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 β-elimination of the selenium moiety, which was found in many cases to lead to considerably improved product yields.

Reduction of Substituted Benzo-Fused Cyclic Sulfonamides with Mg-MeOH: An Experimental and Computational Study

A study involving the use of Mg-MeOH for the double reductive cleavage of both N–S and C–S bonds in a series of 11 benzo-fused cyclic sulfonamides is reported. Examples where the sulfonamide nitrogen atom is part of a pyrrolidine ring effectively undergo reduction, as long as a methoxy substituent is not para-positioned in the aromatic ring, relative to the sulfonyl group. In contrast, if the nitrogen atom is contained within an aromatic ring (pyrrole or indole), the presence of a para-methoxy substituent does not prohibit reduction. If deuterated methanol is used, aromatic ortho-deuterium incorporation was observed. To better understand how structure affects reactivity, density functional theory calculations were performed using three functionals. Results using CAM-B3LYP were found to best correlate with experimental observations, and these demonstrate the impact that the different aromatic substitution patterns and types of N-atom have on the lowest unoccupied molecular orbital (LUMO) energies and adiabatic electron affinities.

The Titanium-Mediated Double Reductive Cleavage of Cyclic Sulfonamides for the Synthesis of Aryl Pyrrolidines

Reduction of a range of benzo-fused cyclic sulfonamides has been accomplished using low-valent titanium. This operationally simple method generates the corresponding aryl-substituted cyclic amines, typically, with good conversion. Notably, unlike our previous Li-NH₃-based method, loss of heteroatom-based substituents (X) on the aromatic ring does not readily occur, and the robustness of this method was demonstrated with a synthesis of the Sceletium alkaloid mesembrane.

Halonium ion triggered rearrangement of unsaturated benzo-annulated bi- and tricyclic sulfonamides

The halonium ion mediated 1,2-Wagner-Meerwein-type rearrangement of a series of benzo-fused bi- and tricyclic sulfonamides is reported. During this rearrangement the carbon-carbon bond that migrates was selectively set in the intramolecular Mizoroki-Heck (IHR) synthesis of the starting materials. Consequently, this method constitutes a means to access the regioisomeric series of cyclic sulfonamides not observed during the Mizoroki-Heck reaction.