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

Abstract 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.