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Selective Copper-NHC Catalyzed Aerobic Cleavage of #-1 Lignin Models to Aldehydes Zhong-zhen Zhou, Mingxin Liu, and Chao-Jun Li ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b00565 • Publication Date (Web): 06 Apr 2017 Downloaded from http://pubs.acs.org on April 6, 2017
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ACS Catalysis
Selective Copper-NHC Catalyzed Aerobic Cleavage of β-1 Lignin Models to Aldehydes Zhong-zhen Zhou, †, §, ‡ Mingxin Liu †, ‡ and Chao-Jun Li †, * †
Department of Chemistry and FQRNT Centre for Green Chemistry Catalysis, McGill University 801 Sherbrooke St. W., Montreal, Quebec H3A 0B8 (Canada). §
School of Pharmaceutical Sciences, Southern Medical University, Guangzhou 510515, China.
ABSTRACT: As fossil resources are undergoing fast depletion, harvesting sustainable aromatic compounds from biorenewable lignin becomes highly appealing nowadays. However, the development of efficient catalyst for lignin depolymerization to high value-added chemicals is challenging. In this paper, a selective copper-NHC catalyzed aerobic cleavage is reported for β-1 lignin model compound, which represents an important family of natural wood lignin. Using this catalyst system, the cleavage of β-1 model compounds gave >99% conversions and satisfactory yields of the corresponding aldehydes, which can serve as versatile starting materials for chemical industry.
KEYWORDS: aerobic cleavage, copper-NHC, β-1 lignin models, C-C bond cleavage, aldehydes Efficient cleavage of lignin linkages for harvesting high value-added aromatic material remains highly challenging nowadays1-4. Among those linkages, the β-1 linkage is a significant component5-6. According to thioacidolysis and 2D-HSQC-NMR studies of the lignin oligomers from cryptomeria japonica (softwood) and eucalyptus globulus (hardwood), β-1 abundancy is up to 30% for the former, whereas up to 50% was observed for the later7. Moreover, other linkages, for example β-O-4 in softwood and hardwood lignin can also be readily converted into β-1 by oxidative cross-linking reaction1, 8. Nevertheless, considerable interests were paid to the cleavage of β-O-4 linkage2, 915 . The cleavage method for β-1 linkage remains very scarce16-21, especially examples utilizing oxygen as a naturally abundant and environmentally benign terminal oxidant.
(a) Baker and Hanson (2013) OMe
HO
O
1 equiv Cu (OTf)/TEMPO 10 equiv 2,6-lutidine MeO O2 (1 atm)
MeO
toluene, 100 C, 48h
MeO OMe
100% conversion
HO
82 %
25% (X = H) 1% (X = OH)
O OMe
MeO
H
H
+
HO
O
MeO OMe 6%
9%
(b) Mariano (2015) h 1 9,10-Dicyanoanthracene R
+R
OH
R1 OH
OH
O
R2 OH
5%H2O-MeCN, O2 100% conversion
1
OH
O
O
16
In 2013, Baker and Hanson reported an aerobic cleavage of β-1 linkage using copper(I) trifluoromethane-sulfonate (CuOTf)/TEMPO system. This discovery is highly important, however the method requires excessive alkali (10 equiv 2,6-lutidine) and even excessive catalyst for certain cases (Figure 1a). The substrate scope was very limited. Moreover, the cleavage resulted in different oxidation product mixture with dramatically varying composition between different substrates. In 2015, Mariano developed a photochemical and enzymatic SET (single electron transfer) promoted C-C bond aerobic cleavage for lignin β-1 model compounds (Figure 1b). Though conversions were high, mixture of products was still obtained using this method. In 2016, Wang developed an acid-promoted C–C bond aerobic cleavage catalyzed by copper catalyst. (Figure 1c)21 However, only one β-1 ketone example was reported. A more efficient and product-selective method towards β-1 cleavage is still highly desirable.
