text: A number of formal [2+2+2] cycloadditions have been realized under Nb catalysis, including alkyne trimerizations and couplings of alkynes with alkenes or nitriles to form cyclohexadienes or pyridines, respectively. Typically a Nb(III) catalyst will form a Nb(V) metallocyclopropene with a terminal alkyne component and then engage in sequential migratory insertions and reductive elimination to furnish the six membered ring and regenerate the Nb(III).
  An organoniobium catalyst has also been developed for (Z)-selective semihydrogenation of alkynes. The mechanistic pathway for this reaction is distinct from other transition metal catalyzed hydrogenations, proceeding through the Nb(V) metallocyclopropene which engages with hydrogen either through direct sigma-bond metathesis or outer sphere 1,2-addition.