The Facile Hydrolysis of Imidazolinium Chlorides (N-Heterocyclic Carbene Precursors) Under Basic Aqueous Conditions.

The hydrolysis of imidazolinium chlorides takes place readily in a basic water/dichloromethane biphasic mixture at room temperature. Experi-mental parameters were optimized to afford full conversions and high yields of γ-amino-formamides starting from twelve symme-trical substrates with alkyl or aryl substituents on their nitrogen atoms, and five unsym-metrical 1-alkyl-3-aryl-imidazol-in-ium chlorides. NMR and XRD analyses showed that the cleavage of unsymmetrical salts led to γ-alkylamino-N-aryl-formamides with a high regio-selec-ti-vity and that bulky alkyl or aryl groups on the formamide moiety led to the isolation of the (E)-isomer in high stereoisomeric purity (>95%), whereas smaller and more flexible alkyl substituents afforded mixtures of (E)- and (Z)-rotamers. Control experiments showed that the hydrolysis of 1,3-dimesityl-imidazolinium chloride (SIMes·HCl) did not occur readily in pure or acidic water and that the presence of bulky aromatic substituents on the nitrogen atoms of 1,3-bis(2,6-diisopropylphenyl)-imidazol-inium chloride (SIDip·HCl) efficiently slowed down its hydrolysis under basic aqueous conditions. Most strikingly, this work highlighted the critical influence of the counteranion on the reactivity of imidazolinium cations. Indeed, the chloride salts underwent a facile hydrolysis in the presence of water and Na2CO3, whereas various other NHC·HX derivatives reacted much slower or remained essentially inert under these conditions.