Palladium-Catalyzed Regioselective Arylalkenylation of Ynamides

Intermediate Control: Unlocking Hitherto Unknown Reactivity and Selectivity in N-Conjugated Allenes and Alkynes

ConspectusControlling selectivity through manipulation of reaction intermediates remains one of the most enduring challenges in organic chemistry, providing novel solutions for selective C-C π-bond functionalization. This approach, guided by activation principles, provides an effective method for selective functional group installation, enabling direct synthesis of organic molecules that are inaccessible through conventional pathways. In particular, the selective functionalization of N-conjugated allenes and alkynes has emerged as a promising research focus, driven by advances in controlling reactive intermediates and activation strategies. In this regard, our group, alongside others, has established some new approaches that have emerged as a suitable platform for the synthesis of functionalized enamides. This Account reviews recent developments in the field, highlighting new modes of reactivity and selectivity, atom-economical functionalizations, and strategies for regio- and stereocontrol, while providing mechanistic insights into related transformations.Our study is systematically organized into two sections based on substrate type and chronological research progression. In the first section, we establish a platform by controlling allenamide-derived intermediates, enabling both allenamide-alkyne (AA) cross-coupling and a few novel electrophile-promoted hydrofunctionalization reactions. The unprecedented selectivity in Pd-catalyzed allenamide-alkyne cross-coupling is achieved through neighboring group chelation, with phosphine ligand selection controlling the reaction outcome. In parallel, the electrophile-promoted functionalizations─including haloalkynylation, hydrooxycarbonylation, hydrodifluoroalkylation, and intermolecular hydroamination─are achieved through strategic selection of electrophiles or their precursors.Additionally, our findings demonstrate how ynamides' reactivity toward both electrophiles and nucleophiles, controlled through activator modulation, expands the scope of accessible transformations. Key findings include: (1) chemoselective [2 + 2 + 2] annulation through efficient trapping of N-arylated nitrilium electrophiles by ynamides, (2) divergent C-H annulation of indole-derived vinylogous ynamides controlled by metal and ligand selection via intramolecular hydroarylation, (3) bromoalkynylation-enabled functional group migration through a novel 1,3-alkynyl shift.The final section explores how N-electron polarization in 1,3-enynes enables new chemoselectivity in metal-free inter- and intramolecular couplings with indole substrates. Our findings demonstrate that modulating N-electron conjugation within the enyne skeleton─through both linear and cross conjugation─can direct activation pathways and control product selectivity.This Account aims to stimulate broader research into the intermediate-controlled functionalization of activated π-systems. Future research directions include advanced activator design, novel functional group migration strategies, and deeper mechanistic studies to enable rational reaction development.

A Precise Route to Tetrasubstituted Allyl Amines via Regioselective Dicarbofunctionalization of Masked Propargyl Amines

Allyl amines are vital components in various biologically important molecules and play a significant role in their function. Presently, most methods are geared toward the preparation of di- and trisubstituted allyl amines, leaving a gap for the development of more versatile approaches. We herein describe an approach to yield tetrasubstituted allyl amines through palladium (Pd)-catalyzed regioselective dicarbofunctionalization of masked N-phthalimide-protected propargyl amines. The cationic Pd-intermediate in conjunction with the masked amine exerts collective control for the reaction regioselectivity. This method accommodates a wide range of alkynes, aryl boronic acids, and aryl diazonium salts offering direct access to a wide range of unusual tetrasubstituted allyl amines.

Rhodium(III)-Catalyzed Intramolecular Cyclization and Sequential Aromatization of Ynamides with Propargyl Esters: Access to 2,5-Dihydropyrroles and Pyrroles

Disclosed herein is a rhodium(III)-catalyzed intramolecular cyclization of ynamides with propargyl esters. A variety of highly functionalized 2,5-dihydropyrroles were obtained in moderate to good yields with high E/Z selectivities. Subsequent oxidation of the products gave valuable pyrrole derivatives. Additionally, scale-up reactions and late-stage derivatizations highlight the potential synthetic utility of this methodology.

Regioselective Twofold Annulation of Propargyl Acetates

The regioselective annulation of unsymmetrical alkynes has always been a focused research topic. A Pd-catalyzed double annulation of unsymmetrical alkynes (i.e., yne-acetates) with aryl diazonium salts for the synthesis of substituted naphthalene derivatives is developed. The process addresses intrinsic regioselectivity challenges in the annulations of unsymmetrical alkynes. Mechanistic investigations shed light on the crucial role of the acetate-directing groups in determining the regiochemical reaction outcome.

