Elucidating the Mechanism of Electrooxidative Allene Dioxygenation: Dual Role of Tetramethylpiperidine N-Oxyl (TEMPO)

The cumulated π system of a nonsymmetric allene contains three distinct unsaturated carbons that imbue it with unique reactivity toward radicals as compared to its alkene and alkyne counterparts. Despite the synthetic potential of these versatile building blocks, electrochemical transformations of allenes have been historically underexplored. Myriad strategies for easy access to allenes, coupled with the resurgence of interest in sustainable oxidative transformations of hydrocarbons, prompted our efforts to conduct an in-depth investigation of a rare example of an electrochemical TEMPO-mediated allene dioxygenation. The resultant vinyl-TEMPO motif is readily postfunctionalized to install a heteroatom at each allene carbon. Mechanistic investigations, including cyclic voltammetry (CV) studies, computations, and monitoring by operando NMR (ReactNMR) were performed to lay the groundwork for future electrochemical allene functionalizations that deliver unique synthetic building blocks.

TCCA/RSeSeR-Mediated Selenoalkoxy of Allenamides via a Radical Process: Synthesis of Selanyl-allylic N,O-Aminals

An efficient TCCA (trichloroisocyanuric acid)/RSeSeR-mediated selenoalkoxy of allenamides for the construction of selanyl-allylic N,OA-aminal derivatives was developed. The reaction exhibits good functional group tolerance and high efficiency, affording the products in good to excellent yields. Mechanistic investigations indicated that a selanyl-allylic radical intermediate was first formed via the RSe radical added to the central carbon of allenamides, which subsequently furnished highly stable selanyl-allylic carbocation intermediate by abstraction of an electron by the chlorine radical. Moreover, this is the first report of using selenium reagent (RSeCl) to activate allenamides via a radical process.

Photoinduced Hydrosilylation of Fluorinated Alkenes

A visible-light-induced synthesis protocol for silylmonofluoroalkanes is described. The silylation of alkenyl fluorides using (trimethylsilyl)silanes as organosilicon reagents proceeds well under mild conditions via a sequential photoinduced single-electron transfer and protonation process. The protocol shows a broad substrate scope, transition-metal-free conditions, and high functional group tolerance. A wide variety of silylmonofluoroalkanes were obtained in generally good yields (up to 82%).

Photocatalyst-free, visible-light-induced regio- and stereoselective synthesis of phosphorylated enamines from N-allenamides via [1,3]-sulfonyl shift at room temperature

Herein, we report the first visible-light-induced strategy for the rapid synthesis of densely functionalized α- and γ-phosphorylated β-sulfonyl enamines in a regio- and stereoselective manner from N-sulfonyl allenamides and H-phosphine oxides. The transformation displays a broad substrate scope, while operating at room temperature under photocatalyst- and additive-free conditions. In this atom-economical process, either terminal or substituted N-sulfonyl allenamides trigger an unprecedented N-to-C [1,3]-sulfonyl shift, relying on a dual radical allyl resonance and α-heteroatom effect in its triplet excited state. A plausible reaction mechanism is proposed which was supported by the outcomes of theoretical approaches based on Density Functional Theory (DFT) calculations.

Cobalt-Catalyzed Allylic Alkylation at sp3-Carbon Centers

The rising demand and financial costs of noble transition metal catalysts have emphasized the need for sustainable catalytic approaches. Over the past few years, base-metal catalysts have emerged as ideal candidates to replace their noble-metal counterparts because of their abundance and easiness of handling. Despite the significant advancements achieved with precious transition metals, earth-abundant cobalt catalysts have emerged as efficient alternatives for allylic substitution reactions. In this review, allylic alkylations at sp3-carbon centers mediated by cobalt will be discussed, with a special focus on the mechanistic features, scope, and limitations.

Chemical fixation of CO2/CS2 to access iodoallenyl oxazolidinones and allenyl thiazolidine-thiones

Constructing heterocyclic compounds by chemical fixation of CO2/CS2 as a C1 building block is a promising approach. An efficient and environmentally friendly synthetic approach has been developed using CO2/CS2 to prepare complicated allenyl heterocycles with high yields and diastereoselectivities in a metal-free manner under mild conditions. NIS promoted CO2 fixation and the cyclization reaction by exclusive 1,4-syn-addition of 1,3-enynes rather than 1,2-addition or 3,4-addition, while CS2 participated in unique 1,4-syn-hydrothiolation of 1,3-enynes to afford allenyl heterocycles with different reaction patterns.

Dual Nickel/Photoredox-Catalyzed Arylsulfonylation of Allenes

The nickel/photoredox dual catalysis system is an efficient conversion platform for the difunctionalization of unsaturated hydrocarbons. Herein, we disclose the first dual nickel/photoredox-catalyzed intramolecular 1,2-arylsulfonylation of allenes, which can accurately construct a C(sp2)-C(sp2) bond and a C(sp3)-S bond. The reaction exhibits excellent chemoselectivity and regioselectivity, allowing modular conformations of a diverse series of 3-sulfonylmethylbenzofuran derivatives. Control experiments showed that the bipyridine ligand is crucial for the formation of a stable σ-alkyl nickel intermediate, providing the possibility for sulfonyl radical insertion. Meanwhile, the electrophilic sulfonyl radical facilitates further oxidative addition of the σ-alkyl nickel intermediate and inhibits addition with allenes. In addition, control experiments, cyclic voltammetry tests, Stern-Volmer experiments, and density functional theory calculations afford evidence for the Ni(0)/Ni(I)/Ni(II)/Ni(III) pathway in this 1,2-arylsulfonylation.

