Rapid assembly of explicit, functional silicones

Article abstract of DOI:10.1039/c0dt00400f

The impressive surface activity of silicones can be enhanced by the incorporation of hydrophilic organic functional groups and polymers. Traditional routes to such compounds, which typically involve platinum-catalyzed hydrosilylation, suffer from incompatibility with certain functional groups. B(C₆F₅)₃-catalyzed condensation of hydrosilanes with alkoxysilanes offers new opportunities to prepare explicit silicone structures. We demonstrate here that conversion of alcohols to silyl ethers competes unproductively with alkoxysilane conversion to disiloxanes. By contrast, a wide range of structurally complex alkyl halide and oligovinyl compounds can be readily made in high yield. Thermal 3+2-cycloadditions and thiol-ene click reactions are used to convert these compounds into surface active materials. The Royal Society of Chemistry.

Full text of DOI:10.1039/c0dt00400f

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New Horizon of Organosilicon Chemistry
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Published in issue 39, 2010 of Dalton Transactions
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PERSPECTIVES:
Silylium ions in catalysis
Hendrik F. T. Klare and Martin Oestreich
Dalton Trans., 2010, DOI: 10.1039/C003097J
Kinetic studies of reactions of organosilylenes: what have they taught us?
Rosa Becerra and Robin Walsh
Dalton Trans., 2010, DOI: 10.1039/C0DT00198H
π-Conjugated disilenes stabilized by fused-ring bulky “Rind” groups
Tsukasa Matsuo, Megumi Kobayashi and Kohei Tamao
Dalton Trans., 2010, DOI: 10.1039/C0DT00287A
Organosilicon compounds meet subatomic physics: Muon spin resonance
Robert West and Paul W. Percival
Dalton Trans., 2010, DOI: 10.1039/C0DT00188K
HOT ARTICLE:
Novel neutral hexacoordinate silicon(IV) complexes with two bidentate monoanionic benzamidinato
ligands
Konstantin Junold, Christian Burschka, Rüdiger Bertermann and Reinhold Tacke
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PAPER
www.rsc.org/dalton | Dalton Transactions
Rapid assembly of explicit, functional silicones†
John B. Grande, Ferdinand Gonzaga and Michael A. Brook*
Received 30th April 2010, Accepted 17th June 2010
DOI: 10.1039/c0dt00400f
The impressive surface activity of silicones can be enhanced by the incorporation of hydrophilic organic
functional groups and polymers. Traditional routes to such compounds, which typically involve
platinum-catalyzed hydrosilylation, suffer from incompatibility with certain functional groups.
B(C6F5)3-catalyzed condensation of hydrosilanes with alkoxysilanes offers new opportunities to prepare
explicit silicone structures. We demonstrate here that conversion of alcohols to silyl ethers competes
unproductively with alkoxysilane conversion to disiloxanes. By contrast, a wide range of structurally
complex alkyl halide and oligovinyl compounds can be readily made in high yield. Thermal
3+2-cycloadditions and thiol-ene click reactions are used to convert these compounds into surface
active materials.
functional, explicit silicones that may be linked to hydrophilic
Introduction
organic moieties.24 Such compounds could be the basis of a new
class of well-defined silicone surfactants. An examination of the
tolerance of B(C6F5)3 to a variety of organic functional groups, and
the conversion of functional silicones into surface active materials
is described below.
Silicone polymers are widely used in commerce. While they have
many attributes that are unmatched by organic polymers, it is
perhaps their surface activity in particular that has been the basis
of most applications. With the exception of some surfactants
that are based on small siloxanes, such as the superwetters1,2
–
commonly used to disperse agricultural chemicals on leaves –
most silicone surfactants involve ill-defined mixtures of silicones
polymers modified by oligo- or poly(ethylene glycol)(PEG) or
propylene glycol (PPG) chains (Fig. 1).3
Experimental
Materials
Allyltriethoxysilane, allyltrimethoxysilane, vinyltriethoxysilane,
methylphenylvinylsilane,
dimethylphenylsilane,
diphenyl-
methylsilane, pentamethyldisiloxane, 1,1,1,3,5,5,5-heptamethyl-
trisiloxane, (3-glycidoxy)propyltrimethoxysilane (3-iodopropyl)-
trimethoxysilane,
1,3-dimethyltetramethoxy-disiloxane,
(3-
chloropropyl)methyldimethoxysilane, and vinyltetramethyl-
disiloxane were purchased from Gelest and used as received.
Vinyltrimethoxysilane (98%), tetramethyl orthosilicate (98%),
tetraethyl orthosilicate (98%), (3-chloropropyl)trimethoxysilane
(97%), (3-chloropropyl)-triethoxysilane (95%), 1-thioglycerol
(98%), (3-aminopropyl)trimethoxysilane (97%), propiolic acid
(95%), poly(ethyleneglycol) methyl ether av. mol. wt: 350, p-
toluenesulfonic acid (98%), 2,2-dimethoxy-2-phenylacetophenone
(99%), allyloxytrimethylsilane (98%), allyl benzyl ether (99%),
chromium(III) acetylacetonate, anhydrous dimethylformamide,
sodium azide and tris(pentafluorophenyl)borane (95%) were
purchased from Aldrich and used as received. Commercial
solvents: hexane, dichloromethane and toluene were dried over
activated alumina prior to use.
Fig. 1 Typical silicone surfactants; A: superwetters; B: rake copolymers.
A much wider range of surface activities is available when
alternate hydrophiles to glycol oligomers are used, including such
functional groups as amines, phosphates, sulfonates, carboxylic
acids,4,5 or clusters of acids6,7 amino acids8,9 and saccharides.10–14
However, in these cases too, a clear correlation between structure
and surface activity is generally not possible because the silicone
constituents are typically poorly defined mixtures.
Recently, we15 and others16–19 have described the preparation
of silicone polymers using the Piers–Rubinsztajn reaction20–23 in
which an alkoxysilane is combined with a hydrosilane in the
presence of B(C6F5)3. Explicit silicone structures can be the
result of such reactions.15 We were interested in establishing
if the Piers–Rubinsztajn process could also be used to create
Instrumentation
1H NMR, 13C NMR and 29Si NMR experiments were recorded at
room temperature and performed on Bruker Avance 200, 500 and
600 MHz nuclear magnetic resonance spectrometers, respectively.
High-resolution MS Spectrometry was performed with a Hi-
Res Waters/Micromass Quattro Global Ultima (Q-TOF mass
spectrometer).
Department of Chemistry and Chemical Biology, McMaster University,
1280 Main St. W., Hamilton ON, L8S 4M1, Canada. E-mail: mabrook@
mcmaster.ca; Fax: +1 905 522 2509; Tel: +1 905 525 9140 ext 23483
† Electronic supplementary information (ESI) available: NMR spectra of
the compounds presented in Fig. 2 and 3. See DOI: 10.1039/c0dt00400f
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 9369–9378 | 9369
©

General synthesis of functional silicones
reaction mixture was filtered over Celite, and residual solvent
and excess starting material was removed in vacuo, affording
pure trimethylsilyl-protected (3-hydroxypropyl)trimethoxy-silane
(4.51 g, 88% yield).
