, Son and Toste showed that oxidation of a substrate similar to 1 proceeded predominantly via C-O bond cleavage, affording phenol and α,β-unsaturated 2 ACS Paragon Plus Environment

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enone products.10, 11 We then showed that oxovanadium catalysts 4 and 7 led to initial cleavage of the benzylic C-H bond of 1 to afford the corresponding ketone which then underwent C-C bond cleavage to aldehyde and acid products.12-14 In contrast, use of copper catalyst 5 gave direct C-C bond cleavage affording predominantly aldehyde. More recently, Hirao and Soo et al. also showed direct C-C bond cleavage under visible light irradiation with catalyst 6b.15 In a complementary study, Stahl and co-workers developed a metal-free organocatalytic system under acidic conditions for the oxidation of β-O-4 lignin models in which initial alcohol oxidation was followed by acid promoted C-C bond cleavage.16 More recently, Wang and coworkers, performed a vanadium sulfate TEMPO oxidation followed by C-C bond cleavage to acids

using

Cu/phen

catalyst

(phen

=

1,10-phenanthroline,

TEMPO

=

2,2,6,6-

tetramethylpiperidine-1-oxyl).17 In a related study Bolm and Corma et al. also obtained acid products using a mixed vanadium-copper hydrotalcite catalyst.18 Although the above studies with complex lignin model 1 demonstrated the wealth of mechanistic possibilities for homogeneous base-metal aerobic oxidation catalysts, carbon balance for the aryloxy group was an issue, presumably due to catalyzed methoxyphenol oxidation.19-21 In order to gain additional insight into the distinct reaction pathways using the vanadium and copper catalysis, we revisited their reactivity with the simple lignin model compounds 2-phenoxyethanol 2 and 1-phenyl-2-phenoxyethanol 3. Previous work using fivecoordinate oxovanadium catalyst precursor 422 has been extended to include six-coordinate bis(oxyquinolate) complex 7 (Figure 2).23,24 The influence of phenyl substitution on the backbone of the lignin model has been explored, and several unusual new reaction products have been identified, shedding further light on the fundamental modes of reactivity of earth-abundant metal catalysts in the aerobic oxidation of lignin model compounds.

RESULTS In previous work using oxovanadium catalyst 4 in air, we obtained 95% conversion of substrate 3 (100 oC, 7 days in DMSO-d6) to benzoic acid (81%), phenol (77%), formic acid (46%) and 2-phenoxy-acetophenone (9%).22 Substrate 2 was even more sluggish to react, affording just 20% conversion to phenol, formic acid, and unidentified products under the same conditions (Scheme 1). These experiments showed that simple lignin models 2 and 3 were oxidized more slowly than the complex lignin model 1.21 In subsequent work, more active 3 ACS Paragon Plus Environment


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versions of both vanadium and copper catalysts have been developed. For example, the 6coordinate oxovanadium complex 7 showed improved oxidation performance over 4 in aerobic alcohol oxidation reactions.24 The Cu catalytic system 5 was optimized using 2,6-lutidine base/co-ligands in toluene solvent (5*).25 Thus, optimized systems 5* and 7 were used to examine in more detail the aerobic oxidation of simple lignin models 2 and 3. OH

air

2

OH

(dipic)V V(O)(OiPr) (10 mol %)

O

O unidentified HO

DMSO-d6 100 oC 7 days 18%

20% conversion

H

6%

2-5%

Scheme 1. Previously reported oxidation of phenoxyethanol.

