Palladium-catalyzed Research Articles

Palladium-Catalyzed Electrooxidative Hydrofluorination of Aryl-Substituted Alkenes with a Nucleophilic Fluorine Source

Herein, we report an electrocatalytic hydrofluorination of aryl-substituted alkenes with a nucleophilic fluorine source. The merger of palladium catalysis with electrooxidation enables the transformation of various substrates ranging from styrenes to more challenging α,β-unsaturated carbonyl derivatives to the corresponding benzylic fluorides. This method can also be applied to the late-stage modification of pharmaceutical derivatives. Mechanistic studies suggest that the generation of a high-valent palladium intermediate via anodic oxidation is the crucial step in this electrocatalytic hydrofluorination.

Construction of Benzo-Fused Polycyclic Heteroaromatic Compounds through Palladium-Catalyzed Intramolecular C-H/C-H Biaryl Coupling

Dibenzo-fused five-membered heteroaromatic compounds, including dibenzofuran, carbazole, and dibenzothiophene, are fundamental structural units in various important polycyclic heteroaromatic compounds. The intramolecular C-H/C-H biaryl coupling of diaryl (thio)ethers and amines based on palladium(II) catalysis under oxidative conditions is known to be one of the most effective, step-economic methods for their construction. Representative examples for the construction of structurally intriguing π-extended polycyclic heteroaromatics through catalytic coupling reactions are briefly summarized in this mini-review.

Hypervalent iodine mediated Pd(II)‐catalyzed ortho‐C(sp2−H) functionalization of azoles deciphering Hantzsch ester and malononitrile as the functional group surrogates

AbstractC−H bond activation has surfaced as a transformative toolbox for the efficient assembly of biologically important molecules. However, despite of major advances in palladium, rhodium and silver catalysis, yet palladium(II)‐ catalyzed, iodobenzene diacetate (PIDA) mediated C−H bond activation in azole skeleton has deluged with interesting wining outcome. Concentrating on that we herein report on the involvement of 4‐functionalized 1,4‐dihydropyridines (R‐DHPs) as the representative surrogates of benzoyl, pivalyl and ethoxycarbonyl groups and in presence of PIDA it releases appropriate functional radical (following single electron transfer oxidation of R‐DHPs under thermal condition)to become the part of ortho‐ functionalized azoles via Pd(II) catalysis. In case of cyanation reaction, malononitrile has served the key source of nitrile functionality by undergoing oxidative C−CN bond cleavage in presence of PIDA and with the help of Pd(II) catalyst it offers ortho‐cyanated azoles. Additionally, the selective formation of C‐3 benzoyl and ester functionalized indazole derivatives in absence of Pd(II)‐catalyst and the unprecedented formation of homo‐coupling products between two identical 2H‐indazoles are also notable outcome of present strategic access. Overall this protocol operates under mild condition, tolerates a wide range of structural moieties and gives versatile azole derivatives in high efficiency. The antioxidant properties of the synthesized 2‐thiophenyl benzoyl selenazole and isoselenazole are further evaluated using a chemically defined method. The Copper‐ascorbate (Cu−As) was utilized as an oxidative stress generator. Out of screened azoles, the 2‐benzoyl isoselenazole compound (3 i) displays magnificent antioxidant property as compared to the 2‐thiophenyl benzoyl selenazole (3 g).

Palladium-Catalyzed Branch- and Z-Selective Allylic C-H Amination with Aromatic Amines.

Allylamines are important building blocks in the synthesis of bioactive compounds. The direct coupling of allylic C-H bonds and commonly available amines is a major synthetic challenge. An allylic C-H amination of 1,4-dienes has been accomplished by palladium catalysis. With aromatic amines, branch-selective allylic aminations are favored to generate thermodynamically unstable Z-allylamines. In addition, more basic aliphatic cyclic amines can also engage in the reaction, but linear dienyl allylic amines are the major products.

Palladium‐Catalyzed Branch‐ and Z‐Selective Allylic C−H Amination with Aromatic Amines

AbstractAllylamines are important building blocks in the synthesis of bioactive compounds. The direct coupling of allylic C−H bonds and commonly available amines is a major synthetic challenge. An allylic C−H amination of 1,4‐dienes has been accomplished by palladium catalysis. With aromatic amines, branch‐selective allylic aminations are favored to generate thermodynamically unstable Z‐allylamines. In addition, more basic aliphatic cyclic amines can also engage in the reaction, but linear dienyl allylic amines are the major products.

Directed Addition of Nitroalkanes Across Unactivated Olefins via Palladium(II) Catalysis

Key words palladium - nitroalkanes - carbopalladation - unactivated alkenes - directed addition

Stereospecific Cross-Coupling of Cyclopropyl Zincates under Dinuclear Palladium(I) Catalysis

Stereospecific Cross-Coupling of Cyclopropyl Zincates under Dinuclear Palladium(I) Catalysis

Synthesis of Fluorinated Chromanes via Palladium-Based Dyotropic Rearrangement

Key words palladium catalysis - dyotropic carbofluorination - fluorinated chromanes

Asymmetric intermolecular allylic C-H amination of alkenes with aliphatic amines.

