Transition-metal-catalyzed immobilization of organic functional groups onto solid supports through vinylsilane coupling reactions

Article abstract of DOI:10.1021/ja102741k

A novel and efficient grafting method for covalent bonding of functional organic molecules to silica or glass surfaces has been developed. The protocol employs transition-metal-catalyzed reactions of vinylsilanes with surface hydroxyl groups. Dimethyldivinylsilane can be used in this procedure as a linker in which one vinyl group is used for direct C-C bond formation with a functional organic molecule and the other is employed to immobilize the alkylsilyl group onto the hydroxyl surface of the solid support.

Full text of DOI:10.1021/ja102741k

Published on Web 05/07/2010
Transition-Metal-Catalyzed Immobilization of Organic Functional Groups onto
Solid Supports through Vinylsilane Coupling Reactions
Jung-Woo Park and Chul-Ho Jun*
Department of Chemistry and Center for BioactiVe Molecular Hybrids, Yonsei UniVersity, Seoul 120-749, Korea
Received April 9, 2010; E-mail: junch@yonsei.ac.kr
Surface modifications of inorganic solids, such as silica or glass,
with organic functional groups have received considerable attention
in materials science research.1 Several protocols for surface
modification of inorganic solids based on use of the postgrafting
method have been devised.2 However, alkoxysilanes1b,2a-e or
chlorosilanes,3 which are commonly used in this modification
technique, are unstable substances that are sensitive to moisture,
hard to transform into the desired functionalized silane precursors,
and difficult to purify by employing chromatographic methods.
Allylsilanes have recently been developed to overcome these
difficulties.4 However, allylsilane derivatives that possess carboxylic
acid groups cannot be prepared because of their facile acid-promoted
destruction by protodesilylation. We recently observed that vinyl-
silanes react with alcohols under very mild conditions in the
presence of rhodium(I)/HCl catalysts to afford alkoxysilanes.5 This
observation stimulated an investigation probing applications of
vinylsilanes in catalytic immobilization of organic substances onto
hydroxyl-containing surfaces of solid supports. One driving force
for this effort was the recognition that vinylsilanes can tolerate both
a variety of functional groups (including carboxylic acids) and
conditions used for chromatographic purification. In this com-
munication, we describe the results of a study that has led to the
development of a vinylsilane-based transition-metal/HCl-catalyzed
immobilization method for covalent attachment of organic func-
tional groups to silica or glass surfaces. In addition, we have found
that divinylsilanes can be used for coupling of functional organic
molecules to solid supports. In this method, one vinyl group of the
divinylsilane is used for immobilization onto a solid surface and
the other participates in a C-C bond-forming reaction [e.g.,
hydroacylation (eq 1)].
Figure 1. (a) Catalytic immobilization of 2a onto silica (1) using various
catalyst mixtures. (b) 13C CP-MAS NMR spectrum of 5 formed by 3b/4.
showed that the 3-chloropropyldimethylsilyl moiety was covalently
incorporated into 1 (Figure 1b).4a,11
This methodology can be employed in a simple inorganic-organic
coupling strategy that relies on the use of divinylsilanes as linker
species. In this process, one vinyl group of the divinylsilane is used
for C-C bond formation with the functional organic substance and
the other vinyl group participates in immobilization to the surface.
Among the many possible types of C-C bond-forming reactions
that occur via C-H bond activation,12 chelation-assisted hydroa-
cylation was chosen for this application.13 Reaction of benzaldehyde
(6a) with dimethyldivinylsilane (2b) in the presence of a mix-
ture of (Ph3P)3RhCl (3c), 2-amino-3-picoline (7), and p-CF3-
C6H4-CO2H (8) produced 3-(dimethyl(vinyl)silyl)phenylpropan-
1-one (9a) in a high 92% yield (Table 1, entry 1). Since the
divinylsilane is much more reactive in the coupling reaction than
a monovinylsilane, 9a was readily obtained without the intervention
of the side reaction involving double hydroacylation.14 This reaction
was performed using various functionalized aldehydes 6, each of
which gave the corresponding functionalized vinylsilane 9 in high
yield (Table 1).
Further investigations demonstrated that the functionalized
vinylsilanes 9 can be catalytically grafted to 1. Specifically, reactions
of 9 in the presence of 3b and 4 led to formation of organic
functional group-immobilized silicas 10 with high loading efficien-
cies (0.58-1.04 mmol g-1). Even the boronate-impregnated vinyl-
silane 9c participated in this process (entry 3). Also, vinylsilane 9e
containing the stable carboxylic group could be prepared using this
protocol through the reaction of 4-formylbenzoic acid (6e) with
2b (entry 5). NHS-ester-functionalized silica 10l was also readily
generated by esterification of 9e with N-hydroxysuccinimide
followed by immobilization of the resulting 9l on 1 in the presence
of 3b and 4. The loading rate of NHS groups in 10l was found to
be 0.70 mmol g-1 (Scheme 1). Furthermore, the resulting 10l can
serve as a useful handle for the introduction of primary-amine-
based organic substances. For example, reaction of 10l with the
dansyl-group-functionalized amine 11 at room temperature gave
the fluorescent, dansyl-group-immobilized silica 12 with a loading
Vinylsilanes can be immobilized onto silica by reaction with
surface hydroxyl groups that is promoted by catalysts (Figure 1a).
For example, when silica6 (1, 100 mg) was treated with 3-chloro-
propyldimethylvinylsilane (2a, 0.75 mmol, 5.8 equiv/OH in 1) in
the presence of [(COE)2RhCl]2 (3a, 3 mol % Rh based on 2a) and
N,N-dimethylacetamide hydrochloride salt7 (DMA·HCl, 4, 6 mol
%) at room temperature for 4 h and then washed with methylene
chloride and methanol, surface-modified silica 5 was produced. The
loading rate of 3-chloropropyldimethylsilyl groups on silica was
determined to be 0.91 mmol g-1.8 Studies exploring the use of other
transition-metal catalysts for the coupling process9 showed that
iridium(I) complex 3b promotes coupling of 4 with an activity of
0.97 mmol g-1 (Figure 1a).10 Inspection of the solid-state 13C cross-
polarization magic-angle-spinning (CP-MAS) NMR spectrum of 5
9
7268 J. AM. CHEM. SOC. 2010, 132, 7268–7269
10.1021/ja102741k  2010 American Chemical Society

