Highly efficient cyanosilylation of sterically bulky ketones catalyzed by tin ion-exchanged montmorillonite

Article abstract of DOI:10.1002/adsc.200800673

Tin ion-exchanged montmorillonite (SnMont) was found to be a powerful catalyst for the cyanosilylation of aromatic and aliphatic ketones having a carbonyl group in sterically congested circumstances with cyanotrimethylsilane, giving the corresponding cyan

Full text of DOI:10.1002/adsc.200800673

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DOI: 10.1002/adsc.200800673
Highly Efficient Cyanosilylation of Sterically Bulky Ketones
Catalyzed by Tin Ion-Exchanged Montmorillonite
Jiacheng Wang,a Yoichi Masui,a Kenya Watanabe,a and Makoto Onakaa,*
a
Department of Chemistry, Graduate School of Arts and Sciences, The University of Tokyo, Meguro-ku, Tokyo 153-8902,
Japan
Fax : (+81)-3-5454-6595; e-mail: conaka@mail.ecc.u-tokyo.ac.jp
Received: November 11, 2008; Revised: February 13, 2009; Published online: March 5, 2009
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200800673.
Abstract: Tin ion-exchanged montmorillonite (Sn-
Mont) was found to be a powerful catalyst for the
cyanosilylation of aromatic and aliphatic ketones
having a carbonyl group in sterically congested cir-
cumstances with cyanotrimethylsilane, giving the
corresponding cyanohydrin trimethylsilyl ethers in
excellent yields.
high yields is a big challenge in the field of current or-
ganic synthesis.
Montmorillonite, a naturally-occurring clay, is com-
posed of stacked, negatively-charged aluminosilicate
layers that hold exchangeable cationic species in the
interlayers.[10]
Normally, multivalent metal cations are introduced
into the interlayer space of Na ion-exchanged mont-
morillonite (Na-Mont),[11,12,13,14] forming the acidic
montmorillonites which have been utilized as solid
acids in various acid-catalyzed organic reactions be-
cause of the environmental compatibility, reusability,
operational simplicity, innocuousness, absence of cor-
rosion, low cost, and easy isolation. Among metal
Keywords: clays; cyanosilylation; heterogeneous
catalysis; ketones; tin
The cyanosilylation of carbonyl compounds with tri- ion-exchanged montmorillonites (M-Monts),[15] we
methylsilyl cyanide (TMSCN) is the most straightfor- have found that Sn-Mont possesses quite a high sur-
ward way to form cyanohydrin trimethylsilyl ethers, face area and very strong acidicity suitable for the Mi-
which can be further transformed into a wide range chael reaction,[11a] the silylation of alcohols,[15] and the
of valuable polyfunctionalized building blocks includ- reduction of carbonyl compounds with hydrosi-
ing a-hydroxy acids, b-amino alcohols, and other bio- lanes.[16]
logically active compounds.[1] Because of its impor-
We have also revealed the catalysis of Sn-Mont for
tance in organic synthesis, different catalysts have the cyanosilylation of aldehydes and ordinary ke-
been developed for the cyanosilylation reaction of tones.[13] And unique catalysts have still been devel-
carbonyl compounds with TMSCN, such as noble oped for the reaction during the last decade.
metal nanoparticles,[2]
a
metal complex,[3] Lewis
In this communication, we address the powerful
acids,[4] base catalysts,[5] organic salts,[6,7] an ionic catalysis of Sn-Mont especially for the cyanosilylation
liquid,[8] and various organic catalysts.[9] Most of these of ketones congested with sterically bulky substituents
catalysts are successful in the cyanosilylation of alde- around the carbonyl group, comparing the catalytic
hydes and simple ketones under mild conditions. effectiveness between Sn-Mont and the recently de-
However, their catalytic activities obviously decrease, veloped catalysts (Scheme 1).
and even disappear in the case of the cyanosilylation
Sn-Mont was prepared through ion-exchange of
of sterically bulky ketones like aromatic ketones Na-Mont (Kunimine Industries Co. Ltd., Japan) with
owing to the congestion around the carbonyl groups.
Thus, the reaction time has to be prolonged to several
or tens of hours, or the reaction must be performed
under heating. Even so, the resultant yields are unsat-
isfactory in most cases. Therefore, to realize the effi-
cient cyanosilylation of sterically congested ketones in
Scheme 1. Cyanosilylation of sterically congested ketone
with Me3SiCN.
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Table 1. Cyanosilylation of benzophenone with TMSCN
an SnCl4 solution. Elemental analysis of Sn-Mont by
inductively coupled plasma (ICP) showed a 97.1% ex-
change of Na+ for Sn4+, indicating that 0.0019 mol of
tin ions are included in one gram of the as-prepared
Sn-Mont. The nitrogen sorption data showed that the
specific surface area greatly increased from 12 m2 gÀ1
of pristine Na-Mont to 280 m2 gÀ1 of Sn-Mont with a
porous structure (see the Supporting Information).
