Emergence of palladium(IV) chemistry in synthesis and catalysis

Fluoride-induced redox-switchable behaviour of a palladium(ii)/(iv) couple

We report the preparation of two Pd(ii) complexes based on (CCC)-pincer-NHC (NHC = N-heterocyclic carbene) ligands. One of these complexes features an NDI unit (NDI = naphthalene-diimide) attached to the CCC-pincer ligand. Reacting these complexes with bromine results in the formation of the corresponding palladium(iv) complexes [PdBr3(NDI-CCC)] and [PdBr3(CCC)]. The NDI-containing pincer complex exhibits a strong sensitivity to fluoride ions, which can induce a one-electron reduction of the naphthalene-diimide moiety. We demonstrate that the addition of fluoride induces the reduction of the NDI moiety via the formation of hydroxide anions, which are the effective reducing agents of the process. The addition of fluoride significantly affects the reactivity of the NDI-containing palladium complexes. For example, the palladium(iv) complex [PdBr3(NDI-CCC)] can transfer bromide to styrene in a stoichiometric manner, but this reaction is inhibited in the presence of fluoride. Similarly, the palladium(ii) complex [PdI(NDI-CCC)] catalyzes the oxidative homocoupling of arylpyridines, but its catalytic activity is quenched when excess fluoride is added. Notably, we demonstrate that this process can be deactivated and reactivated by sequentially introducing an excess of fluoride and NOBF4, revealing a rare instance of a redox-switchable process within a Pd(ii)/Pd(iv) catalytic cycle. In contrast, the (CCC)-pincer palladium(ii) and (iv) complexes lacking the NDI unit show no sensitivity to fluoride. Our study demonstrates that a simple reagent, such as the fluoride anion, can effectively modulate the reactivity of the Pd(ii)/Pd(iv) pair. More broadly, it shows that fluoride serves as a simpler alternative to the metal-based reducing agents commonly used in redox-switchable catalysis.

Chelating 1,2-Bis(2'-picolyl)-o-carborane as a Supporting Ligand for Pd-Catalyzed Selective B(3,6)-H Difunctionalization of o-Carboranes

Transition metal catalyzed selective cage B-H functionalization of carboranes has made significant progress in recent years, giving rise to the efficient synthesis of a large variety of cage B-functionalized carboranes including alkenylation, arylation, alkynylation, borylation, hydroxylation, acyloxylation, amination, and halogenation. However, the mechanisms of these catalytic B-X coupling reactions are not well understood. Herein, we describe the isolation and characterization of the catalytically relevant o-carborane based palladium(II) metallacycle, disclosing the details of Pd-catalyzed B-H functionalization of o-carboranes. As a result, highly selective catalytic B(3,6)-dihalogenation, -dimethylation, and -diarylation of o-carboranes have been achieved.

Chemoselectivity in Pd-Based Dyotropic Rearrangement: Development and Application in Total Synthesis of Pheromones

In the dyotropic rearrangement of molecules with semiflexible structures, characterized by a freely rotating static C-C bond, the formation of a mixture of products is common due to the coexistence of several energetically comparable conformers. Herein, we report that it is possible to modulate the shifting groups by adjusting the metal's coordination sphere in Pd-based dyotropic rearrangement. In the presence of a catalytic amount of Pd(II) salt, the reaction of γ-hydroxyalkenes or γ,δ-dihydroxyalkenes with Selectfluor affords fluorinated tetrahydropyranols or 6,8-dioxabicyclo[3.2.1]octanes (DOBCO), respectively. In this domino process, two C(sp2) and one allylic C(sp3) are sequentially activated and functionalized through a pivotal conformation-controlled chemoselective C(sp3)-C(sp3) and C(sp3)-Pd(IV) bond metathesis reaction. Mechanistic studies suggest a reaction sequence including 5-exo-trig oxypalladation, Pd oxidation, and chemoselective ring expansion 1,2-Csp3/Pd(IV) dyotropic rearrangement, followed by hydroxypalladation of in situ generated dihydropyrans. These findings provide a unique retrosynthetic disconnection for the synthesis of 6,8-DOBCO. We showcase its potential by developing a concise synthesis of three important pheromones. Notably, (+)-frontalin is synthesized from a commercially available 1,5-diene in only two steps, utilizing Sharpless catalytic asymmetric dihydroxylation and Pd-catalyzed domino cyclization developed in this study.

