Amphiphilic silicone architectures via anaerobic Thiol-Ene chemistry

Article abstract of DOI:10.1021/ol202051y

Despite broad application, few silicone-based surfactants of known structure or, therefore, surfactancy have been prepared because of an absence of selective routes and instability of silicones to acid and base. Herein the synthesis of a library of explic

Full text of DOI:10.1021/ol202051y

ORGANIC
LETTERS
2011
Vol. 13, No. 22
6006–6009
Amphiphilic Silicone Architectures via
Anaerobic ThiolꢀEne Chemistry
Daniel J. Keddie,† John B. Grande,‡ Ferdinand Gonzaga,‡ Michael A. Brook,*,‡ and
Tim R. Dargaville*,†
Institute of Health and Biomedical Innovation, Queensland University of Technology,
60 Musk Ave, Kelvin Grove, Queensland, Australia 4059, and Department of Chemistry
and Chemical Biology, McMaster University, 1280 Main Street West, Hamilton,
Ontario, Canada L8S 4M1
t.dargaville@qut.edu.au; mabrook@mcmaster.ca
Received September 10, 2011
ABSTRACT
Despite broad application, few silicone-based surfactants of known structure or, therefore, surfactancy have been prepared because of an
absence of selective routes and instability of silicones to acid and base. Herein the synthesis of a library of explicit silicone-poly(ethylene glycol)
(PEG) materials is reported. Pure silicone fragments were generated by the B(C6F5)3-catalyzed condensation of alkoxysilanes and vinyl-
functionalized hydrosilanes. The resulting pure products were coupled to thiol-terminated PEG materials using photogenerated radicals under
anaerobic conditions.
Silicone surfactants (e.g., Figure 1A) are usually based
on the poly(ethylene glycol) (PEG) modification of short-
chain silicones1,2 or of long-chain polymers modified
randomly along the backbone.3ꢀ6 While such compounds
are considered “exotic” in the surfactant world, due both
to price and composition, the compounds are essential in
certain applications, including cosmetics, paints, and coat-
ings and for the stabilization of bubbles in polyurethane
foam structures.7
Like mostsilicones, silicone surfactants are comprised of
complex mixtures of compounds8ꢀ11 that can vary signifi-
cantly in polydispersity index and, therefore, surfactancy.
Normally, the compounds are prepared by grafting to an
existing hydrosilane an allyl-terminated PEG chain using
platinum-catalyzed hydrosilylation (PEG-CH2CHdCH2 þ
HSiR3 f PEG(CH2)3SiR3). While the reaction is extre-
mely efficient;in ideal cases, less than 10 ppm of Pt is
required;platinum is expensive. In addition, two regioi-
somers are often generated in the hydrosilylation process.
Worse, with the exception of only a very few low molecular
weight materials as shown in Figure 1B, well-defined
hydridosilicones are simply not available. Larger molecu-
lar weight silicone surfactants are sold as broad molecular
weight mixtures of molecules bearing varying degrees of
functionalization. There is, as a consequence, no general
† Queensland University of Technology.
‡ McMaster University.
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Perepelittchenko, L. Appl. Organomet. Chem. 1999, 13, 201–208.
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Rexin, F.; Perepelittchenko, L. Appl. Organomet. Chem. 1999, 13, 611–
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r
10.1021/ol202051y
Published on Web 10/25/2011
2011 American Chemical Society

