Palladium-Catalyzed Aerobic Alkene Arylboration: Coupling Aryl Boronic Acids, Alkenes, and Bis(pinacol)diboron

Alkyl boronic acids and esters are versatile synthetic intermediates that generally require several steps to synthesize. Three-component alkene arylboration reactions allow for the rapid synthesis of alkyl boronic esters. Herein, we report the base-free aerobic Pd-catalyzed three-component alkene arylboration, which allows direct access, in a single step, to alkyl boronic esters from readily available precursors: aryl boronic acids, alkenes, and bis(pinacol)diboron. This approach allows for the formal insertion of an alkene into an Ar–B bond and thus generates an alkyl boronic ester from an aryl boronic acid. The reaction proceeds with both electron-rich and -deficient aryl boronic acids as well as strained cyclic, internal, and terminal olefins. The reactions are regioselective: 1,2-arylboration products are formed with strained cyclic alkenes and β-alkyl-styrenes, while 1,1-arylboration products are generated from terminal alkenes. Forty-five examples are presented with isolated yields of the resulting alkyl boronic esters ranging from 20 to 74%, along with several examples demonstrating the synthetic utility of the products. Mechanistic investigations support the idea that the catalytic cycle occurs through direct arylboration of the alkene. Further, π-benzyl intermediates form when possible, and the rate of borylation is increased with electron-rich arenes relative to electron-poor. Finally, we demonstrate that aryl boroxines, generated in situ, are essential for the transformation, as they rapidly undergo base-free transmetalation with the proposed palladium peroxo intermediate.