Aldehydes as potential acylating reagents for oxidative esterification by inorganic ligand-supported iron catalysis

Article abstract of DOI:10.1039/c9gc02053e

The oxidative esterification of various aldehydes with alcohols could be achieved by a heterogeneous iron(iii) catalyst supported on a ring-like POM inorganic ligand under mild conditions, affording the corresponding esters, including several drug molecules and natural products, in high yields. ESI-MS and control experiments demonstrated that POM-Fe^V(O) was the active catalytic species and the plausible mechanism was presented. More importantly, the 6th run of the iron catalyst recycles shows only a slight decrease in the yield.

Full text of DOI:10.1039/c9gc02053e

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Aldehydes as Potential Acylating Reagents for the Oxidative
Esterification by Inorganic Ligand-Supported Iron Catalysis
Received 00th January 20xx,
Accepted 00th January 20xx
Han Yu,ab‡ Jingjing Wang,b‡ Zhikang Wu,b Qixin Zhao,b Demin Dan,b Sheng Han,b* Jiangjiang Tangc*
and Yongge Weiad*
DOI: 10.1039/x0xx00000x
The oxidative esterification of various aldehydes with alcohols could all be achieved by a heterogeneous iron (III) catalyst
supported on a ring-like POM inorganic ligand under mild conditions, affording the corresponding esters, including several
drug molecules and natural products, in high yields. ESI-MS and control experiments demonstrated that POM-Fev(=O) was
the active catalytic species and the plausible mechanism was presented. More importantly, the 6th run of the iron catalyst
www.rsc.org/
recycles
shows
only
slightly
decrease
in
yield.
combination of recyclable catalyst and green oxidants (O2, air, H2O2,
etc.) under mild conditions should be a better alternative. To the
best of our knowledge, the catalytic system that satisfied the
requirements was quite rare.17
Introduction
Transforming the aldehyde groups to esters via oxidative
coupling with alcohols has become an attractive target for organic
chemists, due to the significance and omnipresence of ester group
in chemistry,1 and more importantly for the convenience of
transformation from aldehydes and alcohols. In comparison, the
traditional esterification methods used to involve costly coupling
reagents or reactive carboxylic halides/anhydrides and undesired
side products from carboxylic acids and alcohols. The use of
stoichiometric reagents, such as KMnO4,2 CrO3,3 would generate
heavy-metal waste. Other strong oxidants, such as H2SO5,4 50%
H2O2,5 TsNBr2,6 TCCA,7 either provided harsh conditions that might
potentially limit substrate scope or produced undesired side
products. So far, many efforts have been devoted to facilitate this
direct oxidation transformation, most of which included the
development of new catalytic systems or new reagents. The
catalytic systems, such as Pd,1f, 1c, 8 Ni,1d Ru,9 NHC,1e, 10 V,11 Fe,12 Zn,13
Au,14 Ti,15 Cu16 could enable the use of mild oxidants in the direct
esterification of aldehyde. However, these systems used to require
high loading of the catalysts, some of which were expensive. In the
meantime, the preparation of some catalysts or their relevant
ligands was quite difficult. The drawbacks have probably hindered
the potential use of these method in industry. Thus, the
O
O
Mo
O
(NH4)3
O
O
O
O
Mo
O
O
O
O
O
O
O
Fe
O
Mo
Mo
O
O
O
O
Mo
O
Mo
O
O
O
O
1:
FeMo
6O18(OH)6]
(NH4)3[
Figure 1 Inorganic ligand-supported iron (III) catalyst
Polyoxometalates (POMs),18 as a class of metal-oxide clusters
with unmatched molecular structural diversity and functionality,
exhibit high redox and acidic properties at the atomic and
molecular levels as well as resistance toward hydrolysis and
oxidative degradation. They thus offer great potentials as stable