Copper-Catalyzed Regio- and Stereoselective Hydroarylation of Ynamide

Presented herein is a copper-catalyzed trans-hydroarylation of ynamides. The reaction showcases the assembly of boronic acids across the carbon-carbon triple bond of ynamides. The reaction proceeds under mild conditions offering a complementary approach for the versatile synthesis of multifunctional (E)-α,β-disubstituted enamides. Moreover, the hydroarylation process is highly regio- and stereoselective. The transformation shows a broad scope (30 examples) and tolerates a wide range of labile functional groups. Control experiments provide substantive evidence supporting the mechanistic cycle and the observed selectivity.

Regiodivergent and Stereoselective Synthesis of Highly Substituted 1,3-Dienes via Arylative Acyloxy Migration of Propargyl Esters

We report the first catalyst-controlled regiodivergent method that enables the synthesis of structurally diverse 1,2,3,4-tetrasubstituted conjugated dienes with excellent regio- and stereochemical outcomes from the same set of readily available propargyl esters and diaryliodonium salts. In this diene chemistry, the in situ generated, highly electrophilic aryl-CuIII complex serves not only as a π-Lewis acid catalyst for alkyne activation/acyloxy migration but also as an aryl electrophile equivalent. The competitive arylative 1,2- and 1,3-acyloxy migration patterns are exquisitely dictated by Cu and Au/Cu relay catalyses, respectively, providing a modular and attractive approach to traditionally inaccessible tetrasubstituted 1,3-dienes in a regiodivergent manner. Finally, the synthetic utility of this method is demonstrated by further synthetic derivatization of 1,3-dienes into an array of useful compounds.

Atom-economic and stereoselective catalytic synthesis of fully substituted enol esters/carbonates of amides in acyclic systems enabled by boron Lewis acid catalysis

Carboacyloxylation of internal alkynes is emerging as a powerful and straightforward strategy for enol ester synthesis. However, the reported examples come with limitations, including the utilization of noble metal catalysts, the control of regio- and Z/E selectivity, and an application in the synthesis of enol carbonates. Herein, a boron Lewis acid-catalyzed intermolecular carboacyloxylation of ynamides with esters to access fully substituted acyclic enol esters in high yield with generally high Z/E selectivity (up to >96 : 4) is reported. Most importantly, readily available allylic carbonates are also compatible with this difunctionalization reaction, representing an atom-economic, catalytic and stereoselective protocol for the construction of acyclic β,β-disubstituted enol carbonates of amides for the first time. The application of the carboacyloxylation products to decarboxylative allylations provided a ready access to enantioenriched α-quaternary amides. Moreover, experimental studies and theoretical calculations were performed to illustrate the reaction mechanism and rationalize the stereochemistry.

Light-mediated sulfonyl-iodination of ynamides and internal alkynes

We synthesized tetrasubstituted olefins regioselectively and stereoselectively from ynamides and internal alkynes with sulfonyl iodides under blue LEDs in few minutes. The key features are being metal-free, easy to handle, simple, broad in scope, and environmentally friendly. Furthermore, a gram-scale experiment was conducted, and the synthesized corresponding sulfonyl-iodinated products were smoothly altered into various other products.

Three Component syn-1,2-Arylmethylation of Internal Alkynes

A Pd-catalyzed three-component syn-1,2-arylmethylation of internal alkynes (ynamides/yne-acetates/alkynes) is described. The readily available and bench stable coupling partners iodo-arenes, and methyl boronic acid are successfully used in this coupling strategy to access the methyl-containing tetra-substituted olefins; the scope is broad showing excellent functional-group tolerance. Notably, the transformation is regio- as well as stereoselective. The biologically relevant motifs (BRM) bearing iodo-arenes and ynamides are also used for the late-stage syn-1,2-arylmethylation of alkynes. Aryl-alkylation, aryl-trideuteriomethylation, alkynyl-methylation, and alkenyl-methylation of ynamides are also presented. The Me-substituted alkenes are further transformed into synthetically important β-amino-indenones and α-fluoro-α'-methyl ketones.

Ligand-controlled stereodivergent alkenylation of alkynes to access functionalized trans- and cis-1,3-dienes

Precise stereocontrol of functionalized alkenes represents a long-standing research topic in organic synthesis. Nevertheless, the development of a catalytic, easily tunable synthetic approach for the stereodivergent synthesis of both E-selective and even more challenging Z-selective highly substituted 1,3-dienes from common substrates remains underexploited. Here, we report a photoredox and nickel dual catalytic strategy for the stereodivergent sulfonylalkenylation of terminal alkynes with vinyl triflates and sodium sulfinates under mild conditions. With a judicious choice of simple nickel catalyst and ligand, this method enables efficient and divergent access to both Z- and E-sulfonyl-1,3-dienes from the same set of simple starting materials. This method features broad substrate scope, good functional compatibility, and excellent chemo-, regio-, and stereoselectivity. Experimental and DFT mechanistic studies offer insights into the observed divergent stereoselectivity controlled by ligands.