Recoverable Palladium Catalysts for Suzuki–Miyaura Cross‐ Coupling Reactions Based on Organic‐Inorganic Hybrid Silica Materials Containing Imidazolium and Dihydroimidazolium Salts

The systems formed by palladium acetate [Pd(OAc)₂] and hybrid silica materials prepared by sol‐gel from monosilylated imidazolium and disilylated dihydroimidazolium salts show catalytic activity in Suzuki–Miyaura cross‐couplings with challenging aryl bromides and chlorides. They are very efficient as recoverable catalysts with aryl bromides. Recycling is also possible with aryl chlorides, although with lower conversions. In situ formation of palladium nanoparticles has been observed in recycling experiments.     

Sulfonated N‐Heterocyclic Carbenes for Pd‐Catalyzed Sonogashira and Suzuki–Miyaura Coupling in Aqueous Solvents

The reactions of the N,N′‐diarylimidazolium and N,N′‐diarylimidazolinium salts with chlorosulfonic acid result in the formation of the respective disulfonated N‐heterocyclic carbene (NHC) precursors in reasonable yields (46–77%). Water‐soluble palladium catalyst complexes, in situ obtained from the respective sulfonated imidazolinium salt, sodium tetrachloropalladate (Na₂PdCl₄) and potassium hydroxide (KOH) in water, were successfully applied in the copper‐free Sonogashira coupling reaction in isopropyl alcohol/water mixtures using 0.2 mol% catalyst loading. The preformed (disulfonatedNHC)PdCl(cinnamyl) complex was used in aqueous Suzuki–Miyaura reactions at 0.1 mol% catalyst loading. The coupling protocol reported here is very useful for Sonogashira reactions of N‐ and S‐heterocyclic aryl bromides and chlorides with aryl‐ and alkylacetylenes.     

Ligand‐Free Buchwald–Hartwig Aromatic Aminations of Aryl Halides Catalyzed by Low‐Leaching and Highly Recyclable Sulfur‐Modified Gold‐Supported Palladium Material

A stable heterogeneous catalyst precursor, sulfur‐modified gold‐supported palladium material (SAPd), has proved to be an excellent source of leached, ligand‐free, Pd for the amination of aryl bromides and chlorides. The reaction‐enabling catalyst is provided in situ as leached Pd in low catalyst loading (0.21±0.02 mol%). This allows the precatalyst (SAPd) to be filtered off and used for a minimum of ten reaction cycles without loss of catalytic activity. SAPd released only trace amounts, less than 0.6 ppm, of highly active Pd during the reaction without any aggregation.     

Direct Amide Synthesis from Alcohols and Amines by Phosphine‐Free Ruthenium Catalyst Systems

Amides are synthesized directly from alcohols and amines in high yields using an in situ generated catalyst from easily available ruthenium complexes such as the (p‐cymene)ruthenium dichloride dimer, [Ru(p‐cymeme)Cl₂]₂, or the (benzene)ruthenium dichloride dimer, [Ru(benzene)Cl₂]₂, an N‐heterocyclic carbene (NHC) ligand, and a nitrogen containing L‐type ligand such as acetonitrile. The phosphine‐free catalyst systems showed improved or comparable activity compared to previous phosphine‐based catalytic systems. The in situ generated catalyst from [Ru(benzene)Cl₂]₂, an NHC ligand, and acetonitrile showed excellent activity toward reactions with cyclic secondary amines such as piperidine and morpholine.     

Palladium‐Catalyzed Efficient and One‐Pot Synthesis of Diarylacetylenes from the Reaction of Aryl Chlorides with 2‐Methyl‐3‐butyn‐2‐ol

An efficient and practical synthetic method has been developed for the preparation of symmetrical diarylacetylenes from the direct reaction of aryl chlorides with 2‐methyl‐3‐butyn‐2‐ol catalyzed by palladium(II) chloride‐bis(tricyclohexylphosphine) [PdCl₂(PCy₃)₂] under mild reaction conditions. Unsymmetrical diarylated acetylenes could be also obtained by using two different aryl chlorides simultaneously. The catalytic procedure includes a novel one‐pot palladium‐catalyzed, double Sonogashira coupling of inactivated aryl chlorides without use of copper(I) as co‐catalyst.     

