Photoinduced Carbon Radical-Medicated C-H Functionalization and Cascade Reactions by Pyridine <i>N</i>-oxide

<p dir="ltr"> Pyridine <i>N</i>-oxides and their derivatives are readily available and highly versatile reagents that have attracted significant attention in organic synthesis. Chapter One first reviews the applications of pyridine <i>N</i>-oxides in traditional two-electron chemistry, such as applications as oxidants, ligands for metal complexes and organocatalysts. However, investigations of unconventional single-electron transfer chemistry of pyridine <i>N</i>-oxides in synthesis and catalysis has long been overlooked until recently. The second section of Chapter One focuses on the recent synthetic development in single-electron transfer chemistry of pyridine <i>N</i>-oxides via the enabling technology of modern photoredox catalysis and photochemistry, including the applications of pyridine <i>N</i>-oxides 1) in the development of visible light-promoted <i>β</i>-oxyvinyl radical generation and corresponding cascade reactions and 2) photoinduced selective C-H functionalization by pyridine <i>N</i>-oxide based hydrogen atom transfer (HAT) catalysis. In Chapter Two, the development of easily accessible pyridine <i>N</i>-oxide derivatives as efficient photoinduced HAT catalysts for selective C–H functionalizations is presented. This method is efficient for a wide range of C-H functionalization and exhibited remarkable versatility. This study further demonstrates that catalyst-controlled site selectivity of C–H functionalization is possible upon alterating the steric structure of pyridine <i>N</i>-oxides. In Chapter Three, the synthetic utility of photocatalytically generated <i>β</i>-oxyvinyl radical from 1,6-enynes and pyridine <i>N</i>-oxides are demonstrated in developing divergent radical cascade annulation reactions. By using an acridinium photocatalyst and 2,6-lutidine<i> N</i>-oxide as a source of oxygen-centered radical, various 1,6-enynes, including oxygen-tethered and nitrogen-tethered ones, 1-allyl-2-ethynylbenzenes, were suitable substrates for producing a series of tetrahydroindeno[2,1-c]pyran-4-ones, tetrahydroindeno[2,1-c]pyridin-4-ones, and 1<i>H</i>-cyclopropa[b]naphthalen-2(1<i>aH</i>)-ones in good to high yields with high diastereoselectivties. In Chapter Four, a photoinduced selective hydroamination of ynamides with nitrogen heteroaromatic nucleophiles is disclosed by using an organocatalytic photoredox system; a direct method to construct a diverse of (Z)-<i>α</i>-azole enamides from ynamides and pyrazoles, as well as triazoles, benzotriazoles, indazoles, and tetrazoles, is developed.</p>