Our studies started with benzophenone as a test
substrate to compare the catalysis between the differ-
ent catalysts in the cyanosilylation with TMSCN, be-
cause benzophenone is much less reactive than alde-
using different catalysts.[a]
Entry
Catalyst
Time (min)
Yield[b] (%)
1
2
3
4
5
No catalyst
Na-Mont
Sn-Mont
SnO2
Sn(OH)4
Sn-MCM-41
Ti-Mont
Fe-Mont
Al-MCM-41
LiCl
30
30
6
0
<0.5
99
<0.5
<0.5
<0.5
92
95
<1
11
30
30
30
30
30
90
90
60
6
hydes and ordinary, simple ketones. The results are 6[c]
7
8
9
summarized in Figure 1 and Table 1. No reaction pro-
10[d]
11[d]
12[d]
ZnI2
LiOMe
0
84 (98[e])
[a]
Reaction conditions: benzophenone (1 mmol); TMSCN
(1.2 mmol); CH2Cl2 (2 mL); catalyst (10 mg); room tem-
perature.
GC yields using n-dodecane as an internal standard.
Sn-MCM-41 prepared by the ion-exchange method or
the direct hydrothermal method (see the Supporting In-
formation).
The reaction was conducted in freshly distilled tetrahy-
drofuran (THF, 2 mL) using 1.9 mol% of a catalyst.
Reaction time was 10 min.
[b]
[c]
[d]
[e]
Figure 1. Time-course of the cyanosilylation of benzophe-
none using Sn-Mont and control heterogeneous catalysts.
Reaction conditions: benzophenone (1 mmol); TMSCN
(1.2 mmol); CH2Cl2 (2 mL); catalyst (10 mg); room temper-
ature.
lytic activity is definitely inferior to that of Sn-Mont
(Figure 1; Table 1, entries 7 and 8).[14] More recently,
Yasuda et al. found that mesoporous aluminosilicate
Al-MCM-41 catalyzed the cyanosilylation of various
aldehydes and ketones with TMSCN under mild reac-
tion conditions.[17] However, it was found that Al-
ceeded in the absence of a catalyst (Table 1, entry 1). MCM-41 had almost no activity for the cyanosilyla-
Firstly, various heterogeneous catalysts were used in tion of benzophenone (Figure 1; Table 1, entry 9).
entries 2–9. Although Na-Mont gave almost no cataly-
Secondly, representative homogeneous catalysts
sis (Table 1, entry 2), Sn-Mont (10 mg) showed the were applied for the cyanosilylation (entries 10–12):
highest catalytic activity for the cyanosilylation to LiCl was active for the cyanosilylation of aldehydes
give the corresponding cyanohydrin trimethylsilyl and ketones under a high molar ratio of substrate to
ether in 99% yield in only 6 min (Figure 1; Table 1, LiCl in THF.[18] However, the cyanosilylation of ben-
entry 3). It should be emphasized that merely filtering zophenone in the presence of LiCl (1.9 mol%) gave a
and condensing the reaction mixture gave the product very poor yield of 11% in 90 min (Table 1, entry 10).
in greater than 98% purity without further purifica- Although ZnI2 was also reported to be highly effi-
tion. In contrast, Sn(OH)4 which was prepared from cient,[19] in our experiment no cyanohydrin trimethyl-
SnCl4 with aqueous NH3 and crystalline SnO2 were in- silyl ether was obtained by use of 1.9 mol% of ZnI2 in
effective for the reaction (Table 1, entries 4 and 5). 60 min (Table 1, entry 11). Among the homogeneous
Sn-MCM-41, which was prepared from silica MCM-41 catalysts, LiOMe[20] (1.9 mol%) turned out to give the
and aqueous SnCl4, or by hydrothermal sol-gel syn- highest yields of 84% and 98% in 6 and 10 min, re-
thesis from SiACHTUNGTRENNUNG(OEt)4 and tin oxalate (see Supporting spectively (Table 1, entry 12). LiOMe is not only
Information), did not promote the reaction (Table 1, basic, but also moisture- and CO2-sensitive so that it
entry 6). Although the other metal ion-exchanged requires special care for handling and storage, and is
montmorillonites such as Ti-Mont and Fe-Mont were not reusable.
also efficient for the cyanosilylation, affording the
Recently, benzyltriphenylphosphonium chloride
product in more than 90% yield in 30 min, their cata- was proposed in the cyanosilylation of benzophenone
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Highly Efficient Cyanosilylation of Sterically Bulky Ketones
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Table 2. Cyanosilylation of various ketones with TMSCN
(0.5 mmol) at 508C for a long reaction period of 48 h
to obtain the corresponding product in 94% yield.[7]
Subsequently, Olah et al. reported that the cyanosily-
lation of benzophenone with 5 mol% of K2CO3 or
phosphate salts in dimethylformamide (DMF) effi-
ciently proceeded to give the cyanosilylated product
in 88–100% yield in 1–2 h at room temperature,[21]
whereas with our catalyst system, use of only 10 mg
of Sn-Mont is enough to complete the reaction in just
6 min with 99% yield (Figure 1; Table 1, entries 1–3).
