Photoreductive Alkylative Dearomatization of N-Alkyl Pyridin-1-ium Salts: Site Selective Access to 4-Alkyl 1,4-Dihydropyridines

A visible light photoreductive dearomatizative alkylation of N-alkyl pyridinium salts for producing 4-alkyl-1,4-dihydropyridines is described. The reaction harnesses N-alkyl pyridinium salts both as the alkyl radical precursors to initiate the reaction and as the 1,4-dihydropyridin-4-yl-based alkylating reagents to terminate the reaction. In contrast to the reported electroreductive procedure via the key dihydropyridine radical intermediates, mechanistic studies demonstrate that this photoreductive dearomatizative alkylation proceeds via directly alkyl radical addition across the pyridinium ring followed by single electron oxidant (SEO) and dearomatization.