A direct, versatile route to functionalized trialkoxysilanes

Article abstract of DOI:10.1039/c3cc48570f

The peroxide initiated radical addition of dithiocarbonates (xanthates) to trialkoxy vinylsilanes leads to functionalized trialkoxysilanes. Prior addition of the dithiocarbonates to an alkene before reaction with the vinylsilane can be used to increase the complexity of the final product. The Royal Society of Chemistry 2014.

Full text of DOI:10.1039/c3cc48570f

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A direct, versatile route to functionalized
trialkoxysilanes†‡
Cite this: Chem. Commun., 2014,
50, 2324
Beatrice Quiclet-Sire,* Yuuki Yanagisawa§ and Samir Z. Zard*
´
Received 9th November 2013,
Accepted 23rd December 2013
DOI: 10.1039/c3cc48570f
www.rsc.org/chemcomm
The peroxide initiated radical addition of dithiocarbonates (xanthates) trialkoxysilane and alkynes has been used.7 Very recently, a
to trialkoxy vinylsilanes leads to functionalized trialkoxysilanes. Prior ‘‘radical click’’ procedure was reported involving the photochemi-
addition of the dithiocarbonates to an alkene before reaction with the cally induced radical addition of 3-mercaptopropyltriethoxysilane
vinylsilane can be used to increase the complexity of the final product. to alkenes.8
We have over the past years developed the radical addition
We describe herein a direct, convergent route to highly functionalized of dithiocarbonates (xanthates) to alkenes, as a method for the
trialkoxysilanes. Trialkoxysilanes have played and continue to play a creation of new carbon–carbon bonds in an inter- or intra-molecular
central role in materials science, surface science, and nanotechnology. fashion.9 A simplified mechanism is outlined in Scheme 1. One of
Among numerous applications, they have been used to form sol–gel the key elements is that the reaction of the intermediate radical Rꢀ
and organogelators;1 to modify various surfaces, especially silica and with its precursor dithiocarbonate 1 is reversible and degenerate
metal oxide surfaces through the controlled formation of mono- (Path A) and therefore does not compete with the desired addition to
layers;2 to create organic–inorganic hybrid materials and nanocompo- the alkene. In other words, radical Rꢀ is continuously regenerated
sites;3 and to allow the assembly of hybrid and supported catalysts.4 from adduct 2 and thus acquires a relatively long effective lifetime,
However, in order to further expand and optimize the utility of even in a concentrated medium, allowing it to participate in
trialkoxysilanes, flexible, versatile and functional group tolerant comparatively slow radical processes not easily accessible using
methods for their concise synthesis must be developed.
The relative hydrolytic fragility of alkoxysilanes in general, and unactivated alkenes (Path B) become routinely possible.
trialkoxysilanes in particular, has considerably restricted the range This addition transfer of dithiocarbonates and related thio-
other methods. In particular, intermolecular additions to ordinary,
of acceptable reaction types and experimental conditions that may carbonylthio derivatives is the basis of the RAFT-MADIX controlled
be harnessed for their preparation. Access to trialkoxysilanes there- polymerization process, which has proved to be an exceedingly
fore hinges on a relatively limited number of approaches. The most powerful technology for the synthesis of block polymers and various
commonly employed process is the hydrosilylation of alkenes and
alkynes, most often catalyzed by rhodium complexes.5 The acyla-
tion of commercially available 3-aminopropyl- and 3-hydroxypropyl-
trialkoxysilane also constitutes a practical route. Both amine and
alcohol, however, arise from the hydrosilylation of allylamine and
allyl alcohol, respectively. The coupling of isocyanate-containing
trialkoxysilanes with amines and alcohols has also been used.6
A click dipolar cycloaddition reaction between an azide-substituted
`
Laboratoire de Synthese Organique, CNRS UMR 7652, Ecole Polytechnique,
F-91128 Palaiseau, France. E-mail: zard@poly.polytechnique.fr;
Fax: +33 169335972; Tel: +33 169335971
† This communication is dedicated with respect to the memory of Professor
´ ´
Manfred Schlosser (Ecole Polytechnique Federale, Lausanne, Switzerland).
‡ Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3cc48570f
Scheme 1 Simplified mechanism for the radical addition of a dithiocar-
bonate (xanthate) to an alkene.
§ Present address: Graduate School of Material Sciences, Nara Institute of Science
and Technology, 8916-5 Takayama, Ikoma, Nara 630-0101, Japan.
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other polymer architectures.10 It has also been applied very recently
in the modification of carbon nanotubes.11 We have now examined
extending this chemistry to the important case of trialkoxysilanes by
using trialkoxy vinylsilanes as the alkene partners. In case of
success, this would open a direct, versatile, and convergent route
to highly functionalized trialkoxysilanes of potential utility in
materials science.
Table 1 Examples of radical additions to trialkoxy-vinylsilanes
Our initial trials involved the lauroyl peroxide initiated
addition of dithiocarbonates to triethoxy vinylsilane 5a (R0 = Et).
