Catalytic Asymmetric Propargylation

Chiral bisphosphine Ph-BPE ligand: a rising star in asymmetric synthesis

Chiral 1,2-bis(2,5-diphenylphospholano)ethane (Ph-BPE) is a class of optimal organic bisphosphine ligands with C2-symmetry. Ph-BPE with its excellent catalytic performance in asymmetric synthesis has attracted much attention of chemists with increasing popularity and is growing into one of the most commonly used organophosphorus ligands, especially in asymmetric catalysis. Over two hundred examples have been reported since 2012. This review presents how Ph-BPE is utilized in asymmetric synthesis and how powerful it is as a chiral ligand or even a catalyst in a wide range of reactions including applications in the total synthesis of bioactive molecules.

Copper-Catalyzed Difunctionalization of Propargylic Carbonates through Tandem Nucleophilic Substitution/Boroprotonation

Highly functionalized organic molecules are in high demand, but their preparation is challenging. Copper-catalyzed transformation of alkynyl- and allenyl-containing substrates has emerged as a powerful tool to achieve this objective. Herein, an efficient copper-catalyzed difunctionalization of propargylic carbonates through tandem nucleophilic substitution/boroprotonation has been developed, affording the formation of thiol-, selenium-, and boron-functionalized alkenes with high yield and stereoselectivity. Two distinct catalytic mechanisms involving a single reaction without any requirement of catalyst change were successfully demonstrated.

Ag-Catalyzed Switchable Synthesis of Site-Specifically Functionalized Pyrroles via Azafulvenium Intermediates

Here, we present a versatile, silver-catalyzed multi-auto-tandem reaction involving enamines, alkynals, and nucleophiles, utilizing the highly reactive intermediate azafulvenium. This method allows for flexible and switchable regiodivergent reactions through either intermolecular or intramolecular nucleophilic attacks, which can be controlled by adjusting the catalytic conditions. A range of site-specific functionalized or polycyclic fused pyrrole products were efficiently produced with a high level of chemocontrol.

Enantioselective and Regiodivergent Synthesis of Propargyl- and Allenylsilanes through Catalytic Propargylic C-H Deprotonation

We report a highly enantioselective intermolecular C-H bond silylation catalyzed by a phosphoramidite-ligated iridium catalyst. Under reagent-controlled protocols, propargylsilanes resulting from C(sp3)-H functionalization, as well the regioisomeric and synthetically versatile allenylsilanes, could be obtained with excellent levels of enantioselectivity and good to excellent control of propargyl/allenyl selectivity. In the case of unsymmetrical dialkyl acetylenes, good to excellent selectivity for functionalization at the less-hindered site was also observed. A variety of electrophilic silyl sources (R3SiOTf and R3SiNTf2), either commercial or in situ-generated, were used as the silylation reagents, and a broad range of simple and functionalized alkynes, including aryl alkyl acetylenes, dialkyl acetylenes, 1,3-enynes, and drug derivatives were successfully employed as substrates. Detailed mechanistic experiments and DFT calculations suggest that an η3-propargyl/allenyl Ir intermediate is generated upon π-complexation-assisted deprotonation and undergoes outer-sphere attack by the electrophilic silylating reagent to give propargylic silanes, with the latter step identified as the enantiodetermining step.

Au-allenylidene promoted decarboxylative annulation to access unsaturated γ-lactams/lactones

A novel Au-allenylidene promoted decarboxylative annulation by intramolecular α-nucleophilic addition has been disclosed. The unsaturated cyclic ethynylethylene carbamates/carbonates can be converted to unique nucleophiles attached with alkylidene ketenes by sequential decarboxylation and oxidation processes. Such alkylidene ketenes can be rapidly trapped by intramolecular α-attacking annulation to generate potential biological active unsaturated γ-lactams/lactones with broad scope, facile post-modification, high regioselectivity and efficiency.

Copper-Catalyzed Enantioconvergent Radical N-Alkylation of Diverse (Hetero)aromatic Amines

The 3d transition metal-catalyzed enantioconvergent radical cross-coupling provides a powerful tool for chiral molecule synthesis. In the classic mechanism, the bond formation relies on the interaction between nucleophile-sequestered metal complexes and radicals, limiting the nucleophile scope to sterically uncongested ones. The coupling of sterically congested nucleophiles poses a significant challenge due to difficulties in transmetalation, restricting the reaction generality. Here, we describe a probable outer-sphere nucleophilic attack mechanism that circumvents the challenging transmetalation associated with sterically congested nucleophiles. This strategy enables a general copper-catalyzed enantioconvergent radical N-alkylation of aromatic amines with secondary/tertiary alkyl halides and exhibits catalyst-controlled stereoselectivity. It accommodates diverse aromatic amines, especially bulky secondary and primary ones to deliver value-added chiral amines (>110 examples). It is expected to inspire the coupling of more nucleophiles, particularly challenging sterically congested ones, and accelerate reaction generality.

