Generating Active "L-Pd(0)" via Neutral or Cationic π-Allylpalladium Complexes Featuring Biaryl/Bipyrazolylphosphines: Synthetic, Mechanistic, and Structure-Activity Studies in Challenging Cross-Coupling Reactions

Palladium-Catalyzed Tsuji-Trost-Type Reaction of 3-Indolylmethylacetates with O, and S Soft Nucleophiles

The chemical valorization of widespread molecules in renewable sources is a field of research widely investigated in the last decades. In this context, we envisaged that indole-3-carbinol, present in different Cruciferae plants, could be a readily available building block for the synthesis of various classes of indoles through a palladium-catalyzed Tsuji-Trost-type reaction with O and S soft nucleophiles. The regiochemical outcome of this high-yielding functionalization shows that the nucleophilic substitution occurs only at the benzylic position. Interestingly, with this protocol, the sulfonyl unit could be appended to the indole nucleus, providing convenient access to new classes of molecules with potential bioactivity.

Synthesis and catalytic properties of palladium(II) complexes with P,π-chelating ferrocene phosphinoallyl ligands and their non-tethered analogues

Hybrid phosphines usually combine a phosphine moiety with another heteroatom secondary donor group in their structures while compounds equipped with hydrocarbyl π-donor moieties remain uncommon. This contribution reports the synthesis and structural characterization of the first P/π-allyl-chelating complexes that were obtained using the structurally flexible and redox-active ferrocene unit as the scaffold, viz. [PdCl(R2PfcCHCHCH2-η3:κP)] (1R; R = Ph and cyclohexyl (Cy); fc = ferrocene-1,1'-diyl). These compounds were synthesized from the respective phosphinoferrocene carboxaldehydes R2PfcCHO via reaction with vinylmagnesium bromide to generate 1-(phosphinoferrocenyl)allyl alcohols, which were subsequently acetylated. The resulting allyl acetates reacted smoothly with [Pd2(dba)3]/[Et3NH]Cl (dba = dibenzylideneacetone) to produce the target compounds. Complexes 1R and their nontethered analogues [PdCl(η3-C3H5)(FcPR2-κP)] (5R; Fc = ferrocenyl) were evaluated as pre-catalysts for the Pd-catalysed allylic amination of cinnamyl acetate with aliphatic amines and Suzuki-Miyaura-type cross-coupling of 4-tolylboronic acid with benzoyl chloride. In these reactions, better results were achieved with compounds 5R (particularly with 5Ph), presumably because they form more stable LPd(0)-type catalysts.

Catalytic 2-Ethylhexanoic Acid Promotes Mild Miyaura Borylations

The Miyaura borylation of aryl and heteroaryl chlorides and bromides using a combination of potassium carbonate and 5 mol % 2-ethylhexanoic acid at 25 °C is reported. The in situ generation of a catalytic amount of potassium 2-ethylhexanoate under these conditions avoids the need for special handling of stoichiometric quantities of hygroscopic potassium 2-ethylhexanoate during the reaction setup as well as difficulties in removing the resulting carboxylic acid during product isolation.

Aqueous Micelles as Solvent, Ligand, and Reaction Promoter in Catalysis

Water is considered to be the most sustainable and safest solvent. Micellar catalysis is a significant contributor to the chemistry in water. It promotes pathways involving water-sensitive intermediates and transient catalytic species under micelles' shielding effect while also replacing costly ligands and dipolar-aprotic solvents. However, there is a lack of critical information about micellar catalysis. This includes why it works better than traditional catalysis in organic solvents, why specific rules in micellar catalysis differ from those of conventional catalysis, and how the limitations of micellar catalysis can be addressed in the future. This Perspective aims to highlight the current gaps in our understanding of micellar catalysis and provide an analysis of designer surfactants' origin and essential components. This will also provide a fundamental understanding of micellar catalysis, including how aqueous micelles can simultaneously perform multiple functions such as solvent, ligand, and reaction promoter.

Ligated Pd-Catalyzed Aminations of Aryl/Heteroaryl Halides with Aliphatic Amines under Sustainable Aqueous Micellar Conditions

Sustainable technology for constructing Pd-catalyzed C-N bonds involving aliphatic amines is reported. A catalytic system that relies on low levels of recyclable precious metal, a known and commercially available ligand, and a recyclable aqueous medium are combined, leading to a newly developed procedure. This new technology can be used in ocean water with equal effectiveness. Applications involving highly challenging reaction partners constituting late-stage functionalization are documented, as is a short but efficient synthesis of the drug naftopidil. Comparisons with existing aminations highlight the many advances being offered.

