Catalysis with Palladium(I) Dimers

Radicalizing CO by Mononuclear Palladium(I)

A mononuclear, T-shaped palladium(I) d9 metalloradical (3), stabilized by a bulky carbazole-based PNP-ligand, was obtained by reduction of palladium chloride or thermal Pd-C bond homolysis of the corresponding neopentyl complex. Pressurizing with CO gave the Pd(I) carbonyl complex, which was structurally characterized by X-ray diffraction. Delocalization of the unpaired electron to the carbonyl carbon was detected by EPR spectroscopy and theoretically modeled by DFT and ab initio methods. The partially reduced and radicalized CO slowly reacts with di(tert-butyl) disulfide under homolytic S-S cleavage and C-S bond formation to give the corresponding metallathioester.

Stereoselective synthesis of thailandamide A methyl ester

A convergent strategy for the stereoselective synthesis of the methyl ester of the structurally challenging and highly labile antibacterial polyene polyketide natural product thailandamide A has been developed. The key steps include the Zincke aldehyde reaction, Stille cross coupling, Negishi reaction, Julia-Kocienski olefination, cross metathesis, and the less explored Pd(I)-based Heck coupling to access different unsaturation bonds. Additionally, Urpi acetal aldol, Evans methylation, and Crimmins acetate aldol reactions were employed to construct four out of six asymmetric centers of the molecule.

Direct α-Arylation of Benzo[b]furans Catalyzed by a Pd3 Cluster

As an interim paradigm for the catalysts between those based on more conventional mononuclear molecular Pd complexes and Pdn nanoparticles widely used in organic synthesis, polynuclear palladium clusters have attracted great attention for their unique reactivity and electronic properties. However, the development of Pd cluster catalysts for organic transformations and mechanistic investigations is still largely unexploited. Herein, we disclose the use of trinuclear palladium (Pd3Cl) species as an active catalyst for the direct C-H α-arylation of benzo[b]furans with aryl iodides to afford 2-arylbenzofurans in good yields under mild conditions. With this method, broad substrate adaptability was observed, and several drug intermediates were synthesized in high yields. Mechanistic studies indicated that the Pd3 core most likely remained intact throughout the reaction course.

Visible light-induced palladium-carbon bond weakening in catalytically relevant T-shaped complexes

Triggering one-electron redox processes during palladium catalysis holds the potential to unlock new reaction mechanisms and synthetic methods not previously accessible in the typical two-electron reaction manifolds that dominate palladium catalysis. We report that T-shaped organopalladium(ii) complexes coordinated by a bulky monophosphine, a class of organometallic intermediate featured in a range of contemporary catalytic reactions, undergo blue light-promoted bond weakening leading to mild and efficient homolytic cleavage of strong Pd(ii)-C(sp3) bonds under ambient conditions. The origin of light-triggered radical formation in these systems, which lack an obvious ligand-based chromophore (i.e., π-systems), was investigated using a combination of DFT calculations, photoactinometry, and transient absorption spectroscopy. The available data suggest T-shaped organopalladium(ii) complexes manifest unusual blue light-accessible Pd-to-C(sp3) transition. The quantum efficiency and excited state lifetime of this process were unexpectedly superior compared to a prototypical (α-diimine)Pd(ii) complex featuring a low-lying, ligand-centered LUMO (π*). These results suggest coordinatively-unsaturated organopalladium(ii) compounds, catalysts in myriad catalytic processes, have untapped potential for one-electron reactivity under visible light excitation.

