Amine-Mediated Bond Cleavage in Oxidized Lignin Models

Article abstract of DOI:10.1002/cssc.202001228

Introducing amines/ammonia into lignin cracking will allow novel bond cleavage pathways. Herein, a method of amines/ammonia-mediated bond cleavage in oxidized lignin β-O-4 models was studied using a copper catalyst at room temperature, demonstrating the effect of the amine source on the selectivity of products. For primary and secondary aliphatic amines, lignin ketone models underwent oxidative C_α?C_β bond cleavage and C_α?N bond formation to generate aromatic amides. For ammonia, the competition between oxygen and ammonia determined the selectivity between C_α?N and C_β?N bond formation, generating amides and α-keto amides, respectively. For tertiary amines, the lignin models underwent oxidative C_α?C_β bond cleavage to benzoic acids. Control experiments indicated that amines act as nucleophiles attacking at the C_α or C_β position of the oxidized β-O-4 linkage to be cleaved. This study represents a novel example that the breakage of oxidized lignin model can be regulated by amines with a copper catalyst.

Full text of DOI:10.1002/cssc.202001228

Chemistry–Sustainability–Energy–Materials
Accepted Article
Title: Amine-mediated Bond Cleavage in Oxidized Lignin Models
Authors: Hongji Li, Meijiang Liu, Huifang Liu, Nengchao Luo, Chaofeng
Zhang, and Feng Wang
This manuscript has been accepted after peer review and appears as an
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To be cited as: ChemSusChem 10.1002/cssc.202001228
Link to VoR: https://doi.org/10.1002/cssc.202001228
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1/2020

ChemSusChem
10.1002/cssc.202001228
FULL PAPER
Amine-mediated Bond Cleavage in Oxidized Lignin Models
Hongji Li,[a,b]+ Meijiang Liu,[a,b]+ Huifang Liu, Nengchao Luo, Chaofeng Zhang, and Feng Wang*
[a]
[a]
[a]
[a]
[a]
H. Li, M. Liu, H. Liu, C. Zhang, Prof. F. Wang
State Key Laboratory of Catalysis (SKLC)
Dalian National Laboratory for Clean Energy (DNL)
Dalian Institute of Chemical Physics (DICP), Dalian 116023, China
E-mail: wangfeng@dicp.ac.cn
[
b]
+]
H. Li, M. Liu
University of Chinese Academy of Sciences, Beijing 100049, China
These authors contributed equally to this work.
[
Supporting information for this article is given via a link at the end of the document.
Abstract: Introducing amines/ammonia into lignin cracking will allow
novel bond cleavage pathways. Herein, method of
The oxidative synthesis of α-ketoamides from lignin models
via C–O bond cleavage was firstly reported by Loh et al. using
CuI/secondary amines, meanwhile amides were obtained as by-
products.[9] Recently Liu et al. reported the anilines assisted C–C
a
amines/ammonia-mediated bonds cleavage in oxidized lignin β-O-4
models was studied using copper catalyst under room temperature,
demonstrating the effect of amine source on selectivity of products.
For primary and secondary aliphatic amines, lignin ketone models
bond
cleavage
via
C–N
bond
formation
for
2-
[10]
phenoxyacetophenones. Difference of product selectivity was
observed in these systems, however, the decisive factor was
unclear due to multiple difference of reaction conditions. In this
work, the effect of amine source on product selectivity was studied
with the same catalyst, solvent and thermal conditions. It was
observed that types of products including amides, α-keto amides
and acids, were switched by choosing different amines/ammonia
(Scheme 1). After control experiments, reaction pathways in the
underwent oxidative C
to generate aromatic amides. For ammonia, the competition between
oxygen and ammonia determined the selectivity between C –N and
–N bond formation, generating amides and α-keto amides,
respectively. For tertiary amines, the lignin models underwent
oxidative C –C bond cleavage to benzoic acids. Control experiments
indicate that amines act as nucleophiles attacking at C or C position
α β α
–C bond cleavage and C –N bond formation
α
C
β
α
β
α
β
of the oxidized β-O-4 linkage to be cleaved. This study represents a
novel example that the breakage of oxidized lignin model can be
regulated by amines with copper catalyst.
presence of amines/ammonia were proposed.
