Copper-catalyzed 1,4-alkylarylation of 1,3-enynes with masked alkyl electrophiles

Photoreductive 1,4-Dicarbofunctionalization of 1,3-Enynes with Organoiodides and Cyanoarenes via Halogen-Atom Transfer

A photoreductive halogen-atom transfer (XAT) strategy for 1,4-dicarbofunctionalization of 1,3-enynes with organoiodides and cyanoarenes is disclosed, enabling access to functionalized allenes in a highly regio-, chemo-, and stereoselective manner. Upon the photoredox catalysis and the activation of Et3N XAT agents, the mild conditions and high functional group tolerance of this protocol enable the formation of two C-C bonds, including a C(sp3)-C(sp3) bond and a C(sp2)-C(sp2) bond, in a single reaction step, and provides a general avenue to polysubstituted allenes and late-stage modification of bioactive compounds.

Copper-Catalyzed sp2 C-H Arylation and Alkynylation of Allenes via Hydrogen Atom Abstraction

Development of methods for the sp2 C-H transformations of allenes has received much attention, and it presents a powerful tool for the synthesis of complicated allene-containing bioactive molecules. With a copper-catalyzed radical relay, sp2 allenic C-H arylation and alkynylation were established herein, using various aryl boronic acids and trimethoxysilyl-substituted alkynes as carbon nucleophiles and using electrophilic N-F reagents as nitrogen-centered radical precursors. These methods featured excellent site selectivity to deliver fully substituted allenes efficiently. Moreover, with silyl-substituted allenes as substrates, a subsequent dual sp2 C-H functionalization process was established as well, which allowed for the divergent synthesis of multifunctionalized allenes, significantly expanding their chemical spaces.

Three-Component Synthesis of Multiple Functionalized Allenes via Copper/Photoredox Dual Catalyzed 1,4-Alkylcyanation of 1,3-Enynes

Efficient access to multiple functionalized allenes via a three component 1,4-alkylcyanation of enynes with cyclic alcohol derivatives in the presence of trimethylsilyl cyanide (TMSCN) under copper/photoredox dual catalysis has been developed. Both easily transformable aldehyde and cyano groups were introduced to tetra-substituted allenes through light-induced C-C bond cleavage of cyclic butanol and pentanol derivatives. The reactions proceeded smoothly under mild conditions with broad functional groups tolerance.

Direct synthesis of conjugated tetraenes from 1,3-enynes with 1,3-dienes

New direct access to conjugated tetraenes has been achieved. A Ru(0)-catalysed reaction of 1,3-enynes with 1,3-dienes gives 1,3,5,7-octatetraene derivatives by formal regioselective insertion of the alkynyl group of 1,3-enynes into the terminal C-H bond in 1,3-dienes. With a silyl substituent on the alkynyl side in 1,3-enynes, the reaction regioselectively proceeds to give the linear cross-dimerisation product having the silyl group at the internal position. Stoichiometric and DFT calculations support the oxidative coupling mechanism for the linear cross-dimerisation. Methyl (2E,4E,6E,8E)-10-hydroxy-2,4,6,8-decatetraenoate, a versatile polyene intermediate, is accessed by this method as a formal synthesis of biologically active compounds.

Palladium-Catalyzed Carbonylative Multicomponent Fluoroalkylation of 1,3-Enynes: Concise Construction of Diverse Cyclic Compounds

Multicomponent reactions, particularly those entailing four or more reagents, have presented a longstanding challenge due to the inherent complexities associated with balancing reactivity, selectivity, and compatibility. In this study, we describe a palladium-catalyzed multi-component fluoroalkylative carbonylation of 1,3-enynes. A series of products featuring three active functional groups-allene, fluoroalkyl, and carboxyl, were efficiently and selectively integrated in a single chemical operation. Furthermore, more intricate fluoroalkyl-substituted pyrimidinones can be constructed by simply altering the 1,3-bisnucleophilic reagent. This approach also provides a valuable strategy for the late-stage modification of naturally occurring molecules and concise construction of diverse cyclic compounds.

