Air-Stable Pd(R-allyl)LCl (L= Q-Phos, P(t-Bu)3, etc.) Systems for C–C/N Couplings: Insight into the Structure–Activity Relationship and Catalyst Activation Pathway

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.

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.

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.

Highly efficient α-arylation of aryl ketones with aryl chlorides by using bulky imidazolylidene-ligated oxazoline palladacycles

α-Aryl derivatives of carbonyl compounds are important building blocks. Herein, we presented an efficient catalytic system for the α-arylation of aryl ketones with inactive aryl chlorides by firstly using N,N'-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene (IPr)-ligated chiral oxazoline palladacycles, and tolerated a wide range of substrates at low catalyst loadings, leading to the desired products in good to excellent yields.

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.

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.

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.

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.

Ylide-Substituted Phosphines: A Platform of Strong Donor Ligands for Gold Catalysis and Palladium-Catalyzed Coupling Reactions

The development of homogeneous catalysts is strongly connected to the design of new, sophisticated ligands, which resolve limitations of a given reaction protocol by manipulating the electronic properties of the metal and its spatial environment. Phosphines are a privileged class of ligands that find applications in many catalytic transformations, ranging from hydrogenation reactions to hydroformylation and coupling chemistry. For many years, chemists have been trying to improve the efficiency, selectivity, and application of coupling reactions. The use of highly electron-rich and bulky phosphines was often associated with increased selectivity and efficiency and led to the development of a vast variety of electron-rich alkyl-substituted phosphines. However, this concept of increasing the ligand donor strength reaches its limits with the use of trialkyl-substituted phosphines with tri-tert-butylphosphine thus being one of the most active ligands for many years. In the course of our research efforts to use the special donor strength of ylides to stabilize electron-deficient, low-valent main group compounds, we realized that ylide-substituted phosphine (YPhos) ligands possess remarkably strong donor abilities. Moreover, the YPhos ligands are highly tunable by changing the nature of the groups on the phosphonium, phosphine, or central ylidic carbon atom. We thus obtained a ligand platform with donor capabilities ranging from PCy₃ to even stronger donor abilities than N-heterocyclic carbenes, while being more sterically demanding than simple phosphines as well as many well-known biarylphosphine ligands.These properties led us to explore the applicability of the YPhos ligands in catalysis. In a series of recent reports, our group applied YPhos ligands in gold and palladium catalyzed reactions at catalytic loadings applicable for medium- to large-scale applications. The increased donor strength and unique architecture allowed for remarkable activities in a series of transformations at mild reactions conditions. For gold(I)-catalyzed reactions, we obtained turnover numbers (TONs) for the hydroamination of phenylacetylene with aniline of over 20 000. Also, more complex reactions were easily catalyzed with efficiencies greater than those of other known gold(I) catalysts. Similar efficacies were found in a series of palladium-catalyzed coupling reactions. In Buchwald-Hartwig aminations, unprecedented activities for the amination of aryl chlorides were reached at room temperature. The speed of formation of the catalytically active mono-YPhos palladium species allowed for some of the amination reactions to be completed in only a few minutes. Adjustment of the ligand design enabled the use of a large variety of different aryl and alkyl amines of different steric demands. Furthermore, the YPhos ligands in general showed high activities and selectivity in the coupling of a variety of carbon nucleophiles with aryl chlorides, bromides, and triflates. This enabled the development of efficient reaction protocols for the α-arylation of unhindered ketones and the coupling of Grignard and zinc reagents as well as the first efficient coupling of chloroarenes with alkyllithium compounds. This Account summarizes the recent development of YPhos ligands and their application in gold and palladium catalysis. We also hope to stimulate further use of this ligand platform in catalysis in the future.

Synthesis of Sterically Encumbered Di- and Triarylamines by Palladium-Catalysed C-N Coupling Reactions at Mild Reaction Conditions

The synthesis of bulky, ortho-substituted triarylamines often represents a synthetic challenge, but is highly desirable due to the use of these compounds in organic electronics. Here, we report on a...

Halogen-Bridged Methylnaphthyl Palladium Dimers as Versatile Catalyst Precursors in Coupling Reactions

Halogen‐bridged methylnaphthyl (MeNAP) palladium dimers are presented as multipurpose Pd‐precursors, ideally suited for catalytic method development and preparative organic synthesis. By simply mixing with phosphine or carbene ligands, they are in situ converted into well‐defined monoligated complexes. Their catalytic performance was benchmarked against state‐of‐the‐art systems in challenging Buchwald–Hartwig, Heck, Suzuki and Negishi couplings, and ketone arylations. Their use enabled record‐setting activities, beyond those achievable by optimization of the ligand alone. The MeNAP catalysts permit syntheses of tetra‐ortho‐substituted arenes and bulky anilines in near‐quantitative yields at room temperature, allow mono‐arylations of small ketones, and enable so far elusive cross‐couplings of secondary alkyl boronic acids with aryl chlorides.

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.

