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.