Heterogeneous Palladium Catalysts Applied to the Synthesis of 2‐ and 2,3‐Functionalised Indoles

Heterogeneous palladium catalysts ([Pd(NH₃)₄]²⁺/NaY and [Pd]/SBA‐15) were applied to the synthesis of 2‐functionalised indoles, giving generally high conversions and selectivities (>89% yield) using only 1 mol % [Pd]‐catalyst under standard reaction conditions (polar solvent, 80 °C). For the synthesis of 2,3‐functionalised indoles by cross‐coupling arylation, the [Pd]/SBA‐15 catalyst was found to be particularly interesting, producing the expected compound with =35% yield after 12 days of reaction, which is comparable to the homogeneous catalyst, Pd(OAc)₂ (=48% yield). In the course of the study, the dual reactivity of the indole nucleus was demonstrated: aryl bromides gave clean C C coupling while aryl iodides led to a clean C N coupling.     

tert‐Butyl Sulfoxides: Key Precursors for Palladium‐Catalyzed Arylation of Sulfenate Salts

The present report describes an efficient and clean generation of sulfenate salts (R¹SO⁻) by pyrolysis of readily available tert‐butyl sulfoxides to give sulfenic acids (R¹SOH) and traceless isobutene, followed by hydrogen abstraction with a weak inorganic base (K₃PO₄). The relevance of this process was exemplified through an in situ palladium‐catalyzed cross‐coupling reaction with aryl halides/triflates leading to aryl sulfoxides. The operationally simple C S bond‐forming protocol developed uses Pd(dba)₂ as catalyst and Xantphos as ligand in toluene or a toluene/H₂O mixture. Further extensions include the use of di‐tert‐butyl sulfoxide as an equivalent for sulfur monoxide dianion (SO²⁻) and the development of diastereoselective versions in the [2.2]paracyclophane and biaryl series.

     

Phosphorus‐Carbon Bond Formation: Palladium‐Catalyzed Cross‐Coupling of H‐Phosphinates and Other P(O)H‐Containing Compounds

Two generally applicable systems have been developed for the cross‐coupling of P(O)H compounds with C_sp2 X and related partners. Palladium catalysis using a ligand/additive combination, typically either xantphos/ethylene glycol or 1,1′‐bis(diphenylphosphino)ferrocene/1,2‐dimethoxyethane, with diisopropylethylamine as the base, proved to be generally useful for the synthesis of numerous P C containing compounds. Routinely, 2 mol% of catalyst are employed (less than half the amount typically employed in most other literature reports). In most cases, excellent results are obtained with a variety of electrophiles (RX, where R=alkenyl, allyl, alkynyl, etc.). The full account of our studies is disclosed, including tandem hydrophosphinylation/coupling and coupling/coupling for doubly catalytic phosphorus‐carbon bond formation. The methodology compares favorably with any existing literature report. The use of an additive appears to be a generally useful strategy to control the reactivity of phosphinylidene compounds.     

A novel resistive‐type humidity sensor based on poly(p‐diethynylbenzene)

Poly(p‐diethynylbenzene) (PDEB) synthesized with nickel catalyst Ni(CC ○ CCH)₂(PPh₃)₂ (Ni C) in dioxane–toluene mixed‐solvent system at 25°C shows a rich trans structure with pendant‐group ( ○ CCH) content of about 35% having higher molecular weight and good solubility. A novel resistive‐type humidity sensor based on PDEB is presented. Its humi‐sensing characteristics are described and discussed. The impedance of the sensor changed from ∼ 10³–10⁷ Ω in almost the whole humidity range [∼ 15–92% relative humidity (RH)], which is low compared with sensors based on other humi‐sensitive conjugate polymers, and hysteresis of no more than 3% RH was observed. The sensor prepared by Langmuir–Blodgett (LB) deposition method shows the best humidity response. An explanation of humi‐sensing behavior of PDEB is attempted by taking into account the interaction between hydrogen protons and super π‐conjugate orbits in PDEB. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2010–2015, 1999     

Catalytic synthesis of 1,6‐dicarbamate hexane over MgO/ZrO2

MgO/ZrO₂ catalyst was prepared for the synthesis of 1,6‐dicarbamate hexane (HDC) using dimethyl carbonate (DMC) and 1,6‐diamine hexane (HDA) as raw materials. When the catalyst is calcined at 600 °C and MgO load is 6 wt%, the catalyst exhibits better activity. When the concentration of catalyst is 2 g (100 mL)^?1 DMC, n(HDA):n(DMC) = 1:10, reaction time is 6 h under reflux temperature, and the yield of 1,6‐dicarbamate hexane is 53.1%. HDC yield decreases from 53.1% to 35.3% after MgO/ZrO₂ being used for three times. The decrease in specific surface area may be attributed to deactivation of MgO/ZrO₂. Copyright © 2007 Society of Chemical Industry     

