Heterogeneous enantioselective catalysts: strategies for the immobilisation of homogeneous catalysts

Enantioselective formation of C-H, C-C, C-O and C-N bonds has been extensively studied using homogeneous asymmetric catalysts for many years. However, these catalysts have yet to make a significant impact in the industrial synthesis of fine chemicals. A central reason is that homogeneous asymmetric catalyst design requires relatively bulky ligands and catalyst re-use through recovery and recycle often causes problems. One mechanism to overcome this problem is to immobilise the asymmetric catalyst onto a support and the resulting heterogeneous asymmetric catalyst can, in principle, be readily re-used. This tutorial review covers the different methodologies for immobilisation, including: adsorption, encapsulation, tethering using a covalent bond and electrostatic interaction and is aimed at both researchers new to the field and those with a wider interest in the immobilisation of homogeneous catalysts. Most importantly, recent studies will be highlighted that demonstrate that immobilised catalysts can give higher enantioselection when compared with their non-immobilised counterparts and the question of how high enantioselection can be achieved is addressed.     

Nickel-based ethylene oligomerization catalysts supported by PNSiP ligands

A series of nickel (II) complexes bearing silicon bridged diphosphines ligands (PNSiP) have been synthesized and characterized. All nickel precatalysts, activated with ethylaluminum dichloride (EtAlCl₂), exhibited moderate to high activities for ethylene dimerization to butylene. The in situ nickel precatalysts formed by mixing N-cyclopentyl-N-((diphenylphosphanyl)dimethylsilyl)-1,1-diphenylphosphanamine (L2) with NiBr₂(DME) showed high catalytic activity (2.40 × 10⁸ g/(mol_Ni·h)) and high product selectivity (88.6%) towards butene using methylcyclohexane as solvent at 1.0 MPa ethylene pressure and 45°C temperature, no polyethylene(PE) was observed. Ligand backbone tuning of PNSiP-based catalytic systems help in precise understanding of steric bulk variation effects on catalytic performance.     

Heterogeneous metallocene catalysts for ethylene polymerization

The catalyst precursor 9-fluorenylidene-1-cyclopentadienylidene-2-hex-5-enylidene zirconium dichloride proved to be highly active in the heterogeneously catalyzed polymerization of ethylene using silica gel/partially hydrolyzed trimethylaluminum (PHT) as cocatalyst. The substitution of position 4 of the fluorenylidene fragment and position 3 of the cyclopentadienylidene ring improves the catalyst activity. The introduction of a phenyl group into the bridge increases the catalyst activity and the molecular weight of the polymer. The prepolymerization of this catalyst system leads to a major change in catalyst and polymer properties. A significant increase in catalyst activity and a lower molecular weight of the produced polyethylene is observed. The presence of hydrogen during prepolymerization or polymerization of ethylene produces a broader molecular weight distribution indicating a higher number of different active centers.     

Sulfated alumina aerogel-based superacid catalysts for 1-hexene oligomerization

Al₂O₃ aerogel samples were synthesized by supercritical drying in two different media: isopropanol and methyl tert-butyl ether. After sulfation in a SO₂Cl₂ vapor, the aerogel acquired superacid properties (–12.4 ≤ H₀ ≤–11.99) and efficiently catalyzed 1-hexene oligomerization and dimerization.     

Kaminsky型催化剂催化乙烯环齐聚反应的研究

研究发现Cp~2ZrL~2(L=Cl,Me,OC~6H~4-p-Me)/EAO(乙基铝氧烷)可同步催化乙烯齐聚-环化反应,不仅给出链状烯烃,而且生成环状剂聚物－－亚甲基环戊烷。环状齐聚物的选择性取决于主催化剂的结构和浓度,反应介质,预反应温度和反应温度,反应时间,Al/Zr比及烷基铝水解程度等因素,加入碱性第三组分对催化活性和选择性亦有一定影响。在优化反应条件下,亚甲基环戊烷的选择性达到37%。     

Our variations on iron and cobalt catalysts toward ethylene oligomerization and polymerization

Iron and cobalt complexes are a new family of catalysts for ethylene oligomerization and polymerization. The extensive researches on bis(imino)pyridyl metal complexes have been carried out with the aim of synthesizing their derivatives and finding suitable reaction parameters for the optimum activity. Beyond the modification works of bis(imino)pyridyl metal complexes, several alternative models with similar coordination sphere have been developed in our group. This review article describes our experiences in innovating new models of iron and cobalt complexes as catalysts for ethylene oligomerization and polymerization.     