Dealkenylative Functionalizations: Conversion of Alkene C(sp3)-C(sp2) Bonds into C(sp3)-X Bonds via Redox-Based Radical Processes

This review highlights the history and recent advances in dealkenylative functionalization. Through this deconstructive strategy, radical functionalizations occur under mild, robust conditions. The reactions described proceed with high efficiency, good stereoselectivity, tolerate many functional groups, and are completed within a matter of minutes. By cleaving the C(sp3)-C(sp2) bond of terpenes and terpenoid-derived precursors, rapid diversification of natural products is possible.

Three-Component Cross-Electrophile 1,4-Alkylarylation of 1,3-Enynes by Merging Nickel and Photoredox Catalysis

A three-component 1,4-alkylarylation of 1,3-enynes with organic halides through the combination of nickel and photoredox catalysis has been established, providing a novel and modular approach for the assembly of tetrasubstituted allenes. This reductive cascade cross-electrophile reaction obviates the need for air-sensitive organometallic reagents and stoichiometric metallic reductants. A diverse range of functional groups are very compatible under mild reaction conditions and give satisfactory yields. Moreover, a reasonable mechanism is presented according to a series of control experiments.

Visible-Light Photoredox Catalyzed Regioselective 1,4-Hydroalkylation of 1,3-Enyne

Described herein is a visible-light photoredox-catalyzed regioselective 1,4-hydroalkylation of 1,3-enynes. Various of di- and tri-substituent allenes were really accessible under the present reaction conditions. The visible-light photoredox activation of the carbon nucleophile to generate its radical species, allowing the addition with un-activated enynes. The synthetic utility for the present protocol was demonstrated by a large-scale reaction, as well as the derivatization of the allene product.

Microwave-promoted radical addition/cyclization of biaryl vinyl ketones with diacyl peroxides in water under metal-free conditions

This communication reports an efficient microwave-promoted radical addition/cyclization reaction of biaryl vinyl ketones with diacyl peroxides in water under metal-free conditions. A series of 10-methyl-10-benzyl(alkyl)phenanthren-9(10H)-ones were obtained in high yields with good functional group tolerance.

Remote Radical 1,3-, 1,4-, 1,5-, 1,6- and 1,7-Difunctionalization Reactions

Radical transformations are powerful in organic synthesis for the construction of molecular scaffolds and introduction of functional groups. In radical difunctionalization reactions, the radicals in the first functionalized intermediates can be relocated through resonance, hydrogen atom or group transfer, and ring opening. The resulting radical intermediates can undertake the following paths for the second functionalization: (1) couple with other radical groups, (2) oxidize to cations and then react with nucleophiles, (3) reduce to anions and then react with electrophiles, (4) couple with metal-complexes. The rearrangements of radicals provide the opportunity for the synthesis of 1,3-, 1,4-, 1,5-, 1,6-, and 1,7-difunctionalization products. Multiple ways to initiate the radical reaction coupling with intermediate radical rearrangements make the radical reactions good for difunctionalization at the remote positions. These reactions offer the advantages of synthetic efficiency, operation simplicity, and product diversity.

When all C-C breaks LO-Ose

Organic peroxides are becoming popular intermediates for novel chemical transformations. The weak O-O bond is readily reduced by transition metals, including iron and copper, to initiate a radical cascade process that breaks C-C bonds. Great potential exists for the rapid generation of complexity, originating from the ability to couple the resulting free radicals with a wide range of partners. First, this review article discusses the history and synthesis of organic peroxides, providing the context necessary to understand this methodology. Then, it highlights 91 examples of recent applications of the radical functionalization of C-C bonds accessed through the transition metal-mediated reduction of organic peroxides. Finally, we provide some comments about safety when working with organic peroxides.

