Amine-free Sonogashira Coupling Research Articles

Chelate Palladium(II) Complexes with Saturated N-Phosphanyl-N-Heterocyclic Carbene Ligands: Synthesis and Catalysis

N-Phosphanyl-N-heterocyclic carbenes (NHCPs) featuring a saturated imidazolin-2-ylidene or tetrahydropyrimid-2-ylidene ring have been synthesized and characterized. The free carbenes exhibit good stability and can be stored in the solid state for months at ambient temperature without decomposition. Contrary to imidazoline-based NHCPs, which decompose by ring opening, N-phosphanyltetrahydropyrimid-2-ylidenes isomerize to 2-phosphanyl tetrahydropyrimidines upon heating. The free carbenes are capable of acting as chelating ligands toward palladium(II), forming very stable mononuclear complexes that have been structurally characterized. The catalytic potential of the complexes has been preliminarily assessed in cross-coupling reactions, most notably in the Suzuki coupling of aryl chlorides, where these complexes display promising activity, and in the copper- and amine-free Sonogashira coupling of aryl bromides.

ChemInform Abstract: Unimolecular Tetrakis‐piperidine‐4‐ol: An Efficient Ligand for Copper and Amine Free Sonogashira Coupling.

A unimolecular tetrakis-piperidine-4-ol has been developed as an efficient ligand in combination with Pd/C for copper and amine free Sonogashira coupling. Various aryl and heteroaryl iodides were successfully coupled with aliphatic and aromatic alkynes to give internal alkynes in good to excellent yields.

Palladium(II) phosphine–ylide complexes as highly efficient pre-catalysts in additive- and amine-free Sonogashira coupling reactions performed under aerobic and low Pd loading conditions

Abstract Moisture and air-stable, robust palladacycle phosphine–ylide complexes as catalyst precursors were used in additive- and amine-free Sonogashira cross-coupling reactions. Various aryl halides were coupled with phenylacetylene in DMF, under air, in the presence of 0.001 mol % of the catalyst to afford the corresponding cross-coupled products in good to excellent yields. Application of the five-membered palladacycle [(P^C)PdCl 2 ] ( C 1 ) in Sonogashira coupling reaction produced comparable catalytic activities of the seven-membered [(C^C)PdCl 2 ] ( C 2 ) analogs.

An efficient protocol for copper- and amine-free Sonogashira reactions catalyzed by mononuclear palladacycle complexes containing bidentate phosphine ligands

Abstract An efficient protocol for copper- and amine-free Sonogashira coupling of aryl halides with phenylacetylene in mild reaction conditions under air is reported using moisture/air-stable and robust palladacycle phosphine complexes as catalyst precursors. The use of 0.001 mol% catalysts in the presence of Cs2CO3 allows the coupling reaction to proceed with moderate to good yields. Also, 31P NMR studies showed that palladacycle 1 can be reduced to zerovalent palladium in methanol, by forming dppe dioxide.

Palladium complexes of the N-fused heterocycle derived abnormal N-heterocyclic carbenes for the much-preferred Cu-free and the amine-free Sonogashira coupling in air

Abnormal N-heterocyclic carbenes of the N-fused heterocycles were employed in stabilizing a series of PEPPSI (Pyridine Enhanced Precatalyst Preparation Stabilization and Initiation) themed complexes of the type (abnormal-NHC)PdI2(pyridine). Specifically, imidazo[1,2-a]pyridine based abnormal complexes, trans-{(1-ethyl-2-(4-R-phenyl)-imidazol-3-ylidene[1,2-a]pyridine)}PdI2(pyridine) [R=F (1b), Cl (2b), Br (3b) and Me (4b)] were synthesized from the reaction of corresponding imidazo[1,2-a]pyridinium iodide salts (1–4)a with PdCl2 in pyridine in the presence of K2CO3 as a base. The (1–4)b complexes carried out the Sonogashira couplings of the aryl bromide and iodide substrates with terminal alkynes in a mixed aqueous medium in air under the Cu-free and amine-free conditions. The density functional theory studies revealed that in the (1–4)b complexes, the Ccarbene–Pd σ-bonding interaction was significantly stronger than the trans Pd–Npyridine σ-bonding interaction.

ChemInform Abstract: Copper‐Free Sonogashira Coupling Reaction in Phosphonium Amino Acid Ionic Liquids.

Phosphonium amino acid ionic liquids were found to be useful for the copper-free and amine-free Sonogashira coupling reaction. A hydrophobic and lipophobic ionic liquid permitted easy separation of...

A copper- and amine-free Sonogashira reaction employing chlorophosphine as new and efficient ligand

An efficient palladium-catalyzed copper- and amine-free Sonogashira coupling reaction of aryl bromides and chlorides was studied using a sterically hindered monooxychlorophosphine as new ligand. The use of 2 mol% Pd(OAc)2 in the presence of K2CO3 allows the coupling reaction to proceed at mild condition with good to excellent yields.

Copper-free Sonogashira Coupling Reaction in Phosphonium Amino Acid Ionic Liquids

Abstract Phosphonium amino acid ionic liquids were found to be useful for the copper-free and amine-free Sonogashira coupling reaction. A hydrophobic and lipophobic ionic liquid permitted easy separation of the product from ionic liquid and catalyst, and the recovered ionic liquid containing Pd catalyst could be reused.

