Rh-catalyzed asymmetric 1,4-addition reactions to α,β-unsaturated carbonyl and related compounds: an update

Diastereodivergence in catalytic asymmetric conjugate addition of carbon nucleophiles

Catalytic asymmetric conjugate additions of carbon nucleophiles have emerged as a potent tool for constructing multi-stereogenic molecules with precise stereochemical control. This review explores the concept of diastereodivergence in such reactions, focusing on strategies to achieve selective access to diverse diastereomeric products upon carbon-carbon bond formation. Drawing from a rich array of examples, we delve into key approaches for controlling the stereochemical outcome of these transformations, including alteration of alkene geometry, fine-tuning of reaction parameters, synergistic catalysis, and isomerization of conjugate adducts. Additionally, we highlight the iterative strategies for conjugate additions, showcasing their potential for diastereodivergent synthesis of methyl-branched stereocenters in 1,3-relationships. By presenting a concentrated overview of this significant topic, this review aims to provide valuable insights into the design and execution of stereodivergent catalytic conjugate additions, offering new avenues for advancing stereoselective synthesis and structural diversity in organic synthesis.

Dual Photoredox/Nickel Catalysis Enables Diastereoselective Synthesis of Multisubstituted γ-Lactams Using Alkyl-GeMe3 as Radical Precursors

Herein, we report a single-step, multicomponent approach to versatile γ-lactams through dual photoredox/nickel-catalyzed dicarbofunctionalization of α,β-unsaturated γ-butyrolactam. This reaction utilized alkyl trimethylgermanium as a radical precursor and acyl chloride as the electrophile, demonstrating remarkable functional group compatibility.

Ligand-controlled regiodivergent arylation of aryl(alkyl)alkynes and asymmetric synthesis of axially chiral 9-alkylidene-9,10-dihydroanthracenes

Transition metal-catalyzed addition of organometallics to aryl(alkyl)alkynes has been well known to proceed with the regioselectivity in forming a carbon-carbon bond at the alkyl-substituted carbon (β-addition). Herein, the reverse regiochemistry with high selectivity in giving 1,1-diarylalkenes (α-addition) was realized in the reaction of arylboronic acids with aryl(alkyl)alkynes by use of a rhodium catalyst coordinated with a chiral diene ligand, whereas the arylation of the same alkynes proceeded with the usual regioselectivity (β-addition) in the presence of a rhodium/DM-BINAP catalyst. The regioselectivity can be switched by the choice of ligands on the rhodium catalysts. This reverse regioselectivity also enabled the catalytic asymmetric synthesis of phoenix-like axially chiral alkylidene dihydroanthracenes with high enantioselectivity through an α-addition/1,4-migration/cyclization sequence.

P-Stereogenic Phosphorus Ligands in Asymmetric Catalysis

Chiral phosphorus ligands play a crucial role in asymmetric catalysis for the efficient synthesis of useful optically active compounds. They are largely categorized into two classes: backbone chirality ligands and P-stereogenic phosphorus ligands. Most of the reported ligands belong to the former class. Privileged ones such as BINAP and DuPhos are frequently employed in a wide range of catalytic asymmetric transformations. In contrast, the latter class of P-stereogenic phosphorus ligands has remained a small family for many years mainly because of their synthetic difficulty. The late 1990s saw the emergence of novel P-stereogenic phosphorus ligands with their superior enantioinduction ability in Rh-catalyzed asymmetric hydrogenation reactions. Since then, numerous P-stereogenic phosphorus ligands have been synthesized and used in catalytic asymmetric reactions. This Review summarizes P-stereogenic phosphorus ligands reported thus far, including their stereochemical and electronic properties that afford high to excellent enantioselectivities. Examples of reactions that use this class of ligands are described together with their applications in the construction of key intermediates for the synthesis of optically active natural products and therapeutic agents. The literature covered dates back to 1968 up until December 2023, centering on studies published in the late 1990s and later years.