Visible-Light-Mediated Three-Component Strategy for the Synthesis of Isoxazolines and Isoxazoles

Isoxazolines and isoxazoles commonly serve as core structures of many therapeutic agents and natural products. However, the metal-free and catalysis-free strategy for the synthesis of these privileged motifs at room temperature remains a challenging task. Herein, we report a three-component strategy to afford diverse isoxazolines and isoxazoles via [3 + 2] cycloadditions of in situ-formed nitronates and olefins/alkynes under visible-light irradiation.

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.

Visible-Light-Mediated Dual Functionalization of Allenes: Regio- and Stereoselective Synthesis of Vinylsulfone Azides

A gentle and effective method for the photocatalytic dual functionalization of allenes with high regio- and stereoselectivity using a nonmetallic catalyst is described. Inexpensive and easily available sulfinates and TMSN3 were employed as sulfone and azido sources, respectively. The method is characterized by satisfactory substrate compatibility and tolerance toward functional groups. The straightforward initial mechanistic experiments suggested that the reaction could follow a radical pathway. The synthesis of vinylsulfone azide derivatives presented here offers a promising scaffold for the future development of vinyl sulfone-based drugs and functional bioorthogonal reagents.

Photoredox-Catalyzed Phosphonocarboxylation of Allenes with Phosphine Oxides and CO2

Phosphonocarboxylation of allenes with diarylphosphine oxides and CO2 via visible-light photoredox catalysis was developed for the first time. This work provided practical and sustainable access to highly valuable but otherwise difficult-to-access linear allylic β-phosphonyl carboxylic acids in moderate yields with exclusive regio- and stereoselectivity. This method was also characterized by step and atom economy and transition-metal free and mild conditions. Preliminary mechanistic studies suggested that allyl-methyl carbanion species are the key intermediates.

Visible-light induced metal-free intramolecular reductive cyclisations of ketones with alkynes and allenes

A visible-light-induced, intramolecular, reductive cyclisation of ketones with an unsaturated hydrocarbon moiety was developed. In contrast to conventional protocols requiring resource precious or hazardous metal sources, this method enables facile access to ketyl radicals under metal-free and mild reaction conditions. By polarity-reversed, ketyl radical hydroalkoxylation of alkynes and allenes, a variety of five-membered (hetero-)cyclic products were generated in good yields with good to excellent stereoselectivities. The embedded homoallylic tertiary alcohol could be transformed into other useful functionalities, highlighting the synthetic utility of this reaction. This efficient and sustainable ketyl-alkyne/allene cross coupling also features broad functional group tolerance and scalability.

Visible Light-Induced Cross-Dehydrocoupling of 3-Cyanocoumarins with Unactivated Aliphatic Aldehydes Enables Access to 4-Acylated Coumarins

We report visible light-induced cross-dehydrocoupling of 3-cyanocoumarins with unactivated aliphatic aldehydes for the direct synthesis of 4-acylated coumarins. Inexpensive and readily available (n-Bu)₄NBr was used as a precursor of the hydrogen atom transfer (HAT) catalyst and [Ir(ppy)₂(dtbbpy)PF₆] as a photocatalyst. A variety of aldehydes including linear, branched, cyclic, and α,β-unsaturated aliphatic aldehydes were suitable for this reaction yielding the desired acylated coumarin derivatives in 41-98% yields. This method provides a mild, atom-economical, and environmentally friendly synthetic route for the direct acylation of 3-cyanocoumarins. This is the first example so far of C-4 aliphatic acylation of coumarins.

Electro-mechanochemical approach towards the chloro sulfoximidations of allenes under solvent-free conditions in a ball mill

An electro-mechanochemical protocol for the synthesis of vinylic sulfoximines has been developed. Utilising mechanochemically strained BaTiO₃ nanoparticles, the catalytic active system is generated in situ by the reduction of copper(II) chloride. Various combinations of electron-donating and -withdrawing groups are tolerated, and the approach leads to products with difunctionalised double bonds in good to excellent yields. Attempts to add a sulfoximidoyl chloride to an alkyne proved difficult. Additions of a sulfonyl iodide to allenes and alkynes proceeded smoothly in the presence of silica gel without the need for activation by a piezoelectric material.

Synthesis of 1,2-diselenides via potassium persulfate-mediated diselenation of allenamides with diselenides

An efficient potassium persulfate-mediated radical addition of allenamides with diselenides was developed to create a workable route to 1,2-diselenide products. The reaction tolerates a wide spectrum of functional groups to deliver the products in good to excellent yields. Mechanistic investigations including a calculation study indicated that the radical cascade proceeds through a vinyl radical intermediate, which is formed via a selenium radical added to the terminal CC double bond of allenamides.

Green and Sustainable Visible Light-Mediated Formation of Amide Bonds: An Emerging Niche in Organic Chemistry

Amide bond is one of the most fascinating functional groups in nature due to its stability, conformational diversity, high bond polarity, and abundance in numerous natural products and drug candidates,...

Copper catalyzed cyano-sulfonylation of allenes via the insertion of sulfur dioxide toward the synthesis of (E)-α-cyanomethyl vinylsulfones

An efficient and practical method for highly regio- and stereo-selective cyanosulfonylation of allenes by inserting sulfur dioxide to synthesize useful (E)-α-cyanomethyl vinylsulfones has been explored.

Blue light enhanced Heck arylation at room temperature applied to allenes

A blue light enhanced synthesis of 2-vinyl pirrolidines and piperidines through a domino Heck arylation–cyclisation applied to allenyl amines is described. Essential is the role of the light in the aryl migration in the carbo-palladation step.

Visible light-mediated radical-cascade addition/cyclization of arylacrylamides with aldehydes to form quaternary oxindoles at room temperature

The visible light-induced oxidative radical cascade coupling of N-arylacrylamides with aldehydes using bromide as the hydrogen atom transfer agent to synthesize functional oxindoles is described.