Pd/C-catalyzed aerobic oxidative esterification of alcohols and aldehydes: A highly efficient microwave-assisted green protocol

Article abstract of DOI:10.3762/bjoc.10.149

We herein describe an environmentally friendly microwave-assisted oxidative esterification of alcohols and aldehydes in the presence of molecular oxygen and a heterogeneous catalysis (Pd/C, 5 mol %). This efficient and ligandless conversion procedure does not require the addition of an organic hydrogen acceptor. The reaction rate is strongly enhanced by mild dielectric heating. Furthermore, it is a versatile green procedure which generally enables the isolation of esters to be carried out by simple filtration in almost quantitative yields.

Full text of DOI:10.3762/bjoc.10.149

Pd/C-catalyzed aerobic oxidative esterification of
alcohols and aldehydes: a highly efficient
microwave-assisted green protocol
Marina Caporaso1, Giancarlo Cravotto*1,2, Spyros Georgakopoulos1,3,
George Heropoulos3, Katia Martina1 and Silvia Tagliapietra1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2014, 10, 1454–1461.
1Dipartimento di Scienza e Tecnologia del Farmaco, University of
Turin, Via P. Giuria 9, 10125 Torino, Italy. Fax +390116707687; Tel:
+390116707684, 2Interdepartmental Centre “Nanostructured
interfaces and surfaces NIS, University of Turin, Via P. Giuria 7–9,
10125 Torino, Italy and 3Institute of Biology, Medicinal Chemistry and
Biotechnology, National Hellenic Research Foundation, 48. Vas.
Constantinou Ave., 11635 Athens, Greece
doi:10.3762/bjoc.10.149
Received: 18 April 2014
Accepted: 26 May 2014
Published: 26 June 2014
Authors are listed in alphabetical order and contributed equally to this
work.
Email:
Giancarlo Cravotto* - giancarlo.cravotto@unito.it
Associate Editor: M. Rueping
* Corresponding author
© 2014 Caporaso et al; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
aerobic oxidation; alcohol; esterification; heterogeneous catalysis;
microwaves
Abstract
We herein describe an environmentally friendly microwave-assisted oxidative esterification of alcohols and aldehydes in the pres-
ence of molecular oxygen and a heterogeneous catalysis (Pd/C, 5 mol %). This efficient and ligandless conversion procedure does
not require the addition of an organic hydrogen acceptor. The reaction rate is strongly enhanced by mild dielectric heating. Further-
more, it is a versatile green procedure which generally enables the isolation of esters to be carried out by simple filtration in almost
quantitative yields.
Introduction
Selective oxidations of alcohols are some of the most important ketones using Au [4-8], Pd [9-12], Ru [13-16] and Cu [17-19]
transformations in organic synthesis. Therefore, reactions that supported catalysts.
employ reusable heterogeneous catalysts and molecular oxygen
are highly desirable from atom economy and environmental An elegant approach to the direct oxidative esterification of
impact point of view [1-3].
alcohols has been described in recent years [20]. The proposed
reaction pathway (Scheme 1) is based on the oxidation of the
A number of methods have been developed for the aerobic oxi- alcohol to an aldehyde, followed by conversion to the respec-
dation of primary and secondary alcohols to aldehydes and tive hemiacetal, which is further oxidized to the corresponding
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Beilstein J. Org. Chem. 2014, 10, 1454–1461.
ester. Relatively little work has been carried out on this so far, for organic synthesis have recently brought to the market high
besides some remarkable results on the catalytic performance of power (up to 1.5 kW) pressure resistant reactors (up to 200 bar)
Au [21-26], Ru [27-29], Ir [30] and Pd [31-35]. Moreover, the that are equipped with separate multiple gas inlets and can be
search for new, less expensive and environmentally friendly considered autoclaves. Only a few publications have described
catalysts has attracted a great deal of interest because of the this technique in heterogeneous gas-phase reactions that are
limited availability and high price of noble metals.
important for industrial processes [51-55]. We have recently
displayed the versatility of these reactors in promoting reac-
tions with gaseous reagents in a closed cavity [56]. In this
present work, we report an efficient Pd/C-catalyzed aerobic
oxidative esterification of aldehydes and alcohols in the pres-
ence of molecular oxygen in a closed MW reactor. The opti-
mized procedure does not require co-catalysts, co-oxidants or
ligands.
