Synthesis of difluoromethylated allenes through trifunctionalization of 1,3-enynes

Regioselective 1,4-/1,3-Difunctionalization of 1,3-Enynes with Selenosulfonates in Water

1,4-/1,3-Regioselective bifunctionalization of 1,3-enynes with selenosulfonates in water under catalyst-free conditions for the construction of sulfonyl allene and 1,3-disulfonyl-conjugated dienes respectively have been developed. The reactions feature mild reaction conditions in aqueous solution and remarkable regioselectivity controlled by substrates.

Photoredox/copper-catalyzed gem-difluoroalkylation-cyanation of 1,3-enynes

A photoredox/copper-catalyzed 1,4-difunctionalization of 1,3-enynes with readily available difluoroalkylating reagents and TMSCN was developed. This reaction proceeded at mild conditions, affording the corresponding difluoroalkylated allenes in good yields with high functional-group tolerance and excellent regioselectivity.

Catalytic (4+2) Annulation via Regio- and Enantioselective Interception of in-situ Generated Alkylgold Intermediate

A regio- and stereoselective stepwise (4+2) annulation of N-propargylamides and α,β-unsaturated imines/ketones has been accomplished with synergetic catalysis by a combination of a gold-complex and a chiral quinine-derived squaramide (QN-SQA), leading to highly functionalized chiral tetrahydropyridines/dihydropyrans in good to high yields with generally excellent enantioselectivity. Mechanistic studies and DFT calculations indicate that the in situ formed alkylgold species is the key intermediate in this transformation, and the amide group served as a traceless directing group in this highly selective transformation. This method complements the enantioselective (4+2) annulation of allene reagents, providing the formal internal C-C π-bond cycloaddition products, which is challenging and remains elusive.

Transition-Metal-Catalyzed 1,2-Diaminations of Olefins: Synthetic Methodologies and Mechanistic Studies

1,2-Diamines are synthetically important motifs in organo-catalysis, natural products, and drug research. Continuous utilization of transition-metal based catalyst in direct 1,2-diamination of olefines, in contrast to metal-free transformations, with numerous impressive advances made in recent years (2015-2023). This review summarized contemporary research on the transition-metal catalyzed/mediated [e. g., Cu(II), Pd(II), Fe(II), Rh(III), Ir(III), and Co(II)] 1,2-diamination (asymmetric and non-asymmetric) especially emphasizing the recent synthetic methodologies and mechanistic understandings. Moreover, up-to-date discussion on (i) paramount role of oxidant and catalyst (ii) key achievements (iii) generality and uniqueness, (iv) synthetic limitations or future challenges, and (v) future opportunities are summarized related to this potential area.

Tunable vicinal, geminal diphosphorylation and C-N bond phosphorylation of enaminones toward divergent phosphorylated ketone derivatives

This paper reports the trifunctionalization reactions of tertiary enaminones in the fashion of selective gem- and vicinal diphosphorylation, leading to the tunable synthesis of α,α- and α,β-diphosphoryl ketones. In addition, the C-N bond phosphorylation with improved substrate tolerance has been achieved.

Conglomerate Crystallization in the Cambridge Structural Database (2020-2021)

Conglomerate crystals are materials capable of undergoing spontaneous resolution and were responsible for the discovery of molecular chirality. Their relevance to modern chemical and crystallographic sciences has been hindered by the difficulty in identifying and searching materials with this characteristic ability to spontaneously bias their own enantioenrichment. With the release of the November 2021 distribution of the Cambridge Structural Database (CSD) (version 5.43), a fresh quantity of chiral conglomerate crystals is expected to have been published in the CSD without identification. Indeed, no crystals in the CSD have been identified as a spontaneously resolving conglomerate crystal in their crystallographic information file since the 2019 release, despite the deposition of over 108,000 new crystal structures into the database over the same time period. A manual inspection of crystals deposited between 2020 and 2021 was conducted to identify 343 new chiral materials which exhibit conglomerate crystallization behavior. It is hoped that the continued manual curation of this list will aid those in the crystallographic and synthetic communities to study and exploit this spontaneous enantioenrichment behavior.

