Facile and selective oxidation of benzylic alcohols to their corresponding carbonyl compounds with sodium nitrate in the presence of Br?nsted acidic ionic liquids

Article abstract of DOI:10.1055/s-2007-973903

A chemoselective and effective procedure for the conversion of benzylic and allylic alcohols into the corresponding carbonyl compounds using sodium nitrate-3-methylimidazolinium hydrogensulfate is reported. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2007-973903

1
118
LETTER
Facile and Selective Oxidation of Benzylic Alcohols to Their Corresponding
Carbonyl Compounds with Sodium Nitrate in the Presence of Brønsted Acidic
Ionic Liquids
lcohols l R. Hajipour,*a,b F. Rafiee, A. E. Ruoho
a
b
F
A
acile andSelectiv
b
e
O
xidation
do f Benzylic
o
a
Pharmaceutical Research Laboratory, College of Chemistry, Isfahan University of Technology, Isfahan 84156, Iran
b
Department of Pharmacology, University of Wisconsin Medical School, 1300 University Avenue, Madison, WI 53706-1532, USA
E-mail: haji@cc.iut.ac.ir
Received 2 January 2007
tems of aqueous 20% nitric acid–nitromethane,12 nitric
acid–dichloromethane,13 nitric acid–ytterbium(III) tri-
Abstract: A chemoselective and effective procedure for the con-
version of benzylic and allylic alcohols into the corresponding
carbonyl compounds using sodium nitrate–3-methylimidazolinium
hydrogensulfate is reported.
14
flate–1,2-dichloroethane and sodium nitrate–p-toluene-
sulfonic acid15 have been successfully utilized for the
oxidation of alcohols. However, all of these oxidative
methods suffer from one or more drawbacks including the
use of toxic heavy metal reagents, long reaction times,
low yields and inconvenient work-up procedures. More-
over, most of the above reactions were conducted in
volatile and toxic organic solvents. Therefore, develop-
ment of a more facile and eco-friendly method for the
oxidation of alcohols to the corresponding carbonyl com-
pounds is still needed. In continuation of our ongoing pro-
Key words: Brønsted acidic ionic liquids, oxidation, alcohols, al-
dehydes, ketones
In recent years, ionic liquids have attracted intensive
interest as possible alternatives to traditional solvents for
organic reactions, particularly in the area of green chem-
istry. This is due to advantageous properties including
negligible vapor pressure and high thermal and chemical
1
,2
gram to develop efficient reagents for oxidizing organic
stability. An ever-increasing interest has focused on
ionic liquids as media for catalytic reactions where the
catalyst is in one phase and the product in another, this
makes the isolation of products easy and reuse of catalyst
convenient. They have also been referred to as ‘designer
solvents’ as their physical and chemical properties can be
compounds,1
3
6–20
in this paper Brønsted acidic ionic liquid
+
–
-methylimidazolinium hydrogensulfate([Hmim] HSO )
4
2
1
has been synthesized and used as both catalyst and
reaction medium in the oxidation of benzylic alcohols
with NaNO3.
+
–
4
adjusted by the careful choice of cation and anion. These Using Brønsted acidic ionic liquid [Hmim] HSO as cat-
compounds also exhibit acidic properties. The unique alyst afforded good to excellent yields in the conversion
properties of ionic liquids mean they are emerging as of benzylic alcohols into the corresponding carbonyl com-
green reaction media’. The use of ionic liquids as reac- pounds (Scheme 1).22
‘
tion medium may offer a convenient solution to both the
solvent-emission and catalytic-recycling problem.3
–5
Me
CH2OH
+
CHO
N
40 °C
Brønsted acidic ionic liquids have been successfully ap-
plied to a variety of reactions including the esterification
NaNO3
+
grinding
N
H
X
6
7
HSO4–
X
of carboxylic acids, protection of aldehydes and ketones
8
and cleavage of ethers.
Scheme 1
The transformation of alcohols to the corresponding car-
bonyl compounds is one of the most fundamental reac- The role of [Hmim] HSO4– in this reaction is both as the
tions in organic synthesis. Various methods have been acidic catalyst and also as the solvent. Use of such a reac-
developed for the oxidation of alcohols to the correspond- tion medium should be appreciated for its easy prepara-
ing carbonyl compounds. Representative oxidants for this tion, convenient separation and eco-friendly nature. The
+
9
purpose include using chromium(VI) reagents, transition ionic liquid can be reused after washing the catalyst with
1
0
metal nitrates on solid supports and dimethyl sulfoxides– diethyl ether to remove the unreacted starting materials
1
1
hydrogen bromide. Among other oxidation agents, nitric and products and dried under high vacuum.
acid has attracted much attention in the conversion of
The results for the oxidation of a variety of benzylic alco-
alcohols into carbonyl compounds mainly due to its ready
hols are summarized in Table 1. The oxidation of various
availability and strong oxidizing power. The reaction sys-
benzylic alcohols gave high yields of carbonyl com-
pounds in short reaction times. The competing over-oxi-
SYNLETT 2007, No. 7, pp 1118–1120
Advanced online publication: 13.04.2007
DOI: 10.1055/s-2007-973903; Art ID: G00407ST
2
4.
0
4.
2
0
0
7
dation of aldehydes to the corresponding carboxylic acids,
aromatic nitration or benzyl nitrate was not observed in
any of the cases studied using the present reaction condi-
tions.
©
Georg Thieme Verlag Stuttgart · New York

