Selective aerobic oxidation of activated alcohols into acids or aldehydes in ionic liquids

Article abstract of DOI:10.1021/jo0707737

(Chemical Equation Presented) Selective aerobic oxidation of activated primary alcohols into acids or aldehydes has been developed in ionic liquids. Under optimal conditions, various alcohols could be selectively converted into their corresponding acids or aldehydes in good to excellent yields. The newly developed catalytic systems could also be recycled and reused for three runs without any significant loss of catalytic activity.

Full text of DOI:10.1021/jo0707737

On the other hand, ionic liquids, composed entirely of ions
with a melting point below 100 °C,7 have attracted increasing
interest in the effort of reduction or replacement of volatile
organic compounds (VOCs) from the reaction media in the area
of Green Chemistry focus.8 Although ionic liquids have been
recognized as alternative green chemistry reaction media because
of their unique properties, including low volatility, high polarity,
good stability over a wide temperature range, and selective
dissolving capacity by a proper choice of cation and anion, the
use of ionic liquids to immobilize and recycle homogeneous
catalysts has become one of the most fruitful areas of ionic
liquids research to date.9 In particular, the advantages of ionic
liquids as reaction media for transition-metal-catalyzed oxidation
have recently been recognized.10
Selective Aerobic Oxidation of Activated Alcohols
into Acids or Aldehydes in Ionic Liquids
Nan Jiang and Arthur J. Ragauskas*
School of Chemistry and Biochemistry, Georgia Institute of
Technology, Atlanta, Georgia 30332
arthur.ragauskas@chemistry.gatech.edu
ReceiVed April 12, 2007
Recently, we reported an efficient vanadium-catalyzed selec-
tive aerobic oxidation of alcohols into aldehydes and ketones
in the ionic liquid, [bmim]PF6.11 In light of our recent success
in the oxidation of primary alcohols into aldehydes, we turned
Selective aerobic oxidation of activated primary alcohols into
acids or aldehydes has been developed in ionic liquids. Under
optimal conditions, various alcohols could be selectively
converted into their corresponding acids or aldehydes in good
to excellent yields. The newly developed catalytic systems
could also be recycled and reused for three runs without any
significant loss of catalytic activity.
(4) For palladium-catalyzed aerobic alcohol oxidation, see: (a) Blackburn,
T. F.; Schwartz, J. Chem. Commun. 1977, 157. (b) Kaneda, K.; Fujii, M.;
Morioka, K. J. Org. Chem. 1996, 61, 4502. (c) Nishimura, T.; Onoue, T.;
Ohe, K.; Uemura, S. Tetrahedron Lett. 1998, 39, 6011. (d) Peterson, K. P.;
Larock, R. C. J. Org. Chem. 1998, 63, 3185. (e) ten Brink, G.-J.; Arends,
I. W. C. E.; Sheldon, R. A. Science 2000, 287, 1636. (f) Steinhoff, B. A.;
Fix, S. A.; Stahl, S. S. J. Am. Chem. Soc. 2002, 124, 766. (g) Schultz, M.
J.; Park, C. C.; Sigman, M. S. Chem. Commun. 2002, 3034. (h) Uozumi,
Y.; Nakao, R. Angew. Chem., Int. Ed. 2003, 42, 194. (i) Jensen, D. R.;
Schultz, M. J.; Mueller, J. A.; Sigman, M. S. Angew. Chem., Int. Ed. 2003,
42, 3810. (j) Iwasawa, T.; Tokunaga, M.; Obora, Y.; Tsuji, Y. J. Am. Chem.
Soc. 2004, 126, 6554. (k) Mueller, J. A.; Cowell, A.; Chandler, B. T.;
Sigman, M. S. J. Am. Chem. Soc. 2005, 127, 14817. (l) Hou, Z.; Theyssen,
N.; Brinkmann, A.; Leitner, W. Angew. Chem., Int. Ed. 2005, 44, 1346.