C O M M U N I C A T I O N S
of 0.65 mmol g-1. Vinylsilanes containing n-alkyl groups with
different chain lengths (9h-k) were also immobilized onto 1 with
high loading efficiencies (0.74-0.94 mmol g-1; entries 8-11).
Table 1. Immobilization of the R Group of Aldehyde 6 onto Silica
(1) Using Dimethyldivinylsilane (2b) as a Covalent Bonding
Mediator
Figure 2. Surface modification of glass slides with vinylsilanes containing
various alkyl chains and the results of contact angle measurements (for
details, see the Supporting Information.)
Acknowledgment. This work was supported by the National
Research Foundation of Korea (NRF) (Grant 2009-0059013) and
the WCU (World Class University) Program through KOSEF
funded by the Ministry of Education, Science and Technology
(MEST), Korea (R32-2008-000-102170). J.-W.P. and C.-H.J.
acknowledge CBMH (R11-2003-019-0000-0) and a fellowship from
the BK21 Program from MEST.
Supporting Information Available: Experimental procedures and
spectroscopic and analytical data for 9a-l and 10a-l. This material is
available free of charge via the Internet at http://pubs.acs.org.
References
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Scheme 1
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(for details of the catalysts used, see the Supporting Information).
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almost removed in the washing step (for details, see the Supporting
Information).
The new grafting immobilization protocol was applied to the
modification of the surface of glass microscope slides. After
treatment with piranha solution (7:3 mixture of H2SO4 and 34.6%
H2O2) for 30 min to generate a large amount of surface hydroxyl
groups, the slides were reacted with n-alkyl-group-impregnated
vinylsilanes 9h-j in the presence of 3b and 4. In each case, the
glass surface became significantly hydrophobic in comparison with
the surface prior to immobilization of the alkyl group, as estimated
from contact angle measurements (7° for the bare surface and
71-94° for the immobilized ones; Figure 2). Studies showed that
the contact angle increased as the length of the alkyl chain increased.
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(14) When the competitive reaction of 6d with 2b and 9a was carried out in the
presence of the corresponding catalyst mixture at 150 °C for 2 h, only 9d
was obtained, leaving 9a unreacted:
In conclusion, the effort described above has led to the
development of a novel and efficient grafting methodology based
on transition-metal-catalyzed reactions of vinylsilanes that enables
the introduction of functional organic molecules onto silica or glass
surfaces. Dimethyldivinylsilane can be used in this procedure as a
linker to covalently bond organic molecules to hydroxyl surfaces
of solid supports.
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