Chiral Iridium Complexes Research Articles

Highly efficient circularly polarized phosphorescent electroluminescence from iridium(iii) complexes with chiral ligands

CP-OLEDs based on new phosphors exhibited an EQEmax of 31.9% and a |gEL| of up to 3.0 × 10−3.

Iridium‐Catalyzed Chemo‐, Diastereo‐, and Enantioselective Allyl‐Allyl Coupling: Accessing All Four Stereoisomers of (E)‐1‐Boryl‐Substituted 1,5‐Dienes by Chirality Pairing

AbstractHere, we report a highly chemo‐, diastereo‐, and enantioselective allyl‐allyl coupling between branched allyl alcohols and α‐silyl‐substituted allylboronate esters, catalyzed by a chiral iridium complex. The α‐silyl‐substituted allylboronate esters can be chemoselectively coupled with allyl electrophiles, affording a diverse set of enantioenriched (E)‐1‐boryl‐substituted 1,5‐dienes in good yields, with excellent stereoselectivity. By permuting the chiral iridium catalysts and the substrates, we efficiently and selectively obtained all four stereoisomers bearing two consecutive chiral centers. Mechanistic studies via density functional theory calculations revealed the origins of the diastereo‐ and chemoselectivities, indicating the pivotal roles of the steric interaction, the β‐silicon effect, and a rapid desilylation process. Additional synthetic modifications for preparing a variety of enantioenriched compounds containing contiguous chiral centers are also included.

Iridium-Catalyzed Chemo-, Diastereo-, and Enantioselective Allyl-Allyl Coupling: Accessing All Four Stereoisomers of (E)-1-Boryl-Substituted 1,5-Dienes by Chirality Pairing.

Here, we report a highly chemo-, diastereo-, and enantioselective allyl-allyl coupling between branched allyl alcohols and α-silyl-substituted allylboronate esters, catalyzed by a chiral iridium complex. The α-silyl-substituted allylboronate esters can be chemoselectively coupled with allyl electrophiles, affording a diverse set of enantioenriched (E)-1-boryl-substituted 1,5-dienes in good yields, with excellent stereoselectivity. By permuting the chiral iridium catalysts and the substrates, we efficiently and selectively obtained all four stereoisomers bearing two consecutive chiral centers. Mechanistic studies via density functional theory calculations revealed the origins of the diastereo- and chemoselectivities, indicating the pivotal roles of the steric interaction, the β-silicon effect, and a rapid desilylation process. Additional synthetic modifications for preparing a variety of enantioenriched compounds containing contiguous chiral centers are also included.

Synthesis and Characterization of Chiral Iridium Complexes Bearing Carbohydrate Functionalized Pyridincarboxamide Ligands and Their Application as Catalysts in the Asymmetric Transfer Hydrogenation of α-Ketoacids in Water

[Cp*IrLnCl] complexes [L1 = (methyl-β-d-glucopyranosid-2-yl)picolinamide, 1; L2 = (methyl-3,4,6-tri-O-acetyl-β-d-glucopyranosid-2-yl)picolinamide, 2; L3 = (2,3,4,6-tetra-O-acetyl-β-d-glucopyranosid-1-yl)picolinamide, 3] have been synthesized and completely characterized in solution, by 1D- and 2D-NMR spectroscopy, and in the solid state, by X-ray single crystal diffractometry. Despite the chirality of the Ln-moiety and metal, a single diastereoisomer is observed for L1 (1) and L2 (2) having a (R)-iridium configuration: the pyranose moiety is oriented in a way to minimize the interactions of the axial protons, vicinal to the amide moiety, and Cp*, with the OMe-group pointing toward the Cp*-ligand and away from Ir–Cl. Such a diastereoisomer is also favored by the establishment of an O–H···Cl–Ir hydrogen bond (2.356 Å) and by the minimization of the steric repulsion between one acetyl moiety of L2 and Cp* and picolinamide ligands in 1 and 2, respectively. DFT calculations computed a stabilization by more than 5.9 and 3.1 kcal/mol of this diastereoisomer with respect to other possible ones. Two interconverting diastereoisomers with different chirality at iridium are instead observed in solution for complex 3 in which −CH2OAc [3a, 63%, (R)] and −OAc [3b, 37%, (S)] moieties, respectively, are oriented toward Cp* and N-arm of picolinamide ligands. Consistently, DFT calculations indicate that 3a and 3b have a comparable stability (ΔE = 1.2 kcal/mol). Complexes 1–3 catalyze the asymmetric transfer hydrogenation of RC(O)C(O)OH to RCH(OH)C(O)OH [R = Ph (PGA), CH2Ph (PPA), CH2(4-OH)C6H4 (HPPA)], using both HCOOH and H3PO3 as hydrogen donor, in water at pH 7 (by phosphate buffer), with excellent chemoselectivity and efficiency (conversion >99%) and moderate to good enantioselectivity (30–70% ee). Utilizing catalyst 3 instead of 2, bearing the pseudoenantiomeric L3 of L2 ligand, causes a reduction of the percentage of the major enantiomer (R) with PGA and an inversion of stereoselectivity from (R) to (S) with PPA and HPPA substrates.

