The introduction of chlorine atoms into organic molecules is fundamental to the manufacture of industrial chemicals,the elaboration of advanced synthetic intermediates and also the fine-tuning of physicochemical and biological properties of drugs,a grochemicals and polymers.W er eport here ag eneral and practical photochemical strategy enabling the site-selective chlorination of sp 3 CÀHb onds.T his process exploits the ability of protonated N-chloroamines to serve as aminium radical precursors and also radical chlorinating agents.U pon photochemical initiation, an efficient radicalchain propagation is established allowing the functionalization of ab road range of substrates due to the large number of compatible functionalities.T he ability to synergistically maximize both polar and steric effects in the H-atom transfer transition state through appropriate selection of the aminium radical has provided the highest knownselectivity in radical sp 3 C À Hc hlorination.

The introduction of chlorine atoms into organic molecules is fundamental to the manufacture of industrial chemicals, the elaboration of advanced synthetic intermediates and also the fine‐tuning of physicochemical and biological properties of drugs, agrochemicals and polymers. We report here a general and practical photochemical strategy enabling the site‐selective chlorination of sp3 C−H bonds. This process exploits the ability of protonated N‐chloroamines to serve as aminium radical precursors and also radical chlorinating agents. Upon photochemical initiation, an efficient radical‐chain propagation is established allowing the functionalization of a broad range of substrates due to the large number of compatible functionalities. The ability to synergistically maximize both polar and steric effects in the H‐atom transfer transition state through appropriate selection of the aminium radical has provided the highest known selectivity in radical sp3 C−H chlorination.