Cu/TEMPO-Catalyzed Direct C2-Quaternization of Pyridines with Aldehydes via Sequential N–N Bond Formation and Cleavage

Abstract Pyridine represents an indispensable class of nitrogen-containing heteroarene, yet regioselective construction of an all-carbon quaternary center at the C2 position has remained a longstanding synthetic challenge. Here, we report a Cu/TEMPO-catalyzed regioselective C2 quaternization of pyridines with readily available aldehydes via sequential formation and cleavage of N–N bonds. Specifically, the reaction involves the following: (i) N–N bond formation from pyridine and O-mesitylenesulfonylhydroxylamine (MSH) to the N-aminopyridinium (isolable or in situ), (ii) dearomatization with aldehydes via a formal [3 + 2] annulation, and (iii) Cu/TEMPO-promoted N–N bond cleavage, affording C2-quaternary products. This method efficiently delivers sterically congested C2 quaternary scaffolds with a broad substrate scope and good functional group tolerance, allowing chiral framework assembly using β-chiral aldehydes. Combined experimental and computational mechanistic studies elucidate a charge density-governed pathway responsible for the high C2 selectivity. Avoiding prefunctionalization of pyridine substrates, the protocol achieves chemo- and regioselective functionalization of electronically distinct pyridine motifs and late-stage modification of bioactive molecules, providing a versatile strategy for the structural diversification of azaarenes toward pharmaceutical and agrochemical applications.