Imidazolium ionic liquid-grafted 2,2′-bipyridine - A novel ligand for the recyclable copper-catalyzed selective oxidation of alcohols in ionic liquid [bmim][PF6]

Article abstract of DOI:10.1246/cl.2005.312

A novel imidazolium ionic liquid-grafted 2,2′-bipyridine ligand has been prepared and successfully employed in the copper-catalyzed selective oxidation of alcohols to the corresponding carbonyl compounds in ionic liquid [bmim] [PF₆] under mild conditions, and the catalytic ionic liquid solution can be recovered and reused without significant loss of catalytic efficiency. Copyright

Full text of DOI:10.1246/cl.2005.312

312
Chemistry Letters Vol.34, No.3 (2005)
Imidazolium Ionic Liquid-Grafted 2,20-Bipyridine—A Novel Ligand for the Recyclable
Copper-catalyzed Selective Oxidation of Alcohols in Ionic Liquid [bmim][PF6]
Xue-E Wu, Li Ma, Meng-Xian Ding, and Lian-Xun Gaoꢀ
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry,
Chinese Academy of Science and Graduate School of Chinese Academy of Sciences, Changchun, 130022, P. R. China
(Received November 29, 2004; CL-041448)
A novel imidazolium ionic liquid-grafted 2,20-bipyridine li-
gand has been prepared and successfully employed in the cop-
per-catalyzed selective oxidation of alcohols to the correspond-
ing carbonyl compounds in ionic liquid [bmim][PF6] under mild
conditions, and the catalytic ionic liquid solution can be recov-
ered and reused without significant loss of catalytic efficiency.
2
3
N
N
N
N
a
b
Br
N
Br
N
The use of ionic liquids (ILs), especially those based upon
the 1-alkyl-3-methylimidazolium cation, as ‘green’ solvents in
organic synthetic processes has gained recognition because of
their solvating ability, negligible vapour pressure, easy recycla-
bility, and reusability.1 Recent advances in ILs research provid-
ed another route for achieving task-specific ionic liquids (TSILs)
in which a functional group is covalently tethered to the cation or
anion of the ILs.2 It was expected that these TSILs might further
enlarge the application scope of ILs in chemistry.
4
1
N
N
N
N
N
N
N
-
-
Br
Br
c
N
-
N
-
N
PF6
PF6
The oxidation of primary and secondary alcohols into the
corresponding carbonyl compounds plays a central role in organ-
ic synthesis.3 Traditionally such transformations have been per-
formed with stoichiometric inorganic oxidants.4 However, from
both economic and environmental viewpoints, there is an urgent
demand for green, atom efficient methods that employ clean ox-
idants, such as O2 or H2O2 and a preferably recyclable catalyst.
Recently it has been demonstrated that ILs can be employed in
the copper-catalyzed oxidation of alcohols employing O2 as ox-
idant.5 However, the immobilization of the catalytic system in
the IL was not successful in their procedure.
In the present communication, we report an efficient synthe-
sis of a new TSIL, see Scheme 1, that contains two 1-methylmi-
dazolium hexafluorophosphate moieties and describe prelimina-
ry results of its catalytic efficiency and reusability in the
copper(I) chloride and 2,2,6,6-tetramethylpiperidine N-oxyl
(TEMPO) catalysed oxidation of alcohols to corresponding alde-
hydes and ketones using the IL [bmim]PF6 as catalyst solvent. IL
[bmim]PF6 has been prepared from [bmim]Cl6 according to a
method described by Fuller et al.7
Scheme 2. The synthesis of ligand 1. a) NBS, CCl4, reflux; b)
1-methylimidazole (2.7 equiv.) 80 ꢁC, 24 h in CH3CN, 98%;
c) KPF6 (1.2 equiv.), rt, 48 h in acetone, 96%.
dine dibromide (4). Further treatment of 4 with KPF6 in acetone
for 48 h provides 1.
The coordination ability of the new IL-ligand has also been
briefly studied and it was found that the new IL-ligand 1 could
form complex with CuCl. The complex was prepared from CuCl
and the IL-bpy ligand in acetonitrile at room temperature and has
been confirmed by element analysis and FTIR spectra.10
O
OH
R2
TEMPO, CuCl, 1
R1
R2
R1
O2, [bmim][PF ], 65 °C
6
Scheme 3. Oxidation of alcohols in ionic liquid.