Dual Inert C-H Bond Site-Selective Activations Enabled by Pd/Norbornene-Mediated Cascade Cyclization toward Medium-Sized Polyheterocyclic Methylene Sulfoximines

A Pd/norbornene-mediated three-component modular one-step reaction facilitated by dual C-H bond activation and cascade cyclization is reported. This procedure uses norbornene as a catalyst in the Catellani-type reaction and as an alkylating building block to accomplish the dual unactivated C-H bond functionalization protocol, which results in the production of polyheterocyclic eight-membered sulfoximines with an indene-fused moiety. This mild, scalable protocol's wide substrate range makes it ideal for site-selective dual C-H functionalization at the highly chemoselective aryl sites.

PdII/CuI-Cocatalyzed Radical Arylation of gem-Difluoroalkenes Using Arylsulfonyl Chlorides

A PdII/CuI-cocatalyzed arylation of gem-difluoroalkenes with arylsulfonyl chlorides, affording various defluorinative arylation/1,2-difunctionalized products, was developed. The interception of aryl radicals generated from the reduction of arylsulfonyl chlorides delivers some hypervalent Pd species, which present high reactivities and chemoselectivities toward the defluorinative arylation product formation. Besides, the nature of the electron-deficient Pd metal center is more prone to reductive elimination under acidic conditions, providing an opportunity to explore new reactivates of fluorinated alkenes into more elaborate substructures.

Palladium-catalyzed remote internal C(sp3)-H bond chlorination of alkenes

C(sp3)-Cl bonds are present in numerous biologically active molecules and can also be used as a site for diversification by substitution or cross-coupling reactions. Herein, we report a remote internal site-selective C(sp3)-H bond chlorination of alkenes through sequential alkene isomerization and hydrochlorination, enabling the synthesis of both benzylic and tertiary chlorides with excellent site-selectivity. This transformation offers exciting possibilities for the late-stage chlorination of derivatives of natural products and pharmaceuticals. We also demonstrate the regioconvergent synthesis of a single alkyl chloride from unrefined mixtures of isomeric alkenes, which can be extracted directly from petrochemical sources.

Selective [3 + 2] C-H/C-H Alkyne Annulation via Dual (Distal) C(β, δ)-H Bond Activation Relay: A Novel Therapeutic Quinazolone-Tethered Benzofulvenes for Oral Cancer

In contrast to proximal C-H bond activations, distal C-H bond activation is fundamentally more challenging and requires distinctly specialized directing partners or techniques. In this context, we report an unprecedented dual (distal) β-C(benzylic)-H and δ-C(aryl)-H bond activation relay protocol for the chemo-, regio-, and stereoselective construction of heterocycle-tethered benzofulvenes via [3 + 2] CH/CH-alkyne annulation under palladium catalysis. The protocol overrides the more favorable [4 + 2] CH/NH annulation and does not follow the vinylic C-H bond activation pathway. Mechanistic studies provide insight into the favored cyclopalladation of key intermediates (resulting from β-C(benzylic)-H bond cleavage) through relay δ-C(aryl)-H cleavage (vs N-H cleavage) prior to reductive elimination, which is the key to desired annulation. The synthesized new chemical entities (NCEs) constitute a novel scaffold with favorable anticancer activity against oral squamous cell carcinoma (OSCC). Detailed biomolecular studies, including RNA-sequencing and analysis, indicate that these compounds (4e and 4w) arrest the cell cycle at the S-phase and target multiple cancer hallmarks, such as the activation of apoptotic pathways and impairment of mitochondrial activity simultaneously, suggesting their chemotherapeutic potential for oral cancer by addressing the complexity and adaptability of cancer cells in chorus.

