Acridine-based photocatalysis in the anti-Markovnikov hydration of unconjugated olefins

Alcohols are a widespread class of organic compounds indispensable to the chemical industry. Although a variety of methods exist for the preparation of alcohols, industrially most alcohols are produced from olefins – an inexpensive and abundant feedstock. Because a monohydric alcohol formally differs from an olefin by one water molecule, alkene hydration represents a natural route to alcohol synthesis. Direct, acid-catalyzed addition of water to olefins, however, follows Markovnikov’s rule and predominantly affords branched isomers. To obtain the linear isomers required for plasticizers, detergents, and other specialty chemicals, indirect strategies have been developed. Their principal limitation lies in the stoichiometric use of reagents other than the alkene and water, leading to wastefulness and reduced cost-efficiency. These shortcomings have motivated the search for catalytic anti-Markovnikov hydration methods. The remarkable advances in photocatalysis made over the past two decades have enabled the realization of numerous anti-Markovnikov hydrofunctionalization reactions. Among these, alkene hydration has been achieved via the intermediate one-electron oxidation of the olefin by potent acridinium photocatalysts. However, the existing systems are significantly restricted in scope, being confined to styrenes, enol ethers, enamines, and other substrates with oxidation potentials below approximately 2.0 V. With most unconjugated alkenes left out of reach, the demand for a general, catalytic anti-Markovnikov hydration of olefins persists. This thesis presents the author’s work aimed at addressing the outlined challenge. Part I focuses on the synthesis of acridine and acridinium catalyst candidates and the evaluation of their photophysical and electrochemical properties. Several structure–property relationships are identified to help better navigate the rational design of photocatalysts based on the acridine scaffold. Part II describes the development of a direct and catalytic anti-Markovnikov hydration of unconjugated tri- and disubstituted olefins. Disubstituted alkenes are hydrated for the first time, while the substrate scope for trisubstituted alkenes is significantly expanded. The advantages and limitations of the method are discussed in detail, along with an analysis of the functional range of olefins within which the catalytic system is effective. Part III outlines several attempts to realize the anti-Markovnikov hydration of monosubstituted alkenes (α-olefins).