Highly efficient reduction of ferrocenyl aldehyde and ketones in ionic liquids

Article abstract of DOI:10.1016/j.jorganchem.2007.10.016

For the first time ferrocenyl aldehyde and ketones were effectively reduced to alkylferrocene using sodium borohydride and aluminum chloride in an ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm]BF₄). Mild reaction conditions,

Full text of DOI:10.1016/j.jorganchem.2007.10.016

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Journal of Organometallic Chemistry 692 (2007) 5687–5689
www.elsevier.com/locate/jorganchem
Communication
Highly efficient reduction of ferrocenyl aldehyde
and ketones in ionic liquids
*
Hongyan Zhang, Zhanxi Bian
Department of Chemistry, Inner Mongolia University, Hohhot 010021, China
Received 31 July 2007; received in revised form 27 September 2007; accepted 5 October 2007
Available online 16 October 2007
Abstract
For the first time ferrocenyl aldehyde and ketones were effectively reduced to alkylferrocene using sodium borohydride and aluminum
chloride in an ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm]BF4). Mild reaction conditions, enhanced rates,
improved yields, and recyclable ionic liquid are the remarkable features exhibited in this process.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Ionic liquid; Reduction; Alkylferrocene; Organometallic synthesis
1. Introduction
vapor pressure, nonflammable, recyclability, etc. [4]. Based
on these outstanding characteristics, the potential indus-
trial applications of RTILs in organic synthesis, gas separa-
tions, liquid–liquid extractions, electrochemical studies,
and catalytic reactions, have been extensively studied
[5–8]. Herein, we report an efficient and environmentally
benign method for the synthesis of alkylferrocenes by
reduction of corresponding ferrocenyl (Fc) aldehyde and
ketones using NaBH4/AlCl3 in ionic liquids without any
other organic solvents (Scheme 1).
Owing to their interesting optical, thermal and redox
properties, ferrocene derivatives have emerged as strong
candidates for materials and catalysts [1]. Alkylferrocenes
are important intermediates in organometallic synthesis.
Consequently, alkylferrocenes are normally prepared via
reduction acylferrocenes [2]. The classical methods for this
transformation include catalytic hydrogenolysis, Clemmen-
son reduction and reductive deoxygenation employing lith-
ium aluminum hydride under anhydrous conditions [3].
However, the usage of zinc amalgam made this process
non-eco-friendly. In addition, the possibility of some func-
tional groups existing in the molecule is limited under the
conventional reaction conditions and excess amount of
toxic solvents such as CH2Cl2, ClCH2CH2Cl was also
required.
2. Results and discussion
Firstly, 10 different solvents, including nine ionic liquids,
were investigated to find out the most suitable one for this
process in which acetylferrocene was used as the substrate.
The results are summarized in Table 1. Under comparable
experimental conditions, the yield was low if THF were
used as solvent, while 94% yield could be obtained in the
presence of [BMIm]BF4. In other words, [BMIm]BF4 can
effectively accelerate this reaction. To our surprise, when
[BMIm]PF6, [HMIm]PF6, [BMIm]NO3, [BMIm]ClO4 and
[Bpy]PF6 were applied, respectively, no product was
detectedÀ. However, we found that the ionic liquids includ-
ing BF4 are only effective mediums (entries 3–5) for the
formation of ethylferrocene, although the mechanism was
In recent years, room temperature ionic liquids (RTILs)
have attracted much interest as a new kind of possible
‘‘green’’ solvents and catalyst media. In comparison with
conventional organic solvents currently being used in the
industry, RTILs have several benefits, for example, they
are tunable polarity, high thermal stability, negligible
*
Corresponding author.
E-mail address: bzx1957@163.com (Z. Bian).
0022-328X/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2007.10.016

