Designing and optimizing tandem bio- and chemo-catalyzed cascade reactions to access industrially relevant chemicals in mild, aqueous conditions

Abstract

Biocatalytic transformations offer a number of advantages over traditional synthesis, including facile conversion of renewable feedstocks, high levels of specificity and stereo-control, and obviate the need for expensive metal catalysts, organic solvents, and high temperatures. However, the scope of products accessible from bioprocesses is limited. In recent years, a compelling complementary strategy has emerged: merging non-enzymatic chemistry with microbial metabolism to direct flux and deliver products that would be challenging to synthesize through biosynthetic means alone. The recognition of organocatalysis as a distinct field in the early 2000s sparked a flood of new research featuring small organic molecules as catalysts. These small molecule catalysts work in ambient conditions and are typically non-toxic and environmentally friendly. Moreover, organocatalysts provide access to reactions and mechanisms similar to enzymatic chemistries with greater stability and fewer restrictions. Taken together, organocatalysts provide a broad reactivity with substrate flexibility and a diverse product profile. These features make organocatalysis a particularly attractive candidate for use in hybrid bioprocesses. However, compared to their metallic and inorganic counterparts, the use of organocatalysts has been underexplored. This thesis begins with a comprehensive literature review encompassing the use of non-enzymatic catalysts in conjunction with whole cell biocatalysts. The studies are organized by type of abiotic catalyst and include examples featuring electrocatalysts, photocatalysts, and chemocatalysts. Throughout the review we take note of the challenges faced when interfacing these two strategies and highlight solutions that allow for tandem one-pot systems. The following two chapters feature foundational work pairing whole-cell biocatalysis and organocatalysis in tandem cascades to furnish commercially relevant products. Our initial work describes a one-pot system capable of converting short chain primary alcohols to α,β-unsaturated aldehydes in aqueous conditions. Alcohol substrates are oxidized to the corresponding aldehyde by the biocatalyst, whole cell Gluconobacter oxidans (G. oxidans), followed by an organocatalyzed aldol addition-condensation reaction to furnish the α,β-unsaturated aldehyde. This methodology was applied to a range C6-C10 products. We further show that the inclusion of an organocatalyst redirects flux in the system. Building on our initial system with G. oxidans, we next explored if other oxidative biocatalysts are compatible with organcatalyzed upgrading of butyraldehyde to 2-ethyl-2-hexenal. Two biocatalysts are compared, whole cell K. pastoris (ATCC® 28485™) and isolated enzyme alcohol oxidase (E.C. 1.1.3.13). The reported system improves upon 2-EH titers by 2.8-3.3-fold at maximal yields from the original G. oxidans system. Finally, we probed if other products can be accessed with our oxidative biocatalyst/amino acid system. In this system, we couple biocatalyzed alcohol oxidation to a Henry reaction, dehydration, and conjugate addition to deliver 1,3-dinitroalkanes in a single flask process. Taken together, the studies in this dissertation build the foundation for merging whole-cell and enzyme-biocatalysis with organocatalysis in mild conditions to expand the scope of bio-sourced chemicals.