Pd(II)-Catalyzed Sequential C–C/C–O Bond Formations: A New Strategy to Construct Trisubstituted Furans

Synthesis of Furans via Rhodium(III)-Catalyzed Cyclization of Acrylic Acids with α-Diazocarbonyl Compounds

An efficient protocol for the synthesis of furans through Rh(III)-catalyzed vinyl C-H activation from acrylic acids and α-diazocarbonyl compounds has been developed. The reaction features broad functional group tolerance and affords a series of furans in moderate to good yields. Moreover, no additives such as copper or silver salts are required. Some control experiments are performed to give insight into the mechanism of this cascade transformation and the decarbonylation process is involved in the formation of the furan product.

Silver-mediated annulation between 5-H-1,2,3-thiadiazoles and 1,3-dicarbonyl compounds to construct polysubstituted furans

An efficient synthesis of polysubstituted furans by Ag(i)-mediated annulation between 5-H-1,2,3-thiadiazoles and 1,3-dicarbonyl compounds is reported.

Palladium-catalyzed synthesis of α-aryl acetophenones from styryl ethers and aryl diazonium salts via regioselective Heck arylation at room temperature

Preparation of α-aryl acetophenones from styryl ethers and aryldiazonium salts is described. The reaction is catalyzed by palladium acetate at room temperature in the absence of ligand and base. The developed method is highly attractive in terms of reaction conditions, substrate scope, functional group tolerance and yields. Synthetic applications of the present method are demonstrated by preparing α-aryl indoles and 3-aryl isocoumarin from styryl ethers.

Domino Decarboxylative Arylation and C-O Selective Bond Formation toward Chromeno[2,3-b]pyridine-2-one Skeletons

Practical Pd-catalyzed 2-pyridones were designed to achieve chromeno[2,3-b]pyridine-2-ones. The reaction proceeds through domino nucleophilic addition and decarboxylative arylation, respectively. This methodology offers a moderately efficient approach to construct the bioactive, fused-heterocyclic skeletons via selective C-O bond formation and decarboxylative arylation in a single step with high selectivity and good yields.

Alkyl Enol Ethers: Development in Intermolecular Organic Transformation

Alkyl enol ethers (AEE) are versatile synthetic intermediates with a unique reactivity pattern. This review article summarizes the synthesis of AEE as well as its reactivity and how enol ether undergoes intermolecular reactions for various bond formation, leading to the construction of several useful organic molecules. The synthetic applications of alkyl enol ethers towards intermolecular bond-forming reactions include metal-catalyzed reactions, cycloaddition and heterocycle formation as well as rwactions in the field of natural products synthesis. The achievement of these impressive transformations prove the countless synthetic potential of AEE. The main objective of this review is to bring attentiveness among synthetic chemists to show how AEE extensively can be used to react with both electrophiles as well as nucleophiles, thereby behaving as an ambiphilic reactant. We trust that the unique reactivity pattern of alkyl enol ethers and the fundamental mechanistic idea can attract chemists in AEE chemistry. Exclusively, intermolecular reactions of AEE with other functionalized moieties have not been reviewed to the best of our knowledge.

Palladium(II)/Lewis Acid-Catalyzed Oxidative Olefination/Annulation of N-Methoxybenzamides: Identifying the Active Intermediates through NMR Characterizations

Although Pd(II)-catalyzed C-H activation in arenes has been widely successful in organic synthesis with many palladacycle compounds isolated as the intermediates in ligand-directed C-H activation, direct identification of the reaction intermediates such as the π-complex prior to the C-H activation is still not successful because of their instability. In the present study, we introduce a Pd(II)/LA (LA: Lewis acid)-catalyzed oxidative olefination/annulation reaction between N-methoxybenzamides and acrylates with oxygen as the oxidant source, in which two intermediates, including an unsymmetrical η⁶-complex and a palladacycle species without the proton releasing to the environment, were identified through NMR characterizations. The in situ formation of the heterobimetallic Pd(II)/LA species such as Pd(II)/Sc(III) may have enhanced the electrophilic properties of the Pd²⁺ cation, thus improving the stability of the π-complex, herein, an unsymmetrical η⁶-complex, and improving its catalytic efficiency. The observed insensitive electronic effect preferred the concerted metalation-deprotonation (CMD) mechanism for this C-H activation, and the detected palladacycle intermediate without the proton releasing to the environment offered an experimental clue to support the proposed CMD mechanism.

Retrosynthetic Analysis of α-Alkenyl-β-Diketones: Regio- and Stereoselective Two-Step Synthesis of Highly Arylated Representatives from Acetylenes, Ketones, and Acyl Chlorides

Highly arylated α-alkenyl-β-diketones are synthesized via a two-step sequence consisting of (i) potassium tert-butoxide/DMSO-catalyzed (E)-stereoselective C-H functionalization of ketones with acetylenes followed by (ii) magnesium bromide etherate/DIPEA-soft enolization of the formed β,γ-unsaturated ketones and regioselective acylation with acyl chlorides. The method is compatible with a broad range of substrates and shown to be applicable as an intermediate stage in the construction of polyarylated heterocycles.

