Propargylic Amination Enabled the Access to Enantioenriched Acyclic α-Quaternary α-Amino Ketones

Asymmetric copper-catalyzed alkynylallylic monofluoroalkylations with fluorinated malonates

The unprecedented copper-catalyzed asymmetric alkynylallylic monofluoroalkylation reaction is described via the use of 1,3-enynes and fluorinated malonates. A series of 1,4-enynes bearing a monofluoroalkyl unit are achieved in high yields, excellent regio- and enantioselectivity and high E/Z selectivity. The asymmetric propargylic monofluoroalkylation is also developed. The reliability and synthetic value of the work are highlighted by a gram-scale test and a couple of downstream transformations. Preliminary mechanistic studies unveil a negative nonlinear effect for the catalytic process.

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.

Enantioselective α-Amination of Amides by One-Pot Organo-/Iodine Sequential Catalysis

An one-pot organo- and iodine sequential catalysis strategy for reactions of amides with pyrazole-based primary amines was described to synthesize chiral α-amino amides with a quaternary stereocenter. This methodology exhibited strong asymmetric induction, resulting in a typical enantiomeric excess value exceeding 99% and diastereoselectivity up to >99:1 dr. Moreover, the reaction was conducted without the use of any metals or strong bases.

Amide-Directed, Rhodium-Catalyzed Regio- and Enantioselective Hydroacylation of Internal Alkenes with Unfunctionalized Aldehydes

Despite the current progress achieved in asymmetric hydroacylations, highly enantioselective catalytic addition of unfunctionalized aldehydes to internal alkenes remains an unsolved challenge. Here, using a coordination-assisted strategy, we developed a rhodium-catalyzed regio- and enantioselective addition of unfunctionalized aldehydes to internal alkenes such as enamides and β,γ-unsaturated amides. Valuable α-amino ketones and 1,4-dicarbonyl compounds were directly obtained with high enantioselectivity from readily available materials.

Rhodium-catalyzed divergent dehydroxylation/alkenylation of hydroxyisoindolinones with vinylene carbonate

Herein, a novel organic transformation involving rhodium-catalyzed divergent dehydroxylation/alkenylation of hydroxyisoindolinone with vinylene carbonate is reported, and a series of architecturally rigid and widely used spirolactams are obtained with excellent functional group tolerance and high selectivity. Remarkably, the promising vinylene carbonate reagent presents a distinct chemical reactivity as a vinyl-oxygen cyclic synthon and first transfers the C-H bond to spiroheterocycle scaffolds. Moreover, another chemoselectivity, direct dehydrogenative coupling with vinylene carbonate, is also presented. This protocol is compatible with green chemistry and only releases H2O and CO2 as byproducts.

Asymmetric Substitution by Alkynyl Copper Driven Dearomatization and Rearomatization

Catalytic asymmetric transformations by dearomatization have developed into a widely applicable synthetic strategy, but heavily relied on the use of arenes bearing a heteroatom. In this case, the dearomatization is facilitated by the involvement of a p-orbital electron of the heteroatom. Different from the conventional substrate-dependent model, here we demonstrate that the activation by a d-orbital electron of the transition-metal center can serve as a driving force for dearomatization, and is applied to the development of a novel asymmetric alkynyl copper facilitated remote substitution reaction. A newly modified PyBox chiral ligand enables the construction of valuable diarylmethyl and triarylmethyl skeletons in high enantioselectivities. An unexpected tandem process involving sequential remote substitution/cyclization/1,5-H shift leads to the formation of the enantioenriched C-N axis. A gram-scale reaction and various downstream transformations highlight the robustness of this method and the potential transformations of the products. Preliminary mechanistic studies reveal a mononuclear Cu-catalyzed remote substitution process.

Green Chemistry Approach toward the Regioselective Synthesis of α,α-Disubstituted Allylic Amines

Molybdenum-catalyzed allylic substitution reactions are known to provide direct and practical ways to construct new carbon-carbon bonds and privileged compounds. However, due to the lack of reports on carbon-heteroatom bond formation as a common deficiency, these reactions still face a great challenge. Described herein is a robust and convenient molybdenum-catalyzed regioselective allylic amination of tertiary allylic carbonates with an amine as the heteroatom nucleophile. Both aromatic and aliphatic amines react with various tertiary allylic alcohol derivatives to deliver the desired α,α-disubstituted allylic amines in high yield with complete regioselectivity. In addition, ethanol as the green solvent, a recyclable catalyst system through simple centrifugation techniques, and simple handling procedures make the current approach green, economic, and sustainable.

