Asymmetric Synthesis of Octahedral Coordination Complexes

Mononuclear Non-Heme Manganese-Catalyzed Enantioselective cis-Dihydroxylation of Alkenes Modeling Rieske Dioxygenases

The practical catalytic enantioselective cis-dihydroxylation of olefins that utilize earth-abundant first-row transition metal catalysts under environmentally friendly conditions is an important yet challenging task. Inspired by the cis-dihydroxylation reactions catalyzed by Rieske dioxygenases and non-heme iron models, we report the biologically inspired cis-dihydroxylation catalysis that employs an inexpensive and readily available mononuclear non-heme manganese complex bearing a tetradentate nitrogen-donor ligand and aqueous hydrogen peroxide (H2O2) and potassium peroxymonosulfate (KHSO5) as terminal oxidants. A wide range of olefins are efficiently oxidized to enantioenriched cis-diols in practically useful yields with excellent cis-dihydroxylation selectivity and enantioselectivity (up to 99% ee). Mechanistic studies, such as isotopically 18O-labeled water experiments, and density functional theory (DFT) calculations support that a manganese(V)-oxo-hydroxo (HO-MnV═O) species, which is formed via the water-assisted heterolytic O-O bond cleavage of putative manganese(III)-hydroperoxide and manganese(III)-peroxysulfate precursors, is the active oxidant that effects the cis-dihydroxylation of olefins; this is reminiscent of the frequently postulated iron(V)-oxo-hydroxo (HO-FeV═O) species in the catalytic arene and alkene cis-dihydroxylation reactions by Rieske dioxygenases and synthetic non-heme iron models. Further, DFT calculations for the mechanism of the HO-MnV═O-mediated enantioselective cis-dihydroxylation of olefins reveal that the first oxo attack step controls the enantioselectivity, which exhibits a high preference for cis-dihydroxylation over epoxidation. In this study, we are able to replicate both the catalytic function and the key chemical principles of Rieske dioxygenases in mononuclear non-heme manganese-catalyzed enantioselective cis-dihydroxylation of olefins.

Catalytic, asymmetric carbon-nitrogen bond formation using metal nitrenoids: from metal-ligand complexes via metalloporphyrins to enzymes

The introduction of nitrogen atoms into small molecules is of fundamental importance and it is vital that ever more efficient and selective methods for achieving this are developed. With this aim, the potential of nitrene chemistry has long been appreciated but its application has been constrained by the extreme reactivity of these labile species. This liability however can be attenuated by complexation with a transition metal and the resulting metal nitrenoids have unique and highly versatile reactivity which includes the amination of certain types of aliphatic C-H bonds as well as reactions with alkenes to afford aziridines. At least one new chiral centre is typically formed in these processes and the development of catalysts to exert control over enantioselectivity in nitrenoid-mediated amination has become a growing area of research, particularly over the past two decades. Compared with some synthetic methods, metal nitrenoid chemistry is notable in that chemists can draw from a diverse array of metals and catalysts , ranging from metal-ligand complexes, bearing a variety of ligand types, via bio-inspired metalloporphyrins, all the way through to, very recently, engineered enzymes themselves. In the latter category in particular, rapid progress is being made, the rate of which suggests that this approach may be instrumental in addressing some of the outstanding challenges in the field. This review covers key developments and strategies that have shaped the field, in addition to the latest advances, up until September 2023.

Improving the Configurational Stability of Chiral-at-Iron Catalysts Containing Two N-(2-Pyridyl)-Substituted N-Heterocyclic Carbene Ligands

Recently, we introduced the first example of chiral-at-iron catalysts in which two achiral N-(2-pyridyl)-substituted N-heterocyclic carbene (NHC) ligands in addition to two labile acetonitriles are coordinated around a central iron, to generate a stereogenic metal center [Hong Y.Chiral-at-Iron Catalyst: Expanding the Chemical Space for Asymmetric Earth-Abundant Metal Catalysis. J. Am. Chem. Soc.2019, 141, 4569-4572]. A more facile synthesis of such chiral-at-iron catalysts was developed, which omits the use of expensive silver salts and an elaborate electrochemical setup. Configurational robustness was improved by replacing the imidazol-2-ylidene carbene moieties with benzimidazol-2-ylidenes. The π-acceptor properties of the altered NHCs were investigated by Ganter's ⁷⁷Se NMR method. The obtained benzimidazol-2-ylidene chiral-at-iron complex is an excellent catalyst for an asymmetric hetero-Diels-Alder reaction under open-flask conditions.

