Oxidative esterification of alcohols by a single-side organically decorated Anderson-type chrome-based catalyst

Article abstract of DOI:10.1039/d1gc00161b

The direct esterification of alcohols with non-noble metal-based catalytic systems faces great challenges. Here, we report a new chrome-based catalyst stabilized by a single pentaerythritol decorated Anderson-type polyoxometalate, [N(C₄H₉)₄]₃[CrMo₆O₁₈(OH)₃C{(OCH₂)₃CH₂OH}], which can realize the efficient transformation from alcohols to esters by H₂O₂oxidation in good yields and high selectivity without extra organic ligands. A variety of alcohols with different functionalities including some natural products and pharmaceutical intermediates are tolerated in this system. The chrome-based catalyst can be recycled several times and still keep the original configuration and catalytic activity. We also propose a reasonable catalytic mechanism and prove the potential for industrial applications.

Full text of DOI:10.1039/d1gc00161b

Volume 23
Number 7
7 April 2021
Pages 2543-2794
Green
Chemistry
Cutting-edge research for a greener sustainable future
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ISSN 1463-9262
PAPER
Feng Jiang, Han Yu, Laurent Ruhlmann, Yongge Wei et al.
Oxidative esterification of alcohols by a single-side
organically decorated Anderson-type chrome-based
catalyst

Green Chemistry
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PAPER
Oxidative esterification of alcohols by a
single-side organically decorated Anderson-type
chrome-based catalyst†
Cite this: Green Chem., 2021, 23,
2652
b
Jingjing Wang,‡a,b Feng Jiang,‡a Chaofu Tao,‡c Han Yu,*c,d,e Laurent Ruhlmann
*
d,e
and Yongge Wei
*
The direct esterification of alcohols with non-noble metal-based catalytic systems faces great challenges.
Here, we report a new chrome-based catalyst stabilized by a single pentaerythritol decorated Anderson-
type polyoxometalate, [N(C4H9)4]3[CrMo6O18(OH)3C{(OCH2)3CH2OH}], which can realize the efficient
transformation from alcohols to esters by H2O2 oxidation in good yields and high selectivity without extra
organic ligands. A variety of alcohols with different functionalities including some natural products and
pharmaceutical intermediates are tolerated in this system. The chrome-based catalyst can be recycled
several times and still keep the original configuration and catalytic activity. We also propose a reasonable
catalytic mechanism and prove the potential for industrial applications.
Received 15th January 2021,
Accepted 10th March 2021
DOI: 10.1039/d1gc00161b
rsc.li/greenchem
selectivity to achieve direct aerobic oxidative esterification of
Introduction
alcohols.4 In 2012, Beller’s group developed the first catalytic
The ester group is an abundant functional group which is acceptorless dehydrogenation of ethanol to ethyl acetate using
widely used in bulk chemicals, fine chemicals, natural pro- a Ru–PNP complex with TONs over 15 000.5 Besides, in 2014,
ducts and polymers.1 Due to their importance and ubiquity, Milstein’s group reported a metal–ligand bifunctional ruthe-
the development of synthetic strategies for esters has been nium catalyst with dual modes of metal–ligand cooperation,
attracted much attention. In general, esters can be obtained for acceptorless dehydrogenative homo-coupling of alcohols.6
from alcohols with carboxylic acid derivatives (carboxylic The above methods all require expensive precious metals (Pd
halides and anhydrides).2 This process uses several steps, pro- or Ru) and toxic organic ligands. This not only increases the
ducing large amounts of undesired side products. In recent cost but also brings challenges for the subsequent separation
years, direct oxidative esterification of alcohols has been of products. In the same year, Jones’ team accomplished an in-
widely investigated using metal–organic catalyst systems expensive iron-based catalyst supported by a cooperative PNP
(Scheme 1A).3 For example, in 2011, Lei’s group reported a pal- pincer ligand to achieve the acceptorless dehydrogenation of
ladium-catalyzed system by using a P-olefin ligand to control
aKey Laboratory of Cardiovascular and Cerebrovascular Disease Prevention and
Control (Gannan Medical University), Ministry of Education, Ganzhou,
Jiangxi 341000, P.R. China
bUniversité de Strasbourg, Institut de Chimie, UMR CNRS 7177, Laboratoire
d’Electrochimie et de Chimie Physique du Corps Solide, 4 rue Blaise Pascal,
CS 90032, 67081 Strasbourg cedex, France. E-mail: Iruhlmann@unistra.fr
cSchool of Chemical and Environmental Engineering, Shanghai Institute of
Technology, Shanghai 201418, P.R. China.
