Air-stable Pd Research Articles

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.

Convenient One-Pot Synthesis of L2Pd(0) Complexes for Cross-Coupling Catalysis

A convenient one-pot synthesis of (tBu3P)2Pd(0) was successfully carried out by using various air-stable Pd(II) salts, such as Pd(cod)Cl2, Pd(nbd)Cl2, Pd(CH3CN)2Cl2, or Pd(PhCN)2Cl2, without the use of an external reducing agent. Using Pd(cod)Cl2 as the precursor, large-scale synthesis of (tBu3P)2Pd(0) was accomplished in excellent yield and purity. Another protocol was also developed for L2Pd(0) complexes via an atom economical one-pot method by reacting in situ generated LPd(R-allyl)Cl complexes (L = tBu3P, Cy3P, (o-Tol)3P, CataXCium A, AmPhos, QPhos: R = H, Me, Ph) with 1 equivalent of L in the presence of a base with yields ranging from 74% to 92%, although the process can be further optimized. These methods are superior to those reported in the literature.

An unusual autocatalysis with an air-stable Pd complex to promote enantioselective synthesis of Si-stereogenic enynes.

Given the powerful potential of chiral-at-silicon chemistry, enantioselective synthesis of Si-stereogenic centers has attracted substantial research interest in recent years. However, the catalytic asymmetric synthesis of Si-stereogenic organosilicon compounds remains an appealing venture and is a challenging subject because of the difficulty in achieving high reactivity and stereoselectivity for "silicon-center" transformations. Herein, we disclose a highly enantioselective palladium-catalyzed hydrosilylation of 1,3-diynes with dihydrosilanes, which enables the facile preparation of Si-stereogenic enynes and an enyne-linked chiral polymer (polyenyne) in good yields and excellent ees (up to >99%) by desymmetrization. The unusual stereoselectivity in this reaction is achieved by precisely controlling the steric hindrance and electronic effect of the newly developed chiral ligands, resulting in a wide range of chiral silanes and a Si-containing polymer bearing a Si-stereogenic center which is otherwise difficult to access. The key to the high enantioselectivity relies on catalyst aggregation-induced non-covalent interaction, which exerts a remarkably positive influence on the Si-H bond activation and enhancement of enantioselectivity, in which the palladium/P-ligand complex was proved to be air-stable and moisture-insensitive in this reaction.

Synthesis, coordination behavior, and catalytic properties of dppf congeners with an inserted carbonyl moiety

Dppf-type ligands, desymmetrized by an inserted carbonyl group coordinate Pd(ii) and Pd(0). Among the air-stable Pd(0) complexes, the compound containing the least electron-donating ligand gives the best catalytic results in the Stille reaction.

Air-stable Pd(0) catalyst bearing dual phosphine ligands: a detailed evaluation of air stability and catalytic property in cross-coupling reactions.

We have synthesised an air-stable Pd(0) catalyst bearing donor and acceptor phosphine ligands (Complex 1). This study revealed the long-term air stability and catalytic property of Complex 1 as a catalyst for cross-coupling reactions, where it was stable in air for eight months. DFT calculations revealed that the acceptor ligands in Complex 1 decreased the HOMO energy level, which provided the observed air stability. Complex 1 successfully served as a catalyst for direct C-H arylation reactions and Suzuki-Miyaura cross-coupling reactions, and catalysed the reaction of a relatively inactive substrate, 2-chrolopyridine, which could not be achieved by conventional, air-stable Pd(0) catalysts. Isolating the intermediates of the coupling reactions revealed that each intermediate possessed the donor ligand (PCy3), which was determined to be responsible for imparting the high catalytic activity exhibited by Complex 1.

C-I-Selective Cross-Coupling Enabled by a Cationic Palladium Trimer.

While there is a growing interest in harnessing synergistic effects of more than one metal in catalysis, relatively little is known beyond bimetallic systems. This report describes the straightforward access to an air-stable Pd trimer and presents unambiguous reactivity data of its privileged capability to differentiate C-I over C-Br bonds in C-C bond formations (arylation and alkylation) of polyhalogenated arenes, which typical Pd0 and PdI -PdI catalysts fail to deliver. Experimental and computational reactivity data, including the first location of a transition state for bond activation by the trimer, are presented, supporting direct trimer reactivity to be feasible.

Buchwald-Hartwig amination using Pd(i) dimer precatalysts supported by biaryl phosphine ligands.

We report the synthesis of air-stable Pd(i) dimer complexes featuring biaryl phosphine ligands. Catalytic experiments suggest that these complexes are competent precatalysts that can mediate cross-coupling amination reactions between aryl halides with both aliphatic and aromatic amine nucleophiles. This work represents an expansion of the air-stable precatalyst toolbox for Pd-catalyzed cross-coupling transformations.

