Heterogeneous Esterification from α-Hydroxy Ketone and Alcohols through a Tandem Oxidation Process over a Hydrotalcite-Supported Bimetallic Catalyst

Article abstract of DOI:10.1021/acs.oprd.8b00265

Heterogeneous aerobic oxidative esterification between α-hydroxy ketone and alcohols catalyzed by a hydrotalcite-supported bimetallic catalyst (CuMn/HT) using O₂ as a green oxidant was achieved. Recyclable CuMn/HT exhibits high catalytic activity due to increased content of oxygen vacancies and a newly generated CuMn₂O₄ crystal phase. This clean esterification proceeds through a tandem oxidation process in the absence of any additives and ligands and affords α-keto esters in good to excellent yields. Moreover, the catalytic system tolerates complicated bioactive molecules as raw materials and can be performed on a multigram scale.

Full text of DOI:10.1021/acs.oprd.8b00265

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Full Paper
Heterogeneous Esterification from #-Hydroxyl Ketone and Alcohols through A
Tandem Oxidation Process over A Hydrotalcite-Supported Bimetallic Catalyst
Xu Meng, Xiuru Bi, Gexin Chen, Baohua Chen, and Peiqing Zhao
Org. Process Res. Dev., Just Accepted Manuscript • DOI: 10.1021/acs.oprd.8b00265 • Publication Date (Web): 08 Oct 2018
Downloaded from http://pubs.acs.org on October 8, 2018
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Heterogeneous Esterification from α-
Hydroxyl Ketone and Alcohols through A
Tandem Oxidation Process over A
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Hydrotalcite-Supported Bimetallic Catalyst
a
Xu Meng,a,* Xiuru Bi, Gexin Chen,a Baohua Chen,b Peiqing
Zhaoa,*
a
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Suzhou
Research Institute of LICP, Lanzhou Institute of Chemical Physics (LICP),
Chinese Academy of Sciences, Lanzhou 730000, China. Fax: + 96 931
8277008; Tel: + 86 931 4968688; E-mail: xumeng@licp.cas.cn,
zhaopq@licp.cas.cn
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b
State Key Laboratory of Applied Organic Chemistry, Lanzhou University,
Lanzhou 730000, China
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Table of Contents graphic:
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ABSTRACT: Heterogeneous aerobic oxidative esterification between α-hydroxy
ketone and alcohols was achieved catalyzed by a hydrotalcite-supported
bimetallic catalyst (CuMn/HT) using O2 as green oxidant. Recyclable CuMn/HT
exhibits highly catalytic activity due to increased content of oxygen vacancies
and newly generated CuMn2O4 crystal phase. This clean esterification proceeds
through a tandem oxidation process in the absence of any additives and
ligands, which leads to α-ketoesters in good to excellent yields. Moreover, the
catalytic system tolerates complicated bio-active molecules as raw materials and
could be performed in multigram-scale.
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KEYWORDS: oxidative esterification, α-ketoesters, hydrotalcite, heterogeneous
synthesis, supported catalyst
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INTRODUCTION
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α-Ketoesters, as key structural motifs, have been found in various biologically
active compounds, pharmaceuticals, anticancer agents.1-3 Moreover, α-ketoesters
are important precursors in organic synthesis, such as in asymmetric reduction,
aminohydrocylation and aldol reaction.4-7 Therefore, efficient synthesis of α-
ketoesters in a straight forward way is of great importance. Traditional synthetic
methods involve esterification of α-ketoacyl derivatives, direct oxidation of α-
hydroxy esters and Pd-catalyzed carbonylation using CO.8-12 In recent years,
acetophenone was directly used as substrate to react with alchohols for the
synthesis of α-ketoesters in the presence of copper salts or I2.13-16 Additionally,
Jiao and coworkers reported a method for α-ketoesters synthesis using α-
carbonyl aldehydes and alcohols as raw catalyzed by copper with pyridine as
additive.17-19 Novel synthetic approaches involving copper-catalyzed C-C or C-N
oxidative cleavage were also emerged.20,21 However, stoichiometric amounts of
iodide reagents, additives, ligands and peroxide (like TBHP) were inevitably
employed in the previous methods. From the viewpoint of a green and
environmental perspective, the development of atom-economic oxidative
esterification for synthesis of α-ketoesters based on a recyclable heterogeneous
catalyst using oxygen as green oxidant is highly desirable and challenging.
