Three-Component Asymmetric Mannich Reaction Catalyzed by a Lewis Acid with Rhodium-Centered Chirality

Dynamic Configuration on a Chiral-at-Rhodium Catalyst Featuring a Flexible Tetradentate Ligand

The unique dynamic configuration of an enantioselective chiral-at-metal catalyst based on Rh(III) and a non-chiral tetradentate ligand is described and resolved. At room temperature, the catalyst undergoes a dynamic configuration process leading to the formation of two interconvertible metal-stereoisomers, remarkably without racemization. Density functional theory (DFT) calculations indicate that this metal-isomerization proceeds via a concerted transition state, which features a trigonal bipyramidal geometry stabilized by the tetradentate ligand. Furthermore, the resolved enantiopure complex shows high catalytic enantioinduction in the Friedel-Crafts reaction, achieving enantiomeric ratios as high as 99 : 1.

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.

Enantioselective Synthesis of α-Aryl-β2 -Amino-Esters by Cooperative Isothiourea and Brønsted Acid Catalysis

The synthesis of α-aryl-β2 -amino esters through enantioselective aminomethylation of an arylacetic acid ester in high yields and enantioselectivity via cooperative isothiourea and Brønsted acid catalysis is demonstrated. The scope and limitations of this process are explored (25 examples, up to 94 % yield and 96:4 er), with applications to the synthesis of (S)-Venlafaxine⋅HCl and (S)-Nakinadine B. Mechanistic studies are consistent with a C(1)-ammonium enolate pathway being followed rather than an alternative dynamic kinetic resolution process. Control studies indicate that (i) a linear effect between catalyst and product er is observed; (ii) an acyl ammonium ion can be used as a precatalyst; (iii) reversible isothiourea addition to an in situ generated iminium ion leads to an off-cycle intermediate that can be used as a productive precatalyst.

Diastereo- and Enantioselective Mannich/Cyclization Cascade Reaction Access to Chiral Benzothiazolopyrimidine Derivatives

An efficient asymmetric Mannich/cyclization cascade strategy was established from 2-benzothiazolimines with N-acylpyrazoles to provide optical active benzothiazolopyrimidine derivatives using a copper-based complex. The mild cascade process constructed various structurally diverse products with broad scope of substrates together with excellent enantioselectivities (up to 99 % ee) and diastereoselectivities (up to 99:1 d.r.).

Direct Asymmetric Three-Component Mannich Reaction Catalyzed by Chiral Counteranion-Assisted Silver

Direct asymmetric three-component Mannich reaction involving simple ketones (such as cyclohexanone, acetone, and acetophenone) as donors and catalyzing by silver tartaric acid-derived phosphate was realized to afford a series of optically active β-amino-ketone derivatives in high yields (up to 96%) and good-to-high enantioselectivities (up to 97%) with moderate-to-good diastereoselectivities. This is the first example of direct catalytic asymmetric three-component Mannich reaction via a chiral counteranion-directed strategy.

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.

Copper-catalyzed enantioselective Mannich reaction between N-acylpyrazoles and isatin-derived ketimines

A copper-catalyzed enantioselective Mannich reaction between N-acylpyrazoles and isatin-derived ketimines is developed.

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).

A chiral nickel DBFOX complex as a bifunctional catalyst for visible-light-promoted asymmetric photoredox reactions

The enantioselective photoredox reaction of α,β-unsaturated carbonyl compounds and tertiary/secondary α-silylamines was enabled by a readily available single NiII-DBFOX catalyst (DBFOX = 4,6-bis((R)-4-phenyl-4,5-dihydrooxazol-2-yl)dibenzo[b,d]furan) under visible light conditions. The non-precious chiral catalyst is involved in the photochemical process to initiate single electron transfer and at the same time provides a well-organized chiral environment for the subsequent radical transformations. Good to excellent enantioselectivities (80-99% ee) were obtained for the formation of chiral γ-amino carboxylic acid derivatives and γ-lactams.

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.

Rhodium(iii)/amine synergistically catalyzed enantioselective Michael addition of cyclic ketones with α,β-unsaturated 2-acyl imidazoles

An enantioselective Michael addition of α,β-unsaturated 2-acyl imidazoles with cyclic ketones catalyzed synergistically by a chiral-at-metal Rh(iii) complex and a secondary amine has been developed, affording the corresponding adducts with up to 98% yield and 99% ee.

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.

Brønsted–Lewis dual acidic ionic liquid immobilized on mesoporous silica materials as an efficient cooperative catalyst for Mannich reactions

Novel mesoporous silica supported ILs have been prepared and successfully applied as a heterogeneous catalyst in Mannich reactions.