Methyltrioxorhenium and sodium bromide-catalyzed oxidation of alcohols to carbonyl compounds with H2O2 using 1-butyl-3- methylimidazolium tetrafluoroborate ionic liquid as a novel recyclable green solvent

Article abstract of DOI:10.1246/bcsj.79.1601

A variety of primary and secondary alcohols have been oxidized to the corresponding aldehydes and ketones in excellent yields with aqueous 30% hydrogen peroxide by using methyltrioxorhenium and sodium bromide as a catalyst system and the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim]BF₄) as a recyclable green solvent.

Full text of DOI:10.1246/bcsj.79.1601

Short Articles
Bull. Chem. Soc. Jpn. Vol. 79, No. 10, 1601–1603 (2006) 1601
ity towards a range of organic and inorganic compounds, ease
of recovery and recyclability, have proven to be excellent
reaction media in organic synthesis.12 In this context various
oxidation reactions, such as epoxidation of alkenes,13 oxida-
tion of aromatic aldehydes,14 and Baeyer–Villiger oxidation,15
have been reported in the literature using MTO–H2O2 system
in ionic liquids. We, herein, report for the first time the use of
the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate
([bmim]BF4) as an efficient, recyclable green solvent for the
oxidation of primary and secondary alcohols to their corre-
sponding aldehydes and ketones with aqueous 30% hydrogen
peroxide using a methyltrioxorhenium (MTO) and NaBr cata-
lytic system (Scheme 1).
Methyltrioxorhenium and Sodium
Bromide-Catalyzed Oxidation of
Alcohols to Carbonyl Compounds
with H2O2 Using 1-Butyl-3-methyl-
imidazolium Tetrafluoroborate
Ionic Liquid as a Novel
Recyclable Green Solvent
A variety of primary and secondary alcohols were selective-
ly oxidized to the corresponding aldehydes and ketones using
the MTO and NaBr catalytic system with aq 30% hydrogen
peroxide in [bmim]BF4 at room temperature. The results are
summarized in Table 1. Among the various alcohols studied,
benzoins were found to be most reactive and required shorter
reaction times for oxidation. Furthermore, aromatic substituted
secondary alcohols were found to be more reactive as com-
pared to the aliphatic/alicyclic alcohols. In the case of primary
alcohols, aliphatic alcohols were found to be less reactive than
benzylic alcohols and gave poor yields of the corresponding
Ã
Suman L. Jain, Vishal B. Sharma, and Bir Sain
Chemical and Biotechnology Division, Indian Institute
of Petroleum, Dehradun-248005, India
Received January 26, 2006; E-mail: birsain@iip.res.in
A variety of primary and secondary alcohols have been
oxidized to the corresponding aldehydes and ketones in
excellent yields with aqueous 30% hydrogen peroxide by
using methyltrioxorhenium and sodium bromide as a catalyst
system and the ionic liquid 1-butyl-3-methylimidazolium
tetrafluoroborate ([bmim]BF4) as a recyclable green solvent.
R1
R2
R1
CH
MTO, NaBr (0.05 mmol, H2O2)
[bmim]BF4, r.t.
OH
C
O
RCH2OH
RCHO
R2
Scheme 1.
The development of new synthetic methodologies using
room temperature ionic liquids as environment friendly recy-
clable solvents has received considerable interest in recent
years due to their unique properties like non-volatile nature,
good solvating capability, high stability in air/moisture, negli-
gible vapor pressure and ability to dissolve a wide range of
organic and inorganic compounds.1 Furthermore, their hydro-
phobicity/hydrophilicity can be easily altered by the modifica-
tion of the cation and anion. Room temperature ionic liquids
have been widely used as solvents in various organic reactions
such as hydrogenation,2 hydroformylation,3 carbon–carbon
bond forming,4 Diels–Alder,5 Heck,6 Friedel–Crafts reactions,7
and oligomerization.8 However, there are only a few reports on
the use of ionic liquids as solvents in the catalytic oxidation
reactions of organic compounds.9
Table 1. Oxidation of Alcohols to Aldehydes and Ketonesa)
Entry
Substrate
Product
Reaction Yield
time/h /%b)
1
2
3
Ph2CHOH
PhCH(OH)Me
Cyclohexanol
Ph2CO
PhCOMe
Cyclohexanone
3.5
4.0
3.0
95
92
80
O
4
5.0
72
OH
5
5.5
70
OH
O
O
OH
The oxidation of alcohols to carbonyl compounds is an im-
portant synthetic transformation both from fundamental and
academic points of view.10 Apart from the use of various con-
ventional oxidants like KMnO4, MnO2, CrO3, SeO2, which
produce copious amounts of undesirable wastes, a variety of
effective catalytic methods using molecular oxygen/hydrogen
peroxide as enviro-economic oxidants with transition-metal-
based catalysts have been reported in the literature for this
transformation.11 The reported catalytic oxidation methods
for the oxidation of alcohols to carbonyl compounds employ
volatile organic compounds (VOCs) like aromatic and halo-
genated hydrocarbons as solvents, which are not only toxic
and/or hazardous but also contribute to serious environmental
problems. Ionic liquids due to their inherent properties, such as
non-volatile nature, high thermal stability, high solvating abil-
6
3.0
85
t
t
Bu
Bu
7
8
9
10
Cyclopentanone
PhCH2OH
p-Me(C6H4)CH2OH p-Me(C6H4)CHO
PhCH(OH)COPh PhCOCOPh
Cyclopentanone
PhCHO
6.0
3.5
3.5
1.5
70
65
75
96
MeO
CH
OH
C
O
OMe
OMe
C
O
C
O
MeO
11
1.5
6.0
95
12 CH3(CH2)2CH2OH CH3(CH2)2CHO
55c)
a) Reaction conditions: alcohol (1 mmol), 30% H2O2 (2
mmol), MTO (0.01 mmol), NaBr (0.05 mmol) in [bmim]BF4
(1 mL) at room temperature under atmospheric pressure.
b) Isolated yields. c) The conversion of n-butanol to n-butanal
was estimated by GC.
Published on the web October 10, 2006; doi:10.1246/bcsj.79.1601