5688
H. Zhang, Z. Bian / Journal of Organometallic Chemistry 692 (2007) 5687–5689
Table 3
O
C
Recycling of the [BMIm]BF4 in the synthesis of ethylferrocenea
Run
Temp. (ꢁC)
Time (min)
Yield (%)b
CH2R
R
NaBH4/AlCl3
1
2
3
4
5
6
7
a
40
40
40
40
40
45
45
55
55
55
60
60
75
75
94
94
93
92
92
93
93
Fe
Fe
Ionic liquid
40˚C
R=H, CH3, C2H5, C6H5, m-ClC6H5, p-ClC6H5, n-C3H6Cl,
p-CH3C6H5, p-BrC6H5, Fc
The products were characterized by 1H NMR and IR.
Isolated yield.
b
Scheme 1. Reduction of ferrocenyl aldehyde and ketones in the presence
of ionic liquids.
With these results we extended our study to other alde-
Table 1
hyde and ketones. Fortunately, the results obtained in
[BMIm]BF4 show, for all the compounds under study,
excellent yields (see Table 2).
Reduction of acetylferrocene by NaBH4/AlCl3 in different solventsa
Entry
Solvents
Temp. (ꢁC)
Time (min)
Yield (%)b
1
2
3
4
5
6
7
8
9
10
a
THF
40
40
40
40
40
40
40
40
40
40
55
55
55
55
300
300
300
300
55
57
86
94
92
One of the most attractive properties of ionic liquids is
that they can provide good to excellent performance in
terms of product separation and reusability when they
are used as either catalysts or reaction medium in synthetic
chemistry. In this transformation, it is also observed that
when the reaction is complete and the product were
extracted, [BMIm]BF4 could be reused once again in the
synthesis of ethylferrocene without a significant decrease
in yield at least seven times except for a small mechanical
loss. The results are shown in Table 3.
[EMIm]BF4
[BMIm]BF4
[HMIm]BF4
[BMIm]PF6
[HMIm]PF6
[BMIm]NO3
[BMIm]ClO4
[Bpy]BF4
40
[Bpy] PF6
300
The products were characterized by 1H NMR and IR.
Isolated yield.
b
3. Conclusions
not clear. On the other hand, the performance of
[EMIm]BF4, [HMIm]BF4 and [BMIm]BF4 is very similar,
although the latter shows slightly higher yield. Subsequent
studies were, therefore, carried out in [BMIm]BF4.
In conclusion, we have demonstrated the reduction of
acetylferrocene to alkylferrocene using NaBH4/AlCl3 in
ionic liquid under mild conditions in excellent yield. The
experimental procedure is very simple and convenient.
The products can be recovered by extraction with ether
and the ionic liquid reused at least seven times.
O
R
CH2
C
R
4. Experimental
NaBH4/AlCl3
[BMIm]BF4
Fe
Fe
All reagents were of AR grade and used without further
purification. Following ionic liquids were used in these reac-
tions: [BMIm]BF4(BMIm = 1-butyl-3-methyl imidazole);
[BMIm]PF6; [BMIm]NO3; [BMIm]ClO4; [EMIm]BF4
(EMIm = 1-ethyl-3-methyl imidazole); [HMIm]BF4 (HMIm =
1-hexyl-3-methyl imidazole) [HMIm]PF6, [Bpy]BF4,
[Bpy]PF6 (Bpy = 1-butyl-pyridinium)All ionic liquids used
in this work were synthesized according to the previous
papers [9] and were dried in vacuo at 90 ꢁC before use.
Table 2
Reduction of ferrocenyl aldehyde and ketone in ionic liquid [BMIm]BF4a
Entry
R
Product
Temp.
(ꢁC)
Time
(min)
Yield
(%)b
1
2
3
4
5
6
7
8
9
10
a
H
CH3
C2H5
C5H6
FcCH3
FcCH2CH3
FcCH2CH2CH3 40
FcCH2Ph
FcCH2PhCl-m
FcCH2PhCl-p
Fc(CH2)4Cl
35
40
60
55
70
55
35
35
35
60
70
110
95
94
92
98
98
98
90
96
96
80
40
40
40
35
40
50
50
4.1. The general procedure for the reaction
m-ClC6H5
p-ClC6H5
n-C3H6Cl
p-CH3C6H5 FcCH2PhCH3
p-BrC6H5
To a solution of acetylferrocene (0.114 g, 0.5 mmol) in
preheated [BMIm]BF4 (25 ml) in a 50 ml common flask
set was slowly added AlCl3 (0.1 g, 0.75 mmol) with stirring
about 30 min at 20 ꢁC. Then NaBH4 (0.0475 g, 1.25 mmol)
was slowly added into the mixture. After the reaction mix-
ture was stirred at 40 ꢁC for 55 min, the mixture was
FcCH2PhBr-p
FcCH2Fc.
Fc
The products were characterized by 1H NMR and IR.
Isolated yield.
b

H. Zhang, Z. Bian / Journal of Organometallic Chemistry 692 (2007) 5687–5689
5689
quenched with cold water (15 ml). The aqueous layer was
separated from the product and extracted with diethyl
ether (3 · 15 ml). The combined organic extract was dried
using anhydrous MgSO4 and filtrated, and then the solvent
was removed under reduced pressure. The residue was
chromatographed on silica (eluent: hexane/diethyl ether =
9:1) to afford the pure ethylferrocene (0.1 g, 94%). On the
other hand, [BMIm]BF4 could be recovered by extraction
with dichloromethane followed by evaporation and drying,
which could be reused directly for the next run.
(2H, m), 2.28–2.31 (2H, m), 3.52–3.55 (2H, t), 4.15–4.20
(9H, m).
p-Methylbenzylferrocene: yellow crystals; m.p. 80–83 ꢁC;
IR: mmax/cmÀ1 3093, 3037, 3016, 2994, 2913, 2851, 1638,
1438, 1102, 1020, 997, 922, 815; 1H NMR (400 MHz,
CDCl3): dH 2.29 (3H, s), 3.64 (2H, s), 4.07–4.08 (4H, m),
4.12 (5H, s), 7.05 (4H, s).
p-Bromobenzylferrocene: orange crystals; m.p. 99–
100 ꢁC; IR: mmax/cmÀ1 3104, 3082, 3048, 2916, 2893,
1
2854, 1638, 1482, 1103, 1002, 813; H NMR (400 MHz,
CDCl3): dH 3.62 (2H, s), 4.06–4.08 (4H, m), 4.12 (5H, s),
7.02–7.04 (2H, m), 7.34–7.37 (2H, m).
4.2. Spectroscopic data for products
Diferrocenylmethane: yellow crystals; m.p. 140–142 ꢁC;
1H NMR spectra were recorded on the spectrometer
operating at 400 MHz. Chemical shifts were reported in
parts per million (ppm d) and referenced to TMS. Infrared
spectra were recorded as a thin film on sodium bromide
and absorptions were reported in wavenumbers (cmÀ1).
Methylferrocene: yellow crystals; m.p. 35–36 ꢁC; IR
(KBr): mmax/cmÀ1 3090, 2922, 2894, 1458, 1102, 998, 816;
1H NMR (400 MHz, CDCl3): dH 1.94 (3H, s), 4.12 (4H,
s), 4.17 (5H, s).
IR: mmax/cmÀ1 3090, 2923, 2853, 1464, 1406, 1103, 1000,
1
816; H NMR (400 MHz, CDCl3): dH 3.36 (2H, s), 4.06–
4.18 (18H, m).
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