Application of ionic liquids in palladium(II) catalyzed homogenous transfer hydrogenation

Article abstract of DOI:10.1016/j.tetlet.2005.07.070

The first application of palladium(II) catalyzed homogenous transfer hydrogenations in ionic liquids is described. Cinnamic acid and its derivatives were reduced in high yields under mild conditions using ammonium formate as hydrogen donor.

Full text of DOI:10.1016/j.tetlet.2005.07.070

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 6203–6204
Application of ionic liquids in palladium(II) catalyzed
homogenous transfer hydrogenation
Zolt a´ n Ba a´ n, Zolt a´ n Finta, Gy o¨ rgy Keglevich and Istv a´ n Hermecz*
Department of Organic Chemical Technology, Budapest University of Technology and Economics,
PO Box 91, H-1521 Budapest, Hungary
Received 17 March 2005; revised 8 July 2005; accepted 15 July 2005
This article is dedicated to Professor Andr a´ s Lipt a´ k on the occasion of his 70th birthday
Abstract—The first application of palladium(II) catalyzed homogenous transfer hydrogenations in ionic liquids is described.
Cinnamic acid and its derivatives were reduced in high yields under mild conditions using ammonium formate as hydrogen donor.
ꢀ
2005 Elsevier Ltd. All rights reserved.
Catalytic transfer hydrogenation is an important syn-
thetic method for the reduction of organic com-
We have found that ionic liquids are suitable solvents
for the palladium(II) catalyzed homogenous transfer
hydrogenation of a,b-unsaturated carboxylic acids
(Scheme 1).
1
–3
pounds.
Such reactions can generally be performed
in various organic solvents, but in some cases water
4
–6
can serve as solvent.
Bertold and co-workers were
7
–11
the first to report the application of ionic liquids
heterogeneous transfer hydrogenation. In their process
in
Cinnamic acid 1a was reduced in high yield under mild
conditions using a catalytic amount of palladium(II)
acetate or palladium(II) chloride and ammonium for-
mate as hydrogen donor in different ionic liquids (Table
1).
1
2
the catalyst was Pd/C, the solvent was [bmim][PF ] and
6
the reaction took place under microwave irradiation.
Geldbach and Dyson reported the asymmetric transfer
hydrogenation of acetophenone with a ruthenium com-
plex in 2-methyl-[bmim][PF ], using triethylammonium
Both Pd(OAc) and PdCl were found to be effective cat-
6
2
2
1
3
formate as the hydrogen donor. Very recently, Ohta
and co-workers published the transfer hydrogenation
of ketones with a modified ruthenium complex in ionic
alysts for the transfer hydrogenation. Of the ionic liq-
TM
uids studied, [bmim][BF4], ECOENG -212 and
TM
ECOENG -500 were the most effective, resulting in
1
4
liquids. In both cases ionic liquids were not only the
solvents of the reactions, but they also had a role in
stabilization of the ruthenium complexes.
>99% hydrogenated product. In contrast, ionic liquids
À
containing the [PF6 ] anion furnished only 2% product
and in typical organic solvents such as ethanol, toluene
O
O
4
OR4
OR
2
palladium(II), HCO NH4
[
bmim][BF4], 65 °C, 5 h
R3
a-i
R3
2a-i
R1
R1
R2
R2
1
Scheme 1.
Keywords: Ionic liquids; Homogenous catalytic transfer hydrogenation; Palladium(II) acetate; Palladium(II) chloride.
*
Corresponding author. Tel.: +36 1 505 2245; fax: +36 1 505 2947; e-mail: istvan.hermecz@sanofi-aventis.com
0040-4039/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.07.070

