Bismuth(III) oxide catalyzed oxidation of alcohols with tert-butyl hydroperoxide

Article abstract of DOI:10.1055/s-0030-1258221

A variety of aromatic, aliphatic and conjugated alcohols were transformed into the corresponding carboxylic acids and ketones with aq 70% t-BuOOH in the presence of catalytic amounts of bismuth(III) oxide. This method possesses a wide range of capabilities, does not involve cumbersome work-up, exhibits chemoselectivity and proceeds under ambient conditions. The resulting products are obtained in good yields within reasonable time. The overall method is green.

Full text of DOI:10.1055/s-0030-1258221

3
736
PAPER
Bismuth(III) Oxide Catalyzed Oxidation of Alcohols with tert-Butyl
Hydroperoxide
Payal Malik, Debashis Chakraborty*
Department of Chemistry, Indian Institute of Technology Madras, Chennai-600 036, Tamil Nadu, India
Fax +91(44)22574202; E-mail: dchakraborty@iitm.ac.in
Received 8 July 2010; revised 13 July 2010
gate the application of bismuth(III) reagents to the oxida-
tion of aldehydes.11 It must be noted that bismuth(III)
Abstract: A variety of aromatic, aliphatic and conjugated alcohols
were transformed into the corresponding carboxylic acids and ke-
tones with aq 70% t-BuOOH in the presence of catalytic amounts of
oxide (Bi O ) is reported to catalyze the oxidation of a-
2
3
12
bismuth(III) oxide. This method possesses a wide range of capabil- hydroxy ketones. We were intrigued to carry out the
ities, does not involve cumbersome work-up, exhibits chemoselec- Bi O -mediated transformation of primary and secondary
2
3
tivity and proceeds under ambient conditions. The resulting
products are obtained in good yields within reasonable time. The
overall method is green.
alcohols into carboxylic acids and ketones, under our pre-
viously developed conditions.
Initial attempts to optimize the reaction conditions for the
oxidation of primary alcohols to the corresponding car-
boxylic acids were performed using (2-methoxyphe-
nyl)methanol as a suitable substrate in the presence of
different solvents, oxidants and 10 mol% bismuth(III)
salts (Table 1).
Key words: oxidations, alcohols, Bi O , t-BuOOH, reaction rates
2
3
The challenge of alcohol oxidation has been of contempo-
rary interest due to its diverse potential in organic synthe-
sis and industrial manufacturing, and it is recognized as a
1
The conversion of (2-methoxyphenyl)methanol into 2-
methoxybenzoic acid is extremely facile in ethyl acetate
in the presence of 10 mol% Bi O and five equivalents of
fundamental reaction. The oxidation of primary alcohols
results in the formation of aldehydes, which are further
oxidized to yield carboxylic acids. Secondary alcohols, on
the other hand, give ketones. The most popular and widely
used oxidant for such transformations is the Jones re-
2
3
aqueous 70% t-BuOOH as the oxidant (Table 1, entry 1)
Table 1 Optimization of the Conversion of (2-Methoxyphe-
2
agent. However, stoichiometric amounts of reagent are
nyl)methanol into 2-Methoxybenzoic Acida
required and the reaction is typically performed under
highly acidic conditions. Substrates with acid-sensitive
functionalities may not tolerate such conditions. In addi-
tion, the generation of Cr-based waste products may be
OMe
OMe O
OH
Bi salt, t-BuOOH
solvent, reflux
OH
3
considered as a potential environmental hazard. Other re-
4
Time (h)b
Yield (%)c
Entry
Catalyst
Solvent
agents that have been used successfully include: Oxone,
5
calcium hypochlorite, and 2-hydroperoxyhexafluoro-2-
propanol, and excellent catalytic methods using metals
have been developed using oxidation reactions. The cat-
1
2
3
4
5
6
7
8
9
0
Bi O
EtOAc
MeCN
toluene
CH Cl
24
29
48
42
36
39
40
35
32
35
41
57
96
88
85
87
89
85
82
83
80
85
84
50
2
3
3
3
3
3
3
3
3
3
6
7
Bi
O
2
alytic oxidation of alcohols employing transition metals
Bi O
8
2
such as Ru, Co, Mo, Pd, V, and W have been reported. In
addition, 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO)
together with NaClO has been used as an efficient combi-
Bi O
2
2
2
Bi O
DMF
9
2
nation for such oxidations. However, the above reagents
and methods have some limitations, which include the use
of superstoichiometric amounts of expensive compounds,
highly basic or acidic reaction conditions and/or high tem-
perature. The search for catalytic processes involving en-
vironmentally benign reagents will always remain an
attractive avenue in this area. Our recent results highlight
the oxidation of aldehydes to carboxylic acids using 30%
H O as the oxidant in the presence of catalytic amounts
Bi O
DMSO
THF
2
Bi O
2
Bi O
EtOH
2
Bi O
MeNO2
EtOAc
EtOAc
EtOAc
2
1
BiCl3
2
2
1
0
11
BiBr3
of AgNO . Our continued interest in catalytically active,
3
environmentally benign processes prompted us to investi-
1
2
Bi(NO ) ·5H O
3
3
2
a
Reaction conditions: aq 70% t-BuOOH (5 equiv), bismuth(III) salt
SYNTHESIS 2010, No. 21, pp 3736–3740
x
x.
x
x
.
2
0
1
0
(
10 mol%).
Advanced online publication: 17.08.2010
DOI: 10.1055/s-0030-1258221; Art ID: Z17610SS
Georg Thieme Verlag Stuttgart · New York
b
Reaction monitored by TLC until all the alcohol was consumed.
Isolated yield after column chromatography.
c
©

