Lewis Acid Rather than Bronsted Acid Sites of Montmorillonite K10 Act as a Powerful and Reusable Green Heterogeneous Catalyst for Rapid Cyanosilylation of Ketones

Article abstract of DOI:10.1055/s-0036-1588640

A practical green protocol was developed for highly efficient cyanosilylation of various ketones catalyzed by commercial montmorillonite K10, with excellent isolated yields (91-99%). The catalyst can be used as received, and its catalytic strength can be

Full text of DOI:10.1055/s-0036-1588640

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©
Georg Thieme Verlag Stuttgart · New York
2016, 27, A–F
A
letter
Synlett
X. Huang et al.
Letter
Lewis Acid Rather than Brønsted Acid Sites of Montmorillonite
K10 Act as a Powerful and Reusable Green Heterogeneous Cata-
lyst for Rapid Cyanosilylation of Ketones
Xiao Huang*
Lin Chen
R1
δ+ δ–
O
M
on-surface
carbonyl
activation
R2
Fengying Ren
Chen Yang
Jianghong Li
Kejin Shi
TMS
NC
M = edge-surface Lewis site
on MMT K10
O
TMSO CN
Xiaojun Gou
Wei Wang*
cat. montmorillonite K10
TMSCN, PhMe, RT
R1
R2
R1
R2
high yield (91–99%)
catalyst recyclable
1
2
R , R = aryl, alkyl
Antibiotics Research and Reevaluation Key Laboratory of
Sichuan Province, Sichuan Industrial Institute of Antibiotics,
Chengdu University, 168 Huaguan Road, 610052, Chengdu,
P. R. of China
huangxiao@cdu.edu.cn
wildwang_5@hotmail.com
6
7
Received: 30.07.2016
Accepted after revision: 10.10.2016
Published online: 11.11.2016
works (MOFs), ionic liquids or phase-transfer catalysts, N-
heterocyclic carbenes, thiourea catalysts,9 Lewis acids,
8
10
11
5c,12
DOI: 10.1055/s-0036-1588640; Art ID: st-2016-u0636-l
base catalysts, dual-activation catalysts,
and support-
13
ed catalysts. In particular, modified MMTs, such as Sn-
Abstract A practical green protocol was developed for highly efficient
cyanosilylation of various ketones catalyzed by commercial montmoril-
lonite K10, with excellent isolated yields (91–99%). The catalyst can be
used as received, and its catalytic strength can be easily restored with-
out loss of activity. Investigations of catalyst recycling and of the active
catalytic sites indicated that Lewis acid sites were mainly responsible for
the cyanosilylation of ketones.
MMT14 and Fe-MMT,15 have been developed as solid-acid
catalysts for cyanosilylation of carbonyls. However, the use
of unmodified commercial MMT (generally known as MMT
K10) has not been well documented for this type of reac-
tion. Here, we report the use of commercial MMT K10 as a
green, powerful, reusable, heterogeneous, solid-acid cata-
lyst for the cyanosilylation of various ketones.
Initially, we examined the cyanosilylation of acetophe-
none (1a) by using commercial MMT K10 (Sigma-Aldrich;
surface area 220–270 m /g) as a catalyst. Acetophenone re-
16
Key words montmorillonite, heterogeneous catalysis, cyanosilyla-
tion, ketones, green chemistry
2
Montmorillonite (MMT), a naturally abundant clay, is
composed of stacked, negatively charged, two-dimensional
layers of aluminosilicate, with exchangeable cationic spe-
acted efficiently with TMSCN in the presence of unmodified
MMT K10 as a catalyst (Table 1, entry 1). To our delight, the
reactivity was improved by increasing the concentration of
the substrate or by carrying out the reaction under solvent-
less conditions (entries 2–4). Among the solvents tested,
hydrocarbon solvents (entries 3 and 5) produced a high re-
activity, whereas moderately or strongly polar solvents gave
relatively poor results (entries 9–11). This might be ex-
plained in terms of masking of acid sites on the catalyst by
the weak Lewis base sites of the solvent (typically oxygen
atoms in these cases). On the other hand, hydrocarbons
cannot act as acceptors in which the leaving carbonyl group
can be smoothly activated by the acid sites on the catalyst.
