New Bifunctional Bis(azairidacycle) with Axial Chirality via Double Cyclometalation of 2,2'-Bis(aminomethyl)-1,1'-binaphthyl.

As a candidate for bifunctional asymmetric catalysts containing a half-sandwich C-N chelating Ir(III) framework (azairidacycle), a dinuclear Ir complex with an axially chiral linkage is newly designed. An expedient synthesis of chiral 2,2'-bis(aminomethyl)-1,1'-binaphthyl (1) from 1,1-bi-2-naphthol (BINOL) was accomplished by a three-step process involving nickel-catalyzed cyanation and subsequent reduction with Raney-Ni and KBH4. The reaction of (S)-1 with an equimolar amount of [IrCl2Cp*]2 (Cp* = η5-C5(CH3)5) in the presence of sodium acetate in acetonitrile at 80 °C gave a diastereomeric mixture of new dinuclear dichloridodiiridium complexes (5) through the double C-H bond cleavage, as confirmed by 1H NMR spectroscopy. A loss of the central chirality on the Ir centers of 5 was demonstrated by treatment with KOC(CH3)3 to generate the corresponding 16e amidoiridium complex 6. The following hydrogen transfer from 2-propanol to 6 provided diastereomers of hydrido(amine)iridium retaining the bis(azairidacycle) architecture. The dinuclear chlorido(amine)iridium 5 can serve as a catalyst precursor for the asymmetric transfer hydrogenation of acetophenone with a substrate to a catalyst ratio of 200 in the presence of KOC(CH3)3 in 2-propanol, leading to (S)-1-phenylethanol with up to an enantiomeric excess (ee) of 67%.

Transfer hydrogenation of carbon dioxide via bicarbonate promoted by bifunctional C-N chelating Cp*Ir complexes.

Metal-ligand cooperative Cp*Ir(iii) complexes derived from primary benzylic amines effectively promote transfer hydrogenation of atmospheric CO2 using 2-propanol at 80 °C. Isotope-labelling experiments strengthen that active Ir species can preferentially reduce bicarbonate congeners formed from CO2. The powerful transfer hydrogenation catalyst exhibits remarkable activity for the conversion of bicarbonates into formate salts with a turnover number up to 3200, even without H2 and CO2.

Half-Sandwich Iridium Complexes Bearing a Diprotic Glyoxime Ligand: Structural Diversity Induced by Reversible Deprotonation.

Synthesis and deprotonation reactions of half-sandwich iridium complexes bearing a vicinal dioxime ligand were studied. Treatment of [{Cp*IrCl(µ-Cl)}2 ] (Cp*=η5 -C5 Me5 ) with dimethylglyoxime (LH2 ) at an Ir:LH2 ratio of 1:1 afforded the cationic dioxime iridium complex [Cp*IrCl(LH2 )]Cl (1). The chlorido complex 1 undergoes stepwise and reversible deprotonation with potassium carbonate to give the oxime-oximato complex [Cp*IrCl(LH)] (2) and the anionic dioximato(2-) complex K[Cp*IrCl(L)] (3) sequentially. Meanwhile, twofold deprotonation of the sulfato complex [Cp*Ir(SO4 )(LH2 )] (4) resulted in the formation of the oximato-bridged dinuclear complex [{Cp*Ir(µ-L)}2 ] (5). X-ray analyses disclosed their supramolecular structures with one-dimensional infinite chain (1 and 2), hexagonal open channels (3), and a tetrameric rhomboid (4) featuring multiple intermolecular hydrogen bonds and electrostatic interactions.

Reductive Amination of Ketonic Compounds Catalyzed by Cp*Ir(III) Complexes Bearing a Picolinamidato Ligand.

Cp*Ir complexes bearing a 2-picolinamide moiety serve as effective catalysts for the direct reductive amination of ketonic compounds to give primary amines under transfer hydrogenation conditions using ammonium formate as both the nitrogen and hydrogen source. The clean and operationally simple transformation proceeds with a substrate to catalyst molar ratio (S/C) of up to 20,000 at relatively low temperature and exhibits excellent chemoselectivity toward primary amines.

