Divergent reactivity of alk-5-ynylidenecyclopropanes in the presence of the 1st or the 2nd generation Grubbs’ catalysts

Alk-5-ynylidenecyclopropanes, by virtue of being equipped with a strained cyclopropane system, can be divergently elaborated into bicyclo[3.3.0]octenes or exocyclopropylidenecycloalkenes depending on whether they react with the first or the second generation Grubbs’ ruthenium carbenes. While the highly reactive second generation system catalyses the formation of ring-closing metathesis products, the less [metathesis] active first generation carbene promotes an intramolecular [3 + 2] cycloaddition to give the bicarbocyclic skeletons.