Atomic-Scale Elucidation of Unusually Distorted Dimeric Complexes Confined in a Zr-Based Metal-Organic Framework

Nanoconfinement in metal-organic framework (MOF) pores can lead to the isolation of unusual or reactive metal complexes. However, MOFs that support the stabilization and precise structural elucidation of metal complexes and small metal clusters are rare. Here, we report a thermally and chemically stable zirconium-based MOF (University of Adelaide Material-1001, UAM-1001) with a high density of free bis-pyrazolyl units that can confine mono- and dinuclear metal complexes. The precursor MOF, UAM-1000, has a high degree of structural flexibility, but post synthetic modification with a bracing linker, biphenyl-4,4'-dicarboxylic acid, partially rigidifies the MOF (UAM-1001). This allows "matrix isolation" and detailed structural elucidation of postsynthetically added dimeric complexes bound within a tetradentate binding site formed by two linkers. Dimeric species [Co2Cl4], [Cu2Cl4], [Ni2Cl3(H2O)2]Cl, and [Rh2(CO)3Cl2] were successfully isolated in UAM-1001 and characterized by single-crystal X-ray diffraction. Comparison of the UAM-1001 isolated species with similar complexes in the solid state reveals that UAM-1001 can significantly distort the structures and enforce notably shorter metal-metal distances. For example, MOF tethering allows isolation of a [Cu2Cl4] complex that rapidly reacts with water in the solid state. The stability, porosity, and modulated flexibility of UAM-1001 provide an ideal platform material for the isolation and study of new dimeric complexes and their reactivity.

Reversible Single-Electron-Transfer to Oxygen in a Stable N-Heterocyclic Carbene Palladium(I) Metalloradical

The chemical and electrochemical one-electron oxidation of [Pd(IPr)2] (1) leads to the formation of mononuclear palladium(I) complex [Pd(IPr)2][PF6] (2). This bench-stable metalloradical has been fully characterized and its structure confirmed by X-ray diffraction analysis. EPR and DFT studies confirm the localization of the unpaired electron onto the metal center. Low temperature NMR and EPR measurements reveal the ability of complex 2 to reversibly coordinate and reduce the dioxygen molecule, leading to the formation of a three-coordinate complex, [PdII(IPr)2(η1-O2)]+ (4), in which the unpaired electron has been transferred to the superoxido ligand.

Switch in Selectivities by Dinuclear Nickel Catalysis: 1,4-Hydroarylation of 1,3-Dienes to Z-Olefins

One of the most challenging tasks in organic synthesis is to control selectivities, especially switching the well-known selectivity to obtain new isomers that were previously inaccessible. Inspired by biological catalysis involving multiple metal centers, catalysis enabled by binuclear metal complexes offers the potential to induce reactivity and selectivity that might not be available to mononuclear catalysts. Herein, we describe that using a macrocyclic bis pyridyl diimine dinickel complex as the catalyst, the commonly observed 4,3-regioselectivity of hydroarylation of 1,3-dienes is switched to 1,4-hydroarylation with thermodynamically less stable Z-stereoselectivity, offering challenging synthetic target Z-olefins. DFT calculations show that the activation of 1,3-diene proceeds through dinuclear Ni-diolefin coordination, and the synergistic effects of two Ni nuclei enable reactivity and selectivity of this binuclear catalysis substantially different from those of mononuclear nickel complexes in the current reaction.

Palladium(I)-Iodide-Catalyzed Deoxygenative Heck Reaction of Vinyl Triflates: A Formate-Mediated Cross-Electrophile Reductive Coupling with cine-Substitution

The first deoxygenative Heck reactions are described, as illustrated by formate-mediated cine-substitutions of vinyl triflates with aryl iodides. The collective data corroborate a mechanism in which Pd(OAc)2 and Bu4NI form the dianionic iodide-bridged dimer [Pd2I6][NBu4]2, which, under reducing conditions, serves as a precursor to the palladium(I) complex [Pd2I4][NBu4]2. Dinculear oxidative addition of aryl iodide forms [Pd2I5(Ar)][NBu4]2, which dissociates to the monometallic complex [PdI2(Ar)][NBu4]. Vinyl triflate migratory insertion-sulfonate elimination delivers a palladium(IV) carbene, which upon β-hydride elimination/C-H reductive elimination gives the product of cine-substitution. These processes are the first efficient formate-mediated cross-electrophile reductive couplings beyond carbonyl addition.