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Heterogeneous catalysis based on solid catalysts is a significant process in
organic transformation since the ease in catalyst handling and recyclability of
catalysts.22-26 The preparation of solid catalysts, such as frequently used
supported catalysts, generally employs wet-impregnation and co-precipitation.
These methods all need suitable solvents in the procedures, while process
employing mechanochemistry for preparation of solid catalysts provides merits
including simplicity, swiftness and solventless conditions.27,28 Particularly,
mechanochemical energy generated from ball-milling (BM) is able to provide
new chemical composition or chemical reactivity on as-synthesized catalysts.29-32
More importantly, various of defects in the as-synthesized catalyst would be
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generated by the centrifugal force and friction during milling, which further
significantly promotes its catalytic performance.
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In continuation of our research on clean organic transformation under
heterogeneous catalytic systems,33-35 herein, we report a heterogeneous aerobic
oxidative esterification between α-hydroxy ketone and various alcohols under
neat conditions. As far as we known, this is the first example that direct using
α-hydroxy ketone as the starting material to synthesize α-ketoesters via a
tandem oxidation process. The reaction is catalyzed by hydrophobic
hydrotalcite-supported bimetallic catalyst (CuMn/HT) and uses O2 as green
oxidant. The catalytic systems can tolerate a variety of alcohols and bio-active
hydroxyl molecules without the help of additives and ligands. The highly active
catalyst can be recovered and reused for 4 times without any loss of catalytic
activity.
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O
O
CuMn/HT, O2
OR
OH
neat conditions
tandem oxidation
O
O
oxidation
H2O
oxidation
H2O
O R
OH
hemiacetal
Scheme 1. CuMn/HT-catalyzed aerobic oxidative esterification.
RESULTS AND DISCUSSION
At the beginning, a series of HT-supported catalysts were obtained by grinding
in an agate mortar for 10 mins followed by ball-milling for 2 h and
calcination.36-40 Nano-MgO (particle size is 20 nm) and Mg-Al oxide obtained
via calcination of HT in air were also used as supports in bimetallic catalysts.
And, regular wet-impregnation in EtOH solution (designated as CuMn/HT-WI)
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and grinding by hand in an agate mortar (designated as CuMn/HT-G) were
employed to prepare bimetallic supported catalysts.
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As-synthesized catalysts were examined in the oxidative esterification
between α-hydroxy ketone 1a and ethanol using O2 as oxidant under neat
conditions (Table 1). Mechanically, 1a firstly is oxidized to the corresponding α-
carbonyl aldehyde which reacts with alcohol to give the intermediate hemiacetal.
Then, the oxidative dehydrogenation occurs on hemiacetal to give the final
product α-ketoester. In this way, a capable catalyst must have excellent redox
ability for this tandem oxidation process. The control experiments prove that the
oxidative esterification did not proceed without any catalysts or in the presence
of hydrophobic HT (Table 1, entries 1 and 2). Then, the reactions using signal
component catalysts show that Cu/HT was catalytically active and manganese
did not play a role as the catalytic metal in this system (Table 1, entries 3 and
4). However, bimetallic supported catalyst CuMn/HT was found to be efficient
for the reaction and full conversion of 1a was observed under neat conditions,
which indicates that cooperative synergistic effect was created between copper
and manganese (Table 1, entry 5). The addition of Co or Fe to Cu/HT did not
show the cooperative effect (Table S1, see SI). The similar influence that the
introduction of manganese enhances the catalytic activity of copper was
reported by Xu`s group and our group also reported that manganese oxide
would transfer electrons from copper to oxidant in an oxidation improving the
catalytic performance.41-45 Next, air and H2O2 were tried to replace O2 as
oxidants, which shows that air led to inferior result and hydrogen peroxide
failed the reaction (Table 1, entry 5). Subsequently, the loadings of supported
metals were investigated and the results show that the loading of copper was
