A Strong Lewis Acidic Diethylsilylium Catalyst for Direct Sulfonamidation of Challenging Ketones

Low‐substitution‐order silylium ions (R 2 HSi + ), as less sterically congested analogs of carbenium ions, exhibit exceptionally high Lewis acidity and distinctive reactivity compared with tertiary counterparts. Herein, we present a strong Lewis acidic diethylsilylium catalysis that enables efficient reductive sulfonamidation of functionalized ketones, including β‐ketoesters, to access alkyl β‐amino esters. The active diethylsilylium ion pair ([Et 2 HSi] + [B(C 6 F 5 ) 4 ] – ), in situ generated via hydride abstraction from diethylsilane by trityl tetrakis(pentafluorophenyl)borate, displays a remarkably enhanced catalytic performance. Diethylsilane functions dually as the reductant and precursor of the silylium species. Operating under solvent‐, metal‐, hydrogen‐, and additive‐free conditions, the protocol offers broad substrate scope and excellent scalability, exemplified by a concise three‐step, multi‐gram synthesis of the antidiabetic drug sitagliptin (> 4 g). Combined computational, NMR, and HR‐MS analyses reveal that the remarkable Lewis acidity of the secondary silylium ion enables activation of challenging substrates and affords exceptional selectivity. A mechanistic understanding of this process provides the foundation for a practical and sustainable catalytic system that expands the frontier of silylium‐mediated C─N bond formation.