Multiple Oxidative Dehydrogenative Functionalization of Arylacetaldehydes Using Molecular Oxygen as Oxidant Leading to 2‐Oxo‐acetamidines

A novel copper‐catalyzed, multiple oxidative dehydrogenative functionalization of arylacetaldehydes leading to 2‐oxo‐acetamidine compounds has been developed. This transformation is highly efficient with dissociation of six hydrogens including two sp³ C H and one sp² C H bond activations. This method not only provides an efficient approach to 2‐oxo‐acetamidine compounds, but also offers a valuable mechanistic insight into this novel copper catalysis.     

Air‐Stable and Highly Active Dendritic Phosphine Oxide‐ Stabilized Palladium Nanoparticles: Preparation,Characterization and Applications in the Carbon‐Carbon Bond Formation and Hydrogenation Reactions

Dendrimer‐stabilized palladium nanoparticles were formed in the reduction of palldium bis(acetylacetonate) [Pd(acac)₂] in the presence of phosphine dendrimer ligands using hydrogen in tetrahydrofuran. The resulting Pd nanoparticles were characterized by TEM, ³¹P NMR and ³¹P MAS NMR. The results indicated that the dendritic phosphine ligands were oxidized to phosphine oxides. These dendrimer‐stabilized Pd nanoparticles were demonstrated to be efficient catalysts for Suzuki and Stille coupling reactions and hydrogenations. The dendritic wedges served as a stabilizer for keeping the nanoparticles from aggregating, and as a vehicle for facilitating the separation and/or the recycling of the Pd catalyst. In the case of the Suzuki coupling reaction, these Pd nanoparticles exhibited high catalytic efficiency (TON up to 65,000) and air stability as compared with the commonly used homogeneous catalyst tetrakis(triphenylphosphine)palladium [Pd(PPh₃)₄]. In addition, the results obtained from the bulky dendritic substrate suggest that the Pd nanoparticles might act as reservoir of catalytically active species, and that the reaction is actually catalyzed by the soluble Pd(0) and/or Pd(II) species leached from the nanoparticle surface.     

Influence of precursors of Li2O and MgO on surface and catalytic properties of Li‐promoted MgO in oxidative coupling of methane

The influence of the catalyst precursors (for Li₂O and MgO) used in the preparation of Li‐doped MgO (Li/Mg = 0.1) on its surface properties (viz basicity, CO₂ content and surface area) and activity/selectivity in the oxidative coupling of methane (OCM) process at 650–750 °C (CH₄/O₂ feed ratio = 3.0–8.0 and space velocity = 5140–20550 cm³ g⁻¹ h⁻¹) has been investigated. The surface and catalytic properties are found to be strongly affected by the precursor for Li₂O (viz lithium nitrate, lithium ethanoate and lithium carbonate) and MgO (viz magnesium nitrate, magnesium hydroxide prepared by different methods, magnesium carbonate, magnesium oxide and magnesium ethanoate). Among the Li–MgO (Li/MgO = 0.1) catalysts, the Li–MgO catalyst prepared using lithium carbonate and magnesium hydroxide (prepared by the precipitation from magnesium sulfate by ammonia solution) and lithium ethanoate and magnesium acetate shows high surface area and basicity, respectively. The catalysts prepared using lithium ethanoate and magnesium ethanoate, and lithium nitrate and magnesium nitrate have very high and almost no CO₂ contents, respectively. The catalysts prepared using lithium ethanoate or carbonate as precursor for Li₂O, and magnesium carbonate or ethanoate, as precursor for MgO, showed a good and comparable performance in the OCM process. The performance of the other catalysts was inferior. No direct relationship between the basicity of Li‐doped MgO or surface area and its catalytic activity/selectivity in the OCM process was, however, observed. © 2000 Society of Chemical Industry     

Palladium‐Catalyzed Synthesis of (Z)‐3‐Arylthioacrylic Acids and Thiochromenones

The three‐component reaction of aryl halides, sodium sulfide pentahydrate (Na₂S⋅5 H₂O), and propiolic acid in the presence of 2.5% bis(triphenylphosphine)palladium chloride [Pd(PPh₃)₂Cl₂], 5% 1,4‐bis(diphenylphosphino)butane (dppb) and 2 equivalents of 1,8‐diazabicycloundec‐7‐ene (DBU) produces stereoselectively (Z)‐3‐arylthioacrylic acids in good yields. A study of the reaction pathway suggested that the C S bond formation between aryl halides and Na₂S⋅5 H₂O proceeded first, and the resulting intermediate reacted with propiolic acid to produce the desired product. In addition, when the resulting product was treated with acid, the respective thiochromenones were formed in good yields.     