ChemInform Abstract: Excellent Suzuki—Miyaura Catalytic Activity of a New Pd(II) Complex with Sulfonamide—Schiff Base Ligand.

AbstractThe novel catalyst exhibits excellent cross‐coupling activity.

ChemInform Abstract: A Robust and Efficient Catalyst Possessing an Electron‐Deficient Ligand for the Palladium‐Catalyzed Direct Arylation of Heteroarenes.

AbstractA unique thermally and air stable Pd(0)‐triarylphosphine catalyst is applied for direct C—H arylation of electronically varied heterocycles with a variety of aryl bromides to give the corresponding biaryls generally in reasonable yields.

PEG-modified N-heterocyclic carbene ligands for highly efficient and recyclable Pd-catalyzed Heck reaction in water

A water-soluble and air-stable Pd(OAc)2/mPEG n MeImI system was found to be a highly efficient and reusable catalyst for the coupling of aryl halides and styrene in neat water using K2CO3 as base. After extraction, the catalytic system could be reused several times with only a slight decrease in its activity.

New Helically Chiral Metallated Complexes: Characterization and Catalytic Activity

New helically chiral and air-stable Pd(II) and Ru(II) complexes were prepared in good yields and fully characterized. The optical properties of these new complexes were investigated, showing interesting behavior. The Ru complex was used as heterogeneous catalyst for the decoloration of calmagite (one of the azoic dyes) solutions at pH 6.5 in the presence of hydrogen peroxide (H2O2). Experiments revealed that the decoloration process was affected by the H2O2 dose, the reaction temperature, and the initial dye concentration. The energetic parameters were also determined and the activation energy, E a, was found to be low, confirming that the prepared catalyst was very efficient for color removal. The new ruthenium complex exhibited excellent catalytic activity and has the advantage of easy separation from the reaction medium. It could remain with good catalytic activity after five cycles.

Synthesis and X-ray Structure Determination of Highly Active Pd(II), Pd(I), and Pd(0) Complexes of Di(tert-butyl)neopentylphosphine (DTBNpP) in the Arylation of Amines and Ketones

The air-stable complex Pd(η(3)-allyl)(DTBNpP)Cl (DTBNpP = di(tert-butyl)neopentylphosphine) serves as a highly efficient precatalyst for the arylation of amines and enolates using aryl bromides and chlorides under mild conditions with yields ranging from 74% to 98%. Amination reactions of aryl bromides were carried out using 1-2 mol % Pd(η(3)-allyl)(DTBNpP)Cl at 23-50 °C without the need to exclude oxygen or moisture. The C-N coupling of the aryl chlorides occurred at relatively lower temperature (80-100 °C) and catalyst loading (1 mol %) using the Pd(η(3)-allyl)(DTBNpP)Cl precatalyst than the catalyst generated in situ from DTBNpP and Pd(2)(dba)(3) (100-140 °C, 2-5 mol % Pd). Other Pd(DTBNpP)(2)-based complexes, (Pd(DTBNpP)(2) and Pd(DTBNpP)(2)Cl(2)) were ineffective precatalysts under identical conditions for the amination reactions. Both Pd(DTBNpP)(2) and Pd(DTBNpP)(2)Cl(2) precatalysts gave nearly quantitative conversions to the product in the α-arylation of propiophenone with p-chlorotoluene and p-bromoanisole at a substrate/catalyst loading of 100/1. At lower substrate/catalyst loading (1000/1), the conversions were lower but comparable to that of Pd(t-Bu(3)P)(2). In many cases, the tri-tert-butylphosphine (TTBP) based Pd(I) dimer, [Pd(μ-Br)(TTBP)](2), stood out to be the most reactive catalyst under identical conditions for the enolate arylation. Interestingly, the air-stable Pd(I) dimer, Pd(2)(DTBNpP)(2)(μ-Cl)(μ-allyl), was less active in comparison to [Pd(μ-Br)(TTBP)](2) and Pd(η(3)-allyl)(DTBNpP)Cl. The X-ray crystal structures of Pd(η(3)-allyl)(DTBNpP)Cl, Pd(DTBNpP)(2)Cl(2), Pd(DTBNpP)(2), and Pd(2)(DTBNpP)(2)(μ-Cl)(μ-allyl) are reported in this paper along with initial studies on the catalyst activation of the Pd(η(3)-allyl)(DTBNpP)Cl precatalyst.

