Heterogeneous allylsilylation of aromatic and aliphatic alkenes catalyzed by proton-exchanged montmorillonite

Article abstract of DOI:10.1021/ol100228t

Figure Presented Allylsilylation of an alkene Is the only known procedure to Install both silyl and allyl groups onto a carbon-carbon double bond directly. Proton-exchanged montmorillonite showed excellent catalytic performances for the allylsilylation of alkenes. For example, the reaction of p-chlorostyrene with allyltrimethylsilane proceeded smoothly to afford the corresponding allylsilylated product In 95% yield. We also attempted to isolate the reaction intermediate on the montmorillonite surface to investigate the reaction mechanism.

Full text of DOI:10.1021/ol100228t

ORGANIC
LETTERS
2010
Vol. 12, No. 7
1508-1511
Heterogeneous Allylsilylation of Aromatic
and Aliphatic Alkenes Catalyzed by
Proton-Exchanged Montmorillonite
Ken Motokura, Shigekazu Matsunaga, Akimitsu Miyaji, Yasuharu Sakamoto, and
Toshihide Baba*
Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259
Nagatsuta-cho, Midori-ku, Yokohama, 226-8502, Japan
tbaba@chemenV.titech.ac.jp
Received January 28, 2010
ABSTRACT
Allylsilylation of an alkene is the only known procedure to install both silyl and allyl groups onto a carbon-carbon double bond directly.
Proton-exchanged montmorillonite showed excellent catalytic performances for the allylsilylation of alkenes. For example, the reaction of
p-chlorostyrene with allyltrimethylsilane proceeded smoothly to afford the corresponding allylsilylated product in 95% yield. We also attempted
to isolate the reaction intermediate on the montmorillonite surface to investigate the reaction mechanism.
Allylsilylation of an alkene is the only known procedure to
install both silyl and allyl groups onto a carbon-carbon
double bond directly (eq 1). Currently, there has been one
report of allylsilylation; aluminum chloride-catalyzed reaction
of aliphatic alkenes.1,2 This route is promising because it
has been reported to show excellent atom efficiency.
However, to install silyl and allyl groups to styrenyl alkenes,
stepwise procedures, which generate a large amount of
byproducts, are still necessary (eq 2).3
organosilane intermediates (Scheme 1, step 1). These inter-
mediates then undergo nucleophilic addition of alkenes (step
2). After the C-Si bond formation, nucleophilic addition of
an allylsilane to the ꢀ-position of the Si atom affords the
desired allylsilylated product (step 3).1
In the first step of the reaction, it is important to avoid
acidic activation of the alkene to increase the selectivity of
the allylsilylated product, because the C-Si bond forms when
the alkene acts as a nucleophile and the cationic organosilane
acts as an electrophile (Scheme 1, step 2). Acidic activation
of alkenes induces alkene oligomerization, especially with
styrene derivatives. According to the hard and soft acids and
bases (HSAB) principle, soft Lewis acids effectively activate
alkenes, which act as soft bases.4 Therefore, the use of a
protonic acid as a hard acid is one means to avoid the
In the AlCl3-catalyzed allylsilylation of alkenes, the key
step is acidic activation of allylsilanes to form cationic
(1) (a) Yeon, S. H.; Lee, B. W.; Yoo, B. R.; Suk, M.-Y.; Jung, I. N.
Organometallics 1995, 14, 2361. (b) Jung, I. N.; Yoo, B. R. Synlett 1999,
519
.
(2) For allylsilylation of alkynes, see: (a) Yoshikawa, E.; Gevorgyan,
V.; Asao, N.; Yamamoto, Y. J. Am. Chem. Soc. 1997, 119, 6781. (b)
Imamura, K.-i.; Yoshikawa, E.; Gevorgyan, V.; Yamamoto, Y. J. Am. Chem.
(4) For soft Lewis acid-catalyzed styrene dimerization, see: (a) Doherty,
S.; Knight, J. G.; Smyth, C. H.; Harrington, R. W.; Clegg, W. Organome-
tallics 2007, 26, 5961. (b) Peng, J.; Li, J.; Qiu, H.; Jiang, J.; Jiang, K.;
Mao, J.; Lai, G. J. Mol. Catal. A: Chem. 2006, 255, 16.
Soc. 1998, 120, 5339
.
(3) Liepins, V.; Ba¨ckvall, J.-E. Chem. Commun. 2001, 265.
10.1021/ol100228t  2010 American Chemical Society
Published on Web 03/03/2010

