Modular Synthesis of Benzocyclobutenes via Pd(II)-Catalyzed Oxidative [2+2] Annulation of Arylboronic Acids with Alkenes

Benzocyclobutenes (BCBs) are highly
valuable compounds in organic
synthesis, medicinal chemistry, and materials science. However, catalytic
modular synthesis of functionalized BCBs from easily accessible starting
materials remains limited. We report herein an efficient synthesis
of diversely functionalized BCBs by a Pd­(II)-catalyzed formal [2+2]
annulation between arylboronic acids and alkenes in the presence of N-fluorobenzene­sulfonimide (NFSI). An intermolecular
carbopalladation followed by palladium oxidation, intramolecular C­(sp2)–H activation by a transient C­(sp3)–Pd­(IV)
species, and selective carbon–carbon (C–C) bond-forming
reductive elimination from a high-valent five-membered palladacycle
is proposed to account for the reaction outcome. Kinetically competent
oxidation of alkylPd­(II) to alkylPd­(IV) species is important to avoid
the formation of a Heck adduct. The reaction forges two C–C
bonds of the cyclobutene core and is compatible with a wide range
of functional groups. No chelating bidentate directing group in the
alkene part is needed for this transformation.