The Role of Boron Reagents in Determining the Site-Selectivity of Pyridine(dicarbene) Cobalt-Catalyzed C-H Borylation of Fluorinated Arenes.

The origin of the - and -to-fluorine site-selectivity in the C(sp)-H borylation of fluorinated arenes with BPin and HBPin promoted by pyridine(dicarbene)cobalt catalysts has been investigated. generation of the cobalt(I)-boryl complex and treatment with three representative fluoroarenes established -selective C(sp)-H oxidative addition to form predominantly the isomers of the corresponding cobalt(I)-aryl complexes. Attempts to observe or isolate the four-coordinate cobalt(I)-boryl complex yielded the cobalt-hydride dimer, , borohydride , or diboryl hydride, depending on the amounts of BPin and HBPin present. The phosphite derivatives and were prepared and crystallographically characterized. In the catalytic borylation of 1,3-difluorobenzene, -to-fluorine cobalt(I)-aryl and borohydride complexes were identified as resting states despite -to-fluorine borylation being the major product of catalysis. Deuterium kinetic isotope effects support irreversible but not turnover-limiting C(sp)-H oxidative addition. Stoichiometric borylation of isolated cobalt(I)-aryl intermediates with BPin established that the cobalt(I)-aryl was more reactive than the isomer and accounts for the observed cobalt(I)-aryl resting states. All cobalt(I)-aryl compounds reacted more quickly with HBPin. While -cobalt(I)-aryl compounds yielded arylboronate products with high site-selectivity, -cobalt-aryl counterparts yielded a mixture of arylboronate isomers and free arene. Deuterium labeling experiments with DBPin confirmed that HBPin mediates reversible C(sp)-H oxidative addition. Thus, the overall site-selectivity arises from two reinforcing effects: () kinetically selective oxidative addition and () faster reaction of the -cobalt-aryl isomer with BPin. As BPin is converted to HBPin, C(sp)-H reductive elimination competes against borylation of the -cobalt-aryl isomer, resulting in increased -selective borylation.