critical for the reaction and 8 wt% of Cu together with 2 wt% of Mn was the
optimal option (Table 1, entries 6-9). On the other hand, other supports were
employed to replace HT in bimetallic supported catalysts. It is found that nano-
MgO and Mg-Al oxide did not give satisfactory results as HT did, which
probably means that layered crystal structure and hydrophobic properties of HT
are important for the activity of the catalyst (Table 1, entries 10 and 11). After
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that, various solvents were used in the catalytic system (0.4 mmol of EtOH
was used in these cases) and the results show that the neat conditions were
the best choice (Table 1, entries 12-15). Furthermore, Cu-Mn oxide was
synthesized via co-precipitation using ammonia and it did not show any
catalytic activity.46 And, the physical mixture of Cu-Mn oxide and HT as the
catalyst failed the reaction as well (Table 1, entry 17). In addition, desired
product 3a was isolated in moderate yields of 70% and 55% over CuMn/HT-WI
and CuMn/HT-G respectively, which indicates that the catalyst preparation
method seems affect the catalytic performance remarkably (Table 1, entries 5,
18 and 19). It is concluded that the best reaction conditions for esterification
involve using CuMn/HT as catalyst under neat conditions (For more
optimization, Table S1, see SI,). It is worth mentioning that the full conversion
of 1a under neat conditions makes purification of 3a very easy and further
column chromatography is unnecessary after removal of catalyst and excessive
EtOH via filtration and distillation respectively.
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Table 1. Optimization of esterification of 1a.a
O
O
catalyst, EtOH
OEt
OH
solvent, 12 h, 70 oC, O2
O
3a
1a
O
oxidation
H2O
oxidation
H2O
O Et
OH
hemiacetal
Entry Catalyst
Metal weight Solvent Isolated
(wt%)b on HT
yield (%)
Cu
Mn
1
2
-
-
-
-
-
EtOH
EtOH
N.R.
N.R.
HT
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3
Cu/HT
8
-
-
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
DMC
DMF
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4
Mn/HT
2
2
8
1
2
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2
-
Trace
5
CuMn/HT
CuMn/HT
CuMn/HT
CuMn/HT
CuMn/HT
CuMn/MgO
CuMn/Mg-Al oxide
CuMn/HT
CuMn/HT
CuMn/HT
CuMn/HT
Cu-Mn oxide
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2
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4
2
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-
95 (57c, 0d)
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0
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16e
17f
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PhMe 0
dioxane 0
EtOH
EtOH
EtOH
EtOH
0
Cu-Mn oxide + HT -
-
0
CuMn/HT-WI
CuMn/HT-G
8
2
2
70
55
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a Reaction conditions: 1a (0.2 mmol), catalyst (20 mg), solvent (1 mL), O2
balloon, 70 oC, 12 h.
b Theoretical loading.
c Under air.
d 1.0 eq of H2O2 was used as oxidant.
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e 5 mg of Cu-Mn oxide was used.
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f The physical mixture of 5 mg of Cu-Mn oxide and 20 mg of HT was used.
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After the optimized catalyst was confirmed, we tried to analyze its
physicochemical properties. First of all, the actual metal loadings were defined
by ICP-AES as 7.1 wt% for Cu and 1.9 wt% for Mn in CuMn/HT catalyst.
Next, the textural properties of support HT and the catalyst were analyzed and
the information is summarized in Table 2. It is found that ball-milling did not
change the textural properties of original HT obviously, while calcination made
the surface area of HT enhance remarkably because HT turns into Mg-Al
mixed oxide partially or fully under calcination via release of CO2 and H2O.47
Meanwhile, calcined HT had larger pore volume and much smaller pore size.
CuMn/HT also showed enhanced surface area compared with original HT and
the pore volume further enhanced significantly, although pore size was similar
as original HT. As we known, great BET surface area and relatively large pore
volume generally are necessary for superior catalytic performance.
Table 2. Textural properties of materials.
BET
surface Total
pore Average
pore
Catalyst
area (m2/g)
volume (cm3/g) size (nm)
HT
6.8
0.03
0.05
0.09
0.17
22.2
32.8
5.3
HT-BMa
7.0
HT-Calcinationb
CuMn/HT
74.5
45.5
19.4
a HT after ball-milling.
b HT after ball-milling followed by calcination at 350 oC for 2h in air.