Cu/Ag-Mediated One-Pot Enantioselective Synthesis of Fully Decorated 1,2,3-Triazolo[1,5-a]pyrazines

The synthesis of chiral triazole-fused pyrazine scaffolds from readily available substrates in a step-economical asymmetric catalytic way is highly appealing. We herein report that an efficient Cu/Ag relay catalyzed protocol employing cascade asymmetric propargylic amination, hydroazidation, and [3 + 2] cycloaddition reaction with high efficiency to access the target enantioenriched 1,2,3-triazolo[1,5-a]pyrazine has been accomplished by applying a novel N,N,P-ligand. The one-pot reaction of three components exhibits high functional group tolerance, excellent enantioselectivities, and a broad substrate scope with readily available starting materials.

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.

Copper-Catalyzed Enantioselective C1,N-Dipolar (3+2) Cycloadditions of 2-Aminoallyl Cations with Indoles

2-Aminoallyl cations are versatile 1,3-dipoles that could potentially be used for diverse (3+n) cycloaddition reactions. Despite some preliminary studies, the asymmetric catalytic transformation of these species is still underdeveloped. We herein report a binuclear copper-catalyzed generation of 2-aminoallyl cations from ethynyl methylene cyclic carbamates and their enantioselective (3+2) cycloaddition reaction with indoles to construct chiral pyrroloindolines. This transformation features a novel C1,N-dipolar reactivity for 2-aminoallyl cations.

Catalytic asymmetric synthesis of α-tertiary aminoketones from sulfoxonium ylides bearing two aryl groups

Disclosed herein is an efficient organocatalytic formal N-H insertion reaction of arylamines with α-keto sulfoxonium ylides bearing two aryl groups, delivering a broad range of α-tertiary aminoketones with good to excellent yields and enantioselectivities (up to 90% yield and 94% ee). The utilities of this protocol were also demonstrated by facile preparation of enantioenriched 2-amino-1,2-diarylethanol bearing two different aryl groups, a type of important building block lacking efficient access.

Nickel-Catalyzed Enantioconvergent Carboxylation Enabled by a Chiral 2,2'-Bipyridine Ligand

In contrast to previous approaches to chiral α-aryl carboxylic acids that based on reactions using hazardous gases, pressurized setup and mostly noble metal catalysts, in this work, a nickel-catalyzed general, efficient and highly enantioselective carboxylation reaction of racemic benzylic (pseudo)halides under mild conditions using atmospheric CO₂ has been developed. A unique chiral 2,2'-bipyridine ligand named Me-SBpy featuring compact polycyclic skeleton enabled both high reactivity and stereoselectivity. The utility of this method has been demonstrated by synthesis of various chiral α-aryl carboxylic acids (30 examples, up to 95 % yield and 99 : 1 er), including profen family anti-inflammatory drugs and transformations using the acids as key intermediates. Based on mechanistic experimental results, a plausible catalytic cycle involving Ni-complex/radical equilibrium and Lewis acid-assisted CO₂ activation has been proposed.

De Novo Construction of Chiral Aminoindolines by Cu-Catalyzed Asymmetric Cyclization and Subsequent Discovery of an Unexpected Sulfonyl Migration

Searching for efficient strategies to access structurally novel aminoindolines is of great significance for drug discovery. However, catalytic asymmetric de novo construction of aminoindoline scaffolds with functionality primed for diversification still remains elusive. Here, we report a Cu-catalyzed asymmetric cyclization of ethynyl benzoxazinones with amines, producing chiral 3-aminoindolines in good yield and with high enantioselectivity (up to 97% yield and 98:2 er). Moreover, a radical-mediated sulfonyl migration of these products was unexpectedly found, further affording new chiral 3-aminoindolines bearing alkenyl sulfonyl groups with retained enantiopurity (up to 84% yield and 98:2 er). Bioactivity evaluations indicate that these 3-aminoindolines show notable antitumor activities and chirality is proven to have a significant impact on their antitumor activity.