X
+
O
O
OH
O
H
R2
H
+ R1 R2 O
+ R1 OH
(c) Wang (2016) O
O
O 20% Cu(OAc) 2/BF3 OEt2 OH
OMe
MeOH, 0.4 MPa O2 100 OC, 3h
H
+
87 %
0%
(d) this work OMe 20% CuCl/SIMes 3 equiv quinoline O2 (1 atm)
OH R
dioxane, 100 oC, 24h
O R
+
H
MeO
OH 1
O H
2a-e
2a
Figure 1. Catalyzed aerobic cleavage of β-1 lignin models to aldehydes.
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Recently, our group has shown that homogeneous copper-NHC-catalysts can catalyze aerobic oxidation of aldehydes via a unique mechanism.22 Motivated by this work and to further explore copper-NHC (Figure 2) catalyzed aerobic oxidation, herein, we would like to report an efficient and highly selective copper-NHC catalyzed aerobic cleavage of β-1 lignin models to aldehydes without using TEMPO. i-Pr N Cu Cl
N
Cu i-Pr Cl
(IPr)CuCl
N
N
Cu Cl
(OSIMes)CuCl
N
N MeO
N Cu Cl
(DPSIMes)CuCl
Cu Cl
Table 1 Optimization of reaction conditions
N
i-Pr
(SIMes)CuCl
ducted in argon (entry 16). In summary, the optimized conditions for the cleavage of β-1 lignin model compounds are (SIMes)CuCl (20%) with quinoline (3 equiv) at 100 oC under O2 (1 atm) for 24 h using dioxane as solvent.
i-Pr N
N
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OMe
(MOPI)CuCl
Entry
Initially, we examined aerobic cleavage of β-1 lignin model compound 1aE using pre-synthesized (SIMes)CuCl as catalyst in dioxane with added quinoline (3 equiv with respect to substrate) under O2 (1 atm) at 100 °C for 24 h (Table 1). To our delight, the yield of 4methoxybenzaldehyde and the conversion of substrate 1aE were 50% and 71%, respectively (entry 1). However, using CuCl (10% mol) without NHC-ligand as catalyst, the yield of 4-methoxy-benzaldehyde was only 21%, and the conversion of substrate 1aE was 52% (entry 2). Using SIMes alone or CuCl and SIMes (without pre-mixing) as catalysts also gave lower conversions and yields. Encouraged by these results, we then examined other NHCs (including IPr, OSIMes, DPSIMes and MOPI), but all resulted in decreased yields (entries 5-8). By increasing the amount of (SIMes)CuCl to 20%, the substrate conversion and the yield were increased to 100% and 90%, respectively (entry 9). The yield of 4-methoxybenzaldehyde was also shown to decrease along with lower amount of quinoline (entry 10). The replacement of either quinoline or dioxane with other bases and solvents also led to lower conversion and yield. Less polar solvents seem preferable to this reaction (Table S1 in the Supporting Information), possibly related to the solubility of β-1 model compound. Lowering the reaction temperature from 100 oC to 80 oC or room temperature led to decreased reaction efficiency (entries 11 and 12). Raising the reaction temperature from 100 oC to 120 oC gave rise to some 4-methoxybenzoic acid (entry 13). Moreover, shortening the reaction time also reduced the yield considerably (entries 14 and 15). The cleavage was not observed when the reaction was con-
Conv. (%)
a
Yield (%)
1
(SIMes)CuCl/10
71
50
2
CuCl/10
52
21
3
SIMes/10
0
0
4
CuCl + SIMes/10
10
7
5
(DPSIMes)CuCl/10
33
14
6
(MOPI)CuCl/10
0
0
7
(OSIMes)CuCl/10
42
17
8
(IPr)CuCl/10
16
7
9
(SIMes)CuCl/20
100
90 (89)
10
c
(SIMes)CuCl/20
100
68
d
(SIMes)CuCl/20
6
0
e
12
(SIMes)CuCl/20
66
60
f
(SIMes)CuCl/20
100
64 (17)
h
(SIMes)CuCl/20
71
54
i
(SIMes)CuCl/20
100
70
j
(SIMes)CuCl/20
1
0
11
Figure 2. The structures of copper-NHCs.
