Palladium-Catalyzed Synthesis of Dibenzosilepin Derivatives via 1,n-Palladium Migration Coupled with anti-Carbopalladation of Alkyne

Synthesis of Inherently Chiral Germepins via Rh-Catalyzed Enantioselective C-H Germylation

A rhodium-catalyzed enantioselective C-H germylation reaction was developed, enabling the synthesis of germepin derivatives with diverse substitutions. This study represents a rare application of E-H/C-H (E = Si or Ge) dehydrogenation coupling reactions in synthesizing inherently chiral molecules, marking the first successful enantioselective synthesis of inherently chiral seven-membered heterocycles containing group 14 elements. Compared to their structurally analogous tribenzosilepins, tribenzogermepins exhibit enhanced stereochemical stability under the reaction conditions. Furthermore, the compatibility of the Si-H bond under identical conditions highlights the distinct reactivity of Si-H and Ge-H bonds.

Divergent Synthesis of Silacycles by Ligand-Controlled Rhodium-Catalyzed Annulation of (2-Hydrosilylaryl)acetylenes

This study demonstrates the rhodium-catalyzed divergent synthesis of 5-, 5,6-, and 8-membered silacyclic compounds, such as benzosilole, silolosiline, and disilocine, from (2-hydrosilylaryl)acetylenes. To achieve high product selectivity, intra- and intermolecular reactions are controlled by tuning the mono- and bidentate phosphine ligands on the rhodium catalyst precursor. Deuterium labeling experiments and computational studies reveal that the annulation reaction proceeds via hydride transfer to the alkyne moiety.

Microwave-Assisted Palladium-Catalyzed Crossover-Annulation: Access to Fused Polycyclic Benzofuran Scaffolds

This study demonstrates quick access to heteroatom-embodied complex fused polycyclic frameworks through a palladium-catalyzed domino process facilitated by microwave-assisted crossover annulation of o-alkynylarylhalides and dihydrobenzofurans derivatives. The overall success of this process lies in the careful design of dihydrobenzofuran precursors that direct the initial palladium-mediated annulation step to proceed in a highly regioselective manner to furnish a single regioisomeric product. Notably, this one-pot method has witnessed good substrate scope and has furnished products with excellent yields.

Controllable 1,4-Palladium Aryl to Aryl Migration in Fused Systems─Application to the Synthesis of Azaborole Multihelicenes

Herein, we report the first 1,4-Pd aryl to aryl migration/Miyaura borylation tandem reaction in fused systems. The Pd shift occurred in the bay region of the dibenzo[g,p]chrysene building blocks, giving rise to a thermodynamically controlled mixture of 1,8- and 1,9-borylated compounds that allowed the preparation of regioisomeric azaborole multihelicenes from the same starting material. The outcome of this synthesis can be controlled by the choice of reaction conditions, allowing the migration process to be turned off in the absence of an acetate additive and the target multiheterohelicenes to be prepared in a regioselective manner. The target compounds show bright green fluorescence in dichloromethane with emission quantum yields (Φ) of up to 0.29, |glum| values up to 2.7 × 10-3, and green or green-yellow emission in the solid state, reaching Φ of 0.22. Single crystal X-ray diffraction analyses gave insight into their molecular structures and the packing arrangement. Evaluation of aromaticity in these multihelicenes revealed a nonaromatic character of the 2H-1,2-azaborole constituent rings.

Divergent Synthesis of Enantioenriched Silicon-Stereogenic Benzyl-, Vinyl- and Borylsilanes via Asymmetric Aryl to Alkyl 1,5-Palladium Migration

Functionalization of Si-bound methyl group provides an efficient access to diverse organosilanes. However, the asymmetric construction of silicon-stereogenic architectures by functionalization of Si-bound methyl group has not yet been described despite recent significant progress in producing chiral silicon. Herein, we disclosed the enantioselective silylmethyl functionalization involving the aryl to alkyl 1,5-palladium migration to access diverse naphthalenes possessing an enantioenriched stereogenic silicon center, which are inaccessible before. It is worthy to note that the realization of asymmetric induction at the step of metal migration itself remains challenging. Our study constitutes the first enantioselective aryl to alkyl 1,5-palladium migration reaction. The key to the success is the discovery and fine-tuning of the different substituents of α,α,α,α-tetraaryl-1,3-dioxolane-4,5-dimethanol (TADDOL)-based phosphoramidites, which ensure the enantioselectivity and desired reactivity.

