Niobium Complexes in Organic Transformations: From Stoichiometric Reactions to Catalytic [2+2+2] Cycloaddition Reactions

摘要

Niobium complexes in organic reactions have received a lot of attention in recent years. Niobium is one of the early transition metals, which possess unique reactivity distinct from that of other transition metals. This review focuses mainly on stoichiometric reactions and catalytic reactions involving niobium complexes. In particular, recent results relating to Nb-V-mediated or Nb-III-catalyzed [2+2+2] cycloaddition of unsaturated compounds (alkyne, alkenes, nitriles, and isocyanates) are mainly presented in this review. In one example of a set of catalytic reactions involving a low-valent niobium species, the reaction between NbCl5 and tris(trimethylsilyl)silane generated a low-valent niobium(III) species, and this promoted [2+2+2] cycloaddition of terminal alkynes and olefins to provide cyclohexa-1,3-dienes. In addition, the use of low-valent niobium(III) species in stoichiometric reactions has also been reported. Many of these reactions are carried out by formation of Nb-alkyne complexes through reactions between low-valent niobium(III) and internal alkynes. Some examples of the use of Nb complexes as Lewis acids have also been reported. In these reactions, the valence of the Nb complexes is V.