Mono and Dinuclear Palladium Pincer Complexes of NNSe Ligand as a Catalyst for Decarboxylative Direct C-H Heteroarylation of (Hetero)arenes

This report describes the synthesis of a new NNSe pincer ligand and its mono- and dinuclear palladium(II) pincer complexes. In the absence of a base, a dinuclear palladium pincer complex (C1) was isolated, while in the presence of Et3 N base a mononuclear palladium pincer complex (C2) was obtained. The new ligand and complexes were characterized using techniques like 1 H, 13 C{1 H} nuclear magnetic resonance (NMR), fourier transform infrared (FTIR), high-resolution mass spectrometry (HRMS), ultraviolet-visible (UV-Visible), and cyclic voltammetry. Both the complexes showed pincer coordination mode with a distorted square planar geometry. The complex C1 has two pincer ligands attached through a Pd-Pd bond in a dinuclear pincer fashion. The air and moisture-insensitive, thermally robust palladium pincer complexes were used as the catalyst for decarboxylative direct C-H heteroarylation of (hetero)arenes. Among the complexes, dinuclear pincer complex C1 showed better catalytic activity. A variety of (hetero)arenes were successfully activated (43-87 % yield) using only 2.5 mol % of catalyst loading under mild reaction conditions. The PPh3 and Hg poisoning experiments suggested a homogeneous nature of catalysis. A plausible reaction pathway was proposed for the dinuclear palladium pincer complex catalyzed decarboxylative C-H bond activation reaction of (hetero)arenes.

Palladium-Catalyzed Carbonylative Difunctionalization of Unactivated Alkenes Initiated by Unstabilized Enolates

This report describes the first example of palladium-catalyzed carbonylative difunctionalization of unactivated alkenes initiated by enolate nucleophiles. The approach involves initiation by an unstabilized enolate nucleophile under an atmospheric pressure of CO and termination with a carbon electrophile. This process is compatible with a diverse range of electrophiles, including aryl, heteroaryl, and vinyl iodides to yield synthetically useful 1,5-diketone products, which were demonstrated to be precursors for multi-substituted pyridines. A PdI -dimer complex with two bridging CO units was observed although its role in catalysis is not yet understood.

Synthesis of Nickel(I)-Bromide Complexes via Oxidation and Ligand Displacement: Evaluation of Ligand Effects on Speciation and Reactivity

Nickel's +1 oxidation state has received much interest due to its varied and often enigmatic behavior in increasingly popular catalytic methods. In part, the lack of understanding about NiI results from common synthetic strategies limiting the breadth of complexes that are accessible for mechanistic study and catalyst design. We report an oxidative approach using tribromide salts that allows for the generation of a well-defined precursor, [NiI(COD)Br]2, as well as several new NiI complexes. Included among them are complexes bearing bulky monophosphines, for which structure-speciation relationships are established and catalytic reactivity in a Suzuki-Miyaura coupling (SMC) is investigated. Notably, these routes also allow for the synthesis of well-defined monomeric t-Bubpy-bound NiI complexes, which has not previously been achieved. These complexes, which react with aryl halides, can enable previously challenging mechanistic investigations and present new opportunities for catalysis and synthesis.

New Insights into the Cooperativity and Dynamics of Dimeric Enzymes

A survey of protein databases indicates that the majority of enzymes exist in oligomeric forms, with about half of those found in the UniProt database being homodimeric. Understanding why many enzymes are in their dimeric form is imperative. Recent developments in experimental and computational techniques have allowed for a deeper comprehension of the cooperative interactions between the subunits of dimeric enzymes. This review aims to succinctly summarize these recent advancements by providing an overview of experimental and theoretical methods, as well as an understanding of cooperativity in substrate binding and the molecular mechanisms of cooperative catalysis within homodimeric enzymes. Focus is set upon the beneficial effects of dimerization and cooperative catalysis. These advancements not only provide essential case studies and theoretical support for comprehending dimeric enzyme catalysis but also serve as a foundation for designing highly efficient catalysts, such as dimeric organic catalysts. Moreover, these developments have significant implications for drug design, as exemplified by Paxlovid, which was designed for the homodimeric main protease of SARS-CoV-2.