For the catalyst, the preferred method was preparation in situ
by mixing 49 mg (0.5 mmol) of CuCl and one equivalent of
ligand 1 (318 mg, 0.5 mmol) in a 5-mL acetonitrile/[bmim][PF6]
(1/1, v/v) mixture. After the acetonitrile was removed in
vacuum, a yellow-colored catalyst solution was obtained. With
5 mol % of this catalyst, TEMPO (5 mol %) as co-catalyst, the
oxidation reactions were carried out at 65 ꢁC under oxygen
atmosphere. The conversion of alcohols was followed by thin
layer chromatography (TLC) and gas chromatography (GC) with
an internal standard. The products were isolated by simple
extraction with diethyl ether. The structures of the products were
confirmed by comparison with authentic standards of carbonyl
compounds. The IL phase could be separated and reused for
further runs.
N
-
N
-
N
N
N
N
PF6
PF6
Scheme 1. Ionic liquid grafted 2,20-bipyridine ligand.
The route to the new bipyridine ligand bearing with pending
imidazolium tags is depicted in Scheme 2.8 The 6,60-bis(bromo-
methyl)-2,20-bipyridine (3), easily prepared from commercially
available 6,60-dimethyl-2,20-bipyridine (2) in one step,9 was re-
acted with 1-methylimidazole in acetonitrile at 80 ꢁC for 24 h
to give 6,60-bis(1-methylimidazolium-3-ylmethyl)-2,20-bipyri-
The results of this study are summarized in Table 1 (Runs 1–
Copyright Ó 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.3 (2005)
313
Table 1. Conversion of alcohols to aldehydes and ketonesa
dary alcohols to their corresponding aldehydes and ketones in
hexafluorophosphate IL under mild conditions. The facile recov-
ery of the catalyst is a great advantage of this method. Work is in
progress to further address the recoverability and recyclability
properties of this new ligand. The approach may therefore be
of interest not only for oxidations but also for many other cata-
lytic reactions in ILs. Studies on the imidazolium ionic tag strat-
egy to increase the reusability of the other catalysts in environ-
mentally benign ILs will be continued.
Time Conversiona
Run
Alcohol
Product
/h
%
1
2
Benzyl alcohol
(4-Methoxy-
phenyl)-methanol benzaldehyde
(4-Nitro-phenyl)- 4-Nitro-
Benzaldehyde
4-Methoxy-
15
>99
15
15
15
15
95
98
98
96
3
4
5
methanol
benzaldehyde
2-Chloro-
phenyl)-methanol benzaldehyde
(2-Chloro-
The work was supported by the National Natural Science
foundation of China (Nos. 20174040 and 50333030).
1-Phenyl-
1-Phenyl-ethanol
ethanone
6
7
8
9
Cyclohexanol
Prop-2-en-1-ol
Heptan-1-ol
Cyclohexanon 48
83
80
70
>99b
References and Notes
Propenal
Heptanal
Benzaldehyde
15
48
15
1
Comprehensive information about this field can be found in
the many reviews that have appeared on this subject: ‘‘Ionic
Liquids in Synthesis,’’ ed. by P. Wasserscheid and T. Welton,
Wiley-VCH, Weinheim (2002); J. Dupont, R. F. de Souza, and
P. A. Z. Suarez, Chem. Rev., 102, 3667 (2002); C. M. Gordon,
Appl. Catal., A, 222, 101 (2001), and references therein.
J. H. Davis, Jr., Chem. Lett., 33, 1072 (2004).
Benzyl alcohol
aSelectivity >99% based on GC. bIn the absence of the IL-bpy
ligand.
2
3
8). As indicated by quantitative GC analysis, aldehydes and
ketones were formed in high yields from the corresponding
primary and secondary alcohols, respectively. However, aliphat-
ic alcohols react slower than benzyl alcohols and allylic alco-
hols. This may be due to the poor solubility of aliphatic alcohols
in ILs.
For reviews on oxidations, see: R. A. Sheldon and J. K. Kochi,
in ‘‘Metal-Catalyzed Oxidations of Organic Compounds,’’
Academic Press, New York (1981); S. V. Ley, J. Norman,
W. P. Griffith, and P. Marsden, Synthesis, 1994, 639; M.
Hudlicky, ‘‘Oxidations in Organic Chemistry,’’ ACS,
Washington, DC (1990).
The recyclability of the used ILs solution containing the cat-
alyst was demonstrated for the oxidation of benzyl alcohol to
benzaldehyde. The results of five consecutive recycling experi-
ments were shown in Table 2. As expected, the reaction was per-
formed five times without a major loss of catalytic activity. The
observed loss of activity was maybe due to mechanical losses
during the extraction procedure. This problem could be mini-
mized by direct distillation of the carbonyl compounds from
the reaction mixture. Indeed, after the carbonyl compounds were
removed by distilling from the IL solution directly, the resulting
catalytic IL solution could be reused for the oxidation of benzyl
alcohol and showed no variation of catalytic activity and selec-
tivity compared to the fresh catalyst, see Table 2.
