Advances in Decarboxylative Palladium-Catalyzed Reactions of Propargyl Electrophiles

Controlled chain-growth polymerization via propargyl/allenyl palladium intermediates

In contrast to allyl palladium complexes, propargylic/allenylic palladium species display complex reactivities that limit their implementation in polymer chemistry, especially for chain-growth polymerizations. Here we report an example of controlled chain-growth polymerization via propargyl/allenyl palladium intermediates. Vinylidenecyclopropane 1,1-dicarboxylate (VDCP), a unique allenylic electrophile, selectively reacts via the σ-allenyl palladium complex rather than the more common π-propargyl pathway, thereby unlocking a chain-growth process. Based on this concept, precise synthesis of alkyne-backbone polymers is realized, featuring fast rate, high molecular weight, narrow dispersity, high chemoselectivity, and excellent end-group fidelity. We demonstrate preparation of unsaturated macromolecules with advanced sequences and architectures using this method, including block, gradient, and graft copolymers.

Palladium-Catalyzed Intermolecular 1,3-Dienylation of Propargyl Esters Involving the Insertion of SO2

A palladium-catalyzed three-component 1,3-dienylation of propargylic esters with DABSO and aryl iodides has been developed. This novel reductive cross-coupling reaction produces 2-sufonylated 1,3-dienes as single products in the presence of reductive metal Mn with high regio- and chemoselectivities. Control experiments demonstrated that the transformation may undergo a radical process.

Palladium-Catalyzed Divergent Synthesis from o-Sulfonamido Trifluoroacetophenones and 2-En-4-ynyl Carbonates

Here we present a palladium-catalyzed divergent transformative reaction between o-sulfonamido trifluoroacetophenones and 2-en-4-ynyl carbonates. A spectrum of enantioenriched 2,5-dihydrofuran-fused tetrahydroquinolines bearing either a (Z)- or (E)-exocyclic double bond are constructed with moderate to high enantioselectivity from 5-aryl-substituted carbonates by tuning palladium sources, while analogous 2,3-dihydrofuran-fused products are furnished by using 5-alkyl-substituted ones.

Palladium-catalysed asymmetric cascade transformations of 4-alken-2-ynyl carbonates to construct complex frameworks

As a class of readily available and multifunctional building blocks, the chemistry of 4-alken-2-ynyl carbonates remains to be explored. Presented herein is a palladium-catalysed cascade transformative reaction between 4-alken-2-ynyl carbonates and ortho-functionalised activated alkenes. Achiral 1,1-bisalkyl-4-alken-2-ynyl carbonates undergo highly regioselective propargylic substitution with ortho-hydroxyphenyl-tethered activated alkenes, and an auto-tandem vinylogous addition, unusual central-carbon Tsuji-Trost alkylation, protonation and β-H elimination process is followed to furnish fused and spirocyclic frameworks with high structural complexity. Even kinetic transformations with racemic 1-monoalkylated 4-alken-2-ynyl carbonates can be accomplished in the assemblies with ortho-aminophenyl-tethered activated alkenes to afford the analogous alkaloid architectures. This palladium-catalysed auto-tandem protocol exhibits excellent chemo-, regio-, stereoselectivity and reaction efficacy, and substantial functionality compatibility is also observed.

Palladium-catalyzed distal γ- and ε-benzylation, allylation and allenylation of enones

We report herein a palladium-catalyzed distal alkylation of silyldienol and silyltrienol ethers of enones through coupling with activated halides to achieve new endo- and exo-alkylated motifs. Additionally, by employing propargyl bromides, synthetically useful linear allenes along with functionalized enones have been synthesized. Low-catalyst loading, and late-stage transformations of pharmaceutically relevant molecules further showcase the importance of the present protocol.

Nickel-Catalyzed Photoredox Allenylation of Alkyl Halides

We report a dual Ni/photoredox-catalyzed cross-coupling method for propargyl carbonates and nonactivated alkyl bromides, facilitating the synthesis of a variety of substituted allenes under mild and practical conditions. Mechanistically, the reaction integrates Ni-catalyzed activation of the propargyl electrophile via SN2' oxidative addition at Ni(I) with silyl radical-induced activation of the alkyl halide through halogen-atom transfer. This methodology provides a gentle approach for introducing allenyl groups into complex halogenated aliphatic molecules, offering further opportunities for derivatization.