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Next, the crystal structures of catalysts were analyzed by X-ray diffraction
(XRD) (Figure 1). We can see that HT transferred into MgO (JCPDS No. 30-
794) and Al2O3 (JCPDS No. 77-2135) to some extent and a few of hydrotalcite
phases (JCPDS No. 22-700) were remained because the calcination is not
higher than 350 oC. And, strong diffraction peaks of CuO (JCPDS No. 45-937)
and relatively weak diffraction peaks of MnOx (JCPDS No. 31-825) were
detected. Interestingly, the diffraction peaks at around 2θ of 18.4o, 34.7o and
62.2o were observed, which proves that crystal phase of CuMn2O4 (JCPDS No.
71-1142) was formed by ball-milling process.48 The new induced chemical
composition probably plays a role as catalytic active sites in the oxidation.
Form the XRD patterns of CuMn/MgO, CuMn/Mg-Al oxide, CuMn/HT-WI and
CuMn/HT-G, CuMn2O4 phase was no longer observed (Figure S1 and Figure
S2).
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Figure 1. XRD patterns of support HT and Cu-Mn/HT catalysts.
For an oxidative esterification over a solid catalyst, the most important
factor that might affect the redox ability is the surface oxygen species because
metal supported catalyst generally proceed the redox reaction via Mars-van-
Krevelen mechanism.49 O1s spectrum mainly deconvoluted into two components:
surface adsorbed oxygen species that contain surface adsorbed water and
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unsaturated oxygen species, and less mobile saturated lattice oxygen species.50
Surface adsorbed oxygen species are more active and mobile than lattice ones,
thereby influencing the aerobic oxidation. As shown in Figure 2 and Table 3,
CuMn/HT had more content of adsorbed oxygen species (85.1%) than
CuMn/HT-WI (72.2%) and CuMn/HT-G (73.4%). In another word, Cu-Mn/HT is
able to offer much more abundant surface oxygen vacancies.51-53 Abundant
oxygen vacancies on catalyst surface induced by mechanochemical energy in
ball-milling process might be responsible for the excellent catalytic performance
of CuMn/HT.
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Figure 2. XPS spectra of O 1s for HT-supported bimetallic catalyst prepared by
different approaches.
Table 3. XPS results of O 1s for CuMn/HT catalysts
Catalyst
Binding energy of
oxygen species in O 1s
(eV)
Oxygen species (%)
Adsorbed O Lattice O Adsorbed Lattice O
O
CuMn/HT
531.6
529.7
529.6
529.6
85.1
72.2
73.4
14.9
27.8
26.6
CuMn/HT-WI 531.3
CuMn/HT-G 531.4
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Finally, morphologies of the support HT and CuMn/HT were observed by
scanning/transmission electron microscope (SEM and TEM). From Figure 3 and
Figure S3, uniform and identical layered morphology was observed in HT and
ball-milled HT, which means ball-milling did not change the HT morphology and
particle size. Nevertheless, calcination of HT made obvious pores (like cracks
between layers) appear in HT, which leads to enhanced surface area. In SEM
images of Cu-Mn/HT, uniform and well-dispersed metal oxides were observed
on support with layered morphology. These results are in agreement with the
BET data and XRD information (Table 2).
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Figure 3. SEM images of original HT (A), HT after ball-milling (B), calcined HT
after ball-milling (C) and CuMn/HT (D).
