and imidazolium and imidazolinium salts and used
directly as transesterification catalysts.5,6 Furthermore,
as some imidazolium salts are ionic liquids, we have
shown that these ionic liquids can act as phase-segre-
gated precatalyst reservoirs for polymerization reactions.6
More recently, active carbene catalysts have been ther-
mally generated from neutral haloalkane adducts of
NHCs and used in the polymerization of lactide.7 How-
ever, only saturated carbenes can be delivered in this
fashion, and under certain conditions, these compounds
were observed to racemize lactide during polymerization.
Unsaturated NHCs have been successfully bound to
silver, and the resulting complex has been used as a
carbene transfer agent to generate transition metal
carbene complexes.8,9 We envisaged that these silver
complexes might provide a convenient source of carbenes
for organocatalytic applications,9 as the metal transfer
reactions of these complexes occur in mild conditions.
Furthermore, the insolubility of silver chloride is expected
to provide a favorable driving force for generation of the
free carbene.
Silver(I)-Carbene Complexes/Ionic
Liquids: Novel N-Heterocyclic Carbene
Delivery Agents for Organocatalytic
Transformations
Alan C. Sentman, Szila´rd Csihony,
Robert M. Waymouth,*,‡ and James L. Hedrick*,†
IBM Almaden Research Center, 650 Harry Road,
San Jose, California 95120, and Department of Chemistry,
Stanford University, Stanford, California 94305
waymouth@stanford.edu; hedrick@almaden.ibm.com
Received August 18, 2004
Three (imidazol-2-ylidene)silver(I) chloride complexes
1a-c were evaluated for their activity as transesterifi-
cation catalysts. Compound 1a is an ionic liquid at room
N-Heterocyclic carbene (NHC) complexes with silver were
investigated as sources of unsaturated NHC carbene cata-
lysts via thermal decomposition. The NHC complex (1-ethyl-
3-methylimidazol-2-ylidene)silver(I) chloride is an ionic
liquid, and was found to catalyze the ring-opening polym-
erization of lactide at elevated temperatures to give narrowly
dispersed polylactide of predictable molecular weight. Silver-
carbene complexes can also be used for the catalysis of small
molecule transesterification reactions. Thermolysis of the
silver complexes in the presence of CS2 yielded the zwitter-
ionic CS2 adducts of the carbene, implicating the inter-
mediacy of the free carbene in these reactions.
Stable N-heterocyclic carbenes (NHCs) have been a
significant area of study since their description by
Wanzlick1 and Arduengo.2 These compounds have found
use as ligands for transition metal catalysts,3 and as
nucleophilic catalysts for various organic transforma-
tions.4-6 However, these carbenes are sensitive to air and
moisture, which can make the isolation and use of these
compounds difficult. It has been recently demonstrated
that carbenes can be generated in situ from tert-butoxide
temperature, and crystallizes slowly at 10 °C. Compounds
1b and 1c are solids at room temperature. The thermal
stability of these compounds was studied by differential
scanning calorimetry (DSC) and thermogravimetric analy-
sis (TGA). Compounds 1b and 1c do not appear to show
any significant thermal decomposition below 250 °C in
the solid state. Compound 1a, however, exhibits a large
† IBM Almaden Research Center.
‡ Department of Chemistry, Stanford University.
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10.1021/jo048555q CCC: $30.25 © 2005 American Chemical Society
Published on Web 02/16/2005
J. Org. Chem. 2005, 70, 2391-2393
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