Dimeric Manganese-Catalyzed Hydroalkenylation of Alkynes with a Versatile Silicon-Based Directing Group

Herein, we present a manganese-catalyzed, branched-selective hydroalkenylation of terminal alkynes, under mild conditions through facile installation of a versatile silanol as a removable directing group. With an alkenyl boronic acid as the coupling partner, this reaction produces stereodefined (E,E)-1,3-dienes with high regio-, chemo- and stereoselectivity. The protocol features mild reaction conditions such as room temperature and an air atmosphere, while maintaining excellent functional group compatibility. The resulting 1,3-dienesilanol products serve as versatile building blocks, as the removal of the silanol group allows for the synthesis of both branched terminal 1,3-dienes for downstream coupling reactions, as well as stereoselective construction of linear (E,E)-1,3-dienes and (E,E,E)- or (E,E,Z)-1,3,5-trienes. In addition, a Diels-Alder cycloaddition can smoothly and selectively deliver silicon-containing pentasubstituted cyclohexene derivatives. Mechanistic investigations, in conjunction with DFT calculations, suggest a bimetallic synergistic activation model to account for the observed enhanced catalytic efficiency and good regioselectivity.

Machine Learning-Guided Development of Trialkylphosphine Ni(I) Dimers and Applications in Site-Selective Catalysis

Owing to the unknown correlation of a metal's ligand and its resulting preferred speciation in terms of oxidation state, geometry, and nuclearity, a rational design of multinuclear catalysts remains challenging. With the goal to accelerate the identification of suitable ligands that form trialkylphosphine-derived dihalogen-bridged Ni^(I) dimers, we herein employed an assumption-based machine learning approach. The workflow offers guidance in ligand space for a desired speciation without (or only minimal) prior experimental data points. We experimentally verified the predictions and synthesized numerous novel Ni^(I) dimers as well as explored their potential in catalysis. We demonstrate C-I selective arylations of polyhalogenated arenes bearing competing C-Br and C-Cl sites in under 5 min at room temperature using 0.2 mol % of the newly developed dimer, [Ni^(I)(μ-Br)PAd₂(n-Bu)]₂, which is so far unmet with alternative dinuclear or mononuclear Ni or Pd catalysts.

Important Role of NH-Carbazole in Aryl Amination Reactions Catalyzed by 2-Aminobiphenyl Palladacycles

2-Aminobiphenyl palladacycles are among the most successful precatalysts for Pd-catalyzed cross-coupling reactions, including aryl amination. However, the role of NH-carbazole, a byproduct of precatalyst activation, remains poorly understood. Herein, the mechanism of the aryl amination reactions catalyzed by a cationic 2-aminobiphenyl palladacycle supported by a terphenyl phosphine ligand, PCyp₂Ar^Xyl2 (Cyp = cyclopentyl; Ar^Xyl2 = 2,6-bis(2,6-dimethylphenyl)phenyl), P1, has been thoroughly investigated. Combining computational and experimental studies, we found that the Pd(II) oxidative addition intermediate reacts with NH-carbazole in the presence of the base (NaO ^t Bu) to yield a stable aryl carbazolyl Pd(II) complex. This species functions as the catalyst resting state, providing the amount of monoligated LPd(0) species required for catalysis and minimizing Pd decomposition. In the case of a reaction with aniline, an equilibrium between the carbazolyl complex and the on-cycle anilido analogue is established, which allows for a fast reaction at room temperature. In contrast, heating is required in a reaction with alkylamines, whose deprotonation involves coordination to the Pd center. A microkinetic model was built combining computational and experimental data to validate the mechanistic proposals. In conclusion, our study shows that despite the rate reduction observed in some reactions by the formation of the aryl carbazolyl Pd(II) complex, this species reduces catalyst decomposition and could be considered an alternative precatalyst in cross-coupling reactions.