Results and Discussion
Pd-catalyzed esterification can be performed with Pd(II) species
[31-35,44]. The challenge of performing aerobic oxidation
under MW irradiation in a preliminary investigation prompted
Scheme 1: Reaction pathway of aerobic oxidative esterification of
alcohols.
Homogeneous and heterogeneous Pd catalysts have been widely us to select benzylalcohol and methanol as substrate for the
used in the selective oxidation of alcohols [36,37]. Most publi- model reaction and PdCl2(PPh3)2 and Pd(OAc)2 as the cata-
cations have focused on the conversion of alcohol to aldehydes lysts. The reactions were performed in a professional multi-
or ketones [38,39]. Several examples of the heterogeneous mode reactor (SynthWave-Milestone/MLS) which was
oxidative esterification of alcohols in the presence of molecular equipped with multiple gas inlets and designed to work over a
oxygen or air have been described [23,25,40-43], however, the wide range of pressures and temperatures. The reaction
main limit of this process is its lack of selectivity, as has already chamber was loaded with oxygen (2.5 bar) and pressurized with
been reported. Several Pd-catalyzed procedures have been opti- nitrogen (up to 20 bar) [57]. Reactions were performed in the
mized and Ag [31,32] or Bi salts [40], hydrosilanes [33] or presence of benzylalcohol, K2CO3 in methanol and numerous
specially designed electron-deficient ligands [9,31] have all different reaction conditions were screened (Table 1).
been added to the catalyst in an attempt to improve conversion
and selectivity. Other examples refer to the use of Pd(II) species As depicted in Table 1, Pd(OAc)2 afforded the methyl ester
in the presence of a cooxidant such as benzyl chloride or desyl (selectivity 51%) after 1 h MW irradiation at 90 oC (Table 1,
chloride [34,44]. The optimized procedures are therefore entry 3) and the optimization of the process was pursued. It is
limited with respect to catalyst accessibility and long reaction noteworthy that longer reaction times at a lower temperature
time. For this reason, the use of inexpensive and easily reusable (Table 1, entry 6), doubling the amount of catalyst (Table 1,
catalysts as well as a reduction in its long reaction time can entry 4) or the addition of an electron-rich ligand such as triph-
make this process cost-effective and environmentally benign. enylphosphine to the reaction mixture (Table 1, entry 5) only
had minimal effects on substrate conversion and selectivity.
In comparison with more expensive and air-sensitive Pd cata- Preliminary sonication of the K2CO3 in methanol, prior to
lysts, the common Pd/C is easily handled and filtered off from adding the catalyst and the substrate, led to a significant
the reaction mixture. It is remarkably stable under acidic and increase in reactivity affording complete conversion and
basic conditions and features a much higher surface area than complete selectivity (compare Table 1, entries 3 and 7).
alumina- and silica-supported catalysts [45]. Moreover, char- Because of the excellent dielectric properties of charcoal, we
coal is rapidly heated under microwave (MW) irradiation thanks tested activated charcoal adsorbed Pd(OAc)2 and observed
to its high electrical and thermal conductivity [46,47]. It is now complete conversion and good selectivity (Table 1, entry 8).
widely accepted that MW can dramatically enhance reaction Based on this result and on a recently published study by Stahl
rates in organic synthesis [48], while the kinetics and yields of et al. [40], it was decided that the investigation be broadened to
any chemical modification are strongly improved by the optimal the readily accessible Pd/C. Gratifyingly, full substrate conver-
heat transfer provided by selective volumetric dielectric heating. sion to the desired ester was afforded without catalyst pre-soni-
MW is surely one of the best techniques for cutting down reac- cation, even at 70 °C (Table 1, entries 9–11). For the sake of
tion time, minimizing catalyst loading and reducing reagent comparison, the oxidative esterification of benzylalcohol in the
excess [49,50]. Design improvements in commercial MW ovens presence of methanol was attempted under conventional
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Beilstein J. Org. Chem. 2014, 10, 1454–1461.
