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 <i>meta</i>- and <i>ortho</i>-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. <i>In situ</i> generation of the cobalt(I)-boryl complex and treatment with three representative fluoroarenes established <i>meta</i>-selective C(sp²)-H oxidative addition to form predominantly the <i>meta</i> 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, <b>[(</b>^<b>iPr</b><b>ACNC)CoH]</b>_<b>2</b>, borohydride <b>(</b>^<b>iPr</b><b>ACNC)CoH</b>_<b>2</b><b>BPin</b>, or diboryl hydride, <b>(</b>^<b>iPr</b><b>ACNC)CoH(BPin)</b>_<b>2</b> depending on the amounts of B₂Pin₂ and HBPin present. The phosphite derivatives <b>(</b>^<b>iPr</b><b>ACNC)CoH(P(O</b>^<b>i</b><b>Pr)</b>_<b>3</b><b>)</b> and <b>(</b>^<b>iPr</b><b>ACNC)CoBPin(P(O</b>^<b>i</b><b>Pr)</b>_<b>3</b><b>)</b> were prepared and crystallographically characterized. In the catalytic borylation of 1,3-difluorobenzene, <i>ortho</i>-to-fluorine cobalt(I)-aryl and borohydride complexes were identified as resting states despite <i>meta</i>-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 <i>meta-</i>cobalt(I)-aryl was more reactive than the <i>ortho-</i>isomer and accounts for the observed cobalt(I)-aryl resting states. All cobalt(I)-aryl compounds reacted more quickly with HBPin. While <i>ortho</i>-cobalt(I)-aryl compounds yielded arylboronate products with high site-selectivity, <i>meta</i>-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>i</i>) kinetically <i>meta-</i>selective oxidative addition and (<i>ii</i>) faster reaction of the <i>meta</i>-cobalt-aryl isomer with B₂Pin₂. As B₂Pin₂ is converted to HBPin, C(sp²)-H reductive elimination competes against borylation of the <i>meta</i>-cobalt-aryl isomer, resulting in increased <i>ortho</i>-selective borylation.