Synthesis of magnetically recoverable imidazolium hydrogen carbonate and its application as an N-heterocyclic carbene catalyst to cyanosilylation of aldehydes and ketones

Article abstract of DOI:10.1016/j.tetlet.2017.03.006

We prepared a magnetically recoverable imidazolium hydrogen carbonate by immobilizing an imidazolium hydrogen carbonate on magnetite and applied it as an N-heterocyclic carbene catalyst to the cyanosilylation of aldehydes and ketones. By employing the magnetite-supported imidazolium hydrogen carbonate as a catalyst, the cyanosilylation proceeded at 60?°C to provide the corresponding trimethylsilylated cyanohydrin in a fair chemical yield. Moreover, after the reaction, the magnetic catalyst was readily recovered by use of an external magnet and could be reused up to five times.

Full text of DOI:10.1016/j.tetlet.2017.03.006

Accepted Manuscript
Synthesis of magnetically recoverable imidazolium hydrogen carbonate and its
application as an N-heterocyclic carbene catalyst to cyanosilylation of aldehydes
and ketones
Akira Fujii, Jun-Chul Choi, Ken-ichi Fujita
PII:
S0040-4039(17)30292-7
DOI:
http://dx.doi.org/10.1016/j.tetlet.2017.03.006
TETL 48707
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
20 January 2017
27 February 2017
2 March 2017
Please cite this article as: Fujii, A., Choi, J-C., Fujita, K-i., Synthesis of magnetically recoverable imidazolium
hydrogen carbonate and its application as an N-heterocyclic carbene catalyst to cyanosilylation of aldehydes and
ketones, Tetrahedron Letters (2017), doi: http://dx.doi.org/10.1016/j.tetlet.2017.03.006
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Synthesis of magnetically recoverable
imidazolium hydrogen carbonate and its
application as an N-heterocyclic carbene
catalyst to cyanosilylation of aldehydes and
ketones
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Akira Fujii, Jun-Chul Choi and Ken-ichi Fujita*

1
Tetrahedron Letters
journal homepage: www.els evier.com
Synthesis of magnetically recoverable imidazolium hydrogen carbonate and its
application as an N-heterocyclic carbene catalyst to cyanosilylation of aldehydes and
ketones
Akira Fujii, Jun-Chul Choi and Ken-ichi Fujita*
National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
ARTICLE INFO
ABSTRACT
Article history:
Received
We prepared a magnetically recoverable imidazolium hydrogen carbonate by immobilizing an
imidazolium hydrogen carbonate on magnetite and applied it as an N-heterocyclic carbene
catalyst to the cyanosilylation of aldehydes and ketones. By employing the magnetite-supported
imidazolium hydrogen carbonate as a catalyst, the cyanosilylation proceeded at 60 °C to provide
the corresponding trimethylsilylated cyanohydrin in a fair chemical yield. Moreover, after the
reaction, the magnetic catalyst was readily recovered by use of an external magnet and could be
reused up to five times.
Received in revised form
Accepted
Available online
Keywords:
Magnetite
2009 Elsevier Ltd. All rights reserved.
Catalysis
Imidazolium hydrogen carbonate
Cyanosilylation
N-heterocyclic carbenes (NHCs) have attracted a lot of
attention not only as ligands of metal complexes, but also as
organocatalysts for various catalytic conversions such as
transesterification, cyanosilylation and polymer synthesis from
the viewpoint of green chemistry.1 Their unique steric and
electronic properties caused by fine tuning of their substituent
pattern leads to their utility as organocatalysts.2 However, the
NHCs are difficult to handle because of their instability in air.3 In
order to bypass this limitation, imidazolium salts are often used
as precatalysts which generate free NHC in situ by the addition
of strong bases.4 In this case, base-tolerant substrates are required
in catalytic conversions. On the other hand, masked NHCs have
also been reported as precatalysts which provide free NHCs
without bases.5 Among them, NHC–CO2 adducts are useful
precatalysts for free NHCs because CO2 is the only byproduct at
the NHC generation step.5a,e However, the preparation of NHC–
CO2 adducts requires free NHCs, which are air- and moisture-
sensitive.
Recently, as a strategy for the generation of free NHCs in situ,
imidazolium hydrogen carbonates were reported.6 In this case,
free NHCs were not required for the preparation of the
imidazolium hydrogen carbonates, because the imidazolium
hydrogen carbonates were prepared from the imidazolium halides
by the anion exchange. The free NHCs are prepared from the
imidazolium hydrogen carbonates only by heating in situ due to
the dissociation of H2CO3 (H2O + CO2) from the imidazolium
hydrogen carbonates.
provides a simple separation of the catalysts without filtration.7
The magnetic separation circumvents time-consuming and
laborious separation steps and allows for practical continuous
catalysis. We have recently reported a magnetite-supported
NHC–gold(I) complex, which catalyzed the hydroamination of
alkynes,8 and a magnetite-supported osmium oxide, which
catalyzed the dihydroxylation of olefins.9 Each magnetic catalyst
could be recovered with simple magnetic decantation by an
external magnet and reused for the subsequent reaction. We
report herein the synthesis of a magnetite-supported imidazolium
hydrogen carbonate by the immobilization of an imidazolium
hydrogen carbonate on magnetite10 and its application as an NHC
catalyst to the cyanosilylation of aldehydes and ketones.11,12
Moreover, this magnetic catalyst was clearly collected from the
reaction mixture by use of an external magnet and could be
reused repeatedly.
Magnetic nanoparticle-supported imidazolium hydrogen
carbonate 5 was synthesized as follows (Scheme 1). Magnetite
(Fe3O4) was chosen for use as a magnetic support, since it could
be readily prepared by a conventional coprecipitation method.13
The silane coupling agent having an imidazolium iodide 3 was
synthesized by stirring of an N,N-dimethylformamide (DMF)
solution of (11-iodoundecyl)trimethoxysilane
1
and 1-t-
butylimidazole 214 at 90 °C for 3 h under an argon atmosphere.
After the evaporation of DMF in vacuo, 3 and magnetite were
refluxed in ethanol for 23 h under an argon atmosphere to afford
the magnetite-supported imidazolium iodide 4, which was
separated by magnetic decantation using an external magnet.
Magnetite-supported imidazolium hydrogen carbonate 5 was
synthesized by the anion exchange of 4 in the presence of
potassium hydrogen carbonate in water at room temperature for
Magnetic nanoparticles offer advantages for use in the
development of clean and green sustainable processes, because
magnetic nanoparticle-supported catalysts can be easily separated
from the reaction medium by use of an external magnet, which

