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.