inorganic ligands for coordination with metal ions, differing from
classical transition-metal complexes of organic ligands.19 Recently,
our group reported that Anderson-type POMs can be used as the
inorganic ligand-supported metal catalysts for the highly efficient
aerobic oxidation of aldehydes to carboxylic acids,20a amines to
imines20b and alcohols to aldehydes.20c
Herein, we report an inorganic ligand-supported iron (III) catalyst,
(NH4)3[FeMo6O18(OH)6] (simplified as FeMo6, Figure 1, Figures S1 to
S3), which can efficiently catalyse oxidative esterification of various
aldehydes with alcohols to afford the corresponding esters directly
in the presence of KCl as an additive, while H2O2 is used as the sole
oxidant. The catalyst, which has a Fe3+ ion core and the planar
arrangement of six MoVI centers at their highest oxidation states
around the central iron atom, enables the adjustment of electrical
character of the catalytic center by the six edge-sharing MO6 units,
in a similar way that organic ligands act in traditional organometallic
complexes. The catalyst could be easily prepared from
NH4Mo7O24·4H2O and Fe2(SO4)3 with high yield (see supporting
information Figure S1) and confirmed by the characterization of FT-
IR and XRD (Figure S2, S3). Mechanistic insights based on the
observation of key intermediate and control reactions are also
studied. Compared with previously reported homogeneous catalyst,
a. Key Lab of Organic Optoelectronics & Molecular Engineering of Ministry of
Education, Department of Chemistry, Tsinghua University, Beijing 100084, P.R.
China. E-mail: yonggewei@ tsinghua.edu.cn
b. School of Chemical and Environmental Engineering Shanghai Institute of
Technology, No. 100 Haiquan Road, Shanghai 201418 (P.R. China) E-mail:
hansheng654321@sina.com
c. Shaanxi Engineering Center of Bioresource Chemistry & Sustainable Utilization,
College of Science, Northwest A&F University, Yangling 712100 (P.R. China) E-
mail: tangjiang11@nwsuaf.edu.cn
d. State Key Laboratory of Natural and Biomimetic Drugs, Peking University, Beijing
100191 (P. R. China) ,E-mail: ygwei@pku.edu.cn
‡These authors contributed equally to this work.
*All correspondences should be addressed.
†Electronic Supplementary Information (ESI) available: experimental conditions,
supplementary table and NMR spectra. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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the use of pure inorganic catalyst avoids the need for even with a prolonged reaction time to 48 h (entries 16 and 17).
complicated/sensitive organic ligands. More importantly, due to its The superior oxidative efficiency of the combined catVaielywsAtrpticllaetOfonrlinme
DOI: 10.1039/C9GC02053E
high stability, this iron catalyst can be recycled and reused at least indicates that every constituent plays a vital role in the inorganic
six times with negligible loss of activity.
ligand-supported iron catalyst. Under 1 atm of O2 or air, methyl
benzoate could also be generated but in much lower yield (32%) or
trace, even after 48 h, this low reactivity might be ascribed to the
[20]
2 Results and discussion
lower activity of molecular O2 (entries 18 and 19).
When the
reaction was performed under nitrogen atmosphere, the desired
product was obtained in trace yield (entry 20), showing that H2O2 is
an essential oxidant for the reaction. Then, we conducted the effect
of temperature, solvent and time on the oxidative coupling of
Initially, our studies began with the oxidative coupling of
benzaldehyde and methanol (as substrate and solvent) with 30%
H2O2 as the sole oxidant in the presence of additives to assess the
effectiveness of the iron catalyst (Table 1). The additives gave
critical influences on the selectivity and the yield (entries 1-10).