In order to determine whether the observed catalysis was from the solid
catalyst or the leached metal species in reaction solution, leaching experiment
was carried out. The oxidative esterification of α-hydroxy ketone 1a and ethanol
was performed under standard neat conditions with CuMn/HT. After the reaction
performed for 6 h, it was stopped and the catalyst was isolated by filtration
from the reaction mixture. At the time, the yield of the reaction was determined
in 50% by NMR. The filtrate was kept running for further 6 h without adding
fresh catalyst, and it is found that the yield of 3a did not increase. Meanwhile,
the filtrate was examined by ICP-AES, which indicates that metal species in
solution were barely detected (Cu and Mn were both below 0.01%). The used
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catalyst was also analyzed by ICP-AES, which shows that the metal loadings
were remained as 7.1 wt% for Cu and 1.9 wt% for Mn. These results indicate
that the observed catalysis was derived from the supported catalyst rather than
leached metal species in solution. Subsequently, the recyclability of CuMn/HT
was investigated in gram-scale due to the need of enough amounts of retrieved
catalyst for the further recycles (Table 4). CuMn/HT-catalyzed oxidative
esterification proceeded smoothly in gram-scale and 95% isolated yield of 3a
was offer. The catalyst was readily isolated from the reaction mixture by
filtration and recovered by treatment in solution of Na2CO3. The XRD patterns
exhibit that recovered catalyst remained the crystal structure successfully
(Figure 1). The experiments prove that retrieved catalyst could be recycled for
4 times without any loss of catalytic activity even in gram-scale synthesis
(Table 4).
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Table 4. The test of recyclability of Cu-Mn/HT in gram-scale.a
O
O
CuMn/HT, EtOH
12 h, 70 oC, O2
OEt
OH
O
1a
3a
Run
1
2
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5
Isolated
(%)
yield 95
94
96
96
94
a Reaction conditions: 1a (10 mmol, 1.36 g), CuMn/HT (1 g), EtOH (50 mL),
70 oC, 12 h, O2 balloon.
Finally, we paid our attention towards the scope of the oxidative
esterification between α-hydroxy ketone and alcohols over Cu-Mn/HT (Table 5).
General speaking, simple aliphatic alcohols, such as methanol, n-butyl alcohol,
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cyclopentanol and cyclohexylethanol, proceeded in the oxidative esterification
smoothly under the neat conditions, which gives the corresponding α-ketoesters
3b-3f in good to excellent yields. When benzyl alcohols and cinnamic alcohol
were used as raw, the reaction hardly proceeded under neat conditions.
Delightedly, toluene was found to be a suited reaction medium for the oxidative
esterification and moderate to good yields of corresponding α-ketoesters 3g-3k
were obtained under standard conditions. In order to further examine the
tolerance and practicability of the catalytic system, more complicated and useful
molecules with bio-activity were tried to synthesize (Scheme 2). The oxidative
esterification of testosterone and cholesterol with α-hydroxy ketone 1a
proceeded successfully and efficiently and corresponding esterified bio-active
products 3l and 3m were isolated in moderate yields. These clean organic
transformations demonstrate the practicability and sustainability of CuMn/HT-
based catalytic system. Moreover, the oxidative esterification could run in multi-
gram scale smoothly. When 0.05 mol of α-hydroxy ketone 1a was catalyzed by
5 g of CuMn/HT in EtOH, full conversion and nearly quantitive yield of pure 3a
without purification by silica gel chromatography were obtained after 24 h
(Scheme 3). The high tolerance and being scalable up to multi-gram of this
catalytic system make it practical and potential for industrial application.
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Table 5. Scope of oxidative esterification of α-hydroxy ketone.a
O
O
OH
OR
CuMn/HT (20 mg)
+
R-OH
OMe
O
solvent-free or in Toluene
12 h, 70 oC, O2
3b-3k
O
O
OBu
O
O
3b, 85%
3c, 95%
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O
O
O
O
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O
3e, 91%
O
3d, 90%
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O
O
O
O
Ph
O
O
3g, 75%b
3f, 89%
O
O
O
O
O
O
3h, 80%b
3i, 78%b
NO2
O
O
O
O
O
Me
O
3j, 81%b
3k, 70%b
a Reaction conditions: 1a (0.2 mmol), alcohol (1 mL), CuMn/HT (20 mg), 70
oC, 12 h, O2 balloon, isolated yields.
b Toluene (1 mL) was used as solvent and alcohol (0.4 mmol) was used as
substrates.
O
OH
O
H3C
H
O
H3C
H
Ph
OH
CuMn/HT
O
H3C
H3C
+
Toluene, 70 oC, 12 h, O2
H
H
H
H
O
O
3l, 49%
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CH3
Me
CH3
Me
Me
H3C
H
H3C
H
Me
CuMn/HT
O
H3C
H3C
H
H
OH
O
+
Toluene, 70 oC
12 h, O2
H
H
H
H
Ph
HO
O
O
3m, 50%
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Scheme 2. Oxidative esterification of testosterone and cholesterol.