Cross-coupling of organic fluorides with allenes: a silyl-radical-relay pathway for the construction of α-alkynyl-substituted all-carbon quaternary centres

Controlling the transformation of versatile and reactive allenes is a considerable challenge. Herein, we report an efficient silylboronate-mediated cross-coupling reaction of organic fluorides with allenes to construct a series of sterically demanding α-ethynyl-containing all-carbon quaternary centers (ACQCs), using catalyst-free silyl-radical-relay reactions to selectively functionalize highly inert C-F bonds in organic fluorides. The key to the success of this transformation lies in the radical rearrangement of an in situ-generated allenyl radical to form a bulky tertiary propargyl radical; however, the transformation does not show efficiency when using the propargyl isomer directly. This unique reaction enables the cross-coupling of a tertiary carbon radical center with a C(sp2)-F bond or a benzylic C(sp3)-F bond. α-Ethynyl-containing ACQCs with (hetero)aromatic substituents and benzyl were efficiently synthesized in a single step using electronically and sterically diverse organic fluorides and allenes. The practical utility of this protocol is showcased by the late-stage functionalization of bioactive molecules and the modification of a liquid crystalline material.

Enantioselective propargylic amination and related tandem sequences to α-tertiary ethynylamines and azacycles

Chiral α-tertiary amines and related azacycles are sought-after compounds for drug development. Despite progress in the catalytic asymmetric construction of aza-quaternary stereocentres, enantioselective synthesis of multifunctional α-tertiary amines remains underdeveloped. Enantioenriched α-disubstituted α-ethynylamines are attractive synthons for constructing chiral α-tertiary amines and azacycles, but methods for their catalytic enantioselective synthesis need to be expanded. Here we describe an enantioselective asymmetric Cu(I)-catalysed propargylic amination (ACPA) of simple ketone-derived propargylic carbonates to give both α-dialkylated and α-alkyl-α-aryl α-tertiary ethynylamines. Sterically confined pyridinebisoxazoline (PYBOX) ligands, with a C4 shielding group and relaying groups, play a key role in achieving excellent enantioselectivity. The syntheses of quaternary 2,5-dihydropyrroles, dihydroquinines, dihydrobenzoquinolines and dihydroquinolino[1,2-α]quinolines are reported, and the synthetic value is further demonstrated by the enantioselective catalytic total synthesis of a selective multi-target β-secretase inhibitor. Enantioselective Cu-catalysed propargylic substitutions with O- and C-centred nucleophiles are also realized, further demonstrating the potential of the PYBOX ligand.

Isolation of Cationic η3-Allenylnickel(II) Key Intermediate Complexes: Origins of Enantioselectivity and Regioselectivity in Nickel(0)-Catalyzed Asymmetric Propargylic Substitutions

Herein, we report the synthesis and isolation of cationic η3-allenylnickel(II) complexes that bear rac-BINAP as a bidentate ligand for the first time via Me3SiOTf-promoted C-O bond cleavage of propargylic tert-butyl carbonate. In contrast, in the presence of the monodentate phosphine ligand PEt3, treatment of propargylic tert-butyl carbonate with Ni(cod)2 resulted in a gradual C-O bond cleavage leading to η1-allenylnickel(II) complexes, i.e., trans-(PEt3)2Ni(η1-CPh═C═CHR)(OBoc). X-ray diffraction and NMR spectroscopy studies of [(η3-RCH-CCPh)Ni(rac-BINAP)](OTf) revealed that the complex adopts an η3-allenyl coordination mode both in the crystal lattice and in solution. A thorough structural comparison between [(η3-RCH-CCPh)Ni(rac-BINAP)](OTf) and palladium and platinum analogues revealed that the η3-allenyl moiety in the nickel complex is similar to that observed in palladium and platinum complexes, albeit that each Ni-C bond is shorter than the corresponding Pd-C and Pt-C bonds due to the smaller ionic radius of nickel to that of Pd or Pt. The reactions of either N-methylaniline or sodium N-methylanilide with [(η3-RCH-CCPh)Ni((R)-BINAP)](OTf) furnished (R)-PhC≡CCH(NMePh)Me as an asymmetric propargylic substitution (APS) product with excellent enantioselectivity. Furthermore, when the nickel-catalyzed APS reaction of propargylic tert-butyl carbonate with N-methylaniline was conducted in DMSO at 60 °C in the presence of 5 mol % of [(η3-RCH-CCPh)Ni((R)-BINAP)](OTf) and 7.5 mol % of sodium N-methylanilide as a catalytic precursor and an additive, respectively, (R)-PhC≡CCH(NMePh)Me was obtained in 79% yield with 90% ee. The experimental results and computational calculations strongly suggest that the nickel-catalyzed APS reaction might proceed via a cationic η3-allenylnickel(II) species as the key reaction intermediate.