Enantioselective Construction of Vicinal Quaternary-Tetrasubstituted Carbon Stereocenters by Copper-Catalyzed Decarboxylative Propargylic Substitution

Enantioselective construction of vicinal tetrasubstituted carbon stereocenters is a formidable challenge in organic synthesis. A copper-catalyzed asymmetric decarboxylative propargylic substitution with 3-amino oxindoles as trisubstituted carbon nucleophiles and propargylic cyclic carbonates as tertiary carbon electrophiles was developed. A range of 3-amino-3,3'-disubstituted oxindoles bearing vicinal quaternary-tetrasubstituted carbon stereocenters were obtained in high yields and good to excellent stereoselectivities (up to 98% yield, >20:1 dr, and 98.5:1.5 er).

Diverse reactivity of alkynes in C-H activation reactions

Alkynes occupy a prominent role as a coupling partner in the transition metal-catalysed directed C-H activation reactions. Due to low steric requirements and linear geometry, alkynes can effectively coordinate with metal d-orbitals. This makes alkynes one of the most successful coupling partners in terms of the number of useful transformations. Remarkably, by changing the reaction conditions and transition-metals from 5d to 3d, the pattern of reactivity of alkynes also changes. Due to the varied reactivity of alkynes, such as alkenylation, annulation, alkylation, and alkynylation, they have been extensively used for the synthesis of valuable organic molecules. Despite enormous explorations with alkynes, there are still a lot more possible ways by which they can be made to react with M-C bonds generated through C-H activation. Practically there is no limit for the creative use of this approach. In particular with the development of new high and low valent first-row metal catalysts, there is plenty of scope for this chemistry to evolve as one of the most explored areas of research in the coming years. Therefore, a highlight article about alkynes is both timely and useful for synthetic chemists working in this area. Herein, we have highlighted the diverse reactivity of alkynes with various transition metals (Ir, Rh, Ru, Pd, Mn, Fe, Co, Ni, Cu) and their applications, along with some of our thoughts on future prospects.

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 Decarboxylative Cascade Cyclization of Propargylic Cyclic Carbonates/Carbamates with Pyridinium 1,4-Zwitterionic Thiolates to Fused Polyheterocyclic Structures

A copper-catalyzed decarboxylative cascade cyclization of propargylic cyclic carbonates/carbamates with pyridinium 1,4-zwitterionic thiolates is developed. A range of fused polyheterocyclic compounds are obtained in moderate to good yields with excellent diastereoselectivities. Of particular note is that four new bonds (two C-C, one C-O/N, one C-S) and four new stereocenters could be efficiently embedded into the tetracyclic fused scaffolds in a single step.

Synthesis of Polydiynes via an Unexpected Dimerization/Polymerization Sequence of C3 Propargylic Electrophiles

Here, we describe the unexpected discovery of a Cu-catalyzed condensation polymerization reaction of propargylic electrophiles (CPPE) that transforms simple C3 building blocks into polydiynes of C6 repeating units. This reaction was achieved by a simple system composed of a copper acetylide initiator and an electron-rich phosphine ligand. Alkyne polymers (up to 33.8 kg/mol) were produced in good yields and exclusive regioselectivity with high functional group compatibility. Hydrogenation of the product afforded a new polyolefin-type backbone, while base-mediated isomerization led to a new type of dienyne-based electron-deficient conjugated polymer. Mechanistic studies revealed a new α-α selective Cu-catalyzed dimerization pathway of the C3 unit, followed by in situ organocopper-mediated chain-growth propagation. These insights not only provide an important understanding of the Cu-catalyzed CPPE of C3, C4, and C6 monomers in general but also lead to a significantly improved synthesis of polydiynes from simpler starting materials with handles for the incorporation of an α-end functional group.

