Cyclic diaryliodonium salts – from synthesis to halogen bond catalysis

Cyclic diaryliodonium salts have emerged as highly adaptable building blocks in organic synthesis and, more recently, as particularly powerful halogen bond (XB) donors in organocatalysis. In the present thesis, both newly designed and previously known cyclic diaryliodonium salts are prepared and examined in a systematic manner with respect to their structural features, their chemical reactivity, and their performance as catalysts.
As a basis for this work, two efficient one‑pot procedures were established that provide access to oxygen‑ and nitrogen‑bridged cyclic diaryliodonium salts. Their utility as synthetic intermediates is illustrated by a broad set of mono‑ and difunctionalization reactions. On this foundation, a broader series of cyclic diaryliodonium salts was investigated in previously unexplored XB‑mediated transformations. In a Pictet–Spengler reaction, a simple dibenzoiodolium salt displayed remarkable catalytic activity even at very low loadings, thus offering an XB‑driven alternative to classical Brønsted or Lewis acid catalysis. In addition, highly strained N‑sulfonyl aziridines were activated by N‑heterocyclic iodonium salts and transformed, via formal [3+2] cycloadditions, with a variety of dipolarophiles, into diverse N‑heterocyclic products. Computational analyses uncovered an unusual dual activation mode involving simultaneous I–N and I–π interactions.

In another part of the thesis, novel chiral triazole-substituted diaryliodonium salts were designed for enantioselective XB catalysis. A series of these chiral iodonium salts was synthesized and evaluated in a vinylogous Mannich reaction, affording up to 99% yield and an outstanding enantiomeric ratio (>99:1). Moreover, these catalysts enabled the efficient functionalization of ketimines with alcohols to form N,O-acetals in up to 99% yield and 90:10 e.r. Computational investigations provided mechanistic insight into the origin of enantioselectivity, validating the concept of asymmetric induction via σ-hole-stabilized chiral moieties in iodine(III)-based XB catalysts.