Cooperative C(sp³)–H and C(sp²)–H Activation of 2‑Ethylpyridines by Copper and Rhodium: A Route toward Quinolizinium Salts

A method for the synthesis of substituted quinolizinium salts from 2-ethylpyridines and alkynes is demonstrated. The transformation is conveniently achieved using 1 mol % of a Rh­(III) catalyst along with an excess amount of copper­(II) salt. The reaction gives high product yields with broad substrate scope and functional group tolerance. Detailed mechanistic studies suggest that 2-vinylpyridine is formed in situ from 2-ethylpyridine by a copper-promoted C­(sp³)–H hydroxylation, followed by dehydration. Later, a Rh­(III)-catalyzed pyridine-directed vinylic C­(sp²)–H activation and annulation with alkynes provided the final product