Reactions of phenolic ester alcohol with aliphatic isocyanates—transcarbamoylation of phenolic to aliphatic urethane: A 13C‐NMR study

Reactions of aliphatic isocyanates with a phenolic ester alcohol (PHEA) were investigated using ¹³C‐NMR spectroscopy. PHEA has two reactive sites: a phenolic  OH group and a secondary aliphatic  OH group. Both  OH groups react with the isocyanate groups. With an organotin catalyst, dibutyltin dilaurate (DBTDL), the aliphatic  OH group reacts first. With a tertiary amine catalyst, 1,4‐diazabicyclo[2.2.2]octane (DABCO), or triphenylphosphine (Ph₃P) or even in the absence of a catalyst at room temperature (RT) the phenolic  OH group reacts first. With the organotin catalyst, the reactions are generally complete in a day at RT. With DABCO or triphenylphosphine or DNNDSA catalysts, the reactions are almost complete only in 3–4 days at RT in ethyl acetate or acetonitrile. Uncatalyzed reactions are slower. With an acid catalyst such as dinonylnaphthalenedisulfonic acid (DNNDSA), both  OH groups react with the isocyanate. When equimolar quantities of PHEA and hexamethylenediisocyanate (HDI) polymerize at RT or reflux in the presence of a catalyst, both  OH groups react, with the phenol reacting slowly. Upon refluxing, the phenolic  OH‐based urethane slowly rearranges (transcarbamoylation) to the aliphatic  OH‐based urethane. DABCO and Ph₃P catalysts effect this rearrangement at a much slower rate than does the acid catalyst. In the presence of a catalytic amount of DBDTL in a refluxing solvent, this rearrangement is complete in 2 h. By refluxing the phenolic–OH‐based urethane in isopropanol, the mechanism of transcarbamoylation was found to be intermolecular. The mechanism is likely to involve deblocking of the phenolic urethane and subsequent reaction of the isocyanate generated, with the aliphatic  OH group. This conclusion was confirmed by differential scanning calorimetry (DSC) experiments. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2212–2228, 2000     

Highly Stereoselective Synthesis of Arylene‐Silylene‐Vinylene Polymers

Stereoregular trans‐arylene‐silylene‐vinylene polymers of M_w=13100–34800 and PDI=1.6–2.9 of the general formulas CH₂CH [ SiMe₂C₆H₄‐SiMe₂CHCH ] ( 16, 17, 18 ) and CH₂CH [ (R)CHCHC₆H₄CHCH ] (where R= Me₂Si‐p C₆H₄‐ SiMe₂ ,  Me₂Si‐m C₆H₄SiMe₂ and  Me₂SiC₆H₄C₆H₄SiMe₂ ) ( 19, 20, 21 ) have been effectively synthesized via silylative coupling (SC) homopolycondensation of bis(vinyldimethylsilyl)arenes ( 10, 12, 14 ) and cross‐polycondensation of 4‐(vinyldimethylsilyl)styrene ( 11 ) as well as cross‐copolycondensation of bis(vinyldimethylsilyl)arenes ( 10, 12 and 14 ) with 1,4‐divinylbenzene ( 9 ) catalyzed by [RuH(Cl)(CO)(PCy₃)₂] ( 7 ). Such highly stereoregular products cannot be synthesized via ADMET polycondensation or ring opening metathesis ROM or polyaddition of hydridosilanes to acetylenes.     

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.     