As a result of a steric effect, the lone pair of electrons of di-
isopropyl ether (entry 8) is almost embraced by the methyl
side-arms, preventing it from acting as an acid acceptor;
consequently, this solvent also gave good results. Although
chloride atoms attached to a carbon can serve as weak acid
1
cies in the interlayers. MMT has been widely used as a het-
2
erogeneous catalyst for many organic reactions, because of
its favorable properties, such as low cost, thermal stability,
the presence of Lewis and Brønsted acid sites, large surface
area, and ease of separation; furthermore, it is environmen-
tally benign.
The cyanosilylation of carbonyls with TMSCN is a pow-
erful protocol for forming new C–C bonds, as well as simul-
taneously protecting an alcohol with a trimethylsilyl group.
The resulting cyanohydrin silyl ethers can be further con-
3
verted into other important organic building blocks, such
as α-hydroxy acids, α-amino acids, or β-amino alcohols.
Many efficient catalytic systems have been reported for cy-
anosilylation of various carbonyl compounds, including al-
4
5
kali metal salts, salen complexes, metal organic frame-
©
Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–F

B
Synlett
X. Huang et al.
Letter
acceptors, they are too weak to compete with the carbonyl
and also by stabilization of the enol form of the substrate
through conjugation with the adjacent carboxylate group,
decreasing the concentration of the ketone form.
oxygen atom. Consequently, CH Cl2 and CHCl3 also gave
2
good results in the reaction (entries 6 and 7).
The turnover frequencies (TOFs) of the substrates were
also calculated and are listed in Table 2. The quantity of ac-
tive acid sites on MMT K10 is about 0.0495 mmol per
100 mg catalyst, as estimated by a poisoning method
using 2-acetylpyridine as a base probe.
Table 1 Cyanosilylation of Acetophenone (1a) under Various Conditionsa
Solventb
1a
mmol/mL] (min)
Time
Yieldc
(%)
14c
Entry
Catalyst
[
The recyclability under the standard conditions was
then investigated. As shown in Figure 1, the catalytic
strength of MMT K10 gradually decreased with successive
cyanosilylation cycles (Figure 1, runs 2–5); at the fifth run,
it was almost completely lost. To our delight, however, the
catalytic strength was restored simply by washing the spent
catalyst with a dilute hydrochloric acid solution (runs 6, 9,
and 12). When the regenerated catalyst was reused, the
pattern of loss and subsequent restoration of activity re-
peated itself (runs 6–12). In addition, when the spent cata-
lyst was washed after each use, its activity was fully regen-
erated (runs 13 and 14). Consequently, by applying this res-
toration method, the catalyst can reused more times than
1
2
3
4
5
6
7
8
9
MMT K10
MMT K10
MMT K10
MMT K10
MMT K10
MMT K10
MMT K10
MMT K10
MMT K10
MMT K10
MMT K10
Sn4+-MMTf
Fe3+-MMTf
Al3+-MMTf
PhMe
PhMe
PhMe
none
1
60
6
90
>98
>98
>98
>98
>98
>98
>98
45e
2
4
5
neatd
2
4
hexane
6
CH
CHCl
i-Pr
Et
2
Cl
2
2
6
3
2
6
2
O
2
6
2
O
2
30
30
30
20
7
10
11
12
13
4
t-BuOMe
THF
2
30e
2
12e
17
PhMe
PhMe
PhMe
2
>98
>98
>98
were tested.
2
Table 2 Cyanosilylation of Various Ketones 1 Catalyzed by MMT K10a
1
2
5
a
Reaction conditions: 1a (1 mmol substrate), MMT K10 (20 mg), TMSCN
1.5 equiv), solvent (0.5 mL), r.t.
All solvents were of analytical grade and used as received.
Isolated yield.
O
cat. MMT K10
TMSO CN
(
b
c
d
e
f
R1
R2
TMSCN, PhMe, RT
R1
R2
1
2
TMSCN (1.2 equiv) was used.
Product Yieldb
TOFc
Acetophenone (1a) was recovered.
Prepared from MMT K10 by a reported procedure.