Expanded Heterogeneous Suzuki–Miyaura Coupling Reactions of Aryl and Heteroaryl Chlorides under Mild Conditions

A mesoporous LTA zeolite (MP‐LTA)‐supported palladium catalyst was developed for the highly efficient Suzuki–Miyaura reaction of aryl and heteroaryl chlorides. The couplings of various aryl chlorides with arylboronic acids in aqueous ethanol were efficiently achieved in the presence of 1.0 mol% of the catalyst. Furthermore, the scope of this catalyst was extended to the coupling of heteroaryl chlorides. Regardless of the substituents, all of the coupling reactions were very clean and highly efficient under mild heating. It shows that our catalyst is one of the most powerful heterogeneous catalysts for the coupling of a wide range of aryl and heteroaryl chlorides. The catalyst could be repetitively used at least 10 times without a significant loss of its catalytic activity. Compared to mesoporous SBA‐15 and MCM‐41 materials, the MP‐LTA support proved to be very stable and robust to prevent degradation upon reuse.     

Aerobic Ligand‐Free Suzuki Coupling Reaction of Aryl Chlorides Catalyzed by In Situ Generated Palladium Nanoparticles at Room Temperature

An aerobic, ligand‐free Suzuki coupling reaction catalyzed by in situ generated palladium nanoparticles in polyethylene glycol with an average molecular weight of 400 Da (PEG‐400) at room temperature has been developed. This catalytic system is a very simple and highly active protocol for the Suzuki coupling of aryl chlorides with arylboronic acids, which proceed smoothly in excellent yields in short times using low catalyst loadings. Control experiments demonstrated that the Suzuki reaction catalyzed by the in situ generated palladium nanoparticles can be carried out much quicker than that using the preprepared particles under the same conditions. The formation of palladium nanoparticles in PEG‐400 was promoted by arylboronic acids.     

Regioselective Heck Reaction of N‐Vinylphthalimide: A General Strategy for the Synthesis of (E)‐N‐Styrylphthalimides and Phenethylamines

The arylation of N‐vinylphthalimide takes place at the β‐position with aryl iodides, bromides and chlorides using palladium acetate [Pd(OAc)₂] or phenone oxime‐derived palladacycles as catalysts under phosphine‐free conditions. The reaction is succesfully carried out in organic solvents, such as DMF, in the presence of an organic base, such as dicyclohexylmethylamine, and with TBAB as additive at 120 °C under conventional or microwave heating. (E)‐N‐Styrylphthalimides are mainly obtained using a rather low palladium loading (0.05–1 mol%). Similar catalytic efficiency is observed using a Kaiser oxime resin‐derived palladacycle, which allows reuse of the polymeric complex for three cycles. The high regioselectivity observed supports that these palladacycles work as a source of Pd(0) species operating mainly through a neutral mechanism. The syntheses of 2‐thienylphenethylamine and mescaline have been performed by subsequent hydrogenation with Wilkinson’s catalyst and hydrazinolysis.     

Pyridin‐, Quinolin‐ and Acridinylidene Palladium Carbene Complexes as Highly Efficient C–C Coupling Catalysts

A series of four new complexes bearing N‐heterocyclic carbene ligands (NHCs) as well as four compounds bearing N‐heterocyclic carbene ligands with remote heteroatoms (rNHCs) of the general types [(NHC)(PPh₃)₂PdCl]⁺BF₄⁻ and [(rNHC)(PPh₃)₂PdCl]⁺BF₄⁻, respectively, have been prepared in high yields. Crystal and molecular structures have been determined for four representative examples. These compounds proved to be efficient catalysts for aryl coupling reactions of the Heck and Suzuki types (reaching TONs of as high as 6,200,000). Both aryl bromides and aryl chlorides can be used as substrates. Like the well known mixed, standard (NHC)(phosphine) compounds, the new six‐numbered, one‐N‐heterocyclic carbene complexes (and in particular certain rNHC‐containing ones) also combine the advantageous stability of bis(carbene) and the high activity of bis(phosphine) complexes. Furthermore, their good catalytic performance and, especially, their easy synthesis based on cheap and commercially available starting materials, make them by far superior when compared to the mixed (NHC)(phosphine) catalysts known thus far.     