O
NH
2
Ar
O
H
=
3
, R
2
, R
1
R
O
1
2
O
Introduction
R
= H, R = alkyl,
= H or alkyl
R1
N
R3
O
3
R
N
R2
Ar
Ar
+
R2
R3
Ar
OH
R
1
Lignin valorization provides a sustainable route to acquire diverse
aromatics as lignin is the most abundant and renewable aromatic
polymers in nature.[ Aiming at breaking the side-chains between
,
R 2
,
R 3
=
a
lk
yl
1]
O
aromatic units, efficient hydrogenolysis, oxidation and hydrolysis
methods have been developed.[2] Hydrogen gas, oxygen gas and
OH
Ar
other active hydrogen and oxygen-containing species were used
to cleave the C–C/C–O bond and cap the fragments, resulting in
the aromatics composed of C/H/O.[3] Besides, heteroatom
Scheme 1. Amines/ammonia-mediated transformation of oxidized lignin models
to three types of products.
(
elements except C/H/O)-containing chemicals emerged as
powerful reagents to depolymerize lignin under mild conditions.[
4]
Meanwhile, heteroatom-containing products were obtained which Results and Discussion
extended the scope of lignin-derived chemicals.
Nitrogen (N)-containing aromatics have become available
from transformation of lignin samples and model compounds
using N-containing reagents.[ Hydroxylamine-mediated bond
cleavage in β-O-4 linkages delivered isoxazoles, amides and
nitriles via a condensation or Beckmann rearrangement.[6] Direct
reaction between sodium azide and lignin benzyl alcohol structure
provided the primary aryl amines.[ Pre-functionalization to
construct acyl oxime in lignin could generate non-phenolic aryl
amines.[8] Despite of these success, the variability of products is
limited because of the adopted nitrogen source. The variety of
amines is much wider than other N-containing reagents, and
transformation of lignin using amines will afford an access to
diverse N-containing products.
To
begin,
2-methoxybenzoxyacetophene
(1a)
and
amines/ammonia were used as substrates, and Cu(OAc) as the
2
5]
catalyst to investigate the oxidative amidation reaction at room
temperature in DMSO and air atmosphere (Table 1). When
ammonia (2a), primary and secondary aliphatic amines (2b-d and
2f-i) were used as the nitrogen source, the C–C bond of 1a was
readily cleaved, generating aromatic amides (4) as major
products with 46-75% yields. Meanwhile, α-keto amides as the
by-products from only C–O bond cleavage were obtained with 2-
19% yields. Guaiacol was efficiently generated and kept stable
7]
(
88-99% yields) under this mild oxidative condition. Aniline (2e)
did not promote the bond cleavage at room temperature.
1
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ChemSusChem
10.1002/cssc.202001228
FULL PAPER
Table 1. Oxidative amidation of lignin model using copper/amines (ammonia).
Table 2. Oxidative amidation of lignin models using copper/amine.
OMe
HO
O
R
3
O
OH
O
Cu(OAc)2
R3
R4
H
N
N
O
OMe
R1
N
+
+
Cu(OAc)
R2 DMSO, air, r.t.
2
3a
1
4
1
O
R
HO
1
3
R
DMSO, air , r.t.