Non-directed C-H arylation of electron-deficient arenes by synergistic silver and Pd3 cluster catalysis

Transition-metal clusters have attracted great attention in catalysis due to their unique reactivity and electronic properties, especially for novel substrate binding and activation modes at the bridging coordination sites of metal clusters. Although palladium complexes have demonstrated outstanding catalytic performance in various transformations, the catalytic behaviors of polynuclear palladium clusters in many important synthetic methodologies remain much less explored so far. Herein, we disclose the use of an atomically defined tri-nuclear palladium (Pd₃Cl) species as a catalyst precursor in Ag(I)-assisted direct C-H arylation with aryl iodides under mild conditions. This catalyst system leads to the formation of synthetically important biaryls in good yields with high site selectivities without the assistance of directing groups.

Sustainable and practical formation of carbon-carbon and carbon-heteroatom bonds employing organo-alkali metal reagents

Metal-catalysed cross-coupling reactions are amongst the most widely used methods to directly construct new bonds. In this connection, sustainable and practical protocols, especially transition metal-catalysed cross-coupling reactions, have become the focus in many aspects of synthetic chemistry due to their high efficiency and atom economy. This review summarises recent advances from 2012 to 2022 in the formation of carbon-carbon bonds and carbon-heteroatom bonds by employing organo-alkali metal reagents.

Rapid and Modular Access to Vinyl Cyclopropanes Enabled by Air-stable Palladium(I) Dimer Catalysis

While vinyl cyclopropanes are valuable functional groups in drugs or natural products as well as established precursors to trigger a rich variety of synthetic transformations, their reactive nature can make their installation via direct catalytic approaches challenging. We herein present a modular access to (di)vinyl cyclopropanes under very mild conditions and full conservation of stereochemistry, allowing access to the cis or trans cyclopropane- as well as E or Z vinyl-stereochemical relationships. Our protocol relies on air-stable dinuclear Pd^I catalysis, which enables rapid (<30 min) and selective access to a diverse range of vinyl cyclopropane motifs at room temperature, even on gram scale.

Emerging Trends in Cross-Coupling: Twelve-Electron-Based L1Pd(0) Catalysts, Their Mechanism of Action, and Selected Applications

Monoligated palladium(0) species, L₁Pd(0), have emerged as the most active catalytic species in the cross-coupling cycle. Today, there are methods available to generate the highly active but unstable L₁Pd(0) catalysts from stable precatalysts. While the size of the ligand plays an important role in the formation of L₁Pd(0) during in situ catalysis, the latter can be precisely generated from the precatalyst by various technologies. Computational, kinetic, and experimental studies indicate that all three steps in the catalytic cycle─oxidative addition, transmetalation, and reductive elimination─contain monoligated Pd. The synthesis of precatalysts, their mode of activation, application studies in model systems, as well as in industry are discussed. Ligand parametrization and AI based data science can potentially help predict the facile formation of L₁Pd(0) species.

Total synthesis of the antibacterial polyketide natural product thailandamide lactone

Stereoselective total synthesis of the structurally intriguing polyketide natural product thailandamide lactone was accomplished, and done so using a convergent approach for the first time to the best of our knowledge. The key features of this synthesis included use of a Crimmins acetate aldol reaction, Evans methylation, Urpi acetal aldol reaction, Sharpless asymmetric epoxidation and subsequent γ-lactonization for the installation of six asymmetric centers and the use of the Negishi reaction, Julia-Kocienski olefination, cross metathesis, HWE olefination and intermolecular Heck coupling for construction of a variety of unsaturated linkages. Pd(i)-based Heck coupling was introduced, for the first time to the best of our knowledge, quite efficiently to couple the major eastern and sensitive western segments of the molecule. The antibacterial activity of thailandamide lactone was also evaluated.