Synthesis of chiral-at-metal rhodium complexes from achiral tripodal tetradentate ligands: resolution and application to enantioselective Diels-Alder and 1,3-dipolar cycloadditions

An improved synthesis of the racemic rhodium compound [RhCl₂(κ⁴ C,N,N',P-L1)] (1) containing an achiral tripodal tetradentate ligand is reported. Their derived solvate complexes [Rh(κ⁴ C,N,N',P-L1)(Solv)₂][SbF₆]₂ (Solv = NCMe, 2; H₂O, 3) are resolved into their two enantiomers. Complexes 2 and 3 catalyze the Diels-Alder (DA) reaction between methacrolein and cyclopentadiene and the 1,3-dipolar cycloaddition reaction between methacrolein and the nitrone N-benzylidenphenylamine-N-oxide. When enantiopure (A _Rh,R _N)-2 was employed as the catalyst, enantiomeric ratios >99/1, in the R at C2 adduct, and up to 94/6, in the 3,5-endo isomer, were achieved in the DA reaction and in the 1,3-dipolar cycloaddition reaction, respectively. A plausible catalytic cycle that accounts for the origin of the observed enantioselectivity is proposed.

Family of Well-Defined Chiral-at-Cobalt(III) Complexes as Metal-Templated Hydrogen-Bond-Donor Catalysts: Effect of Chirality at the Metal Center on the Stereochemical Outcome of the Reaction

A family of well-defined Λ- and Δ-diastereomeric octahedral cationic chiral-at-cobalt complexes were obtained by a simple two-step reaction of (R,R)-1,2-diaminocyclohexane, (R,R)-1,2-diphenylethylenediamine, or (S)-2-(aminomethyl)pyrrolidine and substituted salicylaldehydes with a cobalt(III) salt. It was observed for the first time that the use of an excess of cobalt(III) salt provides both the enantiopure Λ and Δ forms of the corresponding cobalt(III) complexes 1 and 2 in a ratio of diastereomers ranging from 1:1.6 to >20:1 (Λ/Δ) and in 31-95% combined yields. The obtained complexes were robust, air- and bench-stable, soluble in most of organic solvents, and insoluble in water. Through variation of the substituents in the phenyl ring of the salicylaldehyde moiety, it was shown that both steric and electronic effects of substituents have a significant impact on the formation of Λ and Δ isomers. Next, the efficacies of the enantiopure metal-templated complexes 1-3 were investigated in three benchmark asymmetric reactions in order to compare their catalytic activity. The chiral cobalt(III) complexes 1-3 were tested as enantioselective hydrogen-bond-donor catalysts in such important reactions as the Michael addition of the O'Donnell substrate to methyl acrylate, epoxidation of chalcone, and trimethylsilylcyanation of benzaldehyde. It was clearly demonstrated that the chirality at the cobalt center has an impact on the stereochemical outcome of the reactions. In particular, the Λ(R,R)-1 and Δ(R,R)-1 complexes acted as "pseudoenantiomeric" catalysts in the epoxidation and trimethylsilylcyanoation reactions, providing both enantiomers of the products with up to 57% enantiomeric excess.

Enantiopure, luminescent, cyclometalated Ir(III) complexes with N-heterocyclic carbene-naphthalimide chromophore: design, vibrational circular dichroism and TD-DFT calculations

A series of chiral cyclometalated iridium complexes of the type [Ir(C^N)₂(C^C:)], {(C^N) = ppy (2); dfppy (3)} featuring a naphthalimide N-heterocyclic carbene ligand (C^C:) = (Naphthalimide-NHC) are described and fully characterized. The racemic complexes rac-2 and rac-3 were resolved via chiral column chromatography techniques into their corresponding enantiopure species Δ-2, Λ-2, Δ-3, Λ-3 as confirmed by their CD curves. This unique class of molecules containing organic and inorganic chromophores might be used as a platform to probe the stereochemical effect on the photophysical properties. Vibrational circular dichroism (VCD) was used as an important tool to assign successfully the stereochemistry of the enantiomers. TD-DFT calculations are also advanced to support the experimental studies and to rationalize the observed results.