4
G. Cainelli and G. Cardillo, ‘‘Chromium Oxidants in Organic
Chemistry,’’ Springer, Berlin (1984); D. G. Lee and U. A.
Spitzer, J. Org. Chem., 35, 3589 (1970); F. M. Menger and
C. Lee, Tetrahedron Lett., 22, 1655 (1981).
A. Ansari and R. Gree, Org. Lett., 4, 1507 (2002).
J. S. Wilkes, J. A. Levisky, R. A. Wilson, and C. L. Hussey,
Inorg. Chem., 21, 1263 (1982).
5
6
7
8
R. T. Fuller, H. C. Carlin, H. C. de Long, and D. Haworth,
J. Chem. Soc., Chem. Commun., 1994, 299.
Detailed information for the IL-ligand synthesis and catalyst
1
preparation see supporting information. 4: H NMR (DMSO,
400 MHz): ꢀ ¼ 9:38 (s, 2H), 8.23 (d, J ¼ 8:0 Hz, 2 H), 8.05
(t, J ¼ 8:0 Hz, 2 H), 7.90 (s, 2H), 7.79 (s, 2H), 7.57 (d, J ¼
8:0 Hz, 2H), 5.68 (s, 4H), 3.93 (s, 6H); 13C NMR (DMSO,
400 MHz): ꢀ ¼ 154:41, 153.36, 138.78, 137.31, 123.58,
123.12, 122.95, 120.18, 52.81, 35.89. Anal. Calcd. for
C20H22N6Br2: C 47.45, H 4.38, N 16.60, Found C 47.39, H
4.31, N 16.53%. MS (FBA): m=z ¼ 346 [M]þ. IR (KBr, select-
ed data) 1565.63, 1431.88, 1162.03 cmꢂ1. Tg ¼ 299:65 ꢁC. 1:
1H NMR (DMSO, 400 MHz): ꢀ ¼ 9:28 (s, 2H), 8.20 (d,
J ¼ 8:0 Hz, 2 H), 8.03 (t, J ¼ 8:0 Hz, 2 H), 785 (s, 2H), 7.75
(s, 2H), 7.55 (d, J ¼ 8:0 Hz, 2 H), 5.64 (s, 4H), 3.93 (s, 6H);
13C NMR (DMSO, 400 MHz): ꢀ ¼ 154:51, 153.34, 138.80,
137.38, 123.64, 123.20, 122.92, 120.22, 52.96, 35.85. Anal.
Calcd. for C20H22N6P2F12: C 37.74, H 3.48, N 13.21, P 9.73,
Found C 37.69, H 3.41, N 13.20, P 9.68%. MS (FBA):
m=z ¼ 346 [M]þ. IR (KBr, selected data) 1579.73, 1440.07,
1175.23, 832.05 cmꢂ1. mp ¼ 227 ꢁC. Tg ¼ 285:69 ꢁC.
Table 2. Reuse of the catalyst solution for the oxidation of
benzyl alcohol to benzaldehyde
Run
Yield %a
Yield %b
1
2
3
4
5
>99
>99
97
97
90
96
83
95
80
97
aThe carbonyl compounds were removed by extraction
diethyl ether; The carbonyl compounds were removed by
distilling.
b
For comparison, a reaction for the oxidation of benzyl alco-
hol was carried out under the same reaction condition in the ab-
sence of the IL-bpy ligand, see Table 1 (Run 9). Although sim-
ilar results were also obtained, it was observed that the copper
catalyst was extracted from the IL solution along with the polar
products (aldehydes) by the diethyl ether during the extraction
process. The conclusion is substantiated by the observation that
the ether phase turned blue-green.
9
C. R. Woods, M. Benaglia, F. Cozzi, and J. S. Siegel, Angew.
Chem., Int. Ed. Engl., 35, 1830 (1996).
10 Detailed information for this complex preparation see support-
ing information. Elemental analysis calcd for C20H22ClP2F12-
N6Cu: C 32.67, H 3.02, N 11.43, P, 8.42; found C 53.53, H
4.90, N 18.57%. IR (KBr, selected data) 1605.32, 1580.33,
In conclusion, we have shown that the imidazolium IL-
modified bipyridine is an efficient ligand for the performance
of the copper-catalyzed oxidation of various primary and secon-
1469.82, 1438.31, 1183.10, 1169.84, 840.25 cmꢂ1
.
Published on the web (Advance View) January 29, 2005; DOI 10.1246/cl.2005.312