Photocatalytic Cleavage of Aryl Ether in Modified Lignin to Non-phenolic Aromatics

Article abstract of DOI:10.1021/acscatal.9b02719

Depolymerization of lignin meets the difficulty in cleaving the robust aryl ether bond. Herein, through installing an internal nucleophile in the β-O-4′ linkage, the selective cleavage of aryl ether was realized by the intramolecular substitution on aryl rings affording non-phenolic arylamine products. In particular, nitrogen-modified lignin models and lignin samples were employed to generate the iminyl radical under photocatalytic reduction, which acted as the internal nucleophile inducing aryl migration from O to the N atom. The following hydrolysis released primary arylamines and α-hydroxy ketones. Mechanism studies including electron spin resonance (ESR), fluorescence quenching experiments, and density functional theory (DFT) calculations proved the aryl migration pathway. This method enables access to non-phenolic arylamine products from lignin conversion.

Full text of DOI:10.1021/acscatal.9b02719

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Article
Photocatalytic Cleavage of Aryl Ether in
Modified Lignin to Non-phenolic Aromatics
Hongji Li, Anon Bunrit, Jianmin Lu, Zhuyan Gao, Nengchao Luo, Huifang Liu, and Feng Wang
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b02719 • Publication Date (Web): 14 Aug 2019
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Photocatalytic Cleavage of Aryl Ether in Modified Lignin to
Non-phenolic Aromatics
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Hongji Li,†, ‡ Anon Bunrit,† Jianmin Lu,† Zhuyan Gao,†, ‡ Nengchao Luo,†, ‡ Huifang
Liu,† and Feng Wang*, †
† State Key Laboratory of Catalysis (SKLC), Dalian National Laboratory for Clean
Energy (DNL), Dalian Institute of Chemical Physics (DICP), Chinese Academy of
Sciences, Dalian 116023, China.
‡University of Chinese Academy of Sciences, Beijing 100049, China.
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ABSTRACT: Depolymerization of lignin meets the difficulty in cleaving robust aryl
ether bond. Herein through installing an internal nucleophile in β-O-4’ linkage, the
selective cleavage of aryl ether was realized by the intramolecular substitution on aryl
rings affording non-phenolic arylamine products. In particular, nitrogen-modified
lignin models and lignin samples were employed to generate the iminyl radical under
photocatalytic reduction, which acted as the internal nucleophile inducing aryl
migration from O to N atom. The following hydrolysis released primary arylamines and
α-hydroxy ketones. Mechanism studies including electron spin resonance (ESR),
fluorescence quenching experiments and density functional theory (DFT) calculations
proved the aryl migration pathway. This method enables access to non-phenolic
arylamine products from lignin conversion.
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KEYWORDS: lignin • photocatalytic • aryl ether cleavage • amination • aryl migration
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INTRODUCTION
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Lignin, the richest resource of aromatic polymers in nature, has a great potential to
produce various aromatic compounds via sustainable pathways.1-7 Phenolic units
including syringyl (S), guaiacyl (G) and p-hydroxy phenyl (H) units are connected by
many types of linkages, in which the β-O-4’ linkage is the richest and its cleavage is
the key to depolymerize lignin.8-11 Hydrogenolysis using internal hydrogen donors12-14
or external hydrogen donors15-26 and hydrolysis methods27-30 have been developed to
cleave the aliphatic ether bond (Cβ−O) affording the alkyl-substituted phenols, aromatic
ketones, etc. Aerobic oxidation,31-40 retro-aldol reaction,41 decarbonylation42 and
hydrogenation43 methods have been performed to cleave the aliphatic C−C bond and
afford the aromatic acids, aldehydes, etc. Using sodium azide,44 oxidation31 and
rearrangement reactions45 could induce the cleavage of Caryl−Cα bond. Despite their
versatility, most of these transformations did not involve the cleavage of aryl ether bond
(C4’−O) thus the depolymerized products were limited to phenolic monomers.
