Development of Novel Organocatalytic Boration Methods and the Employment of Cycloheptatriene as A Versatile Synthetic Building Block

Abstract

This thesis consists of two main parts. The first part focuses on exploring the application of organocatalysts, small non-metallic organic molecules, to facilitate novel borylation reactions. The second part discusses the development and applications of compounds containing the cycloheptatriene (CHT) moiety, an interesting and valuable building block, in organic synthesis. The two parts of this thesis might seem unrelated at the first glance, however they are connected through the common chemistry of tropylium ion, which is one of the organocatalysts for the first part and the building block for the second part. Chapter 1 provides a brief overview and establishes contexts for the research topics presented in the two subsequent parts of this thesis. This chapter discusses the use of tropylium ions and organic superbases as catalysts in organic reactions, as well as the versality of tropylium ions and cycloheptatriene motifs as synthetic building blocks. It further introduces common approaches for catalytic hydroboration and diboration of unsaturated compounds. Chapter 2 discusses a novel protocol for hydroboration reaction of alkynes, alkenes and epoxides, using tropylium salts as organocatalysts. Organoboron compounds are versatile precursors for cross coupling chemistry. Synthesis of these precursors have traditionally been mediated by transition metal or main group catalysts, which poses several issues with toxicity and product purification, as well as mechanistic understanding. Apart from the synthetic value of this metal-free efficient protocol, tropylium salts proved to be useful probes to explore mechanistic details of hydroboration reactions. This work offers interesting insights into the controversial topic of hidden or true catalysis for hydroboration reactions. Chapters 3 delves into the utilization of the highly efficient phosphazene organocatalyst P1−tBu, renowned for its exceptional strength as an organic Brønsted base, to promote 1,1-diboration reactions of electron-deficient terminal alkynes. A combination of thorough experimental and computational studies suggests interesting mechanistic insights for these phosphazene-catalyzed diboration reactions. Chapter 4 investigates a novel approach that harnesses the tropone oxime tosylate as a privileged synthetic building block. It can react with alcohols, phenols, thiols or terminal alkynes to yield a diverse family of Z,Z,Z-triene products through a rarely reported Nucleophile-intercepted Beckmann fragmentation (NuBFr) reaction. Chapter 5 investigates the ring-contraction rearrangement of cycloheptatrienes to transform them into benzyl groups. Substrate scope studies demonstrate the broad applicability of this method to furans and thiophenes. Simultaneously, the rearrangement of aryl or alkyl cycloheptatrienes into benzyl halides using tellurium(IV) halides as Lewis acid as well as halide source was also explored in this chapter. Chapter 6 offers concluding remarks to the thesis.