Microtubing‐Reactor‐Assisted Aliphatic C−H Functionalization with HCl as a Hydrogen‐Atom‐Transfer Catalyst Precursor in Conjunction with an Organic Photoredox Catalyst

Chlorine radical, which is classically generated by the homolysis of Cl₂ under UV irradiation, can abstract a hydrogen atom from an unactivated C(sp³)?H bond. We herein demonstrate the use of HCl as an effective hydrogen‐atom‐transfer catalyst precursor activated by an organic acridinium photoredox catalyst under visible‐light irradiation for C?H alkylation and allylation. The key to success relied on the utilization of microtubing reactors to maintain the volatile HCl catalyst. This photomediated chlorine‐based C?H activation protocol is effective for a variety of unactivated C(sp³)?H bond patterns, even with primary C(sp³)?H bonds, as in ethane. The merit of this strategy is illustrated by rapid access to several pharmaceutical drugs from abundant unfunctionalized alkane feedstocks.