R
1
O
O
R
N
+
1
H
R2
2d
3
R2
4
R1
R2
1
2
4
1a
2
N
R2
+
R , R , R , R = H or OMe
R , R2 = H, alkyl, aryl
O
4
5
Entry
1
Substrate
Products, yieldsa
Yield of
a, %
Yield of
Yield of
O
O
OH
O
Entry
N reagent
3
4, %
5, %
N
1
2
NH
3
2a
96
89
52
61
15
8
1b
3b, 62%
OH
4a, 56%
O
O
O
OMe
OMe
OMe
NH
2
2b
MeO
OMe
2
3
N
NH
2
9
9
3
5
75
47
13
14
MeO
3
1c
3c, 43%
4a, 49%
2
c
d
O
N
O
OH
NH
NH
2
2
O
O
OMe
4
5
2
MeO
MeO
3a, 86%
1d
4b, 67%
0
8
0
0
6
2
e
O
OH
OH
OH
O
N
H
N
8
63
MeO
MeO
OMe
MeO
MeO
6
4
2f
H
N
1e
3a, 75%
4c, 80%
94
46
55
64
2
7
8
9
2
g
O
O
O
N
NH
NH
90
99
19
14
5
6
2h
1f
OH
3b, 67%
4a, 38%
O
O
OMe
O
N
2
i
O
OMe
MeO
OH
3a, 32%
MeO
4b, 31%
1g
2 2
Conditions: 1a (0.125 mmol), amines/ammonia (5 equiv), Cu(OAc) ·H O (10
mol%), air (1 atm), 12 h, room temperature. Yields were determined by GC with
naphthalene as the internal standard.
Conditions: 1 (0.125 mmol), 2d (40% aqueous, 5 equiv), Cu(OAc)
mol%), in air (1 atm), 9 h, room temperature. Yields were determined by GC
2
·H
2
O (10
with naphthalene as the internal standard.
OH
2 3 2
We chose the system of Cu(OAc) /NH(CH ) /air to examine
substrate scope of lignin models (Table 2). Compared with 1a, the
decrease or increase of methoxy on aryloxy group led to
moderate yields of amides (49-56% yield for 1b and 1c). The
increase of methoxy on acetophenone group (1d and 1e)
exhibited excellent yields of amides (67-80% yield). Phenol and
guaiacol could be released from C–O cleave and kept stable
under the oxidation conditions (62-86% yields). However, 2, 6-
O
H
N
Cu(OAc)
DMSO, r.t.
air or O
2
3b
O
1
R1
R2
O
O
R
N
+
R1
R2
1b
2
2
N
R2
+
O
1
2
R , R = H or alkyl
4
5
H
N
NH
3
2a
NH
2
2b
2f
NH 2h
dimethoxyphenol was prone to over-oxidation (entry 2; CO
formation during reaction, see Figure S2). For lignin models
bearing hydroxymethyl on C (1f and 1g), 31-38% yields of
2
β
100
3b
4
5
amides were obtained during which the formation of α-keto amide
and low conversion of substrate restrained the amide formation
Air O2
8
6
4
0
0
0
O
2
Air
Air
O2
Air
(
Figure S1).
The observation that secondary amines mediated mainly C–
O2
C cleavage over C–O cleavage was different with that in CuI-
catalyzed system.[9] Then air atmosphere was replaced with pure
oxygen atmosphere to check the change of selectivity (Figure 1).
It was observed that the selectivity of α-keto amide obviously
increased in the presence of ammonia under oxygen atmosphere.
However, the product distribution remained in the presence of
primary amine (2b) and secondary amines (2f and 2h) regardless
of atmosphere. The effect of atmosphere on product distribution
was also shown by other lignin models in the presence of
ammonia (Figure 2).
20
0
2
a
2b
2f
2h
Conditions: 1b (0.125 mmol), amines/ammonia (5 equiv), Cu(OAc)
mol%), air or O
2 2
·H O (10
2
(1 atm), 12 h, room temperature. Yields were determined by
GC with naphthalene as the internal standard.
Figure 1. The effect of atmosphere on product distribution in the reaction of
lignin model with amines/ammonia.