Recent Development of Bis-Cyclometalated Chiral-at-Iridium and Rhodium Complexes for Asymmetric Catalysis

The field of asymmetric catalysis has been developing to access synthetically efficacious chiral molecules from the last century. Although there are many sustainable ways to produce nonracemic molecules, simplified and unique methodologies are always appreciated. In the recent developments of asymmetric catalysis, chiral-at-metal Lewis acid catalysis has been recognized as an attractive strategy. The catalysts coordinatively activate a substrate while serving the sole source of chirality by virtue of its helical environment. These configurationally stable complexes were utilized in a large number of asymmetric transformations, ranging from asymmetric Lewis acid catalysis to photoredox and electrocatalysis. Here we provide a comprehensive review of the current advancements in asymmetric catalysis utilizing iridium and rhodium-based chiral-at-metal complexes as catalysts. First, the asymmetric transformations via LUMO and HOMO activation assisted by a chiral Lewis acid catalyst are reviewed. In the second part, visible-light-induced asymmetric catalysis is summarized. The asymmetric transformation via the electricity-driven method is discussed in the final section.

Chiroptical switching behavior of heteroleptic ruthenium complexes bearing acetylacetonato and tropolonato ligands

Four types of tris-chelate ruthenium complexes bearing acetylacetonato (acac) and tropolonato (trop) ligands were synthesized and optically resolved into Δ and Λ isomers: [Ru(acac)₃] (Ru-0), [Ru(acac)₂(trop)] (Ru-1), [Ru(acac)(trop)₂] (Ru-2), and [Ru(trop)₃] (Ru-3). Chiral HPLC chromatograms, electronic circular dichroism (ECD), and vibrational circular dichroism (VCD) of the four ruthenium complexes were systematically investigated. As a result, the absolute configurations of the newly prepared enantiomeric complexes Ru-2 and Ru-3 were determined. For the case of Ru-2, its absolute configuration was also confirmed by single crystal X-ray diffraction analysis. The ECD changes upon chemical oxidation were further investigated for the four complexes. An ECD change in enantiomeric Ru-1 was observed upon oxidation, but the oxidized species soon returned to the neutral state within a few minutes. Enantiomers of Ru-3 also showed explicit ECD changes upon oxidation. Further, the lifetime of the oxidized state was the longest among the four investigated complexes, whereas they racemized in solution at room temperature. In contrast, the enantiomers of heteroleptic complexes (Ru-1 and Ru-2) concurrently exhibited ECD changes, relatively long lifetime of the oxidized state, and nil or quite slow racemization behavior. The coexistence of acac and trop ligands was key to making the competing factors compatible in the resultant ruthenium complexes.

Synthesis and a Catalytic Study of Diastereomeric Cationic Chiral-at-Cobalt Complexes Based on (R,R)-1,2-Diphenylethylenediamine

Here we report the first synthesis of two diastereomeric cationic octahedral Co(III) complexes based on commercially available (R,R)-1,2-diphenylethylenediamine and salicylaldehyde. Both diastereoisomers with opposite chiralities at the metal center (Λ and Δ configurations) were prepared. The new Co(III) complexes possessed both acidic hydrogen-bond donating (HBD) NH moieties and nucleophilic counteranions and operate as bifunctional chiral catalysts for the challenging kinetic resolution of terminal and disubstituted epoxides by the reaction with CO₂ under mild conditions. The highest selectivity factor (s) of 2.8 for the trans-chalcone epoxide was achieved at low catalyst loading (2 mol %) in chlorobenzene, which is the best achieved result currently for this type of substrate.

Enantioselective "organocatalysis in disguise" by the ligand sphere of chiral metal-templated complexes

