Recent advances in directed intramolecular C(sp3)-H amination reactions for the construction of aza-heterocycles

Abstract

Saturated N-heterocycles, such as β-lactam, aziridine, pyrrolidine, piperidine and their benzo-structures indoline, tetrahydroquinoline and tetrahydroisoquinoline, are important "privileged scaffolds" which are of pivotal importance to medicinal chemistry. Intramolecular C-H bond amination reaction has emerged as a straightforward and atom-economical alternative for the synthesis of N-heterocycles. Among the existing C-H amination methodologies, the directed C(sp3)-H amination processes manifested unique reactivity and complementary selectivity profiles in comparison to those of the established methods such as Hoffman-Löffler-Freytag reaction and metal nitrenoid chemistry. These features render directed C(sp3)-H amination an appealing and powerful addition to the amination toolbox available to synthetic chemists. Although this field is still in its infancy, its development has already uncovered fundamental understanding of the reactivity and selectivity patterns of various types of C(sp3)-H bonds, upon which its synthetic applicability was elegantly demonstrated through the synthesis of several natural and artificial chemical entities of high therapeutic relevance. Highlighted herein is the recent important developments in the directing group enabled intramolecular C(sp3)-H amination strategies to construct N-heterocycles, with an emphasis on substrate scope and limitation, selectivity and synthetic applications. The reactions are classified based on the types of directing groups employed. The employment of chelating directing groups compensates entropy loss during catalyst-substrate interaction and enables successful activation of unactivated C(sp3)-H bonds and facile access to azetidines, β-lactams and pyrrolidines via the intermediacy of 5/5 and 5/6 bicyclic metallacycles. Besides palladium catalyst, copper and nickel salts have also been reported to realize similar processes with unique selectivities. Notably, amide directed C(sp3)-H amidation reaction under a novel AgI/AgIII redox catalysis furnished the pyrrolidine scaffold via 6-membered metallacycle. It was applied in the efficient construction of the core structures of natural products(-)-codonopsinine and(-)-Martinellic acid in one operation. The use of innate functionality in the substrate as directing group obviates extra installation and removal steps required for C-H activation reactions. Toward this end, catalytic carbonylation and aziridination reactions were developed via the intermediacy of a novel strained 4-membered palladacycle to afford strained aziridines and β-lactams through PdII/PdIV and Pd0/PdII redox pathways, respectively. Taken together, the above developments have opened new avenues toward more efficient and atom-economic synthesis of important N-heterocycles.