Protic N-Heterocyclic Carbene Versus Pyrazole: Rigorous Comparison of Proton- and Electron-Donating Abilities in a Pincer-Type Framework.

Evaluation of the acidity of proton-responsive ligands such as protic N-heterocyclic carbenes (NHCs) bearing an NH-wingtip provides a key to understanding the metal-ligand cooperation in enzymatic and artificial catalysis. Here, we design a CNN pincer-type ruthenium complex 2 bearing protic NHC and isoelectronic pyrazole units in a symmetrical skeleton, to compare their acidities and electron-donating abilities. The synthesis is achieved by direct C-H metalation of 2-(imidazol-1-yl)-6-(pyrazol-3-yl)pyridine with [RuCl2 (PPh3 )3 ]. 15 N-Labeling experiments confirm that deprotonation of 2 occurs first at the pyrazole side, indicating clearly that the protic pyrazole is more acidic than the NHC group. The electrochemical measurements as well as derivatization to carbonyl complexes demonstrate that the protic NHC is more electron-donating than pyrazole in both protonated and deprotonated forms.

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.

Efficient Access to Chiral Benzhydrols via Asymmetric Transfer Hydrogenation of Unsymmetrical Benzophenones with Bifunctional Oxo-Tethered Ruthenium Catalysts.

A concise asymmetric transfer hydrogenation of diaryl ketones, promoted by bifunctional Ru complexes with an etherial linkage between 1,2-diphenylethylenediamine (DPEN) and η(6)-arene ligands, was successfully developed. Because of the effective discrimination of substituents at the ortho position on the aryl group, unsymmetrical benzophenones were smoothly reduced in a 5:2 mixture of formic acid and triethylamine with an unprecedented level of excellent enantioselectivity. For the non-ortho-substituted benzophenones, the oxo-tethered catalyst electronically discerned biased substrates, resulting in attractive performance yielding chiral diarylmethanols with >99% ee.

Protic Ruthenium Tris(pyrazol-3-ylmethyl)amine Complexes Featuring a Hydrogen-Bonding Network in the Second Coordination Sphere.

We synthesized ruthenium complexes bearing a tris(pyrazol-3-ylmethyl)amine ligand LH3 and revealed that this tripodal ligand allows predictable accumulation of three proton-delivering NH groups around a coordination site. The Brønsted acidity of the NH groups in LH3 led to the formation of multiple hydrogen bonds with the substrate ligand and deprotonation. The chlorido complex ligated by LH3 catalyzed disproportionation of 1,2-diphenylhydrazine.

Enhanced Hydrogen Generation from Formic Acid by Half-Sandwich Iridium(III) Complexes with Metal/NH Bifunctionality: A Pronounced Switch from Transfer Hydrogenation.

By switching the catalytic function from transfer hydrogenation based on the metal/NH bifunctionality, facile dehydrogenation of formic acid was achieved by amido- and hydrido(amine)-Ir complexes derived from N-triflyl-1,2-diphenylethylenediamine (TfDPEN) at ambient temperature even in the absence of base additives. Further acceleration was observed by the addition of water, leading to a maximum turnover frequency above 6000 h(-1). A proton-relay mechanism guided by the protic amine ligand and water is postulated for effective protonation of metal hydrides.

Highly selective carboxylative cyclization of allenylmethylamines with carbon dioxide using N-heterocyclic carbene-silver(I) catalysts.

Silver(I) carboxylate complexes promote the carboxylative cyclization of allenylmethylamines to afford 5-alkenyl-1,3-oxazolidin-2-ones in 2-propanol. The use of an N-heterocyclic carbene ligand (IPr) under pressurized CO2 is effective in suppressing the intramolecular hydroamination that leads to 2,5-dihydropyrroles. The mechanism involving a nucleophilic attack of the carbamate of the allene moiety and a subsequent protonation was realized on the basis of experimental and theoretical results involving a model intermediate, the alkenylgold(I) complex, which was synthesized from Au(OH)(IPr) and 1-methylamino-2,3-butadiene.

Copper-catalyzed formal C - H carboxylation of aromatic compounds with carbon dioxide through arylaluminum intermediates.