Enhancing Tsuji-Trost deallylation in living cells with an internal-nucleophile coumarin-based probe

In recent years, bioorthogonal uncaging reactions have been developed to proceed efficiently under physiological conditions. However, limited progress has been made in the development of protecting groups combining stability under physiological settings with the ability to be quickly removed via bioorthogonal catalysis. Herein, we present a new water-soluble coumarin-derived probe bearing an internal nucleophilic group capable of promoting Tsuji-Trost deallylation under palladium catalysis. This probe can be cleaved by a bioorthogonal palladium complex at a faster rate than the traditional probe, namely N-Alloc-7-amino-4-methylcoumarin. As the deallylation process proved to be efficient in mammalian cells, we envision that this probe may find applications in chemical biology, bioengineering, and medicine.

Nanoparticles as Heterogeneous Catalysts for ppm Pd-Catalyzed Aminations in Water

A general protocol employing heterogeneous catalysis has been developed that enables ppm of Pd-catalyzed C-N cross-coupling reactions under aqueous micellar catalysis. A new nanoparticle catalyst containing specifically ligated Pd, in combination with nanoreactors composed of the designer surfactant Savie, a biodegradable amphiphile, catalyzes C-N bond formations in recyclable water. A variety of coupling partners, ranging from highly functionalized pharmaceutically relevant APIs to educts from the Merck Informer Library, readily participate under these environmentally responsible, sustainable reaction conditions. Other key features associated with this report include the low levels of residual Pd found in the products, the recyclability of the aqueous reaction medium, the use of ocean water as an alternative source of reaction medium, options for the use of pseudohalides as alternative reaction partners, and associated low E factors. In addition, an unprecedented 5-step, one-pot sequence is presented, featuring several of the most widely used transformations in the pharmaceutical industry, suggesting potential industrial applications.

Chiral, air stable, and reliable Pd(0) precatalysts applicable to asymmetric allylic alkylation chemistry

Stereoselective carbon-carbon bond formation via palladium-catalyzed asymmetric allylic alkylation is a crucial strategy to access chiral natural products and active pharmaceutical ingredients. However, catalysts based on the privileged Trost and Pfaltz-Helmchen-Williams PHOX ligands often require high loadings, specific preactivation protocols, and excess chiral ligand. This makes these reactions uneconomical, often unreproducible, and thus unsustainable. Here we report several chiral single-component Pd(0) precatalysts that are active and practically-applicable in a variety of asymmetric allylic alkylation reactions. Despite the decades-long history and widespread use of Trost-type ligands, the precatalysts in this work are the only reported examples of stable, isolable Pd(0) complexes with these ligands. Evaluating these precatalysts across nine asymmetric allylic alkylation reactions reveals high reactivity and selectivity at low Pd loading. Importantly, we also report an unprecedented Pd-catalyzed enantioselective allylation of a hydantoin, achieved on gram scale in high yield and enantioselectivity with only 0.2 mol% catalyst.

A machine-learning tool to predict substrate-adaptive conditions for Pd-catalyzed C-N couplings

Machine-learning methods have great potential to accelerate the identification of reaction conditions for chemical transformations. A tool that gives substrate-adaptive conditions for palladium (Pd)-catalyzed carbon-nitrogen (C-N) couplings is presented. The design and construction of this tool required the generation of an experimental dataset that explores a diverse network of reactant pairings across a set of reaction conditions. A large scope of C-N couplings was actively learned by neural network models by using a systematic process to design experiments. The models showed good performance in experimental validation: Ten products were isolated in more than 85% yield from a range of couplings with out-of-sample reactants designed to challenge the models. Importantly, the developed workflow continually improves the prediction capability of the tool as the corpus of data grows.

Synthesis of 2-Cyanomethyl Indane Derivatives via Pd-Catalyzed Alkene Difunctionalization Reactions of Alkyl Nitriles

The synthesis of indanes bearing substituted cyanomethyl groups at C2 is achieved through Pd-catalyzed coupling reactions between 2-allylphenyl triflate derivatives and alkyl nitriles. Related partially saturated analogues were generated from analogous transformations of alkenyl triflates. The use of a preformed BrettPhosPd(allyl)(Cl) complex as a precatalyst was essential for the success of these reactions.

