A highly efficient and recyclable silica-based scandium(III) interphase catalyst for cyanosilylation of carbonyl compounds

Article abstract of DOI:10.1021/ol0482083

(Chemical Equation Presented) A new silica-based scandium(III) interphase catalyst 2 efficiently catalyzes the cyanosilylation of a variety of aldehydes and ketones. The catalyst shows high thermal stability (up to 300°C) and also is stable in both organi

Full text of DOI:10.1021/ol0482083

ORGANIC
LETTERS
2004
Vol. 6, No. 26
4813-4815
A Highly Efficient and Recyclable
Silica-Based Scandium(III) Interphase
Catalyst for Cyanosilylation of Carbonyl
Compounds
Babak Karimi*,†,‡ and Leila Ma’Mani†
Department of Chemistry, Institute for AdVanced Studies in Basic Sciences (IASBS),
P.O. Box 45195-159, GaVa Zang, Zanjan, Iran, and Institute for Fundamental
Research(IPM), Farmanieh, P.O. Box 19395-5531, Tehran, Iran
karimi@iasbs.ac.ir
Received September 5, 2004
ABSTRACT
A new silica-based scandium(III) interphase catalyst 2 efficiently catalyzes the cyanosilylation of a variety of aldehydes and ketones. The
catalyst shows high thermal stability (up to 300 C) and also is stable in both organic and polar solvents. It also could be recovered and
reused for at least 10 reaction cycles without considerable loss of reactivity.
°
Interest in scandium has increased recently due to the
successful use of its compounds in organic chemistry.1 Along
this line, scandium triflate has been introduced as a promising
mild, powerful, and selective Lewis acid for a variety of
functional group transformations.2 While most Lewis acids
are decomposed or deactivated in the presence of water or
protic solvents, Sc(OTf)3, unlike other traditional Lewis acids,
does not decompose or deactivate under aqueous workup
conditions, and its recovery is often possible.2,3 However,
the most popular recycling methods for recovery of Sc(OTf)3
are time-consuming and include the successive extraction
of organic reaction mixture using deionized water.2,3 There-
fore, there is much room for the design and preparation of
new versions of scandium-based catalysts to circumvent this
problem without considerable loss of its catalytic power.
The heterogenization of inorganic reagents and catalysts
useful in organic reactions is a very important area in so-
called “Green” or sustainable chemistry.4 One of the major
goals is to facilitate easy separation of the final reaction
mixture.5 This feature can lead to improved processing steps,
better process economics, and finally, environmentally
friendly industrial manufacturing. However, traditional het-
erogeneous and supported catalysts are rather limited in the
nature of their active sites and, thus, the scope and reproduc-
ibility of reactions that they can accomplish.6 One way to
overcome this uncertainty of the heterogeneous catalysts is
to covalently incorporate an organic entity (flexible spacer)
onto inorganic solids to create organic-inorganic hybrid
(interphase) catalysts.7 In these types of solid catalysts the
† Institute for Advanced Studies in Basic Sciences.
‡ Institute for Fundamental Research.
(1) (a) Cotton, S. A. Polyhedron 1999, 18, 1691. (b) Cotton, F. A.;
Wilkinson, G. AdVanced Inorganic Chemistry, 5th ed.; Wiley: New York,
1988; p 973.
(2) For reviews, see: (a)Kobayashi, S. Eur. J. Org. Chem. 1991, 1, 15.
(b)Kobayashi, S.; Sugiura, M.; Kitagava, H.; Lam, W. W. L. Chem. ReV.
2002, 102, 2227. For more recent leading references, see: (c) Karimi, B.;
Ma’mani, L. Tetrahedron Lett. 2003, 44, 6051. (d) Karimi, B.; Ma’mani,
L. Synthesis 2003, 2503
(4) (a) Clark, J. H. Catalysis of Organic Reactions using Supported
Inorganic Reagents; VCH: New York, 1994. (b) Chemistry of Waste
Minimization; Clark, J. H., Ed.; Chapman and Hall: London, 1995.
(5) Clark, J. H. Green Chem. 1991, 1.
(6) Bhalay, G.; Dunstan, A.; Glen. A. Synlett 2000, 1846.
(7) (a) Lu, Z. L.; Lindner, E.; Mayer, H. A. Chem. ReV. 2002, 102, 3543.
(b) Wight, A. P.; Davis, M. E. Chem. ReV. 2002, 102, 3589.
(3) Kobayashi, S. Synlett 1994, 689.
10.1021/ol0482083 CCC: $27.50
© 2004 American Chemical Society
Published on Web 11/20/2004