Simple Methodology for Heck Arylation at C‐8 of Adenine Nucleosides

A simple method for the arylation of 8‐vinyladenine nucleoside derivatives is reported. With a broad set of aryl iodides and bromides, the reaction is catalyzed by the simple combination palladium acetate/tris(o‐tolyl)phosphine/triethylamine [Pd(OAc)₂/(o‐tol)₃P/Et₃N]. As expected, aryl chlorides are more difficult coupling partners but some undergo reactions with more exotic catalysts. Although trans‐olefins are the major products, minor amounts of cis‐isomers are detected in some cases, and a post‐arylation mechanism for their formation is proposed. Finally, by subtle catalyst modulation chemoselective N‐arylation of the nucleoside can be achieved in the presence of the vinyl moiety.     

Palladium Nanoparticles Immobilized on Nano‐Silica Triazine Dendritic Polymer (Pdnp‐nSTDP): An Efficient and Reusable Catalyst for Suzuki–Miyaura Cross‐Coupling and Heck Reactions

A new catalyst based on palladium nanoparticles immobilized on nano‐silica triazine dendritic polymer (Pd_np‐nSTDP) was synthesized and characterized by FT‐IR spectroscopy, thermogravimetric analysis, field emission scanning electron microscopy, energy dispersive X‐ray, transmission electron microscopy and elemental analysis. The size of the palladium nanoparticles was determined to be 3.1±0.5 nm. This catalytic system showed high activity in the Suzuki–Miyaura cross‐coupling of aryl iodides, bromides and chlorides with arylboronic acids and also in the Heck reaction of these aryl halides with styrenes. These reactions were best performed in a dimethylformamide (DMF)/water mixture (1:3) in the presence of only 0.006 mol% and 0.01 mol% of the catalyst, respectively, under conventional conditions and microwave irradiation to afford the desired coupling products in high yields. The Pd_np‐nSTDP was also used as an efficient catalyst for the preparation of a series of star‐ and banana‐shaped compounds with a benzene, pyridine, pyrimidine or 1,3,5‐triazine unit as the central core. Moreover, the catalyst could be recovered easily and reused several times without any considerable loss of its catalytic activity.     

Monolith‐ and Silica‐Supported Carboxylate‐Based Grubbs–Herrmann‐Type Metathesis Catalysts

The synthesis of silica‐ and monolith‐supported Grubbs–Herrmann‐type catalysts is described. Two polymerizable, carboxylate‐containing ligands, exo, exo‐7‐oxanorborn‐2‐ene‐5,6‐dicarboxylic anhydride and 7‐oxanorborn‐2‐ene‐5‐carboxylic acid were surface‐immobilized onto silica‐ and ring‐opening metathesis (ROMP‐) derived monolithic supports using “grafting‐from” techniques. The “1^st generation Grubbs catalyst”, RuCl₂(CHPh)(PCy₃)₂, was used for these purposes. In addition, a poly(norborn‐2‐ene‐b‐exo, exo‐norborn‐2‐ene‐5,6‐dicarboxylic anhydride)‐coated silica 60 was prepared. The polymer supported anhydride and carboxylate groups were converted into the corresponding mono‐ and disilver salts, respectively, and reacted with the Grubbs–Herrmann catalyst RuCl₂(CHPh)(IMesH₂)(PCy₃) [IMesH₂=1,3‐bis(2,4,6‐trimethylphenyl)‐4,5‐dihydroimidazol‐2‐ylidene]. Heterogenization was accomplished by exchange of one chlorine ligand with the polymeric, immobilized silver carboxylates to yield monolith‐supported catalysts 4, 5 , and 6 as well as silica‐supported systems 7, 8 and 9 . The actual composition of these heterogenized catalysts was proven by the synthesis of a homogeneous analogue, RuCl[7‐oxanorbornan‐2‐(COOAg)‐3‐COO](CHPh)(IMesH₂)(PCy₃) ( 3 ). All homogeneous and heterogeneous catalysts were used in ring‐closing metathesis (RCM) of diethyl diallylmalonate, 1,7‐octadiene, diallyldiphenylsilane, methyl trans‐3‐pentenoate, diallyl ether, N,N‐diallyltrifluoracetamide and t‐butyl N,N‐diallylcarbamate allowing turnover numbers (TON's) close to 1000. In a flow‐through set‐up, an auxiliary effect of pendant silver carboxylates was observed with catalyst 5 , where the silver moiety functions as a (reversible) phosphine scavenger that both accelerates initiation and stabilizes the catalyst by preventing phosphine elution. Detailed catalytic studies were carried out with the monolith‐supported systems 4 and 6 in order to investigate the effects of temperature and chain‐transfer agents (CTA's) such as cis‐1,4‐diacetoxybut‐2‐ene. In all RCM experiments Ru‐leaching was low, resulting in a Ru‐content of the RCM products ≤3.5 μg/g (3.5 ppm).     