The C - H bond carboxylation of various aromatic compounds with CO2 was achieved by the deprotonative alumination with a mixed alkyl amido lithium aluminate compound iBu3 Al(TMP)Li followed by the NHC-copper-catalyzed carboxylation of the resulting arylaluminum species, which afforded the corresponding carboxylation products in high yield and high selectivity. In addition to benzene derivatives, heteroarenes such as benzofuran, benzothiophene, and indole derivatives are also suitable substrates. Functional groups such as Cl, Br, I, vinyl, amide, and CN could survive the reaction conditions. Some key reaction intermediates such as the copper aryl and isobutyl complexes and their carboxylation products were isolated and structurally characterized by X-ray crystallographic analyses, thus offering important information on the reaction mechanism.

Metal-ligand bifunctional reactivity and catalysis of protic N-heterocyclic carbene and pyrazole complexes featuring ß-NH units.

Metal-ligand bifunctional cooperation has attracted much attention because it offers a powerful methodology to realize a number of highly efficient and selective catalysts. In this article, recent developments in the metal-ligand cooperative reactions of protic N-heterocyclic carbene (NHC) and pyrazole complexes bearing an acidic NH group at the position ß to the metal are surveyed. Protic 2-pyridylidenes as related cooperating non-innocent ligands are also described.

Unsymmetrical pincer-type ruthenium complex containing ß-protic pyrazole and N-heterocyclic carbene arms: comparison of Brønsted acidity of NH groups in second coordination sphere.

A reaction of a 2-(imidazol-1-yl)methyl-6-(pyrazol-3-yl)pyridine with [RuCl2 (PPh3 )3 ] resulted in tautomerization of the imidazole unit to afford the unsymmetrical pincer-type ruthenium complex 2 containing a protic pyrazole and N-heterocyclic carbene (NHC) arms. Deprotonation of 2 with one equivalent of a base led to the formation of the NHC-pyrazolato complex 3, indicating that the protic NHC arm is less acidic. When 2 was treated with two equivalents of a base under H2 or in 2-propanol, the hydrido complex 4 containing protic NHC and pyrazolato groups was obtained through metal-ligand cooperation.

Practical selective hydrogenation of α-fluorinated esters with bifunctional pincer-type ruthenium(II) catalysts leading to fluorinated alcohols or fluoral hemiacetals.

Selective hydrogenation of fluorinated esters with pincer-type bifunctional catalysts RuHCl(CO)(dpa) 1a, trans-RuH2(CO)(dpa) 1b, and trans-RuCl2(CO)(dpa) 1c under mild conditions proceeds rapidly to give the corresponding fluorinated alcohols or hemiacetals in good to excellent yields. Under the optimized conditions, the hydrogenation of chiral (R)-2-fluoropropionate proceeds smoothly to give the corresponding chiral alcohol without any serious decrease of the ee value.

N-N bond cleavage of hydrazines with a multiproton-responsive pincer-type iron complex.

N-N bond cleavage of hydrazines on transition metals is of considerable importance in understanding the mechanism of biological nitrogen fixation under ambient conditions. We found that a metal-ligand-bifunctional complex of iron with a pincer-type ligand bearing two proton-responsive pyrazole arms catalyzes the disproportionation of hydrazine into ammonia and dinitrogen. The NH groups in the pyrazole ligands and hydrazines are crucial for the reaction, which most likely occurs through multiple and bidirectional proton-coupled electron transfer between the iron complex and hydrazine. The multiproton-responsive pincer-type ligand also stabilizes the intermediate diazene complex through a hydrogen-bonding network, as revealed by structural characterization of a κ(1)N-phenylhydrazine complex.

Quantum chemical calculations with the inclusion of nonspecific and specific solvation: asymmetric transfer hydrogenation with bifunctional ruthenium catalysts.

