Comparative study of the retention of catalytic activity of halonium and chalconium cations in the presence of crown ethers

Abstract

Previous studies have established that efficient electrophilic activation can be achieved through σ-hole interactions. However, the behavior of σ-hole donors in the presence of sterically demanding nucleophiles, which are ubiquitous in biological systems, remains insufficiently explored. To address this knowledge gap, we performed ¹H NMR titrations of four crown ethers with the halogen bond donor dibenziodolium triflate. 15-crown-5 was selected for detailed kinetic studies as the most effective σ-hole acceptor. The Schiff base condensation was monitored by ¹H NMR spectroscopy using halonium and chalconium catalysts, both in the presence and absence of 15-crown-5. For comparative purposes, control experiments were conducted with silver triflate as a conventional Lewis acid. The observed rate decrease in the presence of 15-crown-5 was attributed to substrate sequestration by the crown ether. In contrast, AgOTf lost its catalytic activity due to the formation of a 15-crown-5–Ag⁺ complex. Our results demonstrate that while conventional Lewis acids require carefully optimized reaction conditions, σ-hole donors maintain catalytic activity with bulky nucleophiles. These findings highlight the potential of σ-hole donors as robust organocatalysts.