Functionalization on silica gel with allylsilanes. A new method of covalent attachment of organic functional groups on silica gel

Article abstract of DOI:10.1021/ja034691l

Treatment of an (allyl)organosilane with silica gel in refluxing toluene brought about deallylation forming an Si-O-Si bond with the silicon on the silica gel. This Si-O-Si bond formation provides us with a new reliable method for the functionalization of a silica gel surface. Copyright

Full text of DOI:10.1021/ja034691l

Published on Web 04/01/2003
Functionalization on Silica Gel with Allylsilanes. A New Method of Covalent
Attachment of Organic Functional Groups on Silica Gel
Toyoshi Shimada,*,† Kazuko Aoki,† Yo Shinoda,† Tomoaki Nakamura,† Norihito Tokunaga,†
Shinji Inagaki,‡ and Tamio Hayashi*,†
Department of Chemistry, Graduate School of Science, Kyoto UniVersity, Sakyo, Kyoto 606-8502, Japan, and
Toyota Central R&D Laboratories, Inc., Nagakute, Aichi 480-1192, Japan
Received February 17, 2003; E-mail: thayashi@kuchem.kyoto-u.ac.jp
Scheme 1
Binding of an organic functional group to a silica surface via a
covalent bond is the most reliable method of modification and
functionalization of the silica surface.1 The covalent bond used for
the binding is mostly the Si-O-Si bond, where one of the silicon
atoms is on the silica surface and the other comes from organo-
silicon compounds, and the Si-O-Si bond is formed by the
reaction of an Si-OH group on the silica gel surface with
organosilicon compounds containing a leaving group of high
reactivity on the silicon atom. The organosilicon compounds most
Table 1. Loadings of (3-Chloropropyl)silanes 1 and 2 on Silicasa
commonly used are those containing an alkoxy leaving group (Rn-
SiX4-n: X ) OR′).2 Halides, acyloxy, and amino groups on the
organosilane
(mmol)
time
(h)
loadingb
silicon atom (X ) halide, OCOR′, and NR′2 in RnSiX4-n) have
been also used for the Si-O-Si bond formation.1c Unfortunately,
these functional groups are so reactive toward hydrolysis that the
silicon compounds cannot be handled under hydrolytic conditions
or cannot be purified by silica gel chromatography. Here we wish
to report a new method of functionalizing a silica surface, which
is realized by use of (allyl)organosilanes (Scheme 1). They are stable
under regular hydrolytic conditions but undergo deallylation forming
an Si-O-Si bond on the silica under certain conditions.
entry
silica gel
FSM-16
FSM-16
FSM-16
FSM-16
FSM-16
FSM-16
FSM-16
FSM-16
FSM-16
FSM-16
FSM-16
FSM-16
amorphous silica
amorphous silica
(mmol g-1
)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
1 (3)
1 (5)
15
15
15
15
0.5
48
15
15
15
15
0.5
48
15
15
1.1
1.3
1.6
1.7
1.1
1.6
0.8
0.8
1.2
1.3
0.6
1.6
0.3
0.5
1 (10)
1 (15)
1 (10)
1 (10)
2 (3)
2 (5)
2 (10)
2 (15)
2 (10)
2 (10)
1 (10)
2 (10)
We chose 2-propenyl(3-chloropropyl)dimethylsilane (1) as an
organo-functionalized allylsilane,3 which is stable enough to be
purified by silica gel column chromatography at room temperature
(40 °C or lower), and we examined its reactivity toward silica gel
at higher temperature. It was found that the Si-O-Si bond
formation on the silica surface takes place efficiently with the
allylsilane 1 at the temperature of refluxing toluene. The results
obtained for the reaction of allylsilane 1 with FSM-164 and
amorphous silica5 are summarized in Table 1, which also contains
the data obtained with alkoxysilane, methoxy(3-chloropropyl)-
dimethylsilane (2),6 for comparison. As a typical experimental
procedure (entry 2), 1.0 g of FSM-16 was treated with 0.89 g (5.0
mmol, 1.0 equiv of SiOH on the silica gel) of allylsilane 1 in 30
mL of refluxing toluene for 15 h. Filtration and Soxhlet extraction
with methanol for 24 h and with benzene for 48 h followed by
drying at 120 °C for 15 h under 0.2 mmHg gave 1.14 g of the
functionalized FSM-16. Introduction of the 3-chloropropyldimeth-
ylsilyl group onto the silica gel was confirmed by 29Si and 13C
NMR spectra (vide infra), and its amount was determined to be
1.3 mmol per 1.0 g of the starting FSM-16 by elemental analysis
of the chlorine atom. The use of a greater amount of the allylsilane
1 under otherwise the same conditions gave the silica gel containing
a greater amount of the 3-chloropropyldimethylsilyl group, 1.7
mmol per 1.0 g of the FSM-16 being obtained with 15 mmol of 1
(entry 4). It should be noted that the present method using the
allylsilane 1 is more efficient than that using methoxysilane 2, which
has been often used for the functionalization. With a smaller amount
a All reactions were carried out for 1.0 g of silica gel support in 30 mL
of refluxing toluene. b Obtained by elemental analysis of Cl.