LETTER
Facile and Selective Oxidation of Benzylic Alcohols
1119
+
–
Table 1 Oxidation of Alcohols by NaNO –[Hmim] HSO
3
4
Entry
Substrate
Temperature (°C)
Time (min)
Yield (%)a
Mp or bp (°C)/torr
1
2
3
4
5
6
7
8
9
PhCH OH
40
40
40
40
40
40
50
50
50
40
40
40
40
40
80
80
70
60
60
60
5
5
95
97
95
97
95
92
92
87
97
90
94
97
97
97
60
50
70
–
176–178 (177–179)
33–35 (34–35)
100–102/10
2
2-OMeC H CH OH
6
4
2
3-OMeC H CH OH
5
6
4
2
4-OMeC H CH OH
4
245–247 (247–249)
43–45 (42–43)
47–49 (48–50)
209–212 (210–212)
210–212 (212–214)
46–48 (45–47)
106–108/15
6
4
2
3,4-(OMe) C H CH OH
6
2
6
4
2
2,5-(OMe) C H CH OH
6
2
6
4
2
2-ClC H CH OH
10
12
8
6
4
2
3-ClC H CH OH
6
4
2
4-ClC H CH OH
6
4
2
1
1
1
1
1
1
1
1
1
1
0
1
2
3
4
5
6
7
8
9
0
4-MeC H CH OH
7
6
4
2
2-OHC H CH OH
5
195–197 (196–197)
202–204 (202)
46–48 (47–49)
74–76(75–77)
105–107 (105–107)
55–57 (56–59)
94–96 (93–95)
48–50 (47–50)
197
6
4
2
C H CHOHMe
5
6
4
PhCHOHPh
7
4-ClC H CHOHPh
10
15
10
10
10
20
20
6
4
4-NO C H CH OH
2
6
4
2
3-NO C H CH OH
2
6
4
2
PhCHOHCOPh
PhCH CH OH
2
2
Cyclohexanol
Me(CH ) OH
–
2
–
159–162
2
3
a
Yields based on isolated pure products after chromatography and confirmed by comparison with authentic samples (TLC, IR and NMR).
Oxidation of benzylic alcohols using NaNO without
2 HNO2
NO2 + NO + H2O
3
+
–
[
Hmim] HSO was unsuccessful. We also performed the
4
Equation 1
oxidation of benzyl alcohol, 2-chlorobenzyl alcohol and
+
–
4
-methoxybenzylalcohol with NaNO –[Hmim] HSO at
3 4
We also prepared [Hmim] H PO and [Hmim] NO3– as
+
–
+
room temperature and observed that these reactions gave
lower yields and needed longer reaction times.
2
4
acidic ionic – liquids and used them instead of
+
[
Hmim] HSO4 in these reactions. We observed that in
The possible mechanism for oxidation of benzylic alco-
+
–
comparison to [Hmim] HSO the yields of the reactions
in the presence of [Hmim] H PO were less than 40%
+
–
4
hols by sodium nitrate in the presence of [Hmim] HSO4
+
–
2
4
is shown in Scheme 2.
and required longer reaction times (>15 min). This is
probably due to lower Brønsted acidity associated with di-
hydrogen phosphate. The [Hmim] NO3 does not have
any acidic hydrogens, which are needed to catalyze the
oxidation of alcohols.
+
+
–
OH
OH2
NO3–
[
Hmim]HSO4
O
H
O
N
In order to show the chemoselectivity of this method,
competitive reactions have been carried out. Electron-do-
nating substituents on the benzene ring of the benzylic
alcohols accelerate the reaction rate whereas electron-
withdrawing groups reduce the reaction rate dramatically
and also require higher temperatures. Aliphatic alcohols
were not oxidized using this system.
H
O
O
+
HNO2
Scheme 2
As shown in Equation 1, NO3– is converted into HNO ,
2
2
3
then decomposes to NO and NO gases.
2
Synlett 2007, No. 7, 1118–1120 © Thieme Stuttgart · New York

