Abstract
The origin of the meta- and ortho-to-fluorine site-selectivity in the C(sp(2))-H borylation of fluorinated arenes with B(2)Pin(2) 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(sp(2))-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, [((iPr)ACNC)CoH](2), borohydride ((iPr)ACNC)CoH(2)BPin, or diboryl hydride, ((iPr)ACNC)CoH(BPin)(2) depending on the amounts of B(2)Pin(2) and HBPin present. The phosphite derivatives ((iPr)ACNC)CoH(P(O(i)Pr)(3)) and ((iPr)ACNC)CoBPin(P(O(i)Pr)(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(sp(2))-H oxidative addition. Stoichiometric borylation of isolated cobalt(I)-aryl intermediates with B(2)Pin(2) 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(sp(2))-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 B(2)Pin(2). As B(2)Pin(2) is converted to HBPin, C(sp(2))-H reductive elimination competes against borylation of the meta-cobalt-aryl isomer, resulting in increased ortho-selective borylation.