RuII-Catalyzed C-H Activated Diverse Cyclization with Transformation of Substrate-DG to Functional Groups: Synthesis of Functionalized Indoles and Indenones

We present an elegant and efficient method for Ru(II)-catalyzed C-H activation, followed by a diverse range of intermolecular cross-dehydrogenative coupling reactions. This process is facilitated by an intrinsic directing group (DG) and includes the in situ transformation of the DG into common and useful functional groups. Notably, this method avoids the installation and deinstallation of the directing group. Our approach enables the selective functionalization of benzimidate, coupled with the cyclization of o-alkynyl-aniline, resulting in the high-yield synthesis of diverse compounds such as indoles, and indenones. The sequential formation of C-N, C-C, and C-O bonds, followed by hydrolysis, underscores the versatile in situ transformation of the directing group. This work not only broadens the synthetic toolbox for constructing complex heterocyclic structures but also highlights the potential for sustainable and selective synthesis of valuable compounds.

Palladium and copper co-catalyzed chloro-arylation of gem-difluorostyrenes - use of a nitrite additive to suppress β-F elimination

The installation of fluorine and fluorinated functional groups in organic molecules perturbs the physicochemical properties of those molecules and enables the development of new therapeutics, agrichemicals, biological probes and materials. However, current synthetic methodologies cannot access some fluorinated functional groups and fluorinated scaffolds. One such group, the gem-difluorobenzyl motif, might be convergently synthesized by reacting a nucleophilic aryl precursor and an electrophilic gem-difluoroalkene. Previous attempts have relied on forming unstable anionic or organometallic intermediates that rapidly decompose through a β-F elimination process to deliver monofluorovinyl products. In contrast, we report a fluorine-retentive palladium and copper co-catalyzed chloro-arylation of gem-difluorostyrenes that takes advantage of a nitrite (NO2 -) additive to avoid the favorable β-F elimination pathway that forms monofluorinated products, instead delivering difluorinated products.

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.

Hydroxopalladium(IV) complexes prepared using oxygen or hydrogen peroxide as oxidants

The cycloneophylpalladium(II) complexes [Pd(CH2CMe2C6H4)(κ2-N,N'-L)], 1 or 2, with L = RO(CH2)3N(CH2-2-C5H4N)2, with R = H or Me, respectively, react with either dioxygen or hydrogen peroxide in the presence of NH4[PF6] to give rare examples of the corresponding hydroxopalladium(IV) complexes [Pd(OH)(CH2CMe2C6H4)(κ3-N,N',N''-L)][PF6], 3 or 4. The complexes 3 and 4 are stable at room temperature and have been structurally characterized. On heating a solution of 3 or 4 in moist dimethylsulphoxide, selective reductive elimination with C(sp2)-O bond formation is observed, followed by hydrolysis, to give the corresponding pincer complex [Pd(OH)(κ3-N,N',N''-L)][PF6] and 2-t-butylphenol as major products. A more complex reaction occurs in chloroform solution. The mechanisms of reaction are discussed, supported by DFT calculations.

Deciphering complexity in Pd-catalyzed cross-couplings

Understanding complex reaction systems is critical in chemistry. While synthetic methods for selective formation of products are sought after, oftentimes it is the full reaction signature, i.e., complete profile of products/side-products, that informs mechanistic rationale and accelerates discovery chemistry. Here, we report a methodology using high-throughput experimentation and multivariate data analysis to examine the full signature of one of the most complicated chemical reactions catalyzed by palladium known in the chemical literature. A model Pd-catalyzed reaction was selected involving functionalization of 2-bromo-N-phenylbenzamide and multiple bond activation pathways. Principal component analysis, correspondence analysis and heatmaps with hierarchical clustering reveal the factors contributing to the variance in product distributions and show associations between solvents and reaction products. Using robust data from experiments performed with eight solvents, for four different reaction times at five different temperatures, we correlate side-products to a major dominant N-phenyl phenanthridinone product, and many other side products.

Photoexcitation of Distinct Divalent Palladium Complexes in Cross-Coupling Amination Under Air

The development of metal complexes that function as both photocatalyst and cross-coupling catalyst remains a challenging research topic. So far, progress has been shown in palladium(0) excited-state transition metal catalysis for the construction of carbon-carbon bonds where the oxidative addition of alkyl/aryl halides to zero-valent palladium (Pd0 ) is achievable at room temperature. In contrast, the analogous process with divalent palladium (PdII ) is uphill and endothermic. For the first time, we report that divalent palladium can act as a light-absorbing species that undergoes double excitation to realize carbon-nitrogen (C-N) cross-couplings under air. Differently substituted aryl halides can be applied in the mild, and selective cross-coupling amination using palladium acetate as both photocatalyst and cross-coupling catalyst at room temperature. Density functional theory studies supported by mechanistic investigations provide insight into the reaction mechanism.

