Highly Stereoselective Synthesis of Tetrasubstituted Acyclic All-Carbon Olefins via Enol Tosylation and Suzuki–Miyaura Coupling

A highly
stereocontrolled synthesis of tetrasubstituted acyclic
all-carbon olefins has been developed via a stereoselective enolization
and tosylate formation, followed by a palladium-catalyzed Suzuki–Miyaura
cross-coupling of the tosylates and pinacol boronic esters in the
presence of a Pd­(OAc)2/RuPhos catalytic system. Both the
enol tosylation and Suzuki–Miyaura coupling reactions tolerate
an array of electronically and sterically diverse substituents and
generate high yield and stereoselectivity of the olefin products.
Judicious choice of substrate and coupling partner provides access
to either the E- or Z-olefin with
excellent yield and stereochemical fidelity. Olefin isomerization
was observed during the Suzuki–Miyaura coupling. However, under
the optimized cross-coupling reaction conditions, the isomerization
was suppressed to <5% in most cases. Mechanistic probes indicate
that the olefin isomerization occurs via an intermediate, possibly
a zwitterionic palladium carbenoid species.