6204
Z. Ba a´ n et al. / Tetrahedron Letters 46 (2005) 6203–6204
Table 1. Pd(OAc)
ionic liquids using HCO
2
catalyzed transfer hydrogenation of 1a in various
NH as the hydrogen donor
available, nonpyrophoric palladium(II) catalysts. This
method offers an efficient and attractive alternative to
currently available procedures.
2
4
a
Entry
Solvent
Yield (%)
A
B
C
D
E
F
[bmim][BF
[bmim][Cl]
4
]
>99
46
2
2
>99
>99
Acknowledgements
[bmim][PF
[emim][PF
6
]
]
6
TM
b
b
ECOENG -212
Z.B. acknowledges the predoctoral Grant received from
Sanofi-Aventis.
TM
ECOENG -500
a
HPLC yield.
Solvent-innovation GmbH.
b
15
References and notes
Table 2. Pd(OAc)
2
catalyzed transfer hydrogenation in [bmim][BF
4
]
1. Brieger, G.; Nestrick, T. J. Chem. Rev. 1974, 74, 567–
1
2
3
4
a
580.
Compound 1
R
R
R
R
Yield of 2 (%)
2
3
4
5
6
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Rev. 1985, 85, 129–170.
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a
b
c
d
e
f
H
H
H
OH
e
H
H
H
H
H
H
H
H
e
M
H
H
H
>99
>99
69
H
H
>99
52
46
e
OMe
OH
OM
H
H
H
1
0, 2045–2061.
. Tour, J. M.; Pendalwar, S. L. Tetrahedron Lett. 1990, 31,
719–4722.
OM
H
98
>99
e
4
Ph
H
H
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berlin, R. M. Tetrahedron Lett. 2000, 41, 7847–7849.
. Sharma, A.; Joshi, B. P.; Sinha, A. K. Chem. Lett. 2003,
g
h
i
NHCOCH
–NCH(CH
H
3
H
H
3
M
>99
3
2, 1186–1187.
a
HPLC yield.
7. Wasserscheid, P.; Welton, T. Ionic Liquids in Synthesis;
Wiley-VCH, 2003, pp 40–118.
8
9
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. Holbrey, J. D.; Seddon, K. R. Clean Products Process.
1
and chloroform low yields or no reactions were ob-
served. With either sodium formate or triethylammo-
nium formate as hydrogen donor, the reactions
occured with similar yields as when using ammonium
formate. The reduction of cinnamic acid 1a gave the best
999, 1, 223–236.
0. Wasserscheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000,
9, 3772–3789.
1
1
3
1. Hua, Z.; Shanjay, M. V. Aldrichim. Acta 2002, 35, 75–
83.
results at 65 ꢁC for 5 h in [bmim][BF ] using 10 mol % of
12. Bertold, H.; Schotten, T.; H o¨ nig, H. Synthesis 2002, 11,
4
catalyst and 4 equiv of ammonium formate as hydrogen
1607–1610.
1
6,17
1
1
3. Geldbach, T. J.; Dyson, P. J. J. Am. Chem. Soc. 2004, 126,
114–8115.
donor.
8
4. Kawasaki, I.; Tsunoda, K.; Tsuji, T.; Yamaguchi, T.;
Shibuta, H.; Uchida, N.; Yamashita, M.; Ohta, S. Chem.
Commun. 2005, 2134–2136.
After the successful reduction of 1a under these condi-
tions, the method was extended to a variety of a- and
aryl-substituted cinnamic acids (Table 2). Aryl-substi-
tuted compounds bearing methoxy (1b), dihydroxy
TM
TM
1
5. ECOENG -212: [emim][EtSO
CH )N[(CH CH O) H][(CH
4
]; ECOENG -500: [(C13
H27)-
(
3
2
2
2
2
CH O) H]][MeSO ].
2
3
4
(
1c) or dimethoxy (1d) groups were reduced in good
1
6. In a typical procedure, cinnamic acid (0.1 g, 0.66 mmol)
yields. Of the a-substituted derivatives investigated, the
a-methyl (1e) and a-phenyl (1f) substituted examples
provided the corresponding hydrogenated products in
good yields. The a-acetamido (1g) derivative and the
readily available azalactone (1h) were converted directly
to the N-acyl derivatives in moderate yields. The methyl
ester of cinnamic acid (1i) was also hydrogenated in
excellent yield.
and 10 mol % Pd(OAc) (15 mg, 0.066 mol) were added to
2
a stirred solution of 3 mL [bmim][BF ]. Ammonium
4
formate (200 mg, 2.4 mmol) was added and the reaction
mixture was heated to 65 ꢁC for 5 h. After cooling to room
temperature, the mixture was extracted with diethyl ether
(
2 4
2 · 8 mL) and the extract was dried with Na SO . The
pure product, hydrocinnamic acid (0.09 g, 90% yield), was
isolated after removal of the solvent in vacuum.
1
4
7. In some batches of [bmim][BF ], formic acid was a suitable
hydrogen donor, due to the basic impurities of the ionic
liquid.
18. Scammels, P. J.; Scott, J. L.; Singer, R. D. Aust. J. Chem.
2005, 58, 155–169.
In conclusion, we have found a new application of ionic
liquids in the homogenous transfer hydrogenation of
a,b-unsaturated carboxylic acids with commercially
18