(m) Schultz, M. J.; Hamilton, S. S.; Jensen, D. R.; Sigman, Matthew, S. J.
Org. Chem. 2005, 70, 3343. (n) Steinhoff, B. A.; Stahl, S. S. J. Am. Chem.
Soc. 2006, 128, 4348. (o) Chen, T.; Jiang, J.-J.; Xu, Q.; Shi, M. Org. Lett.
2007, 9, 865. (p) Hou, Z.; Theyseeen, N.; Leitner, W. Green Chem. 2007,
9, 127.
(5) For ruthenium-catalyzed aerobic alcohol oxidation, see: (a) Matsu-
moto, M.; Watanabe, N. J. Org. Chem. 1984, 49, 3435. (b) Kaneda, K.;
Yamashita, T.; Matsushita, T.; Ebitani, K. J. Org. Chem. 1998, 63, 1750.
(c) Masutani, K.; Uchida, T.; Irie, R.; Katsuki, T. Tetrahedron Lett. 2000,
41, 5119. (d) Yamaguchi, K.; Mori, K.; Mizugaki, T.; Ebitani, K.; Kaneda,
K. J. Am. Chem. Soc. 2000, 122, 7144. (e) Dijksman, A.; Marino-Gonzalez,
A.; Mairata i Payeras, A.; Arends, I. W. C. E.; Sheldon, R. A. J. Am. Chem.
Soc. 2001, 123, 6826. (f) Pagliaro, M.; Ciriminna, R. Tetrahedron Lett.
2001, 42, 4511. (g) Yamaguchi, K.; Mizuno, N. Angew. Chem., Int. Ed.
2002, 41, 4538. (h) Musawir, M.; Davey, P. N.; Kelly, G.; Kozhevnikov,
I. V. Chem. Commun. 2003, 1414. (i) Shimizu, H.; Onitsuka, S.; Egami,
H.; Katsuki, T. J. Am. Chem. Soc. 2005, 127, 5396. (j) Egami, H.; Onitsuka,
S.; Katsuki, S., T. Tetrahedron Lett. 2005, 46, 6049.
Alcohol oxidation is an important transformation from the
viewpoint of organic synthesis and industrial manufacturing.1
Many oxidations of this type are carried out using stoichiometric
amounts of oxidizing reagents (i.e., KMnO4, MnO2, CrO3, Br2,
etc.)2 with considerable drawbacks such as high cost, waste
byproducts, and serious environmental issues. In comparison,
molecular oxygen may serve as superior oxidant that is of lower
cost, greater abundance, and improved safety. Furthermore, the
use of molecular oxygen as the primary oxidant may also have
the advantage that water is the sole final byproduct. Thus,
catalytic aerobic alcohol oxidation represents a promising
protocol for organic synthesis and industrial applications.
Accordingly, there has been concerted effort directed at
developing various transition metals (mainly copper,3 pal-
ladium,4 ruthenium,5 and vanadium6) to catalyze aerobic alcohol
oxidation.
* To whom correspondence should be addressed.
(1) (a) Hudlick, M. Oxidations in Organic Chemistry; American Chemical
Society: Washington, DC, 1990. (b) Larock, R. C. ComprehensiVe Organic
Transformations; Wiley: New York, 1999.
(2) March, J. AdVanced Organic Chemistry: Reactions, Mechanisms,
and Structure, 4ed.; John Wiley & Sons: New York, 1992.
(6) For vanadium-catalyzed aerobic alcohol oxidation, see: (a) Kirihara,
M.; Ochiai, Y.; Takizawa, S.; Takahata, H.; Nemoto, H. Chem. Commun.