Catalyst/ mol %
13
14 15
16 a
a
b
g
1
The yields were determined by H NMR using 1,3,5b trimethoxybenzene as the internal standard; Isolated yield; c The amount of quinoline is 1.5 equiv with respect to subd e strate. The reaction temperature is rt; The reaction temo f o g perature is 80 C; The reaction temperature is 120 C; The h i yield of 4-methoxybenzoic acid; The reaction time is 10 h; j The reaction time is 20 h; The reaction was performed under argon.
With the optimized conditions in hand, we examined our method for the cleavage of other β-1 lignin model compounds. The results in Table 2 indicate that for all β-1 model compounds, substrate conversions were always >99% under the optimized reaction conditions. The yields of the cleavage aldehyde products are good to excellent for a broad range of substrates. Compared to 1aE (erythro), its threo-diastereomer 1bT (entry 2) gave 4methoxybenzaldehyde in 70% yield. The yields of aldehydes from other erythro-substrates were also higher than their threo- diastereomers, which is consistent with 12 previous study. In addition, erythro- substrates 1c, 1e, 1g and 1j, bearing one or two MeO- groups at other positions of benzene ring A, gave lower yield of the cleavage products than 1aE.
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ACS Catalysis
Table 2 Copper-NHC catalyzed aerobic cleavage of β-1 lignin model compounds
Scheme 2. The cleavage of β-O-4 lignin model compound 1kE using (SIMes)CuCl catalyst system To further demonstrate the practicality of the (SIMes)CuCl catalyst system, 1 mmol reaction of compound 1aE was carried using pure O2 to give 217.5 mg of compound 2a in 80% isolated yield (Scheme 1). In addition, we also performed the cleavage of β-O-4 model compound 3 using (SIMes)CuCl catalyst system. The result showed that the ketone 4 is main product, and the rate of cleavage of compound 3 is only 12% (Scheme 2). Furthermore, under the same conditions, using 1:1 mixture of 1aE (scheme 1) and 3 (Scheme 2) only resulted cleavage of the former together with a small amount of ketone 4.
a
1
The yields were determined by H-NMR using 1,3,5trimethoxybenzene as the internal standard.
Scheme 1. The cleavage of compound 1aE (1 mmol) using (SIMes)CuCl catalyst system
Figure 3 Proposed mechanism of copper-NHC catalyzed aerobic cleavage of β-1 lignin model compounds.
For the (SIMes)CuCl catalyst system, these reactions afforded aldehydes as major products resulted from Cα−Cβ bond cleavage (shown in Figure 3), but no ketone product (see the crude 1H NMR (Figure S1) in Supplementary Information) was observed. Furthermore, the results of entries 3-10 showed that, for asymmetric substrates, the yields of aldehyde 2a were always lower than their cleavage counterparts (2b-2e). Similar data was also observed by Baker and Hanson16 using Cu(I) OTf /TEMPO as catalyst, suggesting that the reaction may undergo similar initial oxidation of the primary alcohol in compounds 1, followed by a retro-aldol reaction to break the Cα−Cβ bond and generate the first product 2a-e, which come
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from A-ring. In our study, it is also possible that the quinoline was oxidized into quinoline-N-oxide and served as TEMPO-like additive. The resulting fragment 7 was then oxidized to 8 (The NMR evidence for 8 presence was attached in Figure S2 in supplementary information). Then 8 was further cleaved to give 2a, which come from B-ring. Aldehyde 2a can be further oxidized into 9, which was detected in trace amount in our experiment. The proposed mechanism was supported by experiments depicted in scheme 3. The cleavage of 1,2-diphenylethan-1-ol (compound 10) did not occur under our catalyst conditions (Scheme 3a). Under the same conditions, ketone 11 (scheme 3b) gave 52% 1-(3,4-dimethoxyphenyl)-2-(4methoxyphenyl)-ethane-1,2-dione (compound 13) and 2% 1-(3,4-dimethoxyphenyl)-2-(4-methoxyphenyl)-ethanone (compound 12), which possibly generated from oxidation of alcohol into carboxylic acid22 followed by decarboxylative process. Only 14% of cleavage products was obtained with 1:1 ratio of 3,4-dimethoxybenzoic acid and 4methoxybenzaldehyde, while no 3,4dimethoxybenzaldehyde or 4-methoxybenzoic acid were not detected in the reaction mixture [see the crude 1H NMR (Figure S4) and crude GC-MS (Figure S5) in Supplementary Information]. Finally, the oxidative cleavage of 4-methoxybenzeneacetic acid (compound 8) was also observed with our reaction conditions (scheme 3c).