Reaction of pentamethyldisiloxane with (3-glycidoxypropyl)-
trimethoxysilane. To
a solution of (3-glycidoxypropyl)tri-
methoxysilane (0.150 g, 0.63 mmol) in dry hexane (5 ml), was
added pentamethyldisiloxane (0.376 g, 2.5 mmol). The mixture
was stirred at room temperature for 5 min before the addition
of B(C6F5)3 (10 ml of a solution containing 40 mg dissolved in
1 ml of toluene, 7.8 ¥ 10-4 mmol). After a 30 s induction time, the
reaction flask became warm, signifying the onset of an exothermic
reaction, lasting roughly 10 min. The mixture was allowed to
cool to room temperature before the addition of neutral alumina
(1.5 g). The resulting solution was filtered and examined via
NMR. The obtained spectrum showed evidence of complete loss
of epoxide, with no evidence of reaction at the methoxy moieties.
The major and minor products deduced by proton and carbon
NMR experiments are shown below.
1H NMR (CDCl3, 500 MHz): d 3.55 (s, 9 H, (H3CO)3Si), 3.52
(t, 2 H, O3SiCH2CH2CH2O–, J = 6.8 Hz), 1.60–1.63 (m, 2 H,
O3SiCH2CH2CH2O–), 0.61–0.64 (m, 2 H, O3SiCH2CH2CH2O),
0.09 ppm (s, 9 H, OSi(CH3)3). 13C NMR (CDCl3 125 MHz): d
64.73, 50.45, 25.72, 4.94, -0.52 ppm. 29Si NMR (CDCl3, 99 MHz,
1% w/v Cr(acac)3): d 16.69 (M), -41.95 ppm (T).
Reduction of trimethylsilyl-protected (3-hydroxypropyl)-
trimethoxysilane with pentamethyldisiloxane. To
a solution
of trimethylsilyl-protected (3-hydroxypropyl)-trimethoxysilane
6 (0.250 g, 0.99 mmol) in dry hexane (5 ml) was added
pentamethyldisiloxane (0.661 g, 4.45 mmol). The mixture was
stirred at room temperature for 5 min before the addition of
B(C6F5)3 (20 ml of a prepared solution containing 40 mg dissolved
in 1 ml of toluene, 1.6 ¥ 10-3 mmol). After a 60 s induction time,
moderate evolution of gas and heat from the solution occurred,
lasting roughly 5 min. The mixture was allowed to cool to
room temperature before the addition of neutral alumina (1 g).
The resulting solution was filtered, concentrated under reduced
pressure and examined via 1H NMR. The obtained spectrum
showed complete cleavage of both methoxy and trimethylsiloxy
groups (see ESI†) consistent with a propyl-modified silicone. As
this product was not of synthetic interest, it was not isolated.
When fewer equivalents of hydrosilane were employed, little to
no control was observed, giving mixtures of products 8, 9 and
related silicones with various degrees of reaction at both methoxy
and trimethylsiloxy moieties.
Major Product: (~ 88% by NMR). 1H NMR (CDCl3,
500 MHz): d 4.01–4.05 (m, 1 H, g, J = 6.0 Hz) 3.56 (s, 9 H’s, a),
3.39–3.39–3.43 (m, 2 H’s, d), 3.37 (dd, 1 H, e, J = 6.0, 9.6 Hz), 3.24
(dd, 1 H, e¢, J = 6.0, 9.6 Hz), 1.66 (m, 2 H’s, c), 1.17 (d, 3 H’s, f, J =
6 Hz), 0.65–0.68 (m, 2 H’s, b), 0.09 (s, 9 H’s, i), 0.07 ppm (s, 6 H’s, h).
13C NMR (CDCl3 125 MHz): d 76.53, 73.56, 67.23, 50.64, 22.93,
20.94, 5.39, 1.92, -0.15 ppm. 29Si NMR (CDCl3, 99 MHz, 1%
w/v Cr(acac)3): d 7.07 (M), -13.56 (D), -42.01 ppm (T). HRMS
(ES Positive mode): m/z [M + H+] calculated = 385.1898, found =
385.1878.
Minor Product: (~ 12% by NMR). 1H NMR (CDCl3,
500 MHz): d 3.72 (t, 2 H, g, J = 6.4 Hz), 3.48 (t, 2 H, e, J =
6.4 Hz), 3.56 (s, 9 H’s, a), 3.39–3.43 (m, 2 H’s, d), 1.79 (m, 2 H, f)
1.66 (m, 2 H’s, c), 0.67 (m, 2 H’s, b), 0.09 (s, 9 H’s, i), 0.06 ppm (s,
6 H’s, h). 13C NMR (CDCl3 125 MHz): d 73.12, 67.56, 59.33, 50.64,
33.61, 22.93, 5.39, 1.92, -0.17 ppm. 29Si NMR (CDCl3, 99 MHz,
1% w/v Cr(acac)3): d 7.31 (M), -13.02 (D), -42.01 ppm (T).
When the same reaction was run in dichloromethane (otherwise
identical protocol), the outcome was somewhat different. Rather
than the explicit reaction observed in hexane at the epoxide, the
methoxy groups also started to react. The reaction was, therefore,
not particularly useful in a synthetic sense. In either solvent, even
with excess hydrosilane it was not possible to convert all OMe
groups into disiloxanes (see ESI†).
Synthesis of benzyl protected (3-hydroxypropyl)trimethoxysilane
7. In a round bottom flask equipped with a stir bar and
water jacket condenser under a nitrogen atmosphere was added
trimethoxysilane (9.07 g, 74.25 mmol) in dry toluene (15 ml).
Allyl benzyl ether (10 g, 67.5 mmol) was then added, followed
by Karstedt’s platinum complex (5 ml, 2% solution in xylenes,
1.0 mmol of Pt). The reaction was then monitored via NMR to
ensure full conversion (16 h). Once complete, solvent was removed
under reduced pressure. The resulting mixture was then subjected
◦
to distillation (150 C, 2 mmHg) yielding pure benzyl-protected
(3-hydroxypropyl)trimethoxysilane. (11.46 g, 88% yield).
1H NMR (CDCl3, 500 MHz): d 7.27–7.29 (m, 4 H, phenyl),
7.23–7.26 (m, 1 H, phenyl), 4.45 (s, 2 H, OCH2(C6H5)), 3.51 (s,
9 H, (H3CO)3Si), 3.40 (t, 2 H, O3SiCH2CH2CH2O–, J = 6.5 Hz),
1.66–1.70 (m, 2 H, O3SiCH2CH2CH2O), 0.63–0.66 ppm (m, 2 H,
O3SiCH2CH2CH2O). 13C NMR (CDCl3 125 MHz): d 138.64,
128.30, 127.57, 127.43, 72.73, 72.43, 50.49, 22.84, 5.34 ppm. 29Si
NMR (CDCl3, 99 MHz, 1% w/v Cr(acac)3): d -42.11 ppm (T).
Protected alcohols
Synthesis of trimethylsilyl protected (3-hydroxypropyl)-
trimethoxysilane 6. In a round bottom flask equipped with a stir
bar and water jacket condenser under a nitrogen atmosphere was
added trimethoxysilane (2.5 g, 20.5 mmol) in dry toluene (8 ml).
Allyloxytrimethylsilane (3.19 g, 24.5 mmol) was then added,
followed by Karstedt’s platinum complex (5 ml, 2% solution
in xylenes, 1.0 mmol of Pt). The reaction was then monitored
via NMR to ensure full conversion (3 h). Once complete,
activated charcoal was added (~ 0.25 g) to the solution. The
resulting mixture was stirred for an additional 2 h. The crude
Reaction of benzyl-protected (3-hydroxypropyl)trimethoxysilane
with pentamethyldisiloxane. To a solution of benzyl-protected
(3-hydroxypropyl)trimethoxysilane (0.100 g, 0.37 mmol) in dry
hexane (5 ml), was added pentamethyldisiloxane 1 (0.33 g,
2.2 mmol). The mixture was stirred at room temperature for 5 min
before the addition of B(C6F5)3 (10 ml of a solution containing
40 mg dissolved in 1 ml of toluene, 7.8 ¥ 10-4 mmol). After
a 30 s induction time, the reaction flask became warm, and
evolution of gas occurred, signifying the onset of an exothermic
9370 | Dalton Trans., 2010, 39, 9369–9378
This journal is
The Royal Society of Chemistry 2010
©

reaction, lasting roughly 5 min. The mixture was allowed to cool
to room temperature before the addition of neutral alumina (1 g).