E-mail: hanyu0220@mail.tsinghua.edu.cn
dKey Lab of Organic Optoelectronics & Molecular Engineering of Ministry of
Education, Department of Chemistry, Tsinghua University, Beijing 100084,
P.R. China. E-mail: yonggewei@mail.tsinghua.edu.cn
eState Key Laboratory of Natural and Biomimetic Drugs, Peking University,
Beijing 100191, P.R. China
†Electronic supplementary information (ESI) available: Experimental con-
ditions, ESI table and NMR spectra. See DOI: 10.1039/d1gc00161b
‡These authors contributed equally to this work.
Scheme 1 Direct synthesis of esters from alcohols.
2652 | Green Chem., 2021, 23, 2652–2657
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alcohols with lower H2 pressure.7 Later, Ding’s group devel-
oped a novel catalytic system with a tripodal cobalt complex
supported by a tetradentate tripodal ligand for efficiently con-
verting primary alcohols to esters.8 Although the problem with
using the noble metal catalyst has been solved, catalytic
systems still need a large number of organic ligands or high
pressure. Recently, non-noble metal (Co,9 Fe,10 Cu,11) catalysts
and transition metal-free12 catalysts for oxidative esterification
were reported successfully. Although the reaction systems are
green and efficient using air or O2 as an oxidant without any
additives, the preparation of composite material present some
challenges like high temperature, serious aggregation and
structural collapses during calcination. Overall, there is still a
great challenge to find a green and sustainable method to
prepare esters.
Polyoxometalates (POMs), as metal–oxide clusters with high
redox and acidic properties, are of great interest in the field of
organic reactions.13 But one type of POMs, the Anderson–
Evans polyoxoanions, which exhibits physical and chemical
properties depending strongly on the heteroatom, counter
cation, has been little studied. As they possess six hydroxy
groups on both sides, Anderson POMs are easy to be grafted
organic groups which results in unique catalytic properties.14
Table 1 Optimization of the reaction conditions on the oxidative ester-
ification of cyclohexanemethanol with methanola
Conv.b
(%)
Yieldb
(%)
Entry
Deviations from standard conditions
1
2
3
4
5
6
7
8
None
Without CrPOM-OH
Without KCl
(NH4)6Mo7O24·4H2O
94
—
<10
<5
<5
<15
—
18
91
—
Trace
—
—
Trace
—
10
Cr(NO3)3·9H2O
(NH4)6Mo7O24·4H2O + Cr(NO3)3·9H2O
N2 (1.0 bar) instead of H2O2 (3.0 equiv.)
O2 (1.0 bar) instead of H2O2 (3.0 equiv.)
a Reaction conditions: Cat.
1 (1.0 mol%), cyclohexanemethanol
(1.0 mmol), 30% H2O2 (3.0 equiv.), CH3OH (2 mL), 65 °C, 36 h.
b Determined by GC-MS analysis.
Recently, our group used a series of Anderson–Evans polyoxoa- entries 4–6). If the reaction was conducted under N2 atmo-
nions with different heteroatoms to realize the conversion of sphere, no target product was detected (Table 1, entry 7). But
aldehydes to carboxylic acids, amines to imines and even alde- when we processed under O2 atmosphere, we could get the
hydes to esters. We also found that the catalytic system can not corresponding ester with 10% yield (Table 1, entry 8). It proved
only be easily recycled but also maintain good catalytic activity that oxidants were essential in the process, and O2 as the
after being used several times.15 Herein, we propose a cheap oxidant was not sufficient for the reaction to proceed. After
single-side organically decorated Anderson-type chrome-based this, various temperatures were tested in methanol with KCl as
catalyst to efficiently convert alcohols to esters in the absence the base (Fig. S5A†). 65 °C showed the best result with 78%
of complicated organic ligands (Scheme 1B).
yield of the target product 2a. Then we explored the influence
of time, oxidant, the amount of oxidant and negative/positive
ion of the additive (Fig. S5B–E†). We concluded that 36 h reac-
tion time, H2O2 (3.0 equiv.) as the oxidant, K+ as the positive
ion and Cl− as the negative ion can get the highest yield (91%).
