Carboxamide-accelerated Chemoselective Borylation of Iodoarenes under Photoirradiation

Borylation of haloarene is one of the most important methodologies to synthesize borylated arenes. Generally, borylation of haloarene occurs smoothly at sterically less-hindered para- or meta-position by using a transition metal, a photoredox catalyst, or basic conditions. This study reports on ortho-specific and chemoselective borylation of ortho-iodoarene possessing carboxamide under visible-light irradiation. When a haloarene containing both C–I and C–X bonds is employed as a substrate, another C–X bond (not ortho) remains intact during the reaction. Mechanistic studies revealed that the key to success of this reaction is to generate diboron-bridged five-membered ring as a transition state, in which the diboron-bridged five-membered ring and the benzene ring in the transition state are perpendicular to each other owing to steric repulsion by the iodine atom at the ortho-position. This chemoselectivity is suitable for the synthesis of borylated building blocks.