Mechanistic Analysis Reveals Key Role of Interchalcogen Multicatalysis in Photo-Aerobic 3-Pyrroline Syntheses by Aza-Wacker Cyclizations.

A light-driven dual and ternary catalytic aza-Wacker protocol for the construction of 3-pyrrolines by partially disulfide-assisted selenium-π-acid multicatalysis is reported. A structurally diverse array of sulfonamides possessing homopolar mono-, di- and trisubstituted olefinic double bonds is selectively converted to the corresponding 3-pyrrolines in up to 95 % isolated yield and with good functional group tolerance. Advanced electrochemical mechanistic investigations of the protocol suggest a dual role of the disulfide co-catalyst. On the one hand, the disulfide serves as an electron hole shuttle between the excited photoredox catalyst and the selenium co-catalyst. On the other hand, the sulfur species engages in the final, product releasing step of the catalytic cycle by accelerating the ß-elimination of the selenium moiety, which was found in many cases to lead to considerably improved product yields.

Stable Mesoionic N-Heterocyclic Olefins (mNHOs).

We report a new class of stable mesoionic N-heterocyclic olefins, featuring a highly polarized (strongly ylidic) double bond. The ground-state structure cannot be described through an uncharged mesomeric Lewis-structure, thereby structurally distinguishing them from traditional N-heterocyclic olefins (NHOs). mNHOs can easily be obtained through deprotonation of the corresponding methylated N,N'-diaryl-1,2,3-triazolium and N,N'-diaryl-imidazolium salts, respectively. In their reactivity, they represent strong σ-donor ligands as shown by their coordination complexes of rhodium and boron. Their calculated proton affinities, their experimentally derived basicities (competition experiments), as well as donor abilities (Tolman electronic parameter; TEP) exceed the so far reported class of NHOs.

Synthesis of Alkynylthiopyridinium Salts and Their Use as Thioketene Equivalents.

A synthetic method has been developed for the preparation of dihalo(pyridinium)sulfuranes and their transformation into alkynylthiopyridinium salts, starting from inexpensive thiopyridones. The reactivity of these salts towards different nucleophiles is evaluated. Most thiols and amines are converted into dithioesters and thioamides, respectively; whereas sterically demanding thiols delivered alkynylthioethers. These results, together with preliminary mechanistic studies reveal that alkynylthiopyridinium salts can be considered synthetic equivalents of unstable thioketenes.