Visible-Light-Induced Radical Transformations
Date : September 3, 2020 09:00 ~
Speaker : Prof. Eun Jin Cho(Chung-Ang University)
Recently, visible light photocatalysis has attracted substantial attention due to its environmental compatibility and mechanistic versatility in promoting a large number of synthetically important reactions. We have developed a variety of radical transformations using Ru-, Ir-, Pt-based photocatalysts and organophotocatalysts under visible light irradiation. Fluoroalkylated organic compounds play significant roles in the pharmaceutical, agrochemical, and material sciences owing to the substantial influence that fluorine substitution has on the physical and chemical properties of substances. Visible light-induced methods allowed access to fluoroalkyl group-containing molecules, such as –CF3, –CF2R, –CF2SPh, and –CF2OPh groups.1 In the studies, electron deficient carbon-centered fluoroalkyl radicals were successfully generated by the appropriate choice of fluoroalkyl source, photocatalyst, additives, and solvent. Notably, we have observed that additives significantly affect the efficiencies and selectivities of these reactions and can even change the outcome of the reaction by playing additional roles during its course. By understanding the roles of additives, we developed several controlled fluoroalkylation reactions where different products were formed selectively from the same starting substrates. We demonstrated an unprecedented approach for the generation of iminyl radicals via a photocatalytic energy transfer process from readily available heterocyclic precursors.2 This method is distinctive to the previous photoinduced electron transfer approaches to N-centered radicals. In combination with singlet oxygen, the in situ generated nitrogen radical undergoes a selective ipso-addition to arenes to furnish remotely double functionalized spiro-azalactam products. The scope of substrates was explored with the mechanistic elucidation to include the computational studies. In addition, we reported another exclusive energy transfer approach for the development of a challenging radical-radical C(sp3)-N cross-coupling process by reactivity-tuning of the catalytic system.3 The homolytic N-O bond cleavage of oxime esters in the presence of an Ir complex produces acyloxy and iminyl radicals, which underwent decarboxylative cross-coupling to yield valuable imines. The synthetic utility of this method was explored by studying highly functionalized oxime esters, including derivatives of biologically active natural products and drug molecules. Furthermore, in situ transformations of the imine products into pharmaceutically important diarylmethylamines was also demonstrated, showcasing the utility of the imine products as valuable amine building blocks.