Recyclable 2nd generation ionic liquids as green solvents for the oxidation of alcohols with hypervalent iodine reagents

Article abstract of DOI:10.1016/j.tet.2003.12.056

Alcohols undergo smooth oxidation with iodoxybenzoic acid (IBX) or with Dess-Martin-Periodinane (DMP) in hydrophilic [bmim]BF₄ and hydrophobic [bmim]PF₆ ionic liquids at room temperature under mild conditions to afford the corresponding carbonyl compounds in excellent yields with high selectivity. IBX and DMP promoted oxidations are faster in ionic liquids when compared to conventional solvents such as DMSO, DMF, EtOAc and H₂O. The recovery of the byproduct iodosobenzoic acid (IBA) is especially simple in ionic liquids. The recovered ionic liquids can be recycled in subsequent reactions with consistent activity.

Full text of DOI:10.1016/j.tet.2003.12.056

Tetrahedron 60 (2004) 2131–2135
Recyclable 2nd generation ionic liquids as green solvents for the
oxidation of alcohols with hypervalent iodine reagents
*
J. S. Yadav, B. V. S. Reddy, A. K. Basak and A. Venkat Narsaiah
Division of Organic Chemistry, Indian Institute of Chemical Technology, Hyderabad 500007, India
Received 7 October 2003; revised 2 December 2003; accepted 18 December 2003
Abstract—Alcohols undergo smooth oxidation with iodoxybenzoic acid (IBX) or with Dess–Martin-Periodinane (DMP) in hydrophilic
[bmim]BF4 and hydrophobic [bmim]PF6 ionic liquids at room temperature under mild conditions to afford the corresponding carbonyl
compounds in excellent yields with high selectivity. IBX and DMP promoted oxidations are faster in ionic liquids when compared to
conventional solvents such as DMSO, DMF, EtOAc and H2O. The recovery of the byproduct iodosobenzoic acid (IBA) is especially simple
in ionic liquids. The recovered ionic liquids can be recycled in subsequent reactions with consistent activity.
q 2004 Elsevier Ltd. All rights reserved.
1. Introduction
made to recycle them, thereby making the process more
convenient, economic and eco-friendly. Recent demand for
eco-friendly chemical processes has led to the development
of several clean and practical oxidations and still awaits
further improvements towards high-yielding, clean, safe and
efficient methods for the oxidation of alcohols.
Hypervalent iodine reagents have attracted increasing
interest as oxidants in organic synthesis due to their mild,
selective and environmentally benign oxidizing properties.1
Among various hypervalent iodine reagents, IBX is a
versatile oxidizing agent because of its high efficiency, easy
availability, mild reaction conditions, and its stability
against moisture and air.2 The wide functional group
tolerance and high-yielding reactions without over oxi-
dation have made IBX very familiar for the oxidation of
primary alcohols.3 IBX also oxidizes vic-diols to
a-diketones without cleaving the glycol C–C bond4 and
allows the selective oxidation of 1,4-diols to g-lactols.5
Recently, IBX has been employed as an efficient oxidizing
agent in DMSO for the clean oxidation of alcohols to
carbonyl compounds even in the presence of thioethers and
amino compounds.6 Subsequently, polymer-supported IBX
reagents have also been developed to promote oxidation
reactions.7 In recent reports, the use of IBX as a mild
oxidant has been extended to many other elegant oxidative
transformations.8 More recently, practical IBX oxidations
have been reported in organic solvents such as acetone,
ethyl acetate, chloroform, benzene and acetonitrile.9
However, IBX oxidations in organic solvents typically
require longer reaction times at high temperature to
accomplish the reaction. The high temperature reaction
conditions are not only detrimental to certain functional
groups, but also to the control of chemoselectivity. Owing to
the volatile nature of organic solvents, no attempt has been
In recent times, ionic liquids have gained recognition as
possible environmentally benign alternatives to the more
volatile organic solvents.10 Ionic liquids possess many
attractive properties, such as wide liquid range, negligible
vapor pressure, high thermal stability and good solvating
ability for a wide range of substrates and catalysts, which
alleviate some of the environmental issues. Their non-
volatile nature can reduce the emission of toxic organic
compounds and facilitate the separation of products and/or
catalysts from the reaction solvents. Furthermore, ionic
liquids are found to be an efficient reaction media for the
immobilization of transition metal based catalysts, Lewis
acids and enzymes.11 The hallmark of such ionic liquids is
the ability to alter their properties as desired by manipulat-
ing their structure with respect to the choice of organic
cation or anion and side chain attached to the organic cation
(Fig. 1).