Palladium-Catalyzed Oxidation Reactions of Alkenes with Green Oxidants

Transition-metal-catalyzed oxidative functionalization of alkenes has emerged as a powerful and valuable tool in modern organic synthesis. Recently, many methods have been established for the assembly of C-C and C-heteroatom bonds, which provides tremendous possibility for application in biology, medicine, and materials science. However, the use of stoichiometric amounts of strong oxidants leads to poor selectivity, low atom economy, and a series of undesired waste products. By contrast, green oxidants, such as O₂ , H₂ O₂ , or tert-butyl hydroperoxide (TBHP), have bright prospects due to their attributes of mild, low cost, and great sustainability in transition-metal-catalyzed oxidation reactions. Based on the great and unique potential for the development of aerobic reactions, this review mainly highlights homogenous palladium-catalyzed green oxidations of alkenes that have been reported in recent years. These methods provide new strategies for the transformation and functionalization of alkenes; some of them have also been successfully applied to the synthesis of the core structures of drugs and natural products. Additionally, through in-depth studies of the reaction mechanisms in this field, it is believed that palladium-catalyzed green oxidation reactions of alkenes with O₂ , H₂ O₂ , or TBHP will create added value for organic synthetic chemistry.

Hypoiodite-catalysed oxidative cyclisation of Michael adducts of chalcones with 1,3-dicarbonyl compounds: a facile and versatile approach to substituted furans and cyclopropanes

Hypoiodite catalysis for selective oxidative cyclization has been developed, yielding either furans or cyclopropanes with excellent diastereoselectivity.

Palladium-catalyzed Heck-type reaction of oximes with allylic alcohols: synthesis of pyridines and azafluorenones

An efficient palladium-catalyzed Heck-type reaction of oximes with allylic alcohol has been developed for the synthesis of pyridines and azafluorenones.

Palladium(ii)-catalyzed hydroxy-involved enolate-type efficient C–C functionalization of hydroxynaphthoquinones at room temperature

A simple, mild, efficient and scalable strategy has been developed for synthesizing hydroxynaphthoquinones via a Pd-catalyzed hydroxy-involved enolate-type reaction.

M2+ and Ln3+-catalyzed synthesis of a [1,2,4]triazine core via intramolecular C–H/N–H functionalization and C–N bond formation (M = Mn, Zn, Cd; Ln = Dy, Tb)

A novel approach for the direct construction of a 1,2,4-triazine core via metal catalyzed intramolecular C–H/N–H functionalization has been developed.

Palladium-catalyzed sequential C-N/C-O bond formations: synthesis of oxazole derivatives from amides and ketones

A highly efficient method for the synthesis of oxazole derivatives from simple amides and ketones has been established via a Pd(II)-catalyzed sp(2) C-H activation pathway in one step. The reaction is supposed to proceed through a C-N bond formation followed by a C-O bond formation closing the ring. Because of the simple and readily available starting materials, easy operation, and high bioactivity of oxazoles, this strategy can be broadly applied to medical chemistry.

Palladium-catalyzed condensation of N-aryl imines and alkynylbenziodoxolones to form multisubstituted furans

A palladium(II) catalyst promotes condensation of an N-aryl imine and an alkynylbenziodoxolone derivative to afford a multisubstituted furan, whose substituents are derived from the alkynyl moiety (2-position), the imine (3- and 4-positions), and the 2-iodobenzoate moiety (5-position), along with an N-arylformamide under mild conditions. The 2-iodophenyl group of the furan product serves as a versatile handle for further transformations. A series of isotope-labeling experiments shed light on the bond reorganization process in this unusual condensation reaction, which includes cleavage of the C-C triple bond and fragmentation of the carboxylate moiety.

DDQ-mediated oxidative coupling: an approach to 2,3-dicyanofuran (thiophene)

A facile oxidative coupling of α-carbonyl radicals to 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) for the synthesis of 2,3-dicyanofurans and thiophenes starting from readily available β-diketones, simple ketones, and β-keto thioamides in up to 95% yield in one step was developed. Mechanistic investigations revealed that a radical process could be involved in this transformation, and a water promoted C-C bond cleavage pathway is proposed for the formation of 2,3-dicyanofurans and thiophenes.

Gold catalysed synthesis of 3-alkoxyfurans at room temperature

3-Alkoxyfurans can be readily prepared from aldehydes via a gold-catalysed cyclisation of propargylic alcohols bearing an acetal unit.