Chiral-at-Metal Rh(III) Complex Catalyzed Asymmetric Conjugate Addition of Unactivated Alkenes with α,β-Unsaturated 2-Acyl Imidazoles

Ligand Enabled Iridium-Catalyzed Enantioselective Hydroalkenylation of α-Olefins and Styrenes with Acrylamides

A new type of chiral spiro diphosphite ligand has been developed. Using these ligands, iridium-catalyzed highly enantioselective hydroalkenylation of α-olefins and styrenes with acrylamides has been realized. A variety of aliphatic and aromatic alkenes were successfully coupled with acrylamides to produce γ-substituted chiral acrylamides. The reaction exhibits excellent branched selectivity and high enantioselectivity along with broad substrate scope and good functional group tolerance. DFT calculations indicate that the methyl groups in the ligand play a crucial role in controlling both regioselectivity and enantioselectivity of the reaction.

Chiral-at-metal catalysts: history, terminology, design, synthesis, and applications

For decades, advances in chiral transition metal catalysis have been closely tied to the development of customized chiral ligands. Recently, however, an alternative approach to this traditional metal-plus-chiral-ligand method has emerged. In this new strategy, chiral transition metal catalysts are composed entirely of achiral ligands, with the overall chirality originating exclusively from a stereogenic metal center. This "chiral-at-metal" approach offers the benefit of structural simplicity. More importantly, by removing the need for chiral elements within the ligand framework, it opens up new possibilities for designing innovative catalyst architectures with unique properties. As a result, chiral-at-metal catalysis is becoming an increasingly important area of research. This review offers a comprehensive overview and detailed insights into asymmetric chiral-at-metal catalysis, encouraging scientists to explore new avenues in asymmetric transition metal catalysis and driving innovation in both fundamental and applied research.

Copper-Catalyzed Propargylation of Terminal Olefins to Skipped Enynes

A highly efficient and easy to operate method for synthesizing skipped enynes catalyzed by copper has been reported for the first time. A wide variety of skipped enynes were obtained in acceptable to good yields employing nonactivated terminal olefins as nucleophiles under mild conditions. This protocol features broad substrate scope and high functional group tolerance, therefore enabling late-stage functionalization and a scale-up experiment further to highlight the synthetic utility.

Bortoluzzi A, Sandjo LP, Braga AL, F. de Assis F. Highly Enantioselective Lewis Acid Catalyzed Conjugate Addition of Imidazo[1,2-a]pyridines to α,β-Unsaturated 2-Acylimidazoles under Mild Conditions

A highly enantioselective protocol for the conjugate addition of 2-arylimidazo[1,2-a]pyridines and other imidazo derivatives to α,β-unsaturated 2-acylimidazoles is described. The method uses a previously reported chiral-at-metal rhodium catalyst and provides the corresponding adducts in yields of 25-98% with enantioselectivities up to er > 99:1. Additionally, the transformation proceeds under mild conditions using ethanol as the solvent at room temperature.

Enantioselective Conjugate Addition of Boronic Acids to α,β-Unsaturated 2-Acyl Imidazoles Catalyzed by Chiral Diols

Herein, we report the enantioselective conjugate addition of organic boronic acids to α,β-unsaturated 2-acyl imidazoles using (R)-3,3'-I₂-BINOL as the catalyst. The catalytic system shows high efficiency and tolerance to alkenylboronic acids and heteroarylboronic acids. The corresponding Michael addition products were obtained in moderate to excellent yields and with moderate to excellent enantioselectivities (up to 97% ee). A gram-scale reaction was also conducted, and the desired product was obtained in high yield with no erosion in enantioselectivity. Finally, the synthetic utility of the methodology was demonstrated by transforming the 2-acyl imidazole moiety to the corresponding aldehyde, carboxylic acid, ester, and amide derivatives.

Catalytic Asymmetric Conjugate Addition of 2-Methyl-3,5-dinitrobenzoates to Unsaturated Ketones

Asymmetric catalytic vinylogous Michael addition of 2-methyl-3,5-dinitrobenzoates to unsaturated ketones catalyzed by chiral rhodium catalysts has been established. This strategy allowed the synthesis of a variety of optically pure compounds containing imidazole and 3,5-dinitrobenzene skeletons in 64-98% yields with 88-98% ee. The developed reaction not only represents highly asymmetric catalytic enantioselective vinylogous Michael addition employing 2-methyl-3,5-dinitrobenzoates as a building block but also enriches the chemistry of catalytic asymmetric vinylogous Michael addition of 2-methyl-3,5-dinitrobenzoates. Furthermore, the protocol showed obvious advantages in reaction activity and enantioselectivity. When the chiral rhodium catalyst was reduced to 0.06 mol %, the gram-level reaction was still achieved to provide the desired product with 95% ee.