Light-driven redox-relay deracemization of cyclic sulfonamides catalyzed by a chiral-at-metal iridium complex

A visible-light-driven redox-relay deracemization of cyclic sulfonamides catalyzed by a chiral-at-metal iridium complex as a single catalyst is described.

The Implications of the Brønsted Acidic Properties of Crabtree-Type Catalysts in the Asymmetric Hydrogenation of Olefins.

Chiral iridium complexes derived from Crabtree’s catalyst are highly useful in modern hydrogenations of olefins attributed to high reactivity, stereoselectivity, and stability. Despite that these precatalysts are pH neutral, the reaction mixtures turn acidic under hydrogenation conditions. This Perspective is devoted to the implications of the intrinsic Brønsted acidity of catalytic intermediates in asymmetric hydrogenation of olefins. Despite that the acidity has often been used only as a rationale for side-product formation, more recent methodologies have started to use this property advantageously. We hope that this Perspective serves as a stimulant for the development of such compelling and new asymmetric hydrogenations. The inherent scientific opportunities in utilizing or annihilating the generated Brønsted acid are enormous, and potential new innovations are outlined toward the end.

Vibrational Circular Dichroism Spectroscopy toward Intercalation Compounds of Sodium Montmorillonite: Evidences for Molecular Packing of Enantiopure Monovalent Ir(III) Complexes within Interlayer Spaces

Solid state vibrational circular dichroism (SD-VCD) spectra were recorded on the intercalation compounds of sodium montmorillonite with a chiral iridium(III) complex. The complex used was Δ- (or Λ-)[Ir(bzq)2(phen)]+ (bzqH = benzo[h]quinoline; phen = 1,10-phenanthroline). The complex formed mono- or bi-molecular layers in interlayer spaces, depending on the adsorption amount to 50% or 100% cation exchange capacity (CEC), respectively. Notably, the VCD signals were sensitive enough to reveal the detailed molecular interactions in adsorption layers. Signal enhancement was achieved by the coherent delocalization of vibrational motions over tightly bound associates. Models of molecular packing were proposed with the help of theoretical simulations.

Cationic Iridium-Catalyzed Asymmetric Decarbonylative Aryl Addition of Aromatic Aldehydes to Bicyclic Alkenes.

We report an unprecedented catalytic protocol for the enantioselective decarbonylative transformation of aryl aldehydes. In this process, the decarbonylation of aldehydes catalyzed by chiral iridium complexes enabled the formation of asymmetric C-C bonds through the formation of an aryl-iridium intermediate. The decarbonylative aryl addition to bicyclic alkenes was fluidly performed without a stoichiometric aryl-metal reagent, such as aryl boronic acid, with a cationic iridium complex generated in situ from Ir(cod)2 (BArF 4 ) and the sulfur-linked bis(phosphoramidite) ligand ((R,R)-S-Me-BIPAM). This reaction has broad functional group compatibility, and no waste is generated, except carbon monoxide.

Highly efficient and enantioselective synthesis of chiral lactones via Ir-catalysed asymmetric hydrogenation of ketoesters.

Owing to the biological significance and great synthetic value of chiral lactones and their derivatives, increasing attention has been paid to developing effective synthetic methods for chiral lactones. We herein report an efficient asymmetric hydrogenation of benzo-fused ketoesters, γ-ketoesters and biaryl-bridged ketoesters catalyzed by chiral iridium complexes bearing ferrocene-based chiral ligands, furnishing a series of chiral lactones in superb yields and excellent enantioselectivities (up to 99% yields and up to 99% ee).

Asymmetric hydrogenation of 1,4-diketones: facile synthesis of enantiopure 1,4-diarylbutane-1,4-diols.

Owing to the biological significance and great synthetic value of 1,4-diarylbutane-1,4-diols and their derivatives, increasingly considerable attention has been paid to developing effective synthetic methods for chiral 1,4-diarylbutane-1,4-diols. We herein report an efficient asymmetric hydrogenation of 1,4-diaryldiketones catalyzed by a chiral iridium complex bearing f-amphox as ligand, furnishing a series of 1,4-diarylbutane-1,4-diols in excellent yields (up to >99%) with exceptional enantioselectivities (up to >99.9% ee) and diastereoselectivities (up to >100 : 1 dr).

