Isonicotinate Ester Catalyzed Decarboxylative Borylation of (Hetero)Aryl and Alkenyl Carboxylic Acids through <i>N</i>‑Hydroxyphthalimide Esters

Abstract

Decarboxylative borylation of aryl and alkenyl carboxylic acids with bis­(pinacolato)­diboron was achieved through <i>N</i>-hydroxyphthalimide esters using <i>tert</i>-butyl isonicotinate as a catalyst under base-free conditions. A variety of aryl carboxylic acids possessing different functional groups and electronic properties can be smoothly converted to aryl boronate esters, including those that are difficult to decarboxylate under transition-metal catalysis, offering a new method enabling use of carboxylic acid as building blocks in organic synthesis. Mechanistic analysis suggests the reaction proceeds through coupling of a transient aryl radical generated by radical decarboxylation with a pyridine-stabilized persistent boryl radical. Activation of redox active esters may proceed via an intramolecular single-electron-transfer (SET) process through a pyridine–diboron–phthalimide adduct and accounts for the base-free reaction conditions