Equilibrium control in enyne metathesis:: Crossover studies and the kinetic reactivity of (E,Z)-1,3-disubstituted-1,3-dienes

The stereoselectivity of diene bond formation in the ruthenium-carbene mediated intermolecular enyne metathesis was studied. Initial reaction between an alkyne and 1-hexene gave mixtures of E- and Z-isomers in the newly formed 1,3-diene. However, over time the mixtures equilibrated to form mostly the diene of the E-configuration. To evaluate individual reactivity of the E- and Z-dienes, they were independently synthesized. The E-diene was found to be kinetically-stable under nominal metathesis conditions while the Z-diene isomerized to the E-isomer. The Z-isomeric dienes were found to react with other alkenes to produce a new diene of E-configuration. A secondary metathesis mechanism involving ruthenium alkylidene intermediates was invoked to explain the dynamic stereochemistry observed in this study.