With the optimized conditions in hand, we studied the
Results and discussion
Initially, we prepared the Anderson–Evans polyoxoanion with
chromium as the heteroatom ((NH4)3[CrMo6O18(OH)6], simpli- scope using a series of alcohols (Table 2). To our delight,
fied as CrIIIMo6) and characterized its structure by FT-IR and various alcohols can be efficiently converted to the corres-
XRD to confirm its Anderson type according to previous ponding methyl esters under the conditions of methanol as
reports (Fig. S1 and S2†).16 Then, pentaerythritol was the solvent, and even inert fatty alcohols can also achieve the
successfully connected to the POMs precursor by a hydro- transformation but with moderate yields. Benzyl alcohols with
thermal method to form the desired compound electron-withdrawing groups or electron-donating groups
([N(C4H9)4]3[CrMo6O18(OH)3{(OCH2)3CCH2OH}], simplified as could be effectively converted into the corresponding methyl
CrIIIMo6-OH, Cat. 1) and conducted FT-IR and XRD character- esters with the highest yield of 92% (compounds 2a–2k). The
ization (Fig. S3 and S4†). Then we screened the optimal reac- yields of electron-withdrawing substrates (F, Cl, Br, NO2, CF3)
tion conditions with cyclohexanemethanol and methanol as were higher than those of electron-donating substrates (CH3,
the model reaction (Fig. S5†). We found when Cat. 1 was the isopropyl, OCH3, OBn, CN). Furthermore, we also explored the
catalyst, KCl (0.2 equiv.) was the additive, and H2O2 (3.0 influence of steric hindrance; ortho-substituents (compounds
equiv.) was the oxidant, we obtained the best yield at 65 °C for 3a–3c) were more difficult to be converted into the corres-
36 h (Table 1, entry 1). Moreover, we gained a low yield in the ponding ester compounds as compared to para-substituents
absence of Cat. 1 or KCl (Table 1, entries 2 and 3). This means (compounds 2b, 2d and 2e). Even for cinnamyl alcohol, it can
such additive compounds play an important role in the be converted into the target product in 72% yield (compound
process. If we replaced Cat. 1 with the catalyst precursor 4a). As for cinnamyl alcohol derivatives with electron-with-
(NH4)6Mo7O24·4H2O or Cr(NO3)3·9H2O or both of them, only a drawing groups (F, Cl) and electron-donating groups (OCH3) as
little amount of the target product was obtained (Table 1, substrates, the desired esters can be isolated in good yields
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Table 2 Scope of methyl estersa,b
Table 3 Other aromatic ester compoundsa,d
a Cat. 1 (1.0 mol%), benzyl alcohol (1.0 mmol), 30% H2O2 (3.0 equiv.),
alcohol (2 mL), KCl (0.2 equiv.) at 65 °C for 36 h. b Silicotungstic acid
(0.1 mmol). c Alcohols (2.0 equiv.) and silicotungstic acid (0.1 mmol).
d Yields were determined by column chromatography.
a Cat. 1 (1.0 mol%), alcohols (1.0 mmol), 30% H2O2 (3.0 equiv.),
CH3OH (2 mL), and KCl (0.2 equiv.) at 65 °C for 36 h. b Isolated yields
were determined by column chromatography. c Conversion yields were
determined by GC-MS.
It should be noticed that the natural product β-hydroxy
alantolactone, a sesquiterpene lactone mainly isolated from
plants of the Inula genus, exhibiting potent anti-inflammatory
and anticancer activities, could also be gained with 47% yield
(Fig. 1, compound 32). Moreover, this catalytic system is also
effective for some drug intermediates (Fig. 1, compounds 33
and 34). The results show that the catalytic system possesses
good substrate tolerance.
Subsequently, we explored the gram scale reaction in oxi-
dative esterification of alcohols. Benzyl alcohol derivatives
(10 mmol) can be reacted under the optimized conditions
leading to a high yield of desired esters (Fig. 2). Moreover, we
also performed the recyclability of Cat. 1 and concluded that
the catalyst can maintain high catalytic activity and the orig-
inal configuration after being used multiple times from the
FT-IR and XRD spectra of the catalyst before and after the reac-
tion (Fig. S6–S8†). Finally, a 100 gram-scale reaction was
(compounds 4b–4d). To our surprise, methyl 2,4,6-trimethyl-
benzoate and 2-naphthalenemethanol can also react with
methanol under optimized conditions and gain the desired
product with 71% and 72% yields, respectively (compounds 5
and 6). For heterocyclic substrates, the reaction proceeded
smoothly with high yields (compounds 7–9). We also tried
using this catalytic system for the inert aliphatic alcohols.
Fortunately, 3-phenylpropionic acid methyl ester and cyclohex-
anecarboxylic acid methyl ester were gained in 71% and 91%
yields by column chromatography (compounds 10 and 11). For
other chained aliphatic hydrocarbons, we also got the corres-
ponding esters with good yield through the detection of the
reaction system using GC-MS (compounds 12–17).
Based on the above work, the oxidative esterifications of
benzyl alcohol with other alcohols were also investigated
(Table 3). We firstly carried out the reaction using ethanol as
the solvent under the standard conditions gaining ethyl benzo-
ate in 80% yield (compound 18). Apart from that, 2-propanol,
isobutyl alcohol, 2-butanol and 1-butanol could also react with
benzyl alcohol to afford the corresponding esters with 79%,
74%, 70% and 80% yields (compounds 19–22), respectively.