With the basic additives, such as K2CO3, NaHCO3, NaOAc, Na2SO3,
NEt3, pyridine, the ester product was suppressed and only low yield
or trace product was obtained. By contrast, the neutral salt
additives could enhance the selectivity but also it depends on the
counterions of the cations, while the potassium salts proved to be
better than sodium salts. We found KCl was optimal and this is most
likely attributed to the Cl− which acts as an electron-transfer
mediator to improve the electron transfer efficiency.[17b] We have
also examined the influence of the loading amount of catalyst and
additive (entries11-14). To our delight, the desired product methyl
benzoate could be obtained with excellent selectivity and yield in
the presence of KCl (0.2 equiv.) and 1 mol% of 1. However, in the
absence of iron catalyst, trace amounts of methyl benzoate were
detected under the same conditions (entry 15), indicating the key
role of the iron catalyst.
o
benzaldehyde with methanol, and found that the reaction at 65 C
for 24 hours with methanol as solvent is the best reaction condition,
gave 90% yield of methyl benzoate(Table S1). Finally, we
investigated the influence of amount of H2O2 for this model
reaction and gained H2O2 (2.0 equiv.) is the optimum condition with
92% target product (Figure S4).
Table 2 Fe-catalyzed oxidative esterification of various aldehydes
with methanola,b
1
Cat. (1.0 mol% )
O
O
30% H2O2 (2.0 equiv.)
KCl (0.2 equiv.)
O
R1
H
R1
CH3OH
1
65 oC, 24 h
2
O
O
O
O
O
O
F
O
O
O
O
O
Cl
Br
F
2a,
2f,
2b,
2c,
2h,
2d,
2i,
2e,
92%
O
90%
O
93%
91%
89%
O
O
Table 1 Optimization of reaction conditionsa
O
O
O
O
O
1
Cat. (1.0 mol% )
OHC
F
F3
C
O2
N
NC
30% H2O2 (2.0 equiv.)
additive (0.1 equiv.)
O
O
2j,
74%
O
2g,
85%
O
86%
O
92%
O
80%
O
H
CH3OH
O
65 oC, 24 h
O
O
O
O
O
MeO
BnO
Entry
Catalyst (mol%)
Additives
(equiv.)
Na2SO3
NaCl
TEA
KBr
NaI
NaHCO3
NaAc
K2CO3
Pyridine
KCl
Yield (%)g
2k,
2l,
2m,
2n,
2o,
70%
89%
83%
72%
81%
O
O
O
O
O
N
O
O
O
O
O
1
2
3
4
5
6
7
8
9
10
11b
12c
13
14
15
16
17
18d
19e
20f
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.5
1.5
-
55
81
17
68
19
Trace
27
Trace
13
87
92
88
83
80
-
Trace
Trace
32
Trace
Trace
O
S
2t,
82%
O
2p,
2u,
2q,
2r,
2s,
90%
86%
O
93%
89%
O
O
O
O
O
c
O
c
O
O
7
c
c
2v,
2w,
2x,
%
2y,
83%
58%
80%
64
53%
O
H2
N
O
N
N
O
O
O2N
O
O
BocHN
O
HN
O
O
O
O
HN
O
NH
2ac,
O
Br
c
2z,
48%
2aa,
43%
61%
2ab,
54%
2ad,
36%
aUnless otherwise stated, all reactions were carried out under
following conditions: Cat. 1 (1.0 mol %), aldehydes (1.0 mmol),
KCl
KCl
KCl
KCl
KCl
KCl
KCl
KCl
o
30% H2O2 (2.0 equiv.), KCl (0.2 equiv.), and CH3OH (2 mL) at 65 C
for 24 h. bIsolated yields. Yields were determined by GC and
c
confirmed by GC-MS.
(NH4)6Mo7O24·4H2O
With the optimized reaction conditions in hand, the scope of
both the benzylic and aliphatic alcohols, and methanol partner,
were investigated (Table 2). To our delight, various substituted
aromatic aldehydes could be employed in this oxidative
esterification with the desired products being obtained in
remarkably high yields. Halogen-substituted aromatic aldehydes
were also amenable to the reaction conditions, giving the desired
products in excellent yields regardless of the location of the
substituent (compounds 2b-2f). Strongly electron-withdrawing
groups, such as -CF3 and -NO2, were also tolerated under the
standard conditions (compounds 2g and 2h). Aromatic aldehydes
Fe2(SO4)3
1.0
1.0
1.0
KCl
KCl
aReaction conditions: Cat. 1 (1.0 mol%), benzaldehyde (1.0
o
b
mmol), 2.0 equiv 30% H2O2, CH3OH (2 mL), 65 C, 24 h. KCl (0.2
equiv.). cKCl (0.3 equiv.). din oxygen atmosphere. ein air
atmosphere. fin nitrogen atmosphere. gYield were determined by
GC-MS analysis.