Rhodium-Catalyzed Asymmetric Addition to 4- or 5-Carbonyl-cycloenones through Dynamic Kinetic Resolution: Enantioselective Synthesis of (-)-Cannabidiol

The reaction of 4/5-carbonyl-cycloalkenone 1 or its achiral isomer 1' with organoboronic acid 2 in the presence of a chiral diene (S,S)-Fc-tfb-rhodium catalyst gave disubstituted trans-cycloalkanone 3 with high diastereo- and enantioselectivity. This highly efficient dynamic kinetic resolution is achieved by fast racemization of 1 through the formation of a dienolate followed by kinetic resolution with the chiral catalyst. The utility is demonstrated by the synthesis of key intermediates en route to (-)-cannabidiol.

Bridging the incompatibility gap in dual asymmetric catalysis over a thermoresponsive hydrogel-supported catalyst

The integration of dual asymmetric catalysis is highly beneficial for the synthesis of organic molecules with multiple stereocenters. However, two major issues that need to be addressed are the intrinsic deactivation of dual-species and the extrinsic conflict of reaction conditions. To overcome these concerns, we have utilized the compartmental and thermoresponsive properties of poly(N-isopropylacrylamide) (PNIPAM) to develop a cross-linked PNIPAM-hydrogel-supported bifunctional catalyst. This catalyst is designed with Rh(diene) species situated on the outer surface and Ru(diamine) species positioned within the interior of the hydrogel. The compartmental function of PNIPAM in the middle overcomes intrinsic mutual deactivations between the dual-species. The thermoresponsive nature of PNIPAM allows for precise control of catalytic pathways in resolving external conflicts by controlling the reaction switching between an Rh-catalyzed enantioselective 1,4-addition at 50°C and a Ru-catalyzed asymmetric transfer hydrogenation (ATH) at 25°C. As we envisioned, this sequential 1,4-addition/reduction dual enantioselective cascade reaction achieves a transformation from incompatibility to compatibility, resulting in direct access to γ-substituted cyclic alcohols with dual stereocenters in high yields and enantio/diastereoselectivities. Mechanistic investigation reveals a reversible temperature transition between 50°C and 25°C, ensuring a cascade process comprising a 1,4-addition followed by the ATH process.

Synthesis of Alkyl and Aryl Boronate Esters via CeO2-Catalyzed Borylation of Alkyl and Aryl Electrophiles Including Alkyl Chlorides

A recyclable protocol using a CeO2-nanorod catalyst for borylation of alkyl halides with B2pin2 (pin = OCMe2CMe2O) is reported. A wide range of synthetically useful alkyl boronate esters are readily obtained from primary and secondary alkyl electrophiles, including unactivated alkyl chlorides, demonstrating broad utility and functional group tolerance. Preliminary investigation revealed an involvement of in situ formed catalytically active boryl species. The catalyst can be reused for up to six runs without appreciable loss in activity. In addition, we have demonstrated the use of this recyclable catalyst for the borylation of aryl halides with B2pin2, providing valuable aryl boronate esters under neat conditions.

Catalytic asymmetric indolization by a desymmetrizing [3 + 2] annulation strategy

A new catalytic asymmetric indolization reaction by a desymmetrizing [3 + 2] annulation strategy is developed. The reaction proceeds via a rhodium-catalyzed enantioposition-selective addition/5-exo-trig cyclization/dehydration cascade between ortho-amino arylboronic acids and 2,2-disubstituted cyclopentene-1,3-diones to produce N-unprotected cyclopenta[b]indoles bearing an all-carbon quaternary stereocenter in high yields with good enantioselectivities. A quantitative structure-selectivity relationship (QSSR) model was established to identify the optimal chiral ligand, which effectively controlled the formation of the stereocenter away from the reaction site. Density functional theory (DFT) calculations, non-covalent interaction analysis, and Eyring analysis were performed to understand the key reaction step and the function of the ligand.

Stereodivergent Construction of 1,3-Chiral Centers via Tandem Asymmetric Conjugate Addition and Allylic Substitution Reaction

Herein, we report a synthesis of cyclohexanones bearing multi-continuous stereocenters by combining copper-catalyzed asymmetric conjugate addition of dialkylzinc reagents to cyclic enones with iridium-catalyzed asymmetric allylic substitution reaction. Good to excellent yields, diastereoselectivity and enantioselectivity can be obtained. Unlike the stereodivergent construction of adjacent stereocenters (1,2-position) reported in the literature, the current reaction can achieve the stereodivergent construction of nonadjacent stereocenters (1,3-position) by a proper combination of two chiral catalysts with different enantiomers.

