Zirconocene reaction with stilbene

The chemistry of 306 differs from that of its seven-membered analogue in that its reaction with rram-stilbene gives reductive coupling product 308 in addition to the stilbene adduct of zirconocene, the sole product of the latter (vide infra). This was attributed to differences in ring strain.116... 

TrisubsHtuted alkenes. Negishi et al have extended the reaction of alkynes with trimethylalane and zirconocene dichloride (this volume) to a corresponding addition reaction with trimethylalane-titanocene dichloride. This latter reagent is somewhat more reactive. It reacts with diphenylacetylene to form (Z)-a-methyl-stilbene (2) in high yield. The yield of 2 is <30% when 1 is replaced by the corresponding zirconocene reagent. 

The unique stereochemistry of the diene results therefore from the elimination step and not from a further isomerization of the dienyl zirconocene with zirconocene derivatives (i.e., Cp2Zr-catalyzed stilbene stereoisomerization) [78]), since the hydrozirconation reaction of several l -ene-3-yne and lZ-ene-3-yne derivatives with Cp2ZrH(Cl) leads only to the (E,E) and (Z, ) isomers, respectively, in good yields (see Scheme 5) [33]. 

---