Preparation of Dienyl Boronates by Tandem Ene–Yne Metathesis/Dienyl Isomerization: Ready Access to Diene Building Blocks for the Synthesis of Polyenes

The ene–yne metathesis of alkenyl boronates with
terminal
alkynes is reported. These challenging metatheses were accomplished
using a Grubbs catalyst bearing the cyclic alkyl amino carbene (CAAC)
ligand, whereas N-heterocyclic carbene (NHC) derived
catalysts gave lower yields. Subsequent dienyl isomerization via a
cobalt-catalyzed hydrogen atom transfer (HAT) furnished the more substituted
dienyl boronate with high EE/EZ ratios.
Finally, the resulting dienyl boronate products were successfully
used in Suzuki–Miyaura cross-coupling reactions and in a Diels–Alder
cycloaddition.