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perb conversions. For the (SIMes)CuCl catalyst system, the Cα−Cβ bond cleavage reaction may proceed by a retro-aldol reaction after the selective oxidation of the primary alcohol.
ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Optimization of reaction conditions; Synthesis of diastereomeric β-1 lignin model compounds; General procedures for aerobic cleavage of β-1 lignin models; The crude NMR of copper-SIMes catalyzed aerobic cleavage of 1aE and 2-(4-methoxyphenyl)acetic acid and ketone 11; Spectra of β-1 lignin model compounds and products. (PDF)
AUTHOR INFORMATION Corresponding Author *Prof. Dr. C.-J. Li Department of Chemistry and FQRNT Center for Green Chemistry and Catalysis McGill University, Montreal, Quebec, H3A 0B8 (Canada) E-mail:
[email protected] Homepage: http://cjli.mcgill.ca/cjpage.htm
Author Contributions ‡These authors contributed equally.
a)
Notes
OH
(SIMes)CuCl (20 mol %) O2 (1 atm)
The authors declare no competing financial interest.
No product
quilinone (3 equiv), dioxane 100oC, 24h
10
EXPERIMENTAL
0% conversion b) OMe
O
MeO
OH OMe
11
O
(SIMes)CuCl (20 mol %) O2 (1 atm) MeO quilinone (3 equiv), dioxane 100oC, 24h
O OH +
H
MeO OMe 14%
14 %
75% conversion
O
O H +
+ MeO
OH
MeO OMe N.D.
N.D. OMe
O + MeO
12, 2% OMe
OMe
O + O
MeO OMe
General procedures for aerobic cleavage of β-1 lignin models. A reaction vessel, charged with (SIMes)CuCl catalyst (8 mg, 0.02 mmol, 20 mol %) and a β-1 lignin model compound (0.1 mmol), was gently flushed with oxygen of ordinary purity using a balloon. After this, distilled dioxane (0.6 mL) and quinoine (36 μL, 3 equiv) were added to the vessel. The reaction mixture was then warmed up to 100 oC, and kept at 100 oC for 24h. After this, water (5 mL) was added, and the pH of the aqueous phase was then adjusted to 2 with 0.1 M HCl. Then, the mixture was extracted with ethyl ether 3 times with a total ether volume of 15 mL and the combined ether phase was dried over anhydrous sodium sulfate, and evaporated in vacuo to obtain a residue, which was chromatographed using a mixture of hexane and ethyl acetate (v/v = 20:1) to give aldehydes.
13, 52%
ACKNOWLEDGMENT
c) COOH (SIMes)CuCl (20 mol %) O2 (1 atm)
COOH +
quilinone (3 equiv), dioxane MeO 100oC, 24h
MeO 8
CHO MeO
9 isolated yield = 55%
2a 10%
Scheme 3. Mechanism investigation In summary, we developed an efficient (SIMes)CuCl catalyst system for the cleavage of β-1 lignin linkages to give aldehydes with good to excellent yields, high selectivity, and su-
We are grateful to the Canada Research Chair Foundation (to C.-J. L.), the Canadian Foundation for Innovation, FRQNT Centre in Green Chemistry and Catalysis, and the Natural Science and Engineering Research Council of Canada for support of our research. Dr. Z. Z. Zhou is grateful for the support from China Scholarship Council (CSC) and Scholarship Council of Southern Medical University.
ABBREVIATIONS
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ACS Catalysis
TEMPO,
2,2,6,6-tetramethylpiperidine-1-oxyl; NHC, N‑heterocyclic carbene; rt, room temperature; N. D., not detected.
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