Obtaining the non-phenolic aromatics will extend the application of lignin
conversion,46,47 however, selective activation of desired aryl ether is retarded by its high
bond energy and co-existed aliphatic ethers.48
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Recently the cation radical-accelerated nucleophilic aromatic substitution was
established to cleave the aryl ether bond, in which single electron oxidation activated
the arene ring and imidazole as the external nucleophile finished the substitution of
methoxy- and benzyloxy-groups.49 Despite its feasibility partly shown by the cleaving
aryl ether in lignin β-O-4’ ketone models, complicated chemical environment
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neighboring β-O-4’ linkage may affect its application in lignin samples. In contrast, one
alternative method using an internal nucleophile in β-O-4’ linkage may overcome the
effect of chemical environment via kinetically-favorable intramolecular reaction.50 Pre-
modification of lignin has emerged as a strategy to deliver functionalized monomers
and polymers,51-53 and we considered that the precursor of internal nucleophile could
be constructed during modification.
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Herein we developed the internal nucleophile to selectively cleave aryl ether in
lignin based on nitrogen-modification (Scheme 1). The lignin β-O-4’ linkage was
modified from an alcohol structure to an acyl oxime structure, namely α-oximated β-
aryl ether, from which the iminyl radical was generated under photocatalytic conditions
and participated in intramolecular amination of aryl ether to deliver non-phenolic
primary arylamines. Aliphatic C−O bond kept stable and the α-hydroxy ketones were
obtained as another part of products. Both lignin models and preliminary modified
lignin conversion exhibited the feasibility of this method in non-phenolic arylamines
generation from lignin.
Otherwise, nitrogen-containing reagents have been utilized to promote the
cleavage of lignin linkages where nitrogen-containing reagents could act as the strong
nucleophile to attack the inert C−O bonds or C−C bonds.44,46,47,54 Meanwhile, the
generated nitrogen-containing products are useful chemicals such as aniline, which is
the key chemical for the manufacture of polyurethane, etc.26,55 In this work, primary
arylamines instead of phenolic products were obtained via amination of aryl ethers and
these products could be readily derived to other nitrogen-containing compounds, thus
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lignin -O-4' linkages

bond cleavage
products
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C1-C (Caryl-Calkyl
)
OH
R'
phenolic
aromatics

1
1
O
O
Ar
Ar
C-C (Calkyl-Calkyl
C-O (Calkyl-O)
)

4'
4'
Ar
Ar
Ar
Ar
HO
4
4
R
O
O
well studied
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O
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
R'

C-O4' (Caryl-O)
non-phenolic
aromatics
R
Y
rarely studied
Our strategy: installing internal nucleophile to selectively cleave aryl ether
AcO
Ar
N
PTH
R'
O
Ar
Ar
h
H2N
R
O
non-phenolic
primary arylamines
lignin models
lignin extracts
N
O
Ar
Ar
R
O
Scheme 1. Cleavage of different bonds in lignin β-O-4’ linkage and the installing
internal nucleophile strategy to cleave aryl ether bond.
this work may connect lignin conversion with production of abundant nitrogen-
containing chemicals.
RESULTS AND DISCUSSION
The α-oximated β-aryl ether models which were prepared via oximation and acetylation
of β-O-4’ ketone models could be used to generate the iminyl radical under
photocatalytic reduction. In previous reports the iminyl radical could act as a versatile
intermediate to trigger useful transformations.56 In this case, we explored the iminyl
radical to induce the aryl migration from oxygen to nitrogen atom which cleaved the
aryl ether bond and constructed aryl C−N bond, and following hydrolysis afforded
primary arylamine and α-hydroxy ketone. It was found that phenothiazine (PTH) used
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as the catalyst in previous photocatalytic reduction (Ered (PTH•+/PTH*) near −2.1 V vs
SCE)57-60 exhibited good results in the reductive cleavage of N−O bond for the
generation of iminyl radical from acetyl oxime 1a. The highest yields of primary
arylamine 2a (70%) and 2-hydroxyacetophenone 3a (80%) were obtained from aryl
ether cleavage when using PTH PC1 as the photocatalyst, ascorbic acid (HA) as the
Table 1. Optimization of reaction conditions.
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PC1 (6 mol%)
AcO
O
N
Ascorbic acid (2 eq.)
NH2
OH
O
HCOOH (4 eq.)