2
24 h.6a 5 was collected by magnetic decantation using an external
magnet. The loading of 5 was 0.126 mmol/g, which was
determined by elemental analysis of magnetite-supported
imidazolium hydrogen carbonate 5.15,16
Figure 2. Cyanosilylation catalyzed by 5 in hexane.
Scheme 1. Preparation of 5.
We first examined the time course of cyanosilylation of
benzaldehyde 6a with trimethylsilyl cyanide catalyzed by
magnetite-supported imidazolium hydrogen carbonate
5 in
various solvents (Figure 1). By employing 0.01 mol% of a
magnetic catalyst 5, the mixture of benzaldehyde 6a and
trimethylsilyl cyanide was stirred at 60 °C for 120 min in each
solvent. The magnetic catalyst 5 was collected using an external
magnet, and the reaction mixture was then transferred out of the
reaction vessel. By carrying out the cyanosilylation of 6a in 2-
propanol, the corresponding trimethylsilylated cyanohydrin 7a
was obtained in an only 2% chemical yield.17 In contrast, the
cyanosilylation of 6a proceeded smoothly in hexane,
tetrahydrofuran, acetonitrile, or 1,2-dichloroethane. Among these
solvents, hexane was found to be the most effective for
cyanosilylation of 6a. Figure 2 shows the reaction mixture of the
cyanosilylation catalyzed by 5 in hexane. After the reaction, the
magnetic catalyst 5 could be rapidly collected from the reaction
mixture by use of an external magnet, as shown in Figure 3.
Figure 3. Magnetic separation of the catalyst 5.
cyanohydrin 7a was obtained in a low chemical yield at 60 °C
(7%; Table 1, entry 2). On the other hand, by employing 0.01
mol% of the magnetite-supported imidazolium hydrogen
carbonate 5, the cyanosilylation of benzaldehyde 6a proceeded
smoothly at 60 °C to provide 7a in a 90% chemical yield (Table
1, entry 3). These results suggest that the cyanosilylation of 6a
requires the imidazolium hydrogen carbonate as a precatalyst for
free NHC.6a
We subsequently examined the temperature effect for the
cyanosilylation of 6a by use of 0.01 mol% of a magnetite-
supported imidazolium hydrogen carbonate 5. As a result, even
by carrying out the cyanosilylation of 6a at room temperature,
the corresponding trimethylsilylated cyanohydrin 7a was
obtained in a 67% chemical yield (Table 1, entry 4). This result
implies that free NHC could be generated even at room
temperature due to the reversible transformation between free
NHC and imidazolium hydrogen carbonate.6b Moreover, by
carrying out the cyanosilylation at room temperature catalyzed by
5 which was preheated at 60 °C for 30 min before the
cyanosilylation, the chemical yield of 7a increased (76%, Table
1, entry 5). This result suggests that preheating of the magnetite-
supported imidazolium hydrogen carbonate 5 has a positive
effect on the generation of free NHC from imidazolium hydrogen
carbonate.6b Finally, by carrying out the cyanosilylation of 6a at
60 °C through the use of 0.02 mol% of 5, the corresponding
trimethylsilylated cyanohydrin 7a was obtained quantitatively
(Table 1, entry 6).
100
90
80
70
60
Hexane
Tetrahydrofuran
Acetonitrile
50
40
30
1,2-Dichloroethane
20
2-Propanol
10
0
0
20
40
60
80
100
120
Time (min)
Table 1. Cyanosilylation of 6a by use of a catalytic amount of
4 or 5.a
Figure 1. Optimization of solvent for cyanosilylation of 6a
catalyzed by 5.a
Reaction conditions: benzaldehyde 6a (1.0 eq.), trimethylsilyl cyanide (1.02
a
eq.), catalyst 5 (0.01 mol%), solvent (1 M based on 6a), carried out at 60 °C
b
under an argon atmosphere. Determined by the integration of 1H NMR
absorptions with reference to an internal standard.
Next, to confirm an active species and the temperature effect
for the cyanosilylation, we carried out the cyanosilylation of
benzaldehyde 6a through the use of a magnetite-supported
imidazolium iodide 4 or a magnetite-supported imidazolium
hydrogen carbonate 516 in hexane for 30 min (Table 1).18 First, in
the absence of both of 4 and 5, the cyanosilylation of
benzaldehyde 6a hardly proceeded at 60 °C (3%; Table 1, entry
1). Then, by employing 0.01 mol% of the magnetite-supported
imidazolium iodide 4, the corresponding trimethylsilylated
Entry Additive
x (mol%)
–
Temperature (°C)
Yield (%)b
1
2
3
none
60
60
60
rt
rt
60
3
7
90
67
76
>99
4
5
5
5
5
0.01
0.01
0.01
0.01
0.02
4
5c
6
a Reaction conditions were the same as those indicated in Figure 1.
Determined by the integration of 1H NMR absorptions with reference to an
b
internal standard. c A magnetic catalyst 5 which was preheated at 60 °C for 30
min in hexane was used for the cyanosilylation at room temperature.

