Silver nitrate-catalyzed selective air oxidation of benzylic and allylic alcohols to corresponding aldehydes or ketones

Article abstract of DOI:10.1002/jccs.201300669

The liquid phase selective air oxidation of benzylic and allylic alcohols to corresponding aldehydes or ketones has been achieved in high yields with catalytic amount of AgNO₃ and in the presence of Na ₂CO₃. The liquid phase selective air oxidation of benzylic and allylic alcohols to corresponding aldehydes or ketones has been achieved in high yields with catalytic amount of AgNO₃ and in the presence of Na₂CO₃.

Full text of DOI:10.1002/jccs.201300669

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Communication
Silver Nitrate-Catalyzed Selective Air Oxidation of Benzylic and Allylic Alcohols to
Corresponding Aldehydes or Ketones
Jiayi Wang,a,b Yuan Li,a Yanqing Penga and Gonghua Songa,*
aShanghai Key Laboratory of Chemical Biology, School of Pharmacy, East China University of Science and Technology,
Shanghai, 200237, P. R. China
bShanghai Key Laboratory of Catalysis Technology for Polyolefins, Shanghai Research Institute of Chemical Industry,
Shanghai, 200062, P. R. China
(Received: Dec. 24, 2013; Accepted: Mar. 7, 2014; Published Online: Apr. 14, 2014; DOI: 10.1002/jccs.201300669)
The liquid phase selective air oxidation of benzylic and allylic alcohols to corresponding aldehydes or ke-
tones has been achieved in high yields with catalytic amount of AgNO3 and in the presence of Na2CO3.
Keywords: Oxidation; Silver nitrate; Selectivity; Alcohols.
INTRODUCTION
The selective oxidation of alcohols to aldehydes or
sponding aldehydes or ketones using AgNO3 as catalyst
and in the presence of Na2CO3 (Scheme 1).
ketones is a very important organic transformation, espe-
cially in preparing for some important precursors and inter-
mediates for drugs, fine chemicals, vitamins and fra-
grances.1 Many catalysts have been developed for this
transformation, such as DDQ-Mn(OAc)3,2 Au-Cs2CO3,3
Ru-4-5 or Pd-based complexes.6-10 But these methods suffer
from some drawbacks, such as expensive metals, tedious
work up procedure, toxic reagents or producing large
amounts of waste. As a green, environmental friendly and
readily available oxidant, O2/air was preferably used in ox-
idation reactions.11 However, the selective oxidation of
benzylic and allylic alcohol with air or oxygen as the only
oxidant is still a challenging task.
Silver-catalyzed oxidation of benzylic and
allylic alcohols
Scheme 1
RESULTS AND DISCUSSION
The air oxidation of benzyl alcohol to benzaldehyde
catalyzed by AgNO3 in toluene was chosen as a model reac-
tion. It was found that a relative weak inorganic base
(Na2CO3), rather than a strong inorganic base (NaOH) or
an organic base (NEt3), resulted in better yield of benz-
aldehyde (Table 1, entry 1-3). No reaction would take place
in the absence of a base (Table 1, entry 4). When the reac-
tion was performed under argon atmosphere, no product
was detected (Table 1, entry 5). Prolonging reaction time to
6 hours, benzyl alcohol was almost completely and selec-
tively converted to benzaldehyde even only 2 mol% of
AgNO3 was used (Table 1, entry 6-9). Further lowering the
amount of AgNO3 or Na2CO3 caused certain decrease in
yield (Table 1, entry 10-11). Ag2CO3 was found to be less
active than AgNO3 (Table 1, entry 12). The yield of benz-
aldehyde versus time with different loading amount of
AgNO3 was also investigated. As shown in Fig. 1, the reac-
tion can proceed smoothly when 2 mol% of AgNO3 was
used, while increasing the catalyst loading amount did not
reduce the reaction time, and decreasing the AgNO3 load-
Silver-based catalyst is widely used in gas-phase oxi-
dation processes, such as the oxidation of ethylene to ethyl-
ene oxide,12 benzyl alcohol to benzaldehyde,13 and so on.
However, high-temperature is inevitable to such a process.