Copper-Catalyzed Enantioselective Formal [4 + 1] and [3 + 3] Cycloaddition of Ethynylethylene Carbonates

Herein we employed ethynylethylene carbonates (EECs) to achieve formal [4 + 1] and [3 + 3] cycloaddition with cyclic 1,3-dicarbonyl compounds. On one hand, EECs with styryl substitution could undergo a remotely controlled enantioselective [4 + 1] cycloaddition reaction. This reaction exhibits good chemoselectivity, regioselectivity, and enantioselectivity. In addition, a [3 + 3] cycloaddition reaction of EECs with cyclic 1,3-dicarbonyl compounds was also achieved, leading to a series of 4H-pyrans with impressive chemoselectivity and enantioselectivity.

Enantio- and Diastereodivergent N-Heterocyclic Carbene/Nickel Dual-Catalyzed Umpolung Propargylic Substitutions of Enals

The creation of full stereoisomers of an organic compound comprising multiple contiguous stereocenters with simultaneous control over both relative and absolute configurations remains a significant challenge in synthetic chemistry. Using a cooperative catalysis strategy, we established an N-heterocyclic carbene/nickel-catalyzed enantio- and diastereodivergent propargylation reaction to access 3,3'-disubstituted oxindoles, enabling the incorporation of internal alkyne functionality and the introduction of a single quaternary or vicinal quaternary/tertiary stereogenic center. By selecting the appropriate combination of catalyst chirality, all four potential stereoisomers of α-quaternary propargylated oxindoles were synthesized in a predictable and precise way with remarkable yields, diastereoselectivities, and enantioselectivities from identical starting materials. The synthetic utility of this method was demonstrated in the concise asymmetric total synthesis of (-)-debromoflustramine B and (-)-C(β-Me)-debromoflustramine B.

Copper-Catalyzed Remote Enantioselective Sulfonylation of Yne-Allylic Esters with Sodium Sulfinates

Impressive progress has been made in the copper-catalyzed asymmetric propargylic substitution (APS) reaction, but its use in remote asymmetric yne-allylic substitution remains a challenging topic. Herein, we report the first remote enantioselective copper-catalyzed sulfonylation of yne-allylic esters with sodium sulfinates. The reaction is assumed to occur via a copper-vinylvinylidene species as the key reactive intermediate. The use of readily available starting materials, the mild reaction conditions, and the excellent regio-, enantio- and stereoselectivity, as well as broad substrate scope (>70 examples), show the practicality and attractiveness of this method.

Copper-Catalyzed Umpolung Reactivity of Propargylic Carbonates in the Presence of Diboronates: One Stone Four Birds

Allylation and propargylation are two powerful synthetic strategies for making new substances that have been of significant importance in chemistry, medicine, and material fields. Conventional tactics employ various preformed allylation and propargylation reagents. In this study, a conceptually novel copper-catalyzed and B2pin2-mediated Umpolung reactivity of propargylic carbonates has been achieved for the first time, realizing both allylation and propargylation of aldehydes and ketones without additional reductants. Three types of allylation products and one type of propargylation product are generated efficiently, and all allylation products are formed with syn-configurations predominantly. The choice of ligands plays a vital role in modulating the Umpolung modes. The synthetic applications have been demonstrated in a myriad of further transformations including natural product synthesis, and systematic mechanistic studies have been conducted to reveal detailed insights into the Umpolung processes.

Catalyst-Free Propargylboration of Ketones with Allenyl-Bpins: Highly Stereoselective Synthesis of tert-Homopropargyl Alcohols Bearing Vicinal Stereocenters

A practical and efficient propargylboration of ketones is presented using general allenylboronic acid pinacol esters (allenyl-Bpins) without a catalyst. This reaction is triggered by in-situ activation of stable allenyl-Bpins through the sequential addition of 1.25 equiv. of n BuLi and the prerequisite 2.0 equiv. of TFAA. Under the optimized reaction conditions, the versatile trisubstituted allenyl-Bpins react with various ketones smoothly to afford a wide range of tert-homopropargyl alcohols bearing vicinal stereocenters in high yields with good to excellent diastereoselectivities. Furthermore, propargylboration of ketones with chiral trisubstituted allenyl-Bpins allows for the asymmetric synthesis of chiral tert-homopropargyl alcohols with a full chirality transfer.

