Lei Wang, Bivas Mondal, Bohan Li, Zhenguo Zhang, Teck‐Peng Loh, Peng Chen, Zhenhua Jia
N, S -Acetals are important motifs in pharmaceuticals, natural products, and functional materials. Herein, we report a metal-free, aqueous-phase synthesis of N, S -acetals through [Ph 3 C] + [B(C 6 F 5 ) 4 ] − -catalyzed generation of highly electrophilic N -acyliminium borates from 3-hydroxyisoindolin-1-ones. This protocol proceeds at room temperature and physiologically relevant pH, exhibits broad substrate scope including sterically hindered and biologically relevant substrates, and enables site-selective cysteine modification of unprotected peptides and proteins. The N -acyliminium borate intermediate was directly characterized in solution, providing mechanistic support. This operationally simple method offers a general and efficient platform for thiol functionalization and late-stage peptide/protein modification.