Isolation and Characterization of Heteroleptic Mononuclear Palladium(I) Complexes

Catalytic transformations involving Pd(0)/Pd(II) catalytic cycles are very well known, and processes involving high-valent Pd(III) and Pd(IV) and low-valent Pd(I) intermediates have also gained interest in recent years. Although low-valent Pd(I) intermediates are proposed in these catalytic cycles, isolated and characterized mononuclear Pd(I) species are very rare. Herein, we report the isolation of two heteroleptic mononuclear Pd(I) complexes stabilized by dithiapyridinophane ligands that were fully characterized by single-crystal X-ray diffraction; EPR, IR, UV-vis spectroscopies; and computational studies. Excitingly, one of these Pd(I) complexes shows Kumada Csp³-Csp² cross-coupling competency, and initial studies of the other shows direct evidence for Csp³-H bond activation proposed to occur at the Pd(I) center.

Heterolytic carbon-iodine bond cleavage by a palladium(I) metalloradical

The well-defined Pd(I) metalloradical [Pd(PtBu₃)₂]⁺ reacts with aryl and alkyl iodides at room temperature, yielding [Pd(PtBu₃)(μ-I)]₂ and phosphonium salts. Pd(II) aryl/alkyl derivates, reflecting net radical oxidative addition of the substrate to the metalloradical, are generated during the reaction and two examples have been isolated and crystallographically characterised.

Palladium-Catalyzed Three-Component Selective Aminoallylation of Diazo Compounds

We describe a Xantphos-containing dinuclear palladium complex-enabled geminal aminoallylation of diazocarbonyl compounds, which selectively provides a range of quaternary α-amino esters. Direct N-H insertion, allylic alkylation of amino nucleophiles, and diene formation were not observed under standard conditions. Mechanistic studies indicated that a relayed pathway via allylation of the N-H insertion product or [2,3]-sigmatropic rearrangement of an ylide intermediate was unlikely.

Changes in ligand coordination mode induce bimetallic C-C coupling pathways

Carbon-carbon coupling is one of the most powerful tools in the organic synthesis arsenal. Known methodologies primarily exploit monometallic Pd⁰/Pd^II catalytic mechanisms to give new C-C bonds. Bimetallic C-C coupling mechanisms that involve a Pd^I/Pd^II redox cycle, remain underexplored. Thus, a detailed mechnaistic understanding is imperative for the development of new bimetallic catalysts. Previously, a Pd^II-Me dimer (1) supported by L1, which has phosphine and 1-azaallyl donor groups, underwent reductive elimination to give ethane, a Pd^I dimer, a Pd^II monometallic complex, and Pd black. Herein, a comprehensive experimental and computational study of the reactivity of 1 is presented, which reveals that the versatile coordination chemistry of L1 promotes bimetallic C-C bond formation. The phosphine 1-azaallyl ligand adopts various bridging modes to maintain the bimetallic structure throughout the C-C bond forming mechanism, which involves intramolecular methyl transfer and 1,1-reductive elimination from one of the palladium atoms. The minor byproduct, methane, likely forms through a monometallic intermediate that is sensitive to solvent C-H activation. Overall, the capacity of L1 to adopt different coordination modes promotes the bimetallic C-C coupling channel through pathways that are unattainable with statically-coordinated ligands.

