Vanadium-catalyzed selective aerobic alcohol oxidation in ionic liquid [bmim]PF6

Article abstract of DOI:10.1016/j.tetlet.2006.11.032

A selective aerobic oxidation of alcohols into the corresponding aldehydes or ketones was developed by using two-component system VO(acac)₂/DABCO in the ionic liquid [bmim]PF₆, and the catalysts can be recycled and reused for three runs without any significant loss of catalytic activity.

Full text of DOI:10.1016/j.tetlet.2006.11.032

Tetrahedron Letters 48 (2007) 273–276
Vanadium-catalyzed selective aerobic alcohol oxidation
in ionic liquid [bmim]PF6
Nan Jiang and Arthur J. Ragauskas*
School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332, USA
Received 26 July 2006; revised 5 November 2006; accepted 6 November 2006
Available online 28 November 2006
Abstract—A selective aerobic oxidation of alcohols into the corresponding aldehydes or ketones was developed by using two-com-
ponent system VO(acac)
any significant loss of catalytic activity.
2006 Elsevier Ltd. All rights reserved.
2
/DABCO in the ionic liquid [bmim]PF
6
, and the catalysts can be recycled and reused for three runs without
ꢀ
The catalytic conversion of alcohols to the correspond-
ing aldehydes or ketones is a fundamental transforma-
tion in both laboratory and industrial synthetic
for an ionic liquid can allow for fine-tuning of their mis-
cibility with water and organic solvents to simplify sep-
aration processes by liquid–liquid extraction, with either
aqueous or conventional organic solvents. Thus, ionic
liquids have not only been extensively studied in the last
1
chemistry. From economic and environmental perspec-
tives, the development of new catalytic oxidation sys-
tems with molecular oxygen as terminal oxidant is
particularly attractive. The use of molecular oxygen as
the primary oxidant has several benefits including low-
cost, safety, abundance, and water as the sole final
byproduct. Accordingly, many transition metals (mainly
1
1
few years as media for organic synthesis and catalysis,
but the use of ionic liquids to immobilize and recycle
homogeneous catalysts has been one of the most fruitful
areas of ionic liquids research to date. Moreover, many
1
2
ionic liquids have become commercially available.
2
3
4
5
copper, palladium, ruthenium, and vanadium ) have
been used as catalysts for aerobic alcohol oxidation.
However, many of these catalytic systems are performed
in aromatic, or halogenated hydrocarbon solvents.
Notwithstanding the advantages of ionic liquids as reac-
tion media for oxidative processes, their potential in
transition-metal catalyzed oxidation has yet to be widely
1
3
recognized.
On the other hand, much attention has been directed
toward the reduction or replacement of volatile organic
compounds (VOCs) from the reaction media in the
In continuation of our interest in exploring green oxida-
1
4
tion of alcohols in ionic liquids, we herein report a
vanadium-catalyzed selective aerobic oxidation of benz-
ylic and allylic alcohols to aldehydes or ketones with an
ionic liquid as the solvent. Furthermore, we demonstrate
that the catalytic system can be readily recycled and
reused for three runs, without any significant loss of
catalytic activity.
6
Green Chemistry focus area. A variety of environmen-
7
8
tally benign media, such as water, ionic liquids, fluor-
9
10
ous solvents, and supercritical fluids, have been
promoted as replacements to VOCs. Ionic liquids, com-
posed entirely of ions with low viscosities and melting
points typically below 100 ꢁC, have been recognized as
promising green solvents due to their unique properties,
including low volatility, high polarity, good stability
over a wide temperature range, and capacity to dissolve
various organic, inorganic, organometallic compounds.
Furthermore, the proper choice of cation and anion
The initial investigation started with aerobic oxidation of
4-methoxybenzyl alcohol. In the presence of 5 mol %
VO(acac) and 10 mol % Et N under 1 atm oxygen in
2
3
the ionic liquid [bmim]BF (1-butyl-3-methylimidazo-
4
lium tetrafluoroborate) at 80 ꢁC, 4-methoxybenzalde-
hyde was obtained in 37% conversion and 24% isolated
yield after 6 h of stirring (Scheme 1). Furthermore, no
overoxidized product (4-methoxybenzoic acid) was
6
Keywords: Aerobic alcohol oxidation; Ionic liquids; [bmim]PF .
*
Corresponding author. Tel.: +1 404 894 9701; fax: +1 404 894
778; e-mail: arthur.ragauskas@chemistry.gatech.edu
1
4
observed by H NMR of the crude reaction mixture.
0040-4039/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.11.032