1999, 1387. (b) Maeda, Y.; Kakiuchi, N.; Matsumura, S.; Nishimura, T.;
Kawamura, T.; Uemura, S. J. Org. Chem. 2002, 67, 6718. (c) Velusamy,
S.; Punniyamurthy, T. Org. Lett. 2004, 6, 217. (d) Figiel, P. J.; Sobczak,
M.; Ziolkowski, J. J. Chem. Commun. 2004, 244. (e) Reddy, S. R.; Das,
S.; Punniyamurthy, T. Tetrahedron Lett. 2004, 45, 3561. (f) Maeda, Y.;
Washitake, Y.; Nishimura, T.; Iwai, K.; Yamauchi, T.; Uemura, S.
Tetrahedron 2004, 60, 9031. (g) Radosevich, A. T.; Musich, C.; Toste, F.
D. J. Am. Chem. Soc. 2005, 127, 1090. (h) Lingaiah, N.; Reddy, K. M.;
Babu, N. S.; Rao, K. N.; Suryanarayana, I.; Prasad, P. S. S. Catal. Commun.
2006, 7, 245. (i) Vasylyev, M.; Neumann, R. Chem. Mater. 2006, 18, 2781.
(7) Welton, T. Chem. ReV. 1999, 99, 2071.
(3) For copper-catalyzed aerobic alcohol oxidation, see: (a) Semmelhack,
M. F.; Schmid, C. R.; Cortes, D. A.; Chou, C. S. J. Am. Chem. Soc. 1984,
106, 4283. (b) Marko, I. E.; Giles, P. R.; Tsukazaki, M.; Brown, S. M.;
Urch, C. J. Science 1996, 274, 2044. (c) Wang, Y.; DuBois, J. L.; Hedman,
B.; Hodgson, K. O.; Stack, T. D. P. Science 1998, 279, 537. (d) Chaudhuri,
P.; Hess, M.; Mueller, J.; Hildenbrand, K.; Bill, E.; Weyhermueller, T.;
Wieghardt, K. J. Am. Chem. Soc. 1999, 121, 9599. (e) Ansari, I. A.; Gree,
R. Org. Lett. 2002, 4, 1507. (f) Gamez, P.; Arends, I. W. C. E.; Reedijk, J.;
Sheldon, R. A. Chem. Commun. 2003, 2414. (g) Marko, I. E.; Gautier, A.;
Dumeunier, R.; Doda, K.; Philippart, F.; Brown, S. M.; Urch, C. J. Angew.
Chem., Int. Ed. 2004, 43, 1588. (h) Zaccheria, F.; Ravasio, N.; Psaro, R.;
Fusi, A. Chem. Commun. 2005, 253. (i) Jiang, N.; Ragauskas, A. J. Org.
Lett. 2005, 7, 3689. (j) Velusamy, S.; Srinivasan, A.; Punniyamurthy, T.
Tetrahedron Lett. 2006, 47, 923. (k) Striegler, S. Tetrahedron 2006, 62,
9109. (l) Jiang, N.; Ragauskas, A. J. J. Org. Chem. 2006, 71, 7087. (m)
Chung, C. W. Y.; Toy, P. H. J. Comb. Chem. 2007, 9, 115.
(8) Anasta, P. T.; Warner, J. C. Green Chemistry: Theory and Practice;
Oxford University Press: New York, 1998.
(9) For recent reviews, see: (a) Wassercheid, P.; Keim, W. Angew.
Chem., Int. Ed. 2000, 39, 3772. (b) Sheldon, R. A. Chem. Commun. 2001,
2399. (c) Dupont, J.; de Souza, R. F.; Suares, P. A. Z. Chem. ReV. 2002,
102, 3667. (d) Miao, W.; Chan, T. H. Acc. Chem. Res. 2006, 39, 897.
(10) Muzart, J. AdV. Synth. Catal. 2006, 348, 275.
(11) Jiang, N.; Ragauskas, A. J. Tetrahedron Lett. 2007, 48, 273.
