Oxidative Esterification of Aldehydes and Alcohols Catalyzed by Camphor-Based Imidazolium Salts

Article abstract of DOI:10.1007/s10562-019-03092-z

Abstract: Sixteen new camphor-based imidazolium salts have been synthesized with renewable camphorsulfonic acid as the starting material. The chemical shifts of the characteristic proton of C₂ on the imidazolium ring (N?C=N) were discussed thoroughly and all of these imidazolium salts exhibit good thermal stability. Furthermore, the excellent catalytic performance of the synthesized imidazolium salts were observed in the oxidative esterification between aromatic or aliphatic aldehydes containing electron-withdrawing or electron-donating groups on aromatic ring and primary or secondary alcohol by air as the sole oxidant. Graphic Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-019-03092-z

Catalysis Letters
https://doi.org/10.1007/s10562-019-03092-z
Oxidative Esterifcation of Aldehydes and Alcohols Catalyzed
by Camphor‑Based Imidazolium Salts
Tiancen Bian1 · Li Feng1 · Danfeng Li1 · Jiaxin Huang1 · Yuxun Zhao1 · Xu Xu1,3 · Yiqin Yang2,3 · Shifa Wang1,3
Received: 8 October 2019 / Accepted: 30 December 2019
© Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract
Sixteen new camphor-based imidazolium salts have been synthesized with renewable camphorsulfonic acid as the starting
material. The chemical shifts of the characteristic proton of C2 on the imidazolium ring (N−C=N) were discussed thor-
oughly and all of these imidazolium salts exhibit good thermal stability. Furthermore, the excellent catalytic performance
of the synthesized imidazolium salts were observed in the oxidative esterifcation between aromatic or aliphatic aldehydes
containing electron-withdrawing or electron-donating groups on aromatic ring and primary or secondary alcohol by air as
the sole oxidant.
Graphic Abstract
Keywords Camphorsulfonic acid · Imidazolium salts · Catalysis · Oxidative esterifcation
1 Introduction
Electronic supplementary material The online version of this
Imidazolium salts, derived from imidazoles, consist of a pair
of cation and anion, and have been found widespread utility
in various felds. They are widely applied in organic syn-
thesis [1–8], antitumor and antimicrobial activities [9–17],
antioxidative activities [18, 19] and probes [20]. Therefore,
more and more attention are being paid to synthesizing novel
kinds of imidazolium salts. Moreover, imidazolium salts can
be tuned and modifed by changing the substituent of the
cation or anion specifcally, which has made imidazolium
salts much more tunable or designable. The direct attach-
ment of functional groups to the nitrogen of the imidazolium
article (https://doi.org/10.1007/s10562-019-03092-z) contains
supplementary material, which is available to authorized users.
* Shifa Wang
wangshifa65@163.com
1
College of Chemical Engineering, Nanjing Forestry
University, Nanjing 210037, People’s Republic of China
2
College of Light Industry and Food, Nanjing Forestry
University, Nanjing 210037, People’s Republic of China
3
Co-Innovation Center of Efcient Processing and Utilization
of Forest Resources, Nanjing Forestry University,
Nanjing 210037, People’s Republic of China
Vol.:(0123456789)
1 3

T. Bian et al.
cation makes it a easy way to prepare task specifc imidazo-
lium salts efciently.
in a pressure tube and N2 was used to exclude oxygen. The
resulting reaction mixture was heated at 130 °C for 5 h.
After the reaction completed, EtOAc (ca. 6 ml) was added
in the mixture. Precipitation was formed and the fltrate was
removed. The crude product was recrystallized in methanol
and dried under vacuum.
Camphor, as a versatile building block, has shown great
value in medicinal chemistry [21–23], material science
[24–26] and sensors [27]. Camphor and their derivatives
are bulky and possess multiple stereocenters, and they are
widely used as source of chiral auxiliaries, chiral reagents
and asymmetric catalyst [28–31]. Camphor-based imida-
zolium salts have been emerging in recent years and can
be applied in enantioselective organic reactions and have
particular thermotropic behavior [32]. However, there were
no camphor-based imidazolium salts with phenyl ring con-
necting to the imidazolium core, which could increase the
rigidity and stability of the salts.
