A copper-free palladium catalyzed cross coupling reaction of vinyl tosylates with terminal acetylenes

Microwave-assisted, Copper-free Sonogashira Coupling Between Aryl Halides and Terminal Alkynes Using Recyclable Ionic Liquid and Catalyst

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An efficient method for the Sonogashira coupling reaction between aryl halides and terminal alkynes has been developed. The reaction was performed in ionic liquid 1-butyl-3- methylimidazolium tetrafluoroborate ([Bmim][BF4]) under microwave irradiation. High yields of products were obtained in a very short reaction time. Moreover, the medium and catalyst could be recovered and reused three times without considerable decrease in reaction yields.

Palladium-Catalyzed Cross-Couplings by C-O Bond Activation

Although palladium-catalyzed cross-coupling of aryl halides and reactive pseudohalides has revolutionized the way organic molecules are constructed today across various fields of chemistry, comparatively less progress has been made in the palladium-catalyzed cross-coupling of less reactive C-O electrophiles. This is despite the fact that the use of phenols and phenol derivatives as bench-stable cross-coupling partners has been well-recognized to bring about major advantages over aryl halides, such as (1) natural abundance of phenols, (2) avoidance of toxic halides, (3) orthogonal cross-coupling conditions, (4) prefunctionalization of phenolic substrates by electrophilic substitution or C-H functionalization, (5) ready availability of phenols from a different pool of precursors than aryl halides. In this review, we present an overview of recent advances made in the field of palladium-catalyzed cross-coupling of C-O electrophiles with a focus on (1) catalytic systems, (2) reaction type, and (3) class of C-O coupling partners. Although the field has been historically dominated by nickel catalysis, it is now evident that the use of more versatile, more functional group tolerant and highly active palladium catalysts supported by appropriately designed ancillary ligands enables the cross-coupling with improved substrate scope and generality, and likely represents a practical solution to the broadly applicable cross-coupling of various C-O bonds across diverse chemical disciplines. The review covers the period through June 2020.

Direct Umpolung Morita-Baylis-Hillman like α-Functionalization of Enones via Enolonium Species

Herein we report on the umpolung of Morita-Baylis-Hillman type intermediates and application to the α-functionalization of enone C-H bonds. This reaction gives direct access to α-chloro-enones, 1,2-diketones and α-tosyloxy-enones. The latter are important intermediates for cross-coupling reaction and, to the best of our knowledge, cannot be made in a single step from enones in any other way. The proposed mechanism is supported by spectroscopic studies. The key initial step involves conjugate attack of an amine (DABCO or pyridine), likely assisted by hypervalent iodine acting as a Lewis acid leading to formation of an electrophilic β-ammonium-enolonium species. Nucleophilic attack by acetate, tosylate, or chloride anion is followed by base induced elimination of the ammonium species to give the noted products. Hydrolysis of α-acetoxy-enones lead to formation of 1,2-diketones. The α-tosyl-enones participate in Negishi coupling reactions under standard conditions.

Synthesis of Enynones from Alkynes, Alkynyl Ketones and Aromatic Aldehydes Using the TiCl4/Et3N Reagent System

Alkynyltitanium species, prepared in situ from alk-1-ynes using the TiCl4/Et3N reagent system, react with aromatic aldehydes to give enynones 2 in 49–38% yields. Reaction of alkynyl aryl ketones with aromatic aldehydes and the TiCl4/Et3N reagent gives the enynones 2 in 78–92% yields.

Oxine based unsymmetrical (O−, N, S/Se) pincer ligands and their palladium(ii) complexes: synthesis, structural aspects and applications as a catalyst in amine and copper-free Sonogashira coupling

Newly synthesized and characterized (single crystal structure) complexes, [Pd(O⁻, N, S/Se)Cl], efficiently catalyse Sonogashira coupling of ArX at 0.5–1 mol%.

Efficient Catalytic Sonogashira Reaction Catalysed by Modified Fibre-Palladium(II) Complex in Air

A hydroxylamine modified polypropylene fibre [Pd(II)-AOFs] was synthesised and used to catalyse the Sonogashira reaction between phenylacetylene and iodobenzene. The results showed that highly efficient catalytic activity can be achieved for the cross-coupling reaction. More importantly, the reaction conditions are moderate and do not require inert gas protection. The catalyst could be readily extracted by simple operations and conveniently recycled four times without an obviously significant decrease in activity.

C-Alkynylation of Chromones by Sonogashira Reaction

Sonogashira reaction of bromochromones and -flavones with a bromine atom on their benzene or heterocyclic ring with various terminal alkynes gave the desired products with nearly the same efficiency as the previously used iodine derivatives. The coupling reactions were performed in the presence of [tetrakis(triphenylphosphine)palladium(0)], copper(i) co-catalyst, and triethylamine, resulting in the formation of numerous hitherto unknown alkynylated oxygen heterocycles, and provide further proof for the applicability of this reaction for these O-heterocycles. Chromones with ethynyl functionality were prepared by removal of the trimethylsilyl protecting group and used as terminal alkynes in a second cross-coupling reaction.