Recently, a few studies reported that supported silver cata-
lysts are catalytically active in the liquid phase oxidation of
alcohols.14-19 Nevertheless, except for the inconvenience of
fussy procedure of preparing supported silver catalysts,
there is some other shortcomings such as low conversion,14
need of co-catalyst,15-16 exclusion of substrates with elec-
tron withdrawing groups,17-18 without the chemoselective
oxidation of benzylic and allylic alcohols,17,19 and so on. To
the best of our knowledge, there is no report yet of success-
fully applying an unsupported silver-based catalyst to the
liquid phase selective oxidation of benzylic and allylic al-
cohols. Herein we report a simple and efficient protocol for
the air oxidation of benzylic and allylic alcohols to corre-
* Corresponding author. Tel: +86-21-64253140; Fax: +86-21-64252603; Email: ghsong@ecust.edu.cn
J. Chin. Chem. Soc. 2014, 61, 517-520
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Wang et al.
Table 1. Optimization of reaction conditions for oxidizing benzyl
alcohol to benzaldehyde[a]
Entry
AgNO3 (mol %)
Base
t (h)
Yield[b] (%)
1
20
20
20
20
20
20
10
5
NaOH
Na2CO3
NEt3
2
2
2
2
2
6
6
6
6
6
6
6
10[c]
2
36
3
trace
4
-
0
0[d]
5
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
6
100
7
100
8
100
96, 80, [e] 45[f]
9
2
10
11
12
1
85
88[g]
74[h]
2
1
[a] Reaction condition: benzyl alcohol (1 mmol), Na2CO3 (2
mmol), AgNO3, air atmosphere, toluene 2.5 mL, reflux.
[b] Based on GC analysis.
[c] Small amount of benzoic acid was detected after treated with
0.1 M HCl.
[d] Under Ar atmosphere.
[e] Second run for the reuse of the catalytic system.
[f] Third run for the reuse of the catalytic system.
[g] 1 mmol Na2CO3 was used as base.
[h] 1 mol% Ag2CO3 was used instead of AgNO3.
ing amount to 1 mol% led to obvious decrease in yield. The
optimized reaction conditions were thus established as fol-
lows: 2 mol% AgNO3 as catalyst, 2 equiv. Na2CO3 as base,
reflux in toluene and in air atmosphere.
plied directly to the next run after dried under vacuum. A
moderate yield of 80% could be obtained in the second run.
Unfortunately, the yield decreased dramatically (Table 1,
entry 9) when the catalyst was reused for second time (third
run).
To demonstrate the recoverability of the catalyst sys-
tem, the solid filtrated from the reaction mixture was ap-
Then, the air oxidation of various alcohols, including
benzylic alcohols, allylic alcohols, aromatic and aliphatic
alcohols, were investigated under optimized conditions.
The oxidation of 4-methoxybenzyl alcohol went smoothly
and very high isolated yield was achieved after 6 hours (Ta-
ble 2, entries 2). Only moderate yields were obtained even
with longer reaction time when less active electron defi-
cient primary benzylic alcohols, 4-chlorobenzyl alcohol,
4-bromo benzyl alcohol and 4-nitrobenzyl alcohol were
used (Table 2, entries 3–5). For secondary alcohols, both
the electron rich and electron deficient secondary benzylic
alcohols delivered the products with good to excellent
yields (Table 2, entries 6–9). Bulky sec-alcohol substrates
such as benzhydrol also gave excellent yield (Table 2, entry
10). For the oxidation of 1-(naphthalen-2-yl) ethanol, lon-
ger time was needed to get satisfied yield (Table 2, entry
11). Allylic alcohol was also tolerated, thereby affording
Fig. 1. Yield of benzaldehyde versus time with differ-
ent loading amount of AgNO3. Reaction condi-
tions: 1 mmol benzyl alcohol, x mol% AgNO3,
2 mmol Na2CO3, refluxed in toluene and in air
atmosphere.