Vinyl‐addition copolymerization of norbornene and polar norbornene derivatives using novel bis(β‐ketoamino)Ni(II)/B(C6F5)3/AlEt3 catalytic systems

Copolymerization of norbornene (NBE) and polar norbornene derivatives undergoes vinyl polymerization by using novel catalyst systems formed in situ by combining bis(β‐ketoamino)Ni(II) complexes {Ni[R₁C(O)CHC(NR₃)R₂]₂ (R_l = R₂ = CH₃, R₃ = naphthyl, 1 ; R₁ = R₂ = CH₃, R₃ = C₆H₅, 2 ; R₁ = C₆H₅, R₂ = CH₃, R₃ = naphthyl, 3 ; R_l = R₂ = CH₃, R₃ = 2, 6‐(CH₃)₂C₆H₃, 4 ; R₁ = R₂ = CH₃, R₃ = 2, 6‐′Pr₂C₆H₃ 5 ; R₁ = C₆H₅, R₂ = CH₃, R₃ = 2, 6‐′Pr₂C₆H₃, 6 )} and B(C₆F₅)₃/AlEt₃ in toluene. The 1 /B(C₆F₅)₃/AlEt₃ catalytic system is effective for copolymerization of NBE with NBE OCOCH₃ and NBE CH₂OH, respectively, and copolymerization activity is followed in the order of NBE CH₂OH > NBE OCOCH₃ > NBE CN. The molecular weights of the obtained poly(NBE/NBE CH₂OH) reached 5.97 × 10⁴ to 2.07 × 10⁵ g/mol and the NBE CH₂OH incorporation ratios reached 7.0–55.4 mol % by adjusting the comonomer feedstock composition. The copolymerization of NBE and NBE CH₂OH also depend on catalyst structures and activity of catalyst followed in the order of 2 > 1 > 3 > 5 > 4 > 6 . The molecular weights and NBE CH₂OH incorporation ratios of poly(NBE/NBE CH₂OH) were adjustable to be 1.91–5.37 × 10⁵ g/mol and 9.5–41.1 mol %  OH units by using catalysts 1 – 6 . The achieved copolymers were confirmed to be vinyl‐addition type, noncrystalline and have good thermal stability (T_d = 380–410°C). © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

User‐Friendly Methylation of Aryl and Vinyl Halides and Pseudohalides with DABAL‐Me3

An extremely technically simple cross‐methylation of aryl and vinyl halides and pseudohalides using an air‐stable adduct of trimethylaluminium with a Pd(0) catalyst supported by commercially available biarylphosphines gives excellent yields of methylated products (mainly >95%). Reactions can be run with either 0.5 mol % catalyst or without requiring the exclusion of atmospheric oxygen or the drying of solvents in some cases. A wide variety of functional groups is tolerated including CN, OH, CO₂R, CHO and NO₂.     

Active and Stable Ni‐MgO Catalyst Coated on a Metal Monolith for Methane Steam Reforming under Low Steam‐to‐Carbon Ratios

A structured reaction system in the form of an Ni‐MgO catalyst reduced to nanoscale particle size and coated on a metallic monolith proved to be an active and stable system for methane steam reforming under a steam‐to‐carbon ratio of 1.5 and a temperature of 700 °C. The catalyst‐coated monolith exhibited higher stability and much higher CH₄ conversion than the same catalyst in a catalyst particle bed reaction system. The high activity is attributed to the properties of the metal monolith and to the small size of the catalyst particles on the coating, while the stability is ascribed to the NiO‐MgO solid solution formed in the Ni‐MgO catalyst. These results are better than the corresponding ones obtained with a conventional Ni‐Al₂O₃ catalyst reported previously [1] and comparable to the ones presented in the literature, with the advantage of working under a low steam‐to‐carbon ratio.     

Oxidative coupling of methane over Sm-promoted MgO: Influence of composition and preparation conditions

Influences of promoter concentration (or Sm/Mg ratio), precursor for MgO (viz. Mg-acetate, Mg-carbonate and Mg-hydroxide), calcination temperature of Sm-promoted MgO catalyst on the catalytic activity/selectivity in the oxidative coupling of methane (OCM) at different temperatures (650–850°C) and CH₄/O₂ ratios in feed (2·0–8·0) at a high space velocity (51600 cm³ g⁻¹ h⁻¹) have been investigated. The catalytic activity/selectivity of Sm–MgO catalysts in the OCM are found to be strongly influenced by the Sm/Mg ratio, precursor used for MgO and catalyst calcination temperature. The catalyst with Sm/Mg ratio of 0·11, prepared using magnesium acetate and magnesium carbonate as a source of MgO and calcining at 950°C, is found to be highly active and selective in the OCM process. A drastic reduction in catalytic activity/selectivity is observed when the catalyst is supported on low surface area porous catalyst carriers, indicating strong catalyst–support interactions. ©1997 SCI     

Regioselective Palladium(0)‐Catalyzed Cross‐Coupling Reactions and Metal‐Halide Exchange Reactions of Tetrabromothiophene: Optimization,Scope and Limitations