Aliphatic allylic amines are found in a great variety of complex and biorelevant molecules. The direct allylic C-H amination of alkenes serves as the most straightforward method toward these motifs. However, use of widely available internal alkenes with aliphatic amines in this transformation remains a synthetic challenge. In particular, palladium catalysis faces the twin challenges of inefficient coordination of Pd(II) to internal alkenes but excessively tight and therefore inhibitory coordination of Pd(II) by basic aliphatic amines. We report a general solution to these problems. The developed protocol, in contrast to a classical Pd(II/0) scenario, operates through a blue light-induced Pd(0/I/II) manifold with mild aryl bromide oxidant. This open-shell approach also enables enantio- and diastereoselective allylic C-H amination.

Palladium-Catalyzed Difunctionalization of Norbornenes via Arylation and Alkynylation

We report the first general and practical method for the addition of aryl halides and alkynes to norbornenes with palladium catalysis. Norbornenes have been used as the unsaturated acceptors of aryl and alkynyl groups to construct saturated bridged C-C bonds. The combination of Pd(OAc)2/PCy3HBF4 has been identified as the optimal system promoting difunctionalization of norbornenes via the C-X/C-H bond cleavage and highly selective C(sp3)-C(sp2)/C(sp3)-C(sp) bond formation. Broad substrate scope and excellent functional group tolerance have been achieved to show the high efficiency of this approach. Mechanism studies based on experiments and DFT have been performed to gain insights into the catalytic mechanism.

Difluoroalkylative carbonylation of alkenes to access carbonyl difluoro-containing heterocycles: convenient synthesis of gemigliptin

Fluorinated heterocycles play a vital role in pharmaceutical and agrochemical industries. Hence, rapid and efficient construction of fluorinated heterocycles remains highly demanded. Herein, a difluoroalkylative carbonylative cyclization of unactivated alkenes and ethylene gas enabled by palladium catalysis has been developed for the first time toward the synthesis of α-carbonyl difluoro-modified glutarimides. This procedure can also be applied to the synthesis of GeMigliptin which is a medicine approved for the treatment of type 2 diabetes mellitus.

Palladium-Catalyzed Remote Hydro-Oxygenation of Internal Alkenes: An Efficient Access to Primary Alcohols.

As a general method for the synthesis of alcohols, the direct oxygenation of alkenes is difficult to afford linear alcohols. Herein, we communicate the remote hydro-oxygenation of alkenes under palladium catalysis, in which both terminal and internal alkenes are suitable to yield the corresponding linear alcohols efficiently. A compatible SelectFluor/silane redox system plays an essential role for the excellent chemo- and regioselectivities. The reaction features a broad substrate scope and excellent functional group compatibility.

Visible‐Light Enabled Late‐Stage, Room‐Temperature Aminocarbonylation of Aryl Iodides with Labeled Carbon Monoxide

AbstractIn recent years, coupling reactions mediated by visible light have been widely studied because such reactions can often be run at significantly lower temperatures. In this work, aryl iodides were coupled with amines at ambient temperature in the presence of stoichiometric amounts of carbon monoxide using visible‐light irradiation and palladium catalysis. A wide range of aryl substrates, both electron‐deficient and rich, as well as heterocycles, were carbonylated to provide the corresponding amides in moderate to good yields based on CO. In addition, the generality of the amine was investigated, and primary and secondary amines yielded the desired products in moderate to good yields. This methodology was also successfully applied to the synthesis of pharmaceuticals and the incorporation of a carbon‐14 label into the carbonyl group, where the use of 9‐methyl‐fluorene‐9‐carbonyl chloride (COgen) as the carbon isotope‐labeled CO source allowed easy translation between the unlabeled and the labeled transformations.

Asymmetric Assemblies of 1,3-Dienes, Sulfuric Diamide, and Aldehydes to Access 1,3-Diamines through Relay Amine/Palladium Catalysis

A relay catalytic protocol using pyrrolidine and palladium catalysis has been developed for asymmetric synthesis of 1,3-diamine derivatives from 3-substituted 1,3-dienes, sulfuric diamide, and aldehydes. This one-pot, three-component reaction features the advantages of a high atom step economy and operational simplicity, providing an efficient and straightforward access to valuable 1,3-diamines incorporating quaternary and tertiary stereogenic centers with moderate to good enantioselectivity.

Use of (E,E)-Dienoic Acids as Switchable (E,E)- and (Z,E)-Dienyl Anion Surrogates via Ligand-Controlled Palladium Catalysis.

