Cyanosilylation of carbonyl compounds with trimethylsilyl cyanide catalyzed by an yttrium-pillared silicotungstate dimer

Article abstract of DOI:10.1002/anie.201200486

An yttrium-pillared silicotungstate dimer (see picture) catalyzes the cyanosilylation of structurally diverse ketones and aldehydes with trimethylsilyl cyanide (TMSCN). The reactions proceed selectively and afford the corresponding cyanohydrin trimethylsi

Full text of DOI:10.1002/anie.201200486

.
Angewandte
Communications
DOI: 10.1002/anie.201200486
Polyoxometalates
Cyanosilylation of Carbonyl Compounds with Trimethylsilyl Cyanide
Catalyzed by an Yttrium-Pillared Silicotungstate Dimer**
Yuji Kikukawa, Kosuke Suzuki, Midori Sugawa, Tomohisa Hirano, Keigo Kamata,
Kazuya Yamaguchi, and Noritaka Mizuno*
Cyanohydrins are a very important class of compounds in
chemistry as well as biology, and have been widely utilized as
important synthetic intermediates for organic compounds,
such as a-hydroxy acids, a-hydroxy aldehydes, and b-amino
alcohols. For the synthesis of cyanohydrins, various cyanat-
ing reagents have been employed, and trimethylsilyl cyanide
as nucleophilic sites as well as stabilizers of cationic inter-
[5,6]
mediates. In particular, it is expected that POMs with large
negative charges can nucleophilically activate TMSCN, thus
resulting in promotion of cyanosilylations, as observed for
[1]
[2h,3h]
Lewis base catalyzed cyanosilylations.
Initially, the cyanosilylation of 2-adamantanone (1a) with
TMSCN was carried out with the À8-charged POM
TBA H [g-SiW O ] (SiW10, TBA = tetra-n-butylammo-
(
TMSCN) is one of the most useful and safe cyanating
reagents for nucleophilic addition to carbonyl compounds to
4
4
10 36
[
1]
give cyanohydrin trimethylsilyl ether. Hence, the develop-
ment of efficient catalysts for cyanosilylation of carbonyl
compounds with TMSCN is a very important subject in
current research, and several efficient catalysts have been
nium) in 1,2-dichloroethane at 308C. SiW10 promoted the
cyanosilylation, giving the corresponding cyanohydrin trime-
[7]
thylsilyl ether 2a in 15% yield after 20 minutes (Scheme 1).
In the case of TBA H [a-SiW O ] (SiW11), 2a was also
4
4
11 39
[
2–4]
developed so far.
Lewis acid catalysts can act as electro-
obtained in 16% yield (Table S2). In the absence of the
catalysts, cyanosilylation was not observed under the present
conditions.
philic catalysts to activate carbonyl compounds and have been
extensively investigated for cyanosilylation (see Table S1 in
[
2,3]
the Supporting Information).
Several nucleophilic cata-
lysts, such as amines, phosphines, phosphazanes, and alkaline-
earth metal oxides, can activate TMSCN and promote
[
2,3]
cyanosilylation (Table S1).
Asymmetric cyanosilylations
have also been successfully developed by employing customly
[
4]
designed chiral ligands.
Polyoxometalates (POMs) are a large family of anionic
metal–oxygen clusters that consist of Group V and VI metals
in their highest oxidation states, and are thermally and
oxidatively stable in comparison with commonly utilized
Scheme 1. Cyanosilylation of 2-adamantanone (1a) with TMSCN. Reac-
[
5]
tion conditions: catalyst (1 mol% for SiW10 and Y(acac) ; 0.5 mol%
3
organometallic catalysts and organocatalysts. The chemical
properties of POMs, for example, redox potentials, (multi)-
electron-transfer properties, acidities, and solubilities, and
negative charges, can be finely tuned by choosing the
constituent anion and countercations, and diverse structures
for Y2), 1a (0.5 mmol), TMSCN (molar ratio TMSCN/1a=1.5:1), 1,2-
dichloroethane (3 mL), 308C, 20 min.
