THE DEVELOPMENT OF NOVEL INTERMOLECULAR C–C BOND FORMING REACTIONS IN ENZYMES

Abstract

Intermolecular C–C bond forming reactions are incredibly useful reactions in organic chemistry due to their ability to generate complexity quickly to form synthetically valuable products. Enzymes are ideal catalysts to make C–C bonds due to their exquisite selectivity and control of reactive intermediates. However, synthetic chemists cannot make full use of the selectivity of enzymes in intermolecular reactions due to the limited variety of reactions enzymes can access. This thesis endeavors to address this problem by demonstrating how enzymes can use novel gating and activation mechanisms to do C–C bond forming reactions and therefore expand the types of disconnections that are available to the synthetic chemist using enzymes. First, a new gating mechanism is discussed for the hydroalkylation of alkenes. ‘Ene’-reductases (EREDs) can template a radical precursor and alkene in the active site to form a ternary charge transfer complex with the reduced flavin cofactor (FMNhq). Since both coupling partners are required for radical initiation to occur, this ensures fidelity for the C–C coupling over early termination of the radical. This method was extended to the formation of sp3–sp2 C–C bonds by using arenes as the coupling partner. Directed evolution was used to tune both the regioselectivity and the electronics of the charge transfer complex. Next, the electron transfer pathways and photophysics of lactate monooxygenase (LMO) were investigated. The mechanism by which substrate oxidation outcompetes quenching by active site residues and how a covalent flavin adduct forms was elucidated. The formation of the covalent adduct was then leveraged for a photo-oxidation of mandelic acid and the formation of C–C bonds through a SH2 type mechanism. Then, the electronics of the charge transfer complex were further tuned by the bioconjugation of dyes to increase the photoefficiency of intramolecular and intermolecular C–C bond formation. Finally, the synergistic use of pyridoxal 5’-phosphate enzymes (PLP) with photocatalysts is discussed to synthesize chiral α-tertiary amino acids from simple amino acids and Katritzky salts. Mechanistic studies indicate the photocatalyst associates with both the protein and the Katritzky salt to ensure radical initiation occurs close to the active site.