PAPER
Bi O -Catalyzed Oxidation of Alcohols with t-BuOOH
3737
2
3
when heated to reflux. Oxidation with aqueous t-BuOOH iphatic substrates (Table 2). The scope of our catalytic
alone in ethyl acetate was found to be negligible (<5%). In system was applicable to a wide range of aromatic, conju-
the presence of 5 mol% Bi O and five equivalents of gated, and aliphatic substrates. The primary alcohols were
2
3
aqueous 70% t-BuOOH as the oxidant in ethyl acetate, the converted into the corresponding carboxylic acids in good
reaction required 36 hours to reach completion with 75% isolated yields in reasonable times (Table 2, entries 1–16).
isolated yield of the product. In ethyl acetate, with It is pertinent to mention here that mild halogenic oxidants
1
3
14
1
0 mol% Bi O and five equivalents of aqueous 70% such as hypochlorites, chlorites, and N-bromosuccin-
2 3
1
5
t-BuOOH as the oxidant, the reaction went to completion imide (NBS) are not suitable for use with substrates with
in a much shorter time. With five equivalents of 5 M electron-rich aromatic rings, olefinic bonds, or secondary
t-BuOOH in decane, the reaction was found to reach com- hydroxy groups. Substitutions at different positions on the
pletion in 30 hours with 90% isolated yield. The reaction phenyl ring did not hinder the reaction, although the reac-
took much longer to reach completion (39 h) when per- tion time was affected. The catalytic system developed
formed with five equivalents of aqueous 30% H O in eth- here is mild and showed sufficient selectivity to enable the
2
2
yl acetate, and yielded 86% of product. Ethyl acetate expected oxidation to take place without affecting other
yielded the best results among the different solvents used functionalities such as phenol and amine groups (Table 2,
for optimization (Table 1, entries 1–9). Other bismuth(III) entries 7 and 8). Oxidation of a,b-unsaturated derivatives
salts (Table 1, entries 10–12) were not as effective as (Table 2, entry 15) resulted in the formation of the expect-
Bi O .
ed acid in very good yield. In addition, the transformation
of secondary alcohols into ketones was also extremely
facile, as indicated by entries 17–20 of Table 2.
2
3
Having established the optimal conditions for oxidation,
we continued our studies with a variety of aromatic and al-
a
Table 2 Bi O -Catalyzed Oxidation of Alcohols
2
3
Bi2O3 (10 mol%)
aq 70% t-BuOOH ( 5 equiv)
O
R1
OH
R2
R1
EtOAc
OH
Bi2O3 (10 mol%)
aq 70% t-BuOOH ( 5 equiv)
OH
O
R1
EtOAc
R1
R2
Entry
1
Alcohol
Product
Time (h)b
18
Yield (%)c
88
CH2OH
COOH
COOH
2
3
23
24
90
96
MeO
CH2OH
MeO
COOH
CH2OH
OMe
OMe
CH2OH
COOH
4
5
29
31
87
89
MeO
MeO
MeO
MeO
CH2OH
COOH
MeO
MeO
OMe
CH2OH
OMe
COOH
6
25
90
MeO
MeO
MeO
MeO
7
8
27
33
87
85
HO
CH2OH
HO
COOH
Me2N
CH2OH
2
Me N
COOH
Synthesis 2010, No. 21, 3736–3740 © Thieme Stuttgart · New York

