Styrylsilane coupling reagents for immobilization of organic functional groups on silica and glass surfaces

Article abstract of DOI:10.1039/c8cc04863k

Styrylsilanes serve as new coupling reagents for introducing organic functional groups on silica and glass surfaces. Functionalized styrylsilanes, which are readily prepared via catalytic hydrosilylation of the corresponding phenylacetylenes with silanes, are immobilized on silica through acid catalyzed processes under mild conditions.

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
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Styrylsilane Coupling Reagents for Immobilization of Organic
Functional Groups on Silica and Glass Surface
Soo-Bin Kim,§ Chang-Hee Lee§ and Chul-Ho Jun*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
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Styrylsilanes serve as new coupling reagents for introducing between the silyl groups on the polymer and hydroxy groups
organic functional groups on silica and glass surfaces. on the silica or glass surfaces (Fig. 1b).10b Finally, the new
Functionalized styrylsilanes, which are readily prepared via protocol was utilized to fabricate a recyclable sensor for
catalytic hydrosilylation of the corresponding phenylacetylenes aromatic nitro compound.
with silanes, are immobilized on silica through acid catalyzed
processes under mild conditions.
Surface modifications of inorganic solids, such as silica or glass,
utilizing covalent bond-forming reactions have received
considerable attention because of their widespread
applications in the fabrication of chemical and bio-sensor1,
drug delivery2 and enzyme immobilization3 systems, DNA
probe films4, and polymer brushes5. Among the various
substances used in silica surface-grafting methods6,
alkoxysilanes7 are the most common. Unfortunately,
alkoxysilanes are unstable, moisture sensitive substances that
are difficult to purify by employing chromatographic methods.
Alkenylsilane such as allyl-8, methallyl-9 and vinyl-silanes10 have
been developed to overcome this problem. However,
stoichiometric reactions of organometallic compounds such as
Grignard reagents with chlorosilanes are required to prepare
these silanes8-10, and this limits their large-scale production
(Fig. 1a).
Below, we describe the preparation and applications of
novel styrylsilane coupling reagents for introducing organic
functional groups on silica and glass surfaces. These
Fig. 1 (a) Previous approach using methallylsilane derivatives. (b) Conceptual basis of
substances are readily prepared in large quantities via catalytic
the new styrylsilane coupling reagent.
hydrosilylation reactions of phenylacetylene with silanes.
Using this protocol, various styrylsilanes were prepared and
Initial exploratory studies focussed on immobilization
employed to generate functionalized silica and glass surfaces.
reactions of benzyldimethyl(styryl)silane
promoted by various acid catalysts (Table 1). The results
show that reaction of 1a (0.08 mmol) with silica (20 mg) in
(
1a
)
on silica
In addition, styrylsilane moieties were linked the backbone of
polybutadiene as part of a sequence to produce a unique
organic-inorganic hybrid materials, which is stable toward
hydrolysis because it possess robust multiple covalent bonds
3
2
the presence of a catalytic amount of TfOH (3a, 5 mol%) at
room temperature for 2 h, followed by washing with CH2Cl2,
produces surface-modified silica 4a (entry 1).11 The amount of
immobilized benzyl(dimethyl)silane moieties loaded on 4a was
determined by using elemental analysis to be 1.02 mmol/g,
corresponding to 1.98 molecules/nm2 of the surface density of
the benzyl(dimethyl)silane moieties on silica 4a 12
Reactions
.
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employing other acid catalysts generate modified silica with substituted (MeO) styrylsilane 1f (entries 4 and 5). Compared
loading amounts that are lower than that of 4a (entries 2-6). to those of -substituted vinylsilanes (entries 1 and 4-5),
The reaction conducted in the absence of an acid catalyst leads reactions of the -phenyl-substituted vinylsilane 1g and
β
α
to silica 4g with a loading extent of only 0.17 mmol/g (entry 7). vinylsilane 1h generate modified silica with very low loading
When the reaction time is increased from 2 h to 6 h under amounts (0.42 and 0.44 mmol/g) (entries 6 and 7). The low
TfOH catalyst, the loading extent of immobilized silane moiety reactivities of 1g and 1h might be caused by the propensity of
is increased to 1.11 mmol/g and a longer reaction times (eg., reactions of 1g and 1h with acids to form α-silyl carbocations
12 h) do not cause a significant enhancement of the loading intermediates which would resist substitution reaction at Si by
efficiency (entries 8 and 9).
a silanol silophile (see below).
Table 1 Immobilization reactions with styrylsilane 1a on silica 2 in the presence of the
various acid catalysts 3a
Scheme 1 (a) Proposed mechanism of immobilization with 1a. (b) Proposed mechanism
of immobilization with α-phenyl-substituted vinylsilane 1g and vinylsilane 1h.