Rh(II)/Pd(0) Dual-Catalyzed Regio-Divergent Three-Component Propargylic Substitution

Regio-divergent propargylic substitution to generate functionally diverse products from identical starting materials remains a formidable challenge, probably due to the unpredictable regiochemical complexity. In practically, the synthesis of α-quaternary propargylic-substituted products is still much less developed, and preprepared nucleophiles are generally applied in this type of reaction with propargylic substrates, which limits the reaction efficiency and diversity of the obtained products. Herein, we disclose unprecedented three-component propargylic substitution of α-diazo esters with amines and propargylic carbonates under dirhodium/palladium dual catalysis. The key to the success of this multicomponent propargylic substitution is to avoid two-component side reactions through a tandem process of dirhodium(II)-catalyzed carbene insertion and palladium-catalyzed regiodivergent propargylic substitution. The judicious selection of a diphosphine (dppf) or monophosphine (tBuBrettphos) as the ligand is crucial for the reaction to generate different products in a switchable way, α-quaternary 1,3-dienyl or propargylated products, with high regio- and chemoselectivities.

Rh(III)-Catalyzed C-H Activation of Benzamides and Chemodivergent Annulation with Benzoxazinanones: Substrate Controlled Selectivity

Decarboxylative annulation of propargyl carbamates with benzamides has been realized via rhodium-catalyzed C-H bond activation under mild conditions, delivering two distinct classes of heterocycles in high efficiency and selectivity under substrate control. This protocol provides a direct synthetic method for the preparation of functionalized 1,8-naphthyridines and isoindolinones.

Lanthanum-Catalyzed Stereospecific Cross-Coupling of Propargylic Substrates with Grignard Reagents

Transition-metal-catalyzed cross-coupling of propargylic electrophiles and Grignard reagents provides densely functionalized products that are extremely useful synthetic intermediates. However, examples of conversion of propargylic derivatives to form propargyl compounds remain limited due to the challenging regioselectivity. We use LaCl3·2LiCl to catalyze propargylation of Grignard reagents in the absence of ligand in high regioselectivity and stereospecificity. The approach shows a wide substrate scope using alkyl or (hetero)aryl Grignard reagents and alkynyl electrophiles with different leaving groups. Our protocol was further applied for the formal synthesis of frondosin B. It is worth exploring methodologies utilizing the naturally abundant and relatively nontoxic lanthanum catalysts.

Copper-catalyzed propargylic C-H functionalization for allene syntheses

Allenenitriles bearing different synthetically versatile functional groups have been prepared smoothly from 5-alkynyl fluorosulfonamides in decent yields with an excellent chemo- and regio-selectivity under redox neutral conditions. The resulting allenenitriles can be readily converted to useful functionalized heterocycles. Based on mechanistic study, it is confirmed that this is the first example of radical-based non-activated propargylic C-H functionalization for allene syntheses.

Photoredox cobalt-catalyzed regio-, diastereo- and enantioselective propargylation of aldehydes via propargyl radicals

Catalytic enantioselective introduction of a propargyl group constitutes one of the most important carbon-carbon forming reactions, as it is versatile to be transformed into diverse functional groups and frequently used in the synthesis of natural products and biologically active molecules. Stereoconvergent transformations of racemic propargyl precursors to a single enantiomer of products via propargyl radicals represent a powerful strategy and provide new reactivity. However, only few Cu- or Ni-catalyzed protocols have been developed with limited reaction modes. Herein, a photoredox/cobalt-catalyzed regio-, diastereo- and enantioselective propargyl addition to aldehydes via propargyl radicals is presented, enabling construction of a broad scope of homopropargyl alcohols that are otherwise difficult to access in high efficiency and stereoselectivity from racemic propargyl carbonates. Mechanistic studies and DFT calculations provided evidence for the involvement of propargyl radicals, the origin of the stereoconvergent process and the stereochemical models.

Evaluation of helicene-derived 2,2'-bipyridine N-monoxide catalyst for the enantioselective propargylation of N-acylhydrazones with allenyltrichlorosilane

Helicene-derived 2,2'-bipyridine N-monoxide was evaluated as a Lewis base catalyst for the enantioselective propargylation of N-acylhydrazones with allenyltrichlorosilane. The helicene-derived catalyst provided moderate-to-good reactivity and enantioselectivity for a range of acylhydrazones. This study represents the first example of the catalytic asymmetric propargylation of non-activated acylhydrazones.