Synthesis, characterization, and reactivity of [Pd(phosphine)(py)Cl2] (PEPPSI) and [Pd(phosphine)Cl2]2 complexes

The synthesis of novel phosphine palladium PEPPSI and dimer complexes bearing RuPhos, SPhos and XPhos phosphines is reported. The crystal structures of XPhos Pd PEPPSI with pyridine, SPhos Pd PEPPSI with 3-chloropyridine as throw-away ligands and the RuPhos palladium dimer were obtained and compared with previously reported congeners. The catalytic activity of these novel complexes was examined via a C-N coupling reaction involving 4-chloroanisole and morpholine. RuPhos complex 2b proved most active, leading to 97% yield with a low (0.2 mol%) catalyst loading, while phosphine palladium dimers showed significantly lower catalytic activity. However, the addition of 3-pentanone as an activator/stabilizer significantly improved the yields of phosphine dimers and PEPPSI complexes when the reactions were performed in THF.

Aqueous micellar technology: an alternative beyond organic solvents

Solvents are the major source of chemical waste from synthetic chemistry labs. Growing attention to more environmentally friendly sustainable processes demands novel technologies to substitute toxic or hazardous solvents. If not always, sometimes, water can be a suitable substitute for organic solvents, if used appropriately. However, the sole use of water as a solvent remains non-practical due to its incompatibility with organic reagents. Nonetheless, over the past few years, new additives have been disclosed to achieve chemistry in water that also include aqueous micelles as nanoreactors. Although one cannot claim micellar catalysis to be a greener technology for every single transformation, it remains the sustainable or greener alternative for many reactions. Literature precedents support that micellar technology has much more potential than just as a reaction medium, i.e., the role of the amphiphile as a ligand obviating phosphine ligands in catalysis, the shielding effect of micelles to protect water-sensitive reaction intermediates in catalysis, and the compartmentalization effect. While compiling the powerful impact of micellar catalysis, this article highlights two diverse recent technologies: (i) the design and employment of the surfactant PS-750-M in selective catalysis; (ii) the use of the semisynthetic HPMC polymer to enable ultrafast reactions in water.

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.

Introducing Savie: A Biodegradable Surfactant Enabling Chemo- and Biocatalysis and Related Reactions in Recyclable Water

Savie is a biodegradable surfactant derived from vitamin E and polysarcosine (PSar) developed for use in organic synthesis in recyclable water. This includes homogeneous catalysis (including examples employing only ppm levels of catalyst), heterogeneous catalysis, and biocatalytic transformations, including a multistep chemoenzymatic sequence. Use of Savie frequently leads to significantly higher yields than do conventional surfactants, while obviating the need for waste-generating organic solvents.

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.

Formation of exceptional monomeric YPhos-PdCl2 complexes with high activities in coupling reactions

The use of well-defined palladium(ii) complexes as precatalysts for C-X cross-coupling reactions has improved the use of palladium catalysts in organic synthesis including large-scale processes. Whereas sophisticated Pd(ii) precursors have been developed in the past years to facilitate catalyst activation as well as the handling of systems with more advanced monophosphine ligands, we herein report that simple PdCl₂ complexes function as efficient precatalysts for ylide-substituted phosphines (YPhos). These complexes are readily synthesized from PdCl₂ sources and form unprecedented monomeric PdCl₂ complexes without the need for any additional coligand. Instead, these structures are stabilized through a unique bonding motif, in which the YPhos ligands bind to the metal through the adjacent phosphine and ylidic carbon site. DFT calculations showed that these bonds are both dative interactions with the stronger interaction originating from the electron-rich phosphine donor. This bonding mode leads to a remarkable stability even towards air and moisture. Nonetheless, the complexes readily form monoligated LPd(0) complexes and thus the active palladium(0) species in coupling reactions. Accordingly, the YPhos-PdCl₂ complexes serve as highly efficient precatalysts for a series of C-C and C-X coupling reactions. Despite their simplicity they can compete with the efficiency of more complex and less stable precatalysts.

