Borane-Catalyzed C(sp3)–F Bond Arylation and Esterification Enabled by Transborylation

The activation and functionalization of carbon–fluorine bonds represent a significant synthetic challenge, given the high thermodynamic barrier to C–F bond cleavage. Stoichiometric hydridoborane-mediated C–F functionalization has recently emerged, but is yet to be rendered catalytic. Herein, the borane-catalyzed coupling of alkyl fluorides with arenes (carbon–carbon bond formation) and carboxylic acids (carbon–oxygen bond formation) has been developed using transborylation reactions to achieve catalytic turnover. Successful C–C and C–O coupling across a variety of structurally and electronically differentiated arenes and carboxylic acids was achieved using 9-borabicyclo[3.3.1]­nonane (H-B-9-BBN) as the catalyst and pinacolborane (HBpin), with broad functional group tolerance. Experimental and computational studies suggest a mechanistic dichotomy for the carbon–carbon and carbon–oxygen coupling reactions. B–F transborylation (B–F/B–H metathesis) between F-B-9-BBN and HBpin enabled catalytic turnover for carbon–carbon bond formation, whereas direct exchange between the alkyl fluoride and acyloxyboronic ester (C–F/B–O metathesis) was proposed for carbon–oxygen coupling, where H-B-9-BBN catalyzed the dehydrocoupling of the carboxylic acid with HBpin.