These structural variations offer flexibility to the chemist to
Keywords: Dess–Martin-Periodinane; Iodoxybenzoic acid; Iodosobenzoic
acid.
*
Corresponding author. Tel.: þ91-402-7193434; fax: þ91-40-27160512;
e-mail address: yadav@iict.ap.nic.in
Figure 1.
0040–4020/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2003.12.056

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J. S. Yadav et al. / Tetrahedron 60 (2004) 2131–2135
Table 1. IBX- and DMP-promoted oxidation of alcohols in ionic liquids
devise the most idealized solvent, catering for the needs of
any particular process. Their unprecedented ability to
solvate a broad spectrum of substrates of organic and
inorganic nature has widened the horizon of their
applicability.12
Entry
Alcohol
Aldehydea
IBX
DMP
1
2
Time Yield Time Yield
(%)b
(h)
(h)
(%)b
a
3.5
97 2.5
90 4.0
95
2. Results and discussions
b
6.5
3.0
93
97
With an ever increasing quest for the exploration of newer
reactions in ionic liquids, herein we wish to report, for the
first time, the use of ionic liquids as novel and recyclable
polar reaction media for hypervalent iodine reagent
promoted oxidation reactions of alcohols (Scheme 1).
c
95 2.0
d
e
4.0
3.5
91 3.0
93 2.5
82c
91
f
5.0
92 3.0
96
g
h
3.5
4.5
95 2.5
94 3.0
89
90
Scheme 1.
For instance treatment of benzyl alcohol with IBX in
hydrophilic [bmim]BF4 ionic liquid afforded benzaldehyde
in 91% yield. The oxidation is very clean and complete
within 4.0 h at room temperature. In a similar manner,
various primary and secondary alcohols underwent smooth
oxidation with IBX to give the corresponding aldehydes and
ketones in high yields. In all cases, the reactions proceeded
readily at room temperature with high efficiency. The
oxidation of chiral primary alcohols proceeds without
epimerization (entry p Table 1). a,b-Unsaturated alcohols
also oxidized to the corresponding carbonyls in high yields
(entries e, h, Table 1). Tertiary alcohols did not undergo
oxidation under these conditions. No over oxidation of
aldehydes to acids was observed in the case of the oxidation
of primary alcohols. The results obtained with alcohols
prompted us to extend this process to the oxidation of vic-
diols. 1,2-Diols such as styrene diol and 1,2-diphenyl
ethane-1,2-diol (pinacol) underwent smooth oxidation with
IBX to the corresponding 1,2-diketones without the
cleavage of the glycol C–C bond (Scheme 2).
i
5.5
6.0
4.5
4.0
6.0
92 3.5
95 4.0
93 3.5
91 3.5
86 4.5
87
85
96
95
89
j
k
l
m
n
6.5
87 5.0
91
Arylcarbinols gave comparatively better yields than ali-
phatic alcohols. This method is highly selective to oxidize
secondary alcohols in the presence of primary alcohols
especially in case of styrene diol (entry i, Table 1). The
method is very mild and compatible with a wide range of
functional groups such as methoxy, methylenedioxy,
phenoxy ethers, carbamates and acetonides present in the
substrate. IBX shows enhanced reactivity and selectivity in
ionic liquids compared to organic solvents. For instance,
treatment of 1-phenyl ethanol with IBX in [bmim]BF4 at
room temperature for 4.5 h afforded acetophenone in 93%
yield whereas the same reaction in refluxing chloroform or
in refluxing ethyl acetate after 8.0 h gave the desired ketone
in 75 and 79% yields, respectively. In these organic
solvents, high temperature reaction conditions and longer
reaction times are typical to achieve comparable yields to
those obtained in ionic liquids at room temperature.