Copper-Catalyzed Enantioconvergent Radical C(sp3)-N Cross-Coupling of Activated Racemic Alkyl Halides with (Hetero)aromatic Amines under Ambient Conditions

The enantioconvergent C(sp³)-N cross-coupling of racemic alkyl halides with (hetero)aromatic amines represents an ideal means to afford enantioenriched N-alkyl (hetero)aromatic amines yet has remained unexplored due to the catalyst poisoning specifically for strong-coordinating heteroaromatic amines. Here, we demonstrate a copper-catalyzed enantioconvergent radical C(sp³)-N cross-coupling of activated racemic alkyl halides with (hetero)aromatic amines under ambient conditions. The key to success is the judicious selection of appropriate multidentate anionic ligands through readily fine-tuning both electronic and steric properties for the formation of a stable and rigid chelating Cu complex. Thus, this kind of ligand could not only enhance the reducing capability of a copper catalyst to provide an enantioconvergent radical pathway but also avoid the coordination with other coordinating heteroatoms, thereby overcoming catalyst poisoning and/or chiral ligand displacement. This protocol covers a wide range of coupling partners (89 examples for activated racemic secondary/tertiary alkyl bromides/chlorides and (hetero)aromatic amines) with high functional group compatibility. When allied with follow-up transformations, it provides a highly flexible platform to access synthetically useful enantioenriched amine building blocks.

Copper-catalysed enantioconvergent alkylation of oxygen nucleophiles

Carbon-oxygen bonds are commonplace in organic molecules, including chiral bioactive compounds; therefore, the development of methods for their construction with simultaneous control of stereoselectivity is an important objective in synthesis. The Williamson ether synthesis, first reported in 18501, is the most widely used approach to the alkylation of an oxygen nucleophile, but it has significant limitations (scope and stereochemistry) owing to its reaction mechanism (SN2 pathway). Transition-metal catalysis of the coupling of an oxygen nucleophile with an alkyl electrophile has the potential to address these limitations, but progress so far has been limited2-7, especially with regard to controlling enantioselectivity. Here we establish that a readily available copper catalyst can achieve an array of enantioconvergent substitution reactions of α-haloamides, a useful family of electrophiles, by oxygen nucleophiles; the reaction proceeds under mild conditions in the presence of a wide variety of functional groups. The catalyst is uniquely effective in being able to achieve enantioconvergent alkylations of not only oxygen nucleophiles but also nitrogen nucleophiles, giving support for the potential of transition-metal catalysts to provide a solution to the pivotal challenge of achieving enantioselective alkylations of heteroatom nucleophiles.

Highly Regioselective Propargylation/Allenylation of Organolanthanum Reagents with Aldehydes

The metal-mediated propargylation or allenylation of carbonyl compounds is well-adapted to the preparation of homopropargylic or allenylic alcohols, which are multifunctional intermediates in synthetic chemistry. However, the regioselectivity of reactions using propargyl or allenyl metal reagents is difficult to control, owing to the equilibrium between the two species. In our study, propargyl or allenyl organolanthanum reagents were prepared using trimethylsilylpropyne or prop-1-yn-1-ylbenzene substrates. The treatment of the organolanthanum reagents with aldehydes yielded the regioselective products, respectively. This study provides a better understanding of structural specificity and the special chemoselectivity of rare earth metal reagents.

Copper(I)-catalyzed asymmetric alkylation of α-imino-esters

Asymmetric alkylation of enolates is one of the most direct and important reactions to prepare α-chiral carbonyl compounds. Except for the classical methods that rely on the use of chiral auxiliaries, asymmetric catalysis emerged as a powerful tool, especially asymmetric phase-transfer catalysis. However, in the field of transition metal catalysis, only limited success with asymmetric alkylation of enolates was achieved. Hereby, we disclose a copper(I)-catalyzed asymmetric alkylation of α-imino-esters with various alkyl halides, including allyl bromides, propargyl bromide, benzyl bromides, α-bromo carbonyl compounds, and alkyl iodides. Both linear and cyclic α-imino-esters serve as competent pronucleophiles in the alkylation, which affords α-amino acid derivatives bearing either a trisubstituted or a tetrasubstituted stereogenic carbon center in high to excellent enantioselectivity. Control experiments indicate that the α-imino-ester is activated by a chiral copper(I)-phosphine complex through coordination, thus enabling facile deprotonation to provide a stabilized copper(I)-enolate in the presence of a mild base. Finally, the mildly basic nature allows the asymmetric alkylation of chiral dipeptides with excellent both chemo- and enantioselectivities.

Recent Advances in the Synthesis of Propargyl Derivatives, and Their Application as Synthetic Intermediates and Building Blocks

The propargyl group is a highly versatile moiety whose introduction into small-molecule building blocks opens up new synthetic pathways for further elaboration. The last decade has witnessed remarkable progress in both the synthesis of propargylation agents and their application in the synthesis and functionalization of more elaborate/complex building blocks and intermediates. The goal of this review is to highlight these exciting advances and to underscore their impact.

