Aerobic oxidative esterification of alcohols catalyzed by polymer-incarcerated gold nanoclusters under ambient conditions

Article abstract of DOI:10.1039/b926877d

Environmentally benign aerobic oxidation of alcohols to methyl esters catalyzed by polymer-incarcerated gold nanoclusters (PI-Au) was developed and reactions proceeded under very mild conditions. The catalyst could be recovered by simple operations without significant loss of activity.

Full text of DOI:10.1039/b926877d

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Aerobic oxidative esterification of alcohols catalyzed by
polymer-incarcerated gold nanoclusters under ambient conditions†
Hiroyuki Miyamura, Tomohiro Yasukawa and Shu¯ Kobayashi*
Received 23rd December 2009, Accepted 8th February 2010
First published as an Advance Article on the web 1st March 2010
DOI: 10.1039/b926877d
Environmentally benign aerobic oxidation of alcohols to
methyl esters catalyzed by polymer-incarcerated gold nan-
oclusters (PI-Au) was developed and reactions proceeded
under very mild conditions. The catalyst could be recovered
by simple operations without significant loss of activity.
esterification reactions from aldehydes using gold nanoclusters
were recently performed by Christensen and coworkers.7a,7b They
showed that oxidation of aldehydes to esters was much faster
than that of alcohols to aldehydes.7,9 However, they could not
achieve quantitative comparison of these two steps because the
first step required relatively harsh conditions.
Recently, we reported that gold nanocluster catalysts im-
mobilized on polystyrene-based polymers with cross-linking
moieties,10 polymer-incarcerated gold nanocluster catalyst (PI-
Au), were effective for aerobic oxidations of alcohols to ketones
and aldehydes and hydroquinones to quinones under mild
conditions.11,12 Moreover, PI-Au can be reused several times with
careful handling without loss of catalytic activity.
We now report selective aerobic oxidation of alcohols and
aldehydes to the corresponding methyl esters catalyzed by PI-
Au under mild conditions. The necessity of water for high
activity and comparison of kinetic analysis between oxidation
from alcohols to aldehydes and from aldehydes to esters is also
described. To the best of our knowledge, this is the first example
of aerobic oxidative methyl esterification from alcohols that
proceeds at room temperature under atmospheric conditions.
PI-Au was prepared following the previously reported proce-
dure (Scheme 1).11 To examine the catalytic activity of PI-Au for
an esterification reaction, oxidation of p-methylbenzyl alcohol
was performed in methanol in the presence of a base under
atmospheric oxygen conditions at room temperature (Table 1).
Environmentally benign organic synthesis making the most of
resources and energy is required today, when the issues of
global warming CO2 emissions and shortage of natural resources
are worsening. Esterification is one of the most important
transformations in organic synthesis; esters are stable and can be
easily transformed to many kinds of functional groups.1 Methyl
esters in particular are very useful from the viewpoints of atom
economy and versatility for further transformations.
Esterification is traditionally a two-step procedure that in-
cludes synthesis of carboxylic acids or activated carboxylic
acid derivatives such as acid anhydrides or acid chlorides.2
While several facile and economical esterification reactions have
been developed, these protocols usually require stoichiometric
amounts of reagents, and thus a large amount of waste is
formed. In this respect, direct oxidative esterification reactions
from alcohols using molecular oxygen catalyzed by reusable
heterogeneous catalysts under mild conditions, such as at-
mospheric pressure and room temperature, are an attractive
and challenging subject for both organic synthesis and green
chemistry.
Since the discovery of them by Haruta and coworkers,
aerobic oxidation reactions catalyzed by gold nanoclusters have
been widely investigated.3,4 In particular, selective oxidation
of alcohols to the corresponding carbonyl compounds such
as aldehydes, carboxylic acids and esters catalyzed by gold
nanoclusters is attractive for both industry and the laboratory,
and has been extensively investigated recently,4–6,8a,8d and several
gold nanocluster catalysts for aerobic oxidation of alcohols or
aldehydes to esters have been reported. Although oxidation of
aldehydes to esters proceeds under mild conditions,7 examples
starting from alcohols under mild conditions (atmospheric pres-
sure and ambient temperature) were very limited.8 Aerobic oxi-
dation of alcohols to esters is divided into three steps: oxidation
of alcohols to aldehydes; hemiacetal formation; and oxidation
of hemiacetals to esters. Substantial mechanistic studies of
Scheme 1 Preparation of PI-Au.
First, bases were examined, and it was found that 0.5 equiv-
alent of potassium carbonate (17.3 mg in 2 ml of MeOH) was
enough to saturate the reaction mixture (Table 1, entries 1–4).
Potassium carbonate was better than sodium methoxide (entry
4). In the absence of water, yield of the methyl ester dropped
(entry 6), suggesting that water was essential as an activator.
Several experiments using various amounts of water were
performed. The excess water generated the carboxylic acid as
