Asymmetric Catalytic Insertion of α-Diazo Carbonyl Compounds into O–H Bonds of Carboxylic Acids

Asymmetric Carbene Transfer: Enhancing Chemical Diversity for Drug Discovery

The quest to explore chemical space is vital for identifying novel disease targets, impacting both the effectiveness and safety profile of therapeutic agents. The tangible chemical space, currently estimated at a conservative 108 synthesized compounds, pales in comparison to the theoretically conceivable diversity of 1060 molecules. To bridge this vast gap, organic chemists are spearheading innovative methodologies that promise to broaden this limited chemical diversity. A beacon of this progressive wave is Asymmetric Carbene Transfer (ACT), a burgeoning strategy that significantly boosts molecular diversity with efficient bond-formation and precise chiral control. This review focuses on the capabilities of ACT in creating pharmaceutically significant molecules, encompassing an array of natural products and bioactive compounds. Through the lens of ACT, we discern its substantial influence on drug discovery, paving the way for novel therapeutic avenues by expanding the boundaries of molecular diversity. This review will shed light on prospective methodological developments of ACT and articulate their conceivable contributions to the medicinal chemistry arena.

Straightforward access to α-carbonyloxy esters and β-keto thioethers from aryldiazoacetates

A metal- and additive-free approach has been described for synthesizing α-carbonyloxy esters and β-keto thioethers from readily available aryldiazoacetates with carboxylic acids and thiol derivatives, respectively. α-Carbonyloxy esters and β-keto thioether derivatives were synthesized in good to high yields from aryldiazoacetates, carboxylic acids, and thiol derivatives decorated with various functional groups. Finally, the potential of the new approach is demonstrated through its application in gram-scale reactions and the synthesis of a few bioactive molecules.

O-H Insertion of Hydrogenphosphate Derivatives and α-Diazo Compounds

An efficient O-H insertion of hydrogenphosphate derivatives and α-diazo compounds has been developed to construct α-phosphoryloxy scaffolds. Diverse α-phosphoryloxy skeletons could be obtained under mild and catalyst-free conditions in good yields. The control experiments suggest a protonation and nucleophilic addition process of α-diazo compounds via a diazonium ion pair for this transformation.

Is Enol Always the Culprit? The Curious Case of High Enantioselectivity in a Chiral Rh(II) Complex Catalyzed Carbene Insertion Reaction

The mechanism of Rh2 (S-NTTL)4 catalyzed carbene insertion into C(3)-H of indole is investigated using DFT methods. Since the commonly accepted enol mechanism cannot account for enantioinduction, a concerted oxocarbenium pathway was proposed in an earlier work using a model catalyst. However, after considering the full catalytic system, this study finds that akin to other reactions, here, too, the enol pathway is of lower energy, which now naturally raises a conundrum regarding the mode of chiral induction. Herein, a new water promoted mechanistic pathway involving a metal-associated enol intermediate hydrogen bonding and stereochemical model are proposed to solve this puzzle. It is shown how the catalyst bowl-shaped structure along with substrate-catalyst binding is crucial for achieving high levels of enantioselectivity. A stereodetermining water-assisted proton transfer is proposed and confirmed through deuterium-labeling experiments. The water molecules are held together by H-bonding interactions with the carboxylate ligands that is reminiscent of enzyme catalysis. Although several previous studies have aimed at understanding the mechanism of metal catalyzed carbene insertion reactions, the origin of high stereoinduction especially with chiral metal complexes remains unclear, and till date there is no transition state model that can explain the high enantioselectivity with such chiral Rh complexes. The metal-associated enol pathway is currently underrepresented in catalytic cycles and may play a crucial role in catalyst design. Since the enol pathway is commonly adopted in other metal-catalyzed X-H insertion reactions involving a diazoester, the presented results are not specific to the current reaction. Therefore, this study could provide the direction for achieving high levels of enantioselectivity which is otherwise difficult to achieve with a single metal catalyst.