Table 1: Oxidative esterification of benzylalcohol with methanol.
Entry
Pd catalyst
Temp, Time [°C, h]
Conv [%]
1 [%]a
2a [%]a
1
2
PdCl2(PPh3)2
PdCl2(PPh3)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2/C
10% Pd/C
10% Pd/C
10% Pd/C
Pd(PPh3)4
10% Pd/C
10% Pd/C
10% Pd/C
MW, 90 °C, 1 h
MW, 70 °C, 2 h
MW, 90 °C, 1 h
MW, 90 °C, 1 h
MW, 90 °C, 1 h
MW, 70 °C, 2 h
MW, 90 °C, 1 h
MW, 90 °C, 1 h
MW, 90 °C, 1 h
MW, 90 °C, 1 h
MW, 70 °C, 2 h
MW, 90 °C, 1 h
60 °C, 22 h
35
48
17
35
49
46
41
46
0
12
11
51
42
36
34
100
89
100
100
100
–
3
100
88
4b
5c
82
6
80
7d
100
100
100
100
100
2.3
2.5
1.2
15
8
11
0
9
10d
11
12d
13d,e
14d,f
15d,f
0
0
2
3
0
90 °C, 3.5 h
1
–
90 °C, 7 h
15
0
aIsolated yield. bThe reaction was performed with 10 mol % Pd. c10 mol % triphenylphosphine was added. dThe mixture of the base in 1 mL MeOH
was sonicated in an ultrasound bath for 10 s prior to the addition of the catalyst and the substrate. eO2 balloon. fThe reaction was performed in Parr
2.5 bar O2/17.5 bar N2.
heating. Reactions were performed under O2 pressure (simple
rubber balloon and Parr reactor at 2.5 bar) at 60 °C and 90 °C
(Table 1, entry 13–15). Poor conversion was detected at low O2
pressure at 60 °C after 22 h and yields were not improved in the
Parr reactor. The optimized protocol under MW was repeated
using the Parr reactor filled with O2 (2.5 bar) and pressurized
with N2 (17.5 bar). The low value of 15% conversion was
observed after 7 h stirring at 90 °C.
These encouraging results prompted us to screen for the influ-
ence of different bases on reaction conversion and selectivity
when the reaction was catalyzed by Pd(OAc)2, Pd(OAc)2/C,
and Pd/C.
The three-dimensional array in Figure 1A highlights that high
conversion was generally achieved under different conditions.
Good results were obtained with Pd (OAc)2 under homogenous
and heterogeneous catalysis. The reactivity of Pd/C was
observed to be more influenced by the kind of base and only
K2CO3, Na2CO3 and KOMe gave good conversion (see also
Supporting Information File 1). The selectivity of experiments
Figure 1: Screening of different catalysts and bases in the catalytic
oxidative esterification of benzylalcohol. A) Comparison of the methyl
benzoate yield. B) Comparison of the benzylalcohol conversion and
that gave the highest conversions was compared in the two-
dimensional array (Figure 1B). As depicted, Pd/C gave
selectivity. Conditions: 1 mmol benzylalcohol, 5 mol % Pd, 2 equiv
base, 1 mL methanol, O2 (2.5 bar), N2 17.5 bar.
complete selectivity in the presence of either Na2CO3 or
K2CO3, while Pd(OAc)2 was much more selective in the pres-
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Beilstein J. Org. Chem. 2014, 10, 1454–1461.
ence of K2CO3 when the reaction was performed in the homo- As depicted in Table 2, the optimized Pd/C catalyzed oxidative
geneous phase. Charcoal supported Pd(OAc)2 showed the esterification protocol exhibits general applicability with
highest selectivity with KOMe.
various aldehydes. High methyl ester yields were obtained in
Table 2: Oxidative esterification of various aldehydes in the presence of methanol.
Entry
1
Substrate
Temp, Time [°C, h]
100, 1
Conv. [%]a
100
Ester [%]a
98
2
3
90, 2
100
75
98
50
100, 1
4
5
100, 1
90, 2
100
100
94b
98c
6
7
100, 1
100, 1
100
100
98
96
8
100, 1
100
98
9
100, 1
23
85
13
63
10
120, 1.5
11
12
13
120, 1.5
120, 1.5
120, 1.5
65
85
70
73
97
100
aIsolated yield; bwhen repeated at 120 °C (1,5 h) 15% of nitrobenzene was detected by GC–MS; cwhen repeated at 120 °C (1,5 h) 22% of nitro
benzene was detected by GC–MS.