1
3
The C NMR spectrum of TMSCN in [D ]1,2-dichloro-
4
[
5]
can be synthesized. In addition to the above-mentioned
properties, the important feature of POMs is the presence of
bare nucleophilic surfaces as a result of external oxygen atoms
ethane (50 mm) showed a methyl signal at À1.95 ppm. Upon
addition of one equivalent of SiW10 to the solution, the signal
almost disappeared, and a new signal was observed at
1.01 ppm and assigned to the methyl groups of trimethyl-
(
MÀOÀM and M=O species, M = W or Mo), which might act
[
8]
silanol, thus suggesting that SiW10 can nucleophilically
activate TMSCN; trimethylsilanol is probably produced
through the nucleophilic activation of TMSCN by SiW10,
followed by the reaction with water (including in SiW10 and/
[*] Dr. Y. Kikukawa, Dr. K. Suzuki, M. Sugawa, T. Hirano, Dr. K. Kamata,
Dr. K. Yamaguchi, Prof. Dr. N. Mizuno
Department of Applied Chemistry, School of Engineering
The University of Tokyo
[
9]
or the solvent). In addition, the positive-ion cold-spray
ionization mass spectrometry (CSI-MS) analysis of a solution
of SiW10 and TMSCN (molar ratio = 1:200) in 1,2-dichloro-
ethane showed a set of signals centered at m/z = 3947 and
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan)
E-mail: tmizuno@mail.ecc.u-tokyo.ac.jp
[
**] This work was supported in part by the Japan Society for the
Promotion of Science (JSPS) through its FIRST Program, Global
COE Program (Chemistry Innovation through Cooperation of
Science and Engineering), and Grants-in-Aid for Scientific
Researches from the Ministry of Education, Culture, Sports,
Science, and Technology. Y.K. and T.H. are grateful for JSPS Research
Fellowship for Young Scientists.
+
corresponding to [TBA TMS SiW O ] (TMS = trimethyl-
5
4
10 36
silyl cation, Figure S1), also supporting the idea.
Although SiW10 could activate TMSCN and promote the
cyanosilylation of 1a, the activity was still very low. If
carbonyl compounds are simultaneously activated at the
neighboring electrophilic Lewis acid centers, reaction rates
for cyanosilylation are increased by the synergetic effect. In
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201200486.
3
686
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Angew. Chem. Int. Ed. 2012, 51, 3686 –3690

Angewandte
Chemie
order to investigate the simultaneous activation of both
TMSCN and carbonyl compounds, we carried out the SiW10-
catalyzed cyanosilylation of 1a in the presence of various
The bond-valence sum (BVS) values of silicon (3.85),
tungsten (5.92–6.09), and yttrium (3.11) atoms in Y2’ indicate
that the respective valences are + 4, + 6, and + 3. In addition,
the oxygen atoms with relatively lower BVS values (1.13 and
1.14 for O23 and O24, respectively) were found in the
3
+
3+
2+
3+
4+
4+
3+
3+
4+
Al , Sc , Zn , Y , Zr , Sn , La , Yb , and Hf salts as
[
10]
Lewis acids (Table S2). Several rare-earth-metal salts were
8
À
found to promote the cyanosilylation of 1a effectively
[g-SiW O ] framework, thus suggesting that protons are
10 36
(
Table S2). For example, addition of Y(acac) (acac = acetyl-
likely located on these oxygen atoms. The O2 (BVS = 0.36)
and O3 (BVS = 0.39) atoms were oxygen atoms of water
molecules (aqua ligands).
3
acetonato) to SiW10 increased the yield of 2a from 15% to
1%, while the activity of Y(acac) was almost negligible (2%
4
3
8
À
yield; Scheme 1). In contrast, such a positive effect of
Yttrium-containing POM Y2’ is a dimer of [g-SiW O ]
10 36
Y(acac) was not observed for the SiW11-catalyzed cyano-
pillared by two yttrium atoms (Figure 1). Each yttrium atom
3
silylation (Table S2). The positive-ion CSI-MS spectrum of
a mixture of SiW10 and Y(acac)3 in 1,2-dichloroethane
exhibited several sets of signals. Among them, two main
sets of signals centered at m/z = 3745 and 7283 were assign-
in Y2’ is heptacoordinated by one acetone molecule, two aqua
8À
ligands, and four terminal oxygen atoms of the [g-SiW O ]
10
36
subunits. Most importantly, acetone molecules are coordi-
nated to the yttrium centers in Y2’, thus suggesting that these
sites might act as Lewis acidic centers to activate carbonyl
2
+
able to [TBA H (SiYW O ) ] and [TBA H (SiYW O ) -
1
0
2
10 36
2
9
2
10 36 2
+
[14]
(
H O) ] , respectively (Figure S2a). This result suggests that
compounds. Each yttrium atom is located “out-of-pocket”
2
2
SiW10 reacts with Y(acac)3 in situ to form an yttrium-
without direct interaction with the internal SiO tetrahedrons.