It should be noted that when Fe2(SO4)3 or (NH4)6Mo7O24 were used
as the catalyst alone, only trace amounts of product were formed,
bearing
electron-donating
groups
(p-methoxybenzyl,
p-
2 | J. Name., 2012, 00, 1-3
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ARTICLE
O
O
O
O
methylbenzyl, p-isopropylbenzyl, p-benzyloxybenzyl, 2-naphthalene
and 2,4,6-trimethylbenzyl alcohol) all underwent completely
oxidative esterification with excellent selectivity to provide the
corresponding esters (compounds 2k-2p). Heteroaromatic
aldehydes containing nitrogen, oxygen or sulfur atoms could also be
transformed into the corresponding esters in excellent yields
(compounds 2q-2s). Even unsaturated aromatic alcohols, including
cinnamyl alcohol, bearing a carbon-carbon double bond, provided
the desired products also in excellent yields (compound 2t). It is
worth noting that aliphatic aldehydes are less reactive than benzylic
aldehydes, but these could also be used in this oxidative
esterification to give the corresponding esters in moderate to good
yields (compounds 2x-2z). We also studied the oxidation of
aldehydes for the preparation of active drug molecules; all
substrates reacted to give the corresponding oxidized derivatives
(compounds 2aa-2ad) in good yields of 36%-61%. Finally, the utility
of our oxidation protocol was convincingly demonstrated by the
scalable oxidation of benzaldehyde in good yield (90%, 40 mmol
scale) (Figure S5).
After successful application of the oxidative esterification to
benzylic aldehydes with methanol, we attempted to extend the
newly developed protocol to long-chain aliphatic alcohols (Table 3).
Ethyl alcohol was the first aliphatic alcohol used in this oxidative
esterification reaction (compound 4a). For longer chain and branch
fatty alcohols such as 1-butanol, isobutyl alcohol, neopentyl alcohol,
2-propanol, 2-butanol and tert-butanol could all be used to afford
their desired products (compounds 4b-4g). By decreasing the pH of
the solution via addition of nitric acid and using an optimized
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O
O
O
O
DOI: 10.1039/C9GC02053E
4a,
4b,
4c,
4d,
91%
71%
73%
68%
O
O
O
O
O
O
O
O
c
4e,
4f,
4g,
4h,
71%
74%
O
57%
73%
O
O
O
O
O
O
O
c
c
c
c
4i,
4j,
4k,
4l,
71%
74%
64%
52%
O
O
O
O
O
O
OH
O
O
O
O
O
OAc
e
c
d
4m,
4n,
85%
91%
4o,
4p,
58%
74%
aReaction conditions: Cat. 1 (1.0 mol%), benzaldehyde (1.0 mmol),
30% H2O2 (2.0 equiv.), KCl (0.2 equiv.), and alcohols (2 mL) at 65 oC
for 24 h. bIsolated yields. R2OH (2.0 equiv) with nitric acid (0.7
c
mmol). dCat. 1 (1.0 mol %), cyclohexanecarboxaldehyde (1.0
mmol) and benzyl alcohol (2.0 equiv.) as substrates with nitric acid
(0.7 mmol). Cat. 1 (1.0 mol%), acetaldehyde (1.0 mmol) and 6-O-
e
acetylbritannilactone (2.0 equiv.) as substrates with nitric acid (0.7
mmol).
the acidic condition (compound 4o). The active natural product, β-
hydroxy alantolactone, containing esters groups, can be easily
prepared with this protocol from the corresponding alcohol
(compound 4p).