Metal-Free Highly Regioselective 1,4-Sulfonyliodination of 1,3-Enynes

Herein, a novel, practical, and green synthetic method using readily available 1,3-enynes with sulfonyl hydrazides and I₂ through tert-butyl hydroperoxide (TBHP)-mediated 1,4-sulfonyliodination has been developed for synthesizing various tetrasubstituted allenyl iodides under metal-free conditions. Notably, the proposed method exhibits a broad substrate scope, operational simplicity, tolerance to air, high functional-group tolerance, satisfactory yields, and excellent regioselectivity as well as involves the use of cost-effective reagents such as green oxidants.

Divergent Synthesis of Fluorinated Alkenes, Allenes, and Enynes via Reaction of 2-Trifluoromethyl-1,3-enynes with Carbon Nucleophiles

Herein, inorganic base K₃PO₄ promoted divergent synthesis of CF₃-substituted allenes, cyclopentenes, alkynes, and fluorinated enynes via regioselective nucleophilic addition of carbon nucleophiles to 2-trifluoromethyl-1,3-enynes was developed. With the choice of different carbon nucleophiles, various fluorinated compounds could be obtained under K₃PO₄/DMF reaction system. When malononitriles were used as nucleophiles, CF₃-substituted allenes, cyclopentenes, and alkynes could be obtained, respectively. By using 1,3-dicarbonyl compounds as nucleophiles, ring-monofluorinated 4H-pyrans could be prepared, and 1,1-difluoro-1,3-enynes could be furnished with the participation of diethyl malonate. Moreover, these five kinds of fluorinated allenes, alkenes, and enynes are valuable building blocks.

Synthesis of tetrasubstituted allenes via a 1,4-palladium migration/carbene insertion/β-H elimination sequence

A palladium-catalyzed synthesis of tetrasubstituted allenes from aryl bromides and aryl diazoacetates is developed. This transformation proceeded via an aryl to alkenyl 1,4-palladium migration/carbene insertion/β-H elimination sequence under mild reaction conditions.

Radical 1,2,3-tricarbofunctionalization of α-vinyl-β-ketoesters enabled by a carbon shift from an all-carbon quaternary center

Herein, we report an intermolecular, radical 1,2,3-tricarbofunctionalization of α-vinyl-β-ketoesters to achieve the goal of building molecular complexity via the one-pot multifunctionalization of alkenes. This reaction allows the expansion of the carbon ring by a carbon shift from an all-carbon quaternary center, and enables further C–C bond formation on the tertiary carbon intermediate with the aim of reconstructing a new all-carbon quaternary center. The good functional group compatibility ensures diverse synthetic transformations of this method. Experimental and theoretical studies reveal that the excellent diastereoselectivity should be attributed to the hydrogen bonding between the substrates and solvent.

Herein, we report an intermolecular, radical 1,2,3-tricarbofunctionalization of α-vinyl-β-ketoesters to achieve the goal of building molecular complexity via the one-pot multifunctionalization of alkenes.

Enantio-, Diastereo- and Regioselective Synthesis of Chiral Cyclic and Acyclic gem-Difluoromethylenes by Palladium-Catalyzed [4+2] Cycloaddition

gem-Difluoromethylene moieties are attractive in medicinal chemistry due to their ability to mimic other more ubiquitous functional groups. Thus, effective asymmetric methods for their construction are highly desirable, especially for the industrial production of chiral drugs. Using a Pd-catalyzed asymmetric [4+2] cycloaddition between substituted-2-alkylidenetrimethylene carbonates and gem-difluoroalkyl ketones, we were able to easily access chiral 1,3-dioxanes that contain a tetrasubstituted difluoroalkyl stereogenic center in cyclic and acyclic skeletons. A novel phosphoramidite ligand, which contains a bulky 1,1-dinaphthylmethanamino moiety, was developed to provide the products in high yield with excellent enantio-, diastereo-, and regioselectivity. Strikingly, the gem-difluoro substitution pattern promotes the reaction, and pentafluoroethylketone, an α,α-difluorinated β-ketoester, and a β-ketosulfone are suitable substrates for this method.

