A highly effective and green procedure for the formation of α-alkylated ketones has been disclosed via the reaction of primary alcohols with secondary alcohols and ketones by using [IrCl(COD)(NHC)] complexes as a catalyst. Various α-alkylated ketones were obtained in high yields from the alkylation of alcohol with alcohol and ketone with alcohol through a borrowing hydrogen reaction by using 0.05-0.5 mol % iridium(I) and a catalytic amount of KOH (5-10 mol %) as the base under air atmosphere and within very short reaction times.

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A new phenoxide chelated [Ir(NHC)Cp*Cl] (NHC = Nheterocyclic carbene; Cp* = pentamethylcyclopentadienyl) complex (3) has been prepared by reaction of [IrCp*Cl 2 ] 2 with an in situ prepared NHC−Ag complex in dichloromethane at ambient temperature. The Ir III complex was stable toward air and moisture and was fully characterized by 1 H, 13 C NMR, HRMS, and single-crystal X-ray diffraction. The new complex was found to be an active catalyst for transfer hydrogenative reductive amination under aqueous conditions with formate as hydrogen source as well as hydrogenative reductive amination reactions using H 2 . Various carbonyl compounds such as aliphatic and aromatic ketones and aldehydes were successfully reacted with amines to give new amines. In comparison with transfer hydrogenative reductive amination, the reductive amination with H 2 is faster and permits higher molar ratios of the substrate to the catalyst (S/ C).

Iridium­(I) complexes having an imidazol-2-ylidene ligand with benzylic wingtips efficiently catalyzed the β-alkylation of secondary alcohols with primary alcohols and acceptorless dehydrogenative cyclization of 2-aminobenzyl alcohol with ketones through a borrowing hydrogen pathway. The β-alkylated alcohols, including cholesterol derivatives, and substituted quinolines were obtained in good yields by using a minute amount of the catalyst with a catalytic amount of NaOH or KOH under the air atmosphere, liberating water (and H2 in the case of quinoline synthesis) as the sole byproduct. Notably, this system demonstrated turnover numbers of 940 000 (for β-alkylation of secondary alcohols with primary alcohols by using down to 0.0001 mol % = 1 ppm of the catalyst) and 9200 (acceptorless dehydrogenative cyclization of 2-aminobenzyl alcohol with ketones).

Two new [Ir(NHC)(COD)Cl] (NHC = N-heterocyclic carbene; COD = 1,5-cyclooctadiene) iridium complexes (2a,b) have been prepared by the reaction of [Ir(COD)Cl]2 with in situ prepared NHC-Ag carbene transfer agents in dichloromethane at ambient temperature. They have been fully characterized by (1)H, (13)C NMR, and elemental analysis. X-ray diffraction studies on single crystals of 2a and 2b confirm the square planar geometry. Complexes of type [Ir(NHC)(CO)2Cl] [NHC = 1,3-diisopropyl(5,6-dimethyl)benzimidazol-2-ylidene] 3 were also synthesized to compare σ-donor/π-acceptor strength of NHC ligands. Transfer hydrogenation (TH) reactions of various aldehydes and ketones have been studied using complexes 2a and 2b as catalysts. The 5,6-dimethyl substituted iridium complex (2b) showed the highest catalytic activity for the TH reaction.

A series of new Ir(III) complexes with carbene ligands that contain a range of benzyl wingtip groups have been prepared and fully characterised by NMR spectroscopy, HRMS, elemental analysis and X-ray diffraction. All the complexes were active in the acceptorless dehydrogenation of alcohol substrates in 2,2,2-trifluoroethanol to give the corresponding carbonyl compounds. The most active complex bore an electron-rich carbene ligand; this complex was used to catalyse the highly efficient and chemoselective dehydrogenation of a wide range of secondary alcohols to their respective ketones, with turnover numbers up to 1660. Mechanistic studies suggested that the turnover of the dehydrogenation reaction is limited by the H2 -formation step.

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N‐[(1‐Phosphanylferrocen‐1′‐yl)methyl]‐N′‐[(2,4,6‐trimethylphenyl)methyl]‐5,6‐di‐X‐benzimidazolium tetrafluoroborate salts (X = H, 5a and Me, 5b), precursors of new phosphane‐benzimidazol‐2‐ylidene bifunctional ligands, and related ferrocenyl 5,6‐di‐X‐benzimidazol‐2‐ylidene iodide salts (X = H, 6a and Me, 6b), precursors of monodentate benzimidazol‐2‐ylidene ligands, have been prepared for the first time. Cationic rhodium(I) complexes 7a and 7b and neutral rhodium(I) complexes 8a and 8b have been obtained in good yields and have been fully characterised. Cationic rhodium(I) complexes 10a and 10b were prepared from 8a and 8b, for comparison with complexes 7, but not isolated. All complexes showed good activities for the catalytic hydrosilylation of acetophenone derivatives. The activities are much greater than for related imidazol‐2‐ylidene systems, and the cationic complexes are more active than the neutral complexes; the highest activity is observed for the more soluble complex 7b. The use of bidentate ligands proved to be essential for obtaining good selectivities of the desired alcohol.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)