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 non-directed C–H borylation is low. Here, we report the dramatic effect exercised by 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 towards the C–H borylation than the unbound arenes. That enabled 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. We could explain the exceptional site selectivity observed for the preferential borylation of (η6-arene)Cr(CO)3 complexes using density functional theory (DFT) calculations. 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. This study will further spearhead the development of non-directed C–H borylation with a bimetallic system to harness the effect of the non-covalent metal-arene π-type interactions on the reactivity and the selectivity of the C–H functionalization.