An N-Fluorinated Imide for Practical Catalytic Imidations

Catalytic imidation using NFSI as the nitrogen source has become an emerging tool for oxidative carbon-nitrogen bond formation. However, the less than ideal benzenesulfonimide moiety is incorporated into products, severely detracting its synthetic value. As a solution to this challenge, we report herein the development of a novel N-fluorinated imide, N-fluoro-N-(fluorosulfonyl)carbamate (NFC), by which the attached imide moiety acts as a modular synthetic handle for one-step derivatization to amines, sulfonamides, and sulfamides. Furthermore, this study revealed the superior reactivity of NFC as showcased in a copper-catalyzed imidation of benzene derivatives and imidocyanation of aliphatic alkenes, overcoming the limitation of NFSI-mediated reactions.

Copper-Catalysed Synthesis of Trifluoromethyl Allenes via Fluoro-carboalkynylation of Alkenes

Allenes and trifluoromethyl motifs are considered as important building blocks in materials and pharmaceuticals. A copper-catalysed synthesis of trifluoromethyl allenes utilizing readily available feedstocks under mild and environmentlly friendly conditions...

Radical transformations for allene synthesis

Allenes are valuable organic molecules that feature unique physical and chemical properties. They are not only often found in natural products, but also act as versatile building blocks for the access of complex molecular targets, such as natural products, pharmaceuticals, and functional materials. Therefore, many remarkable and elegant methodologies have been established for the synthesis of allenes. Recently, more and more methods for radical synthesis of allenes have been developed, clearly emphasizing the associated great synthetic values. In this perspective, we will discuss recent important advances in the synthesis of allenes via radical intermediates by categorizing them into different types of substrates as well as distinct catalytic systems. The mechanistic studies and synthetic challenges will be highlighted.

Recent important advances in the synthesis of allenes via radical strategies are highlighted.

Copper-Catalyzed Direct Allenylation of Inactive Cyclic Ethers

We report here a direct allenylation reaction of inactive cyclic ethers. The reaction proceeds through a copper-catalyzed 1,4-difunctionalization of 1,3-enynes, with cyano group installed at the allenes simultaneously. This methodology shows a broad functional group compatibility to 1,3-enynes. Diversified allene-modified cyclic ether derivatives were synthesized with high regioselectivity under mild conditions.

N-Heterocyclic Carbene-Catalyzed 1,4-Alkylacylation of 1,3-Enynes

The radical relay coupling reaction recently emerged as a powerful synthetic strategy for producing tetrasubstituted allenes. However, bond-forming processes involving the allenyl radical intermediate are mostly limited to those promoted by transition metals. In this report, we describe that a ketyl radical generated from single-electron oxidation of the Breslow intermediate is an excellent coupling partner of allenyl radicals. An organocatalytic 1,4-alkylacylation of 1,3-enynes occurred smoothly in the presence of an aldehyde, a radical precursor, and an N-heterocyclic carbene catalyst. This transformation showed remarkable tolerance to both aromatic and aliphatic aldehydes, enyne substitution, and diversified radical precursors.

Dual Photoredox/Nickel-Catalyzed 1,4-Sulfonylarylation of 1,3-Enynes with Sulfinate Salts and Aryl Halides: Entry into Tetrasubstituted Allenes

A radical-mediated three-component 1,4-sulfonylarylation of 1,3-enynes with aryl iodides and sulfinate salts using cooperative photoredox/nickel catalysis is described. This protocol enables the synthesis of tetrasubstituted sulfonyl-containing allenes under redox-neutral conditions and provides a versatile 1,3-enyne 1,4-difunctionalization platform for the synthesis of a diverse range of tetrasubstituted allenes with high chemo- and regioselectivities, excellent functional group tolerance, and a broad substrate scope.

HFIP-Catalyzed Difluoroalkylation of Propargylic Alcohols to Access Tetrasubstituted Difluoroalkyl Allenes

A hexafluoroisopropanol (HFIP)-catalyzed difluoroalkylation of propargylic alcohols with difluoroenoxysilanes to access structurally diverse tetrasubstituted difluoroalkyl allenes has been developed. This convenient procedure enables the rapid construction of highly functionalized multisubstituted fluorinated allenes in a mild and straightforward way. Furthermore, the synthetic potential of this methodology has been demonstrated by the facile synthesis of various structurally interesting fluorine-containing molecules such as gem-difluorosubstituted dihydropyran, tetrasubstituted CF₂H-allene, and multisubstituted fluorinated cyclopentanone derivatives.

