Metal-Free sp2‑C–H Borylation as a Common Reactivity Pattern of Frustrated 2‑Aminophenylboranes

C–H borylation is a powerful and atom-efficient method for converting affordable and abundant chemicals into versatile organic reagents used in the production of fine chemicals and functional materials. Herein we report a facile C–H borylation of aromatic and olefinic C–H bonds with 2-aminophenylboranes. Computational and experimental studies reveal that the metal-free C–H insertion proceeds via a frustrated Lewis pair mechanism involving heterolytic splitting of the C–H bond by cooperative action of the amine and boryl groups. The adapted geometry of the reactive B and N centers results in an unprecedentently low kinetic barrier for both insertion into the sp2-C–H bond and intramolecular protonation of the sp2-C–B bond in 2-ammoniophenyl­(aryl)- or -(alkenyl)­borates. This common reactivity pattern serves as a platform for various catalytic reactions such as C–H borylation and hydrogenation of alkynes. In particular, we demonstrate that simple 2-aminopyridinium salts efficiently catalyze the C–H borylation of hetarenes with catecholborane. This reaction is presumably mediated by a borenium species isoelectronic to 2-aminophenylboranes.