Synthesis of substituted biaryls via Suzuki,Stille and Hiyama cross‐coupling and homo‐coupling reactions by CN‐dimeric and monomeric ortho‐palladated catalysts

The catalytic activity of dimeric [Pd{C₆H₂(CH₂CH₂NH₂)–(OMe)₂,2,3}(μ‐Br)]₂ and monomeric [Pd{C₆H₂(CH₂CH₂NH₂)–(OMe)₂,2,3}Br(PPh₃)] complexes as efficient, stable and air‐ and moisture‐tolerant catalysts was investigated in the Suzuki, Stille and Hiyama cross‐coupling and homo‐coupling reactions of various aryl halides. Substituted biaryls were produced in excellent yields in short reaction times using catalytic amounts of these complexes. The monomeric complex was demonstrated to be more active than the corresponding dimeric catalyst for the cross‐coupling reaction of unreactive aryl bromides and chlorides. The combination of homogeneous metal catalysts and microwave irradiation gave higher yields of products in shorter reaction times. Copyright © 2013 John Wiley & Sons, Ltd.     

Five-membered cyclopalladated complex containing bidentate phosphine ligands; Synthesis,characterization, and highly efficient Suzuki cross-coupling reactions

A nonsymmetric phosphorus ylide and its palladium(II) complex have been synthesized as potential catalytically active compounds. The reaction of 1 equiv nonsymmetric phosphorus ylide, Ph₂PCH₂PPh₂C(H)C(O)PhBr with [Pd(dppe)Cl₂], followed by treatment with 2 equiv AgOTf led to [(dppe)Pd(Ph₂PCH₂PPh₂C(H)C(O)PhBr)](OSO₂CF₃)₂, which contains a five-membered P,P chelate ring on one side and a five-membered P,C chelate ring on the other side. The palladium complex was synthesized and investigated by fourier transform infrared spectroscopy (FT-IR), UV–visible, multinuclear (¹H, ³¹P and ¹⁹F) nuclear magnetic resonance (NMR), and electrospray ionisation-mass spectroscopic techniques. FT-IR and ³¹P NMR studies revealed that the phosphorus ylide is coordinated to palladium via the terminal phosphorus (P_c) of the ylide and methene group (CH). Suzuki reactions for varying aryl halides using the cyclopalladated complex as an efficient catalyst were performed. Various aryl halides were coupled with arylboronic acids in DMF, under air, in the presence of 0.001?mol% of the homogeneous catalyst to afford the corresponding cross-coupled products in good to excellent yields.     

MCM-41-supported bidentate phosphine palladium(0) complex as an efficient catalyst for the heterogeneous Stille reaction

A Stille coupling reaction of organostannanes with organic halides has been developed in the presence of a catalytic amount of MCM-41-supported bidentate phosphine palladium(0) complex (0.5 mol %) in DMF/H₂O (9:1) under air atmosphere in high yields. This polymeric palladium catalyst exhibits higher activity than Pd(PPh₃)₄ and can be reused at least 10 times without any decrease in activity.     

Atom-efficient cross-coupling reactions of triarylbismuths with acyl chlorides under Pd(0) catalysis

The atom-efficient cross-coupling reaction of triarylbismuths with a variety of aliphatic, aromatic, and hetero-aromatic acyl chlorides was demonstrated to afford high yields of cross-coupled ketones under palladium catalysis. The corresponding cross-coupling reaction with diacid chlorides also furnished bis-coupled ketones in good yields.     

Synthesis of cobalt-containing monodentate phosphine ligand and application toward Suzuki cross-coupling reactions

A bulky, dicobalt complexed, mono-dentate phosphine, [(μ-PPh₂CH₂PPh₂)Co₂(CO)₄](μ,η-PhCCPCy₂) (4), was prepared from the reactions of the bis(diphenylphosphino)methylene (dppm)-bridged dicobalt complex Co₂(CO)₆(μ-Ph₂PCH₂PPh₂) (2) with PhCCPCy₂ (3). Combination of 4 and Pd(OAc)₂ (1:1) gave an active catalyst for the palladium-catalyzed Suzuki coupling of aryl bromides with phenylboronic acid; the catalytic reactions can be performed even under low catalyst loadings (0.1-0.001 mol% 4/Pd(OAc)₂). Compound 4 has been proved to be an authentic and effective mono-dentate phosphine ligand. Crucial factors such as 4/Pd(OAc)₂ ratio, base being used, solvent volume, temperature, and electronic variation of the aryl bromides in reactions were also investigated and results are reported.     