But tert-butanol and neopentyl alcohol can only be converted
into esters under acidic conditions with 69% and 68% yields
(compounds 23 and 24). Under the same conditions, func-
tional glycol, unsaturated fatty alcohols, heterocyclic alcohols
and even cyclic fatty alcohols can process smoothly with good
yields (compounds 25–29). Notably, phenol can be trans-
formed into benzoic acid phenyl ester in 64% yield (compound
30), and the oxidative self-esterification reaction of benzyl
alcohol also can perform at 83% yield (compound 31).
Fig. 1 Natural products and pharmaceutical intermediate substrates.
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Fig. 2 Gram-scale reactions.
carried out in 2000 ml methanol with 10.0 mol catalyst and
108.1 g (1.0 mol) of benzyl alcohol. The analytically pure
methyl benzoate was separated with 74.9% yield after 36 h.
In order to explore the catalytic mechanism of the system,
we performed some control experiments (Fig. 3). We firstly
conducted the optimum reaction conditions, but in an Ar
atmosphere with a low yield of the target product (Fig. 3a),
indicating that Cat. 1 is not an activated oxidant. Under the
optimal reaction conditions, we reduced the reaction time to
24 h and detected the presence of benzaldehyde as an inter-
mediate determined by GC-MS (Fig. 3b), which confirmed the
accuracy of the benzaldehyde as intermediate product hypoth-
esis. When using benzaldehyde as the substrate, we found that
the yield of methyl benzoate can reach 96% within 24 h, so
benzaldehyde was further verified as the intermediate product
(Fig. 3c). Under the same conditions, we also used benzoic
acid as the substrate and found that just a few methyl benzo-
ates were formed, thus benzoic acid was not the intermediate
product, and benzaldehyde was the only intermediate product
in the reaction process (Fig. 3d).
To demonstrate the important role of additives and the
presence of benzaldehyde in the reaction system,17 we tested
1
the reaction process (Fig. 4). From the H NMR spectrum, we
found that the additive (KCl) has a significant influence on the
yield of the ester. In the presence of KCl, benzyl alcohol can be
almost completely converted to methyl benzoate (3.94 ppm)
after 36 h (Fig. 4a). But there is still a lot of benzaldehyde
(10.04 ppm) and benzyl alcohol in the system after 36 h in the
absence of KCl (Fig. 4b). The main reason may be that the
chloride ion can bind with CrIIIMo6-OH through multiple
hydrogen bonds to drive the formation of a supramolecular
compound (CrIIIMo6-OH·Cl), just as the adduct of bromide ion
Fig. 4 Reaction process experiment. (a) In the presence of KCl; (b) in
the absence of KCl; (c) benzaldehyde as substrate.
and FeMo6 we had reported.18 Further studies for only K+
being the effective cation are being actively pursued in our lab-
oratory. In addition, when using benzaldehyde as the sub-
strate, we detected the presence of hemiacetal (Fig. 4c).
However, the hemiacetal was not detected in this catalytic reac-
tion, which may be due to the low concentration of benz-
aldehyde, and less hemiacetal is afforded which can be quickly
converted into ester.
Based on the previous reports and experimental results,19
a
plausible mechanism is proposed in Fig. 5 (In the figure, the
chloride ion is not drawn for the whole reaction process being
clear). In the presence of H2O2, Cat. 1 can be converted into
Cr(V) intermediate A. A can interact with alcohols to form inter-
mediate B. Then B releases the aldehyde through intra-
molecular electron transfer and returns to Cat. 1. The first cata-
lytic cycle is completed. Simultaneously, the second catalytic
cycle is opened with the recycled Cat. 1 being reoxidized to
Fig. 3 Control experiments.
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20192BAB205114), Talent project of Jiangxi Province (No.
jxsq2018106059), Doctoral Fund of the Ministry of Education
of China No. 20130002110042, Tsinghua University Initiative
Foundation Research Program No. 20131089204, and the State
Key Laboratory of Natural and Biomimetic Drugs K20160202.
The start-up fund of Shanghai Institute of Technology is also
gratefully acknowledged. We also acknowledge the support of
China Scholarship Council (CSC).
Notes and references
Fig. 5 Plausible mechanism.
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Conclusions
In conclusion, we have demonstrated that a single-side organic
decorated Anderson-type chrome-based catalyst can be used
for the direct aerobic oxidative esterification of benzylic alco-
hols with methanol and various long-chain aliphatic alcohols
under mild conditions. It also has good catalytic effect on
some natural products and pharmaceutical intermediates.
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in this catalyst system and the system is shelf-stable, commer-
cially available, and inexpensive. The catalyst can be applied to
gram-grade reactions, and still maintain good activity after re-
cycling for many times. So the catalytic system has the poten-
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We are grateful to the National Natural Science Foundation of
China (No. 21961003, 21971134, 21631007 and 21225103),
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