UC Riverside

Abstract

A central goal of modern sustainable chemistry is to convert feedstock chemicals to more functionalized and useful products in a mild and selective manner. The most desirable transformation is the oxidation of linear unactivated hydrocarbons with regioselectivity, i.e. the ability to selectively oxidize one position on the hydrocarbon chain. This is bioinspired: enzymes are capable of selectively transforming one specific C-H bond in the presence of other equivalent bonds in the same molecule via proximity- based interactions. Bioinspired catalysts are known to effect C-H oxidation in mild conditions, but obtaining regioselectivity relies heavily on functional groups to direct the oxidation.

The main goal of this project is to combine the concepts of molecular recognition and oxidation catalysis to create a small molecule mimic of enzymatic CH oxidation processes that is capable of both recognition of a hydrocarbon substrate, and catalyze its mild, regioselective oxidation. This involves the study of both hydrocarbon recognition, and hydrocarbon oxidation catalysis. The first approach was to create an oxidation catalyst containing a multi-metal catalytic center. The preorganized ligand scaffold is capable of coordinating multiple Fe(II) centers to form an electrophilic CH oxidation catalyst. This catalyst oxidizes unactivated hydrocarbons including simple, linear alkanes under mild conditions in good yields with selectivity for the oxidation of secondary CH bonds. To introduce molecular recognition elements to these active catalytic metal complexes, functionalized self-folding cavitands with metal coordinating groups were synthesized. The upper rim donors allow controlled noncovalent binding of suitably sized guest species via both self-complementary hydrogen bonding and space-filling interactions, and metal-mediated self-folding is possible if bidentate coordinators are incorporated. The functionalized cavitands were further explored as biological sensors for trimethylammonium species such as choline and trimethyllysine. In-depth mechanistic studies of the molecular motion of substrates inside the cavity were performed in different environments, notably in lipid micelles and magnetically ordered bicelles using 2D NMR. The in/out exchange mechanism and up/down kinetic motion of guests in cavitand scaffolds could be quantified.