The resulting solution was filtered, concentrated under reduced
1H NMR (CDCl3, 200 MHz): d 3.51 (t, 2 H, O3SiCH2CH2-
CH2Cl, J = 7.1 Hz), 1.83–1.98 (m, 2 H, O3SiCH2CH2CH2Cl), 0.63–
0.72 (m, 2 H, O3SiCH2CH2CH2Cl), 0.12 (s, 54 H, OSi(CH3)3),
0.07 (s, 9 H, (CH3)SiO3). 13C NMR (CDCl3 50 MHz): d
47.65 (O3SiCH2CH2CH2Cl), 26.97 (O3SiCH2CH2CH2Cl), 12.14
(O3SiCH2CH2CH2Cl), 1.91 ((CH3)3SiO), -1.89 ppm ((CH3)SiO3).
29Si NMR (CDCl3, 99 MHz, 1% w/v Cr(acac)3): d 7.71 (M), -66.09
1
pressure and examined via H NMR. When only one equivalent
of hydrosilane was added, both OMe and O-benzyl groups
simultaneously reacted (see ESI†). Even with excess hydrosilane
it was not possible to convert all OMe groups into disiloxanes.
Peaks associated with compound 10 (although compound 10
+
(D), -71.29 ppm (T). HRMS (ES Positive mode): m/z [M + NH4 ]
calculated = 834.2643, found = 834.2667.
1
was not isolated, characteristic peaks were observed via H and
COSY experiments. See ESI for COSY data†) 1H NMR (CDCl3,
500 MHz): d 1.31–1.36 (m, 2 H, (MeO)3SiCH2CH2CH3), 0.853–
0.882 (t, 2 H, (MeO)3SiCH2CH2CH3, J = 7.5 Hz), 0.486–0.519 ppm
(m, 2 H, (MeO)3SiCH2CH2CH3).
Synthesis of (3-chloropropyl)bis(1,1,1,3,5,5,5-heptamethyl-
trisiloxy)methylsilane 14. To a solution of chloropropyldi-
methoxymethylsilane (0.500 g, 2.73 mmol) in dry dichloromethane
(10 ml), was added 1,1,1,3,5,5,5-heptamethyltrisiloxane (1.52 g,
6.84 mmol, 2.5 fold excess). The mixture was stirred at room
temperature for 5 min before the addition of tris(pentafluoro-
phenyl)borane (20 ml of a prepared solution containing 40 mg
dissolved in 1 ml of toluene, 1.6 ¥ 10-3 mmol). After a 5 min
induction time, rapid evolution of gas and heat from the solution
occurred. The mixture was allowed to cool to room temperature
before the addition of neutral alumina (1 g). The resulting
solution was filtered and concentrated under reduced pressure.
The remaining solvent and excess reagents were removed in
vacuo, affording pure (3-choloropropyl)di(1,1,1,3,5,5,5-hepta-
methyltrisiloxy)methylsilane (1.48 g, 91% yield).
Peaks associated with compound 11 (although compound 11
was not isolated, characteristic peaks were observed via 1H
and COSY experiments. See ESI for COSY data†) 1H NMR
(CDCl3, 500 MHz): d 3.51–3.49 (t, 2 H, (MeO)3SiCH2CH2CH2O–,
J = 7 Hz), 1.58–1.53 (m, 2 H, (MeO)3SiCH2CH2CH2O–), 0.57–
0.54 ppm (m, 2 H, (MeO)3SiCH2CH2CH2O–).
Haloalkanes
Synthesis of (3-chloropropyl)tris(pentamethyldisiloxy)silane 12.
To a solution of chloropropyltriethoxysilane (0.500 g, 2.07 mmol)
in dry hexane (10 ml), was added pentamethyldisiloxane (1.38 g,
9.34 mmol). The mixture was stirred at room temperature for 5 min
before the addition of B(C6F5)3 (25 ml of a solution containing
40 mg dissolved in 1 ml of toluene, 1.95 ¥ 10-3 mmol). After a 2 min
induction time, rapid evolution of gas and heat from the solution
occurred. The mixture was allowed to cool to room temperature
before the addition of neutral alumina (1.5 g). The resulting solu-
tion was filtered and concentrated on a rotary evaporator. The re-
maining solvent and excess reagents were removed in vacuo, afford-
ing pure (3-chloropropyl)tris(pentamethyldisiloxy)silane (1.08 g,
1.81 mmol, 87.8% yield).
1H NMR (CDCl3, 500 MHz): d 3.50 (t, 2 H, O2SiCH2CH2-
CH2Cl, J = 7.0 Hz), 1.82–1.88 (m, 2 H, O2SiCH2CH2CH2Cl), 0.62–
0.66 (m, 2 H, O2SiCH2CH2CH2Cl), 0.102 (s, 36 H, OSi(CH3)3),
0.09 (s, 3 H, O2Si(CH3)(CH2CH2CH2Cl)), 0.03 (s, 6 H, (CH3)SiO3).
13C NMR (CDCl3 125 MHz): d 47.81 (O2SiCH2CH2CH2Cl),
26.80 (O2SiCH2CH2CH2Cl), 15.13 (O2SiCH2CH2CH2Cl), 1.81
(CH3)3SiO), -0.58 O2Si(CH3)(CH2CH2CH2Cl)), -2.0 ppm
(CH3SiO3). 29Si NMR (CDCl3, 99 MHz, 1% w/v Cr(acac)3): d
7.29 (M), -24.49 (D), -65.82 ppm (T). HRMS (ES Positive mode):
+
m/z [M + NH4 ] calculated = 595.1837, found = 595.1819.
1H NMR (CDCl3, 500 MHz): d 3.51 (t, 2 H, O2SiCH2CH2-
CH2Cl, J = 7.0 Hz), 1.83–1.86 (m, 2 H, O2SiCH2CH2CH2Cl), 0.62–
0.65 (m, 2 H, O2SiCH2CH2CH2Cl), 0.09 (s, 27 H, OSi(CH3)3),
0.07 (s, 18 H, (CH3)2SiO2). 13C NMR (CDCl3 125 MHz): d
47.70 (O2SiCH2CH2CH2Cl), 27.09 (O2SiCH2CH2CH2Cl), 12.10
(O2SiCH2CH2CH2Cl), 1.95 ((CH3)3SiO), 1.26 ppm ((CH3)2SiO2).
29Si NMR (CDCl3, 99 MHz, 1% w/v Cr(acac)3): d 7.05 (M), -21.90
Synthesis of (3-chloropropyl)tris(dimethylphenylsilyloxy)silane
15. To a solution of chloropropyltriethoxysilane (0.100 g,
0.5 mmol) in dry hexane (4 ml), was added dimethylphenylsilane
(0.308 g, 2.3 mmol). The mixture was stirred at room temperature
for 5 min before the addition of tris(pentafluorophenyl)borane
(10 ml of a solution containing 40 mg dissolved in 1 ml of toluene,
7.8 ¥ 10-4 mmol). After a 2 min induction time, rapid evolution
of gas and heat from the solution occurred. The mixture was
allowed to cool to room temperature before the addition of neutral
alumina (1 g). The resulting solution was filtered and concentrated
on a rotary evaporator. The remaining solvent and excess reagents
were removed in vacuo, affording pure chloropropyltris(dimethyl-
phenylsilyl)silane (0.260 g, 91% yield).