Palladium-Catalyzed Electrooxidative Double C-H Arylation

The electrochemical transition metal-catalyzed cross-dehydrogenative reaction has emerged as a promising platform to achieve a sustainable and atom-economic organic synthesis that avoids hazardous oxidants and minimizes undesired byproducts and circuitous functional group operations. However, a poor mechanistic understanding still prevents the widespread adoption of this strategy. In this regard, we herein present an electrochemical palladium-catalyzed oxidative coupling strategy to access biaryls in the absence of a stoichiometric chemical oxidant. The robust palladaelectrocatalysis considerably suppresses the occurrence of homocoupling and oxygenation, being compatible even with electron-deficient arenes. Late-stage functionalization and Boscalid precursor synthesis further highlighted the practical importance of our electrolysis. Remarkably, mechanistic studies including the evaluation of the reaction order of each component by variable time normalization analysis (VTNA) and initial rate analysis, H/D exchange experiment, kinetic isotope effect, and stoichiometric organometallic experiments provided strong support for the involvement of transmetalation between two organopalladium complexes in the turnover limiting step. Therefore, matching the concentrations or lifetimes of two distinct organopalladium intermediates is revealed to be a pivot to the success of electrooxidative catalysis. Moreover, the presence of cationic copper(II) seems to contribute to the stabilization of the palladium(0) catalyst instead of playing a role in the oxidation of the catalyst.

Palladium-Based Dyotropic Rearrangement Enables A Triple Functionalization of Gem-Disubstituted Alkenes: An Unusual Fluorolactonization Reaction

We report in this paper a Pd(II)-catalyzed migratory gem-fluorolactonization of ene-carboxylic acids. Reaction of 4-methylenealkanoic acid derivatives with Selectfluor in the presence of Pd(OAc)2 (1.0 mol %) at room temperature affords fluorolactones in good to excellent yields. 2-(2-Methylenecycloalkanyl)acetic acids are transformed to bridged fluorolactones under identical conditions. One C-C, one C-O and one tertiary C-F bond were generated along the gem-disubstituted carbon-carbon double bond in this operationally simple transformation. Trapping experiments indicates that the reaction is initiated by a 5-exo-trig oxypalladation followed by Pd oxidation, regioselective ring-enlarging 1,2-alkyl/Pd(IV) dyotropic rearrangement and C-F bond forming reductive elimination cascade. Post-transformations of these fluorolactones taking advantage of the electrophilicity of the 1-fluoroalkylcarboxylate function are also documented.

Palladium K-edge X-ray Absorption Spectroscopy Studies on Controlled Ligand Systems

X-ray absorption spectroscopy (XAS) is widely used across the life and physical sciences to identify the electronic properties and structure surrounding a specific element. XAS is less often used for the characterization of organometallic compounds, especially for sensitive and highly reactive species. In this study, we used solid- and solution-phase XAS to compare a series of 25 palladium complexes in controlled ligand environments. The compounds include palladium centers in the formal I, II, III, and IV oxidation states, supported by tridentate and tetradentate macrocyclic ligands, with different halide and methyl ligand combinations. The Pd K-edge energies increased not only upon oxidizing the metal center but also upon increasing the denticity of the ligand framework, substituting sigma-donating methyl groups with chlorides, and increasing the charge of the overall metal complex by replacing charged ligands with neutral ligands. These trends were then applied to characterize compounds whose oxidation states were otherwise unconfirmed.

