Design, Synthesis and Applications of Biomimetic Flavin-based Organocatalysts

Download or read book Design, Synthesis and Applications of Biomimetic Flavin-based Organocatalysts written by Mohammad Shawkat Hossain and published by . This book was released on 2019 with total page 439 pages. Available in PDF, EPUB and Kindle. Book excerpt: Oxidation reactions are one of the most important transformations in synthetic chemistry. Most commonly used oxidants are metal oxides, which are neither green nor selective towards multi-functional molecules. The efficiency of the reactions can be improved by the introduction of a catalyst by lowering the activation energy. In nature, oxidation reactions are highly specific and are controlled by enzymes, co-enzymes, and availability of a renewable terminal oxidant, molecular oxygen. Among the various enzymatic oxidative transformations, our research was inspired by the oxidations controlled by co-enzymes FMN or FAD in the presence of flavoproteins and molecular oxygen. Flavin mimics were often studied/used to oxidize heteroatoms; however, the goal of this research was initialy focused on mimicking other chemoselective oxidations of flavins, primarily at carbon centers. In chapter one, the function of natural flavins in nature and different approaches to perform nucleophilic and electrophilic oxidation using artificial flavin is described. In chapter two, flavin reactivity was extended to the Dakin oxidation of electron-rich benzaldehydes. This conversion of aryl aldehyde to phenol worked efficiently under mild conditions with either hydrogen peroxide or molecular oxygen as terminal oxidant. The mechanism was understood by various reaction rates studies. Substituent effects of catalysts and substrates aided in the identification of the rate determining step. The order of reagents and extensive pH studies reveal the various way flavin catalysts enable oxo-transfer under mild, more neutral conditions, by altering both the pKa of the active nucleophilic oxygen source, and by lowering the energy of the oxide leaving group, as previously hypothesized by Briuce and coworkers. In chapter three, pre-heteroaromatic compounds (initially investigated as possible reducing agents for flavin mimics) were transformed to their aromatic form aerobically using flavin catalyst in arguably the most mild methods described to date. Chapters four through six describe the preparation and function of various classes of flavins. Chapter four discussed the bioactivity of new flavin species against cancerous and normal breast cells. Chapter five details the preparation of water-soluble classes of flavins and their photochemical properties (O2 activation and Methyl Orange degredation). And Chapter six discusses the preparation of redox active flavins that were prepared for tethering to metal oxide photocatalyst and various electrode surfaces. Chapter seven describes a new preparation of 5-deazaflavin cofactors - deazariboflavin, 8-hydroxydeazariboflavin (an F420-precursor), and 10-methyl deazaflavin. An improved route to these species is described, and their redox activity were investigated in F420-dependent enzymes. Finally, chapter eight shows a new, concise and high yielding synthetic route explored for the synthesis of flavin catalysts, which includes the synthesis of different o-phenylenediamine derivatives and alloxan derivatives.