The influence of the nature of phosphine ligand on palladium catalysts for cross-coupling of weakly nucleophilic potassium pentafluorophenyltrifluoroborate with ArHal and PhCH2Hal (Hal=Br,Cl)

The influence of the ligand nature on catalytic activity of palladium catalysts for cross-coupling of weakly nucleophilic potassium pentafluorophenyltrifluoroborate, which imitates the behavior of electron-deficient organoboron reagents, with aryl halides, ArHal (Hal=Br, Cl) was studied. The activity of the catalysts generated in situ from Pd(OAc)₂ and appropriate phosphorous containing ligands and the reaction selectivity was found to depend on the nature of bulky phosphines used as ligands. As a result, conditions for involving the electron-deficient organoboron reagent—potassium pentafluorophenyltrifluoroborate—in the palladium-catalyzed cross-coupling with aryl bromides and aryl chlorides were identified. It was demonstrated that the chosen conditions are appropriate for the reaction of K[C₆F₅BF₃] with benzyl chloride and benzyl bromide deriving pentafluorophenylarylmethanes, C₆F₅CH₂Ar.     

Oxidative addition of N-halosuccinimides to palladium(0): the discovery of neutral palladium(II) imidate complexes, which enhance Stille coupling of allylic and benzylic halides

The Stille coupling of organostannanes and organohalides, mediated by air and moisture stable palladium(II) phosphine complexes containing succinimide or phthalimide (imidate) ligands, has been investigated. An efficient synthetic route to several palladium(II) complexes containing succinimide and phthalimide ligands, has been developed. cis-Bromobis(triphenylphosphine)(N-succinimide)palladium(II) [(Ph₃P)₂Pd(N-Succ)Br] is shown to mediate the Stille coupling of allylic and benzylic halides with alkenyl, aryl and allyl stannanes. In competition experiments between 4-nitrobromobenzene and benzyl bromide with a cis-stannylvinyl ester, (Ph₃P)₂Pd(N-Succ)Br preferentially cross-couples benzyl bromide, whereas with other commonly employed precatalysts 4-nitrobromobenzene undergoes preferential cross-coupling. Furthermore, preferential reaction of deactivated benzyl bromides over activated benzyl bromides is observed for the first time. The type of halide and presence of a succinimide ligand are essential for effective Stille coupling. The type of phosphine ligand is also shown to alter the catalytic activity of palladium(II) succinimide complexes.     

Pd(0) complex of fuberidazole modified magnetic nanoparticles: A novel magnetically retrievable high-performance catalyst for Suzuki and Stille C-C coupling reactions

Fuberidazole has been successfully immobilized onto nano-Fe₃O₄ supported (3-chloropropyl)trimethoxysilane (3-CPTS) leading to a novel functionalized magnetic nanoparticle (FB/MNP). The Pd(0) complex, Pd-FB/MNP, was prepared by grafting Pd (OAc)₂ on FB/MNP and subsequent reduction of a synthesized Pd (II) complex using NaBH₄. Pd-FB/MNP has been characterized by FT-IR, SEM, TGA, XRD, ICP, EDS, BET and VSM. The Pd(0) complex proved to be an efficient phosphine- and halide-free recyclable heterogeneous catalyst for Suzuki as well as for Stille C-C coupling reactions showing high catalytic activity (up to 98%). Its catalytic activity in both reactions has been studied in PEG-400 as a green solvent. Besides, the selectivity of aryl iodide and aryl bromide over aryl chloride is observed during the C-C coupling reaction. The catalyst could be recovered easily from the reaction mixture using an external magnet device and recycled several times without considerable loss in activity. Additionally, the results of a palladium leaching test of the nano-catalyst demonstrate that no leaching of Pd took place during the C-C coupling process making the procedure environmentally friendly.     

Microwave-assisted Stille and Hiyama cross-coupling reactions catalyzed by ortho-palladated complexes of homoveratrylamine

The activity of dimeric [Pd{C₆H₂(CH₂CH₂NH₂)-(OMe)₂-3,4}(μ-Br)]₂ and monomeric [Pd{C₆H₂(CH₂CH₂NH₂)-(OMe)₂-3,4}Br(PPh₃)] complexes as efficient, air, and moisture tolerant catalysts was investigated in Stille and Hiyama cross-coupling reactions of various aryl halides. Substituted biaryls were produced in excellent yields in short reaction times using these complexes. The monomeric complex had been demonstrated to be more active than the corresponding dimeric catalyst for the cross-coupling of some of aryl bromides and unreactive aryl chlorides. The combination of homogenous metal catalyst, microwave irradiation, and microwave-active polar solvents gave high yields of products in short reaction times.     