Pathway to Construct α-Acyloxy Esters by B(C6F5)3-Catalyzed O-H Insertion of Carboxylic Acids with Sulfoxonium Ylides

We report the first example of B(C6F5)3-catalyzed O-H insertion reaction of sulfoxonium ylides and carboxylic acids, achieving efficient construction of diester moieties under metal-free condition. This protocol is characterized by broad substrate tolerance, particularly for various phenylacetic acids, and good compatibility with water/air condition, which is superior to most other methods.

Construction of α-Acyloxy Ketones via Photoredox-Catalyzed O-H Insertion of Sulfoxonium Ylides with Carboxylic Acids

Herein, a photoredox-catalyzed insertion of sulfoxonium ylides with carboxylic acids was advanced under mild and simple conditions, offering a practical approach for preparing α-acyloxy ketones with a broad scope of carboxylic acids. A combined experimental and computational study suggests that this reaction proceeds via a stepwise proton-assisted electron transfer mechanism.

Catalytic Asymmetric P-H Insertion Reactions

Albeit notable endeavors in enantioselective carbene insertion into X-H bonds (X = C, O, N, S, Si, B), the catalytic asymmetric P-H insertion reactions still stand for a long-lasting challenge. By merging transition-metal catalysis with organocatalysis, we achieve a scalable enantioselective P-H insertion transformation between diazo pyrazoleamides and H-phosphine oxides that upon subsequent reduction delivers a wide variety of optically active β-hydroxyl phosphine oxides in good yields with high enantioselectivity. The achiral copper catalyst fosters the carbenoid insertion into the P-H bond, while the chiral cinchona alkaloid-derived organocatalyst controls the subsequent enantioselective outcome. Density functional theory (DFT) calculations further reveal that the copper catalyst chelates to the organocatalyst, enhances its acidity, and accordingly promotes the enantioselective proton transfer. Our work showcases the potential of combining transition-metal catalysis with organocatalysis to realize elusive asymmetric reactions.

Enantioselective N-Heterocyclic Carbene Catalyzed α-Oxidative Coupling of Enals with Carboxylic Acids Using an Iodine(III) Reagent

The enantioselective α-oxidative coupling of enals with carboxylic acids was developed via the umpolung of an NHC-bound enolate with an iodine(III) reagent. The corresponding α-acyloxyl-β,γ-unsaturated esters were afforded in good yields, with high regio- and enantioselectivities. The key step of the reaction involves the formation of enol iodine(III) intermediate from the enolate with iodosobenzene, which changes the polarity of α-carbon of the enal from nucleophilic to electrophilic, and thus facilitates the subsequent addition of carboxylate.

A rhodium-catalyzed ylide formation/Smiles rearrangement reaction of chalcogenide ether and triazoles

A Rh(ii)-catalyzed highly stereoselective chalcogenide ylide formation/Smiles rearrangement reaction of diaryl thioether/selenoethers and triazoles was successfully developed.

Carboxylic Acid O-H Insertion Reaction of β-Ester Diazos Enabling Synthesis of β-Acyloxy Esters

Generation of non-stabilized β-ester diazos and their applications in carboxylic acid O-H insertion reactions have been reported, which afford β-acyloxy esters in excellent yield. Varieties of aryl- and alkyl-substituted diazos are well tolerated in this insertion reaction under mild and convenient conditions. Moreover, structural modification of the natural product and molecular drug can also be achieved in this reaction. This protocol not only realizes the reaction involving unstable β-ester diazos but also provides an efficient strategy for the synthesis of β-acyloxy esters.

Experimental and Computational Mechanistic Study of Carbonazidate-Initiated Cascade Reactions

A variety of Huisgen cyclization or nitrene/carbene alkyne cascade reactions with different types of termination were investigated. Accessible nitrene precursors were assessed, and carbonazidates were found to be the only effective initiators. Solvents, terminal alkynyl substituents, and catalysts can all impact the reaction outcome. Study of the mechanism both computationally (by density functional theory) and experimentally revealed relevant intermediates and plausible reaction pathways.