LPdCl2(amine) complexes supported by terphenyl phosphanes: applications in aryl amination reactions

Despite the excellent catalytic properties display by NHC-Pd-PEPPSI complexes in cross-coupling, phosphane analogs have been barely screened. In this work, we report the synthesis and characterization of a series of LPdCl₂(amine) complexes bearing dialkylterphenyl phosphanes (PR₂Ar') and pyridine or morpholine ligands. The novel compounds have been tested as precatalysts in aryl amination reactions. The complex [(PCyp₂Ar^Xyl2)PdCl₂(morpholine)] shows the best catalytic activity allowing the room-temperature coupling of aryl bromides and chlorides with aniline.

trans-Dichlorobis(XPhos)palladium(II) Precatalyst for Suzuki-Miyaura Cross-Coupling Reactions of Aryl/Vinyl Sulfonates/Halides: Scope, Mechanistic Study, and Synthetic Applications

Suzuki-Miyaura cross-coupling reactions of aryl/vinyl sulfonates/halides with various boron species were performed using an easily available trans-dichlorobis(XPhos)palladium(II) precatalyst. Under microwave assistance, more than 30 coupling products were obtained with yields ranging from 23 to 99%, including the synthesis of two bioactive compounds, dubamine and tamoxifen. A mechanistic investigation of the Suzuki-Miyaura reaction was conducted notably by nuclear magnetic resonance (NMR) and high-resolution mass spectroscopy, revealing the nature of the active Pd⁰ species and of the reducing entity.

Heterogeneous Metal-Organic Framework Catalysts for Suzuki-Miyaura Cross Coupling in the Pharma Industry

The synthesis of drug substances (DS) requires the continuous effort of the pharma industry to ensure high sustainability standards. The Suzuki-Miyaura cross coupling is a fundamental C-C bond-forming reaction to produce complex DS intermediates. The present contribution points out the way in which the synthesis of DS intermediates by C-C cross coupling can be economically competitive, while minimizing waste by selecting the appropriate heterogeneous catalyst. By comparing homogeneous, immobilized heterogeneous catalysts on silica and metal-organic framework (MOF) catalysts, while considering the perspectives of academia and industry, the critical parameters for a successful industrial application of heterogeneous catalytic Suzuki-Miyaura cross coupling reactions were identified. Heterogeneous catalysts, such as MOFs, may provide a complementary platform for reducing waste and the costs of production related to such transformations.

Shielding Effect of Nanomicelles: Stable and Catalytically Active Oxidizable Pd(0) Nanoparticle Catalyst Compatible for Cross-Couplings of Water-Sensitive Acid Chlorides in Water

Under the shielding effect of nanomicelles, a sustainable micellar technology for the design and convenient synthesis of ligand-free oxidizable ultrasmall Pd(0) nanoparticles (NPs) and their subsequent catalytic exploration for couplings of water-sensitive acid chlorides in water is reported. A proline-derived amphiphile, PS-750-M, plays a crucial role in stabilizing these NPs, preventing their aggregation and oxidation state changes. These NPs were characterized using ¹³C nuclear magnetic resonance (NMR), infrared (IR), and surface-enhanced Raman scattering (SERS) spectroscopy to evaluate the carbonyl interactions of PS-750-M with Pd. High-resolution transmission electron microscopy (HRTEM) and energy-dispersive X-ray spectroscopy (EDX) studies were performed to reveal the morphology, particle size distribution, and chemical composition, whereas X-ray photoelectron spectroscopy (XPS) measurements unveiled the oxidation state of the metal. In the cross-couplings of water-sensitive acid chlorides with boronic acids, the micelle's shielding effect and boronic acids plays a vital role in preventing unwanted side reactions, including the hydrolysis of acid chlorides under basic pH. This approach is scalable and the applications are showcased in multigram scale reactions.

Mechanistic Aspects of the Palladium-Catalyzed Suzuki-Miyaura Cross-Coupling Reaction

The story of C-C bond formation includes several reactions, and surely Suzuki-Miyaura is among the most outstanding ones. Herein, a brief historical overview of insights regarding the reaction mechanism is provided. In particular, the formation of the catalytically active species is probably the main concern, thus the preactivation is in competition with, or even assumes the role of the rate determining step (rds) of the overall reaction. Computational chemistry is key in identifying the rds and thus leading to milder conditions on an experimental level by means of predictive catalysis.