Entry Substrate
Time
(min)
14c
(min–1)
(%)
1
2
1
2
3
4
5
6
7
8
9
1a, R = Ph; R = Me
6
6
2a
2b
2c
2d
2e
2f
99
99
99
99
99
96
98
97
96
97d
17
17
13
11
7
Various cation-exchanged MMTs were also effective cat-
1
2
1b, R = Ph; R = Et
4+
3+
alysts for the cyanosilylation, including Sn -, Fe -, and
1
2
3
+
3+
1c, R = 4-FC
6
H
4
; R = Me
8
10
15
10
15
10
15
10
Al -exchanged MMTs (Table 1, entries 12–14). Fe - and
1
2
3+
1d, R = 4-ClC H ; R = Me
Al -MMT had similar catalytic activities relative to unmod-
6
4
4+
1
2
ified MMT, whereas Sn -MMT showed a slightly lower ac-
tivity.
1e, R = 4-BrC
6
H
4
; R = Me
15b
1
2
1f, R = 4-HOC
6
H
4
; R = Me
11
7
Inspired by the above results, we treated various ke-
tones under the optimized catalytic conditions (Table 2).
Generally, most of the tested ketones were converted into
the corresponding cyanohydrin silyl ethers within 15 min-
utes; these included aliphatic ketones (Table 2, entries 11–
1
2
1g, R = 4-MeOC
6
H
4
; R = Me
2g
2h
2i
1
2
1h, R = 2-HOC
6
H
4
; R = Me
11
7
1
2
1i, R = 4-O
2
NC
6
H
4
; R = Me
1
2
10
1j, R = PhCH=CH; R = Me
2j
11
1
14), aromatic ketones substituted with electron-donating
1k, R = 4-MeOC H CH
6
4
2
1
1
10
2k
96
11
2
R = Me
or electron-withdrawing groups (entries 1–9), α,β-unsatu-
rated ketones (entry 10; the 1,2-adduct was cleanly ob-
tained), and sterically hindered ketones (entries 15 and 16).
For 2-acetylpyridine (1s; entry 19), however, the catalytic
system failed, probably because the active acid sites were
immediately quenched by the basic nitrogen atom of the
pyridine ring. Other ketones bearing heteroatoms (entries
1
2
12
13
1l, R –R = (CH
2
)
5
5
5
5
2l
98
98
98
20
20
20
1
2
1m, R = i-Bu; R = Me
2m
2n
1
2
14
1n, R = n-Pent; R = Me
o,
1
15
O
8
2o
99
13
17 and 18) required slightly longer reaction times. In the
case of ethyl acetoacetate (1t; entry 20), the prolonged re-
action time might be explained by the presence of an extra
acid acceptor, which reduces the activity of the catalyst,
1
2
1
6
7
1p, R = Ph; R = Ph
15
2p
2q
99
91
7
2
1
2
1
1q, R = 2-furyl; R = Me
50
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Synlett
X. Huang et al.
Letter
dral Si4+ ions are isomorphously replaced by other trivalent
Table 2 (continued)
Entry Substrate
3+
Product Yieldb
TOFc
metal cations, such as Al , and a proportion of the octahe-
Time
(
min)
(%)
(min–1)
dral Al ions are replaced by divalent cations, such as Mg ,
which leads to a charge deficiency. To balance this charge
3+
2+
1
2
18
19
20
1r, R = 2-thienyl; R = Me
50
180
180
300
300
300
2r
93
2
–
+
+
2+
+
deficiency, hydrated cations (Na , K , Ca , H O , etc.) occu-
1
2
–e
3
1s, R = 2-pyridyl; R = Me
py the interlayer space of MMT. Accordingly, Brønsted acidi-
ty arises from the presence of terminal hydroxy groups on
the external surface and from the hydrated cations in the
interlamellar space of MMT. Lewis acidity stems from cen-
1
2
1t, R = EtO
2
CCH
2
; R = Me
2t
2a
2a
92
12
<1
–
2 f
1
1a, R = Ph; R = Me
1
2
2g 1a, R = Ph; R = Me
1
2
46
–
2
3+
2+
3+
3h 1a, R = Ph; R = Me
1
2
–d
tral metal ions such as Al , Mg , and Fe in the lattice and
from other metallic cations in the interlamellar space. Nor-
mally, the central metal ions are fully bonded to adjacent
oxygen atoms to form typical octahedra; consequently,
these intact octahedra within the crystal arrays are incapa-
ble of providing much Lewis acidity. However, the small
particle size of MMT K10 (typically 5–10 μm) means that
the sandwich layers are crushed to a remarkable extent,
creating many broken edges of stacked layers, which con-
tribute to both Brønsted and Lewis acidity. In particular,
these surface imperfections with many coordinated metal
ions, which are poised to trap external ligands, should be
the main source of the Lewis acidity of unmodified MMT
K10.
