A dream combination for catalysis: Highly reactive and recyclable scandium(iii) triflate-catalyzed cyanosilylations of carbonyl compounds in an ionic liquid

Article abstract of DOI:10.1039/b900254e

The catalytic activity of lanthanide triflates, particularly scandium triflate, increased dramatically in [bmim][SbF₆], allowing the cyanosilylation of a variety of aldehydes and ketones with a turnover frequency up to 48000 mol h^-1 and a total turnover number of 100000.

Full text of DOI:10.1039/b900254e

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www.rsc.org/greenchem | Green Chemistry
A dream combination for catalysis: highly reactive and recyclable
scandium(III) triflate-catalyzed cyanosilylations of carbonyl compounds
in an ionic liquid†
Boyoung Y. Park, Ka Yeon Ryu, Jung Hwan Park and Sang-gi Lee*
Received 8th January 2009, Accepted 31st March 2009
First published as an Advance Article on the web 9th April 2009
DOI: 10.1039/b900254e
The catalytic activity of lanthanide triflates, particularly
scandium triflate, increased dramatically in [bmim][SbF ],
allowing the cyanosilylation of a variety of aldehydes and
ketones with a turnover frequency up to 48 000 mol h and
a total turnover number of 100 000.
ubiquitous in many Lewis acid-catalyzed reactions, mainly due
7
to the pioneering work by Kobayashi. However, only a few
6
8
9
lanthanide triflates, such as Yb(OTf)
3
and Sc(OTf) , have been
3
-
1
investigated for the cyanosilylation of carbonyl compounds, and
their catalytic activities are lower or merely comparable to those
1
0a
10b
10c
of other active catalysts, such as Cu(OTf)
and LiCl.
2
,
ZnI
2
,
LiClO
4
,
10d
The development of a recyclable catalytic system with high
reactivity that enables economical and environmental sustain-
ability is one of the most exciting challenges in chemistry.
Emerging ionic liquids (ILs) have attracted increasing attention
as novel vehicles for catalyst immobilization. Different types
of metallic catalysts can be immobilized in ILs, allowing the
Moreover, there are no reports on lanthanide
triflate-catalyzed cyanosilylation of carbonyls in ionic liquids.
Although, Loh et al. indicated that ionic liquids have their
own catalytic activity for the cyanosilylation of aldehydes, their
11
efficiencies were quite low. Based on these observations, we
hypothesized that a combination of lanthanide triflates with an
ionic liquid would accelerate the cyanosilylation reaction with
high recyclability.
1
recovery and reuse of both the catalyst and IL. In addition to
these advantages, unique reactivity and selectivity that cannot
be achieved in conventional organic solvents are also observed
The initial experiments on the reaction of benzaldehyde (1a)
and TMSCN were carried out without a catalyst in 1-butyl-
3-methylimidazolium (bmim)-based ionic liquids with different
anions, such as BF , SbF , and PF .‡ Although the efficiency was
2
in ILs. As part of our continuing studies on catalysis in ionic
3
liquids, Song and ourselves collaboratively reported that metal
3
a
triflate-catalyzed reactions, such as three-component reactions,
4
6
6
3b
3h
11
Diels–Alder reactions, and Friedel–Crafts alkenylation, were
much faster in ionic liquids than in conventional organic
solvents. In particular, hydrophobic ionic liquids with a non-
relatively lower than that reported by Loh et al., only the ionic
liquid with the SbF anion, [bmim][SbF ], showed appreciable
6
6
catalytic activity with 34% conversion (entry 1, Table 1). Accord-
ingly, the cyanosilylation was carried out first with 0.5 mol% of
Sc(OTf) in [bmim][SbF ]. All the benzaldehyde was converted
coordinating anion, such as PF
6
and SbF , exhibited positive
6
ionic liquid effects that were attributed to in situ anion exchange
3
6
between the catalyst and ionic liquid causing them to become a
to the corresponding cyanosilyl ether 2a within 5 min (entry 2,
Table 1), even in the presence of 0.1 mol% catalyst (entry 3,
Table 1). 40% conversion was achieved in 5 min when the
3i
more electrophilic Lewis acid. With the aim of exploiting the
4
synergic effects of lanthanide triflates in ionic liquids, this study
developed highly effective and environmentally benign catalytic
cyanosilylation reactions of aldehydes and ketones in an ionic
liquid with a turnover frequency (TOF) up to 48 000 mol h and
a total turnover number (TON) of 100 000.