Zinc Iodide Catalyzed Synthesis of Trisubstituted Allenes from Terminal Alkynes and Ketones

A straightforward, user-friendly, efficient protocol for the one pot, ZnI₂-catalyzed allenylation of terminal alkynes with pyrrolidine and ketones, toward trisubstituted allenes, is described. Trisubstituted allenes can be obtained under either conventional heating or microwave irradiation conditions, which significantly reduces the reaction time. A sustainable, widely available, and low-cost metal salt catalyst is employed, and the reactions are carried out under solvent-free conditions. Among others, synthetically valuable allenes bearing functionalities such as amide, hydroxyl, or phthalimide can be efficiently prepared. Mechanistic experiments, including kinetic isotope effect measurements and density functional theory (DFT) calculations, suggest a rate-determining [1,5]-hydride transfer during the transformation of the intermediate propargylamine to the final allene.

Direct synthesis of pentasubstituted pyrroles and hexasubstituted pyrrolines from propargyl sulfonylamides and allenamides

Multisubstituted pyrroles are important fragments that appear in many bioactive small molecule scaffolds. Efficient synthesis of multisubstituted pyrroles with different substituents from easily accessible starting materials is challenging. Herein, we describe a metal-free method for the preparation of pentasubstituted pyrroles and hexasubstituted pyrrolines with different substituents and a free amino group by a base-promoted cascade addition-cyclization of propargylamides or allenamides with trimethylsilyl cyanide. This method would complement previous methods and support expansion of the toolbox for the synthesis of valuable, but previously inaccessible, highly substituted pyrroles and pyrrolines. Mechanistic studies to elucidate the reaction pathway have been conducted.

Cobalt-Catalyzed Divergent Aminofluorination and Diamination of Styrenes with N-Fluorosulfonamides

A cobalt-catalyzed aminofluorination reaction of styrenes with N-fluorosulfonamides serving as both the amination and fluorination agents has been developed. The switch of selectivity in this catalytic reaction from aminofluorination to diamination could be easily achieved by the addition of 1.0 equiv of PPh₃. Both transformations tolerated a wide array of substrates under mild reaction conditions.

Visible-Light-Induced 1,4-Hydroxysulfonylation of Vinyl Enynes with Sulfonyl Chlorides: The Bridge of Chloride Linking Water and Enynes

A novel visible-light-induced 1,4-hydroxysulfonylation of vinyl enynes with sulfonyl chlorides has been established, providing a highly efficient protocol to access multisubstituted sulfonyl allenic alcohols. Control experiments and mechanistic studies disclose that the target products result from sequential reactions of hydroxyl and tosyl radicals, among which chloride anion plays a key role to generate the requisite ^•OH, thus bridging water and enynes. Moreover, the vinyl pendant is believed to decisively affect the site-selectivity of hydroxyl radical.

Regioselective Three-Component Synthesis of Vicinal Diamines via 1,2-Diamination of Styrenes

The vicinal diamine motif plays a significant role in natural products, drug design, and organic synthesis, and development of synthetic methods for the synthesis of diamines is a long-standing interest. Herein, we report a regioselective intermolecular three-component vicinal diamination of styrenes with acetonitrile and azodicarboxylates. The diamination products can be produced in moderate to excellent yields via the Ritter reaction. Synthetic applications and theoretical studies of this reaction have been conducted.

Synthesis of Aminoallenes via Selenium-π-Acid-Catalyzed Cross-Coupling of N-Fluorinated Sulfonimides with Simple Alkynes

The facile synthesis of aminoallenes, accomplished by a selenium-π-acid-catalyzed cross-coupling of an N-fluorinated sulfonimide with simple, non-activated alkynes, is reported. Until now, aminoallenes were difficult to be accessed by customary means, inasmuch as pre-activated and, in part, intricate starting materials were necessary for their synthesis. In sharp contrast, the current study shows that ordinary internal alkynes can serve as simple and readily available precursors for the construction of the aminoallene motif. The operating reaction conditions tolerate numerous functional groups such as esters, nitriles, (silyl)ethers, acetals, and halogen substituents, furnishing the target compounds in up to 86 % yield.