Aryl C O

cleavage
:
:
1
CH3CN H2O = 1
OMe
OMe
365 nm
2a
, 70%
3a
, 80%
O
1a
OH
Aliphatic C O

cleavage
OMe
4a
5a
, 7%
, 7%
S
N
Phenothiazines (PTH)
HO
H
O
PC1; R = R' = Me
PC2; R = R' = H
R
R
O
HO
PC3; R = H, R' = t-Bu
PC4; R = H, R' = CF3
Ascorbic acid
R'
Yields from aryl C−O
Yields from aliphatic
cleavage, %
C−O cleavage, %
Entry
Deviation from above
2a
67
5
3a
40
0
4a
9
5a
11
1
1
2
CH3CN/H2O (4/1) as solvent
CH3CN as solvent
13
0
3
Without ascorbic acid
Na2S2O3 instead of ascorbic acid
Without HCOOH
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46
66
71
64
66
40
28
0
23
61
77
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65
77
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0
1
4
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1
5
12
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14
7
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6
NaH2PO4 instead of HCOOH
PC2 instead of PC1
PC3 instead of PC1
PC4 instead of PC1
Without PC1
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15
7
7
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9
4
4
10
11
4
4
Dark reaction
0
0
Conditions: substrate (0.05 mmol), solvent (1 mL), argon atmosphere, room temperature, 4 h.
The yields were determined by HPLC with 4’-methylacetophenone as the internal standard.
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reductant, formic acid, and a mixture of acetonitrile and water (1:1, v/v) as the solvent
(Table 1). Only 7% yields of guaiacol and acetophenone, which were main products in
previously reported photo-reduction of β-O-4’ ketones, were generated from aliphatic
C−O bond cleavage.61-63 Substitution of adjacent methoxyl on phenolic motif was not
observed under the optimal conditions, indicating the good selectivity of desired aryl
ether cleavage.
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Then control experiments were performed to identify the roles of each additive
during this reaction. The formation of 2a and 3a were diminished or inhibited with
decrease of water content, which may be ascribed to insufficient hydrolysis of the
possible imine intermediate (Table 1, entries1 and 2). Further solvent screening
indicated that CH3CN/H2O (1/1, v/v) was the better solvent than other solvents in this
system (Supporting Information, Table S1). The desired products were produced in low
yields in the absence of ascorbic acid (10% and 23%, 2a and 3a, respectively; entry 3).
Replacing ascorbic acid with sodium thiosulfate retained the products moderately
(entry 4), indicating the ascorbic acid could act as the reductant to participate in the
oxidative cycling step of PTH catalyst.64 The addition of formic acid was proposed to
promote the hydrolysis of imine intermediate to release arylamine and ketone products.
Without formic acid the yields of desired products slightly decreased (entry 5) and it
could be replaced by sodium dihydrogen phosphate (entry 6). Other phenothiazines
could also act as the efficient photocatalyst (entries 7-9), among which the catalysts
bearing electron-donating groups (Me, t-Bu) behaved better than the catalyst bearing
electron-withdrawing group (CF3). Less overlapping between the UV-Vis absorption
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spectrum of PTH PC4 with the emission of selected lamp than that of PC1, PC2 and
PC3 indicated PC4 might undergo a worse photo excitation under this irradiation
condition (Supporting Information, Figure S1).59 The reaction in the absence of
photocatalyst delivered 28-33% yields of desired products which could be rationalized
as a pure photoreaction since the substrate itself absorbed the LED irradiation (entry
10). No products were obtained under dark condition confirming this transformation to
be photo-triggered (entry 11).
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The effect of leaving group (LG) including ether and esters in oxime derivatives
on this transformation was studied. Poor yields (9% and 10%, 2a and 3a, respectively;
Table 2, entry 1) of desired products were obtained using methoxyl group as LG
indicating methoxyl group was an inferior LG. Simple acetate (2a, 70%), pivalate (2a,
Table 2. Investigation of leaving groups.
PC1 (6 mol%)
LG
Ascorbic Acid (2 eq.)
HCOOH (4 eq.)
N
O
NH2
O
OH
OMe
CH3CN : H2O = 1 : 1
365 nm
OMe
2a
3a
1
3a
Leaving group
Yield of , %
2a
Entry
Yield of , %
1
9
10
O
O
2
80
51
70
76
O
O
3
4
5
O
O
O
73
58
70
59
O
O
CF3
Conditions: substrate (0.05 mmol), solvent (1 mL), argon atmosphere, room temperature, 4 h. The
yields were determined by HPLC with 4’-methylacetophenone as the internal standard. LG, leaving
group.