Sc(OTf) loading was decreased further to 0.01 mol%, indicating
3
a TOF (defied as moles of product per mole of catalyst per hour)
-
1
-1
of 48 000 mol h (entry 4, Table 1). More than 99% conversion
was achieved when the reaction time was extended to 30 min,
thus indicating that the Sc(OTf) in [bmim][SbF ] is one of the
Cyanohydrines have demonstrated considerable synthetic
potential as useful building blocks in organic synthesis.
Hence, many catalytic methods, including asymmetric reactions,
3
6
most reactive catalysts reported (entry 5, Table 1). In contrast,
significantly decreased conversions were observed in other ionic
liquids, [bmim][BF ] (entry 6, Table 1) and [bmim][PF ] (entry 7,
5
have been explored. Among the various catalysts examined,
4
6
Lewis acid-catalyzed cyanosilylations with trimethylsilylcyanide
Table 1) and in CH Cl (entry 8, Table 1). The catalytic
2
2
6
(
TMSCN) have been investigated most extensively. Therefore,
activities of various lanthanide triflates and In(OTf) were also
3
it is not surprising to expect that the Lewis acidic rare-earth
triflates may be effective catalysts for cyanosilylation reactions.
Lanthanide triflates, particularly scandium(III) triflate, are
investigated using a loading of 0.1 mol% in [bmim][SbF ]. As
6
shown in Fig. 1, Gd(OTf) (97%), Ho(OTf) (97%), Er(OTf)
3
3
3
(97%), Tm(OTf) (99%), Y(OTf) (96%), and In(OTf) (96%)
3
3
3
also exhibited excellent activity. Other lanthanide triflates,
such as Ce(OTf) (87%), Sm(OTf) (91%), Eu(OTf) (86%),
Tb(OTf) (93%), Lu(OTf) (94%), and La(OTf) (93%), showed
comparable activity with conversions ranging from 86 to 94%.
On the other hand, Pr(OTf) (<5%), Nd(OTf) (<5%), and
Dy(OTf) (28%) showed extremely low catalytic activity. These
3
3
3
Department of Chemistry and Nano Science (BK 21), Ewha Womans
University, Seoul, 120-750, Korea. E-mail: sanggi@ewha.ac.kr;
3
3
3
Fax: +82 2 3277 3419; Tel: +82 2 3277 4505
3
3
1
†
Electronic supplementary information (ESI) available:
H and
1
3
3
C NMR spectra for 2a–2t, and 3a, 3d, 3i, 3k, 3m. See DOI:
1
0.1039/b900254e
results suggested that an appropriate choice of metal triflate and
9
46 | Green Chem., 2009, 11, 946–948
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Table 1 Sc(OTf)
3
-catalyzed cyanosilylation of benzaldehyde with TM-
SCN in an ionic liquid
Table 2 Sc(OTf)
3
-catalyzed cyanosilylation of various aldehydes and
a
a
ketones in [bmim][SbF
6
]
b
Entry
1
Product
Yield (%)
b
Entry
Sc(OTf)
3
(mol%)
Solvent
Time
Conv. (%)
1
2
3
4
5
6
7
8
—
[bmim][SbF
[bmim][SbF
[bmim][SbF
[bmim][SbF
[bmim][SbF
6
6
6
6
6
]
]
]
]
]
24 h
34
>99
>99 (98)
40
>99
11
<5
7
1
2
3
4
5
6
7
8
9
10
11
12
13
14
5
6
7
8
9
2-CH
3-CH
3
OC
OC
6
6
H
H
4
4
CHO (1b)
CHO (1c)
2b
>99
>99
>99 (94)
>99
>99
96
100
>99 (98)
>99
>99 (95)
>99
>99 (95)
>99
>99
>99
>99
>99
>99
>99
0.5
0.1
0.01
0.01
0.1
0.1
0.1
5 min
5 min
5 min
30 min
5 min
5 min
5 min
3
2c
c
4-CH OC H CHO (1d)
2d (3d)
2e
3
6
4
4-CH
3
C
6
H
4
CHO (1e)
4-ClC
4-PhOC
2-Naphthaldehyde (1h)
2-Furaldehyde (1i)
2-Thiophenecarboxaldehyde (1j)
6
H
4
CHO (1f)
2f
[bmim][BF
[bmim][PF
CH Cl
4
]
]
6
H
4
CHO (1g)
2g
2h
6
2i (3i)
2j
2
2
a
Reaction conditions: benzaldehyde (1.0 mmol), TMSCN (1.2 mmol)
Trans-C
PhCH CH
CH CH
(CH CHCHO (1n)
6
H
4
CH=CHCHO (1k)
2k (3k)
2l
b
1
in [bmim][X] (1.0 mL) at room temperature. Determined by H NMR
analysis with the average of three reactions. Parentheses denote the
isolated yield as cyanohydrine after acidic hydrolysis of the cyanosilyl
ether with 1 N aqueous HCl solution for 2 h at room temperature.
