Applications of Halogen-Atom Transfer (XAT) for the Generation of Carbon Radicals in Synthetic Photochemistry and Photocatalysis

Abstract

The halogen-atom transfer (XAT) is one of the most important and applied processes for the generation of carbon radicals in synthetic chemistry. In this review, we summarize and highlight the most important aspects associated with XAT and the impact it has had on photochemistry and photocatalysis. The organization of the material starts with the analysis of the most important mechanistic aspects and then follows a subdivision based on the nature of the reagents used in the halogen abstraction. This review aims to provide a general overview of the fundamental concepts and main agents involved in XAT processes with the objective of offering a tool to understand and facilitate the development of new synthetic radical strategies.

Transition-metal Free C-N Bond Formation from Alkyl Iodide via Halogen-atom Transfer (XAT): Dual Roles of Diazonium Salts

“…However, >95% recovery of the starting materials 2l and 2m was observed with full decomposition of the diazonium salts under current reaction conditions (Fig 8). Since the in halogen-atom transfer (XAT) of different alkyl halides can be varied according to the bond dissociation energy (BDE) 1 . As a result, the relative rates for halogen atom abstractions was determined by the strength of the C−halogen bonds, and the result of our experiments also support that the trend of halogen atom abstractions is iodides > bromides > chlorides 1,65,66 .…”

“…Carbon radical has been well recognized as an easily-generated, reactive, and variable species in organic synthesis 1 . As a result, numbers of critical transformations via radical process provide wonderfully alternative methods to address the shortcomings in ionic chemistry which one pair of electrons are involved in 1 .…”

“…Carbon radical has been well recognized as an easily-generated, reactive, and variable species in organic synthesis 1 . As a result, numbers of critical transformations via radical process provide wonderfully alternative methods to address the shortcomings in ionic chemistry which one pair of electrons are involved in 1 . For example, homolytic cleavage of C-halogen bonds of a alkyl halide makes it to be possible for the generation of a alkyl radical, which can be further functionalized to access highly valuable alkyl substituted products (Fig 1a).…”

“…Halogen-atom transfer (XAT), which much differs from the above two strategies, provides new avenues for diverse functionalizations based on the bond dissociation energy (BDE) and the polarizability of the C−halogen bond (Fig 1a, down) 1,14,15 . To trigger the homolytic activation of organic halides through XAT process, various abstractors such as -aminoalkyl radicals, catalytic Mn specie, organosiliane, and Tin have been developed recently due to the weak C-halogen energy (50 to 70 kcal/mol for C-I bond) 1,16,17 .…”

Isolated Neutral [4]Helicene Radical Provides Insight into Consecutive Two-Photon Excitation Photocatalysis

“…recently reported that α-aminoalkyl radical, generated via SET between alkyl amines and a close shell excited PC*, can initiate halogen atom transfer (XAT) (Figure 1a, right). 16,17 Therefore, the concept of super photoreducing radicals in conPET systems is weakened due to the common usage of amine as the sacrificial electron donor. Furthermore, a recent study by Nocera and colleagues questioned the viability for radicals generated in-situ during conPET or electrophotocatalysis to act as efficient photocatalysts, due to their short-lived excited state which should hamper their participation in bimolecular reactions with substrates.…”