+
(D), -70.19 ppm (T). HRMS (ES Positive mode): m/z [M + NH4 ]
calculated = 612.2103, found = 612.2079.
Synthesis of (3-chloropropyl)tris(1,1,1,3,5,5,5-heptamethyl-
trisiloxy)silane 13. To a solution of chloropropyltrimethoxy-
silane (0.500 g, 4.15 mmol) in dry hexane (10 ml), was added
1,1,1,3,5,5,5-heptamethyltrisiloxane (1.98 g, 12.4 mmol). The
mixture was stirred at room temperature for 5 min before the
addition of tris(pentafluorophenyl)borane (25 ml of a solution
containing 40 mg dissolved in 1 ml of toluene, 1.95 ¥ 10-3 mmol).
After a 1 min induction time, rapid evolution of gas and heat
from the solution occurred. The mixture was allowed to cool to
room temperature before the addition of neutral alumina (1.5 g).
The resulting solution was filtered and concentrated on a rotary
evaporator. The remaining solvent and excess reagents were
removed in vacuo, affording pure chloropropyltris-(1,1,1,3,5,5,5-
heptamethyltrisiloxy)silane (1.51 g, 90% yield).
1H NMR (CDCl3, 200 MHz): d 7.38–7.54 (m, 6 H, phenyl)
7.27–7.33 (m, 9 H, phenyl), 3.36 (t, 2 H, O2SiCH2CH2CH2Cl, J =
6.8 Hz), 1.54–1.67 (m, 2 H, O2SiCH2CH2CH2Cl), 0.31–0.57 (m,
2 H, O2SiCH2CH2CH2Cl), 0.31 (s, 18 H, OSi(CH3)2(C6H5)). 13C
NMR (CDCl3 125 MHz): d 139.3, 133.2, 129.5, 127.9, 47.7, 26.9,
12.1, 0.64 ppm. 29Si NMR (CDCl3, 99 MHz, 1% w/v Cr(acac)3): d
-1.98 (M), -66.90 ppm (T). HRMS (ES Positive mode): m/z [M +
+
NH4 ] calculated = 576.2009, found = 576.2003.
Synthesis of (3-iodopropyl)tris(diphenylmethylsiloxy)silane 16.
To a solution of iodopropyltrimethoxysilane (1.0 g, 3.45 mmol)
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 9369–9378 | 9371
©

in dry hexane (10 ml), was added methyldiphenylsilane (3.07 g,
15.5 mmol). The mixture was stirred at room temperature for 5 min
before the addition of B(C6F5)3 (25 ml of a solution containing
40 mg dissolved in 1 ml of toluene, 1.95 ¥ 10-3 mmol). After a 6 min
induction time, rapid evolution of gas and heat from the solution
occurred. The mixture was allowed to cool to room temperature
before the addition of neutral alumina (1.5 g). The resulting solu-
tion was filtered and concentrated on a rotary evaporator. The re-
maining solvent and excess reagents were removed in vacuo, afford-
ing pure (3-iodopropyl)tris(diphenylmethylsiloxy)silane (2.18 g,
76% yield).
in vacuo, affording pure tris(phenyldimethylsilyloxy)vinylsilane
(0.79 g, 1.55 mmol, 92%).
1H NMR (CDCl3, 500 MHz): d 7.50–7.52 (m, 6 H, phenyl),
7.34–7.37 (m, 3 H phenyl), 7.28–7.31 (m, 6 H phenyl), 5.78–
5.92 (m, 3 H, vinyl), 0.29 ppm (s, 18 H (C6H6)(H3C)2SiO). 13C
NMR (CDCl3 125 MHz): d 139.45, 134.46, 133.30, 133.21, 129.37,
127.76,.71 ppm. 29Si NMR (99 MHz 1% w/v Cr(acac)3): d -1.58
+
(M), -78.86 (T). HRMS (ES Positive mode): m/z [M + NH4 ]
+
calculated = 526.2085 [M + NH4 ] found = 526.2051.
Synthesis of allyltris(dimethylphenylsilyloxy)silane 21. To a so-
lution of allyltrimethoxysilane (0.500 g, 3.10 mmol) in dry hexane
(10 ml) was added phenyldimethylsilane (1.67 g, 12.3 mmol).
The mixture was stirred at room temperature for 5 min before
the addition of B(C6F5)3 (20 ml of a solution containing 40 mg
dissolved in 1 ml of toluene, 1.6 ¥ 10-3 mmol). After a 1 min
induction time, rapid evolution of gas and heat from the solution
occurred. The mixture was allowed to cool to room temperature
before the addition of neutral alumina (1 g). The resulting solution
was filtered and concentrated under reduced pressure. The remain-
ing solvent and excess reagents were removed in vacuo, affording
pure allyltris(dimethylphenylsilyloxy)silane (1.38 g, 2.6 mmol, 86%
yield)
1H NMR (CDCl3, 500 MHz): d 7.46–7.48 (m, 12 H, phenyl),
7.38–7.41 (m, 6 H, phenyl), 7.25–7.29 (m, 12 H, phenyl),
2.89 (t, 2 H, O3SiCH2CH2CH2I J = 7.0 Hz), 1.59–1.64 (m, 2
H, O3SiCH2CH2CH2I), 0.54–0.55 (m, 2 H, O3SiCH2CH2CH2I),
0.52 ppm (s, 9 H, OSi(CH3)(C6H5)2). 13C NMR (CDCl3 125 MHz):
d 137.21, 134.11, 129.76, 127.89, 27.83, 16.29, 10.77, -0.84 ppm.
29Si NMR (CDCl3, 99 MHz, 1% w/v Cr(acac)3): d -11.48 (M),
+
-67.51 (T) ppm. HRMS (ES Positive mode): m/z [M + NH4 ]
calculated = 854.1834, found = 854.1863.
Synthesis of (3-iodopropyl)tris(methylphenylvinylsilyloxy)silane
17. To a solution of iodopropyltrimethoxysilane (0.500 g,
1.7 mmol) in dry hexane (10 ml), was added methylphenylvinylsi-
lane (1.14 g, 7.75 mmol). The mixture was stirred at room tempera-
ture for 5 min before the addition of tris(pentafluorophenyl)borane
(20 ml of a solution containing 40 mg dissolved in 1 ml of toluene,
1.6 ¥ 10-3 mmol). After a 1 min induction time, rapid evolution
of gas and heat from the solution occurred. The mixture was
allowed to cool to room temperature before the addition of
neutral alumina (1.5 g). The resulting solution was filtered and
concentrated under reduced pressure affording pure iodopropyl-
tris(methylphenylvinylsilyloxy)silane (1.1 g, 93% yield).
1H NMR (CDCl3, 500 MHz): d 7.54–7.56 (m, 6 H, phenyl) 7.38–
7.41 (m, 3 H, phenyl), 7.33–7.36 (m, 6 H, phenyl) 5.70–5.75 (m, 1
H, O3SiCH2CH CH2), 4.85–4.89 (m, 2 H, O3SiCH2CH CH2),
1.55 (d, 2 H, O3SiCH2CH CH2, J = 8 Hz), 0.34 ppm (s, 18 H’s,
(C6H6)Si(CH3)2O). 13C NMR (CDCl3 125 MHz): d 139.47, 133.48,
133.21, 129.41, 127.79, 133.48, 114.53, 22.33, 0.69 ppm. 29Si NMR
(CDCl3, 99 MHz, 1% w/v Cr(acac)3): d -2.14 (M), -70.62 ppm
+
(T). HRMS (ES Positive mode): m/z [M + NH4 ] calculated =
540.2242, found = 540.2267.