Copper- and amine-free Sonogashira–Hagihara coupling reaction catalyzed by Pd(0) nanoparticles supported on modified crosslinked polyacrylamide

Pd(0) nanoparticles supported on modified crosslinked polyacrylamide containing phosphinite ligand was prepared. The complex was a highly efficient catalyst for the Sonogashira– Hagihara coupling reaction in the absence of a copper cocatalyst and an amine base under aerobic condition. This protocol can be applied for different aryl halides including chloroarenes. The catalyst can be reused several times without any considerable decrease in its activity because of low leaching of palladium metal from the support. Transmission electron microscopy (TEM) analysis of the catalyst revealed the formation of stable and polydispersed Pd(0) nanoparticles.

Expeditious Enyne Construction from Alkynes via Oxidative Pd(II) Catalyzed Heck-Type Coupling

The enyne, ubiquitous in natural products, can be a challenge to generate since these moieties require many synthetic transformations to assemble them. We developed a simpler protocol to construct enynes while we found that this oxidative reaction was tolerant in substrate scope. In addition, the utility of this reaction was demonstrated through the attempt in synthesizing antifungal agent Lamisi.

Imidazolylsulfonates: electrophilic partners in cross-coupling reactions

Aryl imidazolylsulfonates participate as electrophilic coupling partners in palladium-mediated cross-coupling reactions. The aryl imidazolylsulfonates display good stability while maintaining good reactivity in a variety of palladium-catalyzed coupling reactions. Imidazolylsulfonates are a practical and economic alternative to triflates.

Highly convenient amine-free sonogashira coupling in air in a polar mixed aqueous medium by trans- and cis-[(NHC)2PdX2] (X=Cl, Br) complexes of N/O-functionalized N-heterocyclic carbenes

Two new trans- and cis-[(NHC)(2)PdX(2)] (X=Cl, Br) complexes of N/O-functionalized N-heterocyclic carbenes employed in a highly convenient amine-free Sonogashira cross-coupling reaction in air in a polar mixed aqueous medium are reported. Specifically, the trans-[{1-benzyl-3-(3,3-dimethyl-2-oxobutyl)imidazol-2-ylidene}(2)PdBr(2)] (3) and cis-[{1-benzyl-3-(N-tert-butylacetamido)imidazol-2-ylidene}(2)PdCl(2)] (4) complexes effectively catalyzed the Sonogashira cross-coupling reaction of aryl iodides with substituted acetylenes in air in a mixed solvent (DMF/H(2)O, 3:1 v/v) under amine-free conditions. Interestingly, these trans- and cis-[(NHC)(2)PdX(2)] (X=Cl, Br) complexes, with two N-heterocyclic carbene ligands, exhibited superior activity compared with the now popular PEPPSI (pyridine enhanced precatalyst preparation, stabilization and initiation)-themed analogues, trans-[(NHC)Pd(pyridine)X(2)] (X=Cl, Br), 3 a and 4 a, with one N-heterocyclic carbene ligand and a "throw away" pyridine ligand in a trans disposition to each other. The higher activities of 3 and 4 compared with PEPPSI analogues 3 a and 4 a are attributed to more-electron-rich metal centers, as revealed by DFT studies, in the former complexes and is in concurrence with a more electron-rich metal center being effective in facilitating the oxidative addition of aryl halide, often a rate-determining step in palladium-mediated cross-coupling reactions. Complexes 3 and 4 were prepared from the corresponding silver analogues by transmetalation with [(cod)PdCl(2)], whereas the corresponding PEPPSI analogues 3 a and 4 a were obtained directly from the imidazolium halide salts by reaction with PdCl(2) in pyridine in the presence of K(2)CO(3) as base.

Palladium-catalyzed Hiyama cross-coupling reactions of aryl mesylates

The combination of palladium acetate with XPhos shows high efficiency in the Hiyama cross-coupling reactions of aryl mesylates with arylsilanes. The reactions proceed smoothly to generate the corresponding biaryl compounds in good yields.

Palladium-catalyzed Suzuki-Miyaura cross-couplings of aryl tosylates with potassium aryltrifluoroborates

Pd-catalyzed Suzuki-Miyaura cross-coupling reaction of aryl tosylates with potassium aryl trifluoroborate in the presence of bulky and electron-rich phosphine ligand is described. In addition, a useful chemoselective coupling of an aryl chloride in the presence of a tosyloxy group was demonstrated.