Copper-Catalysed Rearrangement of Cyclic Ethynylethylene Carbonates: Synthetic Applications and Mechanistic Studies

Transition-metal-catalysed reactions of cyclic ethynylethylene carbonates have been intensively studied because of their robustness in new bond formation and diversified molecule construction. Known reaction modes usually involve a substitution step occurring at either the propargylic or terminal alkyne positions. Here, we report an unprecedented reaction pattern in which cyclic ethynylethylene carbonates first undergo a rearrangement to release allenal intermediates, which subsequently react with diverse nucleophiles to furnish synthetically useful allylic and propargylic allenols, phosphorus ylides, and cyclopropylidene ketones through an addition process rather than a substitution pathway. The products enable various further transformations, and mechanistic studies and theoretical calculations reveal that the reaction does not proceed via a semipinacol type [1,2]-hydride shift, but through base-mediated deprotonation as the key step to induce the rearrangement.

Stereodivergent Synthesis of Allenes with α,β-Adjacent Central Chiralities Empowered by Synergistic Pd/Cu Catalysis

The stereodivergent synthesis of allene compounds bearing α,β-adjacent central chiralities has been realized via the Pd/Cu-catalyzed dynamic kinetic asymmetric alkylation of racemic allenylic esters. The matched reactivity of bimetallic catalytic system enables the challenging reaction of racemic aryl-substituted allenylic acetates with sterically crowded aldimine esters smoothly under mild reaction conditions. Various chiral non-natural amino acids bearing a terminal allenyl group are easily synthesized in high yields and with excellent diastereo- and enantioselectivities (up to >20 : 1 dr, >99 % ee). Importantly, all four stereoisomers of the product can be readily accessed by switching the configurations of the two chiral metal catalysts. Furthermore, the easy interconversion between the uncommon η³ -butadienyl palladium intermediate featuring a weak C=C/Pd coordination bond and a stable Csp² -Pd bond is beneficial for the dynamic kinetic asymmetric transformation process (DyKAT).

Catalytic Enantioselective Allylboration and Related Reactions of Isatins Promoted by Chiral BINOLs: Scope and Mechanistic Studies

An improvement in the catalytic enantioselective allylboration of isatins with 2-allyl-1,3,2-dioxaborolane in the presence of chiral BINOL derivatives is reported, offering an efficient one-step access to enantioenriched N-unprotected 3-allyl-3-hydroxy-2-oxindoles. This catalytic process is also effective for the crotylboration reaction with enantiomeric ratios (er) up to 97:3, as well as for the asymmetric synthesis of homopropargylic alcohols via an allenyl addition to indoline-2,3-diones. Origins of the high enantioselectivity in chiral BINOL-catalyzed allylboration of isatins were examined by DFT calculations. A hypothetical scenario suggested a crucial internal hydrogen bonding between the amide group (C═O···H-O) and the ethylene hydroxyl of the transient chiral mixed boronate ester, generating a rigid and stabilized system that favors the addition of the allylboron species to the Re face of the ketone function. The key role of the alcohol additive (t-BuOH or t-AmOH) in the enantioselective allylboration reaction of isatins has also been shown on the basis of a kinetics study and computational calculations by favoring the transesterification of the 2-allyl-1,3,2-dioxaborolane with BINOL via proton transfer processes.

Chiral aldehyde-nickel dual catalysis enables asymmetric α-propargylation of amino acids and stereodivergent synthesis of NP25302

The combined catalytic systems derived from organocatalysts and transition metals exhibit powerful activation and stereoselective-control abilities in asymmetric catalysis. This work describes a highly efficient chiral aldehyde-nickel dual catalytic system and its application for the direct asymmetric α-propargylation reaction of amino acid esters with propargylic alcohol derivatives. Various structural diversity α,α-disubstituted non-proteinogenic α-amino acid esters are produced in good-to-excellent yields and enantioselectivities. Furthermore, a stereodivergent synthesis of natural product NP25302 is achieved, and a reasonable reaction mechanism is proposed to illustrate the observed stereoselectivity based on the results of control experiments, nonlinear effect investigation, and HRMS detection.