Asymmetric catalysis holds a prominent position among the important developments in chemistry during the 20th century. This was acknowledged by the 2001 Nobel Prize in chemistry awarded to Knowles, Noyori, and Sharpless for their development of chiral metal catalysts for organic transformations. The key feature of the catalysts was the crucial role of the chiral ligand and the nature of the metal ions, which promoted the catalytic conversions of the substrates via direct coordination. Subsequently the development of asymmetric organic catalysis opened new avenues to the synthesis of enantiopure compounds, avoiding any use of metal ions. Recently, an alternative approach to asymmetric catalysis emerged that relied on the catalytic functions of the ligands themselves boosted by coordination to metal ions. In other words, in these hybrid chiral catalysts the substrates are activated not by the metal ions but by the ligands. The activation and enantioselective control occurred via well-orchestrated and custom-tailored non-covalent interactions of the substrates with the ligand sphere of chiral metal complexes. In these metal-templated catalysts, the metal served either as a template (a purely structural role), or it constituted the exclusive source of chirality (metal-centred chirality due to the spatial arrangement of achiral or chiral bi-/tridentate ligands around an octahedral metal centre), and/or it increased the Brønsted acidity of the ligands. Although the field is still in its infancy, it represents an inspiring combination of both metal and organic catalysis and holds major unexplored potential to push the frontiers of asymmetric catalysis. Here we present an overview of this emerging field discussing the principles, applications and perspectives on the catalytic use of chiral metal complexes that operate as "organocatalysts in disguise". It has been demonstrated that these chiral metal complexes are efficient and provide high stereoselective control in asymmetric hydrogen bonding catalysis, phase-transfer catalysis, Brønsted acid/base catalysis, enamine catalysis, nucleophilic catalysis, and photocatalysis as well as bifunctional catalysis. Also, many of the catalysts have been identified as highly effective catalysts at remarkably low catalyst loadings. These hybrid systems offer many opportunities in the synthesis of chiral compounds and represent promising alternatives to metal-based and organocatalytic asymmetric transformations.

Synthesis of Rhodium Complexes with Chiral Diene Ligands via Diastereoselective Coordination and Their Application in the Asymmetric Insertion of Diazo Compounds into E-H Bonds

A new method for the synthesis of chiral diene rhodium catalysts is introduced. The readily available racemic tetrafluorobenzobarrelene complexes [(R₂ -TFB)RhCl]₂ were separated into two enantiomers via selective coordination of one of them with the auxiliary S-salicyl-oxazoline ligand. One of the resulting chiral complexes with an exceptionally bulky diene ligand [(R,R-iPr₂ -TFB)RhCl]₂ was an efficient catalyst for the asymmetric insertion of diazoesters into B-H and Si-H bonds giving the functionalized organoboranes and silanes with high yields (79-97 %) and enantiomeric purity (87-98 % ee). The stereoselectivity of separation via auxiliary ligand and that of the catalytic reaction was predicted by DFT calculations.

Bis-Cyclometalated Indazole and Benzimidazole Chiral-at-Iridium Complexes: Synthesis and Asymmetric Catalysis

A new class of bis-cyclometalated iridium(III) catalysts containing two inert cyclometalated 6-tert-butyl-2-phenyl-2H-indazole bidentate ligands or two inert cyclometalated 5-tert-butyl-1-methyl-2-phenylbenzimidazoles is introduced. The coordination sphere is complemented by two labile acetonitriles, and a hexafluorophosphate ion serves as a counterion for the monocationic complexes. Single enantiomers of the chiral-at-iridium complexes (>99% er) are obtained through a chiral-auxiliary-mediated approach using a monofluorinated salicyloxazoline and are investigated as catalysts in the enantioselective conjugate addition of indole to an α,β-unsaturated 2-acyl imidazole and an asymmetric Nazarov cyclization.

Green-Light Activation of Push-Pull Ruthenium(II) Complexes

Synthesis, characterization, electrochemistry, and photophysics of homo- and heteroleptic ruthenium(II) complexes [Ru(cpmp)₂ ]²⁺ (2²⁺ ) and [Ru(cpmp)(ddpd)]²⁺ (3²⁺ ) bearing the tridentate ligands 6,2''-carboxypyridyl-2,2'-methylamine-pyridyl-pyridine (cpmp) and N,N'-dimethyl-N,N'-dipyridin-2-ylpyridine-2,6-diamine (ddpd) are reported. The complexes possess one (3²⁺ ) or two (2²⁺ ) electron-deficient dipyridyl ketone fragments as electron-accepting sites enabling intraligand charge transfer (ILCT), ligand-to-ligand charge transfer (LL'CT) and low-energy metal-to-ligand charge transfer (MLCT) absorptions. The latter peak around 544 nm (green light). Complex 2²⁺ shows ³ MLCT phosphorescence in the red to near-infrared spectral region at room temperature in deaerated acetonitrile solution with an emission quantum yield of 1.3 % and a ³ MLCT lifetime of 477 ns, whereas 3²⁺ is much less luminescent. This different behavior is ascribed to the energy gap law and the shape of the parasitic excited ³ MC state potential energy surface. This study highlights the importance of the excited-state energies and geometries for the actual excited-state dynamics. Aromatic and aliphatic amines reductively quench the excited state of 2²⁺ paving the way to photocatalytic applications using low-energy green light as exemplified with the green-light-sensitized thiol-ene click reaction.