Di-t-butyl(ferrocenylmethyl)phosphine: air-stability, structural characterization, coordination chemistry, and application to palladium-catalyzed cross-coupling reactions

Di-t-butyl(ferrocenylmethyl)phosphine (1) has been isolated and structurally characterized. This ligand was found to be reasonably air stable as a solid and it has been shown to possess electron donating ability similar to that of tri-i-propylphosphine. A palladium catalyst bearing this ligand performed room temperature Suzuki-Miyaura coupling reactions with aryl bromides. Modest Heck coupling reactivity with aryl bromides was also observed at 100 °C. Complexation of 1 with Pd₂(dba)₃ led to formation of (1)₂Pd⁰. Addition of 4-bromoanisole to solutions containing both 1 and Pd₂(dba)₃ led to formation of an oxidative addition product when 1:Pd ratios were ?1. With a 2:1 ratio of 1:Pd, monophosphine complex formation and oxidative addition were significantly inhibited.     

Tsuji-Trost allylations with palladium recovery by phosphines/Pd(0)-triolefinic macrocyclic catalysts

The allylation of several nitrogen and oxygen based nucleophiles with ethyl cinnamyl carbonate under mild conditions is described. The processes take place in the absence of added base and in the presence of the precatalytic system Pd(0)-triolefinic macrocycle/1,1′-bis(diphenylphosphino)ferrocene. The macrocyclic ligand plays a key role in the recovery of the metal in the form of the initial macrocyclic complex.     

Organostannoxane-supported Pd(0) nanoparticles as efficient catalysts for Heck-coupling reactions

A new functional organostannoxane cage, SnP, that contains phosphine ligands in its periphery has been structurally utilized as support palladium(0) nanoparticles SnPPd. The latter was shown to catalyze the Heck coupling reactions of wide variety of functionalities efficiently.     

Palladium catalyzed Suzuki cross-coupling reactions using N,O-bidentate ligands

Palladium-catalyzed Suzuki cross-coupling reactions employing Schiff-bases as ligands toward a series of substituted arylbromides and boronic acids were pursued. In the presence of a N,O-bidentate ligand, 2-[1-(2,4,6-trimethyl-phenylimino)-ethyl]-phenol 5, the catalytic reactions could be carried out efficiently at room temperature with a wide array of arylbromides, even with electronically deactivated arenes. A deprotonated 5, 5′, chelated palladium acetate complex, [5′Pd(II)(OAc)(solv)] 8, was proposed as a precursor of a genuine catalytically active species.     

Palladium(II) complexes bearing methylene and ethylene bridged pyrido-annelated N-heterocyclic carbene ligands as active catalysts for Heck and Suzuki-Miyaura cross-coupling reactions

The synthesis and characterization of new bidentate N-heterocyclic carbene ligands is described. The ligands are derived from methylene (me) and ethylene (et) bridged imidazo[1,5-a]pyridine-3-ylidenes (impy) and can be synthesized readily from imidazo[1,5-a]pyridine and the respective dihaloalkanes. Palladium(II) dihalide complexes bearing these et(impy)₂ or me(impy)₂ ligands were prepared and also structurally characterized. The angle of the impy plane vs. the C-Pd-C plane is depending on the bridging unit as well as the halide ligands. In the solid state the me(impy)₂PdBr₂ complex forms a dimer by weak intermolecular Pd-hydrogen bridges. The activity of the Pd-complexes as catalysts for the Heck and Suzuki-Miyaura reactions was tested under various conditions. The catalysts show good activity at 120 °C in Heck and at 85 °C in Suzuki-Miyaura reactions.     

Highly active Pd(II) catalysts with pyridylbenzoimidazole ligands for the Heck reaction

The air-, and thermo-stable palladium(II) complexes C1-C10 are prepared by the reaction of PdCl₂(CH₃CN)₂ with pyridylbenzoimidazole. With various substituents on the pyridine ring, palladium atom was coordinated by two pyridylbenzoimidazole molecules via nitrogen atoms of benzoimidazole. The structure of complexes C3, C4, C6, and C7 has been confirmed by X-ray diffraction analysis. Without substituents on the pyridine ring, palladium atom was directly coordinated with two nitrogen atoms of pyridine and benzoimidazole nitrogen via intramolecular chelation (C10). These complexes performed the Heck olefination of aryl bromides in a good to high yield under phosphine-free conditions.     