Nickel-catalyzed switchable 1,3-dienylation and enantioselective allenylation of phosphine oxides

The development of general catalytic methods for the regio- and stereoselective construction of phosphoryl derivatives from identical substrates remains a formidable challenge in organic synthesis. Enabled by the newly developed BDPP-type ligands, we disclosed a nickel-catalyzed allenylation of phosphine oxides rationally and predictably, allowing the construction of versatile chiral allenylphosphoryl derivatives with high enantiopurity (up to 94% e.e.). Alternatively, using an achiral phosphine ligand dcypbz under acidic conditions, we achieved a regiochemical switch of the 1,3-dienylation to afford functionalized phosphinoyl 1,3-butadienes (up to 93% yield). The salient features of this method include switchable reactivity, broad substrate scope, readily available feedstock, single-step preparation, and high asymmetric induction.

Historical perspective on ruthenium-catalyzed hydrogen transfer and survey of enantioselective hydrogen auto-transfer processes for the conversion of lower alcohols to higher alcohols

Ruthenium-catalyzed hydrogen auto-transfer reactions for the direct enantioselective conversion of lower alcohols to higher alcohols are surveyed. These processes enable completely atom-efficient carbonyl addition from alcohol proelectrophiles in the absence of premetalated reagents or metallic reductants. Applications in target-oriented synthesis are highlighted, and a brief historical perspective on ruthenium-catalyzed hydrogen transfer processes is given.

Effect of Propargylic Substituents on Enantioselectivity and Reactivity in Ruthenium-Catalyzed Propargylic Substitution Reactions: A DFT Study

We recently proposed a transition-state model for asymmetric propargylic substitution reactions of propargylic alcohols catalyzed by optically active thiolate-bridged diruthenium complexes [Chem. - Asian J.2021, 16, 3760-3766]. In the present study, we further examined the effects of propargylic substituents on both enantioselectivity and reactivity in the propargylic substitution reactions via ωB97X-D-level density functional theory (DFT) calculations. When the propargylic alcohol bears a methyl group at the propargylic position, we obtained results that contrast with the result of our previous study on propargylic alcohols without methyl groups. This result indicates that methyl group substitution at the propargylic position reverses the stereoselectivity. Substitution of a trifluoromethyl group for a methyl group was suggested to result in higher enantioselectivity. The obtained results are consistent with the experimental study on enantioselective propargylic phosphinylation reactions reported by our group.

Quaternary and Tertiary Carbon Centers Synthesized via Gallium-Catalyzed Direct Substitution of Unfunctionalized Propargylic Alcohols with Boronic Acids

(IPr)GaCl₃/AgSbF₆, AgSbF₆, and GaCl₃ catalyzed substitution of the hydroxyl of secondary and tertiary propargylic alcohols with organoboronic acids via C-C bond formation, and GaCl₃ effectively synthesized all-carbon quaternary propargylic centers. These catalysts performed the substitution at carbons bearing alkyl substituents, which has been problematic for other systems. Highly hindered carbon stereocenters were thus produced, including quaternary centers bearing doubly ortho-substituted aryl rings, that are difficult to access with traditional methods.

Enantioselective and Diastereodivergent Allylation of Propargylic C-H Bonds

An iridium-catalyzed stereoselective coupling of allylic ethers and alkynes to generate 3,4-substituted 1,5-enynes is reported. Under optimized conditions, the coupling products are formed with excellent regio-, diastereo-, and enantioselectivities, and the protocol is functional group tolerant. Moreover, we report conditions that allow the reaction to proceed with complete reversal of diastereoselectivity. Mechanistic studies are consistent with an unprecedented dual role for the iridium catalyst, enabling the propargylic deprotonation of the alkyne through π-coordination, as well as the generation of a π-allyl species from the allylic ether starting material.

Catalytic Asymmetric Vinylogous and Bisvinylogous Propargylic Substitution

Distinct regio- and enantioselectivity control in copper-catalyzed vinylogous and bisvinylogous propargylic substitution has been accomplished by using a novel chiral N,N,P ligand. The developed method provides an efficient and selective approach to an array of highly enantioenriched alkynyl unsaturated carbonyl compounds. Salient features include excellent functional group tolerance and broad substrate scope. The synthetic utility of the developed method is further demonstrated by a gram-scale synthesis and by application to a range of transformations including enantioselective synthesis of unique challenging compounds.

Copper-catalyzed asymmetric propargylic substitution with salicylaldehyde-derived imine esters

Copper-catalyzed asymmetric propargylic substitution with salicylaldehyde-derived imine esters and propargylic carbonates has been successfully realized, generating a wide range of chiral amino acid derivatives containing propargylic groups with excellent results (up to 95% yield and 94% ee). The ortho-hydroxy group of the salicylaldehyde-derived imine esters is crucial to increase the reactivity and stabilize the azomethine ylide, which may be due to the formation of an intramolecular hydrogen bond between the hydroxyl group and the imine group. A series of synthetic transformations were carried out to access other important chiral compounds, which displayed the synthetic versatility.