Copper-free and amine-free Sonogashira coupling in air in a mixed aqueous medium by palladium complexes of N/ O-functionalized N-heterocyclic carbenes

Highly convenient copper-free and amine-free Sonogashira coupling of aryl bromides and iodides with terminal acetylenes under amenable conditions in air and in a mixed aqueous medium are reported using several new, user friendly and robust palladium precatalysts ( 1– 5) of N/ O-functionalized N-heterocyclic carbenes (NHCs). In particular, the precatalysts, 1 and 2, were synthesized from the imidazolium chloride salts by the treatment with PdCl 2 in pyridine in presence of K 2CO 3 as a base while the precatalysts, 3– 5, were synthesized from the respective silver complexes by the treatment with (COD)PdCl 2. The DFT studies carried out on the 1– 5 complexes suggest the presence of strong NHC–Pd σ-interactions arising out of deeply buried NHC–Pd σ-bonding molecular orbitals (MOs) that account for the inert nature of the metal–carbene bonds and also provide insights into the exceptional stability of these precatalysts.

Palladium complexes of abnormal N-heterocyclic carbenes as precatalysts for the much preferred Cu-free and amine-free Sonogashira coupling in air in a mixed-aqueous medium

A series of new PEPPSI (Pyridine Enhanced Precatalyst Preparation Stabilization and Initiation) themed precatalysts of abnormal N-heterocyclic carbenes for the highly desirable Cu-free and amine-free Sonogashira coupling in air in a mixed-aqueous medium is reported. Specifically, the PEPPSI themed (NHC)PdI2(pyridine) type precatalysts, 1b-4b, efficiently carried out the highly convenient Cu-free and amine-free Sonogashira coupling of aryl bromides and iodides with terminal acetylenes in air in a mixed aqueous medium. Complexes, 1b-4b, were synthesized by the direct reaction of the corresponding imidazo[1,2-a]pyridinium iodide salts, 1a-4a, with PdCl2 in pyridine in the presence of K2CO3 as a base while the imidazo[1,2-a]pyridinium iodide salts, 1a-4a, were in turn synthesized by the alkylation reactions of the respective imidazo[1,2-a]pyridine derivatives with alkyl iodides. The density functional theory (DFT) studies revealed that these imidazol-3-ylidene[1,2-a]pyridine derived abnormal carbenes are strongly sigma-donating and consequently significantly weaken the catalytically important labile trans pyridine ligand in 1b-4b.

Immobilization of Pd Catalysts on Mesoporous Silica for Amine- and Copper-Free Sonogashira Coupling Reactions

Immobilization of catalysts on solid supports is a promising approach to combine the advantages of heterogeneous and homogeneous catalysts. Pd(PPh3)2Cl2, known as an extremely active homogeneous catalyst for the Sonogashira coupling reaction, has been immobilized on high-surface-area MCF (mesocellular foams)–type mesoporous silica powder modified with 3-aminopropyltriethoxysilane and subsequently with diphenylphosphine. The functionalized MCF-type silica and supported catalysts have been characterized by x-ray photoelectron spectroscopy (XPS), fourier transform infrared spectroscopy (FTIR), elemental analysis, nitrogen sorption porosimetry, and scanning electron microscopy (SEM). Such supported Pd catalysts have proven to be useful recyclable reagents for copper- and amine-free Sonogashira coupling reactions of haloaromatic compounds with terminal alkynes.

Easy Copper‐, Ligand‐ and Amine‐Free Sonogashira Coupling Reaction Catalyzed by Palladium on Carbon at Low Catalyst Loading and by Exposure to Air.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Easy Copper-, Ligand- and Amine-Free Sonogashira Coupling Reaction Catalyzed by Palladium on Carbon at Low Catalyst Loading and by Exposure to Air

An easy copper-, ligand and amine-free Sonogashira coupling reaction catalyzed by commercially available palladium on carbon with air at low catalyst loading (0.2 mol% Pd) has been developed. Aryl iodides coupled with aromatic alkynes gave good to excellent yields and with aliphatic alkyne gave moderate to good yields. The reaction system is easy to handle.

Easy Copper-, Ligand- and Amine-Free Sonogashira Coupling Reaction Catalyzed by Palladium on Carbon at Low Catalyst Loading and by Exposure to Air

Easy Copper-, Ligand- and Amine-Free Sonogashira Coupling Reaction Catalyzed by Palladium on Carbon at Low Catalyst Loading and by Exposure to Air

A copper- and amine-free Sonogashira coupling reaction promoted by a ferrocene-based phosphinimine-phosphine ligand at low catalyst loading

A palladium(II) complex containing a ferrocene-based phosphinimine-phosphine ligand was applied to the amine- and copper-free Sonogashira coupling of aryl iodides and aryl bromides with terminal alkynes using 1 equiv of tetrabutylammonium acetate as an activator. The corresponding disubstituted alkynes were obtained in high yields and TONs using 0.1 mol % Pd-catalyst.