Catalysts complexes

Ziegler catalysts Complex catalysts prepared by interaction between an organometallic derivative and a transition metal derivative. A typical catalyst is the product of the interaction of TiCU and AIBU3. These catalysts polymerize olefins, particularly ethylene, to polyolefins, the polymerization generally being in a siereoregular manner. 

Herein ko is the second-order rate constant for the uncatalysed reaction and k. is the second-order rate constant for the reaction of the 2.4-catalyst complex. [2.4] is the concentration of free dienophile... 

Dimersol is a commercial process for the dimeri2ation of propylene, butylenes, or a mixture of both, to and Cg olefins this process produces a more linear olefin than the phosphoric acid process. The reaction is conducted at ambient temperature, using a water-soluble catalyst complex (16). 

Liquid-Ph se Processes. Prior to 1980, commercial hquid-phase processes were based primarily on an AIQ. catalyst. AIQ. systems have been developed since the 1930s by a number of companies, including Dow, BASF, Shell Chemical, Monsanto, SociStH Chimique des Charboimages, and Union Carbide—Badger. These processes generally involve ethyl chloride or occasionally hydrogen chloride as a catalyst promoter. Recycled alkylated ben2enes are combined with the AIQ. and ethyl chloride to form a separate catalyst—complex phase that is heavier than the hydrocarbon phase and can be separated and recycled. 

In 1974, Monsanto brought on-stream an improved Hquid-phase AIQ. alkylation process that significantly reduced the AIQ. catalyst used by operating the reactor at a higher temperature (42—44). In this process, the separate heavy catalyst—complex phase previously mentioned was eliminated. Eliminating the catalyst—complex phase increases selectivities and overall yields in addition to lessening the problem of waste catalyst disposal. The ethylben2ene yields exceed 98%. 

Aluminum Chloride-Based All lation. The eadier alkylation processes were variations of the Eriedel-Craft reaction on an aluminum chloride catalyst complex in a Hquid-phase reactor (27), including those developed by Dow Chemical, BASE, Monsanto, and Union Carbide in cooperation with Badger. The Union Carbide-Badger process was the one most widely used during the 1960s and 1970s, with 20 plants built worldwide. 

Two pathways were found for the chiral hydrogenation, and they give products with different stereochemistries (19). One pathway involves the preferred mode of initial binding of the reactant to the catalyst. The other pathway involves an isomer of the reactant—catalyst complex that is formed in only small amounts, but its conversion is energetically favorable and constitutes the kinetically predominant pathway to products (9) (Fig. 4). Thus the chirahty of the product is determined not by the preferred mode of the initial binding, but instead by the more favorable energetics of the pathway involving the minor isomer of the reactant—catalyst complex. 

Esters such as ben2oates and phthalates are also used in the preparation of high activity catalysts for olefin polymeri2ation. They appear to function as electron donors in the catalyst complex, and play a significant role in catalyst performance (115). 

Polystyrene produced by free-radical polymerisation techniques is part syndio-tactic and part atactic in structure and therefore amorphous. In 1955 Natta and his co-workers reported the preparation of substantially isotactic polystyrene using aluminium alkyl-titanium halide catalyst complexes. Similar systems were also patented by Ziegler at about the same time. The use of n-butyl-lithium as a catalyst has been described. Whereas at room temperature atactic polymers are produced, polymerisation at -30°C leads to isotactic polymer, with a narrow molecular weight distribution. 

One other feature of the data in Table 10.10 is worthy of further comment Notice that alkyl substituted acylium ions exhibit a smaller ortho para ratio than the various arpyl systems. If steric factors were dominating the position selectivity, one would expect the opposite result A possible explanation for this feature of the data could be that the aryl compounds are reacting via free acylium ions, whereas the alkyl systems may involve more bulky acyl chloride-catalyst complexes. 

The complexation procedure included addition of an equimolar amount of R,R-DBFOX/Ph to a suspension of a metal salt in dichloromethane. A clear solution resulted after stirring for a few hours at room temperature, indicating that formation of the complex was complete. The resulting solution containing the catalyst complex was used to promote asymmetric Diels-Alder reactions between cyclopen-tadiene and 3-acryloyl-2-oxazolidinone. Both the catalytic activity of the catalysts and levels of chirality induction were evaluated on the basis of the enantio-selectivities observed for the endo cycloadduct. 

Traces of bases such as methylimidazole in the final ionic liquid product can play an unfavorable role in some common applications of ionic liquids (such as bipha-sic catalysis). Many electrophilic catalyst complexes will coordinate the base in an irreversible manner and be deactivated. 