Does an Enol Pathway Preclude High Stereoselectivity in Iron-Catalyzed Indole C-H Functionalization via Carbene Insertion?

C-H functionalization of indoles via Fe carbenoids presents an attractive strategy to obtain biologically important structural motifs. However, obtaining good stereoselectivity with Fe has been a significant challenge. It is unclear whether the low selectivity is due to a radical pathway or an ionic mechanism involving metal-free species. We therefore present a density functional theory (DFT) study of indole alkylation with diazoacetates catalyzed by Fe(ClO₄)TMEDA/spirobisoxazoline and myoglobin. We explore three mechanistic pathways: nucleophilic, radical, and oxocarbenium routes. The nucleophilic pathway is the most feasible with the formation of an enol species that tautomerizes to furnish the alkylated indole. While this mechanism is routinely proposed, the stereochemical model has been conspicuously absent until now. We show that the conventionally invoked enol pathway is not responsible for the low enantiomeric excess. The enol intermediate can stay coordinated to the catalyst via different binding sites placing the enol in proximity to the chiral environment and affecting the stereoselective proton transfer. Both the binding strength and the chiral environment are crucial for obtaining high selectivity. Our study provides the much needed insights for the modest-low selectivities of Fe systems and could help in expediting the discovery of an efficient catalytic system. These mechanistic underpinnings could also be applicable to other metal (Rh, Pd, Cu, etc.)-catalyzed X-H insertion reactions.

Carboxylic Acid Functionalization Using Sulfoxonium Ylides as a Carbene Source

Functionalization of carboxylic acids using sulfoxonium ylides in the presence of [VO(acac)₂] as a catalyst is reported. The usual carbene source, diazo compounds, failed to produce α-carbonyloxy esters in good yield when compared to sulfoxonium ylides. Various standard spectroscopic and analytical techniques were used to characterize the products formed.

Asymmetric synthesis of isochromanone derivatives via trapping carboxylic oxonium ylides and aldol cascade

An efficient asymmetric synthesis of isochromanone derivatives was realized through Z-selective-1,3-OH insertion/aldol cyclization reaction involving acyclic carboxylic oxonium ylides. The combination of achiral dirhodium salts and chiral N,N′-dioxide–metal complexes, along with the use of α-diazoketones instead of α-diazoesters, enables the cascade reaction efficiently. A variety of benzo-fused δ-lactones bearing vicinal quaternary stereocenters were obtained with good to excellent enantioselectivity, bypassing the competitive 1,1-OH insertion and racemic background aldol reaction.

A highly enantioselective cascade Z-selective-1,3-OH insertion/aldol cyclization of ketoacids with diazoketones involving carboxylic oxonium ylides was achieved by using a bimetallic Rh(ii)/chiral N,N′-dioxide-Fe(iii) or Sc(iii) complex catalyst.

Enantioselective [1,2]-Stevens Rearrangement of Thiosulfonates to Construct Dithio-Substituted Quaternary Carbon Centers

An enantioselective [1,2] Stevens rearrangement was realized by using chiral guanidine and copper(i) complexes. Bis-sulfuration of α-diazocarbonyl compounds was developed through using thiosulfonates as the sulfenylating agent. It was undoubtedly an atom-economic process providing an efficient route to access novel chiral dithioketal derivatives, affording the corresponding products in good yields (up to 90% yield) and enantioselectivities (up to 96 : 4 er). A novel catalytic cycle was proposed to rationalize the reaction process and enantiocontrol.

An asymmetric [1,2] Stevens rearrangement was realized via chiral guanidine and copper(i) complexes. A series of novel chiral dithioketal derivatives were obtained with good yields (up to 90% yield) and enantioselectivities (up to 96 : 4 er).

