Active Palladium Colloids via Palladacycle Degradation as Efficient Catalysts for Oxidative Homocoupling and Cross-Coupling of Aryl Boronic Acids

Total Synthesis of Linear Coumarniolignoids (+) and (-)-Sapiumin C, (-)-Moluccanin, and (-)-Hemidesminine

Coumarinolignoids (CLs) are a class of natural products isolated from a diverse range of plant species. Due to their unique structural scaffold, they exhibit a wide range of interesting biological activities including hepatoprotective, antitumor, anti-inflammatory, and antioxidant activities among others. In this research, key intermediate 10 was used to stereoselectively synthesize CLs (7'S,8'S)- and (7'R,8'R)-sapiumin C 1 and 2, (7'S,8'S)-moluccanin 3, and (7'S,8'S) hemidesminine 4 for the first time, establishing a versatile synthetic method for the stereoselective synthesis of linear CLs. The developed method includes a Mitsunobu coupling, a modified Miyaura arylation via a rhodium catalyst, and an acid-catalyzed cyclization in key bond-forming steps. The developed synthetic route allows the synthesis of both enantiomers of a given natural product from the same chiral pool reagent (S)-solketal while allowing easy variation of aromatic substitution and hydroxymethyl/allylhydroxymethyl moieties that are common in this class of natural products.

Pd-Catalyzed cross-coupling synthesis of 4-aryl-3-formylcoumarins

The threefold cross-coupling of triarylbismuth reagents with 4-chloro-3-formylcoumarins furnished the corresponding 4-aryl-3-formylcoumarins in a chemoselective manner with high yields under Pd-catalyzed conditions. This method was successfully applied to electronically different triarylbismuth reagents and 4-chloro-3-formylcoumarins preserving the 3-formyl group in the coumarin scaffold.

Water-Soluble Noble Metal Nanoparticle Catalysts Capped with Small Organic Molecules for Organic Transformations in Water

This article recaps a variety of interesting catalytic studies based on solubilized and freely movable noble metal nanoparticle catalysts employed for organic reactions in either pure water or water-organic biphasic systems. Small organic ligand-capped metal nanoparticles are fundamentally attractive materials due to their enormous potential as a well-defined system that can provide spatial control near active catalytic sites. The nanoparticle catalysts are first grouped based on the synthetic method (direct reduction, phase transfer, and redispersion) and then again based on the type of reaction such as alkene hydrogenation, arene hydrogenation, nitroaromatic reduction, carbon-carbon coupling reactions, etc. The impacts of various ligands on the catalytic activity and selectivity of semi-heterogeneous nanoparticles in water are discussed in detail. The catalytic systems using polymers, dendrimers, and ionic liquids as supporting or protecting materials are excluded from the subject of this review.

Ferrocenyl palladacycles derived from unsymmetrical pincer-type ligands: evidence of Pd(0) nanoparticle generation during the Suzuki-Miyaura reaction and applications in the direct arylation of thiazoles and isoxazoles

A new family of ferrocenyl-palladacycle complexes Pd(L¹)Cl (Pd1) and Pd(L²)Cl (Pd2) were synthesized and characterized by UV-visible, IR, ESI-MS, and NMR spectral studies. The molecular structures of Pd1 and Pd2 were determined by X-ray crystallographic studies. Palladacycle catalyzed Suzuki-Miyaura cross-coupling reactions were investigated utilizing the derivatives of phenylboronic acids and substituted chlorobenzenes. Mechanistic investigation authenticated the generation of Pd(0) nanoparticles during the catalytic cycle and the nanoparticles were characterized by XPS, SEM and TEM analysis. Direct C-H arylation of thiazole and isoxazole derivatives employing these ferrocenyl-palladacycle complexes was examined. The reaction model for the arylation reaction implicating the in situ generation of Pd(0) nanoparticles was proposed.