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P. Malik, D. Chakraborty
PAPER
a
Table 2 Bi O -Catalyzed Oxidation of Alcohols (continued)
2
3
Bi2O3 (10 mol%)
aq 70% t-BuOOH ( 5 equiv)
O
R1
OH
R2
R1
EtOAc
OH
Bi2O3 (10 mol%)
aq 70% t-BuOOH ( 5 equiv)
OH
O
R1
EtOAc
R1
R2
Entry
9
Alcohol
Product
Time (h)b
23
Yield (%)c
88
86
Cl
Cl
CH2OH
Cl
COOH
COOH
CH2OH
Cl
Cl
1
1
1
0
1
2
28
50
48
Cl
CH2OH
NO2
COOH
NO2
82
85
CH2OH
COOH
O2N
2
O N
1
1
3
4
49
25
84
89
O2N
O
CH2OH
O2N
O
COOH
CH2OH
COOH
1
1
5
6
CH2OH
CH OH
COOH
COOH
25
55
87
83
Ph
Ph
2
OH
O
O
1
1
7
8
6
93
90
Ph
Ph
Ph
OH
Ph
Ph
Ph
17
OH
O
1
2
9
0
4
92
89
OH
O
22
O2N
O2N
a
b
c
Reactions performed in EtOAc with 10 mol% Bi O and aq 70% t-BuOOH (5 equiv), reflux.
Reaction monitored by TLC until all the alcohol was consumed.
Isolated yield after column chromatography.
2
3
Kinetic studies on the oxidation of (2-methoxyphe- The concentration of the alcohol decreases steadily, while
nyl)methanol, 3-phenylprop-2-en-1-ol, (4-nitrophe- that of the carboxylic acid increases. The concentration of
nyl)methanol, butanol, and di-4-tolylmethanol were the intermediate aldehyde increases, achieves a steady
explored next. High-pressure liquid chromatography state, and is then progressively converted into the acid.
(
HPLC) was used to determine the various starting mate- We have calculated the rate of such reactions. As an ex-
rials, products and aldehyde intermediates (for primary al- ample let us consider the conversion of butanol into butyr-
cohols) formed during the alcohol oxidation as a function ic acid, using the Van’t Hoff differential method to
of time. The concentration of reactant, intermediate and determine the order (n) and rate constant (k) (Figure 2).
product for the oxidation of butanol is shown in Figure 1.
Synthesis 2010, No. 21, 3736–3740 © Thieme Stuttgart · New York

PAPER
Bi O -Catalyzed Oxidation of Alcohols with t-BuOOH
3739
2
3
In summary, we have developed a simple, efficient,
chemoselective and inexpensive catalytic method for the
oxidation of primary alcohols to carboxylic acids and sec-
ondary alcohols to ketones using the bench-top reagent
Bi O . The overall method is green. It is noteworthy that
2
3
this method does not depend upon elaborate ligands or
other additives.
All substrates used in this study were purchased from Aldrich and
used as received. Solvents were purchased from Ranchem, India
1
13
and purified using standard methods. H and C NMR spectra of
samples in CDCl were recorded with a Bruker Avance 400 instru-
3
ment. Chemical shifts (d) were referenced to residual solvent reso-
nances and are reported as parts per million (ppm) relative to TMS.
HPLC analysis was performed with a Waters HPLC instrument fit-
ted with a Waters 515 pump and Waters 2487 dual l absorbance de-
tector.
General Procedure
To a stirred suspension of Bi O (46 mg, 0.1 mmol) and alcohol (1
2
3
Figure 1 Concentration versus time plot for the oxidation of buta-
nol with 10 mol% Bi O and aq 70% t-BuOOH (5 equiv) in EtOAc
under reflux
mmol) in EtOAc (2.5 mL) was added aq 70% t-BuOOH (0.95 mL,
mmol). The reaction mixture was heated to reflux and the progress
2
3
5
of the reaction was monitored by TLC until all the alcohol was con-
sumed. All the volatiles were removed and the crude product was
treated with sat. aq NaHCO (2 × 1 mL) and extracted with EtOAc
From Figure 1, the rate of the reaction at different concen-
trations can be estimated by evaluating the slope of the
tangent at each point on the curve corresponding to that of
3
(
2 mL). The aqueous layer was acidified with 2 M HCl and extract-
ed with EtOAc (2 × 2 mL). The organic layer was concentrated and
subjected to column chromatography. The spectral data for all com-
pounds were identical to those reported in the literature.
butanol. With these data, log (rate) versus log (concen-
1
0
10
tration) is plotted (Figure 2). The order (n) and rate con-
stant (k) is given by the slope of the line and its intercept
on the log (rate) axis, respectively. From Figure 2, it is
Supporting Information for this article is available online at
1
0
http://www.thieme-connect.com/ejournals/toc/synthesis.
clear that this reaction proceeds with second-order kinet-
–1 –1
ics (n = 2.08) and the rate constant k = 0.7006 L·mol h .
For the other substrates, namely (2-methoxyphenyl)meth-
anol, 3-phenylprop-2-en-1-ol, (4-nitrophenyl)methanol,
and di-4-tolylmethanol, the order of the reactions are also
n ≈ 2, with rate constants (k) of 0.9174, 0.8874, 0.7105,
Acknowledgment
This work was supported by the Department of Science and Tech-
nology, New Delhi. The services from the NMR facility, purchased
under the FIST program and sponsored by the Department of Sci-
ence and Technology, New Delhi, is gratefully acknowledged.
–
1 –1
and 1.293 L·mol h , respectively (see the Supporting In-
formation for details).
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Synthesis 2010, No. 21, 3736–3740 © Thieme Stuttgart · New York