The preparation of unnatural amino acids has been a topic that has remained at the forefront of synthetic chemistry for decades. While many methods for their preparation have been introduced, one of the more general and robust approaches to their synthesis relies on the metal complexation of a glycine Shiff’s base which serves to protect the amino group and carboxyl groups of the glycine. This approach has shown many strengths, opportunity remains to improve this technology, primarily increasing the acidity of the C-H bond of the α-carbon of the glycine. In order to overcome these limitation and to improve the method a series of metal complexed Schiff bases of glycine have been prepared which contain an electron withdrawing moiety in this ligand. This alteration has proved to increase the reactivity of the complexes, however the pKa of the secrecies have not been investigated. Therefore, the goal of this project is to quantify the changes in pKa of previously synthesized ligands with the presence or absence of electron withdrawing groups attached to the molecule in organic solvents. Challenges in determining the pKa’s of these ligands include monitoring the equilibrium of a weak acid-weak base chemical reaction and the use of different methods in determining the acidity of a proton in organic solvent conditions, since common aqueous methods are not possible. We are applying proton nuclear magnetic resonance (1H-NMR) and visible spectroscopy as means to monitoring the weak acid-weak base interaction and to quantify the pKa by proton integration and wavelength absorbance changes, respectively. The results of our studies and potential applications of the findings with respect to the ligand will be discussed from this work.

Method development and determination of metal complex pKa's by NMR and visible spectroscopy

2019 Spring National Meeting

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