10.1021/jo0707737 CCC: $37.00 © 2007 American Chemical Society
Published on Web 08/08/2007
7030
J. Org. Chem. 2007, 72, 7030-7033

SCHEME 1. Vanadium-Catalyzed Aerobic Alcohol
Oxidation
entry 7). Considering the significant effect of ionic liquids on
the reaction rate in our previous study,11 selected imidazolium-
type ionic liquid [hmim]OTf (1-hexyl-3-methylimidazolium
trifluoromethansulfonate), pyridinium-type ionic liquid [bmpy]-
PF6 (1-butyl-4-methylpyridinium hexafluorophosphate), and
pyrrolidinium-type ionic liquid [bmpyr]NTf2 (1-butyl-1-meth-
ylpyrrolidinium bis(trifluoromethylsulfonyl)imide) were also
tested to improve this transformation (Table 1, entries 8-10).
Ionic liquid [hmim]OTf proved to be the optimal, and 4-meth-
oxybenzoic acid was detected as the major product with 77%
isolated yield (Table 1, entry 10). It has to be noted that both
vanadyl acetylacetonate and copper (II) 2-ethylhexanoate as
catalysts are crucial for this oxidation of 4-methoxybenzyl
alcohol into 4-methoxybenzoic acid, and very low conversion
of 4-methoxybenzyl alcohol into acid (8% conversion after 48
h) in the absence of copper (II) 2-ethylhexanoate or no acid in
the absence of vanadyl acetylacetonate was detected by 1H NMR
analysis of the crude reaction mixtures (Table 1, entries 11 and
12).13 Moreover, no ester (4-methoxybenzyl 4-methoxybenzoate)
TABLE 1. Optimization of Aerobic Oxidation of 4-Methoxybenzyl
Alcohola,b
1
was detected by H NMR analysis in all the cases.
yield (%)d
ratioc
Subsequently, the catalytic systems were then applied to
various primary alcohols as summarized in Table 2. It is clear
that all the benzylic alcohols could be selectively oxidized into
aldehydes or acids in good to excellent isolated yields and
electronic variation on the aromatic substients did not diminish
the efficiency and selectivity (Table 2, entries 1-5). trans-
Cinnamyl alcohol was also selectively converted into the
corresponding aldehyde or acid with stereochemical retention
of the double bond (Table 2, entry 6). It has to be noted that
heteroatom-containing (N, S) substrates (2-pyridine methanol
and 2-thiophene methanol) turned out to be compatible with
the employed reaction conditions, and the corresponding alde-
hydes or acids were obtained in good yields (Table 2, entries 7
and 8). Similar to our previous study,11 the aliphatic alcohol,
3-phenyl-1-propanol, is unreactive to the present catalytic
systems, and longer reaction time and elevated temperature
failed to afford the corresponding aldehyde or acid (Table 2,
entry 9). 4-Methylbenzaldehyde can also be smoothly oxidized
into 4-methylbenzoic acid in the presence of VO(acac)2 and
copper (II) 2-ethylhexanoate in 12 h, but the absence of VO-
(acac)2 led to very low conversion of aldehyde into acid (Table
2, entry 10).14 Due to our special interest in oxidation of lignin-
like structures,15 we next examined the aerobic oxidation of two
lignin model compounds,16 3,4-dimethoxybenzyl alcohol and
1-(3,4-dimethoxyphenyl)ethanol. Although, 3,4-dimethoxyben-
entry
cocatalyst 2 mol %
none
CuBr
CuCl
CuBr2
Cu(OAc)2
Cu(acac)2
Cu(II) 2-ethylhexanoate [bmim]PF6
Cu(II) 2-ethylhexanoate [bmpy]PF6
Cu(II) 2-ethylhexanoate [bmpyr]NTf2
Cu(II) 2-ethylhexanoate [hmim]OTf
Cu(II) 2-ethylhexanoate [hmim]OTf
ILs
1a:2a:3a 2a
3a
1
2
3
4
5
6
7
8
[bmim]PF6
[bmim]PF6
[bmim]PF6
[bmim]PF6
[bmim]PF6
[bmim]PF6
0:99:1
0:90:10
0:97:3
0:91:9
0:85:5
0:95:5
90
0:74:26 65
0:89:11
0:66:34
0:13:87
94:6:0
16
77
9
10
11e
12f
none
[hmim]OTf
0:92:8
a Reaction conditions: 2 mmol 4-methoxybenzyl alcohol, 2 mol %
VO(acac)2, 2 mol % cocatalyst, 6 mol % DABCO, 1 atm O2, 0.30 g of
ionic liquid, 95 °C for 15 h. b No ester was detected by H NMR analysis
1
of the crude product mixture in all the cases. c The ratio is determined by
1H NMR analysis of the crude reaction mixture. d Isolated yield by flash
chromatography. e No VO(acac)2 was added. f The reaction was run for
48 h.
