Communications
DOI: 10.1002/anie.200702288
Encapsulated Reactive Groups
Encapsulated N-Heterocyclic Carbenes in Silicones without Reactivity
Modification**
Fabien Bonnette, Tsuyoshi Kato, Mathias Destarac, GØrard Mignani, Fernando P. Cossío, and
Antoine Baceiredo*
[
1]
N-heterocyclic carbenes (NHCs) are recognized as powerful
tools in organometallic chemistry as ligands for transition
a few encapsulations of reactive species in hydrophobic
[
7,8]
cages,
which can be kept even in water without decom-
[
2]
[9]
metals as well as in organic chemistry as nucleophilic
position, have been reported. Bowden et al.reported that
Grubbs catalysts occluded in a silicone resin can be used for
[
3]
organocatalysts.
[
10]
The deprotonation of air-stable imidazolium salts A using
the metathesis reaction in water.
For NHCs, chemical
strong bases is the most commonly used method to prepare
protection using a Lewis acid is also effective.The stability of
NHC ligands on transition metals B is very well known.
Crabtree et al.recently developed an air- and moisture-stable
NHC–CO adduct C which can be used as a useful precursor
2
[
5]
for the synthesis of NHC complexes. However, in all these
cases, the NHC–Lewis acid interactions are so strong that the
release of the NHC requires thermal activation, and, to our
knowledge, there are no reports concerning the use of these
protected carbenes in organocatalysis.
In this context, we chose the hydrophobic, chemically
resistant polydimethylsiloxane (PDMS) as a matrix to protect
NHCs.In addition, since silicon compounds show weak Lewis
acid character depending on the substituents on silicon, an
additional chemical stabilization effect on NHCs can also be
air-sensitive NHCs.The development of storable precursors
[
4,5]
[6]
of NHCs
or of original methods to more easily generate
NHCs is an active research area.Herein we describe the first
efficient stabilization of NHCs in silicone derivatives, without
any significant modification of the reactivity compared to the
free species.
expected.In fact, a stable hypervalent NHC–SiR complex D,
4
using chlorosilanes with relatively strong Lewis acid character
[
11]
(SiR = SiCl , SiPh Cl ), has been reported.
4
4
2
2
To conserve highly reactive species, two types of protec-
tion (physical and chemical) can be considered.Physical
protection in paraffin oils is the usual method for air-sensitive
compounds, such as alkali metals or metal hydrides.Recently,
Silicones containing NHCs were prepared by two meth-
ods: either by dissolving the NHC in already prepared PDMS
(10 wt%) or through the polymerization of octamethylcyclo-
[12]
tetrasiloxane (known as D ) catalyzed by the NHC itself in
4
the presence of hexamethyldisiloxane (known as M ) as a
2
[
*] Dr. F. Bonnette, Dr. T. Kato, Dr. A. Baceiredo
Laboratoire HØtØrochimie Fondamentale et AppliquØe (UMR 5069)
UniversitØ Paul Sabatier
blocking agent.
The decomposition of NHCs 1a–d in different media was
1
monitored by H NMR spectroscopy in C D (Table 1).The
6
6
118, route de Narbonne, 31062 Toulouse Cedex 9 (France)
results clearly show that simple physical protection of NHCs
by polymeric compounds is not sufficient to conserve the
carbene as indicated by the relatively rapid degradation
Fax: (+33)5-61-55-82-04
E-mail: baceired@chimie.ups-tlse.fr
Dr. M. Destarac
Rhodia Recherches et Technologies
Centre de Recherches et Technologies d’Aubervilliers
(hydrolysis) of the NHC 1a into the iminoformamide 2a in a
chemically inert polymer, such as paraffin (Table 1, entry 9).
In contrast, using PDMS (Table 1, entry 5), or silicone
52 rue de la Haie Coq, 93308, Aubervilliers Cedex (France)
oligomer MD M (Table 1, entry 10), no decomposition was
Dr. G. Mignani
4
Rhodia Recherches et Technologies
observed after one week, and only a slight amount of
hydrolyzed product (2a, 5–8%) was observed after one
month.However, the NHC 1a in hexamethyldisiloxane M2
85 avenue des Frres Perret, BP 62,
Saint-Fons, 69192 (France)
Prof. F. P. Cossío
(Table 1, entry 7), which possesses weaker Lewis acid silicon
Departamento de Química Orgµnica I
Universidad del País Vasco-Euskal Herriko Unibertsitatea
Facultad de Química, P. K. 1072,
San Sebastiµn–Donostia (Spain)
centers, decomposes more rapidly (complete hydrolysis after
one week).These results clearly demonstrate the importance
of the chemical protection through a silicon–NHC interaction
for the protection of NHCs.Although carbene 1a is stable to
oxidation, carbenes 1b and 1c are much more reactive.
Particularly, 1c is a highly reactive oily product, which slowly
[
**] We are grateful to the CNRS and RHODIA for financial support of
this work. We also thank SGI/IZO-SGIker UPV/EHU for generous
allocation of computational resources.
[13]
decomposes at room temperature. In fact, 1c, the NHC
Supporting Information for this article is available on the WWW
under http://www.angewandte.org or from the author.
with the smallest substituents, quickly oxidized in PDMS
8632
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2007, 46, 8632 –8635
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