Aryl boronic acids and esters are valuable intermediates for the broad chemistry community as building blocks for synthesis due to the wide variety of valuable transformations the C–B bond. The state-of-the-art methodology for generating these compounds is through iridium-catalyzed C–H activation borylation (CHB). Traditional systems activate the least sterically hinder C–H bond to activate, but many systems have been developed that can selectively activate ortho, meta, and para to several functional groups. To date, however, there are a limited number of systems using iridium that can selectively activate fluorinated aromatics due to its weak electrostatic interactions and small atomic size. The work described within detail a novel dipyridyl hydrazone ligand (dmadph) that can borylate fluoroarenes with increased selectivity for C–H activation meta to fluorine. This ligand also generates catalysts that are significantly more active than those generated using the most common ligand, 4,4′-di-tert-butyl-2,2′-bipyridine (dtbpy). Investigations into this novel ligand framework led to the discovery of an unusual effect of hydrogen pressure generated during CHB on the observed regioselectivity of the reaction, further increasing C–H activation meta to fluorine. The hydrogen pressure generated during CHB enabled an iridium-catalyzed transfer borylation, or isodesmic borylation, of arenes. These are the first examples reported that demonstrate these effects in iridium-catalyzed systems. Due to the activity of the catalysts generated with dmadph, the pre-assembled catalyst was synthesized and isolated and revealed an unusual coordination mode to iridium. Parallel conversion kinetic isotope effect experiments revealed a primary kinetic isotope effect for the C–H borylation. NMR experiments during catalysis, however, identified an Ir–fluoroaryl complex, suggesting it is a resting state for the transfer borylation process and thus operates separate to the canonical CHB mechanism.
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