Sc(OTf)3-mediated silylation of hydroxy functional groups on a solid surface: A catalytic grafting method operating at room temperature

Article abstract of DOI:10.1002/anie.200703112

(Chemical Equation Presented) Surface paradise: Facile catalytic post-grafting of the surface of glass or indium tin oxide (ITO, see picture) with a variety of functional groups has been achieved. Methallylsilanes serve as the grafting reagent and the surfaces show remarkable chemical stability.

Full text of DOI:10.1002/anie.200703112

Angewandte
Chemie
DOI: 10.1002/anie.200703112
Surface Modification
Sc(OTf)3-Mediated Silylation of Hydroxy Functional Groups on a Solid
Surface: A Catalytic Grafting Method Operating at Room
Temperature**
Ye-Rim Yeon, Young Jun Park, Ji-Sung Lee, Jung-Woo Park, Sin-Gun Kang, and Chul-Ho Jun*
In memory of Yoshihiko Ito
À À
The versatility of Si O Si bond-forming reactions has had a
great impact on silica (glass or gel) surface modification,
formation of organosilicon compounds containing organic or
biological functions, and materials science.[1–3] Following the
increased interest in surface technologies, such as dip-pen
lithography,[4] these methods have now been extended to the
preparation of modified surfaces at micrometer to nanometer
scale.[5] Most silica surface-grafting methods are based on the
reaction between the surface and alkoxy-[1,2] or allylsilanes[6]
and are forced by heating to reflux, or very reactive silanes
containing halides, acyloxy, and amino leaving groups are
priate choice not only for their easy preparation and stability,
but also for well-studied O-silylation reactions with alcohols
with a variety of Lewis acid catalysts.[9] Based on this strategy,
we describe herein the first catalytic post-grafting method for
facile chemical modification of the silica surface with a variety
of functional groups using a Sc(OTf)3 catalyst (OTf =
trifluoromethylsulfonate) at room temperature [Eq. (1)].
used.[3] However, there are no Si O Si bond-forming reac-
À À
tions with moisture-stable silylating reagents using a catalyst
in the grafting process onto the silica surface at room
temperature. Such a catalytic grafting method might be
useful because of controllability of the reaction and exper-
imental ease. There is an intriguing analogy between the
grafting procedure and the silylation of alcohols,[7] where the
Silylation of silanols on the amorphous silica gel surface
was carried out with 3-azidopropyldimethallylmethylsilane
(2a, 5 mmol).[10] When the silica gel (1 g) was suspended in
acetonitrile solution in the presence of Sc(OTf)3 (3, 3mol%
based on 2a) at room temperature for one hour, the isobutene
that was liberated during the reaction could be detected
in situ by 1H NMR spectroscopy (see Supporting Information,
Figure S1 ). After the reaction, 3-azidopropylmethylsilyl-
group-grafted silica gel 4a was obtained (1.11 g) and the
amount of 3-azidopropylmethylsilyl moiety loaded on 4a was
À
À
Si OH moiety takes the place of C OH (Scheme 1).
determined by elemental analysis to be 1.34 mmolgÀ1 [11]
.
Scheme 1. Correlation between alcohol silylation and silica surface
grafting.
Characteristic 13C CP-MAS NMR spectra for 4a revealed
that all 13C signals of methallyl group in 2a (Figure 1a) were
now absent but the rest of signals corresponding to the four
carbon atoms of 3-azidopropyl and methyl groups remained
(Figure 1b). This observation shows that the surface formula
proposed for 4a is reasonable.
In our study of RhI-catalyzed O-silylation of alcohols with
vinylsilane,[8] we found that methallylsilanes were an appro-
[*] Y.-R. Yeon, Dr. Y. J. Park, J.-S. Lee, J.-W. Park, S.-G. Kang,
Prof. Dr. C.-H. Jun
Department of Chemistry and Center for Bioactive Molecular Hybrid
(CBMH), Yonsei University, Shin-chon dong, Seodaemun-gu
Seoul 120-749 (Korea)
Fax: (+)82-2-3147-2644
E-mail: junch@yonsei.ac.kr
[**] This work was supported by the CBMH and grant No. R01-2005-000-
10548-0 from the Basic Research Program of the Korea Science &
Engineering Foundation. We thank Dr. J.-S. Bae in KBSI for XPS
measurements. J.-W.P. and S.-G.K. acknowledge the fellowship of
the BK21 program from the Ministry of Education and Human
Resources Development.
Figure 1. 13C NMR spectra of 2a (a) and 13C CP-MAS NMR spectra of
4a (b). The peaks marked with letters m and s correspond to carbon
atoms of the methallyl group and the residual solvent CDCl3, respec-
tively.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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109