reactive center is highly mobile similar to homogeneous
catalysts, and at the same time it has the advantage of
recyclability. Recently, Kobayashi et al. have prepared a new
polymer-supported scandium triflate which shows high
activity in water.8 While this is the first example of polymer-
supported scandium, its preparation suffers from the draw-
backs of the use of expensive starting organic polymers and
reagents. However, to the best of our knowledge there are
no examples of silica-based scandium(III) interphase cata-
lysts. Herein we wish to present the design, synthesis, and
catalytic properties of a novel Sc(OTf)3-immobilized onto
an organically modified silica (Scheme 1).9 Our hypothesis
Table 1. Cyanosilylation of Carbonyl Compounds Using
TMSCN in the Presence of Silica Based Scandium Interphase
Catalyst 2
entry
R1
R2
time (h) yielda,b (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Ph
Ph
Ph
H
3
24
3
4
4
4
2.5
3
8
12
4.5
3
5
5
5
6
6.5
12
95
0c
H
H
H
H
H
H
H
H
H
45d
96
98
95
97
92
90
91
90e
95
94
96
94
92
95
70
80
56e
3-(NO2)C6H4
2-(NO2)C6H4
4-(MeO)C6H4
4-(i-Pr)C6H4
4-(Cl)C6H4
3- Pyridyl
2,6-(Cl)2C6H3
citral
PhCHdCH-
CH3CH2CH2
CH3(CH2)5
4-phenylcyclohexanone
cycloheptanone
cyclopentanone
CH3(CH2)3
Scheme 1
H
H
H
Et
CH3
CH3
PhCH2
4-(Ph)C6H4
12
16
a Isolated yields. b The molar ratio of substrate/TMSCN/2 is 1:1.2:0.0465.
c The reaction was performed in the absence of 2. d The reaction was carried
out in the presence of the solid 2 for 1 h, and at that point the catalyst was
filtered off and further stirring was done in the absence of catalyst for 2 h.
e NMR yields.
In the course of preliminary studies of catalytic properties
of 2, we found that a small amount of 2 (0.15 g, ∼ 4.65 mol
%) catalyzes efficient cyanosilylation of various types of
carbonyl compounds (Table 1, Scheme 2).
was to change the expensive organic polymer chain to silica
chain having spacers composed of a bidentate ligand to
prepare a new immobilized silica-based scandium(III) in-
terphase catalyst. Moreover, the presented bidentate ligand
might be possible to replace by chiral analogues giving rise
the corresponding chiral version of scandium for application
in asymmetric transformations. Quantitative determination
of the functional group contents of the surface-bound
compound 1 was performed with thermogravimetric analysis
(TGA). Typically, a loading at ca. 0.3 mmol‚g-1 is obtained.
Similarly, TGA/DTA analyses of immobilized scandium-
(II) were performed and showed a first peak due to the
desorption of water (centered at 110 °C). This is followed
by a second peak at 375 °C, corresponding to the loss of
triflate groups accompanied by a third peak centered at 535
°C, which corresponds to the loss of the surface-bound
bidentate ligand. Typical loading of scandium was deter-
mined using atomic spectroscopy (AA) and shows a loading
0.29 ( 0.01 mmol.g-1. This result in combination with TGA
analyses demonstrates that 2 corresponds to a 1:1 and 2:1
complex between surface bound and triflate ligands with
scandium, respectively (Schemes 1 and 2).
Scheme 2
Cyanohydrin trimethylsilyl ethers are industrially valuable
and important intermediates in the synthesis of cyanohydrins,
â-amino alcohols, R-hydroxy acids, and other biologically
active compounds.10 They generally have been prepared by
the reaction of a carbonyl with TMSCN in the presence of
Lewis acids,11 lanthanide salts,12 base catalysts,13 and solid
base catalysts.14 However, many of the existing methods for
(10) Kruse, C. G. In Chirality in Industry; Collins, A. N., Sheldrake, G.
N., Crosby, J., Eds.; Wiley: Chichester, 1992; Chapter 14.
(11) (a) Evans, D. A.; Truesdale, L. K.; Carroll, G. L. J. Chem. Soc.,
Chem. Commun. 1973, 55. (b) Birkofer, L.; Muller, F.; Kaiser, W.
Tetrahedron Lett. 1967, 2781. (c) Evans, D. A.; Truesdale, L. K.
Tetrahedron Lett. 1973, 4929. (d) Noyori, R.; Murata, S.; Suzuki, M.
Tetrahedron 1981, 37, 3899. (e) Loh, T. P.; Xu, K. C.; Ho, D. S.; Sim, K.
Y. Synlett 1998, 369. (f) Saravanan, P.; Anand, R. V.; Singh, V. K.
Tetrahedron Lett. 1998, 39, 3823. (g) Shen, Y.; Feng, X.; Li, Y.; Zhang,
G.; Jiang, Y. Synlett 2002, 793.
(8) Nagayama, S.; Kobayashi, S. Angew. Chem., Int. Ed. 2000, 39, 567.
(9) For the details of experimental procedures and characterization of
the catalyst, see the Supporting Information.
(12) (a) Matsubara, S.; Takai, T.; Utimoto, K. Chem. Lett. 1991, 1447.
(b) Vougiokas, A. E.; Kagan, H. B. Tetrahedron Lett. 1987, 28, 5513.
4814
Org. Lett., Vol. 6, No. 26, 2004