2.3 General Procedure for the Synthesis
of Camphor‑Based Imidazolium Salts (3b–3d;
4b–4d; 5b–5d; 6b–6d)
A reported procedure [41] with some modifcations was
as followed: the synthesized iodide salt (2 mmol) was dis-
solved in 10 ml of DCM and 3 equivalent of the metal salts
has been added. The reaction mixture was then stirred for
10 h at room temperature. The solid residue was removed
by fltration and the fltrate was washed with water, and then
concentrated and dried under vacuum to aford good yields
of the targeted camphor-based imidazolium salts (3b–3d;
4b–4d; 5b–5d; 6b–6d).
The traditional ester formation was most accessed via
the carboxylic acid and alcohol in the presence of acidic
catalysts usually sulfuric acid, known as Fischer esterifca-
tion [33]. While with the demand for a higher yield and a
broader substrate scope, a stoichiometric activation of the
carboxylic acid to the corresponding acyl halide was neces-
sary. Other activators like DCC (dicyclohexylcarbodiimide)
and EDC (1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide)
can also achieve this aim smoothly [34]. However, it was
obvious that these activation way was not atom-economic for
the stoichiometric consumption of the activators. Recently,
imidazolium salts have also been reported to catalyze the
oxidative esterifcation of aromatic aldehydes as pre-cata-
lysts. Inspired by the seminal work reported by Feroci et al.
[35–37], herein. We reported the synthesis of sixteen novel
camphor-based imidazolium salts. Their catalytic activities
on oxidative esterifcation of aldehydes and alcohols were
investigated with air as a mild oxidant and short reaction
time and high yield could be obtained in this way.
2.4 General Procedure for Oxidative Esterifcation
of Aldehydes and Alcohols
A three-neck flask was charged with aldehyde 7a–7p
(1.0 mmol), camphor-based imidazolium salts (5 mol%),
Cs2CO3 (163 mg, 0.5 mmol), alcohol (5 mmol) and 10 ml of
toluene in an air atmosphere, then the mixture was stirred for
3 h at 60 °C. The reaction mixture was extracted with diethyl
ether (3×30 ml), concentrated under reduced pressure and
the residue was purifed by fash column chromatography on
silica gel (PE (petroleum ether):EA (ethyl acetate)=20:1)
to give the desired product. All products were identifed by
1H and 13C NMR and in accord with literatures [42–52] (for
spectra see Supplementary Data).
2 Experimental
2.1 General Procedure for Synthesis
of 10‑Iodocamphor (1) and Phenylimidazoles
(2a–2d)
3 Results and Discussion
3.1 Synthesis and Characterization
10-iodocamphor (1) and phenylimidazoles (2a–2d) were
prepared following a modifed literature procedure [38–40]
(see Supplementary Data for details).
In this report, we synthesized sixteen new camphor-based
imidazolium salts with commercially available camphor-
sulfonic acid as starting material, as shown in Scheme 1.