Air-Stable PdI Dimer Enabled Remote Functionalization: Access to Fluorinated 1,1-Diaryl Alkanes with Unprecedented Speed

While remote functionalization via chain walking has the potential to enable access to molecules via novel disconnections, such processes require relatively long reaction times and can be in need of elevated temperatures. This work features a remote arylation in less than 10 min reaction time at room temperature over a distance of up to 11 carbons. The unprecedented speed is enabled by the air‐stable Pd^I dimer [Pd(μ‐I)(PCy₂^tBu)]₂, which in contrast to its P^tBu₃ counterpart does not trigger direct coupling at the initiation site, but regioconvergent and chemoselective remote functionalization to yield valuable fluorinated 1,1‐diaryl alkanes. Our combined experimental and computational studies rationalize the origins of switchability, which are primarily due to differences in dispersion interactions.

Towards determining molecular structure with ESI-MS backed by computational methods: structures of subnanoclusters of Pd and Cu chlorides, ion dynamics in vacuum, and challenges to the methodology

Determining most stable structures of sub-nanoscale ionic clusters in ESI-MS spectra with quantum chemical modeling.

Accelerated dinuclear palladium catalyst identification through unsupervised machine learning

[Figure: see text].

On the Importance of Considering Multinuclear Metal Sites in Homogeneous Catalysis Modeling

In this short review, we provide an account of a number of computational studies of catalytic reaction mechanisms carried out in our groups. We focus in particular on studies in which we came to realize during the course of the investigation that the active catalytic species was a bimetallic complex, rather a monometallic one as previously assumed. In some cases, this realization was in part prompted by experimental observations, but careful exploration based on computation of the speciation of the metal precursor also provided a powerful guide: it is often possible to predict that bimetallic species (intermediates or transition states) lie lower in free energy than a priori competitive monometallic species. In this sense, we argue that in organometallic catalysis, the rule whereby “two is better than one” turns out to be relevant much more often than one might expect.

Reactivity of a Dinuclear PdI Complex [Pd2(μ-PPh2)(μ2-OAc)(PPh3)2] with PPh3: Implications for Cross-Coupling Catalysis Using the Ubiquitous Pd(OAc)2/nPPh3 Catalyst System

[Pd^I₂(μ-PPh₂)(μ₂-OAc)(PPh₃)₂] is the reduction product of Pd^II(OAc)₂(PPh₃)₂, generated by reaction of ‘Pd(OAc)₂’ with two equivalents of PPh₃. Here, we report that the reaction of [Pd^I₂(μ-PPh₂)(μ₂-OAc)(PPh₃)₂] with PPh₃ results in a nuanced disproportionation reaction, forming [Pd⁰(PPh₃)₃] and a phosphinito-bridged Pd^I-dinuclear complex, namely [Pd^I₂(μ-PPh₂){κ₂-P,O-μ-P(O)Ph₂}(κ-PPh₃)₂]. The latter complex is proposed to form by abstraction of an oxygen atom from an acetate ligand at Pd. A mechanism for the formal reduction of a putative Pd^II disproportionation species to the observed Pd^I complex is postulated. Upon reaction of the mixture of [Pd⁰(PPh)₃] and [Pd^I₂(μ-PPh₂){κ₂-P,O-μ-P(O)Ph₂}(κ-PPh₃)₂] with 2-bromopyridine, the former Pd⁰ complex undergoes a fast oxidative addition reaction, while the latter dinuclear Pd^I complex converts slowly to a tripalladium cluster, of the type [Pd₃(μ-X)(μ-PPh₂)₂(PPh₃)₃]X, with an overall 4/3 oxidation state per Pd. Our findings reveal complexity associated with the precatalyst activation step for the ubiquitous ‘Pd(OAc)₂’/nPPh₃ catalyst system, with implications for cross-coupling catalysis.

