Radical-mediated site-selective C−H functionalizations enabled by the unique reactivities of pyridinium salts

Abstract

The importance of introducing functional groups and heteroatoms site-selectively to pyridine derivatives has increased from the past to the present. This thesis describes various radical-mediated reactions using the special properties of pyridinium salts. Pyridinium salts have been used as different roles in the development of five regioselective reactions, respectively. Chapter 1 introduces the selective phosphonation reaction of quinolinones and coumarins using the pyridinium salts as sources of hydrogen atom transfer (HAT) reagents. In chapter 2, the pyridinium salts are used as heterocyclic sources and HAT reagent sources for site-selective phosphonations and carbamoylations. Also the origin of useful regioselectivity is revealed by combining experimental and computational studies. Chapter 3 presents a method for heteroarylation of unactivated remote C–H bonds using the pyridinium salts. In chapter 4, catalyst- and additive-free alkylation, acylation, and carbamoylation by photo-induced charge transfer strategies with the pyridinium salts as electron acceptors and 1,4-DHPs as electron donors. Finally, chapter 5 introduces methods for synthesizing various aryl-alkyl ketones using pyridinium salts as efficient oxidants for deprotonated Breslow intermediates.