A recyclable, self-supported organocatalyst based on a poly(N-Heterocyclic Carbene)

Article abstract of DOI:10.1021/ja200602e

We report the synthesis and catalytic activity of a polymeric imidazolium salt. In contrast to the well-known resin-supported N-heterocyclic carbenes (NHCs), the material described herein affords a polymer with NHCs orthogonally positioned along a main chain upon generation in situ. The unique structural characteristics of the corresponding poly(NHC)s enabled the materials to display catalytic activities that were similar or superior to those displayed by monomeric analogues. Moreover, the new catalyst was successfully recovered and reused with minimal loss of performance over several cycles.(Figure Presented)

Full text of DOI:10.1021/ja200602e

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A Recyclable, Self-Supported Organocatalyst Based on a
Poly(N-Heterocyclic Carbene)
Adam B. Powell,† Yasuo Suzuki,‡ Mitsuru Ueda,‡ Christopher W. Bielawski,† and Alan H. Cowley*,†
†Department of Chemistry and Biochemistry, The University of Texas at Austin, Austin, Texas 78712-0165, United States
‡Department of Organic and Polymeric Materials, Tokyo Institute of Technology, Tokyo 152-8552, Japan
S Supporting Information
b
ABSTRACT: We report the synthesis and catalytic activity
of a polymeric imidazolium salt. In contrast to the well-
known resin-supported N-heterocyclic carbenes (NHCs),
the material described herein affords a polymer with NHCs
orthogonally positioned along a main chain upon generation
in situ. The unique structural characteristics of the corre-
sponding poly(NHC)s enabled the materials to display
catalytic activities that were similar or superior to those
displayed by monomeric analogues. Moreover, the new
catalyst was successfully recovered and reused with minimal
loss of performance over several cycles.
Figure 1. Representative classes of poly(NHC)s.
Scheme 1. Synthesis of Imidazolium Salt 3 and Poly(3) (R =
9,9-Dihexylfluorene)
ecently, intense interest has emerged in the development of
R
recyclable organocatalysts that may be used to facilitate
reactions which typically require transition metals.1 While resin-
supported systems have found success in this regard, the process of
attaching the catalyst to the solid support is often low yielding and
synthetically challenging.2 Furthermore, such heterogeneous sys-
tems frequently suffer from swelling and mass transport issues and
operate via processes that are distinct from those of their homo-
geneous counterparts.3 A promising method for overcoming the
limitations intrinsic to such approaches is to incorporate an
organocatalyst into a soluble, recoverable polymer.4 Considering
that N-heterocyclic carbenes (NHCs) have been proven to be
powerful organocatalysts for promoting a broad range of synthetic
transformations,5 efforts were directed toward the development of
a recyclable polymeric NHC.6
The synthesis of the aforementioned polymer is outlined in
Scheme 1. Alkylation of dibromoimidazole 1, which was obtained
by treating 4,4-dibromobenzil with paraformaldehyde under acidic
conditions,10 afforded 2 in 90% yield. Palladium catalyzed cross-
coupling copolymerization of this monomer with a commercially
available fluorenyl boronic acid diester (4) was used to obtain
poly(2) in high yield.11 The polyimidazole exhibited a weight
average molecular weight (Mw) of approximately 4500 g/mol, as
determined by gel permeation chromatography,12 and displayed
As shown in Figure 1, there are two primary classes of
polymers which contain NHCs as integral components of the
main chains. The first class comprises diametrically opposed,
ditopic NHCs attached to electrophiles or transition metals,7,8
whereas the second features NHC moieties positioned ortho-
gonally with respect to the polymer backbone, often via
metalation.9 The unique architectures inherent in the latter
increase the likelihood of cooperative or synergistic effects
between each repeating unit along the polymer chain and could
ultimately result in higher efficiencies compared to monomeric
analogues. Moreover, modification of such materials may lead to
“self-supported” organocatalysts that are conveniently recycled
and reused. Herein, we describe a recyclable poly(NHC) poly-
mer that exhibits superior catalytic activity compared to a
monomeric analogue and may be recycled repeatedly with only
marginal losses in efficacy.
1
broad H NMR signals, as expected for a high molecular weight
polymer. Treatment of 2 or poly(2) with iodomethane afforded the
monomeric catalyst precursor 3 (which served as a control; see
below) or poly(3), respectively, in high overall yields. In general, the
isolated imidazolium salts were found to be hygroscopic13 but stable
in air.
With 3 and poly(3) in hand, their abilities to catalyze the
benzoin condensation were compared.14 Initial efforts were
Received: January 20, 2011
Published: March 17, 2011
r
2011 American Chemical Society
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Table 1. Selected Data for the Dimerization of Various
Substrates Using Monomeric and Polymeric Imidazolium
Saltsa
was also isolated as a product of this reaction in good yield
(Scheme 2) upon aqueous workup of the filtrate and further
purification by flash column chromatography. This procedure
was repeated twice with only a marginal loss in catalytic efficiency
per cycle (see Scheme 2).18 In contrast, the monomeric analogue 3
could not be recovered followingthe dimerizationreactionusing an
analogous protocol.
In summary, a catalytically active, “self-supported” poly-
(NHC) precursor was synthesized via methylation following
the cross-coupling copolymerization of 2 with a fluorenyl boro-
nic acid diester. The corresponding catalyst was generated in situ
and used to facilitate the dimerization of several benzaldehyde
derivatives in good yields. The results presented herein not only
demonstrate that NHCs can display enhanced catalytic activities
upon incorporation into a polymeric framework but also should
enable other NHC-based catalysts to be conveniently regener-
ated and reused.
% yieldb of benzoin
% yieldb of benzil
product 5
product 6
substrate
X = H
with 3
with poly(3)
with 3
with poly(3)
67
15
9
67
47
43
52
0
6
2
9
0
10
4
X = m-Br
X = p-CF3
X = p-OCH3
’ ASSOCIATED CONTENT
29
9
a In a typical experiment, 2.94 mmol of substrate, to mol % of 3 or the
repeat unit of poly(3), 15 mol % DBU, and 2.0 mL of dry DMSO were
stirred for 40 h at 25 °C under argon. b Isolated yield.
S
Supporting Information. Additional experimental de-
b
tails and spectroscopic data. This material is available free of
charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Scheme 2. Schematic Representation of the Procedure Used
to Recycle and Reuse Poly(3)
Corresponding Author
cowley@mail.utexas.edu
’ ACKNOWLEDGMENT
We are grateful to the Robert A. Welch Foundation (Grants
F-0003 to A.H.C. and F-1621 to C.W.B.) for their generous
support of this work.
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