Intertwining Olefin Thianthrenation with Kornblum/Ganem Oxidations: Ene-type Oxidation to Furnish α,β-Unsaturated Carbonyls

A widely applicable, practical, and scalable synthetic method for efficient ene-type double oxidation of alkenes is reported via a two-step alkenyl thianthrenium umpolung/Kornblum-Ganem oxidation strategy. This chemo- and stereoselective procedure allows easy access to various α,β-unsaturated carbonyls that may be otherwise difficult or cumbersome to synthesize by conventional methods. For α-olefins, this metal-free transformation can be tuned according to synthetic needs to produce either the elusive (Z)-unsaturated aldehydes or their (E) counterparts. Moreover, this strategy has enabled streamlined synthesis of distinct butadienyl pheromones and kairomones.

Chiral Diene Ligands in Asymmetric Catalysis

Asymmetric catalysis has emerged as a general and powerful approach for constructing chiral compounds in an enantioselective manner. Hence, developing novel chiral ligands and catalysts that can effectively induce asymmetry in reactions is crucial in modern chemical synthesis. Among such chiral ligands and catalysts, chiral dienes and their metal complexes have received increased attention, and a great progress has been made over the past two decades. This review provides comprehensive and critical information on the essential aspects of chiral diene ligands and their importance in asymmetric catalysis. The literature covered ranges from August 2003 (when the first effective chiral diene ligand for asymmetric catalysis was reported) to October 2021. This review is divided into two parts. In the first part, the chiral diene ligands are categorized according to their structures, and their preparation methods are summarized. In the second part, their applications in asymmetric transformations are presented according to the reaction types.

Copper Foam as Active Catalysts for the Borylation of α, β-Unsaturated Compounds

The use of simple, inexpensive, and efficient methods to construct carbon–boron and carbon–oxygen bonds has been a hot research topic in organic synthesis. We demonstrated that the desired β-boronic acid products can be obtained under mild conditions using copper foam as an efficient heterogeneous catalyst. The structure of copper foam before and after the reaction was investigated by polarized light microscopy (PM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), and the results have shown that the structure of the catalyst copper foam remained unchanged before and after the reaction. The XPS test results showed that the Cu(0) content increased after the reaction, indicating that copper may be involved in the boron addition reaction. The specific optimization conditions were as follows: CH₃COCH₃ and H₂O were used as mixed solvents, 4-methoxychalcone was used as the raw material, 8 mg of catalyst was used and the reaction was carried out at room temperature and under air for 10 h. The yield of the product obtained was up to 92%, and the catalytic efficiency of the catalytic material remained largely unchanged after five cycles of use.

Rhodium-Catalyzed Desymmetric Arylation of γ,γ-Disubsituted Cyclohexadienones: Asymmetric Synthesis of Chiral All-Carbon Quaternary Centers

The desymmetric arylation of prochiral cyclohexadienones with ArZnCl in the presence of an (R)-segphos-rhodium catalyst gave high yields of the corresponding cyclohexenones, which contain a chiral arylated carbon center at the β-position and a chiral all-carbon quaternary center at the γ-position, with high diastereo- and enantioselectivities. This catalytic system was also applied to the arylation of spirocarbocyclic cyclohexadienones and afforded the corresponding cyclohexenones bearing a chiral spiro quaternary carbon with high dr and ee.

Enantiopure ferrocene-1,2-disulfoxides: synthesis and reactivity

The rational use of directed deprotometallation, sulfur oxidation and sulfoxide/lithium exchange allowed the synthesis of enantiopure ferrocene-1,2-disulfoxide derivatives. Not only do they represent the first members of this original family, but some of them have shown promise as ligands in rhodium-catalysed conjugate addition.