Lowering the reaction temperature was detrimental to the
o
p
7.5
5.5
89 5.0
87 4.0
93
90
a
1
All products were charaterized by H NMR, IR and mass spectroscopy.
Yield refers to pure products after chromatography.
Fifteen percent of benzoic acid was isolated.
b
c
efficiency of this procedure. Although, IBX promoted
oxidations proceed smoothly at room temperature in
DMSO, the reactions typically require long reaction times
(12–15 h) to obtain the products in good yields. Since the
products are fairly soluble in hydrophilic [bmim]BF4 ionic
liquid, they can be easily separated by simple extraction

J. S. Yadav et al. / Tetrahedron 60 (2004) 2131–2135
2133
4. Experimental
4.1. General methods
Melting points were recorded on Buchi R-535 apparatus and
are uncorrected. IR spectra were recorded on a Perkin–
Elmer FT-IR 240-c spectrophotometer using KBr optics. 1H
and 13C NMR spectra were recorded on Gemini-200
spectrometer in CDCl3 using TMS as internal standard.
Mass spectra were recorded on a Finnigan MAT 1020 mass
spectrometer operating at 70 eV. IBX and DMP were
prepared according to the reported procedure in the
literature.13
Scheme 2.
with diethyl ether. Then the rest of the ionic liquid was
diluted with water and filtered to recover the byproduct
iodosobenzoic acid (IBA). The recovered IBA was
reoxidized to IBX and could be reused in subsequent
reactions. The regenerated IBX is indistinguishable from the
freshly prepared one from o-iodobenzoic acid. The aqueous
phase was lyophilized to recover the ionic liquid. The
recovered ionic liquid was recycled in subsequent reactions
with consistent activity. The products were obtained of the
same purity as in the first run, and no decrease in yield was
obtained in runs carried out using recycled ionic liquid. For
instance, treatment of cinnamyl alcohol with IBX in
hydrophilic [bmim]BF4 ionic liquid gave 93, 92 and 93%
yields over three cycles. Although, similar results were also
obtained in hydrophobic [bmim]PF6, the recovery of IBA is
especially simple in [bmim]BF4 due to its hydrophilic
nature. The scope and generality of this process is illustrated
with respect to various alcohols and IBX and the results are
presented in the Table 1. Furthermore, Dess–Martin-
Periodinane (DMP) was also immobilized in ionic liquids
to perform oxidation reactions under mild conditions
(Scheme 3).
4.2. General procedure for the oxidation of alcohols
To a stirred solution of alcohol (1 mmol) in 1-butyl-3-
methylimidazolium tetrafluoroborate ([bmim]BF4, Fluka,
2 mL) ionic liquid was added IBX (1.2 mmol) or DMP
(1.0 mmol) at room temperature and stirring was continued
for the appropriate time (Table 1). After completion of the
reaction, as indicated by TLC, the product was extracted
with diethyl ether (3£10 mL). The combined ether extracts
were concentrated in vacuo and the resulting product was
directly charged on a small silica gel column and eluted with
a mixture of ethyl acetate–n-hexane (2:8) to afford the pure
carbonyl compound. The rest of the ionic liquid was diluted
with water and filtered to remove the IBA. The aqueous
phase was lyophilized to recover the ionic liquid. The
products were characterized by comparison of their NMR,
IR, Mass, TLC, mixed TLC analysis and physical data with
authentic samples. The spectral data of all the products were
identical with those of authentic samples.14 Spectroscopic
data for selected products is as follows.