It can be concluded that the activity of Sn-Mont for
the cyanosilylation of benzophenone is clearly higher
than that of the known catalysts, such as Al-MCM-41,
ZnI2, LiOMe, LiCl, K2CO3, phosphonium salts and
phosphates.
using Sn-Mont as the catalyst.[a]
In order to reveal the scope of the Sn-Mont-cata-
lyzed cyanosilylation, not only common aromatic ke-
tones, but also congested aromatic and aliphatic ke-
tones with a carbonyl group surrounded by phenyl or
tert-butyl substituents were applied (Table 2). In gen-
eral, the cyanosilylation of various ketones (1 mmol)
with TMSCN (1.2 mmol) proceeded efficiently to pro-
vide the desired products in high yields and >98%
purities with only 10 mg of Sn-Mont after filtering
and condensing the reaction mixture (entries 1–10
and 12–14): A simple ketone, 5-nonanone reacted
with TMSCN rapidly in only 30 seconds. Alkyl aryl
ketones underwent the cyanosilylation at room tem-
perature with high yields in very short reaction peri-
ods of 1–6 min, depending on the size of the alkyl
groups (entries 2–5) or the properties of the aromatic
rings (entries 6–9) with an electron-withdrawing or
electron-releasing substituent. It is noteworthy that all
the known catalysts in the literature required several
to tens of hours to complete the cyanosilylation of
alkyl aryl ketones, while Sn-Mont completed the cya-
nosilylation in very short periods. In addition to the
cyanosilylation of benzophenone (entry 10), we also
investigated the cyanosilylation of ketones with a car-
bonyl group under extraordinarily congested circum-
stances (entries 12–15). Excellent yields for the de-
sired adducts were obtained for the cyanosilylation of
9-fluorenone, di-tert-butyl ketone and 2-benzoylnaph-
thalene, although slightly longer reaction times were
required (entries 12–14). In contrast, of note is that
the cyanosilylation of 9-fluorenone by Amberlite
XAD resin needed more than 20 h at 608C.[22] Toward
a further hindered ketone, tetraphenylcyclopenta-
[a]
Reaction conditions: ketone (1 mmol), TMSCN
(1.2 mmol), Sn-Mont (10 mg), CH2Cl2 (2 mL), room tem-
perature.
[b]
Yields were calculated based on the product weight after
filtering and condensing the reaction mixture. Purity of
all products was >98% by 1H NMR.
[c]
The reaction was carried out in the presence of Sn-Mont
ACHTUNGTRENNUNGdienone, the cyanosilylation proceeded with Sn-Mont
for 2 min, and then Sn-Mont was filtered off and the fil-
trate was stirred for 4 min. Yields were determined by
1H NMR using an internal standard.
Reaction time: 2 min.
Reaction was carried out with 0.5 mmol of ketone and
0.6 mmol of TMSCN.
Reaction conditions: ketone (0.25 mmol), TMSCN
(1.25 mmol), Sn-Mont (10 mg), chlorobenzene (2.5 mL),
1008C.
for 1.5 h under heating to obtain a high yield of 93%
(entry 15). Unsurprisingly, no cyanosilylation of tetra-
phenylcyclopentadienone with TMSCN took place
without Sn-Mont even at high temperatures
(entry 15). To the best of our knowledge, the cyanosi-
lylation of congested ketones such as 2-benzoylnaph-
thalene, di-tert-butyl ketone, and tetraphenylcyclopen-
[d]
[e]
[f]
[g]
Reaction was carried out without Sn-Mont.
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Jiacheng Wang et al.
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tadienone with TMSCN represent unprecedented re- Experimental Section
sults.
In the course of the cyanosilylation with Sn-Mont Synthesis of Sn-Mont
in CH2Cl2, some active tin species might migrate from
Sn-Mont was prepared from Na+ ion-exchanged montmoril-
lonite (Na-Mont) “Kunipia-F” (Kunimine Industries Co.