Heterogeneous phase transfer catalysts 1. Synthesis and catalytic properties of polymeric crown-ethers on inorganic supports

A number of new heterogeneous phase transfer catalysts based on polymeric dibenzo-crown-ethers supported on the carbonaceous material Sibunite, ultrafine diamonds, or silica gel are synthesized. The activity of several of the catalysts for the reaction of 1-bromooctane with KI is higher than that of previously known analogs. The catalytic effect depends on the support, linker, and solvent.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1257–1262. June. 1990.     

Monomer-isomerization oligomerization of 2-ethyl-1,3-butadiene by acid catalysts

The cationic oligomerization of 2-ethyl-1,3-butadiene (2EBD) by a superacid (CF₃SO₃H) and a superacid derivative (CH₃COClO₄) accompanied monomer isomerization to 3-methyl-1,3-pentadiene (3MPD) before propagation to yield oligomers of the isomerized monomer as main products in benzene at 50°C. Detection of 3MPD in the reaction mixture and ¹H-NMR structural analysis of the produced oligomers confirmed the occurrence of this “monomer-isomerization oligomerization.” On the other hand, in the presence of a metal halide catalyst (BF₃OEt₂) 2EBD reacted without isomerization and yielded oligomers that were different from those produced by the foregoing superacid catalysts. Monomer isomerization was suppressed in a polar solvent [(CH₂Cl)₂] or at lower temperatures. The mechanism of the oligomerization with monomer isomerization was discussed.     

Heterogeneous catalysts in the alkylation of imidazoles

The reaction of imidazoles with alcohols at 300–400°C was studied in the presence of heterogeneous -Al₂O₃ catalysts and Y zeolites. The major reaction was found to be N-alkylation. This reaction is accompanied by C-alkylation on -Al₂O₃ while the selectivity relative to N-alkylation is close to 100% on zeolite catalysts. The greatest activity with 100% selectivity was found for H zeolite. The alkylation of methanol and ethanol by 2-methylimidazole at 310–320°C gave 1,2-dimethylimidazole in -100% yield and 1-ethyl-2-methylimidazole in 90% yield respectively. The reaction of methanol and imidazole gave 1-methylimidazole in 99% yield. This catalyst displays high stability and capacity for oxidative regeneration.N. D. Zelinskii Institute of Organic Chemistry, Russian Academy of Sciences, 117913 Moscow. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 624–628, May, 1994. Original article submitted April 20, 1994.     

Preparation of chromium catalysts bearing bispyridylamine and its performance in ethylene oligomerization

Transition Metal Chemistry - A series of bispyridylamine ligands and chromium complexes designed for ethylene oligomerization have been synthesized, which was made up of both neutral organic ligand...     

Cryosynthesis of catalysts for propylene oligomerization based on titanium tetrachloride and magnesium

A highly dispersed catalyst for oligo- and polymerization of propylene was synthesized by the interaction of TiCl₄ with magnesium in the cocondensates of their vapor with benzene and pentane. The catalyst contains MgCl₂ and organotitanium and organomagnesium cluster derivatives. The transformations of propylene and hex-1-ene over the catalysts were studied. The direction of catalytic reactions and activity of the catalyst depend on the TiCl₄:Mg molar ratio and the hydrocarbon used. Systems with an equimolar ratio of the reactants obtained in a benzene matrix exhibit the highest activity. Propylene oligomers containing a considerable fraction of unsaturated bonds are formed in the presence of the catalysts at room temperature and a pressure of 300 Torr. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 900–903, May, 1999.     

