Formation of chiral ionic liquids and imidazol-2-ylidene metal complexes from the proteinogenic aminoacid l-histidine

Treatment of the amino acid derivative Bz-His-OMe with excess n-propyl bromide gave the corresponding histidinium salt [Bz-His(n-propyl)2-OMe+Br−]. It features a melting point of 39 °C and may serve as a useful readily available optically active ionic liquid. Its subsequent treatment with silver oxide gave the corresponding l-histidine derived chiral N-heterocyclic carbene complex [“(carbene)2Ag · AgBr2”]. Transmetallation by treatment with Pd(CH3CN)2Cl2 or [Rh(cod)Cl]2 led to the formation of the respective chiral late metal imidazol-2-ylidene complexes [“(carbene)2PdCl2”] and [“(carbene)RhCl(cod)”], respectively. Four diastereomers of the square planar palladium system were observed. Due to the additional chirality center in the l-histidine-derived “Arduengo-carbene ligand” two diastereomers of the rhodium carbene complex were formed.

Graphical abstractThe readily available histidinium salt [Bz-His(n-propyl)2-OMe+Br−] (8a), mp 39 °C, may serve as a useful chiral ionic liquid. Its subsequent treatment with silver oxide followed by transmetallation gave chiral l-histidine-derived N-heterocyclic carbene metal complexes.Figure optionsDownload full-size imageDownload as PowerPoint slide