3 times with retention of the high catalytic performance (89%
yield for third reaction). Other p-substituted styrenes, such
as p-bromo, and p-fuluoro styrenes, with 2a also proceeded
smoothly to afford the corresponding allylsilylated products
in 87 and 91% yields, respectively. This is the first reports
of the allylsilylation of styrene derivatives with allylsilane.
Styrenes with electron-donating groups, such as p-methoxy-
styrene, did not work well as substrates. H+-montmorillonite
promotes allylsilylation of aliphatic alkenes with 2a in good
to excellent yields. In addition to 2a, allyltriethylsilane and
methallyltrimethylsilane were also found to be good allyl-
silylating reagents.
Scheme 1
.
Proposed Reaction Pathway of Allylsilylation of
Alkene with Allylsilane
undesired activation of alkenes. Although allylsilane also has
a CdC bond, the electrons on the allylsilane are not strongly
delocalized due to the electron donating effect of Si atom.
Actually, the calculated proton affinity value of allyltrim-
ethylsilane (205.3 kcal/mol)5 is ∼4 kcal/mol higher than
reported proton affinity values of styrene.6 The protonic acid
may favor allylsilanes over the electron-delocalized styrene
double bonds. If the active Si species forms from allylsilanes
by the acidic activation on protonic acid site,7 the catalytic
allysilylation should be proceeded.
Table 1. Allylsilylation of Alkenes (1) with Allylsilanes (2)
using H+-Montmorillonite Catalysta
Herein, allylsilylation of alkenes with allylsilanes was
examined using protonic acids. Among the protonic acids
evaluated, proton-exchanged montmorillonite (H+-montmo-
rillonite)8 was found to be an excellent catalyst for allylsi-
lylation of both aromatic and aliphatic alkenes. In addition,
we investigated the chemical reactivities of Si species on
montmorillonite surface for mechanistic considerations.
Scheme 2 shows the results of allylsilylation of styrene
(1a) with allyltrimethylsilane (2a) using H+-montmorillonite.
Scheme 2. Allylsilylation of Styrene (1a) with
Allyltrimethylsilane (2a) using H+-Montmorillonite
a Reaction conditions: 1 (1.0 mmol), 2 (3.0 mmol), H+-montmorillonite
(0.10 g), toluene (1 mL), 100 °C. Around 15% of 2a was dimerized to
form [2-(2-propenyl)-1,3-propanediyl]-bis(trimethylsilane). For aliphatic
alkenes, 1 (3.0 mmol) and 2 (1.0 mmol) were used. b Determined by GC
using internal standard technique. Calibration curve was made by use of
isolated products. Yield was based on alkene 1. c At 80 °C. d Conversion
of allylsilane 2a was shown. e Reaction carried out at 60 °C. Yield was
based on allylsilane 2a.
The desired allylsilylated product was obtained in 72% yield
in the presence of H+-montmorillonite. The scope of substrate
in the H+-montmorillonite-catalyzed allylsilylation is shown
in Table 1. The allylsilylation occurred with p-chlorostyrene
(1b) to give the corresponding product in 95% yield. The
heterogeneous H+-montmorillonite could be removed from
the reaction mixture by simple filtration, and reused at least
Catalytic activity of other heterogeneous and homogeneous
protonic acids was evaluated in the reaction of 1b with 2a.
The product yield using commercially available montK10
(7.2%) was lower than that for the H+-montmorillonite.
Proton-exchanged zeolites, such as ZSM-5 and mordenite,
were less active (<1%) due to restricted pore size. The
product yield was very low with strong acids, Amberlyst,
Nafion, trifilic acid, p-toluenesulfonic acid, and H2SO4.
Reported acid strength of H+-montmorillonite (∆H ) 111
kJ mol-1)8 is lower than those of other zeolites (122-160
kJ mol-1). It can be expected that suitable acid strength is
necessary for the allylsilylation.8 The product did not form
(5) The proton affinity was calculated using AM1 method, see: Dewar,
M. J. S.; Dieter, K. M. J. Am. Chem. Soc. 1986, 108, 8075.
(6) Kafafi, S. A.; Meot-Ner, M.; Liebman, J. F. Struct. Chem. 1989, 1,
101.
(7) Onaka and co-workers proposed the formation of cationic silanes at
Bro¨nsted acid sites, see: (a) Higuchi, K.; Onaka, M.; Izumi, Y. Bull. Chem.
Soc. Jpn. 1993, 66, 2016. (b) Wang, J.; Masui, Y.; Watanabe, K.; Onaka,
M. AdV. Synth. Catal. 2009, 351, 553.
(8) (a) Motokura, K.; Fujita, N.; Mori, K.; Mizugaki, T.; Ebitani, K.;
Kaneda, K. Angew. Chem., Int. Ed. 2006, 45, 2605. (b) Motokura, K.;
Nakagiri, N.; Mizugaki, T.; Ebitani, K.; Kaneda, K. J. Org. Chem. 2007,
72, 6006.
Org. Lett., Vol. 12, No. 7, 2010
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when using sodium-exchanged montmorillonite. This result
shows H+ on the montmorillonite surface acts as catalytically
active site.
To estimate the H+-montmorillonite-catalyzed reaction
Scheme 5. Allylsilylation of 1b with Allyltriethylsilane in the
Presence of 1,3-Disilylpropan-2-ol
pathway, we examined the reaction of cyclohexene with 2a.
1
The H NMR study revealed the formation of allylsilylated
product with exclusively trans allyl and silyl groups. There
were no other major products converted from cyclohexene.
This result suggests stepwise addition of silyl and allyl groups
of the allylsilane to alkenes.1
Jung and co-workers proposed two reaction mechanisms
for the AlCl3-catalyzed allylsilylation of alkenes.1 The main
difference between the two mechanisms involves the struc-
ture of the cationic silane intermediate: (i) trimethylsilyl
cation (Si+Me3)1b and (ii) 1,3-disilylpropyl cation
(SiCH2C+HCH2Si).1a We attempted to isolate the silyl
species on the montmorillonite surface in an effort to examine
its reactivity. After the treatment of H+-montmorillonite with
2a, the obtained solid was washed with solvent, dried under
vacuum, and subjected to solid-state NMR analyses (Sup-
porting Information). NMR results revealed the formation
of only trimethylsiloxy species (Me3Si-O-Si(surface)) on
the montmorillonite (Scheme 3A).9 The reaction of 1b with
Scheme 6. Possible Formation Pathway of 1,3-Disilylpropyl
Cation from 2a and H+-Montmorillonite
Scheme 7. Proposed Reaction Mechanism of
H+-Montmorillonite-Catalyzed Allylsilylation
Scheme 3. (A) Formation of SiMe3 Species on Montmorillonite
Surface and (B) Allylsilylation using the SiMe3-Montmorillonite
allyltriethylsilane was then carried out using the treated
montmorillonite (SiMe3-montmorillonite). If the trimethyl-
siloxy species is an active cationic intermediate, the allyl-
silylated product is expected to contain a trimethylsilyl
(SiMe3) group. However, the reaction afforded an allylsily-
lated product only bearing a triethylsilyl (SiEt3) group
(Scheme 3B). This suggests that the surface SiMe3 is not an
active species in the allylsilylation of alkenes.10
During the treatment of H+-montmorillonite with 2a, a
dimerization of 2a took place. The dimerized product was
thought to be obtained from the reaction of the 1,3-
disilylpropyl cation with 2a (Scheme 4). To confirm the
participation of the 1,3-disilylpropyl cation in the catalytic
allylsilylation of alkenes, reaction of 1b with allyltriethyl-
silane was examined in the presence of 1,3-di(trimethylsi-
lyl)propan-2-ol, a precursor to the 1,3-disilylpropyl cation.
Scheme 4
.
Estimated Pathway for Dimerization of 2a on
Montmorillonite Surface
(9) Signals at -0.057 ppm (13C NMR) and -13.68 ppm (29Si NMR)
were observed. These signals are assignable to Me3Si-O-Si(surface) species,
see Supporting Information. See also: (a) Haukka, S.; Root, A. J. Phys.
Chem. 1994, 98, 1695. (b) Derouet, D.; Thuc, C. N. H. J. Appl. Polym. Sci.
2008, 109, 2113.
(10) 29Si NMR shift of cationic trimethylsilyl species (MeSi+) were
generally appeared at down field (30-40 ppm) compared with methoxysilyl
species (∼14 ppm), see: Iley, J.; Bassindale, A. R.; Patel, P. J. Chem. Soc.,
Perkin Trans. 1984, 77. This fact explains the inactivity of the Me3Si species
on montmorillonite surface.
1510
Org. Lett., Vol. 12, No. 7, 2010