Quaternary Carbon Editing Enabled by Sequential Palladium Migration

Peripheral functionalization of a quaternary carbon via C(sp3)-H bond activation has made significant progress in recent years. However, direct editing of a quaternary carbon through Csp3-Csp3 bond cleavage and refunctionalization of nonstrained acyclic molecules remain underexploited. Herein we report a reaction in which a methyl group attached to a quaternary carbon is shifted to its neighboring secondary carbon with concurrent oxidation of the quaternary C-C single bond to the C═C double bond. Specifically, morpholinyl amide of 2,2-dimethyl alkanoic acids is converted to 2-methylene-3-methyl alkanoic acid derivatives in the presence of a catalytic amount of palladium acetate, Selectfluor and sodium carbonate. Control experiments suggest that the reaction proceeds via a sequence of selective C(sp3)-H activation of the methyl group, oxidation of the resulting C(sp3)-PdII to PdIV intermediate followed by unprecedented 1,3-PdIV migration, 1,2-methyl/PdIV dyotropic rearrangement and finally, β-Hydride elimination. In this domino process, palladium migrates successively from the primary to the secondary and finally to the quaternary carbon, leading to the concurrent functionalization of a primary, a secondary, and a quaternary carbon.

Construction of Si-Stereogenic Silanols by Palladium-Catalyzed Enantioselective C-H Alkenylation

The construction of silicon-stereogenic silanols via Pd-catalyzed intermolecular C-H alkenylation with the assistance of a commercially available L-pyroglutamic acid has been realized for the first time. Employing oxime ether as the directing group, silicon-stereogenic silanol derivatives could be readily prepared with excellent enantioselectivities, featuring a broad substrate scope and good functional group tolerance. Moreover, parallel kinetic resolution with unsymmetric substrates further highlighted the generality of this protocol. Mechanistic studies indicate that L-pyroglutamic acid could stabilize the Pd catalyst and provide excellent chiral induction. Preliminary computational studies unveil the origin of the enantioselectivity in the C-H bond activation step.

Expanding the Scope of Alkynes in C-H Activation: Weak Chelation-Assisted Cobalt-Catalyzed Synthesis of Indole C(4)-Acrylophenone via C-O Bond Cleavage of Propargylic Ethers

Herein, we report the facile synthesis of indole C(4)-acrylophenone using a C-H bond activation strategy. For this conversion, an unsymmetrical alkyne (phenylethynyl ether) in the presence of cobalt(III)-catalyst works efficiently. In this process, alkyne gets oxidized in the presence of in situ generated water, which is the key step for this method, for which trifluoroethanol is the water source. The pivaloyl directing group chelates effectively to generate the cobaltacycle intermediate, which was detected through high-resolution mass spectrometry (HRMS). Also, the formation of bis(2,2,2-trifluoroethyl) ether has been confirmed and quantified using 19F NMR. In addition, the applicability of obtained indole C(4)-acrylophenone product has been demonstrated by performing the Nazarov cyclization and conjugate addition to the α,β-unsaturated ketone moiety.

Recent advances in ligand-enabled palladium-catalyzed divergent synthesis

Developing efficient and straightforward strategies to rapidly construct structurally distinct and diverse organic molecules is one of the most fundamental tasks in organic synthesis, drug discovery and materials science. In recent years, divergent synthesis of organic functional molecules from the same starting materials has attracted significant attention and has been recognized as an efficient and powerful strategy. To achieve this objective, the proper adjustment of reaction conditions, such as catalysts, solvents, ligands, etc., is required. In this review, we summarized the recent efforts in chemo-, regio- and stereodivergent reactions involving acyclic and cyclic systems catalyzed by palladium complexes. Meanwhile, the reaction types, including carbonylative reactions, coupling reactions and cycloaddition reactions, as well as the probable mechanism have also been highlighted in detail.