1602 Bull. Chem. Soc. Jpn. Vol. 79, No. 10 (2006)
Ó 2006 The Chemical Society of Japan
Table 2. Effect of Various Solvents on the Oxidation of
Benzhydrol
Me
Re
Me
Re
O
Me
Re
O
O
O
O
O
O
+H2O2
+H2O2
O
O
Yield/%a)
O
O
O
A
Entry
Solvent
Reaction
time/h
B
A or B
H2O
NaBr
NaOBr (OBr)
1
2
3
4
5
Acetonitrile
1,2-Dichloroethane
Dichloromethane
[bmim]BF4
6.5
10
10
3.5
5.0
80
50
40
95
55
+
NaOBr
+
NaOH
HOBr
1
R1
R1
R
Neat
+
HOBr
CHOH
CH OBr
C O
2
2
2
R
R
R
HBr
a) Isolated yields.
Scheme 2.
aldehydes. The effect of bromide was also explored for this
reaction using benzhydrol as the substrate. The oxidation of
benzhydrol (1 mmol) when carried out with aq 30% hydrogen
peroxide (2 mmol) in presence of catalytic amount of methyl-
trioxorhenium (MTO, 0.01 mmol) in [bmim]BF4 at room tem-
perature gave only 20% yield of the benzophenone after 10 h.
However, the same amount of benzophenone was produced
within 3.5 h in the presence of a catalytic amount of bromide.
The use of chloride (NaCl) as a co-catalyst in place of the bro-
mide afforded a poor yield of the benzophenone and required
longer reaction time for the oxidation. The oxidized product
was separated from the reaction mixture by simple extraction
with diethyl ether, and the ionic liquid was used as such for
subsequent experiments after adding fresh substrate and oxi-
dant (aqueous 30% H2O2) under similar reaction conditions.
The benzophenone was isolated in the same yield without fur-
ther addition of either CH3ReO3 or sodium bromide. The cat-
alysis could be repeated up to three times, indicating the reus-
ability and recyclability of both the catalyst and ionic liquid.
The oxidation of benzhydrol using acetonitrile, dichloro-
ethane, dichloromethane, and the ionic liquid as solvent was
studied to evaluate their relative efficiencies, and the results
are summarized in Table 2. Among the different solvents stud-
ied, [bmim]BF4 was found to be the best both in terms of
shorter reaction time and better yield of the product. The effect
of temperature was also examined for the oxidation of benzhy-
drol by carrying out the oxidation at 80 ꢀC under similar reac-
tion conditions. The temperature showed only marginal effects
upon the rate of reaction.
was monitored by TLC (SiO2). After completion, the reaction
mixture was extracted with diethyl ether. The ether layer was
washed with water (two times) and dried over anhydrous MgSO4.
Evaporation of the ether gave a crude mixture, which was purified
by column chromatography (hexane:ethyl acetate = 9:1). The sol-
vent was evaporated under reduced pressure to afford pure benzo-
phenone (95%, 0.17 g). Other alcohols were similarly oxidized
and their reaction times and yields are summarized in Table 1.
We are thankful to the Director IIP for his kind permission
to publish these results. Suman L. Jain and Vishal B. Sharma
are thankful to CSIR, New Delhi for the award of Research
Fellowships.
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