and, in addition, would allow one to probe structureꢀ
surface activity relationships of the surfactants formed.
Four different explicit silicones were produced using the
PiersꢀRubinsztajn coupling of alkoxy- and hydrosilanes
(Scheme 1).16 Catalysis by B(C6F5)3 of the condensation of
vinyltriethoxysilane and the pentamethyldisiloxane,
1,1,1,3,5,5,5-heptamethyltrisiloxane, or phenyldimethylsi-
lane, respectively, led to siloxanes 1, 2, and 3 in good yield.
Alternatively, the tetravinyl material 4 could be made in an
analogous process with Si(OEt)4. These and a commercial
silicone terminated with vinyl groups served as substrates
for the thiolꢀene reaction (Scheme 1).
The PiersꢀRubinsztajn approach constitutes one part of
the synthesis of well-defined surfactants. We sought a simple
and efficient route for grafting these silicones to hydrophiles
under conditions that do not affect silicone architecture:
that is, under conditions that are neither acidic nor basic.17
Several strategies presented themselves including the
modular, metal free “click” reaction between azides and
alkynes.18 We reasoned that a thiolꢀene “click” process19,20
which has been broadly exploited in many technical areas,21
including silicones,22 should efficiently produce a wide range
of molecular structures that can be isolated with limited
workup. Therefore, we examined and describe below the
synthesis of a library of amphiphilic silicones using thiolꢀ
ene chemistry, initiated by photolysis of 2,2-dimethoxy-2-
phenylacetophenone (DMPA), between PEG thiols and
well-defined silicone architectures, prepared by boron-
catalyzed coupling of lower alkoxy- and hydrosiloxanes.
A series of PEG oligomers, terminated at a single end by
thiols, was synthesized from the corresponding PEG mono-
methyl ethers in three steps (see Scheme 2). Synthesis of the
PEG tosylates from the alcohol using the procedure pre-
viously reported by Keegstra et al.23 proved facile and gave
products 5ꢀ8 in high yield (88ꢀ91%) without the need for
purification. Initial attempts to form the PEG thioacetates
9ꢀ12 directly, by refluxing the tosylates in dry MeOH24
in the presence of KSAc, did not produce the desired
Figure 1. Structures of (A) representataive, commercially avail-
able silicone surfactants and (B) readily available functional
hydrosiloxanes.
route to structurally well-defined silicone surfactants and
little knowledge about structure activity relationships.12
Further complicating the situation is the intrinsic reac-
tivity of silicone polymers. Either acids or bases can initiate
a process in which silicones depolymerize in an equilibrium
between chains and rings, for which the equilibrium con-
stant is approximately 1.13 Any processes that involve
linking hydrophiles to well-defined silicones must there-
fore avoid both acidic and basic conditions.
Recently, we described a simple route to explicit, med-
ium sized silicone structures,14 including functional
silicones,15 that involves the B(C6F5)3-catalyzed condensa-
tion of hydrosilanes and alkoxysilanes to give silicones and
alkane byproducts (the PiersꢀRubinsztajn reaction16
(Scheme 1)). The utilization of this route for the creation
of structurally well-defined silicone amphiphiles could
avoid the disadvantages of platinum-catalyzed processes
Scheme 1. Using the PiersꢀRubinsztajn Reaction To Prepare
Explicit Vinylsiloxanes
(12) Hill, R. M. Silicone Surfactants; Dekker: New York, 1999.
(13) Chojnowski, J., Polymerization. In Siloxane Polymers; Clarson,
S. J., Semlyen, J. A., Eds.; Prentice Hall: Englewood Cliffs, NJ, 1993; pp
1ꢀ71.
(14) Thompson, D. B.; Brook, M. A. J. Am. Chem. Soc. 2008, 130,
32–33.
(15) Grande, J. B.; Thompson, D. B.; Gonzaga, F.; Brook, M. A.
Chem. Commun. 2010, 46, 4988–4990.
(16) Brook, M. A.; Grande, J. B.; Ganachaud, F. Adv. Polym. Sci.
2010, 235, 161–183.
(17) Brook, M. A. Silicones. In Silicon in Organic, Organometallic
and Polymer Chemistry; Wiley: New York, 2000; pp 256ꢀ308.
(18) Gonzaga, F.; Yu, G.; Brook, M. A. Chem. Commun. 2009, 1730–
1732.
(19) Killops, K. L.; Campos, L. M.; Hawker, C. J. J. Am. Chem. Soc.
2008, 130, 5062–5064.
(20) Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Angew. Chem., Int.
Ed. 2001, 40, 2004–2021.
(21) Kade, M. J.; Burke, D. J.; Hawker, C. J. J. Polym. Sci. A, Polym.
Chem. 2010, 48, 743–750.
(22) Campos, L. M.; Truong, T. T.; Shim, D. E.; Dimitriou, M. D.;
Shir, D.; Meinel, I.; Gerbec, J. A.; Hahn, H. T.; Rogers, J. A.; Hawker,
C. J. Chem. Mater. 2009, 21, 5319–5326.
(23) Keegstra, E. M. D.; Zwikker, J. W.; Roest, M. R.; Jenneskens,
L. W. J. Org. Chem. 1992, 57, 6678–80.
(24) Bae, J. W.; Lee, E.; Park, K. M.; Park, K. D. Macromolecules
2009, 42, 3437–3442.
Org. Lett., Vol. 13, No. 22, 2011
6007

