Mechanistic Exploration of N-Heterocyclic Carbene Boranes as the Hydrogen Atom Transfer Reagent in Selective Hydrodefluorination Reactions

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