2
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O
OMe
Cu(OAc)2
O
N
Control experiments were performed to explore the reaction
mechanism. Firstly, qualitative isotope labelling experiments
showed the formation of deuterium-labeled formyl butylamine (6-
O
dimethylamine
(2)
DMSO, r.t., 18O
MeO
2
MeO
MeO
1
d
4e
6
h
D), which revealed only one C
conversion of 1-aryl-2-aryloxy ethanone to amide product
Scheme 2, eq. 1). The formation of non-deuterated formyl
butylamine (6) was possibly attributed to partial hydrogen
exchange between 1b-D and 2c. Then 18O
was used to label the
β
–H was abstracted in the
O16 : O18 = 99 : 1
O
OMe
O
(
Cu(OAc)2
ammonia
O
NH
2
(3)
DMSO, r.t., 18
6 h
O
O
2
MeO
2
1
d
5a
oxygen of product. Aromatic amide almost kept all oxygen from
substrate, as no labelling oxygen was introduced into the amide
product (4e) (eq. 2). For α-keto amide product (5a), one of the two
carbonyl oxygen atoms was labeled (eq. 3). Then substrate 7 was
selected as a single-hydrogen-abstracted substrate, and it
smoothly delivered amide with high selectivity (Scheme 3, eq. 4).
Meanwhile, methyl benzoylformate (8) could not be converted into
amide (eq. 5). These tests further confirmed a one-hydrogen-
abstraction process to generate amide, and benzoylformate could
be the intermediate of α-keto amide. In addition, benzaldehyde
and benzoic acid could not be converted to amide (eq. 6),
suggesting the generation of amide directly from nucleophilic
substitution at carbonyl by amine before cleavage of the C–C
bond.
O16 : O18 = 26 : 74
Scheme 2. Isotope labelling experiments.
O
O
OH
O
Cu(OAc)2
NH
2
N
(4)
+
+
H
DMSO,r.t., air
10 h
7
2b
4f, 53%
3b, 69%
O
O
O
Cu(OAc)
2
O
(
5)
+
NH2
O
DMSO,r.t., air
4 h
HN
8
2b
5b, 79%
O
O
O
Cu(OAc)
2
H
NH
2
N
or
OH
10a
+
(6)
H
DMSO,r.t., air
4 h
9
2b
4
f
not detected
Scheme 3. Control experiments with possible intermediate analogues.
R3
HO
O
R3
R1
R2
R1
R2
O
2
3
N
H
R1
N
Cu(OAc)
DMSO, r.t.
air or O
2
O
O
+
NH
3
O
O
O
R1
R2
R1
R2
O
Ar1
- H
O
Ar2
Ar1
R2
2
NH
2
+
Ar OH
Ar1
Ar2
1
2a
NH
2
2
O
O
5
+
O
1
3
4
5
C
3
1
1
1
b
h
d
R , R , R = H
1
R , R3 = H; R2 = OMe
[CuI]
[CuII]
+
[CuI]
[CuII]
R1 R2
R1 = H; R , R3 = OMe
2
O2
O2
O2
N
H
- H+
R1, R2 = OMe; R3 = H
i
1
R1 R2
N
O
O
O
O
H
O
H
R2
O
Ar
R1
1
Ar
2
Ar1
+
H
OAr2
Ar1
Ar2
N
R1
3
4
5
100
N
R2
O
O
O
X
Air
O
X
A
B
4
11
O2
80
60
40
20
0
Air O2
Air O2
Air O2
n
X= [CuL ] or H
Scheme 4.
A
proposed mechanism for amines/ammonia-mediated
transformation of lignin model to amide and α-keto amide.
Based on these investigations and previous reports,[11]
a
plausible reaction mechanism is proposed for amines/ammonia-
mediated transformation of lignin model to amide and α-keto
1
b
1h
1d
1i
II
amide (Scheme 4). Initially the Cu complex reacts with substrate
Conditions: 1 (0.125 mmol), ammonia (5 equiv), Cu(OAc)
or O (1 atm), 12 h, room temperature. Yields were determined by GC with
naphthalene as the internal standard.
