Hydrido Iridium Complexes Research Articles

Asymmetric Transfer Hydrogenative Amination of Benzylic Ketones Catalyzed by Cp*Ir(III) Complexes Bearing a Chiral N-(2-Picolyl)sulfonamidato Ligand

A convenient asymmetric reductive amination of benzylic ketones (α-arylated ketones) catalyzed by newly designed Cp*Ir complexes bearing a chiral N-(2-picolyl)sulfonamidato ligand was developed. Using readily available β-amino alcohols as chiral aminating agents, a range of benzo-fused and acyclic ketones were successfully reduced with formic acid in methanol at 40 °C to afford amines with favorable chemo- and diastereoselectivities. The amino alcohol-derived chiral auxiliary was easily removed by mild periodic oxidants, leading to optically active primary β-arylamines without erosion of the optical purity (up to 97% ee). The excellent catalytic performance was retained even upon lowering the amount of catalyst to a substrate/catalyst (S/C) ratio of 20,000, and the amination could be performed on a large scale exceeding 100 g. The precise hydride transfer to iminium species generated from the ketonic substrate and the chiral amine counterpart was suggested by the mechanistic studies on stoichiometric reactions of isolable hydridoiridium complexes and model intermediates such as N,O-acetal, enamine, and iminium compounds.

Nucleophilic Aromatic Substitution in Hydrodefluorination Exemplified by Hydridoiridium(III) Complexes with Fluorinated Phenylsulfonyl-1,2-diphenylethylenediamine Ligands

In connection with the mechanism of the catalytic reduction of fluoroarenes, the intramolecular defluorinative transformation of a family of iridium hydrides utilized as a hydrogen transfer catalyst is studied. Hydridoiridium(III) complexes bearing fluorinated phenylsulfonyl-1,2-diphenylethylenediamine ligands are spontaneously converted into iridacycles via selective C–F bond cleavage at the ortho position. NMR spectroscopic studies and synthesis of intermediate model compounds verify the stepwise pathway involving intramolecular substitution of the ortho-fluorine atom by the hydrido ligand, i.e., hydrodefluorination (HDF), and the following fluoride-assisted cyclometalation at the transiently formed C–H bond. A hydridoiridium complex with a 2,3,4,5,6-pentafluorophenylsulfonyl (Fs) substituent is more susceptible to HDF than its analog with a 2,3,4,5-tetrafluorophenylsulfonyl (FsH) group. The FsH-derivative clearly shows that C–F bond cleavage occurs in preference to C–H activation. These experimental re...

A Bifunctional Iridium Catalyst Modified for Persistent Hydrogen Generation from Formic Acid: Understanding Deactivation via Cyclometalation of a 1,2-Diphenylethylenediamine Motif

Thermal degradation of a bifunctional Ir complex with a 1,2-diphenylethylenediamine (DPEN) framework was investigated, which is relevant to catalyst deactivation in the acceptorless dehydrogenation of formic acid. The well-defined hydridoiridium complex 1b, derived from N-triflyl-1,2-diphenylethylenediamine (TfDPEN), proved to be solely transformed at the reflux temperature of 1,2-dimethoxyethane (DME) into two iridacycles (2 and 3) via C–H bond cleavage at the ortho carbon atoms of the phenyl substituents on the diamine backbone. These products were successfully isolated and characterized by NMR, elemental analysis, and X-ray crystallography. The iridacycle formation was significantly enhanced in the presence of water, possibly due to facile deprotonative orthometalation via a hydroxidoiridium intermediate. To prevent the deactivation process caused by the cyclometalation of the DPEN moiety, a hydridoiridium complex (5b) without phenyl substituents was synthesized from N-triflylethylenediamine (TfEN). Th...

Comparative Study of Bifunctional Mononuclear and Dinuclear Amidoiridium Complexes with Chiral C-N Chelating Ligands for the Asymmetric Transfer Hydrogenation of Ketones.

A series of new bifunctional C-N chelating Ir complexes possessing a metal/NH group was synthesized by cyclometalation of optically active primary benzylic amines such as O-silylated (S)-2-amino-2-phenylethanols (1 a and 1 a'), (R)-5-amino-6,7,8,9-tetrahydro-5H-benzocycloheptene (1 b), and (R)-1-phenyl-2,2-dimethylpropylamine (1 c). Although treatment of KOtBu with the amine complexes originating from 1 a and 1 a' afforded amido-bridged dinuclear complexes (3 a and 3 a'), more sterically hindered complexes were solely transformed into the coordinatively unsaturated mononuclear amido complexes (3 b and 3 c), which can serve as real catalyst species in the asymmetric transfer hydrogenation. The structural difference in the C-N chelate framework markedly affected the catalytic performance. Among them, amido complex 3 c showed a pronounced ability to catalyze the transfer hydrogenation of acetophenone in 2-propanol, even at a low temperature of -30 °C. A hydridoiridium complex (4 c) was also identified in the reaction of 3 c in 2-propanol, which provides mechanistic insights into the enantiodiscriminating step in the hydrogen transfer to prochiral ketones.

Dehydrogenative Oxidation of Primary and Secondary Alcohols Catalyzed by a Cp*Ir Complex Having a Functional C,N-Chelate Ligand

A new catalytic system for the dehydrogenative oxidation of alcohols using a Cp*Ir complex having a functional C,N-chelate ligand has been developed. With this catalytic system, both primary and secondary alcohols were efficiently converted to aldehydes and ketones, respectively. Mechanistic investigations of this catalytic system have revealed that the catalytically active species is a hydrido iridium complex with a functional C,N-chelate ligand.