B(C6F5)3-catalyzed O-H insertion reactions of diazoalkanes with phosphinic acids

A highly efficient base-, metal-, and oxidant-free catalytic O-H insertion reaction of diazoalkanes and phosphinic acids in the presence of B(C6F5)3 has been developed. This powerful methodology provides a green approach towards the synthesis of a broad spectrum of α-phosphoryloxy carbonyl compounds with good to excellent yields (up to 99% yield). The protocol features the advantages of operational simplicity, high atom economy, practicality, easy scalability and environmental friendliness.

Chemo- and Enantioselective Insertion of Furyl Carbene into the N-H Bond of 2-Pyridones

Asymmetric carbene insertion reactions represent one of the most important protocols to construct carbon-heteroatom bonds. The use of donor-acceptor diazo compounds bearing an ester group is however a prerequisite for achieving high enantioselectivity. Herein, we report a chemo- and enantioselective formal N-H insertion of 2-pyridones that has been accomplished for the first time with enynones as the donor-donor carbene precursors. DFT calculations indicate an unprecedented enantioselective 1,4-proton transfer from O to C. The rhodium catalyst provides a chiral pocket in which the steric repulsion and the π-π interaction of the propeller ligand play a critical role in determining the selectivities.

Natural-Like Spirocyclic Δα,β -Butenolides Obtained from Diazo Homophthalimides

α-Diazo homophotalimides were reacted with various propiolic acids on Rh₂ (esp)₂ catalysis. The resulting propiolate esters were transformed into novel, heterocyclic Δ^α,β -spirobutenolides in good to excellent product yields. The approach represents a fundamentally novel entry into natural-like Δ^α,β -spirobutenolides present in many biologically active natural products as well as fully synthetic compounds endowed with diverse biological activities. The Δ^α,β -spirobutenolides thus obtained were shown to inhibit thioredoxin reductase, a selenocysteine enzyme target for cancer. Moreover, for the best compound in the series (TrxR IC₅₀ 1.49±0.08 μM), by using MALDI-TOF mass-spectrometry it was shown that it selectively binds selenocysteine in the presence of a 10-fold excess of cysteine. This validates the new compound as a promising lead for anticancer therapy development.

Well-Defined, Versatile and Recyclable Half-Sandwich Nickelacarborane Catalyst for Selective Carbene-Transfer Reactions

Catalytic carbene-transfer reactions constitute a class of highly useful transformations in organic synthesis. Although catalysts based on a range of transition-metals have been reported, the readily accessible nickel(II)-based complexes have been rarely used. Herein, an air-stable nickel(II)-carborane complex is reported as a well-defined, versatile and recyclable catalyst for selective carbene transfer reactions with low catalyst loading under mild conditions. This catalyst is effective for several types of reactions including diastereoselective cyclopropanation, epoxidation, selective X-H insertions (X = C, N, O, S, Si), particularly for the unprotected substrates. This represents a rare example of carborane ligands in base metal catalysis.

Brønsted acid-catalyzed homogeneous O–H and S–H insertion reactions under metal- and ligand-free conditions

The economical and accessible CF₃SO₃H successfully catalyzed homogeneous O–H and S–H bond insertion reactions between hydroxyl compounds, thiols and diazo compounds under metal- and ligand-free conditions.

Catalyst-free synthesis of α,α-disubstituted carboxylic acid derivatives under ambient conditions via a Wolff rearrangement reaction

Herein, a hexafluoroisopropanol (HFIP)-promoted Wolff rearrangement reaction was developed, delivering various α,α-disubstituted carboxylic acid derivatives in good to excellent yields.