2
–
2 i
2 j
6k 1a, R = Ph; R = Me
4
1a, R = Ph; R = Me
1
2
300 (60 h) 2a
18 (93)
21
–
1
2
5
1a, R = Ph; R = Me
300
5
2a
2a
2a
–
1
2
2
99
20
25
2 l
7
1a, R = Ph; R = Me
1
2
4
99
a
Reaction conditions: 1 (1.0 mmol), MMT K10 (20 mg), PhMe (0.5 mL),
TMSCN (1.5 mmol), r.t.
b
Isolated yield.
c
TOF = turnover frequency (number of ketone molecules converted per ac-
tive acid site per min).
d
The 1,2-adduct was obtained.
No product was detected by TLC, and the substrate was recovered.
MMT K10 (20 mg) was pretreated with TMSCN (1.5 mmol) in PhMe (0.5 mL)
e
f
for 6h, then 1a (1.0 mmol) was added.
g
TfOH (20 mg) was used as the catalyst.
h
Concd H
2
4
SO (20 mg) was used as the catalyst.
i
17
TMSCN-pretreated MMT K10 was used as the catalyst; the yield in paren-
theses is for a reaction time of 60 h.
j
•
•
OO
•
•
Wet MMT K10 (20 mg containing 20 mg H
MMT K10 was dried in a vacuum at 120 °C for 1 h before use.
MMT K10 was calcined at 400 °C for 1 h before use.
2
O) was used as the catalyst.
T
I
T = Tetrahedral sheet
k
O
2+
OO OH
l
+
+
Na , Ca , H3O , nH2O O = Octahedral sheet
OO
O
H
O
OO
O
H
O
•
•
•
•
I = Interlamellar space
T
O
O
O
O
Typical structure of MMK: a 2:1
aluminosilicate clay mineral;
O
⊗
⊗
⊗
⊗
O
O
•
O
H
O
O
•
O
H
O
typical chemical formula:
T
•
(Na,Ca)0.33(Al,Mg,Fe) (Si O )(OH) •nH O
2
4
10
2
2
OO
• = Si,
Ο = O,
⊗
= Al (predominant), Mg (minor), Fe (rare)
Figure 2 A typical structure of MMT
Although details of the reaction mechanism are not
clear, both Brønsted and Lewis acid sites might act as active
catalytic sites for the cyanosilylation of ketones. A prece-
dent has been reported in which TMSCN is suggested to re-
act with protic sites of acidic MMT to release HCN, possibly
with generation of the TMS cationic Lewis acid, in a manner
similar to that of the TfOH-catalyzed cyanosilylation of hep-
tanal.1 The TMS cationic Lewis acid then activates the car-
bonyl group to produce a carbenium and another TMSCN
molecule that attacks the carbenium intermediate to afford
Figure 1 Recyclability and restoration of the catalyst
4c
MMT K10, as a heterogeneous catalyst that contains
both Brønsted and Lewis acid sites, has a relatively high acid
strength, and its Hammett acidity constant (H ) is similar to
14c
the product and regenerate the TMS cationic Lewis site.
In the case of the ketone, however, concentrated H SO
(~96%) showed no reactivity (Table 2, entry 23), despite its
0
18a
that of concentrated HNO (MMT K10; H = −5.5 to −5.9;
2
4
3
0
18b
HNO : H = −6.3 ). Its relatively high surface area and vari-
3
0
1
8c
able intercalated space make MMT K10 an active acid cata-
lyst. The typical structure of MMT (Figure 2) consists of alu-
minosilicate layers in the form of a sandwich in which cen-
stronger acidity (H
Brønsted acid (H
0
= –9.9 ). TfOH, as a much stronger
= −14.11 ), did catalyze the cyanosilyla-
8b
0
tion (Table 2, entry 22), although its activity was not com-
parable to that of MMT K10. It is probable, therefore, that
the Brønsted acid sites on MMT K10 contribute little to its
catalytic activity.