They indeed proved to be successful with a variety of dithiocarbo-
nates (Table 1, examples 6a, 6c, and 6e), but were also somewhat
frustrated by the tendency of the triethoxysilyl products to partially
hydrolyze during chromatographic separation leading to fluctua-
tions in the yield of products. This problem was remedied by the
simple expedient of using the more hydrolytically robust
triisopropoxy-vinylsilane 5b (R0 = i-Pr) or, even better, tri-t-butoxy-
vinylsilane 5c (R0 = t-Bu). The process now becomes reliable and
allows the introduction of a broad range of functional groups, as
shown by the examples assembled in Table 1. These include
aliphatic and aromatic ketones (6a–j), protected amines (6k and
6m) and amino acid (6l; Boc = t-butoxycarbonyl), amides (6n–p),
imide (6q), heteroaromatics such as pyridine (6r), ester (6s) and even
a complex corticosteroid (6t). In the case of adducts 6h and 6s, the
corresponding O-isopropyl dithiocarbonates were used to perform
the addition, as these were available from another ongoing project.
Imide 6q is in fact an activated amide, which could participate in
crosslinking transformations with polyamines such as a,o-diamines,
triamines, diaminobenzene, etc.12 The possibility of introducing a
simple trifluoromethyl group or a long perfluorinated alkyl chain is
illustrated by examples 6m and 6s. The latter compound would be
very useful for creating super hydrophobic surfaces.13
One implicit condition for the success of the radical additions,
not obvious from the simplified mechanistic scheme (Scheme 1),
is that the initial radical Rꢀ has to be more stable than the adduct
radical 3 (neglecting polar effects, in a first approximation). This
condition has been respected in all the above examples.14 While
keeping in mind this constraint, it is possible in many instances
to take advantage of the presence of the dithiocarbonate group
in the adducts to accomplish a second intermolecular radical
addition and thus obtain even more richly functionalized
products. This modular approach is exemplified by the addition
of dithiocarbonate 7 first to vinylidene carbonate to furnish
intermediate 8,15 followed by a second addition to vinylsilane 5c
to give finally compound 6u containing a protected amine and a
masked diol (Scheme 2). Incidentally, dithiocarbonate 7 was used
to prepare compound 6k (Table 1).
The presence of the dithiocarbonate group in the products
provides a convenient entry into the vast chemistry of organo-
sulfur compounds. Thus, the dithiocarbonate moiety is readily
cleaved by amines to give the corresponding thiol (1,2-diamino-
ethane is especially effective in this respect);16 it may also be
directly oxidized with performic acid to a sulfonic acid;17 or to compound 6v in high yield as a 1 : 1 mixture of diastereoisomers.
simply reductively removed and replaced by a hydrogen atom Reduction with tris-trimethylsilylsilane19 gives sulfur-free bis-
using a number of reducing agents.18 The last transformation is trialkoxysilane 10, a compound that could prove to be useful
illustrated by the two examples shown in Scheme 3. First, a bis- as a crosslinking agent. The second transformation involves
dithiocarbonate 9 is used to perform a double addition leading addition of beta-lactam dithiocarbonate 11 and reduction of
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open new possibilities for surface modification, for supported
catalysts, and for sol–gel formation. It could also simplify the
spectroscopic identification of the modified surfaces through
the inclusion, for example, of easily ‘‘visible’’ aromatic ketones
or fluorinated groups. The presence of the dithiocarbonate
could be exploited not only for further chemical elaboration,
but also as an infrared marker (typical bands at 1220–1240 cmꢁ1
and 1030–1040 cmꢁ1). Elemental analysis for sulfur would further
allow quantitative characterization of the modified structures.
Finally, the possible introduction of a thiol by cleavage of the
thiocarbonate group could be used to construct novel organic–
inorganic hybrid materials, in view of the strong affinity of thiols
for metallic surfaces.20
We thank Nara Institute of Science and Technology for an
internship grant to YY.
Scheme 2 An example of two consecutive intermolecular radical addi-
tions to form a highly functionalized trialkoxysilane.
Notes and references
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the corresponding adduct 6w without prior purification to
afford compound 12 in 61% overall yield.
In summary, we have described a flexible, convergent, atom-
economical, and easily scalable approach for the synthesis of highly
functionalized trialkoxysilanes. Indeed, many of the compounds
obtained in the present study would be very difficult, if not
impossible, to prepare by existing routes. Hitherto inaccessible
trialkoxysilanes and alkoxysilanes in general can now be readily
tailor-made. Even complex and sensitive corticosteroids and
betalactams can be incorporated into the products. The ability
to introduce numerous different functional groups should
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2326 | Chem. Commun., 2014, 50, 2324--2326
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