Z‑Selective Cross-Metathesis and Homodimerization of 3E‑1,3-Dienes: Reaction Optimization, Computational Analysis, and Synthetic Applications

Olefin metathesis reactions with 3E-1,3-dienes using Z-selective cyclometalated ruthenium benzylidene catalysts are described. In particular, a procedure for employing 3E-1,3-dienes in Z-selective homodimerization and cross-metathesis with terminal alkenes is detailed. The reaction takes advantage of the pronounced chemoselectivity of a recently reported ruthenium-based catalyst containing a cyclometalated NHC ligand for terminal alkenes in the presence of internal E-alkenes. A wide array of commonly encountered functional groups can be tolerated, and only a small excess (1.5 equiv) of the diene coupling partner is required to achieve high yields of the desired internal E,Z-diene cross-metathesis product. Computational studies have been performed to elucidate the reaction mechanism. The computations are consistent with a diene-first pathway. The reaction can be used to quickly assemble structurally complex targets. The power of this cross-metathesis reaction is demonstrated by the concise syntheses of two insect pheromones.