Selective C-H Borylation of Polyaromatic Compounds Enabled by Metal-Arene π-Complexation.

The undirected Ir-catalyzed C-H borylation usually occurs preferentially at the least hindered and more acidic C-H bond of the aromatic ring. In the case of polyaromatic compounds possessing multiple unbiased and sterically accessible C-H bonds, the site selectivity for the nondirected C-H borylation is low. Here, we report the dramatic effect exerted by the π-complexation of a chromium tricarbonyl unit on the aromatic ring in the context of Ir-catalyzed C-H borylation. Competition experiments demonstrate that the C-H bonds of an aromatic ring bound to the chromium tricarbonyl unit react on average two orders in magnitude faster toward the C-H borylation than the unbound arenes. This enables an unprecedented C-H borylation with high site selectivity of the aromatic ring π-complexed with a chromium tripod in a series of organic polyaromatic compounds. Besides, the drastic enhancement of the reactivity of C-H bonds induced by the chromium tripod allows the C-H borylation to occur at room temperature with the substrate as a limiting reagent. The DFT studies indicate that the oxidative addition of the C-H bonds has lower activation barriers when the arenes are complexed with a chromium tricarbonyl unit, explaining the observed exceptional site selectivity. This study will further spearhead the development of nondirected C-H borylation with a bimetallic system to harness the effect of the noncovalent metal-arene π-type interactions on the reactivity and the selectivity of the C-H functionalization.