The transformation of alcohols to their corresponding halides is among the most widely utilized reactions in organic synthesis. Various methods employ reagents such as thionyl chloride, phosphorus halides, and Vilsmeier-Haack reagents. The traditional Appel reaction typically uses PPh3 in combination with an electrophilic halogen source such as CCl4, CBr4, or I2. In 2011, the Stephenson group demonstrated the use of ruthenium(II)-based photocatalysts under visible light for the conversion of alcohols to halides. In 2018, Collins and co-workers explored the use of copper (I) complexes for the photocatalytic Appel reaction in continuous flow (Figure ). They evaluated a library of 50 different homoleptic diamine and heteroleptic diamine diphosphine copper(I) catalysts for this transformation and found that [Cu(tmp)(BINAP)]BF4 under purple LED irradiation was the most efficient. This continuous flow method was also applied to synthesizing anhydrides from benzoic acids, achieving yields of 90%.