Niobium-catalyzed Activation of CF3 Group on Alkene: Synthesis of Substituted Indenes

A CF3 group attached to an alkene functionality was activated by a zero-valent niobium catalyst to generate niobium alkenylcarbenoid species. The niobium carbenoid species then underwent insertion to an internal aromatic C-H sigma bond to give indene derivatives in good yields. Isomerization in terms of the alkene geometry was also observed.