2
2
CHO (1l)
c
3
2
CH
2
CHO (1m)
2m (3m)
2n
3 2
)
PhCOCH
PhCOCH
PhCOPh (1q)
3
(1o)
CH
2o
1
1
1
1
1
2
3
(1p)
2p
2q
1-Indanone (1r)
2r
a-Tetralone (1s)
2s
Cyclohexanone (1t)
2t
a
Reaction conditions: carbonyl 1 (1.0 mmol), TMSCN (1.2 mmol), and
(0.1 mol%) in [bmim][SbF ] (1.0 mL) at room temperature for
min for aldehydes and 2 h for ketones. Parentheses denote the isolated
yield as cyanohydrine after acidic hydrolysis of the cyanosilyl ether with
N aqueous HCl solution for 2 h at room temperature.
Sc(OTf)
5
3
6
b
1
toward cyanosilylation of ketones. For example, cyanosilylation
of acetophenone with TMSCN using 5 mol% of Yb(OTf)
in CH Cl provided the corresponding cyanosilyl ether in
1% after 21 h reaction. To our delight, all aromatic and
aliphatic ketones (1o–1t) reacted with TMSCN in the presence
of 0.1 mol% of Sc(OTf) for 2 h to yield the corresponding
3
2
2
10c
4
3
cyanohydrine silyl ethers (2o–2t) quantitatively (entries 14–19,
Table 2).
The excellent recyclability of this catalytic system was ex-
amined with the reaction between benzaldehyde and TMSCN
Fig. 1 Yields obtained with different lanthanide triflates. Reaction con-
ditions: benzaldehyde (1.0 mmol), TMSCN (1.2 mmol), and Ln(OTf)
0.1 mol%) in [bmim][SbF ] (1.0 mL) at room temperature for 5 min. The
3
(
6
1
conversion was determined by H NMR from the average three times
reactions.
in the presence of 0.01 mol% of Sc(OTf)
room temperature for 30 min. After extracting the reaction
mixture with n-hexane, the ionic liquid containing Sc(OTf)
was reused for the next run. No leaching of Sc into organic
layer was observed. As shown in Table 3, the recovered Sc(OTf)
immobilized in [bmim][SbF ] could be reused 10 times without
3
in [bmim][SbF ] at
6
ionic liquid could generate a highly effective catalytic system
for cyanosilylation of carbonyl compounds. The high catalytic
3
activity of metal triflates in [bmim][SbF
6
] is ascribed to the anion
] resulted in a more
3
exchange between Sc(OTf) and [bmim][SbF
3
6
6
3i
Lewis acidic catalyst.
any loss of catalytic activity achieving a total turnover number of
almost 100 000. These results clearly show that a combination of
The present catalytic system worked with a broad range of
substrates. Various aldehydes were investigated in the presence
Sc(OTf) with the ionic liquid increases both its catalytic activity
3
of 0.1 mol% of Sc(OTf)
3
in [bmim][SbF
6
] for 5 min. As shown
and stability.