of the organosilicon compounds and in a shorter reaction time, a
greater amount of the silyl group is introduced on the silica gel
(compare entries 1-6 with entries 7-12). The present method of
functionalization using the allylsilane is also applicable to amor-
phous silica gel, although the amount of the attached silyl group is
lower with allylsilane 1 than with methoxysilane 2 (entries 13 and
14).
The Si-O-Si bond formation between the 3-chloropropyldim-
ethylsilyl group and the FSM-16 surface was demonstrated by 29Si
and 13C CP-MAS NMR spectra. Figure 1 contains the NMR spectra
for allylsilane 1 (a and d), FSM-16 treated with allylsilane 1 (b
and e), and FSM-16 treated with methoxysilane 2 (c and f). The
resonance of 29Si at 1.7 ppm observed for allylsilane 1 (a) was
replaced by the resonance at 15.0 ppm by treatment with FSM-16
(b). This chemical shift is a typical value for trialkylsilyloxy groups
on the silica surface1g,7 and is essentially the same as that in the
FSM-16 (c), which is obtained by a standard procedure for the
modification of a silica surface with a 3-chloropropyldimethylsilyl
group using methoxysilane 2.8 Three 13C singlets at 23.11, 113.16,
and 134.56 ppm which are assigned to the allyl carbons on the
allylsilane 1 (d) disappeared by treatment with FSM-16 (e). The
other four singlets remained on the FSM-16, indicating that the
† Kyoto University.
‡ Toyota Central R&D Laboratories, Inc.
9
4688
J. AM. CHEM. SOC. 2003, 125, 4688-4689
10.1021/ja034691l CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2 a
a Reagents and conditions: (a) H2N(CH2)3SiMe2CH2CHdCH2, EDC,
HOBt, DMF, 83%; (b) FSM-16, toluene, reflux.
on the silica gel as a reliable functionalization method. Immobiliza-
tion of a catalyst on a silica surface is one of the examples.13
Acknowledgment. This work was supported by a Grant-in-Aid
for Scientific Research from the Ministry of Education, Japan.
Supporting Information Available: Experimental procedures,
spectroscopic and analytical data for the substrates and products (PDF).
This material is available free of charge via the Internet at http://
pubs.acs.org.
Figure 1. 29Si and 13C NMR spectra of 2-propenyl(3-chloropropyl)-
dimethylsilane 1 in CDCl3 (a and d), 29Si and 13C CP-MAS NMR spectra
of the reaction products of 1 with FSM-16 (b and e), and those of 2 with
FSM-16 (c and f).
References
(1) (a) Iler, R. K. The Chemistry of Silica; John Wiley & Sons, Ltd.: New
York, 1979; Chapter 6. (b) Pesek, J. J., Leigh, I. E., Eds. Chemically
Modified Surfaces; Royal Society of Chemistry: Cambridge, 1994. (c)
Vansant, E. F.; Van Der Voort, P.; Vrancken, C. In Studies in Surface
Science and Catalysis; Delmon, B., Yates, T., Eds.; Elsevier: Amsterdam,
1995; Vol. 93, Part II. (d) Pesek, J. J., Matyska, M. T., Abuelafiya, R. R.,
Eds. Chemically Modified Surfaces: Recent DeVelopments; Royal Society
of Chemistry: Cambridge, 1996. (e) Vankelecom, I. F. J.; Jacobs, P. A.