Heterogeneous Pd catalysts and microwave irradiation in heck arylation

Pd metal dispersed on supports like γ-Al₂O₃, C, MgO and CaCO₃ is an efficient and recyclable catalyst in Heck arylation of several olefins. This reaction is also effective in microwave environment. IPCL Communication No. 294     

Alkyl-substituted cyclopentadienyl- and phospholyl-zirconium/MAO catalysts for propene and 1-hexene oligomerization

The directed oligomerization of propene and 1-hexene was carried out with a series of Cp′(C₅H₅)ZrCl₂ and Cp₂′ZrCl₂ pre-catalysts (Cp′=C₅HMe₄, C₄Me₄P, C₅Me₅, C₅H₄^tBu, C₅H₃-1,3-^tBu₂, C₅H₂-1,2,4-^tBu₃) together with (C₅H₅)₂ZrCl₂. Oligomers in the molar mass range 300–1500 g/mol for propene and 200–3000 g/mol for 1-hexene were synthesized at 50 °C. The majority of oligomer molecules contain a double-bond end group. Oligomer characterization was carried out by gel permeation chromatography (GPC), ¹H and ¹³C NMR. Vinylidene double bonds (from β-hydrogen elimination) are solely found for the tert-butyl-substituted zirconocenes and for most of the unsymmetrical methyl-substituted Cp′(C₅H₅)ZrCl₂ systems (except Cp′=phospholyl). With (C₄Me₄P)(C₅H₅)ZrCl₂ and with the symmetrical methyl-containing Cp₂′ZrCl₂ pre-catalysts, also vinyl end groups (from β-methyl elimination) are observed in the case of oligopropenes. The vinylidene/vinyl ratio depends on the ligand and the vinyl content increases from C₅HMe₄ (65/35) over C₄Me₄P (61/39) to C₅Me₅ (9/91). The phospholyl zirconocenes and (C₅HMe₄)₂ZrCl₂ also exhibit chain-transfer to aluminum thereby giving saturated oligomers.     

Heterogeneous Ziegler-Natta catalysts: On going progress for the polyolefin industry

A great deal of research and development efforts in Ziegler Natta catalysis has been put within the last ten years into the polyolefin industry. The exploitation of new catalysts, the introduction and consolidation of advanced polymerization process technologies and, finally, the development of a broader range and of a better tailoring of product grades, have made the polyolefin market an expression of a young, dynamic, technology-driven industry. In this context heterogeneous catalysts have played a key role. Many drawbacks of the catalysts of the previous generation have been overcome. However some problems have still to be solved in order to both further improve catalyst performance and reduce the residual areas of empiricism in catalyst design and manufacture.     

Ethylene oligomerization promoted by nickel-based catalysts with silicon-bridged diphosphine amine ligands

Transition Metal Chemistry - A series of nickel complexes [Ni(L1)Br2] (C1), [Ni(L2)Br2] (C2) and [Ni(L3)Br2] (C3)...     

The active forms of palladium in pd-zeolite catalysts for ethylene oligomerization

Conclusions By studying the effects of preliminary treatment of Pd zeolites with various gases and investigating the x-ray photoelectron spectra of the zeolites themselves, it has been shown that it is the cationic form of the metal which is catalytically active in ethylene oligomerization. Metallic Pd is inactive in these catalysts under the conditions of the present work.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2454–2459, November, 1977.The authors would like to thank V. A. Shvets who obtained the ESR spectrum.     

Tuning of nickel 2-phosphinophenolates - catalysts for oligomerization and polymerization of ethylene

This short personal account summarizes recent work concerning the syntheses and structural aspects of neutral and cationic (organo)nickel and (organo)palladium 2-phosphinophenol(ate) complexes and catalysts for the oligo- or polymerization of ethylene.     

Synthesis,characterization, and ethylene oligomerization of three novel dendritic nickel catalysts

Three dendritic nickel complexes C1–C3 were synthesized from three poly(amido amine) dendrimers, salicylic aldehyde and nickel chloride hexahydrate via Schiff base condensation reaction and coordination reaction. The structures of the dendritic ligands and nickel complexes were characterized by FT-IR, UV, ¹H NMR, ESI–MS, and elemental analysis. When activated with aluminum co-catalysts, three complexes C1–C3 were able to catalyze ethylene oligomerization. The catalytic activities and the product distribution of complexes C1–C3 were depended on the reaction parameter, co-catalyst, solvent, and the structure of the pre-catalyst. When using ethyl aluminum sesquichloride (EASC) as co-catalyst in toluene, the catalytic activity of complex C3 containing the longest bridging methylene groups reached the highest value of 1.63 × 10⁶ g·(mol Ni·h)^?1 with 69.15% C₁₁ in the product at 30 min, 25 °C, 0.5 MPa, and Al/Ni ratio of 900.