Formation of β-Oxo-N-vinylimidates via Intermolecular Ester Incorporation in Huisgen Cyclization/Carbene Cascade Reactions

Unusual intermolecular trapping of esters by carbenes generated via a Huisgen cyclization/retroelectrocyclization/dediazotization cascade reaction is presented. β-Oxo-N-vinylimidates could be obtained in one step from propargyl carbonazidates. Mechanistic control experiments suggested reversible dipole formation by ester addition to the carbene, and nitrogen attack to the ester carbonyl was irreversibly followed by stereoselective decarboxylative elimination to give the Z-vinyl imidate. The cross-conjugated enone, imidate, and enamine functional groups in the β-oxo-N-vinylimidates offer novel syntheses of functionalized oxazoles.

Hypervalent Iodine-Mediated Diastereoselective α-Acetoxylation of Cyclic Ketones

A binary hybrid system comprising a hypervalent iodine(III) reagent and BF₃•OEt₂ Lewis acid was found to be effective for the diastereoselective α-acetoxylation of cyclic ketones. In this hybrid system, BF₃•OEt₂ Lewis acid allowed the activation of the hypervalent iodine(III) reagent and cyclic ketones for smooth α-acetoxylation reaction, achieving high diastereoselectivity. This hypervalent iodine-mediated α-acetoxylation of the cyclic ketone reaction plausibly undergoes an S_N2 substitution mechanism via an α-C-bound hypervalent iodine intermediate. The diastereoselectivity of the reaction mainly originates from thermodynamic control.

Rhodium-Catalyzed Sequential Cycloisomerization/Aldol Addition of Cyclopropene Carboxylic Acids with Isatins

A unique, rhodium-catalyzed reaction of cyclopropene carboxylic acids with isatins has been developed, which takes place through a trapping process of transient cyclic carboxylic oxonium ylides. This reaction tolerates a wide variety of cyclopropene carboxylic acids and N-protected or unprotected isatins and represents a new, direct strategy to access valuable γ-substituted γ-butenolide architectures containing an oxindole moiety in very high yields under mild reaction conditions.

Blue Light-promoted Carbene Transfer Reactions of Tosylhydrazones

Metal-free photochemical carbene-transfer reactions of tosylhydrazones were developed under blue light irradiation at room temperature. This reaction constructs C-X (X=C, N, O, S) bonds and cyclopropanes from readily available and stable starting materials.

Highly Enantioselective O-H Bond Insertion Reaction of α-Alkyl- and α-Alkenyl-α-diazoacetates with Water

Catalytic asymmetric reactions in which water is a substrate are rare. Enantioselective transition-metal-catalyzed insertion of carbenes into the O-H bond of water can be used to incorporate water into the stereogenic center, but the reported chiral catalysts give good results only when α-aryl-α-diazoesters are used as the carbene precursors. Herein we report the first highly enantioselective O-H bond insertion reactions between water and α-alkyl- and α-alkenyl-α-diazoesters as carbene precursors, with catalysis by a combination of achiral dirhodium complexes and chiral phosphoric acids or chiral phosphoramides. Participation of the phosphoric acids or phosphoramides in the carbene transfer reaction markedly suppressed competing side reactions, such as β-H migration, carbene dimerization, and olefin isomerization, and thus ensured good yields of the desired products. Fine-tuning of the ester moiety facilitated enantiocontrol of the proton transfer reactions of the enol intermediates and resulted in excellent enantioselectivity. This protocol represents an efficient new method for preparation of multifunctionalized chiral α-alkyl and α-alkenyl hydroxyl esters, which readily undergo various transformations and can thus be used for the synthesis of bioactive compounds. Mechanistic studies revealed that the phosphoric acids and phosphoramides promoted highly enantioselective [1,2]- and [1,3]-proton transfer reactions of the enol intermediates. Maximization of molecular orbital overlap in the transition states of the proton transfer reactions was the original driving force to involve the proton shuttle catalysts in this process.