Dirhodium(II)/Xantphos-Catalyzed Relay Carbene Insertion and Allylic Alkylation Process: Reaction Development and Mechanistic Insights

Although dirhodium-catalyzed multicomponent reactions of diazo compounds, nucleophiles and electrophiles have achieved great advance in organic synthesis, the introduction of allylic moiety as the third component via allylic metal intermediate remains a formidable challenge in this area. Herein, an attractive three-component reaction of readily accessible amines, diazo compounds, and allylic compounds enabled by a novel dirhodium(II)/Xantphos catalysis is disclosed, affording various architecturally complex and functionally diverse α-quaternary α-amino acid derivatives in good yields with high atom and step economy. Mechanistic studies indicate that the transformation is achieved through a relay dirhodium(II)-catalyzed carbene insertion and allylic alkylation process, in which the catalytic properties of dirhodium are effectively modified by the coordination with Xantphos, leading to good activity in the catalytic allylic alkylation process.

Selective Synthesis of Z-Silyl Enol Ethers via Ni-Catalyzed Remote Functionalization of Ketones

We report a remote functionalization strategy, which allows the Z-selective synthesis of silyl enol ethers of (hetero)aromatic and aliphatic ketones via Ni-catalyzed chain walking from a distant olefin site. The positional selectivity is controlled by the directionality of the chain walk and is independent of thermodynamic preferences of the resulting silyl enol ether. Our mechanistic data indicate that a Ni^(I) dimer is formed under these conditions, which serves as a catalyst resting state and, upon reaction with an alkyl bromide, is converted to [Ni^(II)-H] as an active chain-walking/functionalization catalyst, ultimately generating a stabilized η³-bound Ni^(II) enolate as the key selectivity-controlling intermediate.

Base-Free Cross-Couplings of Aryl Diazonium Salts in Methanol: PdII -Alkoxy as Reactivity-Controlling Intermediate

Pd-catalyzed cross-coupling reactions of aryl diazonium salts are generally assumed to proceed via cationic Pd^II intermediates which in turn would be highly reactive in the subsequent transmetalation step. Contrary to this belief, we herein report our observation and rationalization of opposing reactivities of ArN₂ ⁺ in Suzuki (=effective) and Stille (=ineffective) cross-couplings in MeOH. Our systematic experimental and computational studies on the roles of transmetalating agent, solvent, base and the likely involvement of in situ formed diazoether derivatives challenge the currently accepted mechanism. Our data suggest that the observed solvent dichotomy is primarily due to Pd^II -methoxy intermediates being formed, which are unreactive with arylstannanes, but highly reactive with arylboronic acids, complementing the Suzuki "Pd-oxy" mechanism with the direct demonstration of transmetalation of a Pd^II -alkoxy complex. Lewis acids were found to circumvent this reactivity divergence, promoting efficient couplings regardless of the employed conditions or coupling partners.

Photoinduced Palladium-Catalyzed Dicarbofunctionalization of Terminal Alkynes

Herein, a conceptually distinct approach was developed that allowed for the dicarbofunctionalization of alkynes at room temperature using simple, bench-stable alkyl iodides and a second molecule of alkyne as coupling partner. Specifically, the photochemical activation of palladium complexes enabled this strategic dicarbofunctionalization via addition of alkyl radicals from secondary and tertiary alkyl iodides and formation of an intermediate palladium vinyl complex that could undergo subsequent Sonogashira reaction with a second alkyne molecule. This alkylation-alkynylation sequence allowed the one-step synthesis of 1,3-enynes including heteroarenes and biologically active compounds with high efficiency without exogenous photosensitizers or oxidants and now opens up pathways towards cascade reactions via photochemical palladium catalysis.

C4-arylation and domino C4-arylation/3,2-carbonyl migration of indoles by tuning Pd catalytic modes: Pd(i)-Pd(ii) catalysis vs. Pd(ii) catalysis

Efficient C4-arylation and domino C4-arylation/3,2-carbonyl migration of indoles have been developed. The former route enables C4-arylation in a highly efficient and mild manner and the latter route provides an alternative straightforward protocol for synthesis of C2/C4 disubstituted indoles. The mechanism studies imply that the different reaction pathways were tuned by the distinct acid additives, which led to either the Pd(i)-Pd(ii) pathway or Pd(ii) catalysis.