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CHEMICAL SOCIETY
Silver Nitrate-Catalyzed Selective Air Oxidation
(red lines) are also shown in Fig. 2b with binding energy of
368.77 and 367.68 eV for Ag 3d5/2, which implies the exis-
tence of both Ag0 nanoparticles21 and Ag2CO3.22 As shown
in Fig. 3a, during the reaction, Ag(I) from AgNO3 is re-
duced by alcohols to generate Ag0 nanoparticles deposited
on the surface of sodium carbonate. The Ag0 nanoparticles
can also be produced from Ag2CO3 generated by the reac-
tion of Ag+ and sodium carbonate. As AgNO3 exhibited
much higher catalytic activity than Ag2CO3 at same Ag
dosage and the yield with reused catalyst system decreased
remarkably (Table 1, entries 9 and 12), it was speculated
that: (i) higher degree of ionization of AgNO3 makes it
more efficient than Ag2CO3; (ii) the decrease in yield with
recycle catalyst might due to the formation of Ag2CO3, ag-
gregation of Ag nanoparticles and/or leaching of Ag during
the catalyst recovering procedure. With these results in
hand, we proposed that alcohol oxidation occurs on the sil-
ver nanoparticles on the surface of sodium carbonate.15 As
shown in Fig. 3b, after adsorption of benzyl alcohols on the
silver surface, the alcohol is dehydrogenated and then
benzaldehyde is formed in the presence of Na2CO3 and mo-
lecular oxygen. To confirm the hypothesis, we also synthe-
sized Ag nanoparticles according to the literature23 with
some modifications and test their catalytic activities in oxi-
dation of benzyl alcohol to benzaldehyde (See supplemen-
tary material for details). It was found that a moderate yield
of 42.4% can be obtained in 4 hours when 2 mol% Ag
nanoparticles were used as catalyst. Further increasing re-
action time did not improve the yield, which may caused by
the corresponding a,b-unsaturated carbonyl compound
without the disruption of the double bond (Table 2, entry
12). The yield for the oxidation of pyridinemethanol was
poor even under 10 mol% catalyst and longer reaction time
(Table 2, entry 13). As the oxidation of primary and sec-
ondary aliphatic alcohols, such as 2-phenylethanol, cyclo-
hexanol, 1-octanol and 2-octanol, were effectively prohib-
ited (entry 14-17), high chemoselective oxidation of benz-
ylic and allylic alcohols to corresponding aldehydes or ke-
tones was achieved with this oxidation system.
To identify the possible mechanism for the oxidation
of alcohols by AgNO3/Na2CO3 system, the catalyst system
was filtrated after the reaction and characterized by TEM
and XPS (Fig. 2). As shown in Fig. 2a, the silver nano-
particles were well dispersed on the surface of Na2CO3 af-
ter the reaction. In Fig. 2b, the black line shows the Ag 3d
region of the XPS spectrum for the filtered AgNO3/Na2CO3
catalyst. It can be seen that all the Ag 3d peaks are asym-
metric, suggesting that there are at least two kinds of silver
species on the sample surface.20 The curve fitting patterns
Fig. 2. TEM image (a) and Ag 3d region of the XPS
spectrum (b) for filtered AgNO3/Na2CO3 after
reaction.
Fig. 3. Transformation of silver nitrate during the reac-
tion and proposed reaction mechanism.
J. Chin. Chem. Soc. 2014, 61, 517-520
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Communication
Wang et al.
Ag nanoparticles aggregation or agglomeration during the
reaction. Although, the results indicated that Ag nanoparti-
cles could catalyze the reaction, which was in coincidence
with our proposed mechanism.
Research Program of China (973 Program) (Grant No.
2010CB126101), National Natural Science Foundation of
China (Grant 20972052) and Shanghai Key Laboratory of
Catalysis Technology for Polyolefins (LCTP-201301) are
gratefully acknowledged.
CONCLUSIONS
In conclusion, we have developed a simple and effi-
cient method for the selective oxidation of benzylic and al-
lylic alcohols to corresponding aldehydes or ketones using
AgNO3 as catalyst, air as the sole oxidant, and in the pres-
ence of sodium carbonate. Given the highly selective na-
ture of this oxidation, we believe that this method might be
applicable to the chemoselective oxidation of alcohols in
multistep synthesis.
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Financial support for this work from National Basic
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J. Chin. Chem. Soc. 2014, 61, 517-520