The Suzuki reaction of tetrabromothiophene with arylboronic acids provides a regioselective approach to various 5‐aryl‐2,3,4‐tribromothiophenes, symmetrical 2,5‐diaryl‐3,4‐dibromothiophenes, and tetraarylthiophenes. Unsymmetrical 2,5‐diaryl‐3,4‐dibromothiophenes are prepared by Suzuki reaction of 5‐aryl‐2,3,4‐tribromothiophenes. Tetraarylthiophenes containing two different types of aryl groups are obtained by Suzuki reactions of 2,5‐diaryl‐3,4‐dibromothiophenes. During the optimization of the conditions of each individual reaction, the solvent, the catalyst and the temperature play an important role. In several cases, classical conditions [use of tetrakis(triphenylphosphane)palladium(0), Pd(PPh₃)₄, as the catalyst] gave excellent yields. The yields of those transformations which failed or proceeded sluggishly could be significantly improved by application of a new biarylmonophosphine ligand developed by Buchwald and co‐workers. Regioselective metal‐halide exchange reactions of tetrabromothiophene provide a convenient approach to various 2,5‐disubstituted 3,4‐dibromothiophenes. 5‐Alkyl‐2‐trimethylsilyl‐3,4‐dibromothiophenes could be prepared in one pot by sequential addition of trimethylchlorosilane and alkyl bromides. The reaction of tetrabromothiophene with methyl chloroformate and subsequent Suzuki reactions afforded 3,4‐diaryl‐2,5‐bis(methoxycarbonyl)thiophenes.     

Palladium‐Catalyzed Decarboxylative sp‐sp2 Cross‐Coupling Reactions of Aryl and Vinyl Halides and Triflates with α,β‐Ynoic Acids using Silver Oxide

Palladium‐catalyzed decarboxylative sp‐sp² cross‐coupling reactions of aryl and vinyl halides and triflates with α,β‐ynoic acids using silver oxide have been developed. A variety of α,β‐ynoic acids were readily decarboxylated in the presence of silver oxide and then, generated in situ, silver acetylides were coupled with electrophiles in the presence of a palladium(0) catalyst under neutral conditions, producing either symmetrical or unsymmetrical diarylacetylenes, arylalkylacetylenes and arylvinylacetylenes in good to excellent yields.     

New Indenylidene‐Schiff Base‐Ruthenium Complexes for Cross‐Metathesis and Ring‐Closing Metathesis

We here report on the stability and catalytic activity of new indenylidene‐Schiff base‐ruthenium complexes 3a – f through representative cross‐metathesis (CM) and ring‐closing metathesis (RCM) reactions. Excellent activity of the new complexes was found for the two selected RCM reactions; prominent conversion was obtained compared to the commercial Hoveyda–Grubbs catalyst 2 . Moreover, excellent results were obtained for a standard CM reaction. Higher conversions were achieved with one of the indenylidene catalysts compared with Hoveyda–Grubbs catalyst. Unexpectedly, an isomerization reaction was observed during the CM reaction of allylbenzene. To the best of our knowledge, isomerization reactions in this model CM reaction in closed systems have never been described using first generation catalysts, including the Hoveyda–Grubbs catalyst. The first model CM reactions as well as the RCM reactions have been monitored using ¹H NMR. The course of the CM reaction of 3‐phenylprop‐1‐ene ( 8 ) and cis‐1,4‐diacetoxybut‐2‐ene ( 9 ) was monitored by GC. The isomerization reaction was studied by means of GC‐mass spectrometry and in situ IR spectroscopy. All catalysts were structurally characterized by means of ¹H, ¹³C, and ³¹P NMR spectroscopy.     

Structure and Catalytic Performance of Mg‐SBA‐15‐Supported Nickel Catalysts for CO2 Reforming of Methane to Syngas

A series of Mg‐modified SBA‐15 mesoporous silicas with different MgO contents were successfully synthesized by a simple one‐pot synthesis method and further impregnated with Ni. The Mg‐modified SBA‐15 materials and supported Ni catalysts were characterized by N₂ physisorption (BET), X‐ray diffraction (XRD), temperature‐programmed desorption of CO₂ (CO₂‐TPD), temperature‐programmed H₂ reduction (H₂‐TPR), and temperature‐programmed hydrogenation (TPH) techniques and used for methane dry reforming with CO₂. CO₂‐TPD results proved that the addition of Mg increased the total amount of basic sites which was responsible for the enhanced catalytic activity over the Mg‐modified Ni catalyst. The excellent catalytic stability of Ni/8Mg‐SBA‐15 was ascribed to less coking and higher stability of the Ni particle size due to the introduction of Mg.