Slow Reactant–Water Exchange and High Catalytic Performance of Water‐Tolerant Lewis Acids

³¹P nuclear magnetic resonance (NMR) spectroscopic measurement with trimethylphosphine oxide (TMPO) was applied to evaluate the Lewis acid catalysis of various metal triflates in water. The original ³¹P NMR chemical shift and line width of TMPO is changed by the direct interaction of TMPO molecules with the Lewis acid sites of metal triflates. Sc(OTf)₃] and In(OTf)₃] had larger changes in ³¹P chemical shift and line width by formation of the Lewis acid–TMPO complex than other metal triflates. It originates from the strong interaction between the Lewis acid and TMPO, which results in higher stability of Sc(OTf)₃TMPO] and In(OTf)₃TMPO] complexes than other metal triflate–TMPO complexes. The catalytic activities of Sc(OTf)₃] and In(OTf)₃] for Lewis acid‐catalyzed reactions with carbonyl compounds in water were far superior to the other metal triflates, which indicates that the high stability of metal triflate–carbonyl compound complexes cause high catalytic performance for these reactions. Density functional theory (DFT) calculation suggests that low LUMO levels of Sc(OTf)₃] and In(OTf)₃] would be responsible for the formation of stable coordination intermediate with nucleophilic reactant in water.