274
N. Jiang, A. J. Ragauskas / Tetrahedron Letters 48 (2007) 273–276
Table 1. Optimization of the reaction conditions for oxidizing 4-
methoxybenzyl alcohol to 4-methoxybenzaldehyde
OMe
OMe
a
5
mol% VO(acac) , 10 mol% Et N
2
3
b
c
Entry Ionic liquid
Additive Convn./sel. (%) Yield (%)
[
bmim]BF , O , 80 °C, 6 h
4
2
1
2
3
4
5
6
7
8
9
[bmim]BF
[bm im]BF
4
Et
Et N
Et
Et
Et
Et
Pyridine 28/>99
DMAP
DBU
3
N
37/>99
31/>99
52/>99
59/>99
62/>99
64/>99
24
20
43
51
49
58
17
40
—
91
CH OH
CHO
2
2
4
3
[hmim]OTf
[bmpy]PF
[bmpyr]NTf
3
N
3
N
3
N
3
N
Scheme 1.
6
2
[bmim]PF
[bmim]PF
[bmim]PF
[bmim]PF
6
6
6
6
Further efforts were then focused on optimizing the
reaction conditions, and these results are summarized
in Table 1. At first, four imidazolium-type ionic liquids,
51/>99
90/45
[
bmim]BF , [bmim]PF (1-butyl-3-methylimidazolium
10
[bmim]PF6
DABCO 98/>99
4
6
hexafluorophosphate), [bm im]BF (1-butyl-2,3-dimethyl-
imidazolium tetrafluoroborate), and [hmim]OTf (1-
hexyl-3-methylimidazolium trifluoromethansulfonate),
a
b
c
2
4
Employing 2 mmol 4-methoxybenzyl alcohol, 5 mol % VO(acac)
2
,
1
0 mol % additive, 1 atm O , and 0.5 g ionic liquid at 80 ꢁC for 6 h.
2
1
Conversion and selectivity are determined by H NMR analysis of the
crude product mixture.
one pyridinium-type ionic liquid [bmpy]PF (1-butyl-4-
6
methylpyridinium hexafluorophosphate), and one pyr-
Isolated yield by flash chromatography.
a
Table 2. Aerobic oxidation of alcohols in [bmim]PF
6
b
c
Entry
1
Alcohols
Time (h)
6
Products
CHO
Convn. /yield (%)
CH OH
99/96
2
2
6
98/90
CH OH
CHO
CHO
2
CH OH
2
3
4
5
12
6
100/95
98/91
Cl
Cl
MeO
CH OH
MeO
CHO
2
O N
CH OH
12
6
O N
CHO
CHO
22/—
96/85
2
2
2
d
MeO
CH OH
MeO
2
6
7
8
9
12
12
96/94
94/82
99/91
100/87
MeO
MeO
MeO
HO
CH OH
MeO
CHO
2
HO
d
CH2OH
6
CHO
N
N
S
12
9
CH OH
CHO
S
2
1
1
1
0
1
2
CHO
CHO
98/86
14/—
Ph
Ph
OH
OH
Ph
Ph
d
24
OH
12
6
O
35/29
94/90
d
OH
O
O
d
1
1
3
4
6
100/87
Ph
Ph
d
24
<5/—
OH
a
b
c
2 6
Employing 2 mmol alcohol, 5 mol % VO(acac) , 10 mol % DABCO, and 0.5 g [bmim]PF at 80 ꢁC for the appropriate time.
1
Conversion is determined by H NMR analysis of the crude product mixture.
Isolated yield by flash chromatography.
The reaction was carried out at 100 ꢁC.
d

