Radical transformations for allene synthesis

Electrochemically Enabled Hydroxyphosphorylation of 1,3-Enynes to Access Phosphinyl-Substituted Propargyl Alcohols

Catalytic difunctionalization with the direct activation of (O)P-H bonds has been recently established as a potentially robust platform to generate valuable organophosphorus compounds. In terms of 1,3-enynes, despite of the various catalytic methods developed for hydrophosphorylation, the radical-mediated hetero-functionalization of two different atoms has been less explored. In this study, we disclosed an electrochemically induced hydroxyphosphorylation of 1,3-enynes for the construction of phosphinyl-substituted propargyl alcohols. The system involves the direct activation of both arylphosphine oxides and oxygen in ambient air with no external metal or additive needed. The use of electrochemistry ensures the regioselective, atom-economic and eco-friendly for the difunctionalization process. This strategy highlights the advantages of mild reaction conditions, readily available starting materials and broad substrate scope, showing its practical synthetic value in organic synthesis.

Copper-Initiated Regiodivergent Chloropentafluorosulfanylation of 1,3-Enynes under Substrate Control

A copper-catalyzed regiodivergent chloropentafluorosulfanylation strategy for 1,3-enynes using SF5Cl has been developed. The regioselectivity is dictated by the structural and substitution patterns of 1,3-enynes, enabling facile access to three classes of SF5-containing products: propargylic chlorides, 1,3-dienes, and allenes. The reaction systems involve radical species, where the transfer of a chlorine atom from SF5Cl to a carbon radical is considered the predominant pathway. Diverse types of SF5- building blocks can be synthesized through simple functional group transformations.

Total Synthesis of Strigolactones via Palladium-Catalyzed Cascade Carbonylative Carbocyclization of Enallenes

Here we report an efficient route for synthesizing strigolactones (SLs) and their derivatives. Our method relies on a palladium-catalyzed oxidative carbonylation/carbocyclization/carbonylation/alkoxylation cascade reaction, which involves the formation of three new C-C bonds and a new C-O bond while cleaving one C(sp3)-H bond in a single step. With our versatile synthetic strategy, both naturally occurring and artificial SLs were prepared.

Visible-Light-Induced Regioselective Radical-Polar Crossover 1,4-Hydrophosphinylation of 1,3-Enynes: Access to Trisubstituted Allenes Bearing a Phosphine Oxide Group

The radical 1,4-functionalizations of 1,3-enynes have emerged as a powerful strategy for the synthesis of multisubstituted allenes. However, the phosphorus-centered radical-initiated transformations remain largely elusive. Herein, visible-light photoredox catalytic regioselective radical hydrophosphinylation of 1,3-enynes with diaryl phosphine oxides as phosphinoyl radical precursors has been realized. This protocol features mild conditions, a wide substrate scope, and good functional group tolerance, producing a diverse range of phosphinoyl-substituted allenes in moderate to good yields with high atom economy. Detailed mechanistic experiments revealed a radical-polar crossover process in the reaction.

Radical Acylfluoroalkylation of 1,3-Enynes via N-Heterocyclic Carbene/Photoredox Cooperative Catalysis

We report a novel three-component radical acylfluoroalkylation of 1,3-enynes by synergistic N-heterocyclic carbene (NHC)/photoredox catalysis toward various fluorinated allenic aryl ketones. This protocol features a broad substrate scope and excellent functional group tolerability, with examples of late-stage modification of drug molecules and natural products. Notably, seven different fluoroalkyl motifs can be introduced to 1,3-enynes, further demonstrating the robustness and generality of this method. The generation of the fluoroalkyl radical from each sulfinate reagent was individually supported by EPR experiments.

Photoredox-Catalyzed 1,4-Peroxidation-Sulfonylation of Enynones: A Three-Component Radical Coupling Approach for the Synthesis of Highly Functionalized Allenes

An Eosin Y-catalyzed visible light-promoted 1,4-peroxidation-sulfonylation of enynones was achieved to give tetrasubstituted allenes. The photoredox catalysis of Eosin Y allowed the concomitant formation of peroxy and sulfonyl radicals, where the preferential peroxy radical addition to the alkene moiety of enynones resulted in the subsequent α-keto radical-sulfonyl radical cross couplings. The developed photoredox catalysis of Eosin Y demonstrates a regioselective 1,4-diradical addition strategy, opening up a new possibility of diradical functionalization of conjugate systems.