An asymmetric hydrocyanation/Michael reaction of α-diazoacetates via Cu(i)/chiral guanidine catalysis

An asymmetric one-pot hydrocyanation/Michael reaction of α-aryl diazoacetates with trimethylsilyl cyanide, tert-butanol, and N-phenylmaleimides has been realized using a chiral guanidinium salt/CuBr catalyst.

Polycondensation of bis(α-diazo-1,3-dicarbonyl) compounds with dicarboxylic acids: an efficient access to functionalized alternating polyesters

We report an efficient and mild approach to functionalized alternating polyesters on the basis of O–H insertion polycondensation of bis(α-diazo-1,3-dicarbonyl) compounds with dicarboxylic acids.

Enantioselective carbene insertion into the N-H bond of benzophenone imine

Efficient enantioselective insertion of α-diazoesters into the N-H bond of N-sp2-hybridized benzophenone imine was realized by using Rh2(esp)2 and chiral guanidine cooperative catalysis. Both aliphatic and aromatic substituted α-amino esters were obtained in high yields (up to 99%) and good enantioselectivities (up to 95.5 : 4.5 er) under mild reaction conditions.

Cooperative Catalysis of Chiral Guanidine and Rh2(OAc)4 in Asymmetric O-H Insertion of Carboxylic Acid: A Theoretical Investigation

We reported a mechanistic study on asymmetric O-H insertion reaction of α-diazoester with carboxylic acid using Rh₂(OAc)₄/chiral guanidine-amide as the cocatalyst by density functional theory [B3LYP-D3(BJ)/def2-TZVP//B3LYP-D3(BJ)/[6-31G**, SDD] (SMD, Et₂O)]. The catalytic reaction included two stages: (i) formation of Rh-carbene species, subsequently by the construction of C-O bond forming enol and (ii) chiral guanidinium salt-assisted H-transfer to the enol. In cooperative catalysis, Rh₂(OAc)₄ helped to form an enol intermediate via high-reactivity Rh-carbene species, while the in situ-formed guanidium carboxylate acted as a chiral proton shuttle to construct a hydrogen bonding net for the stereo-determinant protonation. The repulsions between the phenyl group of the enol intermediate and the cyclohexyl as well as the ortho-substituted isopropyl group of chiral guanidine played important roles in controlling stereoselectivity. A disadvantageous steric arrangement in si-face attack weakened the stabilizing electrostatic and orbital interaction of reacting species in the H-transfer step, enhancing the pathway to form a predominant product with R-configuration in the two competing pathways. A model was proposed to explain the asymmetric induction of chiral guanidine-amide in protonation.

Silver-Catalyzed Asymmetric Insertion into Phenolic O-H Bonds using Aryl Diazoacetates and Theoretical Mechanistic Studies

An enantioselective insertion reaction of silver carbenes generated from donor-acceptor-substituted diazo compounds into the O-H bond of phenols was developed. A homobinuclear silver complex with a chiral phosphorous ligand was created in situ from AgNTf₂ and (S)-XylylBINAP (in a 2:1 mole ratio). Detailed mechanistic studies using combined experimental and computational techniques revealed that one silver atom center of the catalyst forms a silver carbene and another one works as a Lewis acid for the nucleophilic addition of a phenol. Two counter-anions, two water molecules, and two silver atoms cooperatively mediate the subsequent protonation event to lower the activation energy and control enantioselectivity, affording an array of valuable α-aryl-α-aryloxy esters.

One-Pot Reaction of Carboxylic Acids, Ynol Ethers, and m-CPBA for Synthesis of α-Carbonyloxy Esters

A novel one-pot reaction of carboxylic acids, ynol ethers, and m-CPBA for the synthesis of α-carbonyloxy esters in the presence of Ag₂O is described. This process provides a direct approach to α-carbonyloxy esters with the achievement of formation of three C-O bonds. The protocol is featured with readily available starting materials and broad substrate scope. Control reactions and isotope-labeling reactions were conducted to elucidate a plausible mechanism.