Palladium-catalyzed allene synthesis enabled by β-hydrogen elimination from sp2-carbon

The rational design based on a deep understanding of the present reaction mechanism is an important, viable approach to discover new organic transformations. β-Hydrogen elimination from palladium complexes is a fundamental reaction in palladium catalysis. Normally, the eliminated β-hydrogen has to be attached to a sp³-carbon. We envision that the hydrogen elimination from sp²-carbon is possible by using thoroughly designed reaction systems, which may offer a new strategy for the preparation of allenes. Here, we describe a palladium-catalyzed cross-coupling of 2,2-diarylvinyl bromides and diazo compounds, where a β-vinylic hydrogen elimination from allylic palladium intermediate is proposed to be the key step. Both aryl diazo carbonyl compounds and N-tosylhydrazones are competent carbene precursors in this reaction. The reaction mechanism is explored by control experiments, KIE studies and DFT calculations.

β-Hydrogen elimination is a fundamental reaction in palladium catalysis, however, the eliminated β-hydrogen is usually attached to a sp³-carbon. Here, the authors report a palladium-catalyzed cross-coupling reaction involving a β-vinylic hydrogen elimination from an allylic palladium intermediate.

Metal-free alkynylsulfonylation of vinylarenes

A metal-free two-component alkynylsulfonylation of vinylarenes with aryl alkynylsulfones to afford various β-sulfonyl alkynes in moderate to excellent yields under mild conditions is developed.

Radical Carbonyl Propargylation by Dual Catalysis

Carbonyl propargylation has been established as a valuable tool in the realm of carbon-carbon bond forming reactions. The 1,3-enyne moiety has been recognized as an alternative pronucleophile in the above transformation through an ionic mechanism. Herein, we report for the first time, the radical carbonyl propargylation through dual chromium/photoredox catalysis. A library of valuable homopropargylic alcohols bearing all-carbon quaternary centers could be obtained by a catalytic radical three-component coupling of 1,3-enynes, aldehydes and suitable radical precursors (41 examples). This redox-neutral multi-component reaction occurs under very mild conditions and shows high functional group tolerance. Remarkably, bench-stable, non-toxic, and inexpensive CrCl₃ could be employed as a chromium source. Preliminary mechanistic investigations suggest a radical-polar crossover mechanism, which offers a complementary and novel approach towards the preparation of valuable synthetic architectures from simple chemicals.

Cu(I)-Catalyzed addition-cycloisomerization difunctionalization reaction of 1,3-enyne-alkylidenecyclopropanes (ACPs)

A copper(i)-catalyzed three-component addition-cycloisomerization difunctionalization reaction of 1,3-enyne-ACPs with Togni I reagent and TMSCN under mild reaction conditions has been developed, affording 3-trifluoroethylcyclopenta[b]naphthalene-4-carbonitrile derivatives. The reaction proceeded through a copper(i)-catalyzed 1,4-addition of conjugated 1,3-enynes via a radical relay process and aromatic cycloisomerization of allene-ACP intermediates.

1,4-Fluoroamination of 1,3-Enynes en Route to Fluorinated Allenes

The regioselective synthesis of fluorinated allenes via a metal-free 1,4-fluoroamination of 1,3-enynes is presented. This method employs commercially available N-fluorobenzenesulfonimide serving as both the nitrogen source and the fluorine source, affording access to various tetrasubstituted allenes in a straightforward and atom-economic pathway. Preliminary mechanistic studies and theoretical studies revealed that this reaction might undergo an intimate ion-pair mechanism.

A radical-mediated 1,3,4-trifunctionalization cascade of 1,3-enynes with sulfinates and tert-butyl nitrite: facile access to sulfonyl isoxazoles

An unprecedented indium-promoted three-component 1,3,4-trifunctionalization cascade of 1,3-enynes with sulfinates and tert-butyl nitrite for producing 5-sulfonylisoxazoles via [3+2] annulation is reported.

Trifunctionalization of alkenes and alkynes

In this review, the development of trifunctionalization methods for alkenes and alkynes, including arynes and allenes, over the last decade is disclosed.