Scheme 2. Synthesis of PEG Thiols
Scheme 3. ThiolꢀEne Coupling of PEG Thiols with Divinyl
PDMSa
Figure 2. Explicit PEGꢀsilicone surfactants.
Table 1. SiliconeꢀPEG Compounds
a DMPA = 2,2-dimethoxy-2-phenylacetophenone.
products. After the solvent was changed to dry DMF,25
however, the PEG thioacetates 9ꢀ12 could be isolated in
moderate yield (32ꢀ40%). Subsequent acid hydrolysis of
the thioesters using 10% aq HCl/MeOH furnished the
target PEG thiols 13ꢀ16 in high yield (81ꢀ96%). Note that
the low polydispersity of the original PEG was preserved in
the products. For compounds 14 and 15, for example, the
average PEG oligomer values of 7 ( 1 and 16 ( 4,
respectively, are also reflected in the mass spectra of the
subsequent silicone product following the thiolꢀene process
(see below): high resolution mass spectra confirm the
structure of the compound with 7 or 16 PEG units, respec-
tively (Supporting Information for compounds 22, 28 and
31), and low resolution mass spectra of both the PEG-thiol
and thioether product show the same distributions of
oligomers, separated by 44 mass units.
Initial coupling reactions between discrete vinyl-func-
tional siloxanes and alkyl thiols, such as undecanethiol and
thioglycerol, under aerobic conditions based on the meth-
odology reported by Campos et al.,26 gave the desired
products within 30 min under UV irradiation (data not
(25) Cubero, I. I.; Lopez-Espinosa, M. T. P.; de Buruaga Molina,
A. S. Carbohydr. Res. 1996, 280, 145–50.
(26) Campos, L. M.; Killops, K. L.; Sakai, R.; Paulusse, J. M. J.;
Damiron, D.; Drockenmuller, E.; Messmore, B. W.; Hawker, C. J.
Macromolecules 2008, 41, 7063–7070.
a Isolated yield. b Tetraethyleneglycol dithiol.
6008
Org. Lett., Vol. 13, No. 22, 2011

shown). This was not surprising, as, the literature indicates
that the thiolꢀene reaction should not be significantly
hindered by the presence of oxygen.27 However, thiolꢀene
reactions with silicones are, apparently, not as pliable.28
Survey experiments with a commercially available R,ω-
divinyl PDMS 17 failed to lead to any products in the
presence of oxygen after 2 h irradiation but, after degassing
and reirradiation, the desired ABA block copolymers
18ꢀ20 formed in high yield (66ꢀ79%) within 1 h as shown
by NMR (Scheme 3): single regioisomers were observed.
The stoichiometry used for this reaction was a 1:1.5 ratio of
thiol to vinyl group to ensure complete conversion of the
PEG-thiol, except for high molecular weight materials
where a 1:1 ratio was used to eliminate the need to separate
the polar unreacted PEG from the synthesized amphiphiles.
When an excess of the PEG thiol was used (1.5 equiv thiol
per vinyl group), the resulting products were difficult to
separate from the excess starting material, especially when
the larger PEG compounds were used (Scheme 3).
Analogous reaction conditions were used to convert
vinyl-functional silicones 1ꢀ3 into surface active silicones
21ꢀ29using 3 PEG thiols 13ꢀ15 (Table 1, Figure 2). It was
also possible to prepare the star-shaped silicones 30 and 31
from 4 and the bola-amphiphile 32 from tetraethylenegly-
col dithiol 16 (Figure 2). The explicit silicone fragments
were incorporated into the products unaltered, without the
competing metathesis that accompanies acid or basic
conditions. The formation of both the PEG-thiols and
thioether reactions were in lower than expected yields for
such reactions. This is a consequence of our desire to have
absolutely pure materials, which will permit an under-
standing of structure/surface activity relationships. Several
columns were required in the former case, and use of
narrow distillation cuts in the latter case favored purity
over yield of these polar molecules.
Low concentrations of B(C6F5)3 facilitate the condensa-
tion of alkoxy- and hydridosilanes to give siloxane bonds
under conditions that do not initiate the metathesis of the
silicone: explicit structures can be prepared and isolated.
Silicone compounds, with various 3D profiles;including
linear, branched, or star;are thus readily available from
simple starting materials. The elaboration of these com-
pounds into hydrophilically modified materials can also be
performed without degrading the silicone structures by the
regioselective, photoinduced thiolꢀene click reaction of
thiol-terminated PEG oligomers of various molecular
weights; we have described explicit structures, with differ-
ent three-dimensional profiles, of molecular weights ran-
ging from about 1200 to about 3300 g molꢀ1. Preliminary
studies demonstrate that the products, particularly as the
relative PEG fraction increases, are both soluble and stable
in water. Suchcompounds are expected tohaveprecise and
predictable surface properties that depend upon their 3D
structure and the ratio of hydrophilic to hydrophobic
moieties, which will be the subject of future reports.
Acknowledgment. We thank the Australian Research
Council (grant DP0877988) and the Natural Sciences and
Engineering Research Council of Canada for financial
support.
Supporting Information Available. Experimental pro-
tocols, 1H and 13C NMR, HRMS spectral data for
compounds 21ꢀ32, and LRMS for 22, 28, and 31. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(27) Hoyle, C. E.; Lee, T. Y.; Roper, T. J. Polym. Sci. A, Polym.
Chem. 2004, 42, 5301–5338.
(28) Mueller, U.; Kunze, A.; Herzig, C.; Weis, J. J. Macromol. Sci.
Pure Appl. Chem. 1996, A33, 439–57.
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