N. Jiang, A. J. Ragauskas / Tetrahedron Letters 48 (2007) 273–276
275
rolidinium-type ionic liquid [bmpyr]NTf2 (1-butyl-1-
methylpyrrolidinium bis(trifluoromethylsulfonyl)imide)
Table 3. Recycling of the catalytic system for the aerobic oxidation of
a,b
4
-methoxybenzyl alcohol into aldehyde
were tested with VO(acac) as the catalyst and Et N as
2
3
OMe
OMe
CHO
the additive. It is clear that all the ionic liquids gave
comparable conversions and isolated yields. In addition,
hydrophobic ionic liquids gave better conversion, and
5 mol% VO(acac) , 10 mol% DABCO
2
[
bmim]PF , O , 80 °C
6
2
CH OH
2
[
bmim]PF showed to be optimal (Table 1, entries 1–
6
6
). Next, select additives including, pyridine, DMAP
c
d
Run
Time (h)
Conversion (%)
Yield (%)
(
4-N,N-dimethylaminopyridine), DBU (1,8-diazabicyclo-
1
2
3
6
6
9
98
93
97
91
87
92
[
[
5.4.0]undec-7-ene), and DABCO (1,4-diazabicyclo-
2.2.2]octane) were screened to improve this trans-
formation. While DBU was found to decrease the
a
1
5
Employing 2 mmol 4-methoxybenzyl alcohol, 5 mol % VO(acac)
0 mol % DABCO, and 0.5 g [bmim]PF at 80 ꢁC for the specific
time.
2
,
selectivity significantly, the others gave excellent selec-
tivity toward the aldehyde (Table 1, entries 6–10). DAB-
CO proves to be the best with 98% conversion and 91%
isolated yield.
1
6
b
c
1
Selectivity is over 99% determined by H NMR analysis of the crude
product mixture.
Conversion is determined by H NMR analysis of the crude product
1
Subsequently, the catalytic system16 was then applied to
various benzylic, allylic, and aliphatic alcohols, as sum-
marized in Table 2. It is clear that all primary benzylic
and allylic alcohols were selectively oxidized into the
corresponding aldehydes in excellent conversions and
yields (Table 2, entries 1–10). In addition, conversion
of electron-rich and electron-neutral benzylic alcohols
is faster, and more efficient, with over 98% conversions
mixture.
d
Isolated yield by flash chromatography.
heteroatom-containing (S, N) compounds. Most impor-
tantly, the newly developed catalytic system could also
be recycled and reused for three runs without any signi-
ficant loss of catalytic activity.
(
Table 2, entries 1–4). However, electron-deficient benz-
ylic alcohol was less reactive, and an elevated tempera-
ture was needed to reach good conversion (Table 2,
entry 5). The two lignin model compounds (Table 2,
entries 6 and 7) could be selectively oxidized into the
corresponding aldehydes in excellent conversions and
yields which is relevant to our special interest in the
Acknowledgments
This project was supported by the National Research
Initiative of the USDA Cooperative State Research,
Education and Extension Service, Grant Number
2003-35504-13620.
1
7
oxidation of lignin-like structures. It should also be
noted that the heteroatom-containing (S, N) substrates
2
-pyridine methanol and 2-thiophene methanol, could
References and notes
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1
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16. A general procedure for aerobic oxidation of 4-methoxy-
benzylic alcohol follows: To a solution of 4-methoxybenzyl
alcohol (276 mg, 2 mmol) in 0.5 g ionic liquid [bmim]PF
were added VO(acac) (26.5 mg, 5 mol %) and DABCO
(22.4 mg, 10 mol %). The reaction mixture was stirred at
(1 atm) for 6 h, and then extracted with n-
6
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2
pentane (5 · 6 mL). The combined n-pentane phase was
1
6
concentrated in vacuo. The residue was subjected to H
NMR analysis, and then purified by flash chromatography
(n-pentane/diethyl ether = 8:1) to afford 4-methoxybenz-
aldehyde (colorless oil, 247 mg, yield 91%).
7
2
005, 105, 3095.
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18. It has to be noted that both VO(acac)
2
and DABCO are
1
0. (a) Leitner, W. Top. Curr. Chem. 1999, 206, 107; (b)
insoluble in n-pentane and [bmim]PF
n-pentane.
6
is immiscible with
Patrick, H. R.; Griffith, K.; Liotta, C. L.; Eckert, C. A.;