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Multicomponent Reactions of Pyridines To Give Ring-Fused Pyridiniums: In Situ Activation Strategy Using 1,2-Dichloroethane as a Vinyl Equivalent.

Reported herein is a rhodium(III)-catalyzed three-component annulation reaction of simple pyridines, alkynes, and 1,2-dichloroethane (DCE), affording a streamlined pathway to diverse ring-fused pyridiniums. DCE not only serves as a vinyl equivalent but also as an in situ activating agent for pyridine C2-H activation. A cationic five-membered rhodacycle complex has been isolated and proposed as a possible intermediate. This strategy can be extended to other N-containing heteroarenes for the synthesis of multiring-fused pyridiniums. These multicomponent reactions exhibit excellent regioselectivity for 1,3-diynes, paving a path to the cascade cyclization of 3-fluoropyridine or N-methylpyridin-3-amine with 1,3-diynes for the construction of brand-new tricyclic-fused pyrano- or hydropyridoquinolizinium salts. These ionic fluorophores have been investigated as potential biomarkers.