A plausible mechanism for the immobilization reaction of
styrylsilane 1a
protonation of the styryl group to form a
Regioselective formation of this intermediate occurs because
of the higher stabilities of benzyl13 and -silyl cations.14 The
β-
,
displayed in Scheme 1a, begins with
aAll reactions were carried out with 1a (0.08 mmol), 2 (20 mg) and 3 (5 mol%) in
0.1 mL of CH2Cl2 at room temperature. bDetermined by using the C value derived
from elemental analysis.
β
-silyl cation.
β
silyl carbocation then undergoes substitution at silicon with
silanol groups on silica or glass serving as the nucleophile.8b
This step generates surface modified silica 4a and styrene. In
Table 2 Immobilization reactions with various alkenylsilane 1 on silica 2 in the presence
of 3aa
contrast, protonation of
and vinylsilane 1h should preferentially occur to generate
respective tert- and sec-carbocations, both of which are -silyl
α-phenyl-substituted vinylsilane 1g
α
cations that should not readily undergo substitution at their Si
centers (Scheme 1b).
Scheme 2 Large-scale production of (11-chloroundecyl)dimethyl(styryl)silane (1i).
aAll reactions were carried out with 1 (0.08 mmol), 2 (20 mg) and 3a (5 mol%) in
0.1 mL of CH2Cl2 at room temperature. bDetermined by using the C value derived
from elemental analysis.
The efficiencies of loading other alkenylsilane moieties on
silica were also determined (Table 2, entries 1-7).
Immobilization reaction of 1-pentenylsilane 1b, using TfOH
(
3a, 5 mol%) at room temperature for 2 h, takes place to form
benzyl(dimethyl)silane modified silica 4j with a loading extent
of 0.83 mmol/g (entry 1). Reactions of -disubstituted
α,β
alkenylsilanes 1c and 1d lead to lower loading amounts (0.73
and 0.44 mmol/g) (entries 2 and 3). Reaction of the p-electron
withdrawing group substituted (F) styrylsilane 1e leads to
higher loading efficiency than that of the p-electron donating
Scheme 3 Chemical transformation of the functional groups of 1i.
Functionalized styrylsilane coupling reagents can be readily
prepared in large-scale by sequences involving catalytic
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hydrosilylation of phenylacetylene with the corresponding (1.06 and 0.77 mmol/g, respectively) (entries 2 and 3). Use of
silanes, which are generated by reactions of the distyrylsilane coupling reagent 1q in this process leads to
chlorodimethylsilanes with alkenyl chlorides. This approach is formation of functionalized silica 4t with the higher loading
exemplified by preparation of the (11-chloroundecyl) extent of 0.84 mmol/g (entry 4).
substituted styrylsilane 1i (Scheme 2) and its derivatives
(Scheme 3). The routes begin with reaction of generated using this protocol (entries 5-10). Reactions of the
chlorodimethylsilane ( , 15.0 g) with 11-chloroundec-1-ene ( nitrile, aldehyde, acetate ester and azide containing
15.0 g) in the presence of the Pt catalyst (0.3 mol%) to styrylsilanes 1j 1l and 1n were found to produce the respective
produce chloro(11-chloroundecyl)dimethylsilane, which is immobilized products 4u 4w and 4y with moderate loading
reduced using LiAlH4 (4.5 g) to form (11- extents (0.61, 0.98, 0.68, and 0.70 mmol/g, respectively)
chloroundecyl)dimethylsilane in 86% isolated yield. Silane 1i (entries 5-7 and 9). Interestingly, reaction of the hydroxyl
is then generated in 75% overall yield (20.8 g) by group-functionalized styrylsilane 1m with silica forms the
Many other types of functionalized silica surfaces can be
5
6,
7
-
-
9
hydrosilylation reaction of
9
(17.0 g) and phenylacetylene corresponding functionalized silica with a low loading (0.49
mmol/g) (entry 8). Finally, the NHS-ester group-functionalized
using Rh catalyst 11 (0.6 mol%).
Chloride 1i can be transformed into various functionallized silica 4z for enzyme immobilization is produced with a 0.56
silanes in good yields (Scheme 3). This is demonstrated by mmol/g loading extent (entry 10).9b,9d
reactions of 1i with sodium cyanide, sodium acetate and
sodium azide, which produce the respective silanes 1j, 1l and
1n in 91%, 80%, and 92% yields. Moreover, reduction reactions
of 1j and 1l using DIBAL-H and LiAlH4 produce the aldehyde
and alcohol functionalized styrylsilanes 1k and 1m in 59% and
88% respective yields. Finally, Cu(I)-promoted [3+2]-
cycloaddition reaction of azide 1n with 2,5-dioxopyrrolidin-1-yl
propiolate forms the triazole linked, NHS-ester (N-
hydroxysuccinimide ester) functionalized styrylsilane 1o in 63%
yield. Notably, the styrylsilyl group is preserved in these
processes and the resulting styrylsilanes can be readily purified
by using column chromatography.