Many transition metal complexes dissolve readily in ionic liquids, which enables their use as solvents for transition metal catalysis. Sufficient solubility for a wide range of catalyst complexes is an obvious, but not trivial, prerequisite for a versatile solvent for homogenous catalysis. Some of the other approaches to the replacement of traditional volatile organic solvents by greener alternatives in transition metal catalysis, namely the use of supercritical CO2 or perfluorinated solvents, very often suffer from low catalyst solubility. This limitation is usually overcome by use of special ligand systems, which have to be synthesized prior to the catalytic reaction. 

In the case of ionic liquids, special ligand design is usually not necessary to obtain catalyst complexes dissolved in the ionic liquid in sufficiently high concentrations. 

However, it should be mentioned that the dissolution process of a solid, crystalline complex in an (often relatively viscous) ionic liquid can sometimes be slow. This is due to restricted mass transfer and can be speeded up either by increasing the exchange surface (ultrasonic bath) or by reducing the ionic liquid s viscosity. The latter is easily achieved by addition of small amounts of a volatile organic solvent that dissolves both the catalyst complex and the ionic liquid. As soon as the solution is homogeneous, the volatile solvent is then removed in vacuo. 

In comparison to heterogeneous catalyzed reactions, homogeneous catalysis offers several important advantages. The catalyst complex is usually well defined and can be rationally optimized by ligand modification. Every metal center can be active in the reaction. The reaction conditions are usually much milder (T usually < 200 °C), and selectivities are often much higher than with heterogeneous catalysts. 

Biphasic catalysis in a liquid-liquid system is an ideal approach through which to combine the advantages of both homogeneous and heterogeneous catalysis. The reaction mixture consists of two immiscible solvents. Only one phase contains the catalyst, allowing easy product separation by simple decantation. The catalyst phase can be recycled without any further treatment. However, the right combination of catalyst, catalyst solvent, and product is crucial for the success of biphasic catalysis [22]. The catalyst solvent has to provide excellent solubility for the catalyst complex without competing with the reaction substrate for the free coordination sites at the catalytic center. 

Ionic liquids formed by treatment of a halide salt with a Lewis acid (such as chloro-aluminate or chlorostannate melts) generally act both as solvent and as co-catalyst in transition metal catalysis. The reason for this is that the Lewis acidity or basicity, which is always present (at least latently), results in strong interactions with the catalyst complex. In many cases, the Lewis acidity of an ionic liquid is used to convert the neutral catalyst precursor into the corresponding cationic active form. The activation of Cp2TiCl2 [26] and (ligand)2NiCl2 [27] in acidic chloroaluminate melts and the activation of (PR3)2PtCl2 in chlorostannate melts [28] are examples of this land of activation (Eqs. 5.2-1, 5.2-2, and 5.2-3). 

In cases in which the ionic liquid is not directly involved in creating the active catalytic species, a co-catalytic interaction between the ionic liquid solvent and the dissolved transition metal complex still often takes place and can result in significant catalyst activation. When a catalyst complex is, for example, dissolved in a slightly acidic ionic liquid, some electron-rich parts of the complex (e.g., lone pairs of electrons in the ligand) will interact with the solvent in a way that will usually result in a lower electron density at the catalytic center (for more details see Section 5.2.3). 

Jessop and co-workers studied asymmetric hydrogenation reactions with the catalyst complex Ru(OAc)2(tolBINAP) dissolved in [BMIM][PFg]. In both reactions under investigation - the hydrogenation of tiglic acid (Scheme 5.2.10) and the hydrogenation of the precursor of the anti-inflammatory dmg ibuprofen (Scheme 5.2.11) - no CO2 was present during the catalytic transformation. However, SCCO2 was used in both cases to extract the reaction products from the reaction mixture when the reaction was complete. 

Obviously, the ionic liquid s ability to dissolve the ionic catalyst complex, in combination with low solvent nucleophilicity, opens up the possibility for biphasic processing. Furthermore it was found that the biphasic reaction mode in this specific reaction resulted in improved catalytic activity and selectivity and in enhanced catalyst lifetime. 

The above enantioselectivities are obviously complex functions of many factors, perhaps even more complex than in natural enzymes. Complexity is partly due to the present co-micellar system in which it is difficult to analyze separately the interaction of the substrate with the achiral micelle, and that of the substrate with the catalyst complex. 

Brown and Jensen395 suggested that the rate equation (194) for the reaction of benzene with excess benzoyl chloride could be interpreted according to the mechanisms given by the reactions (201) and (202), (203) and (204) and (205) and (206) which refer to nucleophilic attack of the aromatic upon the polarised acyl halide-catalyst complex, upon the free acylium ion, and upon an ion pair derived from the acyl halide-catalyst complex, viz. 

An explanation for the effect of excess catalyst has been offered by Corriu et al. 16, who measured the rates of the aluminium chloride-catalysed reaction of benzoyl chloride with benzene, toluene, and o-xylene. The observed rate coefficients were analysed in terms of a mixture of second- and third-order reactions (the latter being second-order in the halide-catalyst complex), the following results being obtained benzene