Hafnium Trifluoromethanesulfonate [Hf(OTf)₄] as a Unique Lewis Acid in Organic Synthesis

This microreview summarizes examples of Hf(OTf)₄-mediated or -catalyzed organic reactions. Hafnium triflate possesses four strongly electron-withdrawing OTf groups, together with an ionic radius of intermediate range (Al < Ti < Hf, Zr < Sc < Ln), and has an oxophilic hard character typical of group IV metals. This Lewis acid shows outstanding performance in catalytic Friedel-Crafts acylation reactions and Mannich-type reactions of imines, hydrazones, and N,O-acetals. Several natural products have also been synthesized by use of Hf(OTf)₄-mediated or -catalyzed reactions. Moreover, Hf(OTf)₄ has a number of potent features in carbohydrate syntheses. Hf(OTf)₄ possesses many unique characteristics that distinguish it from other Lewis acids. The unique characteristics of Hf(OTf)₄ should make this compound a major candidate for selection as a Lewis acid in the future.