our attention to the more challenging oxidation of activated
primary alcohols into acids.12 Herein, we wish to report the
selective aerobic oxidation of activated primary alcohols into
acids or aldehydes with ionic liquids as the solvents.
While developing vanadium-catalyzed aerobic alcohol oxida-
tion, we found that copper salt, CuBr, as the cocatalyst could
lead to the overoxidized product 4-methoxybenzoic acid (see
Scheme 1). To explore the scope of this reaction, we examined
various copper complexes as the cocatalyst for this catalysis in
various ionic liquids.
In our initial optimization experiments, 4-methoxybenzyl
alcohol was used as the substrate in the presence of 2 mol %
VO(acac)2 and 6 mol % DABCO (1,4-diazabicyclo[2.2.2]octane)
at 95 °C for 15 h, and the results are summarized in Table 1.
After screening various copper complexes in [bmim]PF6,
copper(II) 2-ethylhexanoate was determined to be the most
effective cocatalyst to yield 4-methoxybenzoic acid (Table 1,
(13) It is noteworthy that the same oxidation proceeded much slower
(11% conversion of alcohol into acid after 12 h) when 2 mol % VO(acac)2
and 2 mol % Ni(acac)2 were employed as the catalysts, despite the fact
that Ni(acac)2 has been reported to catalyze the aerobic oxidation of various
aromatic aldehydes into acids in the ionic liquid [bmim]PF6, see: Howarth,
J. Tetrahedron Lett. 2000, 41, 6627.
(14) In the present catalytic system, copper (II) 2-ethylhexanoate could
work as the oxygen-activating cocatalyst, and the vanadium could be the
active catalyst for the further oxidation of aldehydes into acids, which could
be partly confirmed by the fact that copper (II) 2-ethylhexanoate only led
to very low conversion of 4-methylbenzaldehyde into 4-methylbenzoic acid
in the absence of VO(acac)2 (see Table 2, entry 10). It has to be mentioned
that copper salts have been widely reported as oxygen-activating cocata-
lysts: see (a) Tsuji, J. Synthesis 1984, 369. (b) Lorber, C. Y.; Smidt, S. P.;
Osborn, J. A. Eur. J. Inorg. Chem. 2000, 655. (c) Muldoon, J.; Brown, S.
N. Org. Lett. 2002, 4, 462.
(12) The benzylic or allylic alcohol was oxidized into the aldehyde in
high selectivity due to conjugation of the carbonyl group deactivating the
further oxidation of the intermediate aldehyde, see : van Bekkum, H.;
Lichtenthaler, F. W. Carbohydrates as Organic Raw Meterials; VCH:
Weinheim, 1990; p. 289.
(15) Ragauskas, A. J.; Williams, C. K.; Davison, B. H.; Britovsek, G.;
Cirney, J.; Eckert, C. A.; Frederick, W. J.; Hallett, J. P.; Leak, D. J.; Liotta,
C. L.; Mielenz, J. R.; Murphy, R.; Templer, R.; Tschaplinski, T. Science
2006, 311, 484.