Pd(0) and Pd(II) derivatives with heteroannularly bridged chiral ferrocenyl diphosphine ligands - A stereochemical analysis

The ligands (S_c, S_p)-1-diphenylphosphino-2,1′-(1-dicyclohexylphosphinopropanediyl)ferrocene, (S_c, S_p)-PP^CyPF, and (S_c, S_p)-1-diphenylphosphino-2,1′-(1-diphenylphosphinopropanediyl)ferrocene, (S_c, S_p)-PP^PhPF, have been used in the synthesis of the new Pd(0) and Pd(II) derivatives [Pd(PP^CyPF)(DMFU)] (1) (DMFU = dimethylfumarate), [Pd(PP^CyPF)(MA)] (2) (MA = maleic anhydride), [Pd(η³-2-Me-C₃H₄)(PP)]OTf (PP = PP^CyPF, 3; PP^PhPF, 4) (OTf = triflate), [PdRR′(PP)] (R = Me, R′ = Cl, PP = PP^CyPF, 5, PP^PhPF, 6; R = R′ = Me, PP = PP^CyPF, 7, PP^PhPF, 8; R = R′ = C₆F₅, PP = PP^CyPF, 9, PP^PhPF, 10). The molecular structure of 7 has been determined by X-ray diffraction. In the cases of complexes 1-4 two isomers are formed depending on the orientation of the ancillary ligand with respect to the ferrocenyl core. The stereochemistry of these complexes has been determined. In complex 6 the two possible isomers are obtained whereas in complex 5 the derivative with the Me group trans to PPh₂ is selectively formed. Restricted rotation of the pentafluorophenyl groups with respect to the Pd-C bond has been found in 9 and 10. In all derivatives the conformation of the ferrocenyl ligand is the same as that seen by X-ray diffraction and deduced from NMR data.     

Halogenated-2-pyrones in Sonogashira cross-coupling: limitations, optimisation and consequences for GC analysis of Pd-mediated reactions

The Sonogashira couplings of 4-bromo-6-methyl-2-pyrone (5) with phenylacetylene, mediated by Pd(PPh₃)₂Cl₂ in the presence of a CuI co-catalyst, have been investigated in detail. The concentration of Pd dramatically influences the product yield, with lower Pd-loadings favouring higher conversions and purer cross-coupled product. A post reaction time-dependence in product conversion is seen in samples quenched solely on silica-gel (eluted with CH₂Cl₂). The effect is mirrored in reactions employing 4-nitro-bromobenzene (14) and to a lesser extent (E) and (Z)-ethyl 3-iodo-2-propenonate (16) under similar conditions. A more efficient quenching system (using excess dppe) has been developed to enable accurate determinations in product conversions. Alternatively, solvent and base (Et₃N) removal in vacuo, or quench with saturated aqueous ammonium chloride, prevents further turnover in Sonogashira coupling. An ESI-MS study on samples eluted through silica was undertaken to probe the nature of the soluble Pd/Cu species. The Sonogashira cross-coupling of 4-chloro- and 6-chloro-2-pyrone (18 and 20, respectively) has further been investigated. The former undergoes successful coupling, however the latter decomposes in polar aprotic and protic solvents under standard conditions, through a chlorine substitution process, making Pd-mediated reactions problematic.     

Room temperature Pd(0)/Ad3P-catalyzed coupling reactions of aryl chlorides with bis(pinacolato)diboron

Room temperature Pd(0)/Ad₃P-catalyzed cross-coupling reactions of aryl chlorides with bis(pinacolato)diboron are described. The Pd(0)/Ad₃P catalyst, generated from Ad₃P-coordinated acetanilide-based palladacycle complex, proved to be an efficient catalyst system for the Miyaura borylation reactions of a variety of aryl chlorides with bis(pinacolato)diboron. The mild reaction condition, the easy availability of the catalyst and good coupling yields make these reactions potentially useful in organic synthesis.     

Heterogeneous Suzuki and Stille coupling reactions using highly efficient palladium(0) immobilized MCM-41 catalyst

An efficient palladium(0) immobilized MCM-41 catalytic system for C-C cross-coupling reaction has been developed. Ligand-free Pd(0)-MCM-41 catalyst can be successfully used in coupling reaction between various aryl halides including deactivated chlorobenzene with aryl borane and organotin to give biaryls in excellent yields with high turnover frequency (TOF) (the maximal TOFs are up to 6990 for the reaction of bromobenzene with phenylboronic acid). The catalyst can be recycled and reused without any loss of catalytic activity.     

Pd(II) complexes of novel phosphine ligands: Synthesis,characterization, and catalytic activities on Heck reaction

Novel phosphine oxides, (((3-methylpyridin-2-yl)amino)methyl)diphenylphosphine oxide (1) and diphenyl((pyrazin-2-ylamino)methyl)phosphine oxide (2), were synthesized and characterized. Phosphines ligands (3 and 4) were obtained by the reduction of 1 and 2 with AlH₃, monitored by ³¹P NMR spectroscopy. Pd(II) complexes of 3 and 4 were synthesized and characterized (5 and 6). The catalytic activity of 5 and 6 was tested on the reaction of styrene with both activated and deactivated aryl bromides in air. The results of the catalytic experiments were discussed through DFT calculations.