After reaction completion, the solid iron catalyst could be easily
isolated by filtration and reused directly for subsequent couplings
without further purification. 1 could be recovered and reused at
least six times with little loss in activity (Figures S6 and S7). To
confirm the high stability of the catalyst and its associated
performance, the structure and morphology of the catalyst were
investigated. As confirmed by FT-IR and XRD, the recycled catalyst
remained unchanged (Figures S8 and S9).
catalyst
system
(FeMo6+KCl),
cyclic
fatty
alcohol
(cyclohexanemethanol and cyclohexanol) can reacted with benzyl
alcohol and gained corresponding products in 73% and 74% yield
(compounds 4h and 4i). Additionally, benzyl alcohol could be
effectively converted to its corresponding self-esterification product
with moderate yield (compound 4j). Even phenol could be used as a
coupling partner with the desired ester being obtained in 52% yield
(compound 4k). Furthermore, unsaturated fatty alcohol and
heterocyclic alcohol also undergo facile oxidation to the
corresponding esters (compounds 4l and 4m). Functionalized
alcohols, such as ethylene glycol, could be oxidized
chemoselectively to afford the corresponding esters with 85% yield
(compound 4n). Interestingly, cyclohexanecarboxaldehyde and
benzyl alcohol can get the desired product with 74% yield under
1
Cat. (1.0 mol%)
30% H2O2 (2.0 equiv.)
O
O
A
CH3OH
OH
O
KCl (0.2 equiv.), 65 oC, 24 h
1 (1.0 mmol)
2 mL
2a
Yield: trace (GC-MS)
1
Cat. (1.0 mol%)
Ar atmosphere (1atm)
O
H
O
O
CH3OH
B
KCl (0.2 equiv.), 65 oC, 24 h
1 (1.0 mmol)
2 mL
2a
Yield: trace (GC-MS)
Table 3 Fe-catalyzed oxidative esterification of benzaldehyde with
30% H2O2 (2.0 equiv.)
O
NH4[FeMo6O18(OH)6]
different alcoholsa,b
ESI-MS (m/z)
1099.38239
C
D
POMFev
(POM-FeIII
)
65 o
C
1
Cat. (1.0 mol%)
C
A
30% H2O2 (2.0 equiv.)
KCl (0.2 equiv)
O
4
O
R2
R2OH
O
H
O
O
O
H
65 oC, 24 h
30% H2O2 (2.0 equiv.)
O
POMFev
CH3OH
KCl (0.2 equiv), 65 oC, 24 h
3
C
1 (1.0 mmol)
2 mL
2a
Yield: 92% (GC-MS)
Figure 2 Control experiments
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undergoes O-O bond heterolysis to give the highly reactive high-
View Article Online
valence metal-oxo POM-FeV=O species C asDtOheI: 1a0c.t1i0v3e9/oCx9idGaCn0t2.0T5h3iEs
process resembles the P450s mediated nonheme iron-oxo
complexes-catalyzed hydroxylations.24 This may occur via an
intramolecular Fe-O-Mo charge transfer of the POM cluster. Such
delocalization generates a potent oxidant that is suitable for
coordinating a variety of substrates. The nucleophilic addition of
the hydroxyl group on the hemiacetal to POMFev(=O) C produces
the intermediate D, in which the β-hydrogen elimination occurs to
form the desired ester product and intermediate E. After several
transformations, POMFev(=O) is reformed and next catalytic cycle
begins.
4 Conclusions
Figure 3 Reaction progress of the reaction between PhCHO and
MeOH
In conclusion, we have successfully realized the direct oxidative
esterification of various aldehydes and alcohols/phenol, with POMs
supported heterogeneous iron(III) catalyst and “green” H2O2
oxidant. Moderate to excellent yields were obtained for all the
cases we examined. Control experiments and ESI-MS analyses
displayed that POMFev(=O) be the active catalytic species. The
catalyst was recyclable and the reactivity maintained well after
several runs.
