Mechanistic Studies of Cobalt-Catalyzed C(sp2)–H Borylation of Five-Membered Heteroarenes with Pinacolborane

Studies into the mechanism of cobalt-catalyzed C­(sp2)–H borylation of five-membered heteroarenes with pinacolborane (HBPin) as the boron source established the catalyst resting state as the trans-cobalt­(III) dihydride boryl, (iPrPNP)­Co­(H)2(BPin) (iPrPNP = 2,6-(iPr2PCH2)2(C5H3N)), at both low and high substrate conversions. The overall first-order rate law and observation of a normal deuterium kinetic isotope effect on the borylation of benzofuran versus benzofuran-2-d1 support H2 reductive elimination from the cobalt­(III) dihydride boryl as the turnover-limiting step. These findings stand in contrast to that established previously for the borylation of 2,6-lutidine with the same cobalt precatalyst, where borylation of the 4-position of the pincer occurred faster than the substrate turnover and arene C–H activation by a cobalt­(I) boryl is turnover-limiting. Evaluation of the catalytic activity of different cobalt precursors in the C–H borylation of benzofuran with HBPin established that the ligand design principles for C–H borylation depend on the identities of both the arene and the boron reagent used: electron-donating groups improve catalytic activity of the borylation of pyridines and arenes with B2Pin2, whereas electron-withdrawing groups improve catalytic activity of the borylation of five-membered heteroarenes with HBPin. Catalyst deactivation by P–C bond cleavage from a cobalt­(I) hydride was observed in the C–H borylation of arene substrates with C–H bonds that are less acidic than those of five-membered heteroarenes using HBPin and explains the requirement of B2Pin2 to achieve synthetically useful yields with these arene substrates.