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.

Cooperative Lewis Acid Catalysis for the Enantioselective C(sp3)-H Bond Functionalizations of 2-Alkyl Azaarenes

Herein, we describe the enantioselective C(sp³)-H bond functionalizations of 2-alkyl azaarenes using a cooperative dual Lewis acid catalysis. An achiral Lewis acid activates the unactivated azaarene partner without the need for a strong base. Orthogonally, a chiral-at-metal Lewis acid catalyst enables LUMO lowering and induces chirality. This method tolerates a range of complex molecular scaffolds and exhibits good to excellent yields and selectivity while accepting a wide variety of functional groups.

Fe-BPsalan complex catalyzed highly enantioselective Diels–Alder reaction of alkylidene β-ketoesters

A practical Fe-BPsalan-catalyzed asymmetric Diels–Alder reaction of various alkylidene β-ketoesters and dienes was developed to afford estrone analogues in excellent yields, good to high diastereoselectivities and excellent enantioselectivities.

C-C Bond Cleavage of Unactivated 2-Acylimidazoles

2-Acylimidazoles are widely used as post-transformable carboxylic acid equivalents in chemoselective and enantioselective reactions. Their transformations, however, require pretreatment with highly reactive, toxic methylating reagents to facilitate C-C bond cleavage. Here, we demonstrate that such pretreatment can be avoided and the C-C bond cleaved under neutral conditions without the use of additional reagents or catalysts. The scope of the reaction, including the use of products reported in the literature as substrates, and some mechanistic insights are described.

Asymmetric catalysis with a chiral-at-osmium complex

The first example of a chiral osmium catalyst is reported in which the overall chirality originates exclusively from a stereogenic metal center (metal-centered chirality) with all coordinating ligands being achiral. The non-C2-symmetric chiral-at-metal complex contains two cyclometalated 7-methyl-1,7-phenanthrolinium heterocycles which can be described as two chelating pyridylidene remote N-heterocyclic carbene (rNHC) ligands. The octahedral coordination sphere is completed with one CO and one acetonitrile ligand. A monodentate chiral oxazoline ligand is used as a chiral auxiliary ligand to obtain enantiomerically pure chiral-at-osmium complexes (>99 : 1 e.r.). Finally, it is demonstrated that the developed chiral-at-osmium complex is suitable for ring-closing enantioselective C(sp3)-H aminations, including the first example of catalytic enantioselective cyclizations of azidoformates to chiral 2-oxazolidinones.

Chiral-at-Metal Rh(III) Complex Catalyzed Cascade Reduction-Michael Addition Reaction

An enantioselective three-component cascade reduction-Michael addition reaction catalyzed by chiral-at-metal Rh(III) complexes has been developed. With a Hantzsch ester as the hydride source, a number of malononitrile derivatives were prepared in good yields and excellent enantioselectivities. A model that accounts for the origin and influence factors of the stereoselectivity has been proposed based on experiments.

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).

Kinetic Resolution of Epoxides with CO2 Catalyzed by a Chiral-at-Iridium Complex

Chiral-at-metal bis-cyclometalated iridium(III) complexes are introduced as a new class of chiral catalysts for the reaction of epoxides with CO₂ to form cyclic carbonates under conditions of kinetic resolution. Reactions are typically performed at room temperature in the presence of 1 mol % of iridium catalyst and 1.5 mol % of tetraethylammonium bromide as the nucleophilic cocatalyst to provide selectivity factors of up to 16.6. A variety of monosubstituted epoxides, including styrene epoxide, epoxides with aliphatic side chains, glycidyl ethers, and a glycidyl ester, are found to be suitable substrates. No polymerization side reaction is observed for any of the investigated substrates.

A chiral-at-metal asymmetric catalyzed vinylogous Michael addition of ortho-methyl aromatic nitro compounds for isoxazole derivative synthesis

A chiral-at-metal rhodium(iii) complex catalyzed asymmetric vinylogous Michael addition of 5-methyl 4-nitroisoxazoles with α,β-unsaturated 2-acyl imidazoles has been developed, delivering the corresponding adducts with up to 96% yield and 97% ee.

Enantioselective synthesis of tetrahydroisoquinoline derivatives via chiral-at-metal rhodium complex catalyzed [3+2] cycloaddition

An asymmetric [3+2] cycloaddition of C,N-cyclic azomethine imines with α,β-unsaturated 2-acyl imidazoles catalyzed by a chiral-at-metal rhodium complex has been developed.

Chiral-at-metal rhodium(iii) complex catalyzed enantioselective synthesis of C2-substituted benzofuran derivatives

An enantioselective C2-nucleophilic functionalization of 3-aminobenzofurans has been realized under catalysis of chiral rhodium(iii) complexes, affording a large array of C2-substituted benzofuran derivatives in high yields and enantioselectivities.