3
Table 2. Cyanosilylation of various aldehydes and ketones catalyzed by 5.a
Catalyst 5
x (mol%)
Entry
1
Substrate
Product
Time
Yield (%)b
>99 (97)c
6a
6b
7a
7b
0.02
0.02
30 min
O
H
2
10 min
>99
Cl
3
4
5
6
6c
6d
6e
6f
7c
7d
7e
7f
0.05
0.05
0.05
0.05
3 h
1 h
1 h
1 h
99
93
>99
98
7
6g
7g
0.1
24 h
70
8
9
6h
6i
7h
7i
0.1
0.1
0.1
24 h
24 h
48 h
99 (99)c
97
10
6j
7j
59
a Reaction conditions were the same as those indicated in Figure 1. b Determined by the integration of 1H NMR absorptions with reference to an internal standard.
c Isolated yield.
Then, we performed the cyanosilylation of various aldehydes
and ketones by employing a catalytic amount of the magnetite-
supported imidazolium hydrogen carbonate 5 in hexane at 60 °C,
as shown in Table 2.19 By employing 0.02 mol% of the magnetic
catalyst 5, the cyanosilylation of 4-chlorobenzaldehyde 6b
proceeded within 10 min to provide the corresponding
trimethylsilylated cyanohydrin 7b quantitatively (Table 2, entry
2). In contrast, the cyanosilylation of 4-anisaldehyde 6c took 3 h
even by the use of 0.05 mol% of the magnetic catalyst 5 (99%;
Table 2, entry 3). These results indicate that the electron-
donating group at the phenyl ring is not favorable for the
cyanosilylation of aldehydes.11b In addition, the cyanosilylation
of 3-phenylpropionaldehyde 6d and cyclohexanecarboxaldehyde
6e, which were aliphatic aldehydes, took 1 h to provide the
corresponding trimethylsilylated cyanohydrins 7d and 7e in high
chemical yields through the use of 0.05 mol% of the magnetic
catalyst 5 (93%, >99%; Table 2, entries 4 and 5, respectively). In
contrast, in the case of the cyanosilylation of ketones, 0.1 mol%
of the magnetic catalyst 5 was required because the reactivity of
ketones was lower than that of aldehydes.20 In the case of
acetophenone 6g, by carrying out the cyanosilylation for 24 h, 7g
was obtained in a 70% chemical yield (Table 2, entry 7). On the
other hand, the cyanosilylations of cyclohexanone 6h and 4-
phenyl-2-butanone 6i, which were aliphatic ketones, provided the
corresponding trimethylsilylated cyanohydrins 7h and 7i in high
reaction time was prolonged to 48 h, the corresponding
trimethylsilylated cyanohydrin 7j was obtained in a 59%
chemical yield, probably due to the bulkiness of 6j (Table 2,
entry 10).
Finally, the reusability of the magnetic catalyst 5 was
examined using benzaldehyde 6a and trimethylsilyl cyanide, as
shown in Table 3.21 By employing 0.02 mol% of the magnetite-
supported imidazolium hydrogen carbonate 5 as a catalyst, the
cyanosilylation of 6a was carried out at 60 °C for 30 min in
hexane. In this experiment, it was found that the magnetic
catalyst 5 was readily collected from the reaction mixture by use
of an external magnet, and could be reused up to five times, and
that the corresponding trimethylsilylated cyanohydrin 7a could
be obtained in high chemical yields in all cases.
Table 3. Catalyst recycling in cyanosilylation of 6a catalyzed
by 5.a
Run
Yield (%)b
First
>99
Second
>99
Third
98
Forth
97
Fifth
94
a Reaction conditions were the same as those indicated in Figure 1.
Determined by the integration of 1H NMR absorptions with reference to an
internal standard.
b
chemical yields (99%, 97%; Table 2, entries 8 and 9,
respectively). In the case of benzophenone 6j, although the