Nickel-Catalyzed Asymmetric (3 + 2) Annulations of Propargylic Carbonates and Vinylogous Donors via an Alkenylation Pathway

The transition-metal-catalyzed alkenylation strategy of propargylic alcohol derivatives provides an efficient protocol to access multifunctional products in a double-nucleophilic attack pattern. While limited relevant asymmetric examples have been reported via palladium catalysis, here we first demonstrate that a nonprecious Ni(0)-based chiral complex can efficiently promote the tandem substitution process between propargylic carbonates and N-trifluoroethyl ketimines via consecutive aza-vinylogous activations, finally accomplishing a (3 + 2) annulation reaction to afford products embedding a 4-methylene-3,4-dihydro-2H-pyrrole framework with high regio-, diastereo-, and enantiocontrol. Their assemblies with a few all-carbon-based vinylogous precursors are also successful, and enantioenriched adducts containing a 3-methylenecyclopentene scaffold are furnished effectively. The substitution patterns for both types of substrates are substantial, and an array of synthetic elaborations is conducted to deliver more versatile architectures with high application potential. In addition, density functional theory calculations and control experiments have been conducted to rationalize the catalytic pathways and regio- and enantioselectivity control.

Synthesis of Chiral Endocyclic Allenes and Alkynes via Pd-Catalyzed Asymmetric Higher-Order Dipolar Cycloaddition

A Pd-catalyzed asymmetric higher-order dipolar cycloaddition between allenyl carbonates and azadienes is achieved by exploiting novel alkylidene-π-allyl-Pd dipoles. This research provides a modular platform for the synthesis of challenging chiral endocyclic allenes bearing a medium-sized heterocyclic motif and a centrally chiral stereocenter in good yields with high enantio- and diastereoselectivities (29 examples, up to 97% yield, 97:3 er and >19:1 dr). Experimental and computational studies elucidate the possible reaction mechanism and the observed stereochemical results. Based on the mechanistic understanding, a new π-propargyl-Pd dipole was designed to further extend the success of the higher order dipolar cycloaddition strategy to the synthesis of 10-membered endocyclic alkynes from propargyl carbonates and azadienes (13 examples, up to 98% yield and 94.5:5.5 er).

Palladium-Catalyzed Heteroannulation of Salicylamides with Propargyl Carbonates: Synthesis of 1,4-Benzoxazepin-5-ones

Herein, we report a palladium-catalyzed method to synthesize 1,4-benzoxazepin-5-ones using salicylamides and propargyl carbonates. The heteroannulation provides a wide range of products in good to excellent yields with broad functional group tolerance. In addition, H2O is used as a low-cost, abundant, and safe solvent, which is important in terms of sustainability.

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.

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.

Rh(II)/Pd(0) Dual-Catalyzed Regio-Divergent Three-Component Propargylic Substitution

Regio-divergent propargylic substitution to generate functionally diverse products from identical starting materials remains a formidable challenge, probably due to the unpredictable regiochemical complexity. In practically, the synthesis of α-quaternary propargylic-substituted products is still much less developed, and preprepared nucleophiles are generally applied in this type of reaction with propargylic substrates, which limits the reaction efficiency and diversity of the obtained products. Herein, we disclose unprecedented three-component propargylic substitution of α-diazo esters with amines and propargylic carbonates under dirhodium/palladium dual catalysis. The key to the success of this multicomponent propargylic substitution is to avoid two-component side reactions through a tandem process of dirhodium(II)-catalyzed carbene insertion and palladium-catalyzed regiodivergent propargylic substitution. The judicious selection of a diphosphine (dppf) or monophosphine (tBuBrettphos) as the ligand is crucial for the reaction to generate different products in a switchable way, α-quaternary 1,3-dienyl or propargylated products, with high regio- and chemoselectivities.

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.