Nickel-Catalyzed Three-Component Tandem Radical Cyclization 1,5-Difunctionalization of 1,3-Enynes and Alkyl Bromide

A nickel-catalyzed three-component tandem radical cyclization reaction of aryl bromides with 1,3-enynes and aryl boric acids to construct γ-lactam-substituted allene derivatives has been described. This protocol provides lactam alkyl radicals through the free radical cyclization process, which can be effectively used to participate in the subsequent multicomponent coupling reaction so that 1,3-enynes could directly convert into corresponding poly-substituted allene compounds. In addition, this efficient method enjoys a broad substrate scope and provides a series of 1,5-difunctionalized allenes in a one-pot reaction.

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.

Copper-Catalyzed anti-Selective Radical 1,2-Alkylarylation of Terminal Alkynes

A copper-catalyzed highly anti-selective radical 1,2-alkylarylation of terminal alkynes with aryl boronic acids and alkyl bromides has been established. The reaction exhibits high compatibility with a wide range of terminal alkynes and diverse aryl boronic acids, thus providing facile access to various stereodefined trisubstituted alkenes in high yield under mild reaction conditions. Preliminary mechanistic investigations support the formation of alkyl radicals and their subsequent addition to alkynes in the reaction.

NHC and visible light-mediated photoredox co-catalyzed 1,4-sulfonylacylation of 1,3-enynes for tetrasubstituted allenyl ketones

The modulation of selectivity of highly reactive carbon radical cross-coupling for the construction of C-C bonds represents a challenging task in organic chemistry. N-Heterocyclic carbene (NHC) catalyzed radical transformations have opened a new avenue for acyl radical cross-coupling chemistry. With this method, highly selective cross-coupling of an acyl radical with an alkyl radical for efficient construction of C-C bonds was successfully realized. However, the cross-coupling reaction of acyl radicals with vinyl radicals has been much less investigated. We herein describe NHC and visible light-mediated photoredox co-catalyzed radical 1,4-sulfonylacylation of 1,3-enynes, providing structurally diversified valuable tetrasubstituted allenyl ketones. Mechanistic studies indicated that ketyl radicals are formed from aroyl fluorides via the oxidative quenching of the photocatalyst excited state, allenyl radicals are generated from chemo-specific sulfonyl radical addition to the 1,3-enynes, and finally, the key allenyl and ketyl radical cross-coupling provides tetrasubstituted allenyl ketones.

Palladium-catalyzed selective C-C bond cleavage and stereoselective alkenylation between cyclopropanol and 1,3-diyne: one-step synthesis of diverse conjugated enynes

The stereoselective synthesis of 1,3-enynes from 1,3-diynes is demonstrated by palladium-catalyzed selective C–C bond cleavage of cyclopropanol. Exclusive formation of mono-alkenylated adducts was achieved by eliminating the possibility of di-functionalization with high stereoselectivity. Indeed, this protocol worked very well with electronically and sterically diverse substrates. Several studies, including deuterium labeling experiments and intermolecular competitive experiments, were carried out to understand the mechanistic details. The atomic-level mechanism followed in the catalytic process was also validated using DFT calculations, and the rate-controlling states in the catalytic cycle were identified. Furthermore, preliminary mechanistic investigations with radical scavengers revealed the non-involvement of the radical pathway in this transformation.

Palladium-catalyzed tandem activation and functionalization of readily accessible cyclopropanols have been demonstrated to access valuable conjugated enynes from 1,3-diynes with high stereo-selectivity.

Copper-Catalyzed Regioselective 1,4-Sulfonylcyanation of 1,3-Enynes with Sulfonyl Chlorides and TMSCN

A novel and practical copper-catalyzed reaction for the 1,4-sulfonylcyanation of 1,3-enynes under mild conditions is described. This protocol provides efficient and straightforward access to a variety of 5-sulfonylpenta-2,3-dienenitrile derivatives with...

Nickel-catalysed SET-reduction-based access to functionalized allenes via 1,4-carbohydrogenation of 1,3-enynes with alkyl bromides

Allene synthesis: Using reductive radical–polar crossover as the strategy, functionalized allenes could be easily accessed via the reactions of 1,3-enynes with alkyl bromides enabled by nickel catalysis.

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...