Chiral Iridium Complexes of Anionic NCP Pincer Ligand for Asymmetric Transfer Hydrogenation of 1,1-Diarylethenes with Ethanol.

Chiral iridium complexes ligated by anionic oxazoline-bearing NCP-type pincer ligands were developed and applied to the asymmetric transfer hydrogenation (ATH) of diarylethenes using environmentally benign ethanol as the hydrogen donor. High enantioselectivities could be achieved for substrates bearing ortho-Me, ortho-Cl, or ortho-Br substituents on one of the aryl groups. The ATH of ortho-Br-substituted diarylethenes is particularly attractive due to the propensity of the C(aryl)-Br bond to undergo various new bond-forming events.

Asymmetric Stepwise Reductive Amination of Aryl N‐Heteroaryl Ketones with Benzyl Amines via Iridium Catalysis

AbstractAn asymmetric reductive amination of aryl N‐heteroaryl ketones with Benzyl Amines has been realized via one‐pot two‐step process, using p‐toluenesulfonic acid and chiral iridium complexes as catalysts, a mixture of formic acid/triethylamine as a reductant. Twenty‐three examples of chiral aryl N‐heteroaryl methylamines were obtained with good yields and up to 99% ee.

Tandem Catalytic Indolization/Enantioconvergent Substitution of Alcohols by Borrowing Hydrogen to Access Tricyclic Indoles

AbstractAn efficient tandem catalysis method is achieved for the direct conversion of alcohol‐containing alkynyl anilines to valuable chiral 2,3‐fused tricyclic indoles. This method relies on a tandem indolization followed by enantioconvergent substitution of alcohols via borrowing hydrogen to construct two rings in one step, enabled by relay and cooperative catalysis of a chiral iridium complex with a chiral phosphoric acid. Highly diastereoselective transformations of the tricyclic indole products also provide efficient access to a diverse array of complex polycyclic indoline compounds.

Tandem Catalytic Indolization/Enantioconvergent Substitution of Alcohols by Borrowing Hydrogen to Access Tricyclic Indoles.

An efficient tandem catalysis method is achieved for the direct conversion of alcohol-containing alkynyl anilines to valuable chiral 2,3-fused tricyclic indoles. This method relies on a tandem indolization followed by enantioconvergent substitution of alcohols via borrowing hydrogen to construct two rings in one step, enabled by relay and cooperative catalysis of a chiral iridium complex with a chiral phosphoric acid. Highly diastereoselective transformations of the tricyclic indole products also provide efficient access to a diverse array of complex polycyclic indoline compounds.

Semitransparent Circularly Polarized Phosphorescent Organic Light‐Emitting Diodes with External Quantum Efficiency over 30% and Dissymmetry Factor Close to 10−2

AbstractTo concurrently realize large electroluminescence dissymmetry factor and high device efficiency remains a formidable challenge in the development of circularly polarized organic light‐emitting diode (CP‐OLED). In this work, by introducing a famous chiral resource of R‐camphor, two green chiral iridium(III) isomers of Λ/Δ‐Ir‐(R‐camphor) containing dual stereogenic centers at iridium and ancillary ligand, are efficiently synthesized. Benefiting from their high phosphorescence quantum yields (≈93%) and obvious circularly polarized photoluminescence property with dissymmetry factors (|gPL|) in the 10−3 order of magnitude, the circularly polarized phosphorescent organic light‐emitting diodes using these chiral emitters show excellent performances with the maximum external quantum efficiency (EQEmax) of 30.6%, low efficiency roll‐off, and intense circularly polarized electroluminescence signal with dissymmetry factor (gEL) in the 10−4 order of magnitude. By the novel device engineering with semitransparent cathode, the resulting gEL values are significantly boosted by one order of magnitude, up to 7.70 × 10−3, which is the highest among the CP‐OLEDs with chiral iridium complexes. Noteworthy, the semitransparent devices deliver a record‐high EQEmax of 18.8% among the semitransparent OLEDs. The combination of novel chiral Ir(III) complexes and the semitransparent devices sheds light on the development of high performance CP‐OLEDs with simultaneously high efficiency and large dissymmetry factor.

Iridium Catalysed Asymmetric Allylic Substitution Reaction of Indolizine Derivatives

AbstractA highly efficient direct asymmetric allylic substitution (AAS) reaction of indolizine derivatives with allylic alcohols for accessing enantioenriched indolizine derivatives was realized by combining a chiral iridium complex catalyst with Lewis acid under mild reaction conditions, delivering various chiral allylation products in remarkably high yields and excellent enantioselectivities. This protocol distinguishes itself by availability of the starting materials, mild reaction conditions, broad substrate scope, high yields, excellent selectivity and easy scale‐up in a stereoselective manner, which provides a highly efficient protocol for chiral indolizines.