Palladium(II)/Cationic 2,2’-Bipyridyl System as a Highly Efficient and Reusable Catalyst for the Mizoroki-Heck Reaction in Water

A water-soluble and air-stable Pd(NH3)2Cl2/cationic 2,2’-bipyridyl system was found to be a highly-efficient and reusable catalyst for the coupling of aryl iodides and alkenes in neat water using Bu3N as a base. The reaction was conducted at 140 °C in a sealed tube in air with a catalyst loading as low as 0.0001 mol % for the coupling of activated aryl iodides with butyl and ethyl acrylates, providing the corresponding products in good to excellent yields with very high turnover numbers. In the case of styrene, Mizoroki-Heck coupling products were obtained in good to high yields by using a greater catalyst loading (1 mol %) and TBAB as a phase-transfer agent. After extraction, the residual aqueous solution could be reused several times with only a slight decrease in its activity, making the Mizoroki-Heck reaction “greener”.

Air-Stable, Recyclable, and Regenerative Phosphine Sulfide Palladium(0) Catalysts for C−C Coupling Reaction

Mononuclear phosphine sulfide Pd(0) complexes and a polymer-supported triphenylphosphine sulfide Pd(0) complex were prepared as new air-stable Pd(0) catalysts for C−C coupling reactions. The phosphine sulfide Pd(0) complexes are not decomposed after completion of Suzuki−Miyaura coupling, and the polymer-supported Pd(0) catalyst is practically recyclable, while phosphine Pd(0) complexes are decomposed into inactive Pd(0) black after consuming the substrates. New catalytic activity of Pd(0) that promotes chalcogen atom replacement of phosphine chalcogenides (R3P═X, X = O, S, Se) is reported. A mechanistic study revealed that the new catalytic chalcogen replacement results from activation of the P═X bond as well as promotion of the oxidative chalcogenide formation. The intermediate phosphine was successfully trapped as a phosphine Pd(II) complex, and the P═X bond activation is applicable to regeneration of phosphine or phosphine sulfide from oxidized phosphine.

A Highly Practical and General Route for α-Arylations of Ketones Using Bis-phosphinoferrocene-Based Palladium Catalysts

Well-defined, air-stable Pd complexes of bis-phosphinoferrocene family of catalysts have been studied in the arylation of various ketones with aryl chlorides and aryl bromides. Bis(di-tert-butyl)phosphinoferrocene (DtBPF)-based catalysts such as (DtBPF)PdCl2 and (DtBPF)PdBr2 have been identified as two of the most active catalysts for the α-arylation of a model reaction involving propiophenone and 4-chlorotoluene. The scope of the (DtBPF)PdCl2 catalyst has been efficiently expanded to the arylation of various ketones with aryl chlorides and bromides with up to 97% isolated yields, under relatively mild reaction conditions at low catalyst loadings. The efficacy of the (DtBPF)PdCl2 catalyst was demonstrated at very low catalyst loadings with S/C 10,000 for the difficult aryl bromide, 4-bromoanisole, and 2000 for the electron-neutral aryl chloride, 4-chlorotoluene, on ∼10-g scale with excellent isolated yields and lower Pd in the product (6 and 48 ppm, respectively). Comparative studies on the Pd:DtBPF molar ratios between in situ catalysts and preisolated catalysts revealed that preisolated (DtBPF)PdX2 (X = Cl, Br) are the catalysts of choice due to various practical reasons.

Air-stable Pd(II) catalysts with cryptand-22 ligand for convenient and efficient Suzuki cross-coupling reactions

Air-stable Pd(II) catalysts with cryptand-22 ligand was easily handled in atm air at room temperature for Suzuki cross-coupling reactions of aryl bromides with arylboronic acids both containing electron-withdrawing and electron-donating groups. Generally, reactions proceeded fast to completion in 2 h to give high product yields. Our Pd(OAc) 2/cryptand-22 catalyst is unique, since the reaction solution remained yellow during reaction period and air-stable without formation of Pd black as usually observed for ordinary simple amine ligands.

Applications of an Air Stable Pd Oxazoline Complex for Heck, Suzuki, Sonogashira and Related C—C Bond‐Forming Reactions.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Application of an air stable Pd oxazoline complex for Heck, Suzuki, Sonogashira and related C–C bond-forming reactions

The novel complex trans-[PdCl 2(η 1- N-2-ethyl-2-oxazoline) 2] is shown to be an active and oxidatively robust catalyst for C–C bond-forming reactions (Heck, Sonogashira, Ullman, Suzuki), which can be carried out in air without rigorous solvent/substrate purification and in the absence of additional free ligand.

Pd/C as a Highly Active Catalyst for Heck, Suzuki and Sonogashira Reactions

A specially optimized air-stable Pd on activated carbon catalyst is demonstrated to be a highly active (TON up to 36,000), selective and convenient heterogeneous catalyst for CC couplings of aryl halides in Heck, Suzuki, and Sonogashira reactions. The Pd/C catalyst developed allows extremely low Pd concentrations (down to 0.0025 mol% for Heck coupling, 0.005 mol% for Suzuki coupling) and high conversions of aryl bromides within a few hours. Easy and complete Pd separation and recovery is possible.