Crown Ether-Derived Chiral BINOL: Enantioselective Michael Addition of Alkenyl Boronic Acids to α,β-Unsaturated Ketones

A new class of aza-crown ether-derived chiral BINOL catalysts were designed, synthesized, and applied in the asymmetric Michael addition of alkenylboronic acids to α,β-unsaturated ketones. It was found that introducing aza-crown ethers to the BINOL catalyst could achieve apparently higher enantioselectivity than a similar BINOL catalyst without aza-crown ethers did, although the host-guest complexation of alkali ions by the aza-crown ethers could not further improve the catalysis effectiveness. Under mediation of the aza-crown ether-derived chiral BINOL and in the presence of a magnesium salt, an array of chiral γ,δ-unsaturated ketones were furnished in good enantioselectivities (81-95% ees).

Asymmetric Synthesis of Chiral Bicyclo[2.2.1]hepta-2,5-diene Ligands through Rhodium-Catalyzed Asymmetric Arylative Bis-cyclization of a 1,6-Enyne

A series of novel chiral diene ligands (1R,4S)-L₁, which are based on the bicyclo[2.2.1]heptadiene skeleton and are substituted with methyl and an ester group at the bridgehead carbons, were synthesized through rhodium-catalyzed asymmetric arylative bis-cyclization of 1,6-enyne 1 as a key step. The rhodium catalyst with one of the (1R,4S)-L₁ ligands was used for the asymmetric bis-cyclization of 1 giving bicyclic product (1S,4R)-2 of 99% ee, which is a synthetic precursor of (1S,4R)-L₁ ligands.

Asymmetric Synthesis of Fluorinated Allenes by Rhodium-Catalyzed Enantioselective Alkylation/Defluorination of Propargyl Difluorides with Alkylzincs

The reaction of propargyl difluorides R¹ CF₂ C≡CR² with alkylzincs R³ ZnCl giving axially chiral fluorinated allenes R¹ FC=C=CR² R³ with high enantioselectivity (up to 99 % ee) was found to be catalyzed by a chiral diene/rhodium complex. A key step in the catalytic cycle is selective elimination of one of the enantiotopic fluorides at the β-position of an alkenyl-Rh intermediate, which is generated by regioselective addition of R³ -Rh onto the triple bond of the starting difluorides.

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.

Enantioselective synthesis of tertiary boronic esters through catalytic asymmetric reversed hydroboration

Chiral tertiary boronic esters are important precursors to bioactive compounds and versatile synthetic intermediates to molecules containing quaternary stereocenters. The development of conjugate boryl addition to α,β-unsaturated amide has been hampered by the intrinsic low electrophilicity of the amide group. Here we show the catalytic asymmetric synthesis of enantioenriched tertiary boronic esters through hydroboration of β,β-disubstituted α,β-unsaturated amides. The Rh-catalyzed hydroboration occurs with previously unattainable selectivity to provide tertiary boronic esters in high enantioselectivity. This strategy opens a door for the hydroboration of inert Michael acceptors with high stereocontrol and may provide future applications in the synthesis of biologically active molecules.

Photoinduced Oxidative Alkoxycarbonylation of Alkenes with Alkyl Formates

A photoinduced oxidative alkoxycarbonylation of alkenes initiated by intermolecular addition of alkoxycarbonyl radicals has been demonstrated. Employing alkyl formates as alkoxycarbonyl radical sources, a range of α,β-unsaturated esters were obtained with good regioselectivity and E selectivity under ambient conditions.

Enantioselective Synthesis of Chiral Indane Derivatives by Rhodium-Catalyzed Addition of Arylboron Reagents to Substituted Indenes

Rhodium-catalyzed asymmetric addition of arylboron reagents to indene derivatives proceeded to give 2-arylindanes in good yields with high enantioselectivity. Deuterium-labeling experiments indicated that the present reaction involved a 1,4-Rh shift from an initially formed benzylrhodium to an arylrhodium intermediate before protonation leading to the corresponding addition product. The asymmetric addition was also successful for acenaphthylene, which has a similar skeleton to indene, where it was found that the benzylrhodium intermediate underwent direct protonation without the 1,4-Rh shift.

Are Organozirconium Reagents Applicable in Current Organic Synthesis?

