59th Annual Report on Research 2014

Figure 1: Comparison of the conversion of ester (green, lined) and corresponding acid substrates (blue, solid) demonstrating the positive impact of the acid functional group on product formation.

For the oleic, undecylenic and acrylic acids, epoxide product formed with moderate to high conversions and high selectivities. 4-pentenoic acid and 5-hexenoic acid were oxidized to lactones, most likely via epoxide intermediates.

Scheme 1: Oxidation of 4-pentenoic and 5-hexenoic acids leading to formation of epoxide and lactone.

To probe the impact of ligand structure on the catalysis, we synthesized 5 derivatives of the iron-bpmen complex: 4 complexes with varying substitutions on the pyridine rings and 1 with a substituted piperazine backbone.

Figure 2: Iron complexes synthesized to probe the impact of changes of the ligand framework on oxidation catalysis.

Single crystal analyses showed the impact of the substitution on solid state structures. The iron centers possessed distorted octahedral geometries, owing to the steric requirements of the three 5-membered chelate rings of the tetradentate ligands. The two pyridine donor moieties of the tetradentate ligands were trans to each, and the ligands adopted the cis-a conformation typical of iron complexes with bpmen-type ligands. Of the derivative complexes, only the N,N’-dimethyl-N,N’-bis(5-chloropyridin-2-ylmethyl)-1,2-diaminoethane variant showed appreciable activity. It appeared that increasing the steric bulk of the ligand in the ortho position of the pyridine rings significantly reduced oxidation activity.

Having accomplished the first research objective outlined in the proposal, we started work on the proposal’s second research objective: the incorporation of carboxylic acid functional groups into the catalyst’s ligand framework. Our first targets were the iron complexes of the tripodal ligands shown in Figure 3. We were able to synthesize the alkyl ester variants of these ligands from the corresponding esters of the amino acids glycine, alanine, leucine and phenylalanine.

Figure 3: Ligand framework containing a carboxylic acid functional group for oxidation catalysts.

As expected, iron complexes of these ester ligands did not show any appreciable activity catalyzing the oxidation of the test alkenes cyclohexene and styrene. The syntheses of the carboxylate analogs represent a current effort.

In another ongoing senior thesis project, we started to explore the possibilities of polymerizing the epoxides of fatty esters generated by the epoxidation of oleic acid and esters. Initial efforts have focused on the acrylation of the epoxide moiety and controlling the radical polymerization of the resultant acrylated fatty ester monomers.

The impact of this PRF-funded research project has been multifold. To date, it has permitted me to support the research of 8 undergraduate students during two academic years and over two summers. Of these 8 students, 5 were first-generation students and 4 are from underrepresented minorities.  By many accounts, the experiences have been impactful for the students, and have played important roles in their decisions to continue their studies or explore careers in teaching science. Lilli Morris (funded summer 2013) and Areli Valencia completed their senior thesis research on this project (2013-14). After graduating this summer, Lilli joined the chemistry department at Cornell University as a graduate student, and Areli started teaching high school science in Chicago. Michael Girouard completed a semester-long senior independent research study (fall 2012). After graduation, he joined the Medical Practice Evaluation Center at Massachusetts General Hospital as a clinical research coordinator. Jeff Brewington (summer 2013; other funding) also graduated in 2014 and started his teaching position in Oklahoma for Teach-for-America. Having joined the lab as a sophomore student Tamuka Chidanguro (spring 2013, funded summer 2013) has put his synthetic experience to good use as a research assistant during the 2013-14 academic year, and continues his work in the lab as a current senior thesis research student, albeit on a different project. Jake Huerfano’s experience in the lab during the January term 2014 led him to pursue a senior thesis in my laboratory. He is joined by fellow senior thesis student Claire Lidston (funded summer 2014) and sophomore work-study student Matt Davies (summer 2014, other funding).

The PRF grant has supported our group’s continued efforts of synthesizing classes of closely-related ligands in gram quantities and storing these ligands for future use in other homogeneous catalysis projects, thus contributing to the research infrastructure of my laboratory. During the grant period, I submitted materials for my application for tenure, and the results from this PRF funded research comprised a significant portion of my scholarship. I am happy to report that the application was successful, and I look forward to many years of engaging our students in research activities.