Firstly, 10-iodocamphor (1) and substituded phenylimida-
zoles (2a–2d) were prepared in an efcient way in accord
with the literature [31, 38–40]. The SN2 reaction between
(1) and (2a–2d) occurred in the presence of a small amount
of DMF to give high yield of corresponding iodide salts
(3a, 4a, 5a, 6a). After that, anion exchange reactions were
2.2 General Procedure for the Synthesis
of Camphor‑Based Imidazolium Salts (3a–6a)
To a solution of 10-iodocamphor (5 mmol, 1.39 g) in DMF
(3 ml), substituted phenylimidazoles (5 mmol) were added
1 3

Oxidative Esterifcation of Aldehydes and Alcohols Catalyzed by Camphor‑Based Imidazolium…
N
X
O
I
N
R
SO3H
O
I
N
N
2a-2d
PPh3, I2
MX
R
N
O
R
N
O
DCM, rt, 10 h
toluene, 110 °C, 8 h
DMF, 130 °C, 5 h
1
3a: R=2,4,6-trimethylphenyl,
3b-3d:
4b-4d:
R=2,4,6-trimethylphenyl
X=PF6, BF4,N(Tf)2
R=2,6-diisopropylphenyl
X=PF6, BF4, N(Tf)2
camphorsulfonic acid
4a:
R=2,6-diisopropylphenyl,
5a: R=4-chlorophenyl,
6a:
R=4-bromophenyl;
5b-5d: R=4-chlorophenyl
X=PF6, BF4, N(Tf)2
6b-6d: R=4-bromophenyl
X=PF6, BF4, N(Tf)2
Scheme 1 Synthesis of camphor-based imidazolium salts
Table 1 Chemical shift of C2
Table 2 Physical properties of imidazolium salts (3a–6d)
Salts δ (ppm) Salts δ (ppm)
proton on the imidazolium ring
Salts Abbreviation
Physical state
Tm (°C) T5% (°C)
3a
3b
3c
3d
4a
4b
4c
4d
9.97
10.04
10.00
9.39
9.98
10.09
9.87
5a
5b
5c
5d
6a
6b
6c
6d
11.03
10.82
11.04
9.70
11.02
10.75
10.92
9.87
3a
3b
3c
3d
4a
4b
4c
4d
5a
5b
[MesCamIm]I
[MesCamIm]PF6
[MesCamIm]BF4
[MesCamIm]N(Tf)2 Yellowish solid
[IprCamIm]I
[IprCamIm]PF6
[IprCamIm]BF4
[IprCamIm]N(Tf)2 Yellowish solid
Of-white solid
White solid
White solid
218–220 319
199–201 320
212–214 315
90–92
328
Light-brown solid
Yellowish solid
Light-brown solid
238–239 312
222–224 309
239–240 307
76–78
144–145 310
140–141 313
9.39
312
[4-Br-PhCamIm]I
Light-brown solid
Brown solid
[4-Br-PhCamIm]
performed in DCM at room temperature for 10 h, furnishing
camphor-based imidazolium salts (3b–3d; 4b–4d; 5b–5d;
6b–6d) with 78–86% yields.
PF6
5c
5d
[4-Br-PhCamIm]
BF4
Brown solid
158–160 315
−15.4a 358
1H NMR spectra used here was not only a tool to illumi-
nate the structure and purity of target compounds but also
providing information on the electron density distribution
between cations and anions [53]. In this context, the most
signifcant chemical shifts were observed in the case of the
C2 proton (proton on the carbon of N−C=N) in imidazolium
ring. As shown in the 1H NMR spectrum of these imidazo-
lium salts (for spectra see supplementary data), the signal of
C2 proton is registered in the lowest feld, which means the
C2 proton has the lowest electron density surrounding itself.
The chemical shifts of C2 proton from these camphor-based
imidazolium salts were all listed in Table 1 and investigated
thoroughly. It is obvious to see that the substituents on the
aryl part of these imidazolium salts have a tremendous efect
on C2 proton chemical shifts. The imidazolium salts with
electron-withdrawing groups on the phenyl ring (4-chloro-
phenyl, 4-bromophenyl) have a larger chemical shift than
that with electron-donating groups (2,4,6-trimethylphenyl,
2,6-diisopropylphenyl). This can be understood that the
electron-withdrawing groups make the imidazolium ring
much more electron defcient and the proton prone to be
more acidic [54]. With the cation fxed, the substitution of
[4-Br-PhCamIm]
Brown viscous
liquid
N(Tf)2
6a
6b
[4-Cl-PhCamIm]I
Light-brown solid
Of-white solid
194–195 313
186–187 311
[4-Cl-PhCamIm]
PF6
6c
6d
[4-Cl-PhCamIm]
BF4
Yellowish solid
197–198 309
−11.1a 328
[4-Cl-PhCamIm]
Yellow viscous
liquid
N(Tf)2
aGlass transition temperature (Tg)
the I− anion with PF6− and BF4− resulted in limited changes
in the electron density near the quaternary nitrogen atom.
While a signifcant upfeld variation was observed when the
−
I− was exchanged by the N(Tf)2 , resulting from the increas-
−
ing shielding capacities of N(Tf)2 .