Table 3 Immobilization reactions of various functional group containing styrylsilanesa
Scheme 4 Preparation of pyrene group containing polymer 17 and immobilization
reaction of 17 on glass surface.
A new strategy for introducing organic functional groups on
solid
surfaces,
which
uses
styrylsilyl-impregnated
polybutadiene was explored (Scheme 4). For this purpose, the
dimethylsilyl group-imbedded polymer 13 was first generated
in 76% overall yield by Pt-catalyzed hydrosilylation reaction of
polybutadiene 12 with chlorodimethylsilane (5) followed by
reduction with LiAlH4. Hydrosilylation of 13 with a mixture of
phenylacetylene (10, 0.4 equiv. based on the number of silane
groups in 13) and 11-chloroundec-1-ene (6, 2 equiv.) using Rh
catalyst 11 produces polymer 14 (92%), which contains both
appended styrylsilane and chloroalkyl groups. 1H NMR analysis
shows that this dual-functionalized polymer contains a 2:3
ratio of styrylsilyl and chloroalkyl groups. Reaction of 14 with
aAll reactions were carried out using 1 (0.08 mmol), 2 (20 mg), 3a (5 mol%) in 0.1
mL of CH2Cl2 at room temperature. bDetermined by using derived from C value
from elemental analysis. C1q contains 15%
α-phenyl-substituted vinylsilane
isomer.
sodium azide forms the azide functionalized polybutadiene 15
,
which undergoes Cu(I)-promoted [3+2]-cycloaddition
Immobilization reactions of styrylsilanes were performed to
incorporate various functional groups on silica (Table 3).
reaction15 with 1-(propargyloxymethyl)pyrene (16) to form the
dual-functionalized polybutadiene 17 (76%) containing both
pyrenyl and styrylsilyl groups. Finally, reaction of piranha
solution-treated glass with 17 in the presence of TfOH
2
Reactions of styrylsilanes 1p and 1i produce the respective
immobilized silica 4r and 4s with moderate loading extents
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produces glass 18, which has a surface that is significantly
more hydrophobic than that of untreated glass (contact angles
of surface before and after reaction are 7o and 112o,
respectively).
Conflicts of interest
There are no conflicts to declare.
Notes and references
1
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Fig. 2 (a) Fluorescence spectra of solutions of 18 in CH2Cl2 with different concentrations
of nitrobenzene (NB) (excitation at 330 nm). (b) Fluorescence intensities of solutions of
18 in CH2Cl2 at 482 nm following addition of nitrobenzene (14.4 mM) and washing with
CH2Cl2
It is known that aromatic nitro compounds are efficient
electron transfer quenchers of fluorescent aromatic
compounds.16 To demonstrate potential applications of the
immobilization strategy described above, pyrenyl group-
immobilized glass 18 was utilized as a recyclable fluorescence
sensor of the potential explosive simulant, nitrobenzene (NB).
Inspection of the fluorescence spectra displayed in Fig. 2a
shows that 18 has strong fluorescence with a maximum at 482
nm. Upon addition of NB to a solution of 18 in CH2Cl2, the
intensity of fluorescence at 482 nm decreases, leading to
complete quenching when the concentration of NB reaches
14.4 mM. Pyrenyl group-immobilized glass 18 can be
separated, washed and used for NB detection multiple times
(Fig. 2b).
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6
11 The surface structure of modified silica and its purity were
also examined with 13C CP-MAS NMR and elemental analysis
with washing experiment (see ESI†).
12 The surface density (molecules/nm2) of benzyl(dimethyl)-
silane moiety loaded on 4a can be estimated by using
[loading extent (mmol/g)
is Avogadro’s number and SA is the surface area (310 m2/g)
of the silica
x
NA x 10-21 / SA (m2/g)], where NA
2
.
13 J. McMurry, Organic Chemistry, Brooks/Cole, Pacific Grove,
CA, 5th edn, 2000. pp. 407.
In the investigation described above, we demonstrated that
styrylsilanes serve as a new family of coupling reagents for
immobilization of organic functional groups on solid surfaces.
Styrylsilanes with high purities can be produced on large scales
utilizing catalytic hydrosilylation reactions. This new protocol
enables fabrication of unique functionalized materials, as
exemplified by the preparation of a polymer-based organic-
inorganic hybrid material that serves as a recyclable sensor.
This work was supported by a grant from the National
Research Foundation of Korea (NRF) (Grant 2016-R-1-
A2b4009460).
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