2
·H
2
O (10 mol%), air
and dioxygen to give peroxide species A accompanied by
abstraction of one hydrogen. For ammonia, primary and
secondary amines, intermediate B is formed by the nucleophilic
addition of ketone with amine reversibly. An aminodioxetane
intermediate could be ruled out as the oxygen of ketone remained
in amide product (Scheme 2, eq. 2). Then, intermediate B
undergoes rearrangement to cleave C–C bond, affording amide 4
and formate 11. For ammonia, it could behave as primary and
secondary amines and affords amide product, however, weak
nucleophilicity of ammonia allows the competition of oxygen to
2
Figure 2. The effect of atmosphere on product distribution in the reaction of
different lignin model with ammonia.
OH
O
O
O
N
+
H
D
D
Cu(OAc)
2
3b
(1)
4
d
1b-D
β
abstract another C –H, then α-keto ester C may be generated and
O
O
DMSO, r.t., air
2 h
+
1
undergoes following ammonolysis to give α-keto amide 5.
Compared with primary and secondary amines which attack
carbonyl of ketone to give amides, the tertiary amine without N–H
bond could not act as a nucleophile. Nevertheless, the presence
of tertiary amines led to the C–C bond oxidative cleavage of β-O-
D
N
+
H
N
NH
2
H
H
2
c
6-D
6
3
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ChemSusChem
10.1002/cssc.202001228
FULL PAPER
4
ketone to give benzoic acid (Scheme 5a). Triethylamine,
Isotope labelling experiments showed the formation of
tripropylamine and N, N-dimethylcyclohexylamine afforded
benzoic acid in 58-85% yields and phenol in 56–71% yields.
Bidentate tetramethylethylenediamine behaved less efficiently
deuterium-labeled formate 11a-D (Scheme 6, eq. 7), suggesting
that only one C
β
–H was abstracted in the conversion of 1-aryl-2-
was used, one of two oxygen
aryloxy ethanone to acid. When 18
O
2
atoms in acid was labeled, meanwhile 18O–labeled formate (11a)
was observed, indicating the aerobic oxidative C–C cleavage
process in the copper/amine system (eq. 8). Benzaldehyde (9)
could not be converted to acid (eq. 9), suggesting the generation
of acid directly from nucleophilic substitution at carbonyl by
oxygen species before cleavage of the C–C bond.
For tertiary amines, they are obstructed from attacking the
carbonyl of ketone (Scheme 7). Instead, after the formation of
peroxide species A, the peroxide will attack the carbonyl to give
dioxetane D. Subsequent fragmentation of D affords aromatic
acid 10 and formate.
(
44% yield of benzoic acid). Then other models were tested in the
presence of Cu(OAc) and triethylamine (Scheme 5b). Moderate
to good yields (61–91%) of corresponding acids were obtained.
2
(a)
Cu(OAc)
(tertiary amines)
2
O
OH
O
2
O
OH
5a
+
DMSO, O
2
, r.t.
1b
3b
N
N
N
N
N
2
j
2k
3b, 71%; 5a, 85%
2l
2m
3
b, 60%; 5a, 69%
3b, 56%; 5a, 58%
3b, 56%; 5a, 44%
O
2
[CuII]
CuI]
[
O
O
(
b)
O
Ar2
O
O
R3
Cu(OAc)
2j
DMSO, O
2
OH
O
O
O
Ar1
3
4
R1
Ar1
Ar2
Ar1
X
Ar2
R1
R
R
O
OH
O2
H+
+
O
O O
O
A
D
-
R2
R4
2
, r.t.
R
2
R1 R2
N
1
3
3
10
X = [CuL ] or H
n
1
2
4
R , R , R , R = H or OMe
substrate
R3
Products, yields
O
O
O
O
OH
O
2
+
Ar OH
Ar1 OH
H
OAr2
11
OH
3
10
MeO
1h
MeO
3b, 53%
10b, 78%
Scheme 7. A proposed mechanism for amines-mediated transformation of
lignin model to acid.