Communications
We examined the correlation between immobilization
chemical transformations of the chloro group in 2c were
successfully achieved in good to high yields by various
functional group interconversion reactions and routine
workup processes (Scheme 2). In addition, the products
derived from 2c with a variety of functional groups could be
readily purified by standard silica-gel column chromatogra-
phy.
efficiency and the number of methallyl groups on the silicon
atom using mono- (2b), di- (2c) and trimethallylsilanes (2d)
bearing a 3-chloropropyl group (Table 1) at room temper-
Table 1: Loading efficiency of 4 from the reaction of 1 and silane
derivatives 2 under different reaction conditions.[a]
Entry
n
Sc(OTf)3
[mol%][b]
t [h]
Loading[c]
[mmolgÀ1
]
1
2
3
4
5
2b
2b
2b
2b
2b
0
2
2
3
5
24
1
12
1
0
4b
4b
4b
4b
4b
0.84
0.83
0.70
0.71
1
1
6
7
8
9
2c
2c
2c
2c
2
3
3
5
1
1
12
1
1.02
1.92
1.89
1.93
4c
4c
4c
4c
2
3
Scheme 2. Chemical transformation of the functional groups of 2c.
DIBAH=diisobutylaluminum hydride, Ac=acetyl, Fc=ferrocenyl,
LAH=lithium aluminum hydride.
10
11
2d
2d
3
5
1
1
0.84
1.19
4d[d]
4d[d]
[a] All reactions were performed in 3 mL of MeCN solution and all
surface-modified silica-gel products were purified by Soxhlet extraction
with EtOH for 24 h followed by vacuum drying. R=methallyl. [b] Based
on 2. [c] Determined by carbon elemental analysis. [d] Loading was
calculated based on the assumed structure of n=2 and R2 =methallyl for
4d.
Catalytic grafting reactions of 1 with dimethallylsilanes 2
having various functional groups were performed in the
presence of 3mol% 3 (Table 2). Many different kinds of silica
surface (4) modified by the corresponding functional groups
of 2 could be obtained. The loading efficiencies were between
0.30 and 1.82 mmolgÀ1 depending on the functional groups.
ature. The catalyst 3 appeared to be essential for this
transformation with monoallylsilane 2b, as no reaction
proceeded at all without the catalyst (Table 1, entry 1). The
loading in 4b was 0.84 mmolgÀ1 under 2 mol% 3 and a one-
hour reaction time (Table 1, entry 2). Further increases in the
amount of catalyst (up to 5 mol%) and a prolonged reaction
time (12 h) had no significant effect on the efficiency of
loading (Table 1, entries 3–5). When the grafting reagent was
replaced by dimethallylsilane 2c, loading of the 3-chloropro-
pylsilyl group on silica gel increased by a factor of more than
two, and 3mol% 3 gave the optimal loading in 4c; the loading
efficiency could reach 1.92 mmolgÀ1 (Table 1, entries 6–9).[12]
For trimethallylsilane 2d, as much of methallyl group and its
isomer (internal olefin)[13] were observed at the modified
silica-gel surface in 4d with 13C CP-MAS NMR spectroscopy
(Supporting Information, Figure S2). The exact amount of
methallyl group in 4d could not be determined. Presuming 4d
has one unreacted methallyl group, we estimated the loading
Table 2: Grafting with various functional groups.
Entry
R
4
Loading[a] Entry
[mmolgÀ1
R
4
Loading[a]
[mmolgÀ1
]
]
1
2
3
4
N3 (2a)
4a 1.40
5
6
7
8
OCOCH3 (2h) 4h 1.76
NH2 (2e) 4e 1.37[b]
OH (2i)
4i 1.82
CN (2 f)
CHO (2g) 4g 1.72
4 f 1.43
4-Ph-R’[c] (2j) 4j 0.54
4-Fc-R’[c] (2k) 4k 0.30
[a] Determined by elementary analysis of carbon and nitrogen. [b] Deter-
mined by elementary analysis of nitrogen. [c] R’=1,2,3-triazol-1-yl.
