Copper–sulfone complexes release carbon radicals under visible light via irreversible MLCT

Researchers from the Shanghai Institute of Organic Chemistry (Chinese Academy of Sciences) and ShanghaiTech University, with Xiao-Song Xue and Jinbo Hu as corresponding authors, report a class of isolable copper–heteroaryl sulfone complexes. According to the material from static-content.springer.com, under visible light these complexes undergo irreversible fragmentation driven by metal-to-ligand charge transfer (MLCT). A C–S bond within the coordinated sulfone ligand breaks and carbon-centered radicals form static-content.springer.comstatic-content.springer.comstatic-content.springer.comOpen source ↗. The work was published in Nature.

MLCT is a fundamental photoinduced process underlying a broad range of energy- and electron-transfer events. Without productive substrate engagement, the MLCT excited state usually relaxes to the ground state through photophysical deactivation, so the process is largely reversible static-content.springer.comstatic-content.springer.comstatic-content.springer.comOpen source ↗. In the new complexes the excited state instead leads to bond cleavage.

Mechanism and scope of radicals

The authors report that spectroscopic and computational studies support the formation of MLCT-derived excited states and a low-barrier pathway for C–S bond cleavage static-content.springer.comstatic-content.springer.comstatic-content.springer.comOpen source ↗. The source data include DFT excitation-energy calculations for a copper complex designated Cu-2. The reactivity is largely independent of the reduction potentials of the corresponding free sulfones static-content.springer.comstatic-content.springer.comstatic-content.springer.comOpen source ↗. This allows the generation of fluoroalkyl radicals and of non-fluorinated tertiary, secondary and primary alkyl radicals, including methyl static-content.springer.comstatic-content.springer.comstatic-content.springer.comOpen source ↗.

Synthetic applications

The authors demonstrate the platform in copper-catalyzed C–H fluoroalkylation of arenes and terminal alkynes static-content.springer.comstatic-content.springer.comstatic-content.springer.comOpen source ↗. In a second application, trifluoromethyl radicals generated through MLCT initiate halogen-atom transfer (XAT) from perfluorocyclobutyl iodide (I-FCB). This gives radical perfluorocyclobutylation–iodination of alkenes, which the authors describe as addressing a longstanding challenge in introducing perfluorocyclobutyl motifs static-content.springer.comstatic-content.springer.comstatic-content.springer.comOpen source ↗.

This is a laboratory study of molecular complexes and model reactions. The available material does not give yields, the number of substrates or the reaction conditions.