3.2 Thermal Properties
As observed in Table 2, not all of these imidazolium
salts have low melting points. Similar condition could be
1 3

T. Bian et al.
observed in the case of phenyl/alkyl-substituted pyrazo-
lium, as well as other kinds of imidazolium-based salts
[55–59]. Unlike N,N-dialkyl imidazolium salts, these imi-
dazolium salts have much higher melting points, which can
be attributed to the introducing of aryl moiety and hence
the enhancement of intermolecular π–π stacking occurred
[57]. Four imidazolium salts (3d, 4d, 5d, 6d) among these
16 imidazolium salts could be considered as ionic liquids for
their melting points were below 100 °C.
Considering the efect of the counterion on the melting
points, we found that the anion size was an important fac-
tor that infuence the phase transition temperatures of these
salts. Imidazolium salts containing I− and BF4− displayed
−
highest melting temperatures. While in the case of PF6 , the
melting points are a little lower for a given cation than the
−
imidazolium salts with I− and BF4 , which is correspond-
ing to the trend observed in literature [58]. Moreover, this
efect was more apparent when the weakest coordinating
−
According to the results shown in Table 2, both the elec-
tron-withdrawing substituents (4-chlorophenyl, 4-bromophe-
nyl) and the electron-donating substituents (2,4,6-trimeth-
ylphenyl, 2,6-diisopropylphenyl) on the phenyl ring and the
types of anions afect the melting points of these imidazo-
lium salts. The melting points of imidazolium salts with
electron-donating groups on the phenyl ring are higher than
those of the salts containing electron-withdrawing groups,
when the anions are fxed.
anion N(Tf)2 was applied, and as a result, the imidazo-
−
lium salts with N(Tf)2 as anion have the lowest melting
points below 100 °C. Thus, it can be predicted that the melt-
ing point of studied ILs decreases in the following order:
N(Tf)2− <PF6− <BF4− ≈ I−.
Thermal stability is an important criterion and basis for
evaluating the properties and applications of imidazolium
salts. In this report, thermal stability of these imidazolium
salts was investigated with thermogravimetric analysis and
T5% (temperature at which 5% weight loss of sample was
Fig. 1 TGA curves for camphor-based imidazolium salts 3a–6d with mass weight% against temperature (°C)
1 3

Oxidative Esterifcation of Aldehydes and Alcohols Catalyzed by Camphor‑Based Imidazolium…
Table 3 Catalytic performance of these imidazolium salts with ben-
of aldehyde with alcohol, and the test results were listed in
Table 3. We frst conducted reactions in the following con-
dition: 1 equivalent of benzaldehyde, 5 mol% imidazolium
salts, 0.5 equivalent of Cs2CO3 and 10 equivalent of ethanol
were added to the fask containing toluene equipped with an
oxygen balloon and the mixture was stirred at 80 °C for 3 h.
As comparison, the reaction, in the absence of catalyst, can
hardly go on. In general, several camphor-based imidazo-
lium salts exhibited moderate to good catalytic performance
(3a, 3b, 3d, 4a, 4b, 4c, 4d, 5a, 5c) with yields ranging from
68 to 98%.
zaldehyde and ethanol as model reaction
O
O
5mol% salts, air
Cs2CO3, toluene
H
O
EtOH
+
8a
7a
Entry
Salts
Base
Temp (°C)
Time (h)
Yield (%)a
1
2
3
4
5
6
7
8
None
3a
3b
3c
3d
4a
4b
4c
4d
5a
5b
5c
5d
6a
6b
6c
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
0
98
95
Trace
91
80
95
93
76
85
Trace
68
10
13
Trace
Trace
Trace
The reaction was then optimized by using benzaldehyde
and ethanol as model substrates, and 3a as catalyst. Firstly,
when the temperature was lowered to 60 °C or the amount
of ethanol was decreased to 5 equivalents, the reaction could
still proceed smoothly without any yield loss (Table 4,
entries 2 and 3). Decreasing the catalyst loading to 2 mol%
slightly decreased the product yield (Table 4, entry 2). How-
ever, yield decreasing was observed when temperature was
lowered below 60 °C (Table 4, entries 4 and 5) and time was
reduced less than 3 h (Table 4, entries 6 and 7). The reaction
can be carried out in MeCN and THF with yields as 56%
and 59% respectively (Table 4, entries 8 and 10), while in
TBME, trace of product was obtained (Table 4, entry 8).
