Photochemical Functionalization of Strong C–H Bonds

Photochemical Functionalization of Strong C–H Bonds

Author: ChemistryViews

The direct functionalization of C(sp3)–H bonds could be a useful approach to increasing the value of low-cost chemical raw materials. However, due to their high bond dissociation energies, low polarity, and similar chemical environments, functionalizing C(sp3)–H bonds in alkanes often requires harsh reaction conditions, strong redox systems, or expensive catalysts. This can hamper the industrial application of this type of reaction.

Lei Gong, Xiamen University and Innovation Laboratory for Sciences and Technologies of Energy Materials of Fujian Province (IKKEM), Xiamen, China, and colleagues have developed a photochemical strategy for the functionalization of strong C(sp3)–H bonds using readily available allyl bromides and sodium fluoride under visible light (general reaction pictured). The team reacted various toluene derivatives as well as branched and linear alkanes with allyl bromides that bear different electron-withdrawing groups, such as esters, phosphodiesters, phenylsulfonyl, or a cyano group. The reactions were performed in the presence of NaF using dichloromethane as the solvent at 28 °C under LED light (410 nm). The desired products were obtained in moderate to good yields.

The team proposes that NaF–allyl bromide adducts act as light-active species and initiators for a radical-mediated C–H cleavage. The reaction involves the homolytic cleavage of C–Br bonds and the formation of bromine radicals. The generated bromine radicals activate the C(sp3)–H bonds of alkanes via hydrogen atom transfer to produce alkyl radicals, which undergo radical cross-coupling with an allyl radical to give the allylated products. Overall, the work provides an efficient and cost-effective method for the functionalization of C(sp3)–H bonds, without requiring external photosensitizers or a transition metal.


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