a byproduct and deactivated the reaction (entries 7–10). On the
other hand, the reaction proceeded sluggishly in the presence
of a small amount of water (entry 16); methanol–water (500 : 1)
was the most suitable solvent for this reaction (entry 12). The
reaction almost reached completion under more concentrated
Department of Chemistry, School of Science and Graduate School of
Pharmaceutical Sciences, The University of Tokyo, HFRE Division,
ERATO, JST, Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan.
E-mail: shu_kobayashi@chem.s.u-tokyo.ac.jp; Fax: +81-3-5684-0634
† Electronic supplementary information (ESI) available: Experimental
details. See DOI: 10.1039/b926877d
776 | Green Chem., 2010, 12, 776–778
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Table 1 Optimization of reaction conditions
Table 3 Recovery and reuse of catalyst
Conversion (%)c Aldehyde (%)c Ester (%)c
Run
Yield (%)b
Run
Yield (%)b
Entry Solvent
1d
2e
3
MeOH (wet)a
>99
>99
>99
86
91
71
0
0
0
12
8
35
22
23
5
>99
>99
>99
63
82
35
18
4
85
1
2
3
4
5
6
97
97
97
>99
92
7
86
97
95
74
94
8c
9
MeOH (wet)a
MeOH (wet)a
MeOH (wet)a
MeOH (wet)a
MeOH (dried)a
4f
5g
6
10
11c
90
7
8
9
MeOH–H2O 2 : 1a 57
MeOH–H2O 10 : 1a 38
MeOH–H2O 50 : 1a 92
a 0.125 M. b Determined by GC. c Collected catalyst was heated at 170 ◦C
for 4 h without solvent before using.
10
MeOH–H2O
88
4
74
100 : 1a
11
12
13
14h
15i
16
MeOH–H2O
200 : 1a
>99
>99
96
1
99
The catalyst could be recovered by simple filtration and
washing with water and MeOH. It could be reused several times
without significant loss of activity (Table 3, runs 1–4). Catalysts
with decreased activity after repeated use were also revived by
heating at 170 ◦C for 4 h (runs 8 and 11).
MeOH–H2O
0
>99
93
500 : 1a
MeOH–H2O
500 : 1b
MeOH–H2O
500 : 1
MeOH–H2O
500 : 1
1
97
1
86
This esterification was divided into three steps: oxidizing
alcohols to aldehydes; hemiacetal formation; and second oxi-
dation to form methyl esters (Scheme 2). Kinetic analysis was
performed by using p-methylbenzyl alcohol or p-tolualdehyde
under the optimized conditions in Table 1. An aliquot reaction
mixture was taken by microsyringe and analyzed by GC at each
time step (Fig. 1a and 1b). According to these results, each
step was a first-order reaction and the second step was 30 times
faster than the first step (Fig. 1a-inset and 1b-inset). Moreover,
no hemiacetal formation was observed from the mixture of an
aldehyde and a base in d-MeOH/D2O by NMR measurement
(see ESI†). Therefore, hemiacetal formation and the following
rapid oxidation to esters are considered to occur within the
interior of the solid catalyst.
63
6
54
MeOH–H2O
49
14
17
1000 : 1a
a 0.125 M. b 0.5 M, 5 h. c Determined by GC. d 1.0 eq. of K2CO3 was used.
e 0.75 eq. of K2CO3 was used. f 0.1 eq. of K2CO3 was used. g NaOMe was
used instead of K2CO3. h PI-Au (0.25 mol%), 0.5 M. i PI-Au (0.1 mol%),
1.25 M, K2CO3 (0.05 eq.).
Table 2 Substrate scope for aerobic esterifications
Entry
R
Conversion (%)b
Yield (%)b
1
2
3
4
5
6
7
8
9
Ph
>99
>99
>99
95
>99
>99
>99
12
95
>99
92
60
95
>99
69
4-MeOC6H4
4-BrC6H4
4-NO2C6H4
1-Naphthyl
2-Pyridyl
Ph–CH CH–
PhCH2CH2CH2–
Scheme 2
=
In summary, we have developed aerobic oxidative esterifica-
tion reactions of alcohols using a polymer-incarcerated gold
nanocluster catalyst. While most of the reported esterification
reactions of alcohols mediated by gold nanoclusters require
harsh conditions such as elevated temperature and pressurized
conditions, it is noteworthy that the reactions proceeded under
very mild conditions such as atmospheric pressure and room
temperature. Moreover, catalysts could be reused several times
by simple operations. Thus PI-Au-catalyzed aerobic oxidative
methyl esterification reactions have advantages in terms of
energy efficiency and green chemistry. We also examined kinetic
measurements and revealed that oxidation of hemiacetal to ester
is much faster than oxidation of alcohol to aldehyde. Further
mechanistic study, especially on hemiacetal formation and the
following rapid oxidation inside the PI-Au, is now in progress.
This work was partially supported by a Grant-in-Aid for
Scientific Research from Japan Society of the Promotion of
Science (JSPS). H. M. thanks the JSPS fellowship for Japanese
Junior Scientist.
10
60c
=
CH2 CH–
>99
a 0.125 M. b Determined by GC. c Methyl 3-methoxypropionate was
obtained.
conditions in 5 h (entry 13). TON of PI-Au reached 540 over
24 h (entry 15). It is noted that leaching of gold was not observed
by inductively coupled plasma (ICP) analysis.
Several examples of aerobic oxidative esterification catalyzed
by PI-Au are summarized in Table 2. Aromatic alcohols
containing both electron-donating and -withdrawing groups
were converted to the corresponding methyl esters in good to
high yields (Table 2, entries 1–5, 11). 2-Pyridyl methanol and
cinnamyl alcohol were also transformed to the corresponding
esters in high yields (entries 6 and 7). However, an aliphatic
alcohol was not oxidized smoothly (entry 8). In the oxidation of
allyl alcohol, further Michael addition of methanol to the ester
occurred (entry 9).
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778 | Green Chem., 2010, 12, 776–778
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The Royal Society of Chemistry 2010
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