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Beilstein J. Org. Chem. 2014, 10, 1454–1461.
most cases. The catalytic process proved to be substrate depen-
dent to an extent, so an appropriate reaction temperature,
ranging from 90 to 120 °C, had to be applied for each substrate.
Thiophene carboxaldehyde, for instance, afforded only a 45%
desired ester yield at 100 °C, while the yield increased to 85%
in the absence of any byproducts when the reaction temperature
was raised to 120 °C (Table 2, entry 12). However, significant
amounts of decarbonylated analogues were detected when the
reaction was performed at 120 °C with the nitrobenzaldehyde
derivatives (Table 2, entries 4 and 5). The aldehyde decarbony-
lation reaction in Pd catalytic systems has recently been
reported in the literature [58,59]. High conversion and selec-
tivity were achieved in most cases, apart from 3,5-dimethoxy-
benzaldehyde and 4-chlorobenzaldehyde.
Table 3: Oxidative esterification of various alcohols.
Temp. [°C],
Time [h]
Entry
Substrate
R’
Yield [%]a
Me
Et
100, 1
120, 1.5
98
98
79
78
76
1
2
n-Pr 120, 1.5
iPr 120, 1.5
n-Bu 120, 1.5
Me
Me
120, 1.5
120, 1.5
98
Pd-catalyzed dehalogenation reaction of 4-chlorobenzaldehyde
(120 °C for 1.5 h) gave methyl benzoate as main product
besides traces of the desired product (detected by GC–MS
analysis). Aiming to reduce this undesired reaction, the
temperature was decreased to 100 °C, in this way, despite the
low conversion, 13% of product was obtained.
3
36
4
5
6
7
Me
Me
Me
Me
90, 2
44
58
97
89
Table 3 depicts the results of the oxidative esterification,
performed with a series of alcohols. In most cases, the
substrates were quantitatively oxidized to the corresponding
methyl esters within 1–2 hours. Reaction conditions and
temperature were optimized for each substrate. The oxidative
esterification of benzylalcohol in the presence of aliphatic alco-
hols other than methanol was then studied (Table 3). As
depicted in Table 3 entry 1 complete conversion was achieved
with high selectivity for the corresponding esters of benzylal-
cohol with ethanol, n-PrOH, iPrOH and n-BuOH. When the
reaction was performed with n-BuOH we observed the pres-
ence of butyl butyrate.
100, 2
120, 1.5
120, 1.5
8
Me
Me
Me
120, 1.5
100, 1
96
8
9
When the reaction was performed with benzylalcohol in
n-BuOH (Table 3, entry 1) the main product detected by
GC–MS was the butyl butyrate. Esterification reactions using
other linear alcohols as substrate, such as pentanol and hexade-
canol carried out in methanol, gave in both cases less than 10%
yield of the corresponding esters (data not shown).
10
11
12
120, 1.5
85
Me
Me
Me
120, 1.5
120, 1.5
120, 1.5
35
75
29
The recycling of the catalyst was evaluated in order to better
investigate and optimize the protocol. The catalyst was filtered
on sintered glass and washed three times with methanol. The
recovered Pd/C was reused in the esterification of benzalde-
hyde and benzylalcohol, however only starting material was
detected after MW irradiation at 120 °C for 1.5 h. Notably, the
Pd(0) species can be oxidized to Pd-peroxo complexes [60].