Bis-Cyclometalated Indazole Chiral-at-Rhodium Catalyst for Asymmetric Photoredox Cyanoalkylations

A new class of bis‐cyclometalated rhodium(III) catalysts containing two inert cyclometalated 6‐tert‐butyl‐2‐phenyl‐2H‐indazole ligands and two labile acetonitriles is introduced. Single enantiomers (>99 % ee) were obtained through a chiral‐auxiliary‐mediated approach using a monofluorinated salicyloxazoline. The new chiral‐at‐metal complex is capable of catalyzing the visible‐light‐induced enantioselective α‐cyanoalkylation of 2‐acyl imidazoles in which it serves a dual function as the chiral Lewis acid catalyst for the asymmetric radical chemistry and at the same time as the photoredox catalyst for the visible‐light‐induced redox chemistry (up to 80 % yield, 4:1 d.r., and 95 % ee, 12 examples).

Photochemical Resolution of a Thermally Inert Cyclometalated Ru(phbpy)(N-N)(Sulfoxide)+ Complex

In this work a photosubstitution strategy is presented that can be used for the isolation of chiral organometallic complexes. A series of five cyclometalated complexes Ru(phbpy)(N−N)(DMSO-κS)](PF₆) ([1]PF₆-[5]PF₆) were synthesized and characterized, where Hphbpy = 6′-phenyl-2,2′-bipyridyl, and N–N = bpy (2,2′-bipyridine), phen (1,10-phenanthroline), dpq (pyrazino[2,3-f][1,10]phenanthroline), dppz (dipyrido[3,2-a:2′,3′-c]phenazine, or dppn (benzo[i]dipyrido[3,2-a,2′,3′-c]phenazine), respectively. Due to the asymmetry of the cyclometalated phbpy^– ligand, the corresponding [Ru(phbpy)(N–N)(DMSO-κS)]⁺complexes are chiral. The exceptional thermal inertness of the Ru–S bond made chiral resolution of these complexes by thermal ligand exchange impossible. However, photosubstitution by visible light irradiation in acetonitrile was possible for three of the five complexes ([1]PF₆-[3]PF₆). Further thermal coordination of the chiral sulfoxide (R)-methyl p-tolylsulfoxide to the photoproduct [Ru(phbpy)(phen)(NCMe)]PF₆, followed by reverse phase HPLC, led to the separation and characterization of the two diastereoisomers of [Ru(phbpy)(phen)(MeSO(C₇H₇))]PF₆, thus providing a new photochemical approach toward the synthesis of chiral cyclometalated ruthenium(II) complexes. Full photochemical, electrochemical, and frontier orbital characterization of the cyclometalated complexes [1]PF₆-[5]PF₆ was performed to explain why [4]PF₆ and [5]PF₆ are photochemically inert while [1]PF₆-[3]PF₆ perform selective photosubstitution.

Two-step chiral transfer from d-penicillamine to metallosupramolecular ionic crystals

Treatment of a racemic RhIII4ZnII4 complex, (Δ)4/(Λ)4-[Zn4O{Rh(aet)3}4]6+ (aet = 2-aminoethanethiolate), with d-penicillaminate (d-pen) gave optically pure ionic crystals composed of (Λ)4-[Zn4O{Rh(aet)3}4]6+ and ΛD-[Rh(d-pen)3]3-via intramolecular and intermolecular chiral transfers from d-pen.