Palladium-Catalyzed Stitching of 1,3-C(sp3)-H Bonds with Dihaloarenes: Short Synthesis of (±)-Echinolactone D

Palladium-catalyzed C(sp3)-H functionalization presents an efficient strategy to construct a variety of carbon-carbon bonds. However, application of this approach toward the preparation of five-membered benzo-fused carbocycles via the most simplifying C-H activation logic has not been realized. In this Article, we report a palladium-catalyzed annulation reaction between gem-dimethyl-containing amides and 1-bromo-2-iodoarenes that effectively constructs two Calkyl-Caryl bonds and provides access to a variety of five-membered benzo-fused compounds. In this transformation, the dihaloarene is stitched to the gem-dimethyl moiety via two sequential β-C(sp3)-H arylations utilizing the differential reactivity of the 1,2-difunctionalized electrophile. This annulation reaction is enabled by a dual-ligand system comprising of an N-acyl glycine and a pyridine-3-sulfonic acid that synergistically promotes the palladium stitching and provides the bicyclic products. This method displays a broad substrate scope and shows excellent amide compatibility. We also demonstrate the synthetic potential of this annulation by synthesizing echinolactone D.

Optimizing the Synthetic Potential of O2: Implications of Overpotential in Homogeneous Aerobic Oxidation Catalysis

Molecular oxygen is the quintessential oxidant for organic chemical synthesis, but many challenges continue to limit its utility and breadth of applications. Extensive historical research has focused on overcoming kinetic challenges presented by the ground-state triplet electronic structure of O2 and the various reactivity and selectivity challenges associated with reactive oxygen species derived from O2 reduction. This Perspective will analyze thermodynamic principles underlying catalytic aerobic oxidation reactions, borrowing concepts from the study of the oxygen reduction reaction (ORR) in fuel cells. This analysis is especially important for "oxidase"-type liquid-phase catalytic aerobic oxidation reactions, which proceed by a mechanism that couples two sequential redox half-reactions: (1) substrate oxidation and (2) oxygen reduction, typically affording H2O2 or H2O. The catalysts for these reactions feature redox potentials that lie between the potentials associated with the substrate oxidation and oxygen reduction reactions, and changes in the catalyst potential lead to variations in effective overpotentials for the two half reactions. Catalysts that operate at low ORR overpotential retain a more thermodynamic driving force for the substrate oxidation step, enabling O2 to be used in more challenging oxidations. While catalysts that operate at high ORR overpotential have less driving force available for substrate oxidation, they often exhibit different or improved chemoselectivity relative to the high-potential catalysts. The concepts are elaborated in a series of case studies to highlight their implications for chemical synthesis. Examples include comparisons of (a) NOx/oxoammonium and Cu/nitroxyl catalysts, (b) high-potential quinones and amine oxidase biomimetic quinones, and (c) Pd aerobic oxidation catalysts with or without NOx cocatalysts. In addition, we show how the reductive activation of O2 provides a means to access potentials not accessible with conventional oxidase-type mechanisms. Overall, this analysis highlights the central role of catalyst overpotential in guiding the development of aerobic oxidation reactions.

Diverting the 5-exo-Trig Oxypalladation to Formally 6-endo-Trig Fluorocycloetherification Product through 1,2-O/Pd(IV) Dyotropic Rearrangement

Pd-catalyzed cyclizative functionalization of γ-hydroxyalkenes affords tetrahydrofuran derivatives via a key 5-exo-trig oxypalladation step. Herein, we report a palladium(II)-catalyzed, Selectfluor-mediated formal 6-endo-trig fluorocycloetherification of γ-hydroxyalkenes for the synthesis of functionalized tetrahydropyrans. Mechanistically, an σ-alkyl-Pd(II) intermediate resulting from the 5-exo-trig oxypalladation process is isolated and characterized by X-ray crystallographic analysis. Its oxidation with Selectfluor to Pd(IV) triggers the chemoselective 1,2-O/Pd(IV) dyotropic rearrangement affording, after C-F bond-forming reductive elimination, the tetrahydropyrans with concurrent generation of a tertiary carbon-fluorine bond. The occurrence of this 1,2-positional interchange is further evidenced by trapping the rearranged quaternary C(sp³)-Pd bond by an internal nucleophile that is materialized by the development of a Pd(II)-catalyzed oxidative bis-heterocyclization of alkenes.

Palladium-catalyzed and norbornene-mediated C-H amination and C-O alkenylation of aryl triflates

The C-H amination and C-O alkenylation of aryl triflates was achieved through Pd/norbornene (NBE) cooperative catalysis. By this strategy, various ortho-alkenyl tertiary anilines including those bearing functional groups were produced in good to excellent yields. This reaction represents a new conversion model for phenoxides. It expands the scope of Catellani-type reactions and the application of phenoxides in organic synthesis.