Ylide-Substituted Phosphines with a Cyclic Ylide-Backbone: Angle Dependence of the Donor Strength

Ylide-substituted phosphines (YPhos) have been shown to be highly electron-rich and efficient ligands in a variety of palladium catalyzed transformations. Here, the synthesis and characterization of novel YPhos ligands containing a cyclic backbone architecture are reported. The ligands are easily synthesized from a cyclic phosphonium salt and the chlorophosphines Cy2PCl (L1) and Cy(FluMe)PCl (L2, with FluMe = 9-methylfluorenyl) and were characterized in both solution and solid states. The smaller PCy2-substituted ligand, L1, readily formed the biscoordinate L1 2 Pd species when treated with Pd2(dba)3 and showed no activity in palladium-catalyzed amination reactions even when applied as defined palladium(II) η3-allyl, t-Bu-indenyl, or cinnamyl precursors. Bulkier fluorenyl-substituted ligand L2 similarly was inactive, despite its ability to form the stable monophosphine complex L2·Pd(dba). Assessment of the electronic properties by experimental and computational methods revealed that L1 and L2 are considerably less electron-rich than previously synthesized YPhos ligands. This was shown to be the result of the small P-C-S bond angle, which is sterically enforced due to the cyclic nature of the backbone. Density functional theory calculations revealed that the small angle results in an increased s-character of the lone pair at the ylidic carbon atom and leads to a polarization of the C-P bond toward the carbon atom, thus decreasing the electron density at the phosphorus atom. The results demonstrate the tunability of the donor strength of YPhos ligands by modification of the ligand backbone beyond simple changes of the substitution pattern and are thus important for future ligand design, with a careful balance of many factors to be considered to achieve catalytic activity.

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.

Application of organometallic catalysts for the synthesis of o-tolyl benzonitrile, a key starting material for sartans

This review describes methods for the synthesis of o-tolyl benzonitrile, such as Pd-, Ni-catalyzed Suzuki, Negishi and Kumada couplings reactions.

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.

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 .

Sterically enriched bulky 1,3-bis(N,N′-aralkyl)benzimidazolium based Pd-PEPPSI complexes for Buchwald–Hartwig amination reactions

A simple and efficient synthesis of a series of unexisting Pd-PEPPSI complexes is summarized. These complexes are exploited for their high catalytic activity towards Buchwald–Hartwig amination.

Sustainable ppm level palladium-catalyzed aminations in nanoreactors under mild, aqueous conditions

A 1 : 1 Pd : ligand complex, [t-BuXPhos(Pd-π-cinnamyl)]OTf, has been identified as a highly robust pre-catalyst for amination reactions leading to diarylamines, where loadings of metal are typically at 1000 ppm Pd, run in water at temperatures between rt and 45 °C. The protocol is exceptionally simple, is readily scaled, and compares very favorably vs. traditional amination conditions. It has also been shown to successfully lead to key intermediates associated with several physiologically active compounds.

Stereocontrolled Synthesis of Arylomycin-Based Gram-Negative Antibiotic GDC-5338

We report herein an efficient, stereocontrolled, and chromatography-free synthesis of the novel broad spectrum antibiotic GDC-5338. The route features the construction of a functionalized tripeptide backbone, a high-yielding macrocyclization via a Pd-catalyzed Suzuki-Miyaura reaction, and the late-stage elaboration of key amide bonds with minimal stereochemical erosion. Through extensive reaction development and analytical understanding, these key advancements allowed the preparation of GDC-5338 in 17 steps, 15% overall yield, >99 A % HPLC, and >99:1 dr.

Palladium-mediated intramolecular dearomatization of ligated dialkylterphenyl phosphines

Thermal dearomative rearrangement of coordinated dialkylterphenyl phosphines mediated by Pd(II).

A concise Friedländer/Buchwald–Hartwig approach to the total synthesis of quindoline, a bioactive natural indoloquinoline alkaloid, and toward the unnatural 10-methylquindoline

Friedländer and Buchwald–Hartwig reactions were used to achieve an efficient total synthesis of quindoline and a new synthesis of the unnatural 10-methylquindoline. DFT calculations were employed to explain the outcome of a failed key transformation.

Macrolactamization Approaches to Arylomycin Antibiotics Core

Two practical entries to arylomycin antibiotics core structures are investigated. In route A, the activation of l-Hpg for the key macrolactamization step is achieved in 89% yield in the presence of unprotected phenol and amine functionalities. Alternatively, a propanephosphonic acid anhydride (T3P)-promoted coupling between thel-Tyr and l-Ala moieties in route B led to a facile macrolactamization in 68% yield with a marked reduction in competing oligomerization.

Room-Temperature, Copper-Free Sonogashira Reactions Facilitated by Air-Stable, Monoligated Precatalyst [DTBNpP] Pd(crotyl)Cl

A novel application of [DTBNpP] Pd(crotyl)Cl (DTBNpP = di-tert-butylneopentylphosphine) (P2), an air-stable, commercially available palladium precatalyst that allows rapid access to a monoligated state, has been identified for room-temperature, copper-free Sonogashira couplings of challenging aryl bromides and alkynes. The mild reaction conditions with TMP in dimethyl sulfoxide afford up to 97% yields, excellent functional group tolerability, and broad reaction compatibility with access to one-pot indole formation.

Faster and Greener Chemical Reaction Workup Using Silicone Elastomer-Coated Glass Powders

We have developed a substantially faster and greener technique for routine chemical reaction workups. We use silicone elastomer-coated glass powder ("FastWoRX") as an absorbent to extract organic products from a quenched aqueous reaction mixture. After filtration, FastWoRX powder loaded with organic products can serve as a convenient input for flash chromatographic separations. With this technique, tedious and solvent-consuming liquid-liquid extractions are replaced by a simple filtration. Use of FastWoRX reduces solvent use and the E-factor by up to an order of magnitude compared to traditional liquid-liquid extraction-based workup.

A palladium nanoparticle-catalyzed aryl–amine coupling reaction: high performance of aryl and pyridyl chlorides as the coupling partner

A carbon nitride-supported Pd nanoparticle-catalyzed C–N cross coupling reaction.