NHC-CDI Betaine Adducts and Their Cationic Derivatives as Catalyst Precursors for Dichloromethane Valorization

Abstract

Zwitterionic adducts of N-heterocyclic carbene and carbodiimide (NHC-CDI) are an emerging class of organic compounds with promising properties for applications in various fields. Herein, we report the use of the ICyCDI(p-Tol) betaine adduct (1a) and its cationic derivatives2aand3aas catalyst precursors for the dichloromethane valorization via transformation into high added value products CH2Z2(Z = OR, SR or NR2). This process implies selective chloride substitution of dichloromethane by a range of nucleophiles Na+Z-(preformed or generatedin situfrom HZ and an inorganic base) to yield formaldehyde-derived acetals, dithioacetals, or aminals with full selectivity. The reactions are conducted in a multigram-scale under very mild conditions, using dichloromethane both as a reagent and solvent, and very low catalyst loading (0.01 mol %). The CH2Z2derivatives were isolated in quantitative yields after filtration and evaporation, which facilitates recycling the dichloromethane excess. Mechanistic studies for the synthesis of methylal CH2(OMe)2rule out organocatalysis as being responsible for the CH2transfer, and a phase-transfer catalysis mechanism is proposed instead. Furthermore, we observed that1aand2areact with NaOMe to form unusual isoureate ethers, which are the actual phase-transfer catalysts, with a strong preference for sodium over other alkali metal nucleophiles.