C(alkyl)-C(vinyl) bond cleavage enabled by Retro-Pallada-Diels-Alder reaction

Activation and cleavage of carbon-carbon (C-C) bonds is a fundamental transformation in organic chemistry while inert C-C bonds cleavage remains a long-standing challenge. Retro-Diels-Alder (retro-DA) reaction is a well-known and important tool for C-C bonds cleavage but less been explored in methodology by contrast to other strategies. Herein, we report a selective C(alkyl)-C(vinyl) bond cleavage strategy realized through the transient directing group mediated retro-Diels-Alder reaction of a six-membered palladacycle, which is obtained from an in situ generated hydrazone and palladium hydride species. This unprecedented strategy exhibits good tolerances and thus offers new opportunities for late-stage modifications of complex molecules. DFT calculations revealed that an intriguing retro-Pd(IV)-Diels-Alder process is possibly involved in the catalytic cycle, thus bridging both Retro-Diels-Alder reaction and C-C bond cleavage. We anticipate that this strategy should prove instrumental for potential applications to achieve the modification of functional organic skeletons in synthetic chemistry and other fields involving in molecular editing.

Regio- and enantioselective remote dioxygenation of internal alkenes

Methods for the enantioselective direct oxygenation of internal alkenes have provided chemists with versatile and powerful toolboxes for the synthesis of optically pure alcohols, one of the most privileged structural motifs. Regioselectivity, however, remains a formidable challenge in the functionalization of internal alkenes. Here we report a palladium-catalysed highly regio- and enantioselective remote 1,n-dioxygenation (n ≥ 4) of internal alkenes with engineered pyridine-oxazoline (Pyox) ligands. The reactions proceed efficiently and exhibit a broad substrate scope with excellent regio- and enantioselectivity, affording optically pure 1,n-diol acetates as the key synthons for important bioactive molecules. Experimental studies and density functional theory calculations provide evidence that the regioselectivity is governed by the reactivity disparity of two allylic C-H bonds, where the oxypalladation is reversible and the first palladium migration step proves to be the regioselectivity-determining step, enabled by the modified phenyl-substituted Pyox ligands.

Nickel(II) N-Heterocyclic Carbene Complex for the Hydrogenation of 2-Acetylpyridine under Mild Conditions

Catalyst-mediated hydrogenation of ketones via hydride transfer can be directly used in the synthesis of alcohols which can exhibit great potential in the practical synthesis of pharmaceuticals. The application of Ni-NHC complexes in the hydrogenation of ketones is still limited. In a pursuit to study the effect of Ni-NHC-based complexes in the reactivity towards hydrogenation, we have studied the catalytic efficiency of a pendent-type nickel complex [Ni(NHC)2](PF6)2 constructed from a benzimidazole moiety. The hydrogenation of 2-acetylpyridine was studied with respect to catalyst loading, reaction temperature, reaction time, and solvent medium. The complex was broadly characterized by X-ray crystallography, ESI-MS, NMR, UV-Vis, and IR spectral studies.

Mixed-valent palladium(IV/II)-oxoanion, [PdIVO6PdII6((CH3)2AsO2)6]2

We report on the first example of a Pd^IV-containing polyoxopalladate(II). The discrete mixed-valent polyoxopalladate(IV/II), [Pd^IVPdII6O₆((CH₃)₂AsO₂)₆]^2-, comprising a central Pd^IV ion that is surrounded by a six-membered Pd^II-oxo ring capped by six dimethylarsinate groups, was synthesized and structurally characterized in the solid state, in solution and in the gas phase by multiple analytical techniques.

Sila-spirocyclization involving unstrained C(sp3)-Si bond cleavage

C - Si Bond cleavage is one of the key elemental steps for a wide variety of silicon-based transformations. However, the cleavage of unstrained Si-C(sp3) bonds catalyzed by transition metal are still in their infancy. They generally involve the insertion of a M - C(sp2) species into the C - Si bond and consequent intramolecular C(sp2)‒Si coupling to exclusively produce siloles. Here we report the Pd-catalyzed sila-spirocyclization, in which the Si-C(sp3) bond is activated by the insertion of a M - C(sp3) species and followed by the formation of a new C(sp3)‒Si bond, allowing the construction of diverse spirosilacycles. This reactivity mode, which is strongly supported by DFT calculations may open an avenue for the Si-C(sp3) bond cleavage and silacycle synthesis.