Recent advances in the synthesis and derivatization of N-heterocyclic carbene metal complexes

N-heterocyclic carbene (NHC) metal complexes have gained an incredible amount of attention in the course of the last two decades and have become indispensable as an intricate part of a plethora of applications. The areas of their synthesis and derivatization are constantly evolving and bring new, more sustainable, cost-effective and simpler approaches to the design of existing and next generation catalysts and materials. This article provides an overview of the latest developments, focusing on those which have appeared during the last two years.

Conversion of Pd(I) off-cycle species into highly efficient cross-coupling catalysts

We report on the facile conversion of [Pd2(μ-Cl)(μ-η3-R-allyl)(NHC)2] complexes, which are commonly considered undesirable off-cycle species in cross-coupling reactions, into active [PdCl(μ-Cl)(NHC)]2 pre-catalysts. All reactions proceed under mild conditions (40 °C, 1-2 hours in acetone) using inexpensive HCl as both an oxidant and chloride source. DFT calculations were performed to explore the possible mechanism of this transformation, which appears to involve a combination of two different pathways. Moreover this study provides insights into factors favoring and hindering Pd(i) dimer formation undesirable in catalysis.

Crystal structure of the RuPhos ligand

Palladium 2-di-cyclo-hexyl-phosphanyl-2',6'-diisopropoxybiphenyl (Pd-RuPhos) catalysts demonstrate high catalytic activity for Negishi cross-couplings of sterically hindered aryl halides, for Suzuki-Miyaura cross-couplings of tosyl-ated olefins, and for Buchwald-Hartwig amination of sterically hindered amines. The solid-state structure of the free RuPhos ligand, C30H43O2P, is reported herein for the first time. RuPhos crystallizes in a triclinic cell containing two independent mol-ecules of the phosphine without any lattice solvent. Pertinent bond metrics and comparisons to other phosphine ligands are presented. The structure of RuPhos will be of assistance in the use of this ligand in the design of cross-coupling catalysts.

A General and Selective Synthesis of Methylmonochlorosilanes from Di-, Tri-, and Tetrachlorosilanes

Direct catalytic transformation of chlorosilanes into organosilicon compounds remains challenging due to difficulty in cleaving the strong Si-Cl bond(s). We herein report the palladium-catalyzed cross-coupling reaction of chlorosilanes with organoaluminum reagents. A combination of [Pd(C₃H₅)Cl]₂ and DavePhos ligand catalyzed the selective methylation of various dichlorosilanes 1, trichlorosilanes 5, and tetrachlorosilane 6 to give the corresponding monochlorosilanes.

Pairing Suzuki–Miyaura cross-coupling and catalyst transfer polymerization

Borylation strategies to make AB Suzuki–Miyaura monomers for use in catalyst-transfer polymerization with nickel or palladium catalysts.

Improvement in the Palladium-Catalyzed Miyaura Borylation Reaction by Optimization of the Base: Scope and Mechanistic Study

Aryl boronic acids and esters are important building blocks in API synthesis. The palladium-catalyzed Suzuki-Miyaura borylation is the most common method for their preparation. This paper describes an improvement of the current reaction conditions. By using lipophilic bases such as potassium 2-ethyl hexanoate, the borylation reaction could be achieved at 35 °C in less than 2 h with very low palladium loading (0.5 mol %). A preliminary mechanistic study shows a hitherto unrecognized inhibitory effect by the carboxylate anion on the catalytic cycle, whereas 2-ethyl hexanoate minimizes this inhibitory effect. This improved methodology enables borylation of a wide range of substrates under mild conditions.

Palladium-catalyzed Tsuji-Trost-type reaction of benzofuran-2-ylmethyl acetates with nucleophiles

The palladium-catalyzed benzylic-like nucleophilic substitution of benzofuran-2-ylmethyl acetate with N, S, O and C soft nucleophiles has been investigated. The success of the reaction is dramatically influenced by the choice of catalytic system: with nitrogen based nucleophiles the reaction works well with Pd2(dba)3/dppf, while with sulfur, oxygen and carbo-nucleophiles [Pd(η3-C3H5)Cl]2/XPhos is more efficient. The regiochemical outcome shows that the nucleophilic substitution occurs only on the benzylic position of the η3-(benzofuryl)methyl complex. The high to excellent yields and the simplicity of the experimental procedure make this protocol a versatile synthetic tool for the preparation of 2-substituted benzo[b]furans.