Oxidation of 2-Phenoxyethanol When a pyr-d5 solution of 2 was heated under air with the vanadium catalyst 7 (10 mol %), 69% conversion was observed after 48 h at 100 oC. The major products of this reaction were phenol, formic acid and the formylated substrate 2-phenoxyethyl-1-formate 8 (Scheme 2). Throughout this manuscript product yields are reported relative to the theoretical maximum yield

Scheme 2. Aerobic oxidation of phenoxyethanol using oxovanadium catalyst 7.

using an external standard. No other products were detected in the reaction mixture by 1H NMR spectroscopy, suggesting that formation of phenol is accompanied by oxidation of the C2H4 backbone to give formic acid and CO2. In a separate experiment, the ability of 7 (10 mol %) to catalyze the oxidation of formic acid to CO2 was investigated; after 48 h of heating under air (100 oC, pyr-d5 solvent), 23% of the formic acid was consumed. Note also that under these reaction conditions, no phenol oxidation products are observed. The possible role of 2phenoxyacetaldehyde 9 (see Scheme 3) as an intermediate in this reaction could not be rigorously confirmed, as control experiments heating independently prepared26 9 in DMSO-d6 4 ACS Paragon Plus Environment

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(with an internal standard) for 24 h at 100 oC showed that 9 decomposed thermally to afford phenol, formic acid and several unidentified products (see Experimental Section for details). Stoichiometric reaction of complex 4 with substrate 2 in pyridine for 10 min at 100°C afforded 0.5 equivalents each of 2 and 2-phenoxyacetaldehyde 9.22 The combination of CuCl/TEMPO/2,6-lutidine 5* was also tested as a catalyst for the oxidation of 2-phenoxyethanol 2. When a solution of 2 in toluene was heated under oxygen with catalyst mixture 5* (20 mol %, 100 oC, 40 h), 40% conversion was observed (Scheme 3). In a

Scheme 3. Aerobic oxidation of phenoxyethanol using copper catalyst 5.

control experiment, no oxidation was observed when 2 was heated under the same conditions, but without the CuCl and TEMPO. While the major products, phenol and formylated substrate 8, were similar to those obtained using the vanadium catalyst, additional products 10 and 11 were observed, resulting from substrate functionalization by TEMPO. Carbon dioxide is also likely formed as a product, as a separate experiment revealed that complete conversion of formic acid occurred when it was heated with 5* under O2 in toluene at 100 oC for 18 h. The products of the copper-catalyzed reaction were identified by 1H and

13

C NMR

spectroscopy, GC/MS, and LC-MS analysis as 2-phenoxy-2-(2,2,6,6-tetramethylpiperidin-1yloxy)acetaldehyde 10 (3%), and a mixture of diastereomers 11 in which the nature of X has yet to be determined. Compound 10 was isolated from a large-scale reaction and fully characterized by high resolution mass spectrometry and 1H and 13C NMR spectroscopy. Characteristic features in the 1H NMR spectrum of 10 include a formate O=CH signal at δ 9.56 and an aliphatic CH doublet at δ 5.56 (J = 3 Hz). The four TEMPO methyl groups are inequivalent due to its asymmetric O-alkyl substituent and restricted rotation due to steric hindrance.27 We were unable to isolate compound 11 using column chromatography, but the aliphatic protons were characterized by 1H NMR spectroscopy (Figure 3) and inspection of the TEMPO methyl group resonances in the

13

C NMR spectra of the reaction mixture indicated that X is not TEMPO. 5 ACS Paragon Plus Environment


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(Figure S8). Further attempts to more fully characterize 11 by 2D 1H-13C NMR spectroscopy were unsuccessful. 2

10 11

5.50

ppm

8 11

11 8

5.6

5.4

5.2

5.0

4.8

4.6

4.4

4.2

4.0

3.8

3.6

3.4 ppm

Figure 3. 1H NMR (300 MHz, CDCl3) of the reaction mixture from oxidation of 2 by catalyst 5*, showing aliphatic protons of major and minor diastereomers of 11.