Ligand-Enabled β-C(sp3 )-H Lactamization of Tosyl-Protected Aliphatic Amides Using a Practical Oxidant

The development of C(sp3 )-H functionalization reactions that use common protecting groups and practical oxidants remains a significant challenge. Herein we report a monoprotected aminoethyl thioether (MPAThio) ligand-enabled β-C(sp3 )-H lactamization of tosyl-protected aliphatic amides using tert-butyl hydrogen peroxide (TBHP) as the sole oxidant. This protocol features exceedingly mild reaction conditions, reliable scalability, and the use of practical oxidants and protecting groups. Further derivatization of the β-lactam products enables the synthesis of a range of biologically important motifs including β-amino acids, γ-amino alcohols, and azetidines.

Migrative Carbofluorination of Saturated Amides Enabled by Pd-Based Dyotropic Rearrangement

Directly editing an all-carbon quaternary carbon itself of nonstrained acyclic molecules remains underexploited despite the recent advances in the fields of both C-H and C-C bond activation. Herein, we report a palladium-catalyzed migrative carbofluorination of saturated amides enabled by the activation of both the C(sp³)-H and the C_quaternary-Cσ bonds. In this transformation, the α-quaternary carbon of Weinreb amides is converted to α-tertiary fluoride with concurrent migration of an aryl or an amido group from the α- to β-carbon. DFT calculations indicate that the dyotropic rearrangement proceeds through an unusual anti-selective [2.1.0] bicyclic transition state. The reaction, compatible with a broad range of functional groups, is stereospecific and is applicable to the synthesis of enantioenriched products.

Deracemization of Binaphthyl by Suzuki Diarylation: The Role of Electronic and Steric Effects

We report a Suzuki 2,2'-diarylation of the racemic 2,2'-diiodo-1,1'-binaphthyl which proceeds with deracemization via a pallada(IV)cyclic intermediate, induced by a simple chiral ligand─BINAP [2,2'-bis(diphenylphosphino)-1,1',-binaphthyl]. A systematic study of the reaction scope, using 45 arylboronic acids, reveals that the diarylated product is formed when meta- and/or para-substituted phenylboronic acids are functionalized with a substituent with the Hammett constant from -0.5 to +0.4. Multiparametric analysis accounting for the effect of geometry on the reactivity using Boltzmann-weighted Sterimol parameters and electronic effects described by Hammett descriptors shows that the enantioselectivity depends on steric effects only, with enhanced enantioselectivity observed for substituents with a larger length, wL, and reduced for substituents with a larger maximum width, wB₅. We show that careful tuning of these parameters, with the aid of the presented mathematical model, can lead to excellent enantioselectivity. Additional factors that are investigated and found to affect the stereoselective course of the reaction include the reaction temperature, palladium source, palladium to ligand ratio, and the type of boronic acid derivative. During the chromatographic separation of diarylated products on an achiral silica gel, we observed a rare phenomenon: the diarylated products undergo self-disproportionation of enantiomers, with the major enantiomer being eluted first.

Modular Synthesis of Benzocyclobutenes via Pd(II)-Catalyzed Oxidative [2+2] Annulation of Arylboronic Acids with Alkenes

Benzocyclobutenes (BCBs) are highly valuable compounds in organic synthesis, medicinal chemistry, and materials science. However, catalytic modular synthesis of functionalized BCBs from easily accessible starting materials remains limited. We report herein an efficient synthesis of diversely functionalized BCBs by a Pd(II)-catalyzed formal [2+2] annulation between arylboronic acids and alkenes in the presence of N-fluorobenzenesulfonimide (NFSI). An intermolecular carbopalladation followed by palladium oxidation, intramolecular C(sp²)-H activation by a transient C(sp³)-Pd(IV) species, and selective carbon-carbon (C-C) bond-forming reductive elimination from a high-valent five-membered palladacycle is proposed to account for the reaction outcome. Kinetically competent oxidation of alkylPd(II) to alkylPd(IV) species is important to avoid the formation of a Heck adduct. The reaction forges two C-C bonds of the cyclobutene core and is compatible with a wide range of functional groups. No chelating bidentate directing group in the alkene part is needed for this transformation.