O
O
CuMn/HT (5g)
EtOH (300 mL)
OEt
OH
24 h, 70 oC, O2
O
1a,
50 mmol, 6.8 g
3a
, 95%, 8.45 g
Scheme 3. CuMn/HT-catalyzed oxidative esterification in multi-gram scale.
CONCLUSION
In conclusion, heterogeneous oxidative esterification between α-hydroxy ketone
and alcohols via a tandem oxidation process using oxygen as green oxidant
was developed. The catalytic system based on the recyclable catalyst CuMn/HT
with rich oxygen vacancies and CuMn2O4 active composition is highly tolerant
and scalable up to gram level. More importantly, the use of ball-milling method
under solvent-free conditions provides an environmentally-friendly pathway for
preparation of solid catalysts.
EXPERIMENTAL SECTION
Preparation of CuMn/HT
Copper(II) nitrate trihydrate (ranging from 0.74 g to 2.99 g),
Manganese(II) nitrate tetrahydrate (ranging from 0.56 g to 4.52 g)
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and HT (10 g) were mixed in an agate mortar. Then, the mixture
was transferred into a 100 mL agate grinding jar with 5-15 mm
diameter agate grinding balls. A planetary ball mill was employed to
mill the raw for 2 h and rotation speed was set as 500 rpm. Next,
the milled materials was collected and dried under air at 80 oC for
5 h followed by calcination under air at 350 oC for 2 h. Other HT-
supported bimetallic catalysts that were listed in Table S1 were
prepared by the same way.
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General esterification of α-hydroxy ketone 1a with alcohols under
neat conditions (3a-3f)
A Schlenk tube was charged with 20 mg of CuMn/HT and 0.2
mmol of α-hydroxy ketone 1a. Then, the tube was sealed with a
rubber septum and degassed and recharged with O2 (5 times).
Next, 1 mL of alcohol was injected into the reaction tube under O2.
o
The reaction mixture was stirred at 70 C for 12 h. When the
reaction was finished, the catalyst was removed by filtration. The
filtrate was removed by distillation under reduced pressure to afford
the desired product.
General esterification of α-hydroxy ketone 1a with alcohols in
toluene (3g-3m)
A Schlenk tube was charged with 20 mg of CuMn/HT and 0.2
mmol of α-hydroxy ketone 1a. Then, the tube was sealed with a
rubber septum and degassed and recharged with O2 (5 times).
Alcohol (0.4 mmol) and toluene (1 mL) were injected into the
reaction mixture under O2 respectively. The reaction mixture was
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o
stirred at 70 C for 12 h. When the reaction was finished, the
catalyst was removed by filtration. The filtrate was removed by
distillation under reduced pressure to offer the crude product which
was further purified by silica gel chromatography to yield
corresponding pure product.
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Recovery of retrieved catalyst
After each reaction, CuMn/HT was separated by filtration followed
by washing and drying. Then, it was stirred in 0.5 M Na2CO3
solution under reflux conditions for 24 h. After that, the recovered
o
catalyst was filtrated from the Na2CO3 solution and dried at 80 C
for 5 h. Finally, it was collected for the next run after calcination
under air at 350 oC for 2 h. The reconstruction of HT occurs via a
simple dissolution-reprecipitation process.47
AUTHOR INFORMATION
Corresponding Author
xumeng@licp.cas.cn
zhaopq@licp.cas.cn
Author Contributions
The manuscript was written through contributions of all authors. All authors
have given approval to the final version of the manuscript.
Funding Sources
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The Natural Science Foundation of China (No. 21403256, 21573261) and Youth
Innovation Promotion Association of Chinses Academy of Sciences (No.
2018456).
9
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENT
We gratefully acknowledge the Natural Science Foundation of China (No.
21403256, 21573261) and Youth Innovation Promotion Association of the
Chinses Academy of Sciences (No. 2018456).
SUPPORTING INFORMATION
Experimental information, characterization of catalyst, optimization and copies of
1H and 13C NMR for all products.
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