In Chiral Catalyst Immobilization and Recycling; De Vos, D. E.,
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3-chloropropyldimethylsilyl group was attached to the silica surface
while the allyl group was lost from the allylsilane 1.8 It is worth
noting that the allylsilane 1 is a better modifying reagent than
methoxysilane 2 with respect to the chemical purity of the silica
surface. With methoxysilane 2, which releases the methanol at the
Si-O-Si bond formation, a considerable amount of methoxy group
on the silica gel is observed at 49.3 ppm (f).9 In the reaction with
allylsilane 1, the leaving molecule from the silicon atom is
propene,10 which is not reactive toward silica gel and is readily
removed from the reaction mixture. It is likely that the reaction
proceeds, as the protodesilylation of allylic silanes,11 by way of a
â-silyl cation intermediate which is formed by the protonation of
the allyl group with silanol on the silica surface and undergoes
nucleophilic attack by the silanol oxygen leaving propene.
The present method makes it possible to modify the silica gel
surface with heavy organo-functional groups, which cannot be
purified by distillation due to their nonvolatility. For example, the
BINAP skeleton was attached to the FSM-16 surface by use of
allylsilane 4, which was obtained by the amide bond formation of
BINAP-carboxylic acid 312 with 2-propenyl(3-aminopropyl)dim-
ethylsilane. The BINAP-allylsilane 4 (3.96 g, 4.7 mmol), which
was purified by silica gel chromatography (hexane/ethyl acetate )
2/1), was treated with FSM-16 (0.91 g) in refluxing toluene for 15
h to give the FSM-16 5 containing 0.3 mmol/g of the BINAP unit
(Scheme 2).
(3) Voronkov, M. G.; Kirpichenko, S. V.; Abrosimova, A. T.; Albanov, A.
I.; Keiko, V. V.; Lavrent’yev, V. I. J. Organomet. Chem. 1987, 326, 159.
(4) (a) Inagaki, S.; Fukushima, Y.; Kuroda, K. J. Chem. Soc., Chem. Commun.
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Inagaki, S.; Fukushima, T.; Kondo, S. J. Chem. Soc., Faraday Trans.
1996, 92, 1985. The sample used here has a surface area of 966 m2 g-1
,
pore diameter of 2.8 nm, and 5.0 mmol silanol groups g-1, and it was
dried at 140 °C for 17 h under vacuum at 10-5 mmHg.
(5) Silica gel (Davisil), with a particle diameter range of 100-200 mesh and
a surface area of 480 m2 g-1, was purchased from Aldrich.
(6) Jung, M. E.; Xia, H. Tetrahedron Lett. 1988, 29, 297.
(7) Maciel, G. E.; Sindorf, D. W.; Bartuska, V. J. J. Chromatogr. 1981, 205,
438.
(8) 13C CP-MAS NMR for the 3-chloropropylsiloxy group on silica surface
has been reported: Hays, G. R.; Clague, A. D. H.; Huis, R.; Van Der
Velden, G. Appl. Surf. Sci. 1982, 10, 247.
(9) Schenk, U.; Hunger, M.; Weitkamp, J. Magn. Reson. Chem. 1999, 37,
S75.
(10) The generation of propene was confirmed by the detection of 1,2-
dibromopropane, which is formed by treatment of the evolved gas with
bromine.
(11) (a) Sommer, L. H.; Tyler, L. J.; Whitmore, F. C. J. Am. Chem. Soc. 1948,
70, 2872. (b) Chan, T. H.; Fleming, I. Synthesis 1979, 761. (c) Morita,
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(12) Bayston, D. J.; Fraser, J. L.; Ashton, M. R.; Baxter, A. D.; Polywka, M.
E. C.; Moses, E. J. Org. Chem. 1998, 63, 3137.
(13) As a pertinent review on supported chiral catalysts on inorganic materi-
als: Song, C. E.; Lee, S.-G. Chem. ReV. 2002, 102, 3494.
To summarize, we found a new method for the modification of
the silica gel surface by use of (allyl)organosilanes. In refluxing
toluene, deallylation on the allylsilane takes place to form the Si-
O-Si bond with the silicon on the silica gel. The present method
will have broad applications in the surface-modifying technology
JA034691L
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J. AM. CHEM. SOC. VOL. 125, NO. 16, 2003 4689