Interplay of stereo-electronic, vibronic and environmental effects in tuning the chiroptical properties of an Ir(III) cyclometalated N-heterocyclic carbene

Chiroptical spectra are among the most suitable techniques for investigating the ground and excited electronic states of chiral systems, but their interpretation is not straightforward and strongly benefits from quantum chemical simulations, provided that the employed computational model is sufficiently accurate and deals properly with stereo-electronic, vibrational averaging and environmental effects. Since the synergy among all these effects is only rarely accounted for, especially for large and flexible organometallic systems, the main aim of this contribution is to illustrate the latest developments of computational approaches rooted into the density functional theory for describing stereo-electronic effects and complemented by effective techniques to deal with vibrational modulation effects and solvatochromic shifts. In this connection, chiral iridium complexes offer an especially suitable case study in view of their bright phosphorescence, which is particularly significant for building effective light emitting diodes (OLEDs) and biomarkers and can be finely tuned by the nature of the metal ligands. For instance, a recently synthesized family of cycloiridiated complexes, KC and KD, bearing a pentahelicenic N-heterocyclic carbene (KB), has shown an enhanced long-lasting, bright phosphorescence. Deeper insights into the still unclear nature and origin of the enhancement could be gained by the interpretation of the chiroptical spectra, which is quite challenging in view of the presence of two sources of chirality, the chiral center on Ir and the chiral axis related to the helicene ligand, in addition to the relativistic effects related to the presence of the Ir center. At the same time, the large dimensions of KC and KD hamper the use of the most sophisticated (but prohibitively expensive) computational models, so that more approximate approaches must be validated on a suitable model compound. To this end, after optimizing the computational scheme on a model system devoid of the helicene moiety (KA), we have performed a comprehensive investigation of the KC and KD spectra, whose interpretation is further aided by novel graphical tools. The discussion and analysis of the results will not be focused on the theoretical background, but, rather, on practical details (specific functional, basis set, vibronic model, solvent regime) with the aim of providing general guidelines for the use of last-generation computational spectroscopy tools also by non-specialists.

Five-coordinate iridium(III) complex with ΔΛ chirality.

The coordinatively unsaturated bis-chelated iridium(III) complex, [Ir(2-Bubzq)2Cl] (2-BubzqH = 2-butyl-benzo[h]quinoline), denoted as complex 1, was obtained by reacting iridium(III) trichloride with 2-BubzqH in a 1 : 2 molar ratio. The results were in contrast to the common view that a chlorine-bridged dimer, [Ir(L)2Cl]2 (L = bis-chelate ligand), is formed under the corresponding conditions. A single-crystal X-ray diffraction structural analysis revealed that complex 1 has a five-coordinate geometry with a distorted square pyramidal configuration. The optical resolution of complex 1 was measured chromatographically on a chiral column, yielding Δ and Λ as enantiomers. The resolved enantiomers were stable enough against racemization in CDCl3 as confirmed by the vibrational circular dichroism measurements. Complex 1 reacted with carbon monoxide (CO) to give [Ir(2-Bubzq)2(CO)Cl] and with 1,10-phenanthroline (phen) to give [Ir(2-Bubzq)2(phen)]Cl within a minute with its absolute configuration (ΔΛ chirality) maintained.

Catalytic enantioselective intramolecular Tishchenko reaction of meso-dialdehyde: synthesis of (S)-cedarmycins.

The first successful example of a catalytic enantioselective intramolecular Tishchenko reaction of a meso-dialdehyde in the presence of a chiral iridium complex is described. Chiral lactones were obtained in good yields with up to 91% ee. The obtained enantioenriched lactones were utilized for the first synthesis of (S)-cedarmycins A and B.

Catalytic Enantioselective Functionalizations of C-H Bonds by Chiral Iridium Complexes.

The development of catalytic enantioselective transformations, enabling the construction of complex molecular scaffolds from simple precursors, has been a long-standing challenge in organic synthesis. Recent achievements in transition-metal catalyzed enantioselective functionalizations of carbon-hydrogen (C-H) bonds represent a promising pathway toward this goal. Over the last two decades, iridium catalysis has evolved as a valuable tool enabling the stereocontrolled synthesis of chiral molecules via C-H activation. The development of iridium-based systems with various chiral ligand classes, as well as studies of their reaction mechanisms, has resulted in dynamic progress in this area. This review aims to present a comprehensive picture of the enantioselective functionalizations of C-H bonds by chiral iridium complexes with emphasis on the mechanisms of the C-H activation step.