Mechanistic Exploration\nof <i>N</i>‑Heterocyclic\nCarbene Boranes as the Hydrogen Atom Transfer Reagent in Selective\nHydrodefluorination Reactions

In the modern era of organic synthesis, mechanisms centered\non\nradical intermediates have become increasingly impactful. Among all\nof these, hydrogen atom transfer (HAT) represents one of the most\nfundamental chemical reaction steps and has found applications in\ndesigning practical transformations. Herein, we present a detailed\ncase study on selective hydrodefluorination of trifluoromethylarenes\nutilizing <i>N</i>-heterocyclic carbene boranes (NHC-boranes)\nas the hydrogen atom donor. Under the optimal conditions featuring\nan acridine-based photocatalyst, complete selectivity for mono-hydrodefluorination\nwas achieved across a wide array of substrates. Comprehensive mechanistic\nstudies combining experimental and computational approaches disproved\na chain process involving fluorine atom transfer but rather pointed\nto a HAT non-chain mechanism, where the key step involves the difluorobenzylic radical abstracting\na hydrogen atom from the NHC-borane to generate a boryl radical in\na polarity-matched fashion. Evaluation of a selection of Lewis base-ligated\nboranes revealed molecular descriptors critical to the outcomes of\nthis reaction, and a classification model was built to explain the\nstructure–reactivity relationship and how various elementary\nsteps can be influenced. These results collectively provide valuable\ninformation for future reaction design to increase the utility of\nboranes in organic radical chemistry.