Study Goals / Objectives• The overall goal of this work is to develop catalytic systems that can be used in the

synthesis of α,ω-difunctionalized monomers.

• Specific objectives include the synthesis and testing of three NHCs in a metal-based

system.

The material presented here is based upon work supported by the National Science Foundation under Award No. EEC-0813570. Any opinions, findings,

and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National

Science Foundation.

Using N-Heterocyclic Carbenes in Palladium Catalyst SynthesisThrust 3: Chemical Catalyst Design, High-throughput Catalyst Evolution

Maureen Griffin, Eric Hall, Gina Roberts and Dr. Keith Woo

Research Methods• The NHC we used during this experience was an imidazole-based compound.

Although unfunctionalized imidazole does not prove to be a desirable ligand choice,

variations in the R-group can lead to active, Pd-based catalytic systems. Our

challenge is to find the right combination of tethered, substituted imidazole ligands to

produce an active and steric-friendly palladium-based catalytic system.

• As shown in figure 3, imidazole synthesis uses primary amines, glyoxal and

formaldehyde in refluxing methanol. The resulting imidazole (4) was purified by

basifying to pH 9, washing with ether, removal of solvent and recrystallization with

toluene.4

• The tethered imidazole rings (5) were formed via SN2 chemistry by refluxing 4 in

CH2Br2 (figure 4). An alternative synthesis pathway was also used – with limited

success – by reacting 4 with CH2Br2 in the presence of toluene. The product is

purified via filtration of the precipitate and washing with toluene.5

• Pd complexes are formed by refluxing 5 with Pd(OAc)2 in DMSO. Removal of the

DMSO and washing the crude residue with water results in a light yellow powder (6).5

• All structures were verified using H1 NMR.

Study’s Role in Strategic Plan• This project integrates work across all CBiRC research thrusts for the purpose of

creating a catalyst system that uses “green” chemistry strategies.

Expected Milestones and Deliverables• At the conclusion of our research experience, we hope to have a functional NHC-

based palladium catalyst.

Impact• The development of “greener” chemical catalysts will be used in conjunction with the

biological catalysts developed from Thrusts 1 and 2 creating a framework for

producing industrial chemicals from biorenewable resources.

• This work will result in the production of biodegradable monomers that may be used to

synthesize polymers that may be cheaper to produce than the oil-based version.

Introduction & Research Background• Polymers are a class of chemicals known for their wide range of use in industry,

medicine and nearly every other aspect of our society. Traditionally, polymers are

created using petroleum as a starting material. Since the resulting plastics are

extremely durable, accumulation in landfills has become an increasingly significant

problem. Ideally, polymer synthesis should be derived from a renewable biomass

source as well as undergo an environmentally friendly decomposition pathway.1

• One possible way to achieve this is to focus upon the formation of α,ω-

difunctionalized molecules, particularly diesters (2) via transesterification of fatty acid

methyl esters (1) with methanol and CO (figure 1). These monomers have been

applicable in the formation of polymers such as nylon and biodegradable polyesters.

• Established research has achieved this transformation using organophosphines (3),

but these ligands tend to be expensive and sensitive to atmospheric conditions.2

• N-heterocyclic carbenes have proven to be electronically equivalent to

organophosphines in many aspects.3 Our goal is a NHC-Pd system which will prove

to be as reactive at the established phosphine system.

Discussion of Results• During our summer research experience we successfully synthesized three imidizole-

based palladium catalysts using the processes explained above. Each was

characterized and its identify verified using a combination of H1-NMR and gas

chromatography.

• The resulting Pd catalysts were tested, however preliminary results show little to no

activity. This catalyst synthesis pathway will continue to be developed.

Figure 3: Imidazole synthesis

Figure 2: Organophosphine

3

Figure 1: α,ω-difunctionalized molecule synthesis

21

Figure 4: Palladium catalyst synthesis

45

Liquid nitrogen used to cool

during recrystallization.

Air- and water-free “box” used

during Pd catalyst synthesis

reactions

References1. (a) Flieger, M.; Kantorová, M.; Prell, A.; Řezenka, T.; Votruba, J.; Folia Microbiol. 2003, 1, 27-44. (b) Gross, R. A.; Kalra, B.;

Science 2002, 297, 803-807. 2. Rodriguez, C. J.; Eastham, G.R.; Cole-Hamilton, D.; Inorg. Chem. Commun. 2005, 8, 878-881.3. Diez-Gonzalez, S.; Marion, N.; Nolan, S. P.; Chem. Rev. 2009, 109, 3612–3676.4. Liu, J.; Chen, J.; Zhao, J.; Zhao, Y.; Li, L.; Zhang, H.; Synthesis 2003, 17, 2661-2666. 5. Gardiner, M.G.; Hermann, W. A.; Reisinger, C.; Schwartz, J.; Spiegler, M.; J. Organomet. Chem. 1999, 572, 239-247.

6

AcknowledgementsWe would like to thank Iowa State University, the National Science Foundation and

CBiRC for the opportunity to participate in this RET. In addition, we would like to extend

a special “thank you” to Gina Roberts, Dr. Keith Woo and his lab group, Dr. Adah

Leshem-Ackerman and the CBiRC staff.