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76%) and benzoate (2a, 70%) behaved better than 4-(trifluoromethyl) benzoate (2a,
59%) even though electron-deficient benzoate exhibited the unique validity in previous
Iridium photocatalytic systems,65 which could be explained by the more negative
reduction potential of excited PTH catalyst.58
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After confirmation of optimal conditions, diverse α-oximated β-aryl ethers based
on lignin models were investigated (Table 3). Methoxyl group on the benzene ring Ar2
adjacent to oxime (1b and 1c) showed a minor effect on the aryl migration. It is well
accepted that σ-type unpaired electron of iminyl radical could not be delocalized into
π-systems of aryl iminyl.66 Therefore limited effect from substituents on benzene ring
Ar2 will be shown on the property of iminyl radical. However, substituents on migrated
benzene ring Ar1 obviously affected the intramolecular amination. Substrate 1d
delivered 2b in poor yield (14%), where less steric phenolic motif might be not
favorable to formation of spiro intermediate. In contrast, dimethoxy substituted Ar1 (1e)
proceeded the amination smoothly (2c, 64% yield). Model 1f containing an ortho-
phenyl on Ar1 was a modest substrate compared to 1a. Then complex model 1g was
also tested. Gratifyingly, selective cleavage of aryl ether bond proceeded over cleavages
of aliphatic ether and ester to deliver 54% yield of 3d and 60% yield of 2a.
Model 1h and 1i were prepared to mimic the possible fragments during modified
lignin conversion. 47-50% yields of corresponding primary arylamines were afforded
indicating the conjugated structure induced by carbonyl or cyano group did not affect
the migration of Ar1 motif. Product 2f is of particular note, as the ketone carbonyl group
of 2f would not be tolerated under the reductive amination conditions.67,68 During the
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Table 3. Amination of lignin models.
PC1 (6 mol%)
Ascorbic Acid (2 eq.)
HCOOH (4 eq.)
6
7
8
9
AcO
O
N
O
NH2
OH
Ar1
Ar1
Ar2
Ar2
R
R
CH3CN : H2O = 1 : 1
365 nm
2
3
1
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Products and yieldsa
Substrates
Substrates
Products and yieldsa
AcO
N
OMe
O
NH2
AcO
N
NH2
O
OH
b
O
3a
, 31%
O
OMe
OMe
MeO
1f
2a
, 59%
3b
2d
, 31%
, 60%
1b
AcO
N
AcO
O
OMe
N
MeO
MeO
OH
MeO
MeO
OH
b
O
OMe
OMe
O
OMe
OMe
2a
, 60%
OMe
2a
, 47%
OAc
OMe
OAc
b
3d
1g
, 54%
3c
, 53%
1c
AcO
N
AcO
O
N
OH
NH2
NH2
O
O
3a
, 52%
NC
NC
OMe
1d
2b
, 14%
3a
, 16%
, 70%
b
1h
2e
, 47%
AcO
AcO
OMe
N
OMe
N
OMe
NH2
O
O
NH2
3a
3a
, 60%
OMe
OMe
OMe
b
1e
2c
, 64%
O
2f
, 50%
1i
O
a HPLC yields on 0.05 mmol scale. b Isolated yields on 0.3 mmol scale.
amination reaction, α-hydroxy ketones were generated in similar yields with primary
arylamines.
After the test of lignin models, we explored the application of this amination
method in modified lignin. The modified poplar lignin was prepared via pre-oxidation,
oximation and acylation based on dioxasolv poplar lignin (Supporting Information).
Among these steps, the oxidation step was carried out using the stoichiometric 2,3-
dichloro-5,6-dicyano-1,4-benzoquinone (DDQ),69 which could provide the consecutive
oxidized β-O-4’ motif avoiding the possible disconnected oxidized lignin species in
other oxidation systems.35 The modified lignin exhibited a better solubility in
acetonitrile than dioxasolv lignin and oxidized lignin (Figure 1, a-c). Previous studies
indicated during the acylation step all hydroxyl groups including aliphatic and phenolic
hydroxyls could be acetylated leading to decreased exposed-hydroxyl groups and thus
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Figure 1. N-modification of lignin samples. (a-c) The pictures of samples and
corresponding CH3CN solutions from dioxasolv lignin, pre-oxidized lignin and
modified lignin, respectively. (d-l) 2D NMR spectra of lignin samples in DMSO-d6. (d,
g and i) from dioxasolv lignin. (e, h and k) from pre-oxidized lignin. (f, i, and l) from
N-modified lignin.