Synthesis of bis(methylphenylvinylsiloxy)methyldisiloxane 22.
To a solution of 1,3-dimethyltetramethoxydisiloxane (0.250 g,
1.1 mmol) in dry hexane (5 ml), was added methylphenylvinylsi-
lane (0.90 g, 6.1 mmol). The mixture was stirred at room tempera-
ture for 5 min before the addition of tris(pentafluorophenyl)borane
(15 ml of a solution containing 40 mg dissolved in 1 ml of toluene,
1.2 ¥ 10-3 mmol). After a 4 min induction time, rapid evolution of
gas and heat from the solution occurred. The mixture was allowed
to cool to room temperature before the addition of neutral alumina
(1.5 g). The resulting solution was filtered and concentrated
under reduced pressure affording pure bis(methylphenylvinyl-
siloxy)methyldisiloxane (0.63 g, 76% yield).
1H NMR (CDCl3, 500 MHz): d 7.49–7.51 (m, 6 H, phenyl),
7.36–7.39 (m, 3 H, phenyl), 7.29–7.32 (m, 6 H, phenyl), 6.15–6.23
(m, 3 H, R3SiCHCHH), 6.02–6.06 (m, 3 H, R3SiCHCHH), 5.75–
5.80 (m, 3 H, R3SiCHCHH), 3.03 (t, 2 H, O3SiCH2CH2CH2I,
J = 7.0 Hz), 1.71–1.77 (m, 2 H, O3SiCH2CH2CH2I), 0.57–
0.60 (m,
2 H, O3SiCH2CH2CH2I), 0.371 ppm (t, 9 H,
OSi(CH3)(C6H5)(CHCH2), J = 3.5 Hz). 13C NMR (CDCl3
125 MHz): d 137.36, 136.89, 134.20, 133.74, 129.69, 127.84, 27.93,
16.16, 10.94, -1.18 ppm. 29Si NMR (CDCl3, 99 MHz, 1% w/v
Cr(acac)3): d -13.38 (M), -67.49 ppm (T). HRMS (ES Positive
+
mode): m/z [M + NH4 ] calculated = 704.1365, found = 704.1349.
1H NMR (CDCl3, 500 MHz):
d 7.52–7.54 (m, 8 H,
phenyl), 7.34–7.37 (m, 4 H, phenyl), 7.28–7.30 (m, 8 H,
phenyl) 6.16–6.23 (m, 4 H, SiCH CHH), 5.99–6.02 (m, 4 H,
SiCHC HH), 5.78–5.80 (m, 4 H, SiCH CHH), 0.370 (s, 12
H, (C6H5)(CH CH2)(CH3)SiO), 0.04 ppm (d, 6 H, O3SiCH3).
13C NMR (CDCl3 125 MHz): d 137.7, 137.1, 133.9, 133.7, 129.5,
127.8, -1.12, -2.01 ppm. 29Si NMR (CDCl3, 99 MHz, 1% w/v
Cr(acac)3): d -13.81 (M), -65.83 ppm (T). HRMS (ES Positive
Vinylsilanes
Synthesis of tris(phenyldimethylsilyloxy)vinylsilane 20. To a
solution of vinyltrimethoxysilane (0.250 g, 1.68 mmol) in
dry hexane (10 ml) and dichloromethane (5 ml) was added
dimethylphenylsilane (0.919 g, 6.74 mmol). The mixture was
stirred at room temperature for 5 min before the addition of
tris(pentafluorophenyl)borane (15 ml of a solution containing
40 mg dissolved in 1 ml of toluene, 1.2 ¥ 10-3 mmol). After a
40 s induction time, rapid evolution of gas and heat from the
solution occurred. The mixture was allowed to cool to room
temperature before the addition of neutral alumina (1.5 g). The
resulting solution was filtered and concentrated under reduced
pressure. The remaining solvent and excess reagents were removed
+
mode): m/z [M + NH4 ] calculated = 772.2624, found = 772.2624.
Synthesis of ((MViD)2T)2 23. To
a solution of 1,3-
dimethyltetramethoxydisiloxane (0.250 g, 1.10 mmol) in dry
hexane (10 ml), was added vinyltetramethyldisiloxane (0.97 g,
0.061 mol). The mixture was stirred at room temperature for 5 min
before the addition of tris(pentafluorophenyl)borane (15 ml of a
9372 | Dalton Trans., 2010, 39, 9369–9378
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solution containing 40 mg dissolved in 1 ml of toluene, 1.2 ¥
10-3 mmol). After a 90 s induction time, rapid evolution of gas
and heat from the solution occurred. The mixture was allowed to
cool to room temperature before the addition of neutral alumina
(1 g). The resulting solution was filtered and concentrated under
reduced pressure. The remaining solvent and excess reagents were
removed in vacuo, affording pure ((MViD)2T)2 (0.85 g, 1.1 mmol,
96% yield).
99 MHz, 1% w/v Cr(acac)3): d 7.90 (M), -21.60 ppm (D),
-45.01(T), -110.0 (Q) ppm.
Coupling to hydrophilic moieties
Thermal cycloaddition.
Synthesis of (3-azidopropyl)tris(pentamethyldisiloxy)silane 18.
To a 10 ml round bottom flask equipped with a magnetic
stir bar was added a solution of (3-iodopropyl)tris(penta-
methyldisiloxy)silane (0.500 g, 0.72 mmol) in anhydrous DMF
(2 ml). To the solution was added sodium azide (0.095 g,
1.45 mmol). The mixture was then allowed to stir for 5 h at
90 ◦C. To the mixture was added 20 ml of water and the
desired product was extracted with 25 ml of hexane. The aqueous
layer was washed three times with hexane (10 ml) to ensure
maximum product recovery. The organic layers were combined
and dried over magnesium sulfate (10 g). The resulting solution
was filtered and concentrated under reduced pressure afford-
ing pure (3-azidopropyl)tris(pentamethyldisiloxy)silane (0.38 g,
88% yield).
1H NMR (CDCl3, 600 MHz): d 3.42 (t, 2 H, O3SiCH2CH2-
CH2N3 J = 7.0 Hz), 1.65–1.72 (m, 2 H, O3SiCH2CH2CH2N3), 0.55–
0.58 (m, 2 H, O3SiCH2CH2CH2N3), 0.09 (s, 27 H’s, OSi(CH3)3),
0.07 ppm (s, 18 H’s, (CH3)2SiO2). 13C NMR (CDCl3 150 MHz): d
54.20, 23.19, 11.61, 1.96, 1.28 ppm. 29Si NMR (CDCl3, 99 MHz,
1% w/v Cr(acac)3): d 7.06 (M), -21.88 (D), -70.20 ppm (T).
HRMS (ES Positive mode): m/z [M + Na+] calculated = 624.2061,
found = 624.2064.