Palladium-Catalyzed Skeletal Rearrangement of Substituted 2-Silylaryl Triflates via 1,5-C-Pd/C-Si Bond Exchange

A palladium-catalyzed skeletal rearrangement of 2-(2-allylarylsilyl)aryl triflates has been developed to give highly fused tetrahydrophenanthrosilole derivatives via unprecedented 1,5-C-Pd/C-Si bond exchange. The reaction pathways can be switched toward 4-membered ring-forming C(sp2 )-H alkylation by tuning the reaction conditions to give completely different products, fused dihydrodibenzosilepin derivatives, from the same starting materials. The inspection of the reaction conditions revealed the importance of carboxylates in promoting the C-Pd/C-Si bond exchange.

Copper-Catalyzed Regio- and Stereoselective Formal Hydro(borylmethylsilyl)ation of Internal Alkynes via Alkenyl-to-Alkyl 1,4-Copper Migration

Catalytic reactions involving 1,n-metal migration from carbon to carbon enable a nonclassical way of constructing organic molecular skeletons, rapidly providing complex molecules from relatively simple precursors. By utilization of this attractive feature, a new and efficient synthesis of alkenylsilylmethylboronates has been developed by formal hydro(borylmethylsilyl)ation of unsymmetric internal alkynes with silylboronates under copper catalysis. The reaction proceeds regioselectively and involves an unprecedented alkenyl-to-alkyl 1,4-copper migration. The reaction mechanism has been investigated by a series of kinetic, NMR, and deuterium-labeling experiments.

Rapid Access to Fused Tetracyclic N-Heterocycles via Amino-to-Alkyl 1,5-Palladium Migration Coupled with Intramolecular C(sp3)-C(sp2) Coupling

An unprecedented route for the preparation of fused tetracyclic N-heterocycles is presented through the palladium-catalyzed cyclization of isocyanides with alkyne-tethered aryl iodides. In this transformation, a novel amino-to-alkyl 1,5-palladium migration/intramolecular C(sp3)-C(sp2) coupling sequence was observed first. More importantly, isocyanide exhibited three roles, serving simultaneously as a C1 synthon, a C1N1 synthon, and the donor of C(sp3) for C(sp3)-C(sp2) coupling, and the reaction was the sole successful example that achieved C(sp3)-H activation of isocyanide.

A theoretical study of the ligand-controlled palladium-catalysed regiodivergent synthesis of dibenzosilepin derivatives

Palladium-catalysed ligand-controlled 1,n-palladium migration of silicon-tethering substrates provides a regiodivergent synthesis strategy for constructing silicon-bridged π-conjugated compounds possessing a 6,6-fused or a 5,7-fused scaffold. Density functional theory (DFT) calculations were carried out to elucidate the detailed mechanism of this 1,n-palladium migration involving syn- or anti-carbopalladation. The computational results suggest that alkyne insertion is the regioselectivity-determining step. Upon catalysis without the BINAP ligand, the 1,2-insertion of an alkyne into the Pd-aryl bond leads to the formation of 6,6-fused benzophenanthrosiline, which is more favorable than the 2,1-insertion of alkyne by 4.2 kcal mol^-1. The selective formation of 5,7-fused benzofluorenosilepins via the 2,1-alkyne insertion is facilitated by the BINAP ligand. The 1,2-alkyne insertion with the BINAP ligand is disfavoured due to the steric repulsion between the phenyl group of the substrate and the naphthalene group of the BINAP ligand. The 2,1-alkyne insertion with the BINAP ligand orients the ligand away from the phenyl group of the substrate, which can avoid steric repulsion.

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.