4.2.1. 2a: 3,4-(Methylenedioxy)-benzaldehyde. Low melt-
ing solid, mp 35–37 8C, (lit.14e 36–37 8C), IR (KBr): n
3074, 2849, 1704, 1600, 1548, 1436, 1229, 1116, 1083, 912,
860, 781 cm21. 1H NMR (CDCl3, 400 MHz): d 6.01 (s, 2H),
6.82–7.50 (m, 3H), 9.81 (s, 1H). 13C NMR (CDCl3,
100 MHz, proton decoupled): d 102.3, 105.2, 108.0,
128.2, 131.7, 148.5, 153.6, 190.2. EIMS: m/z (%): 150
(Mþ, 11), 136 (32), 107 (100), 79 (43), 51 (61).
Scheme 3.
In contrast to IBX, the oxidation reactions are faster with
DMP in ionic liquids and results are summarized in Table 1.
Ionic liquids used in this study were obtained from Fluka.
The purity of [bmim]PF6 is $97.0 (NMR).
4.2.2. 2b: 4-Nitro-benzaldehyde. Light yellow solid, mp
104–105 8C (lit.15a 106–107 8C), IR (KBr): n 3093, 2978,
2864, 1713, 1605, 1540, 1343, 1294, 1197, 853, 820,
740 cm21
.
1H NMR (CDCl3, 300 MHz): d 8.07 (d,
J¼8.5 Hz, 2H), 8.36 (d, J¼8.5 Hz, 2H), 10.25 (s, 1H). 13C
NMR (CDCl3, 75 MHz): d 123.6, 129.9, 139.4, 150.4,
189.8. EIMS: m/z (%): 151 (Mþ, 45), 150 (100), 77 (16), 51
(24).
3. Conclusion
In this paper, we have demonstrated that hydrophilic
[bmim]BF4 ionic liquid is an efficient and polar alternative
to conventional solvents for the IBX and DMP promoted
oxidations. This method avoids the use of polar organic
solvents such as DMSO or DMF and high temperature
reaction conditions for IBX oxidations. Enhanced reaction
rates, high conversions and greater selectivities are the
notable features observed in ionic liquids. The use of an
easily accessible and recyclable ionic liquid makes this
procedure quite simple, more convenient and environmen-
tally benign.
4.2.3. 2c: 4-Methoxy-benzaldehyde. Colourless liquid
(lit.15a), IR (neat): n 1701, 1607, 1386, 1308, 1207, 1169,
847, 808 cm21. 1H NMR (CDCl3, 200 MHz): d 3.95 (s, 3H),
7.33 (d, J¼7.9 Hz, 2H), 7.78 (d, J¼7.9 Hz, 2H), 9.97 (s,
1H). EIMS: m/z (%): 136 (Mþ, 60), 135 (100), 107 (26), 92
(18), 77 (45), 63 (21).
4.2.4. 2d: Benzaldehyde. Colourless liquid (lit.15d), IR
(neat): n 3064, 3031, 2852, 2820, 2732, 1702, 1654, 1597,
1584, 1455, 1391, 1311, 1204, 1167, 1023, 828, 746, 688,

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J. S. Yadav et al. / Tetrahedron 60 (2004) 2131–2135
1
650 cm21. H NMR (CDCl3, 300 MHz): d 7.45–7.67 (m,
3H), 7.87–7.90 (m, 2H), 9.90 (s, 1H). 13C NMR (CDCl3,
75 MHz): d 129.0, 129.6, 134.4, 136.4, 192.3. EIMS: m/z
(%): 106 (Mþ, 34), 105 (74), 77 (100), 51 (22).
7.44 (m, 4H), 7.51–7.57 (m, 2H), 7.87 (m, 4H). 13C NMR
(CDCl3, 75 MHz): d 127.7, 129.9, 132.8, 136.5, 196.5.
EIMS: m/z (%): 182 (Mþ, 19), 105 (68), 77 (100), 51 (27).
