Stereospecific synthesis and catalytic activity of L-histidylidene metal complexes

• Published in: Dalton transactions : an international journal of inorganic chemistry Vol. 41; no. 29; pp. 8813 - 8821
• Main Authors: Monney, Angèle, Alberico, Elisabetta, Ortin, Yannick, Müller-Bunz, Helge, Gladiali, Serafino, Albrecht, Martin
• Format: Journal Article
• Language: English
• Published: England 07.08.2012
• Subjects: Amino acids; Asymmetry; Catalysis; Catalysts; Coordination Complexes - chemical synthesis; Coordination Complexes - chemistry; Crystallography, X-Ray; Heterocyclic Compounds - chemistry; Histidine - analogs & derivatives; Iridium - chemistry; Ligands; Methane - analogs & derivatives; Molecular Conformation; NMR spectroscopy; Phosphines; Rhodium; Rhodium - chemistry; Stereoisomerism; Synthesis; Temperature
• ISSN: 1477-9226; 1477-9234
• DOI: 10.1039/c2dt30799e

We report on the synthesis, metal coordination, and catalytic impact of histidylidene, a histidine-derived N-heterocyclic carbene (NHC) ligand. The histidinium salt 3, comprising methyl substituents at both heterocyclic nitrogens and protected at the C- and N-terminus of the amino acid, was rhodated and iridated by a transmetallation protocol using Ag(2)O. Ambient temperature and short reaction times were pivotal for full retention of configuration at the α-carbon. The stereospecificity of the reaction was conveniently probed by (31)P NMR spectroscopy after transmetallation with rhodium(I) and coordination of enantiopure (S)-Ph-binepine. The histidylidene rhodium complexes are highly efficient catalysts for the mild hydrosilylation of ketones. For the cationic complexes [Rh(cod)(histidylidene)(phosphine)](+), lowering the temperature shifted the rate-limiting step of the catalytic reaction to an earlier stage that is not enantioselective. Hence the asymmetric induction-which is governed by the chiral phosphine-did not improve at low temperature.