The bases as another important factor were also investigated
including organic and inorganic bases, including KOH,
K2CO3, DBU (1,8-diazabicyclo[5.4.0]undec-7-ene), DIPEA
(N,N-diisopropylethylamine) and DABCO (1,4-diazabicy-
clo[2.2.2]octane). It was found that the organic base DBU
and the inorganic base K2CO3 could promote the reaction in
a moderate yield (Table 4, entries 12 and 13), while KOH,
DIPEA and DABCO only obtained low yield of products
(Table 4, entries 11, 14 and 15).
9
10
11
12
13
14
15
16
17
6d
aYields determined by GC analysis in the presence of internal stand-
ard
observed) was chosen as the onset decomposition tempera-
ture, which was especially important for TAAILs (in this
text 3d, 4d, 5d, 6d). As shown in Table 2 and Fig. 1, ther-
mal decomposition temperatures of all these imidazolium
salts were found higher than 300 °C (T5% >300 °C). With
regard to the anion efect, the thermal stability of I− contain-
ing imidazolium salts were almost the same with PF6− and
BF4− containing imidazolium salts. While the most thermal
stable imidazolium salts were obtained when the anions was
The substrate scope of aldehydes and alcohols were
then investigated and the results were listed in Table 5. As
shown in Table 5, all the primary, including benzyl alco-
hol, and secondary alcohols were suitable for this catalytic
system and achieved good yields (Table 5, entries 1, 2, 3
and 4). However, when a more steric hindered t-BuOH
(Table 5, entry 5) was applied, only trace of product could be
obsreved, indicating that the steric t-butyl group signifcantly
afected the activity of the reaction. The aromatic aldehydes
with electron-withdrawing substituents, including nitro
(NO2), chloro (Cl), bromo (Br), fuoro (F), trifuoromethyl
(CF3) and cyano (CN) groups had much higher yields than
the aromatic aldehydes with electron-donating substituents.
Increasing the equivalent of alcohol or prolonging the reac-
tion time to 5 h could still not promote the yield of the cor-
responding ester signifcantly (Table 5, entries 15, 16 and
17). This makes sense because electron-withdrawing groups
make the carbonyl much more electrophilic and thus more
−
exchanged to N(Tf)2 (up to 358 °C). Surprisingly, there
were no much diference in thermal stability when electron-
donating groups on the phenyl ring were switched by elec-
tron-withdrawing groups (excepted 5d, 358 °C).
3.3 Catalytic Performance in Aldehyde Oxidative
Esterifcation
With these camphor-based imidazolium salts in hand, we
examined the catalytic potential in oxidative esterifcation
1 3

T. Bian et al.
Table 4 Condition optimization with benzaldehyde and ethanol as model reaction
O
O
5mol% 3a, air
base, solvent
H
O
EtOH
+
8a
7a
Entry
EtOH (mmol)
Base
Solvent
Temp (°C)
Time (h)
Yield (%)a
1
2
3
4
5
6
7
8
10
10
5
5
5
5
5
5
5
5
5
5
5
5
5
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
KOH
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
THF
TBME
MeCN
Toluene
Toluene
Toluene
Toluene
Toluene
80
60
60
40
rt
60
60
60
60
60
60
60
60
60
60
3
3
3
3
98
98 (86b)
98
77
80
45
75
56
Trace
59
Trace
51
78
Over night
1
2
3
3
3
3
3
3
3
3
9
10
11
12
13
14
15
K2CO3
DBU
DIPEA
DABCO
Trace
37
Unless otherwise noted, reactions were performed with following optimized conditions: benzaldehyde (1 mmol), imidazolium salts 3a (5 mol%),
base (0.5 mmol), solvent (10 ml)
aYields determined by GC analysis in the presence of internal standard
b2mol% catalyst loading
easily attacked by alcohols in the basic condition. Excellent
yields were observed for various substitutions at the ortho-,
meta- and para positions of the phenyl ring, when the func-
tional group on the phenyl ring was fuoride (Table 5, entries
10, 11 and 12). As a contrast, a poor yield was observed with
the nitro group located at the ortho-position (Table 5, entry
7). We deemed that the nitro group, as a larger functional
group than fuoro group, had a larger steric hindrance and
thus reduced the reaction activity. Other kinds of aromatic
aldehyde, including 1-pyrenecarboxaldehyde and 2-thenal-
dehyde, could also proceed smoothly with yield over 79%
(Table 5, entries 18 and 19). Compared to aromatic alde-
hyde, aliphatic aldehydes could also achieve good yields in
such condition (Table 5, entries 20 and 21).
