Mechanistic Evaluation of Benzoin Catalysis Enabled by Triazolium Salt Application in the Preparation of Benzil and Heterocycles

Abstract

A library of functionalized pyrrolidine‐based triazolium salts containing a BF4 counterions have been prepared from readily accessible 2‐pyrolidinone via one‐pot reaction protocol. By separating imine‐ether intermediates, a detailed mechanistic pathway has been investigated. Studied mechanistic investigation improved the synthetic process, resulting in higher yields for pyrrolidine‐based triazolium NHC structures and the large‐scale synthesis of chiral NHCs pre‐catalyst with free OH‐group. The presence of a hydroxyl functional group on the NHCs has the potential for further functionalization and for non‐covalent control over catalytic reactions in which the NHCs can serve as organocatalysts or donor ligands for organometallic catalysis. This benzoin catalytic system was investigated by testing different solvents and aldehydes, NHC‐catalysts loading and time variation effects to prepare a broad range of functionalized benzoin compounds with high yields. The highly reactive pyrrolidine‐based triazolium salts (TACF5) showed significant potential application of organocatalysts in benzoin condensation‐oxidation. EDG and EWG substituted α‐hydroxy ketone derivatives were directly converted to corresponding benzil scaffolds from an aldehyde through one‐pot reaction condition with MnO2 acting as an oxidating reagent in open‐air atmosphere. Selective heterocycles, quinoxaline, imidazole and pyrazine precursor were achieved from simple benzil under green conditions.