Diverse reactivity of alkynes in C-H activation reactions

Alkynes occupy a prominent role as a coupling partner in the transition metal-catalysed directed C-H activation reactions. Due to low steric requirements and linear geometry, alkynes can effectively coordinate with metal d-orbitals. This makes alkynes one of the most successful coupling partners in terms of the number of useful transformations. Remarkably, by changing the reaction conditions and transition-metals from 5d to 3d, the pattern of reactivity of alkynes also changes. Due to the varied reactivity of alkynes, such as alkenylation, annulation, alkylation, and alkynylation, they have been extensively used for the synthesis of valuable organic molecules. Despite enormous explorations with alkynes, there are still a lot more possible ways by which they can be made to react with M-C bonds generated through C-H activation. Practically there is no limit for the creative use of this approach. In particular with the development of new high and low valent first-row metal catalysts, there is plenty of scope for this chemistry to evolve as one of the most explored areas of research in the coming years. Therefore, a highlight article about alkynes is both timely and useful for synthetic chemists working in this area. Herein, we have highlighted the diverse reactivity of alkynes with various transition metals (Ir, Rh, Ru, Pd, Mn, Fe, Co, Ni, Cu) and their applications, along with some of our thoughts on future prospects.

Tandem Reactions involving 1,4-Palladium Migrations

Transition-metal-catalyzed tandem reactions have become a mainstay in organic chemistry owing to their high atom- and step-economies. Metal-migration-based tandem reactions allow the engagement of simple starting materials for incorporating functional groups into certain positions and constructing complex scaffolds, which provide novel means that are complementary to traditional cross-coupling or C-H activation processes. In light of the broad utility of the 1,4-Pd migration reaction, this paper reviews its progress in the past two decades, summarizing the tandem process and classifying it based on insertion, elimination, transmetalation, and C-H bond activation. Special emphasis is placed on the driving force of Pd migration and different migration mechanisms. Moreover, this review also attempts to summarize common strategies for improving the regio- and site-selectivities of the migration process.

Copper Foam as Active Catalysts for the Borylation of α, β-Unsaturated Compounds

The use of simple, inexpensive, and efficient methods to construct carbon–boron and carbon–oxygen bonds has been a hot research topic in organic synthesis. We demonstrated that the desired β-boronic acid products can be obtained under mild conditions using copper foam as an efficient heterogeneous catalyst. The structure of copper foam before and after the reaction was investigated by polarized light microscopy (PM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), and the results have shown that the structure of the catalyst copper foam remained unchanged before and after the reaction. The XPS test results showed that the Cu(0) content increased after the reaction, indicating that copper may be involved in the boron addition reaction. The specific optimization conditions were as follows: CH₃COCH₃ and H₂O were used as mixed solvents, 4-methoxychalcone was used as the raw material, 8 mg of catalyst was used and the reaction was carried out at room temperature and under air for 10 h. The yield of the product obtained was up to 92%, and the catalytic efficiency of the catalytic material remained largely unchanged after five cycles of use.

Intermolecular C-H silylation through cascade carbopalladation and vinylic to aryl 1,4-palladium migration

A palladium-catalyzed remote C-H silylation reaction has been developed through vinylic to aryl 1,4-palladium migration. By using alkyne-tethered aryl iodides as the starting materials and hexamethyldisilane as the silylating reagent, the reaction involves cascade intramolecular carbopalladation, 1,4-palladium migration, and silylation with hexamethyldisilane, and leads to the formation of exocyclic alkene-containing 5-silylisoquinolines as the final products.

Quadruple Role of Pd Catalyst in Domino Reaction Involving Aryl to Alkyl 1,5-Pd Migration to Access 1,9-Bridged Triptycenes

A Pd-catalyzed domino reaction of 1,8,13-tribromo-9-methoxytriptycenes is reported. Under conventional Suzuki coupling conditions, the triptycenes underwent multiple transformations to give 1,9-bridged triptycenes. Based on mechanistic investigations, a single Pd catalyst functions as Pd⁰ , Pd^II and Pd^IV species to catalyze four distinct processes: (1) aryl to alkyl 1,5-Pd migration, (2) intramolecular arylation, (3) homocoupling of phenylboronic acid and (4) Suzuki coupling. DFT calculations revealed that 1,5-Pd migration likely proceeds via both concerted Pd^II and stepwise Pd^IV routes. Asymmetric synthesis of the chiral triptycenes, as well as optical resolution, and further transformation are also reported.

Facile synthesis of tribenzosilepins from terphenyls and dihydrosilanes by electrophilic double silylation

Tribenzosilepins were synthesized from terphenyls and dihydrosilanes via a facile approach using a double sila-Friedel–Crafts reaction.