Enantioselective Mukaiyama-Michael Reaction Catalyzed by a Chiral Rhodium Complex Based on Pinene-Modified Pyridine Ligands

The rhodium complex Λ-Rh1 containing chiral pinene-modified pyridine ligands is prepared through a two-step synthetic procedure; it exhibits excellent reactivity and enantiocontrol towards the enantioselective Mukaiyama-Michael reaction of α,β-unsaturated 2-acyl imidazoles with silyl enol ethers, affording enantioenriched 1,5-dicarbonyl compounds in good yields (up to 99 %) with excellent enantioselectivities (up to 99 % ee).

Copper(II)-Catalyzed Tandem Decarboxylative Michael/Aldol Reactions Leading to the Formation of Functionalized Cyclohexenones

This work describes the development of a new single-pot copper(II)-catalyzed decarboxylative Michael reaction between β-keto acids and enones, followed by in situ aldolization, which results in highly functionalized chiral and achiral cyclohexenones. The achiral version of this Robinson annulation features a hitherto unprecedented Michael reaction of β-keto acids with sterically hindered β,β'-substituted enones and provides access to all carbon quaternary stereocenter-containing cyclohexenones (11 examples, 43-83% yield). In addition, an asymmetric chiral bis(oxazoline) copper(II)-catalyzed single-pot Robinson annulation has been devised for preparing chiral cyclohexenones, including some products that contain vicinal stereocenters (5 examples, 65-85% yield, 84-94% ee). This latter protocol has been successfully applied to the enantioselective formation of the oxygenated 10-nor-steroid core from readily available starting materials.

Preparation of chiral-at-metal catalysts and their use in asymmetric photoredox chemistry

Asymmetric catalysis is a powerful approach for the synthesis of optically active compounds, and visible light constitutes an abundant source of energy to enable chemical transformations, which are often triggered by photoinduced electron transfer (photoredox chemistry). Recently, bis-cyclometalated iridium(III) and rhodium(III) complexes were introduced as a novel class of catalysts for combining asymmetric catalysis with visible-light-induced photoredox chemistry. These catalysts are attractive because of their unusual feature of chirality originating exclusively from a stereogenic metal center, which offers the prospect of an especially effective asymmetric induction upon direct coordination of the substrate to the metal center. As these chiral catalysts contain only achiral ligands, special strategies are required for their synthesis. In this protocol, we describe strategies for preparing two types of chiral-at-metal catalysts, namely the Λ- and Δ-enantiomers (left- and right-handed propellers, respectively) of the iridium complex IrS and the rhodium complex RhS. Both contain two cyclometalating 5-tert-butyl-2-phenylbenzothiazoles in addition to two acetonitrile ligands and a hexafluorophosphate counterion. The two cyclometalated ligands set the propeller-shaped chiral geometry, but the acetonitriles are labile and can be replaced by substrate molecules. The synthesis protocol consists of three stages: first, preparation of the ligand 5-tert-butyl-2-phenylbenzothiazole; second, preparation of salicylthiazoline (used for iridium) and salicyloxazoline (used for rhodium) chiral auxiliaries; and third, the auxiliary-mediated synthesis of the individual enantiopure Λ- and Δ-configured catalysts. This class of stereogenic-only-at-metal complexes is of substantial value in the field of asymmetric catalysis, offering stereocontrolled radical reactions based on visible-light-activated photoredox chemistry. Representative examples of visible-light-induced asymmetric catalysis are provided.

Chiral-at-Metal Rh(III) Complex-Catalyzed Michael Addition of Pyrazolones with α,β-Unsaturated 2-Acyl Imidazoles

An efficient enantioselective conjugate addition of pyrazolones with α,β-unsaturated 2-acyl imidazoles catalyzed by chiral-at-metal rhodium complex is reported. The corresponding adducts were obtained in good yields (85%-96%) with excellent enantioselectivities (up to >99%). This protocol exhibits extraordinary reactivity, because of the fact that a complex with as little as 0.05 mol % Rh(III) can catalyze the title reaction on a gram-scale with excellent enantioselectivity.

Catalytic asymmetric synthesis of diphenylbutazone analogues

The asymmetric Michael addition of diphenylbutazone and its analogues to α,β-unsaturated 2-acyl imidazoles has been developed with a chiral-at-metal Rh(iii) complex as a catalyst.

Sequential asymmetric hydrogenation and photoredox chemistry with a single catalyst

A single chiral iridium catalyst promotes two mechanistically distinct reaction types in a sequential fashion, namely asymmetric hydrogenation (two-electron mechanism) and photoredox chemistry (one-electron mechanism).

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.