A Novel (2,2‐Diarylvinyl)phosphine/Palladium Catalyst for Effective Aromatic Amination

A new series of diarylvinylphosphine ligands was designed and synthesized. A catalyst system, consisting of the ligands and palladium species, effectively catalyzed the coupling reaction of aryl bromides and chlorides with amines to afford the corresponding products in good to excellent yields. The efficiency is likely derived from an interaction between the palladium center and the cis‐aryl moiety on the diarylvinylphosphine ligand stabilizing a catalytic intermediate during the coupling reaction.     

Highly Efficient Suzuki Cross‐Coupling Catalyzed by Palladium/Phosphine‐Imidazolium Carbene System

Novel phosphine‐imidazolium salts 2 have been synthesized and successfully used in palladium‐catalyzed Suzuki cross‐coupling. A combination of 0.05 mol % of [Pd(η‐C₃H₅)Cl]₂ and 0.1 mol % of 2b in the presence of 2 equivs. of K₃PO₄ as base provided coupling products in excellent yields in the reaction of aryl bromides and chlorides with aryl boronic acids.     

Asymmetric,Regioselective Bromohydroxylation of 2‐Aryl‐2‐propen‐1‐ols Catalyzed by Quinine‐Derived Catalysts

The asymmetric bromohydroxylation of 2‐aryl‐2‐propen‐1‐ols catalyzed by quinine‐derived bifunctional catalyst has been developed. The regioselectivity was controlled by employing a boronate ester as tether which was formed in situ and enantioselectivity was introduced by taking advantage of a quinine‐derived bifunctional catalyst which activated the boronate ester and N‐bromosuccinimide (NBS) at the same time. Chiral bromohydrin, which is a useful feedstock in organic synthesis, was produced in moderate to excellent enantioselectivity in a two‐step reaction sequence.     

Mizoroki–Heck Reactions Catalyzed by Dichloro{bis[1‐(dicyclohexylphosphanyl)piperidine]}palladium: Palladium Nanoparticle Formation Promoted by (Water‐Induced) Ligand Degradation

The palladium‐based dichlorobis[1‐(dicyclohexylphosphanyl)piperidine] complex – [(P{(NC₅H₁₀)(C₆H₁₁)₂})₂Pd(Cl)₂] is readily prepared in quantitative yield from the reaction of [Pd(cod)(Cl)₂] (cod=cycloocta‐1,5‐diene) with two equivalents of 1‐(dicyclohexylphosphanyl)piperidine in toluene under N₂ within only a few minutes at room temperature. This complex is a highly active Heck catalyst with excellent functional group tolerance, which reliably operates at low catalyst loadings. Various activated, non‐activated, deactivated, functionalized, sterically hindered, and heterocyclic aryl bromides, which may contain nitro, chloro or trifluoromethane groups, nitriles, acetales, ketones, aldehydes, ethers, esters, lactones, amides, anilines, phenols, alcohols, carboxylic acids, and heterocyclic aryl bromides, such as pyridines and derivatives, as well as thiophenes and aryl bromides containing methylsulfanyl groups have been successfully coupled with various (also functionalized) alkenes in excellent yields and selectivities (the E‐isomers are typically exclusively formed) at 140 °C in the presence of 0.05 mol % of the catalyst in DMF. Even though lower catalyst loadings could be used for many electronically activated, non‐activated and some electronically deactivated aryl bromides without noticeable loss of activity, the great advantage of the reaction protocol presented here lies in its reliability and general applicability, which allows its direct adoption to other aryl bromides without the neccessity of its modification. Experimental observations indicated that palladium nanoparticles are the catalytically active form. Consequently, whereas comparable levels of activity were observed for dichloro‐bis(aminophosphine) complexes of palladium, a dramatic drop in activity was found for their phosphine‐based analogue [(P(C₆H₁₁)₃)₂Pd(Cl)₂].     