The search for mild, user-friendly, easily accessible, and robust organometallic reagents is an important feature of organometallic chemistry. Ideally, new methodologies employing organometallics should be developed with respect to practical applications in syntheses of target compounds. In this short review, we investigate if organozirconium reagents can fulfill these criteria. Organozirconium compounds are typically generated via in situ hydrozirconation of alkenes or alkynes with the Schwartz reagent. Alkyl and alkenylzirconium reagents have proven to be convenient in conjugate additions, allylic substitutions, cross-coupling reactions, and additions to carbonyls or imines. Furthermore, the Schwartz reagent itself is a useful reducing agent for polar functional groups.

1 Introduction

2 Synthesis and Generation of the Schwartz Reagent

3 Structure and Properties of Cp2Zr(H)Cl

4 Reactivity of Organozirconium Reagents

4.1 Asymmetric Conjugate Addition

4.2 Asymmetric Allylic Alkylations

4.3 Desymmetrization Reactions

4.4 Cross-Coupling Reactions

4.5 1,2-Additions

5 Conclusions

Asymmetric Synthesis of 1-Tetralones Bearing A Remote Quaternary Stereocenter through Rh-Catalyzed C-C Activation of Cyclopentanones

Herein, we describe the preparation of 1-tetralones bearing a remote quaternary stereocenter in a highly enantioselective manner. A sequence of Pd-catalyzed asymmetric 1,4-addition and Rh-catalyzed enantiospecific C-C/C-H activation delivers diverse 1-tetralones with a C4 quaternary stereocenter, which are prepared in good overall yields and high enantioselectivity.

Copper-catalyzed asymmetric silyl addition to alkenyl-substituted N-heteroarenes

Asymmetric conjugate addition of PhMe₂SiBPin to a wide range of N-heteroaryl alkenes proceeded in the presence of a copper catalyst coordinated with a chiral phosphoramidite ligand to afford useful β-silyl N-heteroarenes in high yields and ees.

Enantioselective Conjugate Addition of Aryl Halides and Triflates to Electron-Deficient Olefins via Nickel- and Rhodium-Catalyzed Sequential Relay Reactions

Asymmetric conjugate addition of aryl halides or aryl triflates to electron-deficient olefins was realized by sequential Miyaura borylation and Hayashi-Miyaura conjugate addition in one pot. A nickel-catalyzed borylation of aryl halides or triflates and a rhodium-chiral diene complex catalyzed enantioselective conjugate addition was executed as a pair of relay reactions as a more efficient and greener protocol.

Highly Enantioselective Hiyama Cross-Coupling via Rh-Catalyzed Allylic Arylation of Racemic Allyl Chlorides

Highly enantioselective Hiyama cross-coupling reactions have been achieved through rhodium(I)-catalyzed dynamic kinetic asymmetric transformations between aryl siloxanes and cyclic racemic allyl halides. This process affords valuable enantiomerically enriched aryl-substituted cyclic allyl products and is compatible with heterocyclic allyl chloride electrophiles.

Highly enantioselective rhodium-catalyzed cross-coupling of boronic acids and racemic allyl halides

Although Csp²-Csp² Suzuki-Miyaura couplings (SMCs) are widely used in small-molecule synthesis, related methods that allow the incorporation of Csp³-hybridized coupling partners, particularly in an asymmetric manner, are less developed. This protocol describes catalytic asymmetric SMC reactions that provide access to enantiomerically enriched cyclic allylic products. The method couples racemic allyl halide starting materials with sp²-hybridized boronic acid derivatives and is compatible with heterocyclic coupling partners. These reactions are catalyzed by a rhodium-ligand complex and typically display very high levels of enantioselectivity (>95% enantiomeric excess (ee)). In this protocol, we detail a procedure using a dihydropyridine-derived allyl chloride for the synthesis of (-)-(S)-tert-butyl-3-(4-bromophenyl)-3,6-dihydropyridine-1(2H)-carboxylate, an intermediate in the synthesis of the anticancer drug niraparib. This procedure affords 1.17 g (86% yield) of the coupling product with 96% ee. The initial experimental setup of the reaction takes 45-50 min, and the reaction is complete within 4-5 h.