After the reaction, formation of allylsilylated product with
SiMe3 was observed (Scheme 5). These results suggest that
the 1,3-disilylpropyl cation forms from allylsilane on the
montmorillonite surface and acts as a reaction intermediate.
The possible pathway of formation of the 1,3-disilylpropyl
cation from 2a and H+- montmorillonite is shown in Scheme
6. Formation of propylene by reaction of allylsilane with
H+-montmorillonite also supports this pathway. The 1,3-
disilylpropyl cation might be easily decomposed to 2a and
the inactive SiMe3 species, which can be detected by the
solid-state NMR analyses.11
attack of an allylsilane to the intermediate to form a new
C-C bond, and (iii) formation of allylsilylated product and
disilyl cation by nucleophilic addition of another allylsilane.
In summary, proton-exchanged montmorillonite was found
to be an excellent catalyst for the allylsilylation of aromatic
and aliphatic alkenes. The corresponding allylsilylated
products were obtained in up to 95% yield. The formation
of a disilyl cationic intermediate on the montmorillonite
surface was proposed in the catalytic cycle. Investigations
of a detailed reaction mechanism and spectroscopic analysis
of the active intermediate are currently underway.
The catalytic cycle for the allylsilylation of alkenes using
H+-montmorillonite is proposed, as shown in Scheme 7; (i)
nucleophilic attack of an alkene to disilyl cation to form
allylsilane and a secondary carbocation intermediate stabi-
lized by the silyl group at the ꢀ-position, (ii) nucleophilic
Acknowledgment. This work was supported by JSPS
(19760541, 21760626).
Supporting Information Available: Proton affinity cal-
culation, experimental procedures, product characterizations,
and solid-state NMR spectra. This material is available free
of charge via the Internet at http://pubs.acs.org.
(11) Because of the highly reusability of H+-montmorillonite, the inactive
SiMe3 species may form during solid-state NMR measurment or pretreat-
ment for the NMR analysis.
OL100228T
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