Remarkable Positive Effect of Silver Salts on Asymmetric Hydrogenation of Acyclic Imines with Cp*Ir Complexes Bearing Chiral N-Sulfonylated Diamine Ligands

A family of bifunctional Ir, Rh, and Ru complexes bearing chiral monosulfonylated diamine ligands has been evaluated for asymmetric hydrogenation of acyclic imines (1) to the corresponding amines (2). A chiral Ir complex, [Cp*IrCl{(S,S)-Tscydn}] (3a), combined with silver salts caused a marked improvement in the catalyst performance in terms of the activity and selectivity. The use of an excess amount of the silver salt, AgSbF6, increased enantioselectivity up to 72% ee in the asymmetric hydrogenation of N-(1-phenylethylidene)benzylamine (1a). A stoichiometric reaction of 3a with AgSbF6 in acetonitrile afforded an isolable cationic complex, [Cp*Ir(Tscydn)(CH3CN)]+SbF6− (4) which was fully characterized by NMR spectroscopy and X-ray crystallography. The resulting cationic complex 4 readily reacted with H2 under ambient conditions in the presence of triethylamine to give a hydridoiridium complex, [Cp*IrH{(S,S)-Tscydn}] (5) and showed comparable catalytic behavior to that for the catalyst system generated in situ from 3a and AgSbF6. On the basis of the additive effect on the outcome of the hydrogenation as well as the 13C{1H} NMR spectrum of a reaction mixture of imine 1a and AgSbF6, the mechanism of the imine hydrogenation including heterolytic bond cleavage of H2 on a cationic complex to generate a hydrido intermediate and the following H− transfer to the imine substrates activated by the silver cation was proposed.

Deprotonation of the Transition Metal Hydride (η5-C5Me5)(PMe3)IrH2. Synthesis and Chemistry of the Strongly Basic Lithium Iridate (η5-C5Me5)(PMe3)Ir(H)(Li)

Treatment of (η5-C5Me5)(PMe3)IrH2 (1) with tert-butyllithium gives (η5-C5Me5)(PMe3)Ir(H)(Li) (2) as a bright yellow solid. NMR evidence indicates that the lithium iridate 2 is aggregated in benzene, is converted to a single symmetrical species in THF, and is present as a dimer in DME. Treatment of 2 with 3,3-dimethylbutane trifluoromethanesulfonate-1,2-syn-d2 (3-syn-d2) gave the alkylated hydridoiridium complex 4a-anti-d2, which was converted to the corresponding chloride Cp*(PMe3)Ir(CHDCHDCMe3)(Cl) (4c-anti-d2) by treatment with CCl4. Analysis of this material by NMR spectroscopy showed that it was contaminated with ≤15% syn isomer. The alkylation therefore proceeds with predominant inversion of configuration at carbon, indicating that the major pathway is an SN2 displacement and not an outer-sphere electron-transfer reaction. Protonation studies carried out on iridate 2 with organic acids of varying pKa allowed us to estimate that the pKa of the dihydride 1 falls in the range 38−41, making it less acidic than DMSO and more acidic than toluene. This represents the least acidic transition metal hydride whose pKa has been quantitatively estimated. Treatment of 2 with main group electrophiles allowed the preparation of several other hydridoiridium derivatives, including Cp*(PMe3)Ir(SnPh3)(H) (5a), Cp*(PMe3)Ir(SnMe3)(H) (5b), and Cp*(PMe3)Ir(BR2)(H) (6a, R = F; 6b, R = Ph). Reaction of 2 with acid chlorides and anhydrides leads to acyl hydrides Cp*(PMe3)Ir(COR)(H), and fluorocarbons also react, giving products such as Cp*(PMe3)Ir(C6F5)(H) in the case of hexafluorobenzene as the electrophile.

Synthesis and Characterization of New Hydridoiridium Complexes Containing Carboxylate Ligands

We thank the DGICYT (Project PB 92-0092, Programa de Promocion General del Conocimiento) and EU (Projet: Selective Processes and Catalysis involving Small Molecules) for financial support. M.M. and E.O.thank the DGA (Diputacion General de Aragon) for a grant.

Carbon-hydrogen addition followed by alkyne insertion into the metal-hydrogen bond of an iridium complex

Thermal C-H activation of benzene is observed with [Ir(COD)(PMe 3 ) 3 ]Cl leading to the formation of mer-IrH(Ph)(Cl)(PMe 3 ) 3 (1). 1 will undergo further reaction with 2-butyne upon chloride removal to yield [Ir-(Ph)(methallyl)(PMe 3 ) 3 ] [PF 6 ] (2). 2 has been characterized by X-ray crystallography. Insertion of 2-butyne into the Ir-H bond of 1 followed by β-hydride elimination to yield an (allene)hydridoiridium complex appear to be key features in the formation of 2. This reaction sequence brings us a step closer to fully functionalizing hydrocarbons following C-H addition

Aspects of catalysis. Part IV. Hydrogen transfer from propan-2-ol to αβ-unsaturated ketones catalysed by a soluble hydridoiridium complex. Isolation of a compound containing a carbon–iridium bond and a chelated keto-group

Double bonds in some αβ-unsaturated ketones are reduced by propan-2-ol in the presence of soluble iridium–sulphoxide catalysts. Chalcone gives (2-benzoyl-1-phenylethyl-O)dichlorobis(dimethyl sulphoxide)iridium as a crystalline intermediate, which is converted into dihydrochalcone on treatment with acids.