this transformation have disadvantages such as prolonged
reaction time,13 the use of nonrecoverable catalysts,10 and
poor yields of the corresponding cyano trimethylsilyl ether
especially in the case of ketones. Carbonyl compounds that
were cyanosilylated in this way are benzaldehydes with both
electron-withdrawing and electron-releasing groups including
sterically hindered ones, aliphatic and R,â-unsaturated al-
dehydes, and cyclic, open-chain, and aromatic ketones.
Representative results are shown in Table 1.
To show the catalyst’s activity we have also performed
cyanosilylation reaction of benzaldehyde using TMSCN in
the absence of 2 under other similar reaction parameters.
The reaction did not progress even after 24 h (Table 1, entry
2).
Figure 1. Recyclability of scandium catalyst 2 for the cyanosily-
lation of benzaldehydes using TMSCN.
As expected, owing to the steric bulk, the yields of the
corresponding cyano trimethylsilyl ethers were moderate in
the case of aromatic ketones (Table 1, entry 20). In contrast
to aromatic ketones, however, the catalyst is quite effective
for cyanosilylation of both cyclic and open-chain aliphatic
ketones (Table 1, entries 15-19).
In conclusion, the novel silica-based scandium(III) inter-
phase catalyst 2, which can be prepared by simple operation
from commercially available and relatively cheap starting
materials, efficiently catalyzes the cyanosilylation of a variety
of aldehydes and ketones. The catalyst shows high thermal
stability (up to 300 °C) and also is stable in both organic
and protic solvents. It also could be recovered and reused
for several reaction cycles without considerable loss of
reactivity. Owing to the bidentate model of the surface bound
ligand in 2, it might be possible to prepare chiral analogues
of 2 using a chiral bidentate ligand. To our knowledge this
is the first example of scandium-based interphase catalyst.
The work on other applications of 2 and also preparation of
its chiral analogues is currently ongoing in our laboratories.
When using a supported catalyst two points become
increasingly crucial issues. The first is the possibility that
some active metal migrates from the solid to liquid phase
and that this leached Sc3+ would become responsible for a
significant extent of the catalytic activity. To rule out the
contribution of homogeneous catalysis in the results shown
in Table 1, the reaction was carried out in the presence of
the solid 2 for 1 h, and at that point the catalyst was filtered
off. The liquid filtrate was then allowed to react, but no
significant conversion was observed after 3 h under the
presented reaction conditions (Table 1, entry 3). This clearly
indicates that no active species were present in the super-
natant. Furthermore, the percent of scandium leaching was
less than 1% of the surface-bound scandium even upon
washing of the catalysts with deionized water. The second
point is the deactivation and reusability of the 2. It should
be noticed that while most Lewis acids are decomposed or
deactivated in the presence of protic solvents, the new
heterogenized scandium catalyst is stable both in water and
organic solvents and could be easily recovered and reused
for at least 10 cycles without loss of yields of products.
Recycling experiments were examined for the cyanosilylation
of benzaldehyde. Thus, after the first run, which gave the
corresponding cyanosilyl ether in 95% yield, after recovery
the catalyst was subjected to a second cyanosilylation
reaction from which it gave the cyanosilyl ether in 98% yield;
the average chemical yield for 10 consecutive runs was 96%,
which clearly demonstrates the practical recyclability of this
catalyst (Figure 1).
Acknowledgment. We acknowledge the Institute for
Fundamental Research (IPM) and Institute for Advanced
Studies in Basic Sciences (IASBS) Research Councils for
support of this work. We also acknowledge Dr. E. Shams
for his effort concerning the AA of samples.
Supporting Information Available: Experimental pro-
cedures and 1H and 13C NMR spectra for new products and
TGA for catalyst 2. This material is available free of charge
via the Internet at http://pubs.acs.org.
(13) (a) Kobayashi, S.; Tsuchiya, Y.; Mukaiyama, T. Chem. Lett. 1991,
537. (b) Comber, R. N.; Hosmer, C. A.; Brouillette, W. J. J. Org. Chem.
1985, 50, 3627.
(14) Kantam, M. L.; Sreekanth, P.; Santhi, P. L. Green Chem. 2000, 47.
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