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

Abstract

The origin of the meta- and ortho-to-fluorine site-selectivity in the C(sp²)–H borylation of fluorinated arenes with B₂Pin₂ and HBPin promoted by pyridine(dicarbene)cobalt catalysts has been investigated. In situ generation of the cobalt(I)-boryl complex and treatment with three representative fluoroarenes established meta-selective C(sp²)–H oxidative addition to form predominantly the meta 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, [(^iPrACNC)CoH]₂, borohydride (^iPrACNC)CoH₂BPin, or diboryl hydride, (^iPrACNC)CoH(BPin)₂ depending on the amounts of B₂Pin₂ and HBPin present. The phosphite derivatives (^iPrACNC)CoH(P(OⁱPr)₃) and (^iPrACNC)CoBPin(P(OⁱPr)₃) were prepared and crystallographically characterized. In the catalytic borylation of 1,3-difluorobenzene, ortho-to-fluorine cobalt(I)-aryl and borohydride complexes were identified as resting states despite meta-to-fluorine borylation being the major product of catalysis. Deuterium kinetic isotope effects support irreversible but not turnover-limiting C(sp²)–H oxidative addition. Stoichiometric borylation of isolated cobalt(I)-aryl intermediates with B₂Pin₂ established that the meta-cobalt(I)-aryl was more reactive than the ortho-isomer and accounts for the observed cobalt(I)-aryl resting states. All cobalt(I)-aryl compounds reacted more quickly with HBPin. While ortho-cobalt(I)-aryl compounds yielded arylboronate products with high site-selectivity, meta-cobalt-aryl counterparts yielded a mixture of arylboronate isomers and free arene. Deuterium labeling experiments with DBPin confirmed that HBPin mediates reversible C(sp²)–H oxidative addition. Thus, the overall site-selectivity arises from two reinforcing effects: (i) kinetically meta-selective oxidative addition and (ii) faster reaction of the meta-cobalt-aryl isomer with B₂Pin₂. As B₂Pin₂ is converted to HBPin, C(sp²)–H reductive elimination competes against borylation of the meta-cobalt-aryl isomer, resulting in increased ortho-selective borylation.