Differences in the Performance of Allyl Based Palladium Precatalysts for Suzuki-Miyaura Reactions

Palladium(II) precatalysts are used extensively to facilitate cross-coupling reactions because they are bench stable and give high activity. As a result, precatalysts such as Buchwald's palladacycles, Organ's PEPPSI species, Nolan's allyl-based complexes, and Yale's 1-tert-butylindenyl containing complexes, are all commercially available. Comparing the performance of the different classes of precatalysts is challenging because they are typically used under different conditions, in part because they are reduced to the active species via different pathways. However, within a particular class of precatalyst, it is easier to compare performance because they activate via similar pathways and are used under the same conditions. Here, we evaluate the activity of different allyl-based precatalysts, such as (η³-allyl)PdCl(L), (η³-crotyl)PdCl(L), (η³-cinnamyl)PdCl(L), and (η³-1-tert-butylindenyl)PdCl(L) in Suzuki-Miyaura reactions. Specifically, we evaluate precatalyst performance as the ancillary ligand (NHC or phosphine), reaction conditions, and substrates are varied. In some cases, we connect relative activity to both the mechanism of activation and the prevalence of the formation of inactive palladium(I) dimers. Additionally, we compare the performance of in situ generated precatalysts with commonly used palladium sources such as tris(dibenzylideneacetone)dipalladium(0) (Pd₂dba₃), bis(acetonitrile)dichloropalladium(II) (Pd(CH₃CN)₂Cl₂), and palladium acetate. Our results provide information about which precatalyst to use under different conditions.

An Efficient Palladium-Catalyzed α-Arylation of Acetone Below its Boiling Point

The monoarylation of acetone is a powerful transformation, but is typically performed at temperatures significantly in excess of its boiling point. Conditions described for performing the reaction at ambient temperatures led to significant dehalogenation when applied to a complex aryl halide. We describe our attempts to overcome both issues in the context of our drug-discovery program.

Palladium Complexes Based on Ylide-Functionalized Phosphines (YPhos): Broadly Applicable High-Performance Precatalysts for the Amination of Aryl Halides at Room Temperature

Palladium allyl, cinnamyl, and indenyl complexes with the ylide-substituted phosphines Cy3 P+ -C- (R)PCy2 (with R=Me (L1) or Ph (L2)) and Cy3 P+ -C- (Me)PtBu2 (L3) were prepared and applied as defined precatalysts in C-N coupling reactions. The complexes are highly active in the amination of 4-chlorotoluene with a series of different amines. Higher yields were observed with the precatalysts in comparison to the in situ generated catalysts. Changes in the ligand structures allowed for improved selectivities by shutting down β-hydride elimination or diarylation reactions. Particularly, the complexes based on L2 (joYPhos) revealed to be universal precatalysts for various amines and aryl halides. Full conversions to the desired products are reached mostly within 1 h reaction time at room temperature, thus making L2 to one of the most efficient ligands in C-N coupling reactions. The applicability of the catalysts was demonstrated for aryl chlorides, bromides and iodides together with primary and secondary aryl and alkyl amines, including gram-scale applications also with low catalyst loadings of down to 0.05 mol %. Kinetic studies further demonstrated the outstanding activity of the precatalysts with TOF over 10.000 h-1 .

The Quest for the Ideal Base: Rational Design of a Nickel Precatalyst Enables Mild, Homogeneous C-N Cross-Coupling

Palladium-catalyzed amination reactions using soluble organic bases have provided a solution to the many issues associated with heterogeneous reaction conditions. Still, homogeneous C-N cross-coupling approaches cannot yet employ bases as weak and economical as trialkylamines. Furthermore, organic base-mediated methods have not been developed for Ni(0/II) catalysis, despite some advantages of such systems over those employing Pd-based catalysts. We designed a new air-stable and easily prepared Ni(II) precatalyst bearing an electron-deficient bidentate phosphine ligand that enables the cross-coupling of aryl triflates with aryl amines using triethylamine (TEA) as base. The method is tolerant of sterically congested coupling partners, as well as those bearing base- and nucleophile-sensitive functional groups. With the aid of density functional theory (DFT) calculations, we determined that the electron-deficient auxiliary ligands decrease both the pK_a of the Ni-bound amine and the barrier to reductive elimination from the resultant Ni(II)-amido complex. Moreover, we determined that the preclusion of Lewis acid-base complexation between the Ni catalyst and the base, due to steric factors, is important for avoiding catalyst inhibition.