3
+
tral Al
ions occupy an octahedral sheet that is
4+
symmetrically embedded between Si ions in two tetrahe-
dral sheets. In these layers, a small fraction of the tetrahe-
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D
Synlett
X. Huang et al.
Letter
When MMT K10 was merely exposed to TMSCN for six
hours before addition of the substrate (Table 2, entry 21),
only 12% of the product was isolated after five hours, and a
large amount of the ketone was recovered. This suggested
TMSCN was adsorbed onto the catalyst, blocking its catalyt-
ic ability. It has been reported that MMT can efficiently ad-
sorb HCN, so it is reasonable to infer that TMSCN might
also be gradually adsorbed physically and chemically onto
MMT.
edge-surface
edge-surface
Lewis site
activation
T
O
T
OH Lewis site
R1 HO
M = Al, Mg, Fe
M
restoration
δ+
δ–
Cl
may not be
the same
formula
Brønsted site
R2
O
H3O+
OTMS+
O
CN
NC TMS
+
H O , H O
3
2
diluted HCl
wash
R1 2OTMS
TMSCN
HCN, TMSOH
R
19
TMSO
NC
T
O
T
OTMS
CN
edge-surface
Lewis site
inactivation
may be more than one
cyanide anion as ligand
M = Al, Mg Fe
M
edge-surface
CN
NC
Lewis site
inactivation
On the basis of the above observations, the Lewis acid
sites on MMT K10 appear to be mainly responsible for its
catalytic activity. As shown in Figure 3, the carbonyl group
of the ketone is activated by a Lewis site to generate a high-
ly reactive carbenium species that is simultaneously at-
tacked by TMSCN to afford a cyanohydrin silyl ether. It is
known that cyanide has a high affinity for metals, and
therefore it adsorbs onto the Lewis acid sites of MMT K10,
not only physically, but also chemically, thereby reducing
the Lewis acidity of the MMT. The results for TMSCN-pre-
treated MMT K10 (Table 2, entry 21) and the fifth run in
catalyst recycling (Figure 1) confirm this catalyst-activity-
diminishing effect. Interestingly, when methanol-washed
MMT K10 pretreated with TMSCN was used in the cyanosi-
lylation of acetophenone (Table 2, entry 24), a poor result
was nevertheless obtained. Cyanide anion was detected in
the methanol eluate by a rapid colorimetric technique using
Figure 3 Possible catalyst behaviors in cyanosilylation of ketone
In conclusion, commercially unmodified MMT K10, as a
heterogeneous solid acid, proved to be a powerful, green,
low-cost, and recyclable catalyst for cyanosilylation of vari-
ous ketones22 (except 2-1-pyridin-2-ylethanone). In addi-
tion, an investigation of catalyst recyclability suggested that
MMT can be used repeatedly without loss of activity if it is
subjected to a simple acid wash between cycles. Further ev-
idence has been found to confirm that adsorption of TMSCN
and coordination of cyanides occur at Lewis acid sites on
MMT K10,23 implying that Lewis acid sites, rather than
Brønsted acid sites, play a critical role in the cyanosilylation
of ketones. Further studies on various types of MMT-cata-
lyzed organic reactions are in progress.
20
ninhydrin; further cyanide was detected in the diluted
HCl washing eluate by the same technique. We therefore in-
ferred that methanol, as a weak ligand, can wash off weakly
adsorbed cyanides, but leaves strongly coordinated cya-
nides nearly intact; at this stage, Lewis acidity is not fully
restored. Consequently, a long time was required for com-
pletion of the reaction [Table 2, entry 24 (parentheses)].
Further washing with dilute hydrochloric acid breaks the
coordinating bond of the cyanides to release HCN and re-
store Lewis acid sites on the MMT (Figure 2); however,
these sites might not have the same structure as the origi-
nal ones, as the color in solution of the catalyst at this stage
was dark green, rather than the original earth yellow. Fur-
thermore, the catalytic strength was similar to the original
value. Additionally, when wet MMT K10 was used as a cata-
lyst (Table 2, entry 25), the active Lewis acid sites were
probably coordinated to water instead of carbonyl groups,
resulting in poor performance. When MMT K10 was pre-
dried in a vacuum at 120 °C for one hour to remove surface-
absorbed and interlamellar water, a slightly higher catalytic
activity was observed (Table 2, entry 26). However, precal-
cination of the MMT K10 at 400 °C for one hour resulted in
a collapse of the interlayer structure, as all the water was
driven out, leading to a decrease in the Brønsted acidity but
Acknowledgment
We wish to express our appreciation to the Antibiotics Research and
Reevaluation Key Laboratory of Sichuan Province, Sichuan Industrial
Institute of Antibiotics, Chengdu University for funding (projects No.