Tosyl/pyrrolyl-capped 1,3-enynes via t-BuOK-assisted reaction of TosMIC with acylethynylpyrroles: a new feature of this popular reagent

1-Tosyl-4-pyrrolyl-1,3-enynes have been synthesized in 37–68% yields by t-BuOK-assisted chemo selective reaction (reflux in THF, 1 h) of 2-(acylethynyl)pyrroles with tosylmethylisocyanide (TosMIC), a widespread reagent, which here manifests a new functionality.

Photoinduced C–O bond cleavage for copper-catalyzed allenyl radical cyanation

The synthesis of allenyl nitriles was realized through the combination of photoinduced C–O bond cleavage and copper catalysis. Both 1,3-disubstituted and trisubstituted allenyl nitriles were accessed with good yields and high regioselectivity.

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.

1,2-Bis-perfluoroalkylations of alkenes and alkynes with perfluorocarboxylic anhydrides via the formation of perfluoroalkylcopper intermediates

A novel, Cu-mediated protocol toward the 1,2-bis-perfluoroalkyaltion of alkenes/alkynes was developed. The method proceeded with perfluorocarboxylic anhydrides as inexpensive and readily available perfluoroalkyl sources. Diacyl peroxide was generated in situ from the perfluorocarboxylic anhydrides and H₂O₂. The key step in this reaction is the formation of a stable perfluoroalkylcopper intermediate that is achieved with the aid of a bipyridyl ligand. Subsequent reaction of the intermediate with perfluoroalkyl-containing alkyl or vinyl radicals affords the desired products.

Regiodivergent sulfonylarylation of 1,3-enynes via nickel/photoredox dual catalysis

Catalytic difunctionalization of 1,3-enynes represents an efficient and versatile approach to rapidly assemble multifunctional propargylic compounds, allenes and 1,3-dienes. Controlling selectivity in such addition reactions has been a long-standing challenging task due to multiple reactive centers resulting from the conjugated structure of 1,3-enynes. Herein, we present a straightforward method for regiodivergent sulfonylarylation of 1,3-enynes via dual nickel and photoredox catalysis. Hinging on the nature of 1,3-enynes, diverse reaction pathways are feasible: synthesis of α-allenyl sulfones via 1,4-sulfonylarylation, or preparation of (E)-1,3-dienyl sulfones with high chemo-, regio- and stereoselectivity through 3,4-sulfonylarylation. Notably, this is the first example that nickel and photoredox catalysis are merged to achieve efficient and versatile difunctionalization of 1,3-enynes.

Modular access to 1,2-allenyl ketones based on a photoredox-catalysed radical-polar crossover process

Herein, a new protocol dealing with the preparation of 1,2-allenyl ketones has been successfully developed via the reactions of enynes with radicals enabled by dual photoredox/copper catalysis. Based on the results of a deuteration experiment and the competition reaction between cyclopropanation and allenation, the mechanism based on a photoredox-neutral-catalysed radical-polar crossover process has been proposed. Synthetic applications of allenes have also been demonstrated.

Copper-Catalyzed Regioselective 1,4-Selenosulfonylation of 1,3-Enynes to Access Cyanoalkylsulfonylated Allenes

By employing CuOAc as the catalyst, we realize a four-component reaction of 1,3-enynes, diselenides, DABCO·(SO₂)₂, and cycloketone oxime esters, providing facile access to diverse cyanoalkylsulfonylated allenyl selenides in moderate to good yields. This reaction features high functional group tolerance and a broad substrate scope, enabling the regioselective, sequential formation of C-SO₂ and C-Se bonds under mild reaction conditions. Moreover, the utility of this methodology is further illustrated through the late-stage functionalization of drug-based molecules.

Photoredox/nickel dual-catalyzed regioselective alkylation of propargylic carbonates for trisubstituted allenes

Herein, a highly regioselective alkylation of propargylic carbonates for trisubstituted allenes with alkyl 1,4-dihydropyridine derivatives (1,4-DHPs) is developed via a photoredox/nickel dual-catalyzed process, which represents the first direct approach to access alkylated allene products without alkyl organometallic reagents. This method features a broad substrate scope and mild conditions. A hypothetical mechanism with an alkyl radical and an allenyl Ni(III) species is proposed. Benzylation products were also obtained to be the complement building blocks for the potential synthesis of pharmaceuticals.