4
1
0À [11]
containing POM [(SiYW O ) ]
,
which would simulta-
The distances between oxygen atoms that are connected to
yttrium atoms (O2 and O3) and nearest-neighbor oxygen
1
0
36 2
neously activate both TMSCN and carbonyl compounds.
These results led us to synthesize the yttrium-containing POM
8
À
atoms of the [g-SiW O ] framework are in the range of
1
0
36
by employing Y(acac) (as an yttrium source) and SiW10 (as
an inorganic ligand).
2.71–2.96 ꢀ (Table S3), thus suggesting the existence of
hydrogen-bonding networks between these oxygen atoms.
3
[15]
The yttrium-containing POM Y2 was successfully synthe-
Thus, the unique local structure around yttrium atoms is likely
stabilized by the hydrogen-bonding networks. The slipped
configuration of the dimer of [g-SiW O ] is probably
10 36
sized by the reaction of SiW10 with Y(acac) (molar ratio =
3
8
À
1
:1) in acetone (85% yield based on SiW10, see the
Supporting Information). The positive-ion CSI-MS spectrum
a result of the steric bulk of four aqua ligands located in the
[16]
of Y2 in 1,2-dichloroethane exhibited only the set of signals
interior.
To date, various yttrium-containing POMs have been
1
0À
that corresponds to the POM [(SiYW O ) ] (Figure S2b).
1
0
36 2
2
9
[17]
9À
The Si NMR spectrum of a solution of Y2 in [D ]1,2-
reported,
e.g., [YW O ] , [{Y(a-SbW O (OH) )-
4
10 36
7À
9
31
2
nÀ
1
dichloroethane showed a signal at À82.1 ppm (Figure S3a),
thus suggesting that Y2 is a single species in 1,2-dichloro-
ethane. From the results of elemental analysis and thermog-
ravimetry, the formula of Y2 was determined to be
(CH COO)(H O)} (WO )] , [{Y(H O)} (b-XW O )CO ]
3
2
3
4
2
3
9
34
3
(X = Ge, Si, As, and P, n = 13 and 12), [Y(GeW O )-
1
1
39
5
À
15À
(H O) ] , [Y (H O) (SbW O )(W O ) ] , [(PY W O ) -
2
2
2
2
2
9
33
5
12À
18
2
2
10 38 4
3
0À
(W O )]
,
[Y(H O)P W O110]
,
and [{Y (m -OH) -
4 3 4
(H O) }(a-P W O ) ] . To the best of our knowledge,
2 8 2 15 56
3
14
2
5
30
[
12]
16À
TBA H [(SiYW O ) ]·7H O.
Fortunately, single crystals
8
2
10 36
2
2
2
of the yttrium-containing POM with the formula
yttrium-containing POMs with g-Keggin frameworks have
not been reported to date.
[12]
TBA H [(SiYW O ) ]·3(acetone)·6H O (Y2’)
suitable
6
4
10 36
2
2
for X-ray crystallographic analysis were successfully obtained
by recrystallization in a mixture of acetone and acetonitrile
Finally, we investigated the catalytic property of Y2 for
[18]
cyanosilylation of carbonyl compounds with TMSCN. The
catalytic performance of Y2 was higher than those of SiW10,
Y(acac) , and a mixture of SiW10 and Y(acac) (Scheme 1). In
(
see the Supporting Information). The ORTEP representa-
[
13]
tion of the anion of Y2’ is shown in Figure 1.