4
eutectic solvent: (a) Lee KS, Anisur RM, Kim KW, Kim WS,
Park T-J, Kang EJ, Lee IS. Chem. Mater. 2012; 24: 682–687. (b)
Azizi N, Rahimi Z, Alipour M. RSC Adv. 2015; 5: 61191–61198.
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Howson SE, Allan LEN, Chmel NP, Clarkson GJ, Deeth RJ,
Faulkner AD, Simpson DH, Scott P. Dalton Trans. 2011; 40:
10416–10433.
In conclusion, we prepared
a
magnetite-supported
imidazolium hydrogen carbonate. By employing the magnetite-
supported imidazolium hydrogen carbonate as an NHC catalyst,
the cyanosilylation of aldehydes and ketones proceeded at 60 °C.
Moreover, the magnetic catalyst was readily recovered by use of
an external magnet and could be reused up to five times for
cyanosilylation.
15. Selected Data for Compound 5
Black powder. IR (KBr): 3402, 2924, 2855, 1767, 1636, 1504,
Acknowledgments
-1
1373 cm . Anal.; found (%): C, 4.31; H, 0.77; N, 0.44; I, 0.82.
The loading of 5 was estimated from the difference between the
residual iodine content of 5 and the iodine content of 4, which is
shown in the Supporting Information. An X-ray diffraction pattern
is also shown in the Supporting Information.
This work was supported by JSPS KAKENHI Grant Number
26410200 and the New Energy and Industrial Technolohy
Development Organization (NEDO).
16. From the results of an elemental analysis of iodine of 5,15 it was
found that 5 contains a small amount of a residual imidazolium
iodide unit.
Supplementary Material
17. 2-Propanol is the most effective solvent for the NHC-catalyzed
22
carboxylative cyclization of propargylic amine with CO2 and for
Supplementary data (experimental procedures, analytical data
1
of 1, 3, 4 and 5, H and 13C NMR data of 1, 3 and 7, X-ray
diffraction patterns of magnetite, 4 and 5) associated with this
the
NHC-catalyzed
tautomerization
of
5-alkylidene-2-
oxazolidinone to 2-oxazolone.23 Thus, the low chemical yield of
the trimethylsilylated cyanohydrin 7a is probably because
trimethylsilyl cyanide, rather than the catalyst 5, was consumed by
an excess amount of 2-propanol.
article can be found, in the online version , at http://
.
References and Notes
18. Typical Procedure (Table 1, entry 6)
To
a mixture of benzaldehyde 6a (637 mg, 6.01 mmol),
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5
Manuscript number: TETL-D-17-00136
Authors: Akira Fujii, Jun-Chul Choi and Ken-ichi
Fujita*
Title: Synthesis of magnetically recoverable
imidazolium hydrogen carbonate and its application as
an N-heterocyclic carbene catalyst to cyanosilylation of
aldehydes and ketones
Highlights
A magnetically recoverable imidazolium hydrogen
carbonate was prepared.
It was applied as an N-heterocyclic carbene catalyst
to cyanosilylation of aldehydes.
The magnetic catalyst was recovered by magnetic
decantation and could be reused.