Palladium-Catalyzed Three-Component 1,4-Alkoxyarylation Reaction of [60]Fullerene

The palladium-catalyzed three-component alkoxyarylation reaction of [60]fullerene with primary/secondary alcohols and aryl iodides generates a series of 1,4-(alkoxy)(aryl)[60]fullerene derivatives. Plausible reaction pathways for the formation of 1,4-(alkoxy)(aryl)[60]fullerenes are proposed. In addition, the electrochemical properties of the synthesized 1,4-alkoxyarylation adducts are investigated.

Palladium-catalyzed three-component 1,4-aminoarylation of [60]fullerene with aryl iodides and N-methoxysulfonamides, and further transformations

The palladium-catalyzed three-component 1,4-aminoarylation of [60]fullerene afforded 1,4-(aryl)(sulfonamide)[60]fullerenes, of which the sulfonamide group could be replaced by a (hetero)aryl, malonate ester or allyl group in the presence of FeCl3.

The continuum of carbon-hydrogen (C-H) activation mechanisms and terminology

As a rapidly growing field across all areas of chemistry, C-H activation/functionalisation is being used to access a wide range of important molecular targets. Of particular interest is the development of a sustainable methodology for alkane functionalisation as a means for reducing hydrocarbon emissions. This Perspective aims to give an outline to the community with respect to commonly used terminology in C-H activation, as well as the mechanisms that are currently understood to operate for (cyclo)alkane activation/functionalisation.

Pd-Catalyzed Indole Synthesis via C-H Activation and Bisamination Sequence with Diaziridinone

This work describes an efficient Pd-catalyzed indole synthesis. A wide variety of indoles can be obtained in good yields from readily available vinyl bromides. The reaction likely proceeds through a sequential aryl C-H activation and bisamination of a resulting pallada(II)cycle with diaziridinone.

Pd-Catalyzed Alkene Diamination Reactions with O-Benzoylhydroxylamine Electrophiles: Evidence Supporting a Pd(II/IV) Catalytic Cycle, the Role of 2,4-Pentanedione Derivatives as Ligands, and Expanded Substrate Scope

This article describes continued studies on Pd-catalyzed alkene diamination reactions between N-allylguanidines or ureas and O-benzoylhydroxylamine derivatives, which serve as N-centered electrophiles. The transformations generate cyclic guanidines and ureas bearing dialkylaminomethyl groups in moderate to good yield. We describe new mechanistic experiments that have led to a revised mechanistic hypothesis that involves a key oxidative addition of the electrophile to a Pd^II complex, followed by reductive elimination from Pd^IV to form the alkyl carbon-nitrogen bond. In addition, we demonstrate that acac, not phosphine, serves as a key ligand for palladium. Moreover, simple acac derivatives bearing substituted aryl groups outperform acac in the catalytic reactions, and phosphines inhibit catalysis in many cases. These discoveries have led to a significant expansion in the scope of this chemistry, which now allows for the coupling of a variety of cyclic amines, acyclic secondary amines, and primary amines. In addition, we also demonstrate that these new conditions allow for the use of amide nucleophiles, in addition to guanidines and ureas.

Complexes of Platinum Group Metals with a Conformationally Locked Scorpionate in a Metal-Organic Framework: An Unusually Close Apical Interaction of Palladium(II)

We report synthetic strategies for installing platinum group metals (PGMs: Pd, Rh, Ir, and Pt) on a scorpionate-derived linker (TpmC*) within a metal-organic framework (MOF), both by room-temperature postsynthetic metalation and by direct solvothermal synthesis, with a wide range of metal loadings relevant for fundamental studies and catalysis. In-depth studies for the palladium adduct Pd(II)@Zr-TpmC* by density-functional-theory-assisted extended X-ray absorption fine structure spectroscopy reveals that the rigid MOF lattice enforces a close Pd(II)-N_apical interaction between the bidentate palladium complex and the third uncoordinated pyrazole arm of the TpmC* ligand (Pd-N_apical = 2.501 ± 0.067 Å), an interaction that is wholly avoided in molecular palladium scorpionates.