O
O
OH
OH
O
MeO
MeO
O
O
MeO
MeO
OH
1i
3
b, 57%
10c, 91%
Conclusion
OMe
O
OMe
OH
MeO
1d
MeO
In summary, we reported a strategy of cleaving oxidized lignin C–
C/C–O bonds, which is directed by primary, secondary and
tertiary aliphatic amines, and ammonia in the presence of a
copper catalyst towards aromatic amide, α-ketoamide and acid
products. This strategy provides the possibility to switch desired
product from lignin models by choice of amines, and further
exploration on real lignin conversion is undergoing.
3
a, 93%
10b, 79%
O
OMe
OH
O
O
MeO
OMe
OH
1
c
MeO
3c, 12%
10a, 61%
2 2 2
Conditions: 1 (0.125 mmol), amines (5 equiv), Cu(OAc) ·H O (10 mol%), O (1
atm), 12 h, room temperature. After the reaction, aromatic acids were esterified
using methanol. The yields were determined by GC with naphthalene as the
internal standard.
Scheme 5. Oxidation of lignin models to aromatic acid using copper/tertiary
amines.
Experimental Section
O
OH
Typically, a 15 mL pressure bottle was charged with lignin model (0.125 mmol),
copper acetate (0.0125 mmol), amine or ammonia (5 equiv), and DMSO (1 mL),
and the reaction was conducted at 25 °C under air or oxygen atmosphere. Then
a solution of naphthalene (16 mg in 0.5 mL DMF) was added. The mixture was
analysed by GC-MS. The analysis of lignin models and products is available in
the Supporting Information.
O
OMe
Cu(OAc)2
triethylamine
MeO
OMe
10b
O
D
D
OMe
(7)
MeO
DMSO,r.t., O
2 h
2
OH
O
O
1
1d-D
+
D
3a
11a-D
O
OMe
O
OMe
Cu(OAc)
triethylamine
2
O
O
OH
(
8)
Acknowledgements
DMSO, r.t., 18
O
O
MeO
2
MeO
1d
6
h
10b
11a
This work was supported by the National Natural Science
Foundation of China (Project No. 21802137 ， 21721004,
21690082, 21690080, 21905275) and the "Strategic Priority
Research Program of the Chinese Academy of Sciences"
(XDB17000000) and the CAS-NSTDA Joint Research Project
O16 : O18 = 40 : 60 O16 : O18 = 19 : 81
O
Cu(OAc)
triethylamine
2
H
no reaction
(9)
DMSO,r.t., O
2 h
2
9
1
(
GJHZ2075).The authors thank Joseph S. M. Samec for technical
assistance in mechanism studies.
Scheme 6. Isotope labelling experiments and control experiment.
4
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FULL PAPER
[
8]
H. Li, A. Bunrit, J. Lu, Z. Gao, N. Luo, H. Liu, F. Wang, ACS Catal. 2019,
Keywords: lignin model • amine • copper catalyst • bond
9
, 8843-8851.
J. Zhang, Y. Liu, S. Chiba, T. P. Loh, Chem. Commun. 2013, 49, 11439-
1441.
cleavage • amide
[9]
1
[10] X. Liu, H. Zhang, C. Wu, Z. Liu, Y. Chen, B. Yu, Z. Liu, New J. Chem.
2018, 42, 1223-1227.
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ChemSusChem
10.1002/cssc.202001228
FULL PAPER
O
NH
2
Ar
O
Lignin model
-keto amide
H
=
3
O
, R2 , R
1
R
O
Ar
Ar
R1 = H, R = alkyl,
2
O
OH
R3 = H or alkyl
R3
N
+
Ar
R2
R1
Cu-catalyzed
R
1
,
R 2
,
amide
2N
R3
R
3
R
=
al
kyl
ammonia/amines
O
OH
acid
Ar
Three types of products from lignin models, including amides, α-keto amides and acids, were switched by choosing different
amines/ammonia.
6
This article is protected by copyright. All rights reserved.