1
120
A. R. Hajipour et al.
LETTER
In conclusion, we have developed a simple and efficient
method for the oxidation of benzylic alcohols into their
Indian J. Chem., Sect. B: Org. Chem. Incl. Med. Chem.
999, 38, 461. (e) Hajipour, A. R.; Mallakpour, S. E.;
Imanzadeh, G. Chem. Lett. 1999, 99. (f) Hajipour, A. R.;
Hantehzadeh, M. J. Org. Chem. 1999, 64, 8475.
1
corresponding carbonyl compounds using NaNO in the
3
presence of a catalytic amount of the Brønsted acidic ionic
(
g) Hajipour, A. R.; Mallakpour, S. E.; Backnejad, H. Synth.
+
–
liquid [Hmim] HSO with high yields and short reaction
4
Commun. 2000, 30, 3855. (h) Hajipour, A. R.; Mallakpour,
S. E.; Afrousheh, A. Phosphorus, Sulfur Silicon Relat. Elem.
times.
2000, 160, 67. (i) Hajipour, A. R.; Mallakpour, S. E.; Khoee,
S. Chem. Lett. 2000, 120. (j) Hajipour, A. R.;
Mohammadpoor-Baltork, I.; Nikbaghat, K.; Imanzadeh, Gh.
Synth. Commun. 1999, 29, 1697.
Acknowledgment
We gratefully acknowledge support received for this project from
the Isfahan University of Technology (IUT), IR Iran (ARH) and
Grants GM 033138, MH 065503, NS 033650 (AER) from the
National Institutes of Health, USA. Further financial support from
the Center of Excellency in Chemistry Research (IUT) is gratefully
acknowledged.
(
17) Hajipour, A. R.; Mahboubkhah, N. J. Chem. Res., Synop.
1998, 122.
(
18) (a) Mohammadpoor-Baltork, I.; Hajipour, A. R.;
Mohammadi, H. Bull. Chem. Soc. Jpn. 1998, 16, 71.
(b) Hajipour, A. R.; Mahboubkhah, N. Synth. Commun.
1998, 28, 3143. (c) Hajipour, A. R.; Mohammadpoor-
Baltork, I.; Kianfar, G. Bull. Chem. Soc. Jpn. 1998, 71,
2655. (d) Hajipour, A. R.; Mohammadpoor-Baltork, I.;
Kianfar, G. Indian J. Chem., Sect. B: Org. Chem. Incl. Med.
Chem. 1998, 37B, 607. (e) Hajipour, A. R.; Mahboubkhah,
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(
(
(
(
(
(
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(
(
(
19) Hajipour, A. R.; Ruoho, A. E. Org. Prep. Proced. Int. 2005,
3
7, 279.
20) Hajipour, A. R.; Adibi, H.; Ruoho, A. E. J. Org. Chem. 2003,
8, 4553.
21) Preparation of [Hmim] HSO : 1-methylimidazole (0.82 g,
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6
7) Wu, H. H.; Yang, F.; Pg, C.; Tang, J.; He, M. Y. Tetrahedron
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+
–
4
0.01 mol) was placed in a two-necked flask with stirrer and
(
(
8) Driver, G.; Johnson, K. E. Green Chem. 2003, 5, 163.
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was cooled to 0 °C. MeCN (10 mL) was added and then
H SO (0.98 g, 0.01 mol) was added slowly with stirring.
2
4
7
40.
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The mixture was stirred for 30 min and the MeCN was
removed by simple decanting.
(
(
(
2
(
22) General procedure: In a mortar a mixture of benzylic alcohol
(
1.0 mmol), sodium nitrate (0.085 g, 1.0 mmol), and
+
–
[Hmim] HSO (0.25 mmol) was ground with a pestle. The
4
12) Gasparrini, F.; Giovannoli, M.; Misiti, D.; Natile, G.;
Palmieri, G. Synth. Commun. 1988, 18, 69.
reaction times and temperatures are shown in Table 1. After
completion of the reaction (monitored by TLC, EtOAc–
cyclohexane, 25:75), the mixture was cooled to r.t., extracted
into Et O and washed with H O. The organic layer was dried
(
(
13) Strazzolini, P.; Runcio, A. Eur. J. Org. Chem. 2003, 526.
14) Barrett, A. G. M.; Braddock, D. C.; McKinnell, R. M.;
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2
2
over Na SO and evaporated under reduced pressure.
2
4
(15) Lee, J. C.; Lee, J. Y.; Lee, J. M. Bull. Korean Chem. Soc.
Purification of the residue using flash column
chromatography (silica gel, EtOAc–cyclohexane, 20:80)
provided the pure carbonyl compounds.
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(
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Synlett 2007, No. 7, 1118–1120 © Thieme Stuttgart · New York

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