Molecular Oxygen Beyond Oxidation: Non-Oxidative and Borderline Transformations in Organic Synthesis

Dioxygen is vital for life and a key oxidant in biological systems. Its importance, however, extends beyond living processes; it has been harnessed for diverse applications since antiquity. In synthetic chemistry, oxygen-mediated oxidations are fundamental transformations. Nevertheless, molecular oxygen can also facilitate reactions without a net oxidative outcome. Intriguingly, this role, which defies conventional portrayal of oxygen as an oxidizing agent, has occasionally been noted in the literature. In some cases, oxygen mediates or induces reactions in which the oxidation state of the target organic compound remains unchanged, or the overall transformation does not involve the net incorporation of oxygen or the removal of hydrogen (or hydrogen equivalents) from the organic substrate. In these transformations, oxygen facilitates formally redox-neutral reactions or processes in which the combined number of heteroatoms, π bonds, and ring structures in the product does not exceed that in the reactant. This review covers oxygen-mediated or -induced reactions that meet these criteria. By presenting mechanistic insights and experimental investigations, it highlights behavior of dioxygen in these transformations. This analysis broadens our understanding of versatility of oxygen in synthetic chemistry and supports the development of more sustainable chemical processes.