Visible Light-Mediated Installation of Halogen Functionalities into Multiple Bond Systems

Abstract

Photoredox catalysis has emerged as a useful tool for organic, synthetic, and material chemists to approach target molecules with new functionalities and structural complexity. This review focuses on the recent advances in the visible light‐promoted halogenation and trifluoromethylation/difluoromethylation of multiple bonds. Halogen functionalities are important chemical handles and building blocks in organic synthesis. Both electrophilic addition of halogens and radical halogenations are widely utilized in organic synthesis to install halogen functionalities. However, harsh conditions and low regioselectivity could present challenges. Despite these concerns, recent efforts have led to obvious advances in the visible light‐promoted halogenation of multiple bonds. In this context, we address the recent achievements in the visible light‐mediated installation of different types of halogen‐containing functionalities, which include (i) halogenation of unsaturated carbon‐carbon bonds, (ii) trifluoromethylation and difluoromethylation of π‐systems, and (iii) photoredox cyclization by insertion of fluorine sources. A variety of active catalytic species in the photoredox halogenations are also discussed in terms of their unique reactivity for the future development of highly practical and efficient processes.

Visible-Light-Induced Catalytic Selective Halogenation with Photocatalyst

“…Traditional halogenation methods include addition reactions to multiple bonds, nucleophilic substitution, or radical substitution reactions [ 12 , 13 , 14 , 15 , 16 , 17 , 18 ]. One of the most fundamental halogenation reactions in organic chemistry is the addition of halide reagents to C-C multiple bonds.…”

“…Multiple-bond compounds (alkenes and alkynes) are easily transformed directly to halogenated products by reacting with halogen molecules or hydrohalic acids. However, this technique has significant weaknesses, such as low selectivity, extremely volatility, and the toxic nature of some halogens, and environmental risks [ 13 ].…”

“…To produce a carbon-halogen bond, the C-H bond was broken, and then the hydrogen atom was replaced by a halide anion or radical. Halide substitution reactions often require harsh reaction conditions such as high temperature, inert pressure, or an excess of halogen agent and initiator compounds [ 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 ]. These requirements have increased the purification and treatment costs of the actual halogenation process.…”

“…However, the existence of special reagents has limited the application scope and decreased functional group tolerance. Besides, these reactions have been generally carried out under difficult circumstances with poor atom economy [ 13 , 14 ].…”