Iso-Pentadienyl Carbonate as a Five Carbon Synthon in Manganese(I)-Catalyzed Selective Linear 1,3-Dienylation

Selective linear 1,3-dienylations are essential transformations, and numerous synthetic efforts have been documented. However, a general method enabling access to electron-rich, -poor, and biologically relevant dienyl molecules is in high demand. Hence, we report a straightforward method of manganese(I)-catalyzed C-H dienylation of arenes by using iso-pentadienyl carbonate as a five carbon synthon. This is a highly unprecedented report for selective linear 1,3-dienylation using manganese C-H activation catalysis. Our method facilitates the synthesis of varieties of dienes, including those suitable for normal or inverse electron demand Diels-Alder reactions, dienyl glycoconjugates, and unnatural amino acids. Extensive mechanistic studies, including isolation of C-H activated organo-manganese complex and isotopic analyses, have supported the proposed mechanism of this dienylation. The synthetic applicability of this method eased to deliver a 6/6/5-fused tricyclic nagilactone scaffold.

Copper-catalyzed propargylic C-H functionalization for allene syntheses

Allenenitriles bearing different synthetically versatile functional groups have been prepared smoothly from 5-alkynyl fluorosulfonamides in decent yields with an excellent chemo- and regio-selectivity under redox neutral conditions. The resulting allenenitriles can be readily converted to useful functionalized heterocycles. Based on mechanistic study, it is confirmed that this is the first example of radical-based non-activated propargylic C-H functionalization for allene syntheses.

Photo and copper dual catalysis for allene syntheses from propargylic derivatives via one-electron process

Different from the traditional two-electron oxidative addition-transmetalation-reductive elimination coupling strategy, visible light has been successfully integrated into transition metal-catalyzed coupling reaction of propargylic alcohol derivatives highly selectively forming allenenitriles: specifically speaking, visible light-mediated Cu-catalyzed cyanation of propargylic oxalates has been realized for the general, efficient, and exclusive syntheses of di-, tri, and tetra-substituted allenenitriles bearing various synthetically versatile functional groups. A set of mechanistic studies, including fluorescence quenching experiments, cyclic voltammetric measurements, radical trapping experiments, control experiments with different photocatalyst, and DFT calculation studies have proven that the current reaction proceeds via visible light-induced redox-neutral reductive quenching radical mechanism, which is a completely different approach as compared to the traditional transition metal-catalyzed two-electron oxidative addition processes.

AgNTf2-Catalyzed Regioselective C-H Alkenylation of N,N-Dialkylanilines with Ynamides

Regioselective C-H alkenylation of N,N-dialkylanilines with ynamides was developed using AgNTf₂ as a catalyst. This approach represents a facile hydroarylation of ynamides, allowing for the introduction of an alkenyl group exclusively at the para position of aniline derivatives. As a result, a series of 4-alkenyl N,N-dialkylanilines were synthesized with excellent regioselectivities.

Mechanistic Studies of the Pd- and Pt-Catalyzed Selective Cyclization of Propargyl/Allenyl Complexes

Following the discovery of an unusual transition-metal-catalyzed reaction, the elucidation of the underlying mechanism is essential to understand the characteristic reactivity of the metal. We previously reported a synthetic method for tricyclic indoles using Pt-catalyzed Friedel-Crafts-type C-H coupling. In this reaction, the Pt catalyst selectively formed a seven-membered ring, but the Pd catalyst only afforded a six-membered ring. However, the reasons for the different selectivities caused by Pd and Pt were unclear. We performed density functional theory (DFT) calculations and experimental studies to reveal the origin of the different behaviors of the two metals. The calculations revealed that the formation of the six- and seven-membered rings proceeds via η¹-allenyl and η³-propargyl/allenyl complexes, respectively. A molecular orbital analysis of the η³-propargyl/allenyl complex revealed that, for the platinum complex, the energy required to convert the unoccupied molecular orbital on the reactive carbon into the lowest unoccupied molecular orbital (LUMO) was lower than that for the palladium complex. In addition, DFT calculations revealed that the combination of platinum and bis[2-(diphenylphosphino)phenyl] ether (DPEphos) reduced the activation energy of the seven-membered cyclization in comparison with palladium or PPh₃. Additional experimental studies, including NMR studies and stoichiometric reactions, support the aforementioned examination.

Umpolung coupling of pyridine-2-carboxaldehydes and propargylic carbonates via N-heterocyclic carbene/palladium synergetic catalysis

The umpolung cross-coupling reaction of pyridine-2-carboxaldehydes and propargylic carbonates has been developed for the first time through N-heterocyclic carbene/palladium cooperative catalysis with the judicious selection of the palladium catalyst, ligand and N-heterocyclic carbene, giving the propargylic ketones regioselectively.