Oxidation of 1-Phenyl-2-Phenoxyethanol Using 10 mol % oxovanadium complex 7, substrate 3 was oxidized somewhat more slowly than 2, with 58% conversion obtained after 48 h (100 oC, pyr-d5). The slower oxidation of 3 is consistent with previous studies which showed that oxidation of sterically hindered secondary benzylic alcohols is slower than that of primary alcohols using 7.24 The major products of the reaction were benzoic acid, phenol and the ketone 2-phenoxyacetophenone 12, along with a small amount of formic acid (Scheme 4). The identity of ketone 12 was confirmed by compari-

Scheme 4. Aerobic oxidation of lignin model 3 using oxovanadium catalyst 7.

son with an independently prepared sample.28 The ketone is likely an intermediate in the formation of the benzoic acid and phenol co-products using catalysts 4 and 7. Indeed, heating an 6 ACS Paragon Plus Environment

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isolated sample of the ketone 12 with 7 (10 mol %) in pyr-d5 solvent (48 h, 100 oC) afforded a similar mixture of benzoic acid, phenol and formic acid with 70% overall conversion (Scheme 5).

Scheme 5. Oxidation of ketone intermediate 12 using oxovanadium catalyst 7.

Additional experiments showed lower conversions of 12 using catalyst 7 (10 mol %) in DMSO-d6 compared with those in pyr-d5 as observed previously using catalyst 4 and substrate 2.22 To a lesser extent the oxidation of formic acid to CO2 in DMSO-d6 (vs. pyr-d5) was also observed in these reactions. As previously discussed, the salen-vanadium catalyst 6a is highly selective for the C-O bond cleavage of arylglycerol β-aryl ether.10 Here, the ability of the 6a catalyst was tested to determine if its reactivity for complex lignin models translates to simple phenoxyethanol lignin models. The aerobic oxidation of 3 using only 1 mol % 6a afforded 97% conversion (105 oC, toluene, 18 h) to a mixture of C-H (ketone 12) and C-O bond cleavage products, acetophenone (30%) and phenol (40%) (Scheme 6). While the higher activity of the salen-vanadium catalyst is evident

Scheme 6. Aerobic oxidation of 3 using 6a.

when comparing the conversions for 3 (> 95% conversion using 10 mol % 7 vs. 1 mol % 6a), the selectivity was significantly lower, presumably due further reactivity of 12. Indeed, using 1 mol % 6a, under the same reaction conditions (105 oC, toluene, 18 h), 98% conversion of 2phenoxyacetophenone was observed affording benzoic acid, benzaldehyde, formic acid and several unidentified peaks corresponding to aldehydes and formate products (Scheme 7). This result suggests that initial selectivity for C-H and C-O bond cleavage are comparable.

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Scheme 7. Aerobic oxidation of ketone using salen-vanadium catalyst 6a.

Oxidation of substrate 3 with Cu catalyst 5* (20 mol %) gave 67% overall conversion after 40 h at 100 oC. The major products were the formylated substrate 2-phenoxy-1-phenylformate 13, and the TEMPO-functionalized ketone 2-phenoxy-1-phenyl-2-(2,2,6,6-tetramethylpiperidin-1yloxy)ethanone 14 (Scheme 8). Phenol, benzoic acid and 2-phenoxyacetophenone were obtained

Scheme 8. Oxidation of 3 using copper catalyst 5.

only as minor products. Compound 14 was isolated from a large-scale reaction and characterized by high resolution mass spectrometry and 1H and

13

C NMR spectroscopy. Control experiments

showed no conversion of 3 under the same reaction conditions (toluene, 2,6-lutidine, 100 oC, 40 h) but without catalyst 5*. For the copper-catalyzed reactions, the substrate bearing the secondary alcohol 3 reacted faster than the substrate with the primary alcohol group 2, in contrast to the vanadium-catalyzed reactions, where 2 reacted faster than 3. DISCUSSION Oxovanadium Catalysis In previous work involving the oxidation of the complex lignin model compound 1 using vanadium catalyst 4, it was found that the predominant reaction in basic solvent such as pyridine is the oxidation of the secondary alcohol22 (Scheme 9), presumably by a 2e- base-assisted 8 ACS Paragon Plus Environment

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Solvent DMSO-d6 pyr-d5

Conv.(%) i > 95 65 > 85 27

ii 5 14

iii iv V vi 14 ND 2 11 29 ND 4 6

vii 4

Aerobic Oxidation of 2-Phenoxyethanol Lignin ... - ACS Publications

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