J. Org. Chem, Vol. 72, No. 18, 2007 7031

TABLE 2. Aerobic Oxidation of Alcohols into Acids or Aldehydes in Ionic Liquidsg
a Reaction conditions A: 2 mmol alcohol, 2 mol % VO(acac)2, 6 mol % DABCO, 1 atm O2, 0.30 g of [bmim]PF6, 95 °C for the specific time; reaction
conditions B: 2 mmol alcohol, 2 mol % VO(acac)2, 2 mol % Cu(II) 2-ethylhexanoate, 6 mol % DABCO, 1 atm O2, 0.30 g of [hmim]OTf, 95 °C for the
specific time. b Isolated yield by flash chromatography. c Reaction was run in 100 mmol scale in 10.0 g of ionic liquid at 95 °C for the specific time. d 4
mol % VO(acac)2 was used. e The reaction was run at 110 °C. f No VO(acac)2 was added. g Determined by 1H NMR analysis of the crude reaction mixture.
zylic alcohol turned out to be readily oxidized into aldehyde in
good isolated yield, its oxidation into acid proved to be
inefficient with the acid only as minor product after 24 h (17%
isolated yield, Table 2, entry 11). Furthermore, 1-(3,4-dimethoxy-
phenyl)ethanol could be successfully oxidized into the ketone,
which coincided with our previous observation that secondary
benzylic and allylic alcohols could be oxidized into the
corresponding ketones.11
Next, we examined the recyclability of the two catalytic
systems for aerobic oxidation of benzyl alcohol in ionic liquids
(Table 3). The recovery and reuse of both the catalytic systems
was accomplished after full extraction of benzaldehyde or
benzoic acid with ether. It is important to stress that the catalytic
(16) It has to be mentioned that the lignin model compound, coniferyl
alcohol (4-hydroxy-3-methoxycinnamyl alcohol) has also been tested for
the aerobic oxidation without success.
7032 J. Org. Chem., Vol. 72, No. 18, 2007

TABLE 3. Recycling of the Catalytic Systems for the Aerobic
Oxidation of Benzyl Alcohol
systems were easy to handle and proved to be readily recyclable
for two additional runs with only slight drop in activity.
In conclusion, selective aerobic oxidation of activated primary
alcohols into acids or aldehydes has been achieved in ionic
liquids. The catalytic systems show good compatibility with
heteroatom-containing (S and N) substrates. Furthermore, both
copper complexes and the ionic liquids play crucial roles in
the product selectivity. Most importantly, the catalysts are easy
to handle and could be recycled and reused for three runs
without any significant loss of catalytic activity.
conditions B
7conditions A
8
PhCH2OH
PhCO H
PhCHO
2b
2
1b
3b
yield (%)b
run
1
time h
conditionsa
2b
3b
8
12
8
12
10
15
A
B
A
B
A
B
90
89
88
76
2
3
84
83
Experimental Section
General Procedure for Catalytic Aerobic Oxidation of Al-
cohols into Aldehydes in [bmim]PF6. Into a 16-mL vial was added
a mixture of alcohol (2 mmol), vanadyl acetylacetonate (VO(acac)2,
10.6 mg, 0.04 mmol), 1,4-diazabicyclo[2,2,2]octane (DABCO, 13.4
mg, 0.12 mmol), and 0.30 g [bmim]PF6. The vial was capped with
a rubber septum, and the reaction mixture was stirred at 95 °C under
1 atm O2 (around 3 bubble/s by a needle through the rubber septum
and vented through a mineral oil bubbler) for the specific time,
and then extracted with ethyl ether (3 × 3 mL). The combined
ether phase was concentrated in vacuo. The residue was purified
by flash chromatography (n-pentane/diethyl ether ) 8:1) to afford
aldehyde.
