Synthesis of Nitrogen Heterocycles by Intramolecular Radical Addition to Alkynes: A Systematic Study

Abstract We report in this letter a systematic study of the synthesis of saturated nitrogen heterocycles by intramolecular radical addition to alkynes. In this perspective, a series of sulfonamides containing both an alkyne and an iodoalkyl subunit was synthesized and subjected to a copper-based photoredox catalyst, [Cu(bcp)DPEphos)]PF6, in the presence of Hunig’s base and under blue light irradiation. In all cases, and regardless of the position of the nitrogen atom with respect to both the alkyne and the iodide, an exclusive exo-dig cyclization was observed, with no traces of competing endo-dig products detectable. This radical cyclization enables the synthesis of azetidines, pyrrolidines, piperidines, and azepanes and, all together, these results provide a useful systematic study of radical digonal cyclizations to nitrogen heterocycles.