Decarboxylative 1,4-carbocyanation of 1,3-enynes to access tetra-substituted allenes via copper/photoredox dual catalysis

We disclose herein the first example of merging photoredox catalysis and copper catalysis for radical 1,4-carbocyanations of 1,3-enynes. Alkyl N-hydroxyphthalimide esters are utilized as radical precursors, and the reported mild and redox-neutral protocol has broad substrate scope and remarkable functional group tolerance. This strategy allows for the synthesis of diverse multi-substituted allenes with high chemo- and regio-selectivities, also permitting late stage allenylation of natural products and drug molecules.

Diacyl peroxides: practical reagents as aryl and alkyl radical sources

Diacyl peroxides, which can be easily synthesized from corresponding carboxylic acids, are commonly utilized as radical initiators and one electron oxidants. Under thermal, transition-metal catalysis or irradiation conditions the cleavage of relatively weak O-O bonds would occur followed by CO₂ extrusion to generate the corresponding aryl or alkyl radicals. Thus, diacyl peroxides can be employed as ideal arylating and alkylating reagents in organic synthesis, including C-H/N-H arylation/alkylation, aryl/alkyl radical addition to unsaturated bonds, hetero arylation/alkylation, eliminative arylation/alkylation, perfluoroalkylation etc. Moreover, these arylation/alkylation protocols have been successfully utilized in the synthesis and late-stage functionalization of natural products as well as bioactive molecules. In this review, recent advances on arylation and alkylation using diacyl peroxides as aryl and alkyl radical sources are summarized and discussed.

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.

Deciphering the Chameleonic Chemistry of Allenols: Breaking the Taboo of a Onetime Esoteric Functionality

The allene functionality has participated in one of the most exciting voyages in organic chemistry, from chemical curiosities to a recurring building block in modern organic chemistry. In the last decades, a special kind of allene, namely, allenol, has emerged. Allenols, formed by an allene moiety and a hydroxyl functional group with diverse connectivity, have become common building blocks for the synthesis of a wide range of structures and frequent motif in naturally occurring systems. The synergistic effect of the allene and hydroxyl functional groups enables allenols to be considered as a unique and sole functionality exhibiting a special reactivity. This Review summarizes the most significant contributions to the chemistry of allenols that appeared during the past decade, with emphasis on their synthesis, reactivity, and occurrence in natural products.

Recent advances in reactions using diacyl peroxides as sources of O- and C-functional groups

This review summarizes recent advances in reactions utilizing diacyl peroxides as O- and C-sources, with examples illustrating how the reactivity of diacyl peroxides in organic reactions can be controlled.

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.

Copper-catalyzed functionalization of enynes

The copper-catalyzed functionalization of enyne derivatives has recently emerged as a powerful approach in contemporary synthesis. Enynes are versatile and readily accessible substrates that can undergo a variety of reactions to yield densely functionalized, enantioenriched products. In this perspective, we review copper-catalyzed transformations of enynes, such as boro- and hydrofunctionalizations, copper-mediated radical difunctionalizations, and cyclizations. Particular attention is given to the regiodivergent functionalization of 1,3-enynes, and the current mechanistic understanding of such processes.

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.

Palladium-Catalyzed Fluoroarylation of gem-Difluoroenynes to Access Trisubstituted Trifluoromethyl Allenes

Various new transformations of gem-difluoroalkenes leading to trifluoromethyl substituted compounds have been well established in the past years. However, the development of new transformations of gem-difluoroenynes lags much behind. Herein is reported the fluoroarylation of 1,1-difluoro-1,3-enynes with aryl halides in the presence of silver fluoride affording trisubstituted trifluoromethyl allenes under the catalysis of palladium. The reaction features mild conditions, high functional-group tolerance, and high regioselectivity.

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.