1H NMR (CDCl3, 500 MHz):
d 6.13 (dd, 4H, OSi-
(CH3)2(CHCHH), J = 15, 19.8 Hz), 5.92 (dd, 4 H, OSi(CH3)2-
(CHCHH), J = 4.6, 15 Hz), 5.72 (dd, 4 H, OSi(CH3)2(CHCHH),
J = 4.6, 19.8 Hz), 0.153 (s, 24 H, OSi(CH3)2(CHCH2)), 0.071 ppm
(s, 30 H (CH3)SiO3, (CH3)2SiO2). 13C NMR (CDCl3 50 MHz): d
139.48, 131.80, 1.29, 0.43, -2.09 ppm. 29Si NMR (99 MHz 1% w/v
Cr(acac)3): d = -4.47 (M), -21.35 (D), -68.35 (T) ppm.
Synthesis of tetrakis(vinyltetramethyldisiloxy)silane 24. To a
solution of tetraethyl orthosilicate (0.200 g, 0.96 mmol) in dry
hexane (5 ml), was added vinyltetramethyldisiloxane (0.85 g,
5.28 mmol). The mixture was stirred at room temperature
for 5 min before the addition of tris(pentafluorophenyl)borane
(10 ml of a solution containing 40 mg dissolved in 1 ml of
toluene, 7.81 ¥ 10-4 mmol). After a 2 min induction time,
rapid evolution of gas and heat from the solution occurred.
The mixture was allowed to cool to room temperature before
the addition of neutral alumina (1 g). The resulting solution
was filtered and concentrated under reduced pressure. The re-
maining solvent and excess reagents were removed in vacuo,
affording pure tetrakis(vinyltetramethyl-disiloxy)silane (0.520 g,
75% yield).
Synthesis of (3-azidopropyl)tris(1,1,1,3,5,5,5-heptamethyltri-
siloxy)silane 19. To a 10 ml round bottom flask equipped
1H NMR (CDCl3, 500 MHz): d 6.13 (dd, 4 H, R3SiCHCHH,
J = 15.0, 20.5 Hz), 5.93 (dd, 4 H, R3SiCHCHH, J = 4.0, 15.0 Hz),
5.74 (dd, 4 H, R3SiCHCHH, J = 4.0, 20.5 Hz), 0.166 (s, 24H,
O(H3C)2Si(CHCH2)), 0.09 ppm (s, 24 H, (CH3)2SiO2)). 13C NMR
(CDCl3 125 MHz): d 139.47 (CH vinyl), 131.82 (CH2 vinyl), 1.17
(SiCH3), 0.45 ppm (SiCH3). 29Si NMR (CDCl3, 99 MHz, 1% w/v
Cr(acac)3): d -4.34 (M), -20.95 (D), -110.18 ppm (D). HRMS (ES
with
a magnetic stir bar was added a solution of (3-
iodopropyl)tris(1,1,1,3,5,5,5-heptamethyltrisiloxy)silane (1.0 g,
1.1 mmol) in anhydrous DMF (2 ml). To the solution was
added sodium azide (0.141 g, 2.18 mmol). The mixture was then
allowed to stir for 5 h at 90 ◦C. To the mixture was added
40 ml of water and the desired product was extracted with
25 ml of hexane. The aqueous layer was washed three times
with hexane (10 ml) to ensure maximum product recovery.
The organic layers were combined and dried over magnesium
sulfate (10 g). The resulting solution was filtered and concen-
trated under reduced pressure affording pure pale yellow (3-
azidopropyl)tris(1,1,1,3,5,5,5-heptamethyltrisiloxy)silane (0.78 g,
87% yield).
+
Positive mode): m/z [M + NH4 ] calculated = 746.2555, found =
746.2564.
Synthesis of (MYD)4Si, Y = CH2CH2Si(OEt)3 25. In a
round bottom flask equipped with a stir bar and water jacket
condenser with drying tube (Drierite desiccant) was added
tetrakis(vinyltetramethyldisiloxy)silane (0.50 g, 0.68 mmol) in
dry toluene (5 ml). Triethoxysilane (0.56 g, 3.4 mmol) was
then added, followed by Karstedt’s platinum complex (5 ml, 2%
solution in xylenes, 1.0 mmol of Pt). The reaction flask was then
immersed in an oil bath at 50 ◦C. The reaction was allowed to
proceed for 48 h, at which time activated charcoal was added
(~ 0.25 g). The resulting solution was stirred for an additional
2 h. The crude reaction mixture was filtered over Celite, and
residual solvent and excess starting material was removed in
vacuo, affording pure (MYD)4Si, Y = CH2CH2Si(OEt)3 (0.74 g,
80% yield).
1H NMR (CDCl3, 600 MHz): d 3.23 (t, 2 H, O3SiCH2CH2-
CH2N3 J = 7.2 Hz), 1.70–1.73 (m, 2 H, O3SiCH2CH2CH2N3),
0.57–0.60 (m,
2 H, O3SiCH2CH2CH2N3), 0.11 (s, 54 H,
OSi(CH3)3), 0.06 ppm (s, 9 H, (CH3)SiO3). 13C NMR (CDCl3
150 MHz): d 53.68, 22.40, 10.89, 1.44, -2.54 ppm. 29Si NMR
(CDCl3, 99 MHz, 1% w/v Cr(acac)3): d 7.81 (M), -66.01 (T),
-71.28 ppm (T).
Synthesis of propiolate terminated monomethoxy poly-
(ethyleneoxide), (av. mol. wt: 350) 26. In a round bottom flask
containing monomethoxypoly(ethylene-oxide) (av. mol. wt 350)
(7.0 g, 20 mmol), was successively added propiolic acid (2.8 g,
40 mmol), toluene (60 ml) and a catalytic amount of para-
toluene sulfonic acid (0.2 g). The flask was equipped with a
Dean Stark apparatus, and heated with azeotropic removal of
1H NMR (CDCl3, 500 MHz): d 3.81 (q, 24 H, (CH3CH2O)3-
SiO, J = 7.0 Hz), 1.22 (t, 36 H, (CH3CH2O)3SiO, J
=
7.0 Hz), 0.54 (s, 16 H, O3SiCH2CH2Si(CH3)2O), 0.07 (s,
24 H, CH2Si(CH3)2OSi(CH3)2OSi), 0.06 ppm (s, 24 H,
CH2Si(CH3)2OSi(CH3)2OSi). 13C NMR (CDCl3 125 MHz): d
58.50, 18.45, 9.21, 1.93, 1.17, -0.42 ppm. 29Si NMR (CDCl3,
1
water. Completion of the reaction was monitored by H NMR,
by comparison of the 3 protons of terminal methoxy compared
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©

to the appearance of the methylenic esters protons at 4.32 ppm
(ca. 20 h). The mixture was then concentrated in vacuo, and the
crude product directly loaded onto a chromatography column
packed with silica gel. Elution started with pure dichloromethane,
then increasing amounts of methanol were added to the eluent
(up to 3%, v:v). The fractions containing the propiolate ester
were combined, evaporated under reduced pressure to afford pure
yellow propiolate, methyl-terminated poly(ethyleneoxide) (7.3 g,
91% yield).
1H NMR (CDCl3, 200 MHz): d 4.32 (t, 2 H, COOCH2, J =
4.6 Hz), 3.74 to 3.50 (m, 29H, OCH2CH2), 3.35 (s, 3H, OCH3),
2.96 to 2.93 (m, 1H, HCCCOO). 13C NMR (CDCl3, 50 MHz): d
75.44, 74.60, 71.99, 70.65, 68.62, 65.32, 59.12.
light (254 nm) and monitored via NMR until complete disap-
pearance of the allyl functionality was observed (typically 30–
40 min). Once complete, the mixture was subjected to column
chromatography (10% acetone, 90% dichloromethane) to remove
the photoinitiator and excess starting materials. The collected
fractions were combined and concentrated under reduced pressure
yielding the sulfide-modified silicone (0.392 g, 69% yield).