Challenging Task of Ni-Catalyzed Highly Regio-/Enantioselective Semihydrogenation of Racemic Tetrasubstituted Allenes via a Kinetic Resolution Process

The first earth-abundant transition metal Ni-catalyzed highly regio- and enantioselective semihydrogenation of racemic tetrasubstituted allenes via a kinetic resolution process as a challenging task was well established. This protocol furnishes expedient access to a diversity of structurally important enantioenriched tetrasubstituted allenes and chiral allylic molecules with high regio-, enantio-, and Z/E-selectivity. Remarkably, this semihydrogenation proceeded with one carbon-carbon double bond of allenes, which was regioselective complementary to the Rh-catalyzed asymmetric version. Deuterium labeling experiments and density functional theory (DFT) calculations were carried out to reveal the reasonable reaction mechanism and explain the regio-/stereoselectivity.

Pd-Catalyzed Enantioselective Creation of All-Carbon Quaternary Center with 2,3-Allenylic Carbonates

Enantioselective construction of all-carbon quaternary centers has been achieved via the palladium-catalyzed highly enantioselective allenylation of oxindoles with 2,3-allenylic carbonates to afford a variety of optically active allene products, which contain oxindole units with different functional groups, in high ee. The corresponding synthetic applications have also been demonstrated.

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.

Nickel-Catalyzed Reductive Cross-Coupling of Propargylic Acetates with Chloro(vinyl)silanes: Access to Silylallenes

Because of their various reactivities, propargyl acetates are refined chemical intermediates that are extensively applied in pharmaceutical synthesis. Currently, reactions between propargyl acetates and chlorosilanes may be the most effective method for synthesizing silylallenes. Nevertheless, owing to the adaptability and selectivity of substrates, transition metal catalysis is difficult to achieve. Herein, nickel-catalyzed reductive cross-coupling reactions between propargyl acetates and substituted vinyl chlorosilanes for the synthesis of tetrasubstituted silylallenes are described. Therein, metallic zinc is a crucial reductant that effectively enables two electrophilic reagents to selectively construct C(sp2)-Si bonds. Additionally, a Ni-catalyzed reductive mechanism involving a radical process is proposed on the basis of deuteration-labeled experiments.

Copper-Catalyzed Radical Allene C(sp2 )-H Cyanation

While the hydrogen atom abstraction (HAA) from C(sp3 )-H bond has been well explored, the radical-mediated chemo- and regio-selective functionalization of allenic C(sp2 )-H bond via direct HAA from C(sp2 )-H bond of allene remains an unsolved challenge in synthetic chemistry. This is primarily due to inherent challenges with addition of radical intermediates to allenes, regioselectivity of HAA process, instability of allenyl radical toward propargyl radical et al. Herein, we report a copper catalyzed allenic C(sp2 )-H cyanation of an array of tri- and di-substituted allenes with exceptional site-selectivity, while mono-substituted allene was successfully cyanated, albeit with a low yield. In the developed strategy, steric N-fluoro-N-alkylsulfonamide, serving as precursor of hydrogen atom abstractor, plays a crucial role in achieving the desired regioselectivity and avoiding addition of N-centered radical to allene.

Highly Selective 1,4-Diacylation/Cycloisomerization of 1,3-Enynes: De Novo Synthetic Strategy to Polysubstituted Furans

The development of a de novo synthetic strategy for rapid assembly of biologically relevant multisubstituted furans is an appealing but challenging task. Herein, we disclose NHC and organophotocatalysis cocatalyzed three-component radical 1,4-diacylation/cycloisomerization cascade process of readily available 1,3-enynes, which provides an efficient and straightforward entry to a wide range of polysubstituted furans with good yields and excellent regio- and chemoselectivities. The reaction features mild conditions, broad substrate scopes, and good functional group compatibilities.

Copper(I)-Catalyzed Dearomatization of Benzofurans with 2-(Chloromethyl)anilines through Radical Addition and Cyclization Cascade

Herein, we described a copper(I)-catalyzed dearomatization of benzofurans with 2-(chloromethyl)anilines to prepare various tetrahydrobenzofuro[3,2-b]quinolines and 2-(quinolin-2-yl)phenols in good to excellent yields through radical addition and an intramolecular cyclization process. Mechanistic studies revealed that 2-(chloromethyl)anilines served as radical precursors. The present method features broad substrate scope, good functional group tolerance, quinoline scaffold diversity, and radical addition dearomatization of benzofurans.

Regioselective Hydro(deutero)silylation of 1,3-Enynes Enabled by Photoredox/Nickel Dual Catalysis

In the presence of visible light irradiation, organophoto/nickel dual catalysts, and the mild base K2HPO4, 1,3-enynes react with silanecarboxylic acids to give the corresponding α-silylallenes with high selectivity. In this uniquely decarboxylative hydrosilylation of 1,3-enynes, a silyl radical process is involved and diverse electron-rich and -poor substrates proceed smoothly in moderate to excellent yields. This transformation is particularly synthetically worthwhile when using MeOD as the solvent, which furnishes new access to α-silyldeuteroallenes.