Carboxylic acids are not readily applied as carbon-based nucleophiles due to their intrinsic acidic group. Here, we demonstrate that free (E,E)-2,4-dienoic acids form electron-neutral and highest occupied molecular orbital-raised η2-complexes with Pd(0) and undergo Friedel-Crafts-type additions to imines with exclusive α-regioselectivity, giving formal dienylated products after decarboxylation. Unusual and switchable (E,E)- and (Z,E)-selectivity, along with excellent enantioselectivity, is achieved via ligand-controlled outer-sphere or inner-sphere reaction modes, respectively, which are well supported by comprehensive density functional theory calculation studies. An unprecedented formal reductive Mannich reaction between (E,E)-dienoic acids and imines is also developed to furnish enantioenriched β-amino acid derivatives.

Enantioselective Double Carbonylation Enabled by High-Valent Palladium Catalysis.

Palladium-catalyzed carbonylation reactions are efficient methods for synthesizing valuable molecules. However, realizing a carbonylation with excellent yield and chemo-, regio-, and enantioselectivities by classical low-valent palladium catalysis is highly challenging. Herein, we describe an enantioselective carbonylation reaction using a high-valent palladium catalysis strategy and employing a chiral sulfoxide phosphine (SOP) ligand. This double aminocarbonylation reaction begins with the formation of a carbamoylpalladium(II) species, which undergoes enantioselective oxidative addition with a cyclic diaryliodonium salt to generate a palladium(IV) intermediate, followed by a second CO insertion and reductive elimination. The mechanism has been illustrated with experimental and computational studies.

Carbonylative Cross-Electrophile Coupling between Aryl Bromides and Aryl Triflates Enabled by Palladium and Rhodium Cooperative Catalysis and CO as Reductant

Ketones are among the most useful functional groups in organic synthesis. Here, we report a carbonylative cross-electrophile coupling reaction that utilizes carbon monoxide gas as both carbonyl source and reductant. The use of Pd/Rh cooperative catalysis enables the carbonylative coupling of easily accessible aryl triflates and aryl bromides. Unlike previous carbonylative cross-electrophile coupling reactions, the method does not require addition of a stoichiometric metal reductant. Notably, density functional theory (DFT) calculations and isotope-labeling indicate that CO serves as the reductant.

Deuteration of Arylthianthren-5-ium Salts in CD3OD.

Deuteration of arylthianthren-5-ium triflates with CD3OD or CD3OD/CD3COCD3 in the presence of Cs2CO3 by palladium catalysis or photoirradiation allowed the convenient synthesis of deuterated arenes in good yields. The Pd-catalyzed reaction generally gave better yields than the photoinduced deuteration, but exceptions also exist. They could complement each other in some cases. These reactions featured eco-friendly conditions, simplicity, inexpensive deuterium sources, good functional group tolerance, and a range of substrates. Since arylthianthren-5-ium salts could be readily synthesized from arenes and thianthrene 5-oxide, this protocol provided a formal aromatic C-H deuteration with high selectivity, enabling efficient deuterium labeling of multifunctionalized arenes and drug molecules.

Catalytic degradation of methyl orange using beta cyclodextrin modified polyvinylidene fluoride mixed matrix membranes imbedded with in‐situ generated palladium nanoparticles

AbstractAzo dyes effluent can cause severe and chronic effects to living organisms since they are toxic, carcinogenic, and mutagenic. They color water, reduce dissolved oxygen, block light penetration, decrease the rate of photosynthesis and adversely affect the whole aquatic biota. Catalysis is one of the methods used in degrading azo dyes to less harmful species. Catalytic palladium nanoparticles (Pd NPs) are effective in the degradation of azo dyes owing to their large surface area and unique electronic properties. However, they are costly, unstable and easily aggregate; hence, a substrate is necessary to enhance their stability, reusability, durability, catalytic efficiency, and cost effectiveness. Herein, Pd NPs were in‐situ synthesized and immobilized in beta cyclodextrin (β‐CD) modified polyvinylidene‐fluoride (PVDF) membranes. The fabricated membranes were characterized using different techniques. Pd NPs slightly improved the properties and performance of the membranes. A complete catalytic degradation of methyl orange azo dye was carried out within 20 min in the presence of sodium borohydride and the increase in Pd NP content increased the rate of degradation. The membranes were recycled and reused five times with no considerable loss of catalytic performance. Therefore, PVDF/β‐CD‐Pd membranes proved to be self‐cleaning, reusable, and efficient in removal of methyl orange from water.

Differentiation of enynes and alkynes in asymmetric multicomponent reactions by Pd catalysis

In this issue of Chem Catalysis , Chen and co-workers describe a chemo- and stereodivergent asymmetric multicomponent reaction (AMCR) involving two different alkynes under palladium catalysis. Through the elaborate manipulation of catalytic conditions, the reactions selectively afford chiral tetra-substituted alkenes or N -heterocycles bearing an exocyclic double bond in either E - or Z -configuration. In this issue of Chem Catalysis , Chen and co-workers describe a chemo- and stereodivergent asymmetric multicomponent reaction (AMCR) involving two different alkynes under palladium catalysis. Through the elaborate manipulation of catalytic conditions, the reactions selectively afford chiral tetra-substituted alkenes or N -heterocycles bearing an exocyclic double bond in either E - or Z -configuration.