Stereospecific synthesis and catalytic activity of L-histidylidene metal complexes.
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.
School of Chemistry and Chemical Biology, University College Dublin, Belfield, Dublin 4, Ireland.
Angèle
A
Monney
Elisabetta
E
Alberico
Yannick
Y
Ortin
Helge
H
Müller-Bunz
Serafino
S
Gladiali
Martin
M
Albrecht
D002384:Catalysis; D056831:Coordination Complexes / Q000138:chemical synthesis / Q000737:chemistry*; D018360:Crystallography, X-Ray; D006571:Heterocyclic Compounds / Q000737:chemistry; D006639:Histidine / Q000031:analogs & derivatives*; D007495:Iridium / Q000737:chemistry; D008024:Ligands; D008697:Methane / Q000031:analogs & derivatives; D008968:Molecular Conformation; D010720:Phosphines; D012238:Rhodium / Q000737:chemistry; D013237:Stereoisomerism; D013696:Temperature
D056831:Coordination Complexes; D006571:Heterocyclic Compounds; D008024:Ligands; D010720:Phosphines; C030011:carbene; D007495:Iridium; D006639:Histidine; D012238:Rhodium; C044646:phosphine; D008697:Methane