This project will develop novel dinuclear catalysts for asymmetric ring opening reactions such as hydrolytic kinetic resolution (HKR) of terminal epoxides. It employs a highly attractive supramolecular approach which enables rapid construction of multinuclear systems in solution via self-assembly of monomeric units. It aims to develop a highly efficient, green process of HKR of epoxides by enabling solvent-free conditions, very low catalyst loading and catalyst recovery/recycle. First, new Schiff base ligands capable of self-assembly through urea-urea hydrogen bonding will be designed and synthesized. Second, the effectiveness of the self-assembled dinuclear catalysts in hydrolytic kinetic resolution of epoxides will be evaluated. Third, detailed self-association studies will be conducted on the bis-urea functionalized Schiff base metal complexes in solution to investigate the correlation between self-assembly and HKR reaction rate. In addition, organometallic gelation will be explored by introducing additional noncovalent bonding interactions to the bis-urea functionalized Schiff-base metal complexes.
With the support of this award from the Chemical Synthesis Program, Professor Sukwon Hong, of the Department of Chemistry at the University of Florida, is developing highly efficient, environmentally friendly processes to convert inexpensive feedstocks to valuable synthetic building blocks under solvent free conditions using small amounts of recyclable catalysts. The proposed research takes a highly innovative approach by merging two different disciplines of chemistry: supramolecular chemistry with transition-metal catalyzed transformations. This interdisciplinary strategy using hydrogen bonding to assemble metal catalysts will provide a new efficient way to bring two metal active sites in close proximity which is a prerequisite for the bimetallic transformations to occur. This novel strategy can be applicable to a wide range of asymmetric reactions and can have a significant impact on the development of more efficient, environmentally friendly industrial processes for important chiral molecules. In addition, this project will provide an excellent opportunity to educate and train undergraduate and graduate students including underrepresented groups owing to its multidisciplinary nature.
Discovering new catalysts and ligands for the development of new reaction technology is of key importance to the scientific community as it can impact a multitude of fields by enabling the preparation of advanced materials. While homogenous catalysts are characteristically assumed to be monomeric in solution when they are developed, they are often found to be of higher order upon further study. With this grant from the National Science Foundation, we established a new ligand design that incorporates structural elements that promote catalyst-catalyst interactions to enhance the association of these species when needed in specific reactions. More explicitly, our group developed novel self assembled dinuclear catalysts through reversible hydrogen bonding for the production of important chiral building blocks (see inset picture). This supramolecular approach proved to be highly efficient for the generation of multimetallic catalysts in solution simply by mixing the monomer units. These unique self-assembled catalysts provide an efficient strategy for overcoming the inherent limitations of bimetallic transformations that obviates the need for covalently linking two catalysts or designing complex ligands with separate binding sites for two metal centers. Two common limitations of traditional bimetallic transformations include the requirement for dilute reaction conditions with respect to substrate and/or high catalyst loadings to increase the interactions between two catalysts. This method is particularly powerful because successfully achieving these goals would enable solvent-free reaction conditions with very low catalyst loadings (0.01 mol %), and possibly the ability to recycle the catalyst. Addressing these limitations will help the chemical industry become more green by substantially reducing the carbon footprint (by eliminating the need for large amounts of solvent) and sustainable (by reducing the need for large amounts of catalysts). A series of cobalt complexes have been developed, for reactions that need activation by two cobalt metal centers. These bis-urea (salen)Co catalysts resulted in rate acceleration (up to 13 times in comparison to the monomeric catalyst) in the hydrolytic kinetic resolution of epichlorohydrin in THF by facilitating cooperative activation through intermolecular urea-urea hydrogen bonding interactions. In addition, the bis-urea (salen)Co(III) catalyst efficiently resolves various terminal epoxides even under solvent-free conditions, which required much shorter reaction time at low catalyst loading (0.03-0.05 mol %). Results from kinetic and mechanistic studies are consistent with the idea that self-assembly through urea-urea hydrogen bonding is responsible for the observed rate enhancement. The self-assembly study with the bis-urea (salen) Co by FT-IR and with the corresponding (salen)Ni complex by 1H NMR showed that intermolecular hydrogen bonding interactions existed between bis-urea scaffolds in THF. This work demonstrates that hydrogen bonding can be utilized to construct chiral bimetallic catalysts. The most important aspects of this work are the establishment of a new ligand design that can be employed in a variety of other systems and the training and professional development opportunities that it provided for graduate students and postdoctoral researchers.