3
3
the presence of Y2 (using only 1 mol%), several structurally
diverse ketones could be converted into the corresponding
cyanohydrin trimethylsilyl ethers, and desilylated products
(
(
cyanohydrins) were not formed in any of these cases
Table 1). The cyanosilylation of linear aliphatic ketones 1e
and 1 f proceeded smoothly and gave the corresponding
cyanohydrin trimethylsilyl ethers in high yields. Sterically
hindered aliphatic cyclic ketones, such as 2-adamantanone
(1a) and cyclooctanone (1d), also reacted with TMSCN to
afford the corresponding products in high yields, although the
steric effect was very significant. Compound 2b was selec-
tively obtained in 89% yield along with a small amount of 2d
(8% yield) when the Y2-catalyzed competitive cyanosilyla-
tion of 1b (C5, 0.25 mmol) and 1d (C8, 0.25 mmol) with
TMSCN (0.375 mmol) was carried out for 0.3 hours under the
conditions shown in Table 1. Such a size-selective cyanosily-
lation might be caused by the steric hindrance around the
Figure 1. ORTEP representation of the anion part of Y2’. The thermal
ellipsoids are shown at 50% probability.
[
19]
active site(s).
Angew. Chem. Int. Ed. 2012, 51, 3686 –3690
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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3687

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Angewandte
Communications
[
a]
Table 1: Scope of the Y2-catalyzed cyanosilylation of ketones.
sponding cyanohydrin trimethylsilyl ether without formation
of the 1,4-addition product. The TOF value for the Y2-
catalyzed cyanosilylation of 1o was the highest among
Entry Substrate
t
Product
Yield
[%]
[h]
[
2–4]
previously reported values (Table S1).
Notably, when the
Y2-catalyzed competitive cyanosilylation of aldehyde 1n
1
1a
1.5
2a
91
(
(
0.25 mmol) and ketone 1h (0.25 mmol) with TMSCN
0.375 mmol) and Y2 (0.05 mol%) was carried out under
2
3
1b
1c
0.3
0.5
2b
2c
90
97
the conditions shown in Scheme 2, 2n was chemoselectively
obtained in 97% yield in just one minute without formation of
2h.
4
1d
8
2d
80
5
6
1e
1 f
2
2
2e
2 f
83
93
[
b]
7
1g
10
2g
59
8
9
1
1
1
1
X=p-H
1h
1i
1j
1k
1l
1m
3
10
10
20
2
X=p-H
2h
2i
2j
2k
2l
2m 90
80
79
80
61
92
Scheme 2. Cyanosilylation of aldehydes. Reaction conditions: aldehyde
X=p-Me
X=o-Me
X=p-OMe
X=p-Cl
X=p-Me
X=o-Me
X=p-OMe
X=p-Cl
(0.5 mmol for 1n and 1o, 0.25 mmol for 1p), TMSCN (molar ratio
0
1
2
3
TMSCN/aldehyde=1.5:1), 1,2-dichloroetnane (0.5 mL), 258C.
X=p-CN
1
X=p-CN
[
(
3
a] Reaction conditions: Y2 (1 mol%), ketone (0.25 mmol), TMSCN
molar ratio TMSCN/ketone=1.5:1), 1,2-dichloroethane (1.5 mL),
08C. Yields were determined by GC analysis using naphthalene as an
A CSI-MS analysis showed that the structure of Y2 was
preserved after cyanosilylation (Figure S5). The isolation of
the product and the recovery of the catalyst were very easy
internal standard. [b] 1,4-Addition product (5% yield).
(
see the Supporting Information). The IR spectrum of the
recovered Y2 was identical to that of the freshly prepared Y2
Figure S6). In addition, the recovered Y2 could be reused for
(
In the case of the a,b-unsaturated ketone 2-cyclohexen-1-
one (1g), the 1,2-addition of TMSCN mainly took place to
give 1-cyano-1-trimethylsilyl-2-cyclohexene (2g) in 59%
yield along with a small amount of the 1,4-addition product
cyanosilylation without an appreciable loss of its high
catalytic performance; for example, 2b was obtained in
97% yield when recovered Y2 was used.