Migratory insertion of copper-allenylidene from propargyl ester

The highly efficient copper-catalyzed homo-dimerization and cross-coupling of propargyl esters have been developed. Various 1-en-3,5-diynes, [5]cumulenes and 1,3-diynes were successfully furnished via the copper-allenylidene intermediates with moderate to excellent yields. Migratory insertion is proposed as the key step to achieve the selectivity at the carbene carbon of the copper-allenylidene.

Recent Advances in Decarboxylative Conversions of Cyclic Carbonates and Beyond

In recent years, functionalized cyclic organic carbonates have emerged as valuable building blocks for the construction of interesting and useful molecules upon decarboxylation under transition-metal catalysis. By employing suitable catalytic systems, the development of chemo-, regio-, stereo- and enantioselective methods for the synthesis of useful and interesting compounds has advanced greatly. On the basis of previous research on this topic, this short review highlights the synthetic potential of cyclic carbonates under transition-metal catalysis over the last two years.

1 Introduction

2 Transition-Metal-Catalyzed Decarboxylation of Vinyl Cyclic Carbonates

3 Zwitterionic Enolate Chemistry Based On Transition-Metal Catalysis

4 Decarboxylation of Alkynyl Cyclic Carbonates and Dioxazolones

5 Conclusions and Perspectives

Kinetically Controllable Construction of Nine-Membered Carbocycles via Pd-Catalyzed Decarboxylative Cycloaddition

A kinetically controllable strategy toward the construction of otherwise challenging nine-membered carbocycles is reported. This Pd-catalyzed decarboxylative procedure utilizes vinyl methylene cyclic carbonates as the C5-dipole and allylidenemalononitriles as C4-building blocks. The protocol features user-friendly operations with controllable regioselectivity and generates CO₂ as the sole byproduct. The formation of synthetically valuable and thermodynamically favored seven-membered carbocycles was also investigated.

Photoinduced, Copper-Catalyzed Enantioconvergent Alkylations of Anilines by Racemic Tertiary Electrophiles: Synthesis and Mechanism

Transition-metal catalysis of substitution reactions of alkyl electrophiles by nitrogen nucleophiles is beginning to emerge as a powerful strategy for synthesizing higher-order amines, as well as controlling their stereochemistry. Herein, we report that a readily accessible chiral copper catalyst (commercially available components) can achieve the photoinduced, enantioconvergent coupling of a variety of racemic tertiary alkyl electrophiles with aniline nucleophiles to generate a new C-N bond with good ee at the fully substituted stereocenter of the product; whereas this photoinduced, copper-catalyzed coupling proceeds at -78 °C, in the absence of light and catalyst, virtually no C-N bond formation is observed even upon heating to 80 °C. The mechanism of this new catalytic enantioconvergent substitution process has been interrogated with the aid of a wide array of tools, including the independent synthesis of proposed intermediates and reactivity studies, spectroscopic investigations featuring photophysical and EPR data, and DFT calculations. These studies led to the identification of three copper-based intermediates in the proposed catalytic cycle, including a chiral three-coordinate formally copper(II)-anilido (DFT analysis points to its formulation as a copper(I)-anilidyl radical) complex that serves as a persistent radical that couples with a tertiary organic radical to generate the desired C-N bond with good enantioselectivity.

Chiral Primary Amine Catalyzed Enantioselective Tandem Reactions Based on Heyns Rearrangement: Synthesis of α-Tertiary Amino Ketones

Herein, we disclose a new catalytic asymmetric tandem reaction based on the Heyns rearrangement for the synthesis of chiral α-amino ketones with readily available substrates. The rearrangement is different from the Heyns rearrangement in that the α-amino ketones were obtained without the shift of the carbonyl group. The key to success is using chiral primary amine as a catalyst by mimicking glucosamine-6-phosphate synthase in catalyzing the efficient Heyns rearrangement in organisms.