In particular, dimethallylsilane 2m (not shown in Table 2)
bearing a red dye, DABS (4-(dimethylamino)azobenzene-4’-
sulfonyl), was noteworthy because the covalent immobiliza-
tion of 2m on 1 could be inspected with the naked eye. To
discriminate covalently grafted DABS from physisorbed dye,
a control experiment was carried out with 5 and 1 as shown in
Scheme 3. During Soxhlet extraction with methanol for 24 h,
the initial red color of the silica gel 4m did not change
significantly, implying that the DABS dye moiety was
covalently bonded to the silica gel surface. In contrast, the
efficiency to be 0.84 mmolgÀ1 with 3mol%
3 and
1.19 mmolgÀ1 with 5 mol% 3 (Table 1, entries 10 and 11).
Other silica materials were also found to be suitable. The
loading rate (mmolgÀ1) was 3.84 for SBA-15, 1.54 for MCF-
5F, and 1.15 for ultrapure spherical silica balls, where 2c was
used as a grafting reagent.
The dimethallylsilane moiety was chosen as a model
system to immobilize a variety of organic functional groups
on 1 and its chemical compatibility was investigated. Facile
110
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30 minutes to activate surface hydroxy groups. When the
microscope slide glass was reacted with octadecyldimethal-
lylmethylsilane (2n) in the presence of 3, the glass surface of
4n became significantly hydrophobic relative to the surface
before the reaction (estimated from the measurement of
contact angle of each surface, 358 and 968; Figure 2a).
In the case of ITO glass, the resulting ferrocene-function-
alized glass of 4o (Figure 2b) was investigated as an electrode
by cyclic voltammetry (CV) in phosphate-buffered aqueous
solution (pH 7.0). The CV revealed that the ferrocenyl group
on the ITO surface is electrochemically active and shows
reversible Fc0/+ redox behavior, as expected for ferrocene
oxidation (Figure 2c). Interestingly, the loading amount of
the ferrocenyl group on ITO glass could be controlled by the
used amounts of the catalyst 3. For example, when the mole
percent of 3 relative to 2o was varied from 2 to 10, the loading
of 4o increased from 0.15 ” 10À7 to 0.28 ” 10À7 mmol per 1 cm2
of ITO glass.[14]
Scheme 3. Immobilization of 1 with a DABS group using DABS-
dimethallylsilane 2m.
redness of another silica gel derived from 1 and 5 completely
disappeared.
The grafting method could be applied to physically or
chemically different surfaces, such as microscope slides and
indium tin oxide (ITO) glass, which had both been soaked in
piranha solution (7:3mixture of H 2SO4 and 34.6% H2O2) for
In summary, we have developed a new catalytic grafting
system composed of Sc(OTf)3 and methallylsilane derivatives
for the rapid surface modification of the hydroxy-terminated
solids at room temperature.
In addition, we believe that
this catalytic grafting proce-
dure has great potential in
the development of special
materials based on SiO2 or
ITO surfaces. Such efforts
are underway in our labora-
tory.
Received: July 12, 2007
Revised: September 18, 2007
Published online: November 8,
2007
Keywords: glasses ·
.
methallylsilane · scandium ·
silicon · surface chemistry
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Angew. Chem. Int. Ed. 2008, 47, 109 –112
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111

Communications
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