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acylated lignin showed a good solubility in most organic solvents.70
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Then qualitative and semi-quantitative analysis was conducted on lignin samples
by 2D 13C-1H HSQC NMR, where the peaks were assigned based on the lignin model
(Supporting Information, Figure S2) and previous literatures.54,69,71,72 The lignin
sidechain regions indicated that β-carbon and γ-carbon in β-O-4’ linkage were stepwise
shifted after oxidation and acylation steps (Figure 1, d-f). The amount of total β-O-4’
linkage per 100 aromatic units decreased from 27 to 21 after modification, which
suggested the theoretical yield of monomers based on β-O-4’ linkage cleavage would
decrease from 7.3% to 4.4%. The aromatic regions indicated these lignin samples were
rich in S and G units, and S/G ratio was 1.1-1.2 (Figure 1, g-i). Meanwhile, the amount
of H unit per 100 aromatic units was 10-12 in the form of p-hydroxybenzoate (PB). In
addition, strong methyl peaks appeared in the aliphatic regions which proved the
modified lignin was successfully acetylated (Figure 1, j-l).
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Then modified lignin was subjected to the amination conditions. To dissolve the
modified lignin sufficiently, CH3CN/H2O/HCOOH (18:1:1, v/v/v) was used as the
solvent. Under this condition, 1.9 wt% primary arylamine monomers (1.5 wt% G
monomer and 0.4 wt% S monomer) were obtained (Supporting Information, Figure S3).
By-products were phenol monomers (2.7 wt%) and 4-acetoxybenzoic acid (1.8 wt%)
as the product of PB unit. The moderate yields of desired arylamine monomers could
be ascribed to low percentage of α-oximated β-aryl ethers after modification and more
restricted polymerized structures in real lignin than free dimer models. Besides, the
isolated large fragments also contained trace amount of arylamine motifs which could
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Figure 2. Mechanism studies. (a) and (b) Stern-Volmer quenching study with substrate
1a (quencher) and PC1 (3.15 mM in CH3CN). (c) The ESR spectra of DMPO capturing
radical produced by substrate 1a and PC1 with or without light irradiation. (d) Cyclic
Voltammetry curves of ascorbic acid, product 2a and 3a.
not be further depolymerized under this system (Supporting Information, Figure S4).
Several experiments were conducted to get insight into the mechanism of the
reaction. Firstly, the electron transfer event between PC1 and substrate 1a was studied.
The emission intensity of the excited state of PC1 was weakened in the presence of 1a
which is followed by the linear Stern–Volmer behavior (Figure 2, a-b). Photo-induced
generation of radical was observed using electron spin resonance (ESR) in the presence
of 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as the trapping agent compared with the
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Figure 3. DFT calculations for the aryl migration pathway from oxygen to nitrogen.
signal under dark condition (Figure 2, c).73 Analysis for the mixture of ESR experiments
by HPLC-MS (Supporting Information, Figure S5) proved the existence of several
DMPO adducts with the radicals (MW. 240). To further prove the role of HA, the
oxidation potentials were measured by cyclic voltammetry (Figure 2, d). Compared to
2a and 3a, HA exhibited a lower oxidation potential, implying HA could act as the
reductant to protect the products from oxidation in the cycling of PTH catalyst.
The aryl migration from oxygen to nitrogen was further investigated by DFT
calculations and comparison of migration pathways between 1a and 1d was used to
elucidate their different reaction behaviors (Figure 3). The spiro-cyclization and aryl
C−O bond cleavage for both substrates are exothermic. However, the whole migration
pathway of substrate 1a is more thermodynamically favorable. Compared to 1a, the
spiro cyclization of substrate 1d exhibits a higher activation barrier (TS1 and TS2), and
activation energies of both following C−O bond cleavages are similar (TS3 and TS4).