Investigation of the C-N Bond-Forming Step in a Photoinduced, Copper-Catalyzed Enantioconvergent N-Alkylation: Characterization and Application of a Stabilized Organic Radical as a Mechanistic Probe

Whereas photoinduced, copper-catalyzed couplings of nitrogen nucleophiles with alkyl electrophiles have recently been shown to provide an attractive approach to achieving a variety of enantioselective C-N bond constructions, mechanistic studies of these transformations have lagged the advances in reaction development. Herein we provide mechanistic insight into a previously reported photoinduced, copper-catalyzed enantioconvergent C-N coupling of a carbazole nucleophile with a racemic tertiary α-haloamide electrophile. Building on the isolation of a copper(II) model complex whose EPR parameters serve as a guide, we independently synthesize two key intermediates in the proposed catalytic cycle, a copper(II) metalloradical (L*Cu^II(carb')₂) (L* = a monodentate chiral phosphine ligand; carb' = a carbazolide ligand), as well as a tertiary α-amide organic radical (R·); the generation and characterization of R· was guided by DFT calculations, which suggested that it would be stable to homocoupling. Continuous-wave (CW) and pulse EPR studies, along with corresponding DFT calculations, are among the techniques used to characterize these reactive radicals. We establish that these two radicals do indeed combine to furnish the C-N coupling product in good yield and with significant enantiomeric excess (77% yield, 55% ee), thereby supporting the chemical competence of these proposed intermediates. DFT calculations are consistent with R· initially binding to copper(II) via a dative interaction from the closed-shell carbonyl oxygen atom of the radical, which positions the α-carbon for direct reaction with the copper(II)-bound carbazole N atom, to generate the C-N bond with enantioselectivity, without the formation of an alkylcopper(III) intermediate.

Organic photoredox catalytic amino-heteroarylation of unactivated olefins to access distal amino ketones

Here we describe a metal-free amino-heteroarylation of unactivated olefins via organic photoredox catalysis, providing a concise and efficient approach for the rapid synthesis of various δ (β, ε)-amino ketones under mild conditions. This protocol demonstrates that the new photocatalyst Cz-NI developed by our group has an excellent photoredox catalytic performance. Finally, a series of mechanistic experiments and DFT calculations indicate that this transformation undergoes a photoredox catalytic sequential radical addition/functional group migration process.

Zwitterionic Metal-Enolate or Equivalent: Generation and Capture

Functionalized ketones and their derivatives are very important building blocks in organic synthesis and material chemistry. The development of novel methodology for the chemo-, regio-, diastereo-, stereo- and enantioselective synthesis of functionalized ketones and their derivatives is the continuous endeavor of organic chemists. Herein we highlight the new approach that was recently initiated and developed by our group for the synthesis of (enantioenriched) ketones and related derivatives based on zwitterionic metal-enolate (ZME) chemistry.

Practical asymmetric amine nucleophilic approach for the modular construction of protected α-quaternary amino acids

We report the first amine nucleophilic approach for the modular construction of enantioenriched protected α-quaternary amino acids. The key to success is the use of an alcohol solvent, which makes a rationally designed COOMe-bonded Cu-allenylidene electrophilic intermediate stable enough to couple with amine nucleophiles before its decomposition. The reaction features wide functional group tolerance with high enantioselectivity, typically >90% ee, and is amenable to the modification of commercially available bioactive molecules. The resultant protected α-amino acids could be readily converted into a number of precious enantioenriched amines featuring α-hindered tertiary carbon centers, which are otherwise synthetically quite challenging, including those of α-amino aldehyde, peptides or α-vinyl amino ester with >92% ee in excellent yields. This protocol could be utilized for the synthesis of the protected bioactive α-ethylnorvaline in 3 steps, a significant advancement in comparison to an 11-step sequence reported previously.

We report the first amine nucleophilic approach for the modular construction of enantioenriched protected α-quaternary amino acids.

Access to azonanes via Pd-catalyzed decarboxylative [5 + 4] cycloaddition with exclusive regioselectivity

An efficient approach for the synthesis of azonanes via Pd-catalyzed decarboxylative [5 + 4] cycloaddition has been developed. The reactions feature wide functional group tolerance with exclusive regioselectivity.

Theoretical insight into the origins of chemo- and diastereo-selectivity in the palladium-catalysed (3 + 2) cyclisation of 5-alkenyl thiazolones

The mechanism of the palladium-catalysed (3 + 2) cyclisation of 5-alkenyl thiazolones and VECs has been investigated from a computational perspective, and the origins of unique chemoselectivity and excellent diastereoselectivity have been disclosed.