1H NMR (CDCl3, 500 MHz): d 3.76 (m, 1 H, SCHH¢CH-
(OH)CHH¢¢(OH)), 3.73 (m, 1 H, SCHH¢CH(OH)CHH¢¢(OH)),
3.52–3.55 (m, 1 H, SCHH¢CH(OH)CHH¢¢(OH)), 2.68–2.72
(m, 1 H, SCHH¢CH(OH)CHH¢¢(OH)), 2.56–2.60 (m, 1 H,
SCHH¢CH(OH)CHH¢¢(OH)), 2.53 (t, 2 H’s, O3SiCH2CH2CH2S,
J = 7.5 Hz), 1.64–1.73 (m, 2 H’s, O3SiCH2CH2CH2S), 0.58–
0.69 (m, 2 H’s, O3SiCH2CH2CH2S), 0.10 (s, 54 H’s, (CH3)3SiO),
0.047 ppm (s, 9 H’s, (CH3)SiO3). 13C NMR (CDCl3 150 MHz):
d 69.60, 65.57, 36.01, 35.72, 23.63, 14.07, 1.91, -1.88 ppm. 29Si
NMR (CDCl3, 119 MHz, 1% w/v Cr(acac)3): d 7.75 (M), -66.01
Thermal cyclization of 18 with propiolate-modified PEG 27. To
a 5 ml round bottom flask equipped with a magnetic stir bar and
previously prepared azidopropyltris(pentamethy-ldisiloxy)silane
(0.280 g, 0.47 mmol), was added propiolate, methyl-PEG (0.178 g,
0.47 mmol; av. mol. wt: 350, mass calculated via NMR 382g/mol)
and dry toluene (1 ml). The mixture was then stirred at 65 ◦C and
monitored via NMR for completion (typical time required is 48
h). Once complete, solvent was removed under reduced pressure,
yielding pure 17 as two isomers, with a ratio of roughly 1 : 4.
(0.324 g, 99.6% yield)
+
(T), -71.20 ppm (T). HRMS (ES Positive mode): m/z [M + NH4 ]
calculated = 906.3145 found = 906.3204.
Results
Incorporation of functional synthetic handles
3-(Glycidoxy)propyltrimethoxysilane was reacted with one equiv-
alent of pentamethyldisiloxane 1 in the presence of catalytic
amounts of B(C6F5)3. No evidence of disiloxane formation was
observed: instead, selective reductive epoxide ring opening oc-
curred (Fig. 2), but only in hexanes as solvent. Surprisingly, there
was no further reaction with the addition of excess 1: the major
product remained 2A (A : B, 88 : 12). In the more polar solvent
dichloromethane some competition occurred between the epoxide
and the alkoxysilane: small amounts of disiloxane 3 were also
formed. This trend in selectivity as a function of solvent polarity
was also seen in the reaction of hydrosilanes with thiols (R3SiH +
HS(CH2)3Si(OR)3 → R3SS(CH2)3Si(OR)3 + H2).24
1,4-Isomer (~75%)
1H NMR (CDCl3, 500 MHz): d 8.05 (s, 1 H, d), 4.49 (t, 2 H,
e, J = 5.0 Hz), 4.38 (t, 2 H, c, J = 7.3 Hz), 3.82 (t, 2 H, f, J =
5.0 Hz), 3.62–3.68 (m, ~ 24H, –OCH2CH2O–), 3.53 (t, 2 H, –
(CO)OCH2CH2OCH2CH2O–, J = 4.8 Hz), 3.36 (s, 3 H, OCH3),
1.96–2.03 (m, 3 H, b), 0.49–0.53 (m, 2 H, a), 0.063 (s, 27 H, g),
0.05 ppm (s, 18 H, h). 13C NMR (CDCl3 125 MHz): d 159.10,
140.49, 128.08, 72.70, 71–28–71.48, 69.70, 64.81, 59.77, 53.69,
25.30, 11.90, 2.57, 1.90 ppm. 29Si NMR (CDCl3, 99 MHz, 1%
w/v Cr(acac)3): d 7.24 (M), -21.52 (D), -70.99 ppm (T).
1,5-Isomer (~25%):
1H NMR (CDCl3, 500 MHz): d 8.13 (s, 1 H, d¢), 4.68 (t, 2 H,
c¢, J = 7.5 Hz), 4.44 (t, 2 H, e¢, J = 5.0 Hz), 3.79 (t, 2 H, f¢,
J = 5.0 Hz), 3.62–3.68 (m, ~ 24H, –OCH2CH2O–), 3.53 (t, 2 H,
–(CO)OCH2CH2OCH2CH2O–, J = 4.8 Hz), 3.36 (s, 3 H, OCH3),
1.93–1.97 (m, 3 H, b¢), 0.49–0.53 (m, 2 H, a¢), 0.051 (s, 27 H, g¢),
0.03 ppm (s, 18 H, h¢). 13C NMR (CDCl3 125 MHz): d 161.45,
138.78, 128.08, 72.70, 71.28–71.48, 69.52, 65.26, 59.77, 53.56,
25.30, 11.99, 2.57, 1.85 ppm. 29Si NMR (CDCl3, 99 MHz, 1%
w/v Cr(acac)3): d 7.06 (M), -21.85 (D), -70.74 ppm (T).
Fig. 2 Reductive epoxide ring opening.
The relative efficiency of silylation between free alcohols and
alkoxysilanes in the presence of B(C6F5)3 was examined using 4,
which was prepared in low yield by platinum-catalyzed hydrosily-
lation of allyl alcohol with HSi(OEt)3.‡ Exposure of 4 to B(C6F5)3
and one equivalent of 1 led preferentially to reaction at the alcohol
giving 5 – the same process as occurs with platinum catalysts
(Fig. 2). By contrast, once the alcohol was protected as a silyl
6 or benzyl ether 7, reactions of both methyl and propyl ethers
occurred competitively. In the case of 6, the propyloxy group was
Thiol-ene click 29
To a solution of allyltris(1,1,1,3,5,5,5-heptamethyltrisiloxy)-silane
(0.500 g, 0.64 mmol) in dry THF, previously passed through
neutral alumina (1 ml), was added 1-thiogylcerol (0.103 g,
0.96 mmol) and 2,2-dimethoxy-2-phenylacetophenone (0.016 g,
0.0625 mmol). The mixture was then irradiated under ultraviolet
‡ Problems with hydrosilylation in the presence of unprotected alcohols are
well known: a reaction between the alcohol and silane can occur, leading
to H2 formation (R3SiH + H2C CHCH2OH → H2C CHCH2OSiR3 +
H2.43 Nevertheless, this direct route was attempted as it avoided protec-
tion/deprotection sequences.
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reduced competitively with the SiOMe group to give mixtures of
8, 9 and related products: tris(trimethylsiloxy)propylsilane was the
product with an excess of 1. The benzyl ether 7 gave an unhelpful
mixture of regioisomeric cleavage products, 10 and 11. Complete
conversion to trifunctional propylsilicones could not be induced
with an excess of 1 (Fig. 3).
Fig. 4 Alkyl halide-derived silicones.
of B(C6F5)3 used here, ~0.5%, very clean disiloxane formation
occurred leading to highly functionalized silicones with both
single and multiple alkene groups (Fig. 5) or mixed alkenes/alkyl
halides 17 (Fig. 4): no silylation of the olefins were observed
during the Piers–Rubinsztajn silicone formation. The yields are
excellent and complex materials, including silicone crosslinkers
20, 21, 22, 23 and 24 (see also Fig. 6), are readily prepared in a few
steps.