Details of the mechanism of asymmetric transfer hydrogenation of ketones catalyzed by two chiral bifunctional ruthenium complexes, (S)-RuH[(R,R)-OCH(Ph)CH(Ph)NH(2)](η(6)-benzene) (Ru-1) or (S)-RuH[(R,R)-p-TsNCH(Ph)CH(Ph)NH(2)](η(6)-mesitylene) (Ru-2), were studied computationally by density functional theory, accounting for the solvation effects by using continuum, discrete, and mixed continuum/discrete solvation models via "solvated supermolecules" approach. In contrast to gas phase quantum chemical calculations, where the reactions were found to proceed via a concerted three-bond asynchronous process through a six-membered pericyclic transition state, incorporation of the implicit and/or explicit solvation into the calculations suggests that the same reactions proceed via two steps in solution: (i) enantio-determining hydride transfer and (ii) proton transfer through the contact ion-pair intermediate, stabilized primarily by ionic hydrogen bonding between the cation and the anion. The calculations suggest that the proton source for neutralizing the chiral RO(-) anion may be either the amine group of the cationic Ru complex or, more likely, a protic solvent molecule. In the latter case, the reaction may not necessarily proceed via the 16e amido complex Ru[(R,R)-XCH(Ph)CH(Ph)NH](η(6)-arene). The origin of enantioselectivity is discussed in terms of the newly formulated mechanism.

Synthesis and asymmetric hydrogenation of (3E)-1-benzyl-3-[(2-oxopyridin-1(2H)-yl)methylidene]piperidine-2,6-dione.

The synthesis of (3E)-1-benzyl-3-[(2-oxopyridin-1(2H)-yl)methylidene]piperidine-2,6-dione 5 from N-benzylglutarimide was achieved in three steps. The asymmetric hydrogenation of 4 gave either the product of partial reduction (10) or full reduction (13), depending on the catalyst which was employed, in high ee in each case. Attempts at asymmetric transfer hydrogenation (ATH) of resulted in formation of a racemic product.

Synthesis, structures, and reactivities of pincer-type ruthenium complexes bearing two proton-responsive pyrazole arms.

A new metal-ligand bifunctional, pincer-type ruthenium complex [RuCl(L1-H(2))(PPh(3))(2)]Cl (1; L1-H(2)=2,6-bis(5-tert-butyl-1H-pyrazol-3-yl)pyridine) featuring two proton-delivering pyrazole arms has been synthesized. Complex 1, derived from [RuCl(2)(PPh(3))(3)] with L1-H(2), underwent reversible deprotonation with potassium carbonate to afford the pyrazolato-pyrazole complex [RuCl(L1-H)(PPh(3))(2)] (2). Further deprotonation of 1 and 2 with potassium hexamethyldisilazide in methanol resulted in the formation of the bis(pyrazolato) complex [Ru(L1)(MeOH)(PPh(3))(2)] (3). Complex 3 smoothly reacted with dioxygen and dinitrogen to give the side-on peroxo complex [Ru(L1)(O(2))(PPh(3))(2)] (4) and end-on dinitrogen complex [Ru(L1)(N(2))(PPh(3))(2)] (5), respectively. On the other hand, the reaction of [RuCl(2)(PPh(3))(3)] with less hindered 2,6-di(1H-pyrazol-3-yl)pyridine (L3-H(2)) led to the formation of the dinuclear complex [{RuCl(2)(PPh(3))(2)}(2)(µ(2)-L3-H(2))(2)] (6), in which the pyrazole-based ligand adopted a tautomeric form different from L1-H(2) in 1 and the central pyridine remained uncoordinated. The detailed structures of 1-6 were determined by X-ray crystallography.

Efficient dynamic kinetic resolution of racemic secondary alcohols by a chemoenzymatic system using bifunctional iridium complexes with C-N chelate amido ligands.

The combined catalyst system of bifunctional amidoiridium complexes derived from benzylic amines with CALB was found to provide a range of chiral acetates from racemic secondary alcohols in excellent yields with nearly perfect enantioselectivities via dynamic kinetic resolution.

Oxo-tethered ruthenium(II) complex as a bifunctional catalyst for asymmetric transfer hydrogenation and H2 hydrogenation.

Newly developed oxo-tethered Ru amido complexes (R,R)-1 and their HCl adducts (R,R)-2 exhibited excellent catalytic performance for both asymmetric transfer hydrogenation and the hydrogenation of ketonic substrates under neutral conditions without any cocatalysts to give chiral secondary alcohols with high levels of enantioselectivity.