Chapter 10: Reactions Involving Carbenes and Nitrenes

Carbenes are generated through multiple pathways including photochemical decomposition, thermal activation, and diazo compound fragmentation, each method producing species with distinct electronic configurations. The chapter distinguishes between singlet carbenes, which feature paired electrons and exhibit characteristic insertion behavior, and triplet carbenes, which possess unpaired electrons and engage in different reaction pathways. Cyclopropanation of alkene substrates represents one of the most synthetically valuable carbene transformations, forming three-membered rings through stereospecific addition mechanisms. Beyond cyclopropanation, carbenes insert into carbon-hydrogen bonds and heteroatom-hydrogen bonds, enabling the construction of novel carbon-carbon and carbon-heteroatom linkages that would be difficult to achieve through conventional synthetic routes. Nitrenes, the nitrogen analogues of carbenes, are similarly short-lived species generated from azide precursors or isocyanate substrates, and they participate in aromatic substitution reactions, insertion processes comparable to those of carbenes, and ring expansion transformations that enlarge existing ring systems. The chapter emphasizes mechanistic understanding by analyzing how electronic structure and spin state determine reactivity patterns and selectivity outcomes. Additionally, the treatment of transition-metal coordinated carbene and nitrene complexes highlights how metal coordination stabilizes these intermediates, enabling catalytic cycles for asymmetric synthesis and regioselective transformations. Throughout the discussion, practical synthetic examples demonstrate how these reactive intermediates overcome limitations of traditional methods, facilitating the construction of complex molecular architectures essential for pharmaceutical and materials applications.