ARRLKF16-07 and ARRLKF16-09).
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(
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21
an increase in the Lewis acidity. The calcined MMT K10
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(f) Kurono, N.; Yamaguchi, M.; Suzuki, K.; Ohkuma, T. J. Org.
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©
Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–F

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Synlett
X. Huang et al.
Letter
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Izumi, Y. Bull. Chem. Soc. Jpn. 1993, 66, 2016. (d) Higuchi, K.;
Onaka, M.; Izumi, Y. J. Chem. Soc., Chem. Commun. 1991, 1035.
(15) (a) Onaka, M.; Higuchi, K.; Sugita, K.; Izumi, Y. Chem. Lett. 1989,
18, 1393. (b) Sn-MMT has been reported to be superior to any
other ion-exchanged MMT (see ref. 14b), but we obtained a dif-
ferent result. This might be caused by different sources of
unmodified MMT.
(6) (a) Bhunia, A.; Dey, S.; Moreno, J. M.; Diaz, U.; Concepcion, P.;
Van Hecke, K.; Janiak, C.; Van Der Voort, P. Chem. Commun.
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7) (a) Martín, S.; Porcar, R.; Peris, E.; Burguete, M. I.; García-Ver-
dugo, E.; Luis, S. V. Green Chem. 2014, 16, 1639. (b) Park, B. Y.;
Ryu, K. Y.; Park, J. H.; Lee, S.-g. Green Chem. 2009, 11, 946.
(16) To the best of our knowledge, there is only one report of an
incomplete investigation of cyanosilylation of aldehydes by
using MMT K10 as catalyst, see: Somanathan, R.; Rivero, I. A.;
Gama, A.; Ochoa, A.; Aguirre, G. Synth. Commun. 1998, 28, 2043.
(17) Catalyst Recycling and Restoration; General Procedure
Catalyst Recycling: When the reaction was complete, the cata-
lyst was collected by filtration with suction, washed thoroughly
with PhMe, and then suction-dried uner air for reuse.
Catalyst Reactivation: After being washed with PhMe, the spent
catalyst was washed slowly with dilute HCl, prepared by adding
1 volume of concd HCl to 20 volumes of MeOH. The catalyst was
then washed successively with anhydrous MeOH and PhMe.
Finally, suction-drying under air fully restored the activity of
the catalyst.
(c) Dekamin, M. G.; Mokhtari, J.; Naimi-Jamal, M. R. Catal.
Commun. 2009, 10, 582. (d) Shen, Z.-L.; Ji, S.-J.; Loh, T.-P. Tetra-
hedron Lett. 2005, 46, 3137.
(8) (a) Zhao, H.; Li, X.; Li, L.; Wang, R. Small 2015, 11, 3642.
(b) Pawar, G. M.; Buchmeiser, M. R. Adv. Synth. Catal. 2010, 352,
917. (c) Song, J. J.; Gallou, F.; Reeves, J. T.; Tan, Z.; Yee, N. K.;
Senanayake, C. H. J. Org. Chem. 2006, 71, 1273.
(
9) (a) Zhang, Z.; Lippert, K. M.; Hausmann, H.; Kotke, M.;
Schreiner, P. R. J. Org. Chem. 2011, 76, 9764. (b) Zuend, S. J.;
Jacobsen, E. N. J. Am. Chem. Soc. 2007, 129, 15872. (c) Fuerst, D.
E.; Jacobsen, E. N. J. Am. Chem. Soc. 2005, 127, 8964.
(
10) (a) Pourmousavi, S. A.; Salahshornia, H. Bull. Korean Chem. Soc.
(18) (a) Laszlo, P. Science 1987, 235, 1473. (b) Mathers, R. T.; LeBlond,
C.; Damodaran, K.; Kushner, D. I.; Schram, V. A. Macromolecules
2008, 41, 524; and references therein. (c) Rochester, C. H.