The peroxo complexes can be reduced back to Pd(0) using
hydrogen, therefore we decided to regenerate the catalyst by
13
aIsolated yield.
hydrogenation. As depicted in the Scheme 2, to our delight, the
oxidative esterification of benzaldehyde gave methyl benzoate
in 80% yield. Based on this evidence, we can assume that Pd(0)
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Beilstein J. Org. Chem. 2014, 10, 1454–1461.
can be oxidized by oxygen to obtain unreactive species. Based Wave (MLS GmbH, Milestone S.r.l.). The sonochemical device
on Beller at al. [31] the Pd-peroxo complex does not catalyze was developed in collaboration with Danacamerini sas (Torino,
the oxidative esterification of benzylalcohol, but the mecha- Italy).
nism for the Pd catalysed aerobic oxidative esterification has
not still been fully elucidated. Additional mechanistic studies General procedure for aerobic aldehyde and alcohol
will be needed to support a mechanistic rationale.
esterification
Na2CO3 (2 equiv) was dissolved in MeOH (1 mL) and soni-
cated with a US bath for 10 sec (20.3 kHz, 60 W). The sub-
strate (aldehyde or alcohol, 1 mmol) and 10% Pd/C (5% Pd/mol
of substrate) were added to this mixture. The reaction was
carried out under magnetic stirring in a MW reactor Synth-
Wave. The 1 L pressure-resistant PTFE cavity (up to 200 bar)
equipped with a 15 position vial rack was loaded with O2
(2.5 bar) followed by the addition of N2 up to 20 bar total pres-
sure. The reaction was irradiated for an appropriate reaction
temperature ranging from 90 to 120 °C (average power 300 W),
and for 1 to 2 hours (see Table 2 and Table 3). The mixture was
then filtered off through celite, the catalyst washed with MeOH
and the solvent evaporated under vacuum. Isolated yields for all
substrates reported were obtained using these conditions.
Scheme 2: Catalyst regeneration and oxidative esterification of
benzaldehyde (2nd cycle).
Conclusion
In summary, we have described a highly efficient, versatile and General procedure for Pd/C catalyst regeneration
eco-friendly protocol for the aerobic esterification of benzylic Pd/C, filtered after a first cycle of oxidative esterification of
alcohols and aldehydes to methyl esters. 5 mol % Pd/C under benzylalcohol, was regenerated. 50 mg were dispersed in
MW irradiation has proven itself to be an effective catalytic toluene (1.45 mL). The reaction was carried out under magnetic
system. The reaction proceeded with high conversion and selec- stirring in the MW reactor (SynthWave). The reactor was
tivity without the addition of ligands and organic hydrogen loaded with H2 (6 bar) pressure followed by the addition of N2
acceptors. MW irradiation promoted the reaction and reaction up to 20 bar total pressure. The reaction was left at 60 °C for 8 h
time was dramatically cut. A preliminary evaluation of catalyst (average power 150 W). The mixture was then filtered and the
recycling showed that Pd/C can be reused for the esterification catalyst was recovered (0.042 g).
of benzaldehyde after hydrogenation.
Supporting Information
Experimental
General methods
Supporting Information File 1
All commercially available reagents and solvents were used
without further purification. Pd sources from Sigma-Aldrich
included 10 wt % Pd/C (Cat. No.75990) and Pd(OAc)2 and
PdCl2(PPh3)2 from Merck. NMR spectra were recorded with a
Bruker 300 Avance (300 MHz and 75 MHz for 1H and 13C, res-
pectively) at 25 °C. Chemical shifts were calibrated to the
residual proton and carbon resonances of the solvent; CDCl3
(δH = 7.26, δC = 77.16). GC–MS analyses were performed in a
GC Agilent 6850 (Agilent Technologies, Palo Alto, CA, USA)
that was fitted with a mass detector Agilent Network 5973,
using a 30 m long capillary column HP 5-MS (5% phenyl
methyl siloxane, i.d. 0.25 mm, film thickness 0.25 μm). GC
conditions were: injection split 1:20, injector temperature
250 °C, detector temperature 280 °C. Gas carrier: helium
(1.2 mL/min), temperature program: from 50 °C (held for
3 min) to 80 °C at 3 °C/min and to 300 °C at 10 °C/min. Reac-
tions were carried out in a professional MW reactor Synth-
Copies of GC–MS chromatograms, MS, 1H NMR and
13C NMR spectra.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-10-149-S1.pdf]
Acknowledgements
This work was funded by the University of Turin (fondi ricerca
locale ex 60%) and by the European Union's Seventh Frame-
work Programme (FP7-REGPOT-2009-1) under grant agree-
ment no. 245866.
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