Ltd.; Na, 2.69; Al, 11.8; Fe, 1.46; Mg, 1.97%. Cation-ex-
change capacity=1.19 mequivgÀ1) according to our previ-
ously report.[16] Na-Mont (8 g) was ion-exchanged with
aqueous SnCl4·5H2O (0.3M, 80 mL) at room temperature
for 2 h and then this exchange procedure was repeated
again. Then the collected clay was washed with water
(80 mL) twice, with a mixture of water (40 mL) and metha-
nol (40 mL) 6 times until the washing filtrate became almost
neutral. Then the solid cake was rinsed with absolute metha-
nol (80 mL) once. Finally Sn-Mont was dried in vacuum (0.5
mmHg) at room temperature for 12 h, followed by being
ground in a mortar with a pestle, passed through a 60-mesh
screen and stored in a glass vial.
the solid into the liquid phase and the leached tin spe-
cies in solution could be partly contributing to the cat-
alysis. To investigate the possibility of such a homoge-
neous catalysis in the cyanosilylation of benzophe-
none, Sn-Mont was filtered off after the reaction mix-
ture had been stirred for 2 min at room temperature,
and then the filtrate was again stirred under the same
conditions. No increase in the yield was observed
after solid Sn-Mont was removed, proving that the cy-
anosilylation was catalyzed heterogeneously (Table 2,
entry 11).
Thus, Sn-Mont is superior to homogeneous catalysts
in the way that Sn-Mont can be easily separated from
the products and can be used in fixed-bed processes.
Generally, the reusability is another important aspect
for a heterogeneous catalyst. Re-use of Sn-Mont was
examined for the cyanosilylation of acetophenone and
benzophenone. Sn-Mont was able to be easily recov-
ered by simple filtration and re-used in at least 3 reac-
tion cycles under the same conditions without any sig-
nificant loss of the activity (Figure 2), confirming that
Sn-Mont is truly reusable and highly stable. Actually,
The synthesis and characterization of the control catalysts
such as Al-MCM-41 and Sn-MCM-41 are shown in the Sup-
porting Information.
Catalytic Reaction
Dichloromethane and 1-chlorobenzene were distilled with
CaH2 and stored in a glass bottle containing activated mo-
lecular sieves 4 A. The experimental procedures for the cya-
as-prepared Sn-Mont was stored in a glass vial under nosilylation of various ketones catalyzed by Sn-Mont are
shown as follows:
ambient conditions for 6 months, and showed no de-
crease in catalytic activity.
The reaction procedures in entries 1–10 and 12–13 in
Table 2: The ketone (1.0 mmol) was added to a CH2Cl2
(2 mL) suspension of Sn-Mont (10 mg) which had been pre-
treated under vacuum at 1208C for 1 h, and then TMSCN
(160 mL, 1.2 mmol) was added. The mixture was stirred at
room temperature under a nitrogen atmosphere. The prog-
ress of the reaction was monitored by TLC. After the reac-
tion was complete, the catalyst was filtered off and washed
with CH2Cl2. Evaporation of the filtrate under vacuum gave
the corresponding cyanohydrin trimethylsilyl ether, the
1
purity of which was analyzed by H NMR to be more than
98%. Thus, no further purification was needed.
The recovered catalyst was separated, washed with a large
amount of water and methanol, dried under vacuo, and used
again in another reaction.
The reaction procedure in entry 14 in Table 2: The reac-
tion procedure is the same as the above one except that a
half amount of ketone (0.5 mmol) and TMSCN (0.6 mmol)
are used.
Figure 2. Reusability of Sn-Mont for the cyanosilylation of
acetophenone and benzophenone, respectively.
The reaction procedure in entry 15 in Table 2: Tetraphe-
In conclusion, Sn-Mont, easily prepared from com- nylcyclopentadienone (0.25 mmol) was added to a 1-chloro-
benzene (2.5 mL) suspension of Sn-Mont (10 mg), which
had been pretreated under vacuum at 1208C for 1 h, and
then TMSCN (1.25 mmol) was added. The mixture was
stirred at 1008C under a nitrogen atmosphere. The progress
of the reaction was monitored by TLC. After the reaction
was complete, the catalyst was filtered off and washed with
CH2Cl2. Evaporation of the filtrate under vacuum gave the
corresponding crude cyanohydrin trimethylsilyl ether. The
pure product was obtained by recrystallization from MeOH.
Products were characterized by spectral analysis
(1H NMR, 13C NMR, and FT-IR) and HR-MS, and the spec-
mercially available montmorillonite clay, was found to
be the most efficient heterogeneous catalyst generally
applicable for the cyanosilylation of various ketones –
from simple to highly congested ones. Further appli-
cation of Sn-Mont as a solid strong acid catalyst to a
variety of fine chemicals syntheses is now in progress.
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tral data and representative NMR images are included in
the Supporting Information.
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We are grateful to Grant-in-Aid for Scientific Research from
the Ministry of Education, Culture, Sports, Science and Tech-
nology, Japan for support of our research.
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