Acidity Functions; Academic Press: London, 1970.
(19) (a) Colin-Garcia, M.; Heredia, A.; Negron-Mendoza, A.; Ortega,
F.; Pi, T.; Ramos-Bernal, S. Int. J. Astrobiol. 2014, 13, 310.
(b) Jamis, J.; Drljaca, A.; Spiccia, L.; Smith, T. D. Chem. Mater.
1995, 7, 2078.
(20) Nagaraja, P.; Hemantha Kumar, M. S.; Yathirajan, H. S.; Prakash,
J. S. Anal. Sci. 2002, 18, 1027.
(21) Gallo, J. M. R.; Teixeira, S.; Schuchardt, U. Appl. Catal., A 2006,
311, 199.
2011, 32, 1575. (b) Kim, S. S.; Rajagopal, G.; George, S. C. Appl.
Organomet. Chem. 2007, 21, 368. (c) George, S. C.; Kim, S. S. Bull.
Korean Chem. Soc. 2007, 28, 1167; and references therein. (d) Li,
Q.; Liu, X.; Wang, J.; Shen, K.; Feng, X. Tetrahedron Lett. 2006, 47,
4011. (e) Chen, F.-X.; Feng, X. Curr. Org. Synth. 2006, 3, 77.
(f) Shen, Y.; Feng, X.; Li, Y.; Zhang, G.; Jiang, Y. Eur. J. Org. Chem.
2
2
004, 129. (g) Shen, Y.; Feng, X.; Li, Y.; Zhang, G.; Jiang, Y. Synlett
002, 793.
(
11) (a) Wang, W.; Liu, X.; Lin, L.; Feng, X. Eur. J. Org. Chem. 2010,
751. (b) Kantam, M. L.; Mahendar, K.; Sreedhar, B.; Kumar, K.
V.; Choudary, B. M. Synth. Commun. 2008, 38, 3919.
c) Gawronski, J.; Wascinska, N.; Gajewy, J. Chem. Rev. 2008, 108,
227. (d) Prakash, G. S.; Vaghoo, H.; Panja, C.; Surampudi, V.;
Kultyshev, R.; Mathew, T.; Olah, G. A. Proc. Natl. Acad. Sci. U.S.A.
007, 104, 3026. (e) Yamaguchi, K.; Imago, T.; Ogasawara, Y.;
Kasai, J.; Kotani, M.; Mizuno, N. Adv. Synth. Catal. 2006, 348,
516. (f) Wang, L.; Huang, X.; Jiang, J.; Liu, X.; Feng, X. Tetrahe-
dron Lett. 2006, 47, 1581. (g) Chen, F.-X.; Feng, X. Synlett 2005,
92. (h) He, B.; Li, Y.; Feng, X.; Zhang, G. Synlett 2004, 1776.
i) Li, Y.; He, B.; Feng, X.; Zhang, G. Synlett 2004, 1598.
4
(
5
(22) 2-Phenyl-2-(trimethylsiloxy)propanenitrile (2a); Typical
Procedure
A test tube was charged with PhCOMe (1a; 120 mg, 1.0 mmol),
MMT K10 (20 mg), and PhMe (0.5 mL). TMSCN (190 μL, 1.5
mmol) was added dropwise with stirring at r.t. After 6 min, TLC
indicated that the reaction was complete, and the mixture was
filtered, washed with PhMe, and concentrated under reduced
2
1
1
8
(
pressure to afford 2a as a colourless oil; yield: 218 mg (99%). H
NMR (400 MHz, CDCl , TMS): δ = 0.18 (s, 9 H), 1.87 (s, 3 H),
3
13
(
12) (a) Xiong, Y.; Huang, X.; Gou, S.; Huang, J.; Wen, Y.; Feng, X. Adv.
Synth. Catal. 2006, 348, 538. (b) Chen, F.-X.; Qin, B.; Feng, X.;
Zhang, G.; Jiang, Y. Tetrahedron 2004, 60, 10449. (c) Chen, F.-X.;
Liu, X.; Qin, B.; Zhou, H.; Feng, X.; Zhang, G. Synthesis 2004,
7.33–7.40 (m, 3 H), 7.53–7.57 (m, 2 H). C NMR (100 MHz,
CDCl , TMS): δ = 1.1, 33.6, 71.6, 121.6, 124.6, 128.5, 128.6,
3
141.6.