General Procedure for Catalytic Aerobic Oxidation of Al-
cohols into Acids in [hmim]OTf. Into a 16-mL vial was added a
mixture of alcohol (2 mmol), vanadyl acetylacetonate (VO(acac)2,
10.6 mg, 0.04 mmol), copper (II) 2-ethylhexanoate (14.0 mg, 0.04
mmol), 1,4-diazabicyclo[2,2,2]octane (DABCO, 13.4 mg, 0.12
mmol), and 0.30 g of [hmim]OTf. The vial was capped with a
rubber septum, and the reaction mixture was vigorously stirred at
95 °C under 1 atm O2 (around 3 bubble/s by a needle through the
rubber septum and vented through a mineral oil bubbler) for the
specific time, and then extracted with ethyl ether (3 × 5 mL). The
combined ether phase was concentrated in vacuo. The residue was
purified by flash chromatography (n-pentane/diethyl ether ) 2:1)
to afford the acid.
a Reaction conditions A: 2 mmol 4-benzyl alcohol, 2 mol % VO(acac)2,
6 mol % DABCO, 1 atm O2, 0.30 g of [bmim]PF6, 95 °C for the specific
time; reaction conditions B: 2 mmol benzyl alcohol, 2 mol % VO(acac)2,
2 mol % Cu(II) 2-ethylhexanoate, 6 mol % DABCO, 1 atm O2, 0.30 g of
[hmim]OTf, 95 °C for the specific time. b Isolated yield by flash chroma-
tography.
adding fresh benzyl alcohol (2 mmol) to the catalytic system under
the same experimental conditions for the specific reaction time (see
Table 3 for the reaction time).
General Procedure for Recycling of the Catalytic System for
Aerobic Oxidation of Benzyl Alcohol into Benzoic Acid. A
mixture of benzyl alcohol (216 mg, 2 mmol), vanadyl acetyl-
acetonate (VO(acac)2, 10.6 mg, 0.04 mmol), copper (II) 2-ethyl-
hexanoate (14.0 mg, 0.04 mmol), and 1,4-diazabicyclo[2,2,2]octane
(DABCO, 13.4 mg, 0.12 mmol) in 0.30 g ionic liquid [hmim]OTf
was vigorously stirred at 95 °C under 1 atm O2 (around 3 bubble/s
by a needle through the rubber septum and vented through a mineral
oil bubbler) for 12 h, and then extracted with ethyl ether (3 × 5
mL). The combined ether phase was concentrated in vacuo. The
residue was purified by flash chromatography to afford benzoic
acid. The next run was performed by adding fresh benzyl alcohol
(2 mmol) to the catalytic system under the same experimental
conditions for the specific reaction time (see Table 3 for the reaction
time).
General Procedure for Recycling of the Catalytic System for
Aerobic Oxidation of Benzyl Alcohol into Benzaldehyde. A
mixture of benzyl alcohol (216 mg, 2 mmol), vanadyl acetyl-
acetonate (VO(acac)2, 10.6 mg, 0.04 mmol), and 1,4-diaza-
bicyclo[2,2,2]octane (DABCO, 13.4 mg, 0.12 mmol) in 0.30 g of
ionic liquid [bmim]PF6 was stirred at 95 °C under 1 atm O2 (around
3 bubble/s by a needle through the rubber septum and vented
through a mineral oil bubbler) for 8 h, and then extracted with ethyl
ether (3 × 3 mL). The combined ether phase was concentrated in
vacuo. The residue was purified by flash chromatography to afford
benzaldehyde. The next run was performed by
Acknowledgment. This project was supported by the
National Research Initiative of the USDA Cooperative State
Research, Education and Extension Service, Grant Number
2003-35504-13620.
Supporting Information Available: 1H and 13C NMR data and
1H NMR spectra for all isolated products. This material is available
free of charge via the Internet at http://pubs.acs.org.
JO0707737
J. Org. Chem, Vol. 72, No. 18, 2007 7033