Mechanistic Origins of Regioselectivity in Cobalt-Catalyzed C(sp2)-H Borylation of Benzoate Esters and Arylboronate Esters.

Synthetic and mechanistic investigations into the C(sp2)-H borylation of various electronically diverse arenes catalyzed by bis(phosphine)pyridine (iPrPNP) cobalt complexes are reported. Borylation of various benzoate esters and arylboronate esters gave remarkably high selectivities for the position para to the functional group; in both cases, this regioselectivity was found to override the ortho to fluorine regioselectivity previously reported for (iPrPNP)Co borylation catalysts which arises from thermodynamic control of C(sp2)-H oxidative addition. Mechanistic studies support two distinct pathways that result in para-to-ester and para-to-boronate ester regioselectivity by thermodynamic and kinetic control, respectively, of C(sp2)-H oxidative addition. Borylation of a particularly electron-deficient fluorinated arylboronate ester resulted in acceleration of C(sp2)-H oxidative addition and concomitant inversion of regioselectivity, demonstrating that subtle changes in the relative rates of individual steps of the catalytic cycle can enable unique and switchable site selectivities.