in Table 2 (entries 1–13, Table 2), all aromatic (1b–1h) and
heteroaromatic aldehydes (1i and 1j), a,b-unsaturated (1k) and
aliphatic aldehydes (1l–1n) were uniformly transformed into
the corresponding cyanohydrine trimethylsilyl ethers (2b–2n)
in quantitative yields. Some of the selected cyanosilyl ethers
In summary, this study examined the catalytic activity of
lanthanide triflates in the cyanosilylation of benzaldehyde with
TMSCN in an ionic liquid. A highly reactive, recyclable and
environmentally benign catalytic system consisting of Sc(OTf)
in [bmim][SbF ] was developed. With this catalytic system,
3
6
(
2a, 2d, 2i, 2k, and 2m) were hydrolyzed to the corresponding
cyanohydrine with excellent yields (entry 3, Table 1 and entries
, 8, 10, 12, Table 2). The reaction can also be extended to
various cyanohydrine silyl ethers were obtained from a wide
range of aldehydes and ketones in quantitative yield. Anion
3
exchange between Sc(OTf)
the catalyst more Lewis acidic, might be responsible for the
excellent catalytic activity of Sc(OTf) in [bmim][SbF ]. The ionic
3
and [bmim][SbF
6
], which makes
a variety of ketones. It has been reported that, compared to
aldehydes, the lanthanide triflates are known to be less reactive
3
6
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a
3 6
Table 3 Recycling of Sc(OTf) and [bmim][SbF ] in the cyanosilylation of benzaldehyde with TMSCN
b
Run
1st
2nd
3rd
98
4th
96
5th
6th
7th
97
8th
97
9th
>99
10th
Conv.(%)
>99
>99
>99
>99
>99
a
Reaction conditions: benzaldehyde (5.0 mmol), TMSCN (6.0 mmol) and Sc(OTf)
0 min. The product was extracted with n-hexane (50 mL ¥ 3) and the ionic liquid containing Sc(OTf) was reused for the next run.
3
(0.01 mol%) in [bmim][SbF
6
] (5.0 mL) at room temperature for
b
3
3
liquid containing Sc(OTf)
3
could also be recovered and reused
Korean Chem.Soc., 2002, 23, 1367; (d) C. E. Song, D.-u. Jung, E. J.
Rho, S.-g. Lee and D. Y. Chi, Chem. Commun., 2002, 3038; (e) C. R.
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Studies aimed at applying this combination of lanthanide triflate
with an ionic liquid to asymmetric cyanosilylations and to other
catalytic reactions are currently under way.
2
003, 2624; (h) C. E. Song, D.-u. Jung, S. Y. Choung, E. J. Rho and
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This work was supported by the Korean Research Foundation
4
5
3
(
KRF-2006-312-C00587) and the Center for Intelligent Nano-
Chem. Rev., 2007, 107, 2592.
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Notes and references
‡
General procedure for the cyanosilylation of carbonyl compounds: a
solution of Sc(OTf) (0.1 mol%) in [bmim][SbF ] (1.0y) was stirred for
0 min at room temperature. A carbonyl compound (1.0 mmol) and
3
6
6
Selected recent papers, see: (a) J. B. King and F. P. Gabbai,
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1
TMSCN (1.2 mmol) were added successively, and the reaction mixture
was stirred for 5 min for aldehydes (2 h for ketones) at room temperature
under a nitrogen atmosphere. The reaction mixture was extracted with n-
hexane (10 mL ¥ 3), and the combined n-hexane layer was concentrated
to give the corresponding cyanosilylated 2. The results are shown in
Tables 1 and 2. For catalyst recycling, the reaction was carried out in
(
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3
, and the recovered ionic liquid
1
layer containing Sc(OTf) was dried under a vacuum pump, and reused
3
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from the above reaction was treated with 1 N HCl (3.0 mL), and stirred
for 2 h at room temperature. The reaction mixture was then extracted
7
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a saturated aqueous NaHCO
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2
Cl
2
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8
9
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9
48 | Green Chem., 2009, 11, 946–948
This journal is © The Royal Society of Chemistry 2009