Intramolecular Copper-Catalyzed Asymmetric Propargylic [4 + 2]- Cycloaddition toward Optically Active Tetrahydroisoindolo[2,1-a]quinoxalines

An intramolecular Cu-catalyzed asymmetric propargylic [4 + 2] cycloaddition of bis-N-nucleophile-functionalized propargylic esters has been realized in the support of a chiral tridentate N-ligand, (S,S)-Pybox-diOAc, leading to chiral tetrahydroisoindolo[2,1-a]quinoxalines in high yields and with good to excellent enantioselectivities. The reaction features high efficiency, simplicity, and broad substrate scope, thus providing a powerful and concise strategy for stereoselective access to optically active polycyclic heterocycle frameworks that are otherwise difficult to synthesize.

A Biocatalytic Platform for the Synthesis of Enantiopure Propargylic Alcohols and Amines

Propargylic alcohols and amines are versatile building blocks in organic synthesis. We demonstrate a straightforward enzymatic cascade to synthesize enantiomerically pure propargylic alcohols and amines from readily available racemic starting materials. In the first step, the peroxygenase from Agrocybe aegerita converted the racemic propargylic alcohols into the corresponding ketones, which then were converted into the enantiomerically pure alcohols using the (R)-selective alcohol dehydrogenase from Lactobacillus kefir or the (S)-selective alcohol dehydrogenase from Thermoanaerobacter brokii. Moreover, an enzymatic Mitsunobu-type conversion of the racemic alcohols into enantiomerically enriched propargylic amines using (R)-selective amine transaminase from Aspergillus terreus or (S)-selective amine transaminase from Chromobacterium violaceum was established. The one-pot two-step cascade reaction yielded a broad range of enantioenriched alcohol and amine products in 70–99% yield.

Doubly stereoconvergent construction of vicinal all-carbon quaternary and tertiary stereocenters by Cu/Mg-catalyzed propargylic substitution

The construction of vicinal, congested stereocenters with high selectivities is of general utility in chemistry. To build two such stereocenters in one step from readily available starting materials is very desirable, but remains challenging. We report here a doubly stereoconvergent, Cu/Mg-catalyzed asymmetric propargylic substitution reaction to convert simple starting materials to products with vicinal tertiary and all-carbon quaternary stereocenters in high yields and excellent diastereo- and enantioselectivities. Both the nucleophiles and the electrophiles employed in this transformation are racemic. This reaction uses earth abundant metal catalysts, operates under ambient conditions, and demonstrates broad substrate scope. The products of this reaction are functional group rich and synthetically versatile. Key to the success of this development is the devise of a Cu/Mg dual catalytic system and the identification of a bulky tridentate pyridinebisimidazoline (PyBim) ligand.

The construction of vicinal, congested stereocenters with high selectivities is of general utility in chemistry. Here the authors report a doubly-stereoconvergent, Cu/Mg-catalyzed asymmetric propargylic substitution reaction to convert simple starting materials to products with vicinal tertiary and all-carbon quaternary stereocenters in high yields and excellent diastereo- and enantioselectivities

Synthesis of Allenyl-Bdan via Cu(I)-Catalyzed Borylation of Propargyl gem-Dichlorides

A copper-catalyzed borylation of propargyl dichlorides was developed under mild reaction conditions. The corresponding chloro-substituted allenyl-Bdan products were obtained in good yields. The utilities of allenyl-Bdan products were examined by their diverse derivatizations.

Stereodivergent propargylic alkylation of enals via cooperative NHC and copper catalysis

Despite that asymmetric stereodivergent synthesis has experienced great success to provide unusual processes for the creation of chirality complexity, concepts appliable to asymmetric stereodivergent catalysis are still limited. The dependence on the unusual capacity of each catalyst to precisely control the reactive site planar in the region poses unparalleled constraints on this field. Here, we first demonstrate that the chiral Cu-allenylidene species can participate in the stereodivergent propargylic alkylation of enals, in concert with chiral N-heterocyclic carbenes (NHCs). Thus, all four stereoisomers were obtained with excellent enantioselectivity and diastereoselectivity (up to >99% e.e. and >95:5 d.r.) from the same starting materials by simply altering chiral Cu-Pybox complex and NHC combinations. The rich chemistry workable in the products enables the structurally diverse synthesis of chiral functional molecules and holds great potential in alkaloid synthesis, as showcased by the preparation of the key building block to access (-)-perophoramidine.

The ability to construct multiple stereocenters in a modular fashion is an important goal of synthetic organic chemistry. Here the authors present a method to construct oxindoles in four stereoisomers with high enantioselectivity and diastereoselectivity from the same starting materials by using cooperative copper- and organocatalysis.