A General and Practical Palladium‐Catalyzed Direct α‐Arylation of Amides with Aryl Halides

An efficient system for the direct catalytic intermolecular α‐arylation of acetamide derivatives with aryl bromides and chlorides is presented. The palladium catalyst is supported by Kwong’s indole‐based phosphine ligand and provides monoarylated amides in up to 95% yield. Excellent chemoselectivities (>10:1) in the mono‐ and diarylation with aryl bromides were achieved by careful selection of bases, solvents, and stoichiometry. Under the coupling conditions, the weakly acidic α‐protons of amides (pK_a up to 35) were reversibly depotonated by lithium tert‐butoxide (LiO‐t‐Bu), sodium tert‐butoxide (NaO‐t‐Bu) or sodium bis(trimethylsilyl)amide [NaN(SiMe₃)₂].

     

Resorcin[4]arene‐Derived Mono‐ and Diphosphines in Suzuki Cross‐Coupling

Three resorcin[4]arene cavitands ( 1 – 3 ) having either one or two resorcinolic C‐2 atoms substituted by a  CH₂PPh₂ podand arm were assessed in the Suzuki arylation of aryl bromides with phenylboronic acid. Using P:Pd ratios of 2:1 and operating in dioxane at 100 °C with a catalyst loading of 0.001 mol% resulted in highly efficient catalytic systems. For example, TOFs up to 34570 mol(converted ArBr)⋅mol(Pd)⁻¹⋅h⁻¹ were obtained with the proximally‐disubstituted cavitand 3 when using 4‐bromotoluene as substrate. The performance was shown to vary in the following order: monophosphine 1 <diphosphine 2 <diphosphine 3 (where 2 is the distally disubstituted cavitand). A comparison of the catalytic properties of monophosphine‐cavitand 1 with those of benzyldiphenylphosphine and o‐anisylmethyldiphenyl phosphine suggests that 1 functions as a hemilabile phosphine, the oxygen atoms close to the phosphorus atom behaving as donors able to temporarily increase the electron density on the metal and/or favour the formation of mono‐ligand Pd(0) species.     

Ethylene polymerization with iron‐based diimine catalyst supported on MCM‐41

A mesoporous molecular sieve MCM‐41 supported iron‐based diimine catalyst ( MC ) was prepared for the first time. The kinetic behavior of ethylene polymerization with MC was studied. The effects of Al/Fe molar ratio and various cocatalysts on the catalytic activity and properties of the polyethylene obtained were investigated. The results showed that good catalytic activities can be reached with cocatalyst methylaluminoxane (MAO) and triethylaluminium (TEA). Ethylene polymerization with MC gave polymers with higher molecular weight, melting temperature and onset temperatures of decomposition (T_onset) and better morphology than those obtained with the corresponding homogeneous catalyst. Copyright © 2004 Society of Chemical Industry     

Catalytic activity during the preparation of PE/clay nanocomposites by in situ polymerization with metallocene catalysts

Catalytic activity during the formation of polyethylene (PE)/clay nanocomposites by in situ polymerization with metallocenes was studied. Ethylene polymerization was carried out with the homogeneous metallocene in the presence of the clay particles and using the clay‐supported metallocene catalyst. It was found that the catalytic activity of the homogeneous metallocene does not decrease in the presence of the clay particles and only a slight decrease of activity occurs using the clay‐supported catalyst. The modification of the clay with MAO cocatalyst as well as its intercalation with ODA surfactant were found to play an important role during the in situ formation of the PE/clay nanocomposite. ODA‐intercalated clay apparently facilitates the activation and monomer insertion processes on zirconocene centers located in internal sites of the clay structure. Although metallocene supported on MAO‐treated clay exhibited somewhat lower catalytic activity than that supported directly on the ODA‐intercalated clay, both systems favored the production of PE nanocomposites containing highly exfoliated clay particles. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009