4.2.13. 2m: Menthone. Colorless liquid (lit.15e), IR (neat): n
2956, 2928, 2871, 1711, 1457, 1711, 1367, 1286, 1246,
1155, 1117, 1093, 1044, 994, 866, 747, 608 cm21. 1H NMR
(CDCl3, 300 MHz): d 0.86 (d, J¼6.7 Hz, 3H), 0.91 (d,
J¼6.7 Hz, 3H), 1.01 (d, J¼6.2 Hz, 3H), 1.34–1.40 (m, 2H),
1.80–2.20 (m, 6H), 2.18–2.37 (m, 1H). 13C NMR (CDCl3,
75 MHz): d 18.7, 21.2, 22.3, 25.9, 27.9, 33.9, 35.5, 50.8,
55.8, 212.0. EIMS: m/z (%): 154 (Mþ, 18), 111 (35), 83
(100), 55 (43).
4.2.5. 2e: trans-Cinnamaldehyde. Pale yellow liquid
(lit.15a), IR (neat): n 3078, 2986, 2834, 1706, 1605, 1534,
1455, 1263, 1075, 876, 687 cm21 1H NMR (CDCl3,
.
300 MHz): d 6.65 (d, J¼16.7 Hz, 1H), 6.69 (dd, J¼6.5,
16.7 Hz, 1H), 7.43–7.37 (m, 3H), 7.54–7.50 (m, 2H), 9.65
(d, J¼6.5 Hz, 1H). 13C NMR (CDCl3, 75 MHz): d 128.5,
128.6, 129.2, 131.4, 134.1, 152.8, 193.7. EIMS: m/z (%):
132 (Mþ, 31), 90 (59), 77 (100), 51 (39).
4.2.6. 2f: 3,4,5-Trimethoxy benzaldehyde. Light yellow
solid, mp 73–74 8C (lit.15b 73–75 8C), IR (KBr): n 3069,
2987, 2842, 1705, 1600, 1567, 1459, 1384, 1226, 892,
764 cm21. 1H NMR (CDCl3, 200 MHz): d 3.80 (s, 3H), 3.90
(s, 6H), 7.10 (s, 2H), 9.86 (s, 1H). EIMS: m/z (%): 196 (Mþ,
40), 165 (68), 103 (100), 74 (27), 49 (32).
4.2.14. 2n: 2-Methyl cyclohexanone. Colourless liquid
(lit.7a), IR (neat): n 2930, 2863, 1721, 1450, 1428, 1375,
1314, 1156, 1053, 995, 866, 734, 650 cm21 1H NMR
.
(CDCl3, 300 MHz): d 1.00 (d, 3H, J¼6.5 Hz), 1.34–1.37
(m, 1H), 1.64–1.82 (m, 3H), 2.10–2.42 (m, 5H). 13C NMR
(CDCl3, 75 MHz): d 14.6, 25.3, 28.1, 36.1, 41.7, 45.4,
213.7. EIMS: m/z (%): 112 (Mþ, 11), 97 (21), 84 (38), 69
(43) 56 (100).
4.2.7. 2g: 2-Furfuraldehyde. Pale yellow liquid (lit.14c), IR
(neat): n 2985, 1698, 1605, 1519, 1168, 1032, 826,
785 cm21
.
1H NMR (CDCl3, 300 MHz): d 6.60 (d,
4.2.15. 2o: 2-Octanone. Colourless liquid (lit.15d), IR
(neat): n 2959, 2932, 2856, 1716, 1593, 1467, 1413, 1363,
J¼5.0 Hz, 1H), 7.25 (d, J¼5.0 Hz, 1H), 7.70 (s, 1H), 9.70
(s, 1H). EIMS: m/z (%): 96 (Mþ, 17), 67 (11), 84 (38), 43
(30), 41 (100), 26 (26).