Electroreductive Carboxylation of Propargylic Acetates with CO2: Access to Tetrasubstituted 2,3-Allenoates

An electroreductive carboxylation of propargylic alcohols with CO2 and then workup with TMSCHN2 to construct tetrasubstituted 2,3-allenoates is developed. This method allows the incorporation of an external ester group into the resulting allene system through electroreduction, carboxylation, and deacetoxylation cascades. Mechanistically, electricity on/off experiments and cyclic voltammetry analysis support the preferential generation of the CO2 radical anion or the 3-aryl propargylic acetate radical anion based on the electron nature of the aryl rings.

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.

Nickel-catalysed asymmetric hydromonofluoromethylation of 1,3-enynes for enantioselective construction of monofluoromethyl-tethered chiral allenes

An unprecedented nickel-catalysed enantioselective hydromonofluoromethylation of 1,3-enynes is developed, allowing the diverse access to monofluoromethyl-tethered axially chiral allenes, including the challenging deuterated monofluoromethyl (CD2F)-tethered ones that are otherwise inaccessible. It represents the first asymmetric 1,4-hydrofunctionalization of 1,3-enynes using low-cost asymmetric nickel catalysis, thus opening a new avenue for the activation of 1,3-enynes in reaction development. The utility is further verified by its broad substrate scope, good functionality tolerance, mild conditions, and diversified product elaborations toward other valuable fluorinated structures. Mechanistic experiments and DFT calculations provide insights into the reaction mechanism and the origin of the enantioselectivity.

Cobalt-Promoted Electroreductive Cross-Coupling of Prop-2-yn-1-yl Acetates with Chloro(vinyl)silanes

An electroreductive cross-coupling of prop-2-yn-1-yl acetates with chloro(vinyl)silanes for producing tetrasubstituted silylallenes is developed. The method enables the formation of a new C─Si bond through the cathodic reduction formation of the silyl radical, radical addition across the C≡C bond, the alkenyl anion intermediate formation, and deacetoxylation and represents a mild, practical route to the synthesis of silylallenes. Mechanistic studies reveal that CoCl2 acts as the mediator to promote the formation of the alkenyl anion intermediate via electron transfer.

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.

Diversified access to di- and trisubstituted allenes via nickel-catalysed reactions of 1,3-enynes with alkyl N-hydroxyphthalimide esters

Herein, a new nickel-catalysed protocol for the preparation of di- and trisubstituted allenes has been successfully developed via the reactions of 1,3-enynes with alkyl N-hydroxyphthalimide esters. The new method based on a reductive radical-polar crossover (RPC) process features broad substrate scope, wide functional group tolerance, and a simple catalyst system. The late-stage allenylation of drugs has also been illustrated.

Access to functionalized alkynylcyclopropanes via reductive radical-polar crossover-based reactions of 1,3-enynes with alkyl radicals

Herein, using a single-electron-transfer reduction-based radical-polar crossover process as a strategy, protocols dealing with the preparation of functionalized alkynylcyclopropanes have been successfully developed via the reactions of 1,3-enynes with alkyl radicals. In addition to redox-neutral photocatalysis, nickel catalysis with zinc as the reductant is also an alternative to enable reactions of 1,3-enynes with redox-active N-hydroxyphthalimide esters. The synthetic application of alkynylcyclopropane has also been demonstrated.

CF2Br2 as a Source for Difluoroolefination of 1,3-Enynes via N-Heterocyclic Carbene Catalysis

Commercially available CF₂Br₂ has been used as a convenient source for the rapid and reliable incorporation of the gem-difluorovinyl motif into an allene framework via an N-heterocyclic carbene catalyzed difluoroolefination of 1,3-enynes. The reaction proceeds through a cascade three-component radical relay/elimination process. This protocol is distinguished by its mild conditions, readily accessible starting materials, wide substrate scope, and ease of late-stage functionalization, thus unlocking an untraditional strategy to construct a new class of functionalized gem-difluorovinyl allenes.

Tunable Construction of Multisubstituted 1,3-Dienes and Allenes via a 1,4-Palladium Migration/Carbene Insertion Cascade

Efficient palladium-catalyzed vinylic C-H alkenylation and allenylation of gem-disubstituted ethylenes with N-tosylhydrazones of aryl alkyl and diaryl ketones were achieved to access trisubstituted 1,3-dienes and tetrasubstituted allenes, respectively. An aryl to vinyl 1,4-palladium migration/carbene insertion/β-hydride elimination sequence proceeded to switch the chemo- and regioselectivities to give structurally diverse products. Use of 2-FC₆H₄OH additive enables enhancement of the reaction efficiency through accelerating the key 1,4-palladium migration process.