3.4 Recycling of Catalyst 3a and Characterization
The hypothesis of the reaction mechanism has been
described in Supplementary data (Scheme S1). To test the
reusability of our synthesized catalyst 3a, we frst con-
ducted the parallel reactions with/without substrate (with
3-nitrobenzaldehyde and ethanol as model reaction) and the
corresponding 1H NMR of 3a were almost identical with the
original spectrum as shown in Fig. 2, which demonstrated
the stability of the catalyst 3a under the reaction condition.
Then we put our eforts on the recycling of the catalyst 3a.
The solvent was frst evaporated under vacuum and diethyl
ether (ca. 20 ml) was poured into the fask to form a suspen-
sion. Catalyst 3a along with the base could be obtained by
1 3

Oxidative Esterifcation of Aldehydes and Alcohols Catalyzed by Camphor‑Based Imidazolium…
Table 5 Catalytic performance of 3a with variation of substrates and alcohols
O
O
5 mol% 3a, air
R2 OH
R2
+
R1
O
R1
H
Cs2CO3, toluene, 60 °C, 3 h
7b-7r
8b-8r
Entry
R1
R2
Product
Yielda
1
2
3
4
5
6
7
8
3-NO2 Phenyl
3-NO2 Phenyl
3-NO2 Phenyl
3-NO2 Phenyl
3-NO2 Phenyl
4-NO2 Phenyl
2-NO2 Phenyl
4-Cl Phenyl
4-Br Phenyl
2-F Phenyl
Et
8b
8c
8d
8e
–
95
95
95
92
Trace
94
Trace
95
90
94
94
92
92
95
Me
i-Pr
Benzyl
t-Bu
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
Me
Et
Et
8f
–
8g
8h
8i
9
10
11
12
13
14
15
16
17
18
19
20
21
3-F Phenyl
4-F Phenyl
8j
8k
8l
3-CF3 Phenyl
3-CN Phenyl
4-OMe Phenyl
4-OMe Phenyl
4-OMe Phenyl
2-Thienyl
1-Pyrenyl
CH3CH2
CH3(CH2)8
8m
8n
8n
8n
8o
8p
8q
8r
73
75b
75c
88
79
84
80
aYields of isolated product, relative to the starting aldehyde
b10 eq of alcohol was used
cReaction time was prolonged to 5 h
fltration and the resulting solid was washed completely with
diethyl ether. Regenerated base and 3a could be reutilized in
four runs (20% of original amount of 3a was added addition-
ally but no additional base was needed each run) with no
signifcant decline in yields. The results were listed in Fig. 3
can be rendered as TAAILs (3d, 4d, 5d, 6d). All these salts
have been fully characterized using 1H, 13C, 19F NMR and
HR-MS techniques. These imidazolium salts exhibited good
thermal stability, all of which have T5% larger than 300 °C.
Several of these new synthesized camphor-based imidazo-
lium salts (3a, 3b, 3d, 4a, 4b, 4c, 4d, 5a, 5c) showed mod-
erate to good catalytic performance in aldehyde oxidative
esterifcation. Furthermore, 3a was successfully employed
in such reaction and high yields were obtained with a broad
scope of aldehyde.
4 Conclusion
To summarize, a series of camphor-based imidazolium
salts have been synthesized by an efcient method, with
−
−
−
anions as I−, PF6 , BF4 and N(Tf)2 . Four of these salts
1 3

T. Bian et al.
Fig. 2 1H NMR of catalyst 3a. a Regenerated with substrate under reaction condition; b regenerated without substrate under reaction condition;
c original spectrum
100
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Key Lab for the Chemistry and Utilization of Agro-Forest Biomass, the
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