Highly Efficient N‐Heterocyclic Carbene Precursor from Sterically Hindered Benzimidazolium Salt for Benzoin Condensation

Abstract

A diverse array of simple benzoin compounds featuring aryl and heteroaryl substituents were prepared from corresponding aldehydes, achieving moderate to excellent yields. To achieve this, a library of NHC precursors was designed and synthesized from imidazole and benzimidazole. Our custom‐designed NHC precursors (L1–L10) varied in size and structure. Built on imidazolinium and benzimidazolinium salts, these catalysts sported substituents at the N1 and N3 positions ranging from compact groups like butyl or benzyl to the much bulkier anthracenyl group. Treatment with a base transforms these salts into highly effective NHC precursors. After being activated, these NHCs become highly effective catalysts for the benzoin condensation reaction. We tested all the NHC precursors we made and found that L10, the one containing the large and bulky anthracenyl group attached to a benzimidazolium ring, worked the best for making benzoin derivatives.