1
1278, 1227., 1167, 1115, 1034, 945, 720 cm21. H NMR
(CDCl3, 200 MHz): d 0.90 (t, 3H, J¼7.0 Hz), 1.30 (m, 6H),
1.56 (m, 2H), 2.16 (s, 3H), 2.44 (m, 2H). 13C NMR (CDCl3,
50 MHz): d 14.3, 22.5, 24.0, 29.1, 29.9, 31.8, 43.8, 209.2.
EIMS: m/z (%): 128 (Mþ, 39), 85 (100), 57 (72), 29 (21).
4.2.8. 2h: 3,7-Dimethyl-2,6-octadienal (citral). Colorless
liquid (lit.15e), IR (neat): n 2850, 1720, 1609, 1533, 1470,
.
1414, 1370, 1060, 895, 723 cm21 1H NMR (CDCl3,
300 MHz): d 1.60 (s, 3H), 1.90 (s, 3H), 2.20 (s, 3H), 2.60
(m, 4H), 5.10 (s, 1H), 5.85 (d, 1H, J¼7.0 Hz), 9.80 (d, 1H,
J¼7.0 Hz). EIMS: m/z (%): 152 (Mþ, 21), 123 (9), 110 (72),
68 (100), 41 (38).
4.2.16. 2p: N-Boc-D-serinal acetonide (Garner aldehyde).
Colourless oil (lit.14d), IR (neat): n 2948, 2835, 1712, 1604,
1
1546, 1438, 1222, 1064, 932, 878, 764 cm21. H NMR
(CDCl3, 200 MHz): d 1.37 (s, 3H), 1.41 (s, 3H), 1.43 (s,
9H), 3.52 (dd, J¼8.3, 8.7 Hz, 1H), 3.73 (dd, J¼2.9, 8.7 Hz,
1H), 3.90 (m, 1H), 9.34 (brs, 1H). 13C NMR (CDCl3,
50 MHz): d 23.8, 24.7, 25.8, 26.7, 28.3, 63.5, 64.7, 81.1,
81.4, 94.4, 95.1, 151.3, 152.6, 199.5. EIMS: m/z (%): 229
(Mþ, 16), 156 (21), 128 (47), 86 (100), 72 (41), 54 (29).
4.2.9. 2i: a-Hydroxy-acetophenone. Pale yellow solid, mp
85–87 8C (lit.15c 89–90 8C), IR (KBr): n 3421, 1689, 1600,
1
1456, 1409, 1301, 1231, 1106, 970, 761, 683 cm21. H
NMR (CDCl3, 300 MHz): d 3.50 (brs, 1H, OH), 4.89 (s,
2H), 7.49–7.67 (m, 3H), 7.92–7.95 (m, 2H). EIMS:
m/z(%): 136 (Mþ1), 105 (77), 77 (100), 51 (17).
4.2.10. 2j: Benzil. Yellowish solid, mp. 94–95 8C (lit.15a
93–95 8C), IR (neat): n 2930, 2863, 1721, 1695, 1450,
1428, 1375, 1314, 1156, 1053, 995, 866, 734, 650 cm21. 1H
NMR (CDCl3, 300 MHz): d 7.53 (t, 4H, J¼8.2 Hz), 7.62 (t,
2H, J¼8.2 Hz), 8.0 (d, 4H, J¼8.2 Hz). 13C NMR (CDCl3,
75 MHz): d 128.8, 130.2, 132.8, 134.9, 194.5. EIMS: m/z
(%): 210 (Mþ, 11), 133 (27), 105 (100), 77 (60), 51 (40).
Acknowledgements
B. V. S. and A. K. B. thank CSIR, New Delhi for the award
of fellowships.
References and notes
4.2.11. 2k: Acetophenone. Colourless liquid (lit.15c), IR
(neat): n 3060, 3005, 2924, 1686, 1599, 1582 1359, 1266,
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1182, 1078, 1024, 955, 760, 690 cm21. H NMR (CDCl3,
300 MHz): d 2.55 (s, 3H), 7.38–7.46 (m, 2H), 7.48–7.55
(m, 1H), 7.90–7.95 (m, 2H). 13C NMR (CDCl3, 75 MHz):
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