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 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. Cl‐ever cats: HCl was used as a hydrogen‐atom‐transfer catalyst precursor and activated to give chlorine radicals by an acridinium photoredox catalyst under visible‐light irradiation in a microtubing reactor, which was effective in retaining the volatile HCl catalyst. This photomediated C−H activation strategy enabled the efficient alkylation and allylation of a variety of unactivated, even primary, C(sp3)−H bonds (see scheme).