Synthesis and Catalytic Activity of Atrane-type Hard and Soft Lewis Superacids with a Silyl,Germyl, or Stannyl Cationic Center

The synthesis and isolation of atrane-type molecules 1 E⁺ (E=Si, Ge, or Sn) having a cationic group 14 elemental center are reported. The cations 1 E⁺ act as hard and soft Lewis superacids, which readily interact with various hard and soft Lewis basic substrates. The rigid atrane framework stabilizes the localized positive charge on the elemental center and assists the formation of the well-defined highly coordinated states of 1 E⁺. The cations were applied to the hydrodefluorination, Friedel-Crafts reaction, alkyne cyclization, and carbonyl reduction as Lewis acid catalysts. Most notably, 1 Si]ClO₄] exhibits unique chemoselectivity that depends on a solvent in the competitive reaction of silyl enol ether with a mixture of benzaldehyde dimethyl acetal and benzaldehyde. Our findings indicate the potential of hard and soft Lewis superacids in organic synthesis.