1,3,3-Trimethyl-2-(trimethylsiloxy)bicyclo[2.2.1]heptane-2-
carbonitrile (2o)
2266. (d) Chen, F.-X.; Zhou, H.; Liu, X.; Qin, B.; Feng, X.; Zhang,
G.; Jiang, Y. Chem. Eur. J. 2004, 10, 4790. (e) Shen, Y.; Feng, X.; Li,
Y.; Zhang, G.; Jiang, Y. Tetrahedron 2003, 59, 5667.
colorless oil; yield: 250 mg (99%). 1H NMR (300 MHz, CDCl3,
TMS): δ = 0.26 (s, 9 H), 0.91 (s, 3 H), 0.99 (s, 3 H), 1.02 (s, 3 H),
1.12–1.25 (m, 1 H), 1.60–1.88 (m, 4 H), 2.06 (d, J = 14.1 Hz, 1 H),
(f) Hamashima, Y.; Kanai, M.; Shibasaki, M. Tetrahedron Lett.
13
2001, 42, 691. (g) Hamashima, Y.; Kanai, M.; Shibasaki, M. J. Am.
2.23 (ddd, J = 14.1, 3.4, 3.0 Hz, 1 H). C NMR (75 MHz, CDCl3,
TMS): δ = 1.0, 10.5, 20.4, 21.1, 26.5, 31.7, 45.1, 47.8, 48.8, 54.1,
78.4, 121.8.
Chem. Soc. 2000, 122, 7412.
(13) (a) Rostami, A.; Atashkar, B. Catal. Commun. 2015, 58, 80.
(
b) Strappaveccia, G.; Lanari, D.; Gelman, D.; Pizzo, F.; Rosati, O.;
2,2-Diphenyl-2-(trimethylsiloxy)acetonitrile (2p)
1
Curini, M.; Vaccaro, L. Green Chem. 2013, 15, 199. (c) Atashkar,
B.; Rostami, A.; Tahmasbi, B. Catal. Sci. Technol. 2013, 3, 2140.
colourless oil; yield: 281 mg (99%).
H NMR (300 MHz,
©
Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–F

F
Synlett
X. Huang et al.
Letter
CDCl ,TMS): δ = 0.12 (s, 9 H), 7.31–7.39 (m, 6 H), 7.47–7.51 (m,
The MeOH-washed catalyst was further rinsed thoroughly with
an additional 20 mL of anhydrous MeOH to ensure a negative
response to ninhydrin (the resulting catalyst was used in Table
2, entry 24, after washing with PhMe and suction-drying under
air). The catalyst was washed again with 5 mL of dilute HCl
solution, prepared by adding 1 volume of concd HCl to 20
volumes of MeOH, and the filtrate was trapped in 2 M aq NaOH
(5 mL). Active charcoal (100 mg) was added to the resulting
solution, and the mixture was agitated for 10 min then filtered.
The final filtrate was subjected to colorimetric ninhydrin
cyanide detection, as described above. The positive blue color
obtained within 2 min proved that cyanide anion was present in
this solution, showing that cyanide anions had been coordi-
nated by Lewis sites and washed off with the dilute HCl.
3
1
3
4
1
H). C NMR (75 MHz, CDCl , TMS): δ = 1.0, 76.2, 120.5, 125.7,
28.3, 128.5, 141.7.
3
(
23) Rapid Colorimetric Ninhydrin Detection of Cyanide (see also
ref. 18)
MMT K10 (100 mg) was treated with TMSCN (400 μL) for 6 h,
collected by filtration, washed thoroughly with PhMe, and
suction-dried under air for a few minutes. The filtrate was dis-
carded. The resulting catalyst was washed slowly with anhyd
MeOH (10 mL), while the filtrate was trapped simultaneously in
1
M aq NaOH (5 mL). Active charcoal (100 mg) was added to the
resulting solution, and the mixture was agitated for 10 min then
filtered. To the filtrate was added 1 wt% aq ninhydrin solution
(1 mL) with stirring. The color of the solution gradually became
blue within 2 min, indicating the presence of cyanide anion.
©
Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–F