Recent Applications of Niobium Catalysts in Organic Synthesis

Niobium catalysts generally possess a good Lewis acidity and have received increased attention in recent years. The applications of niobium pentachloride as Lewis acid in organic synthesis have been recently reviewed. Since then, there has been an increasing number of examples on the use of niobium catalysts in organic reactions such as Biginelli reactions, Friedel-Crafts acylation and Sakurai-Hosomi reactions of acetals, Knoevenagel condensation, acetylation of alcohols and phenols, among others. Also, chiral niobium complexes have been reported to give high enantiomeric excesses in asymmetric Mannich-type reactions. In this review, recent applications of these versatile reagents in organic synthesis, including our own results, will be disclosed.