Ruthenacycles and Iridacycles as Catalysts for Asymmetric Transfer Hydrogenation and Racemisation

Ruthenacycles, which are easily prepared in a single step by reaction between enantiopure aromatic amines and [Ru(arene)Cl2]2 in the presence of NaOH and KPF6, are very good asymmetric transfer hydrogenation catalysts. A range of aromatic ketones were reduced using isopropanol in good yields with ee’s up to 98%. Iridacycles, which are prepared in similar fashion from [IrCp*Cl2]2 are excellent catalysts for the racemisation of secondary alcohols and chlorohydrins at room temperature. This allowed the development of a new dynamic kinetic resolution of chlorohydrins to the enantiopure epoxides in up to 90% yield and 98% enantiomeric excess (ee) using a mutant of the enzyme Haloalcohol dehalogenase C and an iridacycle as racemisation catalyst.

Nickel complexes of 1,2,4-triazole derived amido-functionalized N-heterocyclic carbene ligands: Synthesis, theoretical studies and catalytic application

A series of nickel complexes (1-3)b of 1,2,4-triazole derived amido-functionalized N-heterocyclic carbene ligands were synthesized and structurally characterized. In particular, the [1-(R)-4-N-(furan-2-ylmethyl) acetamido-1,2,4-triazol-5-ylidene](2)Ni [R = Et (1b), i-Pr (2b) and Bn (3b)] complexes were obtained by the direct reaction of the corresponding triazolium chloride salts (1-3) a by the treatment with NiCl2 center dot 6H(2)O in presence of K2CO3 as a base. The density functional theory studies performed on these complexes reveal highly polar character of the NHC-Ni sigma-bonding interaction with corresponding molecular orbital having a maximum contribution (59-69%) from the NHC ligand fragments while that of a minimum contribution (4%) from the central nickel atom. The (1-3)b complexes were found to be moderately active for the catalytic borylation reactions of bromoaryl derivatives by bis(pinacolato) diboron reagent (B(2)pin(2)) in the presence of Cs2CO3 as a base at 70 degrees C. (C) 2015 Elsevier B.V. All rights reserved

Cationic iron(II) complexes of the mixed cyclopentadienyl (Cp) and the N-heterocyclic carbene (NHC) ligands as effective precatalysts for the hydrosilylation of carbonyl compounds

A series of iron(II) complexes of N-heterocyclic carbene ligands was synthesized and fully structurally characterized. Specifically, the benzimidazole based {Cp[1,3-di-R-benzimidazol-2-ylidene]-Fe(CO)(2)}I [R = Et (1b), i-Pr (2b) and n-Bu (3b)] and the imidazole based {Cp[1-benzyl-3-R-imidazol-2-ylidene] Fe(CO)(2)}PF6 [R - Me (4b) and Et (5b)] type of complexes were synthesized from their respective benzimidazolium iodide (1-3)a and their imidazolium hexafluorophosphate (4-5)a salts by the reaction with CpFe(CO)(2)I in the presence of MN(SiMe3)(2) (M = Li or K) as a base. The molecular structures of the (1-5)b complexes reveal that the metal center display a conventional piano stool structure. More importantly, the (1-5)b complexes, when irradiated with visible light, effectively catalyzed the hydrosilylation reaction of carbonyl compounds namely, of the aldehyde and ketone substrates, using organosilane reagents. Specifically, the (1-5)b complexes performed the hydrosilylation of a representative benzaldehyde substrate using phenylsilane in ambient conditions at 30 degrees C while that of the representative acetophenone substrate at a more elevated temperature of 70 degrees C. The benzimidazole derived complexes (1-3)b displayed superior activity than the imidazole derived (4-5)b complexes. (C) 2014 Elsevier B. V. All rights reserved