Fig. 3 Competitive reduction of benzyl and silyl ethers (products in ‘[]’
were removed during workup at low vacuum and are inferred).
Compatible functional groups
Haloalkanes. Alkoxysilane compounds that also contain alkyl
halides, including both iodo and chloro compounds, react cleanly
in high yields with hydrosilanes in the presence of B(C6F5)3 to
give functional siloxanes 12, 13, 14, 15, 16 and 17 (Fig. 4). The
alkyl halide can be further functionalized, as shown in the simple
substitution of 12, 13 to give organoazides 18 and 19, respectively.
The latter compounds provide a facile linkage to hydrophilically
modified and functional materials, as is discussed further below.
The latter substitution (and cyclization) processes do not affect
the structure of the silicone framework.
Fig. 5 Oligoalkene preparation.
Alkenes.
Oligoalkene-functionalized silicones. The hydrosilylation of
alkynes, and particularly of alkenes, is broadly used in silicon
chemistry to incorporate organic residues.25 It has previously
been reported that B(C6F5)3 will catalyze the hydrosilylation of
(thio)ketones, imines26 and alkenes.27,28 In addition, the hydrosi-
lylation of alkenes catalyzed by B(C6F5)3 has been described.27
Therefore, it was not initially anticipated that vinyl-functional
silicones could be prepared under the reaction conditions of the
Piers–Rubinsztajn reaction. However, C C hydrosilylation typi-
cally requires catalyst loadings of ~5wt%. At the concentrations
Fig. 6 Iterative Piers–Rubinsztajn/hydrosilylation.
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Silicone functionalization – strategies for polymer coupling
As noted above, the key objective of this work was finding
appropriate functional groups and reaction conditions that would
permit organofunctionalization of silicones. We show three com-
plementary strategies that demonstrate this point.
Iterative hydrosilylation. Silicones bearing both SiH and
SiCH CH2 residues will self-polymerize in the presence of
platinum or radical catalysts.29,30 By contrast, the same compounds
will selectively form siloxanes in the presence of B(C6F5)3 without
touching the alkenes. However, they can be converted into
new alkoxysilanes 25 with platinum and compounds such as
HSi(OEt)3. Thus, a short iterative procedure can lead to large,
highly functional compounds (Fig. 6) while maintaining silicone
structural integrity. This obvious route to dendrimeric structures
is currently being developed.
Fig. 8 Thiol-ene click.
oxygen-based nucleophiles, leads of an oxonium ion 31 that
subsequently degrades to a disiloxane and alkane, regenerating
the catalyst (Fig. 9). However, B(C6F5)3 is a reasonably strong
Lewis acid that can form Lewis acid–base complexes with
other heteroatoms present in the reaction mixture 32.42 The
interplay between formation of the required SiHB complex and
other complexes controls the outcome of the reactions described
above.
Azide “click” chemistry. Click chemistry31 has become a
standard method to link disparate molecules. Initially, a thermally
activated process described by Huisgen.32–34 It was reinvented by
Sharpless who used a copper catalyst to both increase reaction
selectivity and decrease reaction temperatures.31,35–37 We have
previously examined both the copper-catalyzed and thermal
versions to functionalize38 and crosslink silicones.39
The reaction between azide 18 and poly(ethylene glycol) propi-
olate ester 26 occurred in the absence of a copper catalyst at 65 ◦C
over 48 h to give the surface active graft copolymer 27 (Fig. 7);
the silicone structure was not affected during the process. Because
of this accessibility of well-defined silicone materials, it should be
possible to prepare a wide range of polymeric surfactants using
this protocol. An examination of the surface activity of these and
related compounds is currently under way and will form the basis
of a future report.
Fig. 9 Proposed mechanism for B(C6F5)3-catalyzed siloxane formation.
Neither halides nor olefins provide sufficiently basic Lewis bases
to compete with alkoxysilanes. Thus, at the low concentrations of
B(C6F5)3 used in this study (0.5 wt%), only activation via silylox-
onium formation occurs, leading to clean disiloxane formation.
The process can be repeated multiple times without loss of fidelity
to produce complex structures in high yield in one or two steps
(Fig. 4–6).
Less efficient reactions were observed when more powerful
Lewis bases were present. Piers, in his examination of the
hydrosilylation of imines, for example, showed that strong Lewis
bases such as nitrogen can shut down any reaction involving
SiH, because the equilibrium of N–B vs. H–B complexes lie
almost exclusively on the side of the BN complex (Fig. 9, LB =
RR¢C NR¢¢).26 As the N basicity is reduced, the equilibrium with
SiH is established, and reduction occurs (33 → 34).
Fig. 7 Concise preparation of organofunctional silicones.
Thiol-ene “click” capable silicones thioglycerol. More recently,
the “click” moniker has been applied to the thiol-ene reaction
(Fig. 8).40,41 Normally, this describes a radical chain addition
of a thiol across an alkene to generate a sulfide. Thioglyc-
erol cleanly added to 2824 in the presence of 2,2-dimethoxy-
2-phenylacetophenone to give 29: the process is photoinitiated
at 254 nm. The thiol-ene reaction can therefore also be used
to hydrophilize silicones under conditions that don’t affect the
silicone backbone.
A similar set of complex equilibria will present in each of the
cases studied here. The outcomes, in particular, the efficiency with
which silicones are prepared, depend on the degree to which SiH–
B complexes and silylated oxonium ions can form in the presence
of other Lewis bases.
No reaction was observed to occur between 1 and amino-
propyltriethoxysilane (APTS) in the presence of B(C6F5)3. We
ascribe this, following Piers’ results, to essentially irreversible
complexation between boron and the nitrogen (Fig. 9, LB =
H2N(CH2)3Si(OR)3). Under such circumstances formation of
the necessary B–H–Si complex is precluded. To examine this
Discussion
The Piers–Rubinsztajn reaction is understood to involve the
formation of a Si–H–B complex 30 which, in the presence of
9376 | Dalton Trans., 2010, 39, 9369–9378
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The Royal Society of Chemistry 2010
©

in more detail, proton NMR of the reaction medium contain-
ing tris(pentafluorophenyl)boron with aminopropyltriethoxysi-
lane (APTS) and 1 over 3.5 h, a very long time when compared to
the ca. 5 min normally needed for completion of Piers–Rubinsztajn
reactions. No change in the multiplicity and integration of the dif-
ferent signals of APTS was observed. The only minor change was a
small chemical shift of the amino protons, which can be attributed
to the formation of the B–N complex, from 1.09 ppm in the free
form to 1.32 in the presence of catalyst. It would be reasonable to
expect that such complexes 35 could be relatively strong Brønsted
acids and lead to silicone degradation/redistribution. However, as
noted, there was no evidence of this.
The epoxide opened exclusively in hexane, and reacted com-
petitively with the SiOMe groups in dichloromethane. The major
reaction in both cases can be considered to arise from a B–H
complex analogous to 30 and a silyloxonium ion: the Piers–
Rubinsztajn reaction is as much about silicon-based Lewis acids
as about borohydrides. Reduction of the silyl complex 36 pref-
erentially occurs at the least hindered carbon (Fig. 10a), as
would expected for nucleophilic driven reactions. In the more
polar DCM, the less basic oxygen atoms of the SiOMe groups
begin to compete, leading to some disiloxane formation (giving 3,
Fig. 2).
Acknowledgements
We thank the Natural Sciences and Engineering Research Council
of Canada for financial support and Silcotech Canada and Siltech
Inc. for support and helpful discussions.
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