Therefore, the methoxyl substituent at ortho position could favor the twisting of phenol
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OAc
Ar
N
O
A + HA + H+
Ar
Ar
PTH
- e-
+ e-
- e-
+ e-
-OAc
2 HA
N
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O
Ar
O
Ar
NH2
*
PTH
PTH
Ar
OH
h
H+, H2O
Ar
*
- e-
PTH
PTH
N
N
Ar
N
Ar
O
Ar
Ar
+ e-
+ H+
O
OH
Scheme 2. Proposed pathway for amination of aryl ether to primary arylamine.
segment to achieve spiro-cyclization. These calculation results accorded well with the
experimental results that both aminations occurred while 1a afforded better yields of
desired products. When the electron-withdrawing groups coexist with ortho-methoxyl
on the migrated ring (1h and 1i), moderate yields of desired products could be still
obtained which suggested the effect of ortho-methoxyl could be mainly ascribed to the
steric effect rather than the electronic effect.
Based on the mechanism studies and previous reports,50,56,74 a possible pathway for
the intramolecular amination of aryl ether was proposed (Scheme 2). This reaction starts
with the reduction of N−O bond by the excited PTH, then the generated iminyl radical
attacks the aryl C−O bond where ortho-substituent promotes the spiro-cyclization.
After aryl C−O bond cleavage, primary arylamine and α-hydroxy ketone are released
via further hydrogen abstraction and hydrolysis. HA could act as the final reductant and
protect the products from being oxidized.
CONCLUSIONS
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In conclusion, we developed the aryl ether cleavage to obtain non-phenolic
products by installing an internal nucleophile in modified lignin models and extracts.
Through constructing the acetyl oxime in modified lignin, the internal iminyl radical
was generated under photocatalytic reduction and induced intramolecular spiro
cyclization to cleave the aryl ether bond in α-oximated β-aryl ethers. The obtained non-
phenolic primary arylamines are readily derived to other nitrogen-containing
compounds thus this method enables the access of non-phenolic nitrogen-containing
aromatics from lignin conversion.
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EXPERIMENTAL SECTION
Chemicals and Materials. All chemicals were of analytical grade, purchased from
Aladdin Chemicals and used without further purification. The synthesis of lignin
models and modified lignin, NMR spectra and HRMS were provided in the Supporting
Information.
Catalytic Test. Typically, a 6 mL quartz reactor tube equipped with a stir bar was
loaded with substrate (0.05 mmol), catalyst, and additives. Solvent was added and the
atmosphere was changed to argon before sealing the tube. The reaction mixture was
irradiated at room temperature with lamps (λmax= 365 nm) for specified time. After the
completing of the reaction, internal standard was added. Reaction mixture was analyzed
by HPLC after 5 times dilution. Quantification of 2-hydroxyacetophenone, 2-
methoxyaniline, 2, 6-dimethoxyaniline, 2-phenylaniline, and aniline were performed
using corresponding standard work curves. Quantification of other compounds were
performed using isolated yields.
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As to the modified lignin conversion, a 6 mL quartz reactor tube equipped with a
stir bar was loaded with lignin sample (30 mg), PC1 (1 mg), and ascorbic acid (17.6
mg). Solvent (CH3CN/H2O/HCOOH, 18:1:1, 2 mL) was added and the atmosphere was
changed to argon before sealing the tube. The reaction mixture was irradiated at room
temperature with lamps (λmax = 365 nm) for 8 h. After the completing of the reaction,
internal standard (vanilline) was added. Reaction mixture was analyzed by GC-MS
after 5 times concentration.
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AUTHOR INFORMATION
Corresponding Author
*E-mail for F.W.: wangfeng@dicp.ac.cn
ASSOCIATED CONTENT
Supporting Information
ESR, fluorescence quenching experiments, UV-Vis absorption spectra, cyclic
voltammetry, HPLC-MS and DFT calculations were provided in the Supporting
Information.
ACKNOWLEDGEMENTS
This work was supported by the National Natural Science Foundation of China
(21711530020, 21721004, 21690084, 21690082, 21690080), the "Strategic Priority
Research Program of the Chinese Academy of Sciences" Grant No. XDB17020300,
XDB17000000.
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Our strategy: installing internal nucleophile to selectively cleave aryl ether
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AcO
N
PTH
R'
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O
Ar
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Ar
Ar
h
H2N
R
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O
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lignin models
lignin extracts
non-phenolic
primary arylamines
N
O
Ar
Ar
R
O
The photocatalytic amination of aryl ether in modified lignin models and extracts to
non-phenolic primary arylamines was developed. Through constructing the acetyl
oxime in modified lignin, internal iminyl radical was generated under photocatalytic
reduction and induced intramolecular aryl migration to cleave the aryl ether bond in β-
O-4’ linkages.
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