Pd-Colloids-Catalyzed/Ag2 O-Oxidized General and Selective Esterification of Benzylic Alcohols

Palladium colloids obtained from the degradation of Hermann-Beller palladacycle proved to be an efficient catalytic system in combination with silver oxide as a selective oxidant for the oxidative esterification of differently substituted benzyl alcohols in MeOH as solvent. Excellent reactivity exhibited by the catalytic system also allowed the alcoholic coupling partner to be changed from MeOH to a wide range of alcohols having diverse functionalities. The mildness of the developed protocol also made it possible to employ propargyl alcohol as the coupling partner without any observation of any interference of the terminal alkyne. Selective oxidative coupling of a primary alcoholic functional group over secondary in the case of glycols and glycerols was also made possible using the developed catalyst system. To test the relevancy of Pd/Ag combined catalysis mixed Pd/Ag colloids were synthesized, characterized by TEM, XRD and XPS and applied to oxidative-esterification successfully.

An Active Palladium Colloidal Catalyst for the Selective Oxidative Heterocoupling of (Hetero)Aryl Boronic Acids

A highly selective oxidative heterocoupling protocol for (hetero)aryl boronic acids with an active palladium colloidal catalyst was developed. The judicious choice of electronically different aryl boronic acids made possible such couplings under mild conditions, with air as oxidant, while embracing a wide substrate scope. This successful approach further allowed the development of a unique one-pot sequential oxidative heterocoupling/Suzuki-Miyaura cross-coupling tandem process for accessing substituted terphenyls.

Greener Biogenic Approach for the Synthesis of Palladium Nanoparticles Using Papaya Peel: An Eco-Friendly Catalyst for C-C Coupling Reaction

The development of a green and sustainable synthetic methodology still remains a challenge across the globe. Encouraging the prevailing challenge, herein, we have synthesized Pd nanoparticles (Pd NPs) in a green and environmentally viable route, using the extract of waste papaya peel without the assistance of any reducing agents, high-temperature calcination, and reduction procedures. The biomolecules present in the waste papaya peel extract reduced Pd(II) to nanosize Pd(0) in a one-pot green and sustainable process. As a catalyst, the new Pd NPs offer a simple and efficient methodology in direct Suzuki-Miyaura and Sonogashira coupling with excellent yields under mild reaction conditions.

Ir(COD)Cl]2/Tris(2,4-di-t-butylphenyl)phosphite-Catalyzed Addition Reactions of Arylboronic Acids with Aldehydes

[Ir(COD)Cl]2/tris(2,4-di-t-butylphenyl)phosphite-catalyzed addition reactions of arylboronic acids with aldehydes were described. The Ir(I) catalyst, generated from [Ir(COD)Cl]2 and tris(2,4-di-t-butylphenyl)phosphite, was an efficient catalyst system for the addition reactions of a variety of arylboronic acids with aromatic and aliphatic aldehydes. The easy availability of the catalyst and good yields make these reactions potentially useful in organic synthesis.

Palladium-mediated radical homocoupling reactions: a surface catalytic insight

In this work, we report a palladium nanoparticle-promoted reductive homocoupling of haloarenes, exhibiting a broad functional group tolerance. A mechanistic study was carried out, suggesting single-electron transfer processes on the metal surface.

Palladacycle-Catalyzed Triple Suzuki Coupling Strategy for the Synthesis of Anthracene-Based OLED Emitters

The development of the site-selective Suzuki-Miyaura cross-coupling of dibromoanthracene as an efficient strategy toward organic light emitting diodes (OLEDs) is disclosed in this article. An unprecedented step-economic palladacycle-promoted triple Suzuki coupling protocol allowed the synthesis of three new OLED emitters and could prove to be a useful general strategy for researchers working in this field. Characterization of the synthesized molecules by UV-vis spectroscopy and thermogravimetric analysis-differential scanning calorimetry followed by density functional theory studies of the different properties strongly confirms the derivatives possess more significant hole mobility character than electron transfer capability.