Functionalization of the C−H Bond of N‐Heteroaromatics Assisted by Early Transition‐Metal Complexes

This review focuses on direct C−H bond functionalization reactions of N‐heteroaromatic compounds by group 3 and 4 organometallic complexes. Early transition‐metal‐carbon and ‐nitrogen bonds are highly reactive to C−H bond activation via a σ‐bond metathesis pathway to form new organometallic species. By further inserting other unsaturated molecules into the new metal‐carbon bond, C−H bond functionalized products are selectively obtained. To clarify the reaction mechanism, we categorize the reaction by the chelation size of the metallacycle intermediate generated from the organometallic species with N‐heteroaromatic compounds, and include both stoichiometric and catalytic reactions with their reaction mechanism. Alkyl and amido complexes of group 3 and 4 metals show unique reactivity towards C−H bond activation of N‐heteroaromatics to give three‐ to five‐membered metallacycle intermediates via σ‐bond metathesis. Following insertion reactions of various unsaturated bonds, such as C≡C, C=C, and C=N, affords new carbon–carbon bond forming products.