Stereogenic-Only-at-Metal Asymmetric Catalysts

Chirality is an essential feature of asymmetric catalysts. This review summarizes asymmetric catalysts that derive their chirality exclusively from stereogenic metal centers. Reported chiral-at-metal catalysts can be divided into two classes, namely, inert metal complexes, in which the metal fulfills a purely structural role, so catalysis is mediated entirely through the ligand sphere, and reactive metal complexes. The latter are particularly appealing because structural simplicity (only achiral ligands) is combined with the prospect of particularly effective asymmetric induction (direct contact of the substrate with the chiral metal center). Challenges and solutions for the design of such reactive stereogenic-only-at-metal asymmetric catalysts are discussed.

Naked d-orbital in a centrochiral Ni(II) complex as a catalyst for asymmetric [3+2] cycloaddition

Chiral metal catalysts have been widely applied to asymmetric transformations. However, the electronic structure of the catalyst and how it contributes to the activation of the substrate is seldom investigated. Here, we report an empirical approach for providing insights into the catalytic activation process in the distorted Ni(II)-catalysed asymmetric [3+2] cycloaddition of α-ketoesters. We quantitatively characterize the bonding nature of the catalyst by means of electron density distribution analysis, showing that the distortion around the Ni(II) centre makes the dz² orbital partially ‘naked', wherein the labile acetate ligand is coordinated with electrostatic interaction. The electron-deficient dz² orbital and the acetate act together to deprotonate the α-ketoester, generating the (Λ)-Ni(II)–enolate. The solid and solution state analyses, together with theoretical calculations, strongly link the electronic structure of the centrochiral octahedral Ni(II) complex and its catalytic activity, depicting a cooperative mechanism of enolate binding and outer sphere hydrogen-bonding activation.

Understanding the interplay between electronic structure and performance and how it relates to mechanism is important for catalysis. Here the authors report an asymmetric [3+2] cycloaddition and show the Ni(II) catalyst possesses a weakly bound acetate ligand, leaving the dz² orbital partially vacant.

Octahedral Ruthenium Complex with Exclusive Metal-Centered Chirality for Highly Effective Asymmetric Catalysis

A novel ruthenium catalyst is introduced which contains solely achiral ligands and acquires its chirality entirely from octahedral centrochirality. The configurationally stable catalyst is demonstrated to catalyze the alkynylation of trifluoromethyl ketones with very high enantioselectivity (up to >99% ee) at low catalyst loadings (down to 0.2 mol%).

Stereospecific control of the metal-centred chirality of rhodium(iii) and iridium(iii) complexes bearing tetradentate CNN′P ligands

Complexes of formula [MCl₂(κ⁴C,N,N′,P-L)] (M = Rh, Ir) were diastereoselectively obtained with predetermined absolute configuration from MCl₃·xH₂O and tripodal tetradentate ligands.

Half-sandwich complexes of Ir(iii), Rh(iii) and Ru(ii) with the MaxPhos ligand: metal centred chirality and cyclometallation

The MaxPhos ligand forms half-sandwich Rh, Ir and Ru complexes diastereoselectively. The Ir complex cyclometallates through C(sp³)–H activation.

The diastereoselective synthesis of octahedral cationic iridium hydride complexes with a stereogenic metal centre

We report herein the highly diastereoselective synthesis of octahedral cationic Ir(iii) hydride complexes with a stereogenic metal centre following various strategies. The configurational stability of these compounds has also been investigated.

Expanding the family of bis-cyclometalated chiral-at-metal rhodium(iii) catalysts with a benzothiazole derivative

Synthetic access to previously elusive single enantiomers of an octahedral chiral-at-metal rhodium(iii) complex containing two cyclometalated 2-phenylbenzothiazoles and two acetonitrile ligands is reported. The complex is a superior chiral Lewis acid catalyst compared to its benzoxazole congener which can be rationalized with a higher steric congestion around the coordination sites.

Enantioselective β-alkylation of pyrroles with the formation of an all-carbon quaternary stereocenter

A substitutionally and configurationally inert octahedral chiral-at-metal iridium complex is reported to be an efficient catalyst for the enantioselective Friedel–Crafts alkylation of 2,5-disubstituted pyrroles at the β-position using nitroacrylates as electrophiles.

Molecular tectonics: heterometallic (Ir,Cu) grid-type coordination networks based on cyclometallated Ir(III) chiral metallatectons

A chiral-at-metal Ir(III) organometallic metallatecton was synthesised as a racemic mixture and as enantiopure complexes and combined with Cu(II) to afford a heterobimetallic (Ir,Cu) grid-type 2D coordination network.