To understand the mechanism of the direct oxidation
esterification of aldehyde, several control experiments were
conducted (Figure 2). When benzoic acid was used instead of
benzaldehyde, only trace product was observed (Figure 2A). This is
consistent with the deduction that hemiacetal formed in situ acted
as the intermediate during the reaction.21 The similar result was
obtained when no oxidant was added, which ruled out the
dehydrogenation coupling process in the catalytic cycle (Figure 2B).
To track the active catalytic species in the oxidation,
(NH4)3[FeMo6O18(OH)6] 1 was reacted with H2O2 at 65 oC and the
mixture was analyzed by ESI-MS (Figure 2C, 2D and Figures S10-S13).
It gave a clear evidence that POMFev (=O) had been formed
({Na2[FeMo6O24H6]+O}1-·H2O = 1100.61, see Figure S13). The defined
POMFev(=O) has also proved to be as active as
(NH4)3[FeMo6O18(OH)6] in the reaction, which supports the
deduction that POMFev(=O) was the active catalytic species in cycle.
At the same time, we conducted the reaction progress detaction
between PhCHO and MeOH and found the presence of active
intermediate hemiacetal (Figure 3).
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (Nos. 21871183, 21471087, 21631007,
21225103, 21221062), Doctoral Fund of 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.
O
OH
+
R1 OH
R1
R2
H
R2
O
H
1H NMR
OH
H
v
R2
POM
Fe
O
O
Notes and references
D
R1
-hydride

O
1． a) M. Liu, Z. Zhang, H. Liu, Z. Xie, Q. Mei, B. Han, Sci. Adv.
2018, 4(10), eaas9319; b) Angew. Chem. Int. Ed., 2017, 56,
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Patel, J. Org. Chem., 2003, 68, 2944-2947.
elimination
POMFev
O
O
O
Mo
O
(NH4
)
3
O
O
O
O
C
R1
ESI-MS
Mo
R2
O
O
O
O
O
O
O
Fe
O
Mo
Mo
O
O
O
O
O
Mo
O
Mo
O
O
O
OH
O
v
POM
Fe
H
(NH4)3[FeMo6O18(OH)6]
(POM-Fe)
POMFe-OOH
B
E
-H2O
H2O
POM-FeIII
A
H2O2
2． A. Abiko, J. C. Roberts, T. Takemasa and S. Masamune,
Tetrahedron Lett., 1986, 27, 4537-4540.
Figure 4 Proposed mechanism for Fe-catalyzed esterification of
aldehydes with alcohols
3． B. O'Connor and G. Just. Tetrahedron lett., 1987, 28, 3235-
3236.
Based on the experimental results and previous
investigations,22,23 a tentative mechanism that rationalizes the
product formation is illustrated in Figure 4. For the iron catalyst, the
two oxidizing equivalents optimized for oxidations are not only
stored at the metal center but can be delocalized on the six edge-
sharing MO6 units. Thus, the POM-Fe catalyst A firstly is activated 6.
by H2O2 to generate an active POM-FeIV-OOH species B, which
4. A. Nishihara and I. Kubota. J. Org. Chem., 1968, 33, 2525-2526.
5. R. Tank, U. Pathak, M. Vimal, S. Bhattacharyya and L. K. Pandey,
Green Chem., 2011, 13, 3350-3354.
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Green Chemistry
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View Article Online
DOI: 10.1039/C9GC02053E
O
O
Mo
O
(NH4)3
O
O
O
O
Mo
O
O
O
O
O
O
O
Fe
O
Mo
Mo
O
O
O
O
Mo
O
Mo
O
O
O
O
1:
FeMo
6O18(OH)6]
(NH4)3[
1
Cat. (1.0 mol% )
30% H2O2 (2.0 equiv.)
KCl (0.2 equiv.)
O
O
46 examples
up to 93% yield
R2OH
R2
O
R1
65 oC, 24 h
R1
H
An inorganic ligand supported iron catalyst which can effectively catalyze oxidative
esterification of various aldehydes and alcohols with excellent yields.