The CSI-MS spectrum of a solution of Y2 and TMSCN in
1,2-dichloroethane (molar ratio = 1:200) showed sets of
signals centered at m/z = 3834, 7355, and 7427, which were
(
5% yield; Michael addition). Not only aliphatic ketones but
also less reactive acetophenone derivatives (1h–1m) could be
converted into the corresponding cyanohydrin trimethylsilyl
ethers. The cyanosilylation of acetophenone derivatives with
electron-withdrawing substituents (1l and 1m) proceeded
smoothly. In contrast, acetophenone derivatives with elec-
tron-donating substituents (1i–1k) required longer reaction
times. The Hammett plot for the competitive cyanosilylation
of para-substituted acetophenone derivatives gave the pos-
itive 1 value of + 1.92 (Figure S4), thus suggesting that the
2
+
assigned to [TBA TMS (SiYW O ) (H O) ] , [TBA9-
1
0
2
10 36
2
2
2
+
TMSH(SiYW O ) (H O) ] , and [TBA TMS (SiYW O ) -
(H O) ] , respectively (Figure S7). In addition, the yttrium
1
0
36
2
2
2
9
2
10 36 2
+
2
2
centers might act as Lewis acids to activate carbonyl
[14]
compounds (Figure 1).
Therefore, it is likely that both
TMSCN and carbonyl compounds are activated by surface
oxygen atoms (nucleophilically) and the yttrium centers
(electrophilically), respectively, thus resulting in coexistence
of activated coupling partners on the same Y2 molecules.
Cyanosilylation would therefore be efficiently promoted by
the presence of Y2. The above-mentioned size- and chemo-
selectivity of the cyanosilylation resulting from steric inter-
actions also supports the hypothesis.
À
nucleophilic attack of CN species to a carbonyl carbon atom
is part of the rate-limiting step.
Notably, Y2 showed extremely high catalytic activities for
cyanosilylation of sterically less hindered aldehydes (in
comparison with ketones). The cyanosilylation of benzalde-
hyde (1n) with 0.01 mol% of Y2 gave 2n in 94% yield after
In conclusion, we successfully synthesized a novel yttrium-
pillared silicotungstate dimer by the reaction of SiW10 with
1
5 minutes. In the case of n-hexanal (1o), the cyanosilylation
proceeded very efficiently with only 0.005 mol% of Y2, the
turnover frequency (TOF) was 540000 h , and the turnover
number (TON) reached 18000. The a,b-unsaturated aldehyde
trans-cinnamaldehyde (1p) also selectively gave the corre-
Y(acac) in an organic medium (acetone). In the presence of
3
À1
Y2, cyanosilylation of several structurally diverse ketones and
aldehydes with TMSCN selectively proceeded to afford the
corresponding cyanohydrin trimethylsilyl ethers. In particu-
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688
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Angewandte
Chemie
lar, the catalytic performance for aldehydes was very signifi-
cant; for example, TON = 18000 and TOF = 540000 h were
observed for n-hexanal.
[9] The formation of trimethylsilanol was hardly observed in the
À1
presence of excess amounts of carbonyl compounds (catalytic
turnover conditions).
[
10] For several efficient functional-group transformations catalyzed
by Lewis acidic POMs, see: a) C. Boglio, G. Lemiꢂre, B.
Hasenknopf, S. Thorimbert, E. Lacꢃte, M. Malacria, Angew.
Chem. 2006, 118, 3402; Angew. Chem. Int. Ed. 2006, 45, 3324;
b) C. Boglio, K. Micoine, P. Rꢂmy, B. Hasenknopf, S. Thorim-
bert, E. Lacꢃte, M. Malacria, C. Afonso, J.-C. Tabet, Chem. Eur.
J. 2007, 13, 5426; c) N. Duprꢂ, P. Rꢂmy, K. Micoine, C. Boglio, S.
Thorimbert, E. Lacꢃte, B. Hasenknopf, M. Malacria, Chem. Eur.
J. 2010, 16, 7256; d) M. Bosco, S. Rat, N. Duprꢂ, B. Hasenknopf,
E. Lacꢃte, M. Malacria, P. Rꢂmy, J. Kovensky, S. Thorimbert, A.
Wadouachi, ChemSusChem 2010, 3, 1249.
Received: January 18, 2012
Published online: February 29, 2012
Keywords: carbonyl compounds · cyanosilylation ·
.
polyoxometalate · trimethylsilyl cyanide · yttrium
[
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[
11] We have reported similar in situ prepared catalysts, such as
TBA [{Zn(OH )(m -OH)} {Zn(OH ) } (g-HSiW O ) ]·9H O
8
2
3
2
2
2
2
10 36
2
2
[11a]
(
Zn(acac) + SiW10)
and
TBA [Ag (DMSO) (g-H Si-
2
8 4 2 2
[
2] For homogeneously catalyzed cyanosilylation (see the Support-
ing Information for more details), see: a) B. Y. Park, K. Y. Ryu,
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dimethyl sulfoxide, OAc = acetate):
(AgOAc + SiW10)
(DMSO =
1
0
36
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[
12] The IR spectrum of Y2 was very similar to that of Y2ꢀ
(Figure S8). Although the complete structural determination of
Y2 by X-ray crystallographic analysis has so far been unsuc-
cessful because of the severe disorder around the yttrium centers
and TBA molecules, Y2 was determined to be a dimer of [g-
[
3] For heterogeneously catalyzed cyanosilylation (see the Support-
ing Information for more details), see: a) Y. Ogasawara, S.
Uchida, K. Yamaguchi, N. Mizuno, Chem. Eur. J. 2009, 15, 4343;
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Chem. Int. Ed. 2005, 44, 1826; f) K. Yamaguchi, T. Imago, Y.
Ogasawara, J. Kasai, M. Kotani, N. Mizuno, Adv. Synth. Catal.
8
À
SiW O ] pillared by two yttrium atoms, and the structure was
1
0
36
almost identical to that of Y2’. In addition, the positive-ion CSI-
MS analysis of Y2’ in acetonitrile showed only the sets of signals
1
0À
resulting from the POM [(SiYW O ) ] (Figure S9). All these
1
0
36 2
results show that the anion structures of Y2 and Y2’ are
intrinsically identical, and the difference between them is the
number of TBA cations.
[
[
13] CCDC 860121 (compound Y2’) contains the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
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1993, 66, 2016.
14] The stretching vibration of the C=O bond of coordinated
acetone molecules on the yttrium centers was observed at
À1
À1
1
696 cm (that of non-coordinated acetone: 1715 cm ), thus
[
4] For asymmetric cyanosilylation, see: a) N. H. Khan, R. I. Kure-
suggesting that acetone is electrophilically activated on the
yttrium centers.
shy, S. H. R. Abdi, S. Agrawal, R. V. Jasra, Coord. Chem. Rev.
2008, 252, 593; b) Y. Shen, X. Feng, Y. Li, G. Zhang, Y. Jiang,
[
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[
[
16] F. Xin, M. T. Pope, Inorg. Chem. 1996, 35, 5693.
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5] a) C. L. Hill, C. M. Prosser-McCartha, Coord. Chem. Rev. 1995,
1
4
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[
[
[
6] Y. Ogasawara, S. Itagaki, K. Yamaguchi, N. Mizuno, ChemSus-
Chem 2011, 4, 519.
7] The cyanosilylation of 1a hardly proceeded in the presence of
a À4-charged POM TBA [a-SiW O ] (Table S2).
[18] The scope of cyanosilylation of various ketones was examined
with Y2 (Table 1) because of the very low solubility of Y2’ in 1,2-
dichloroethane (below 0.05 mm). The cyanosilylation of alde-
hyde 1o was carried out with Y2’ (0.005 mol%) in 1,2-dichloro-
4
12 40
8] The formation of trimethylsilanol was also confirmed by GC-MS
analysis of the solution.
Angew. Chem. Int. Ed. 2012, 51, 3686 –3690
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.
Angewandte
Communications
ethane under the conditions described in Scheme 2. Although
Y2’ was partially insoluble under this conditions, 2o was
obtained in 78% yield.
70% and 27% yields, respectively. MgO selectively gave 2b in
79% yield along with a small amount of 2d (2% yield). The ratio
of the yields of 2b and 2d with Y2 (89/9) was much higher than
that with the homogeneous catalyst Sc(OTf)3 (70/27) and
comparable to that with the heterogeneous catalyst MgO (79/2).
[
19] Competitive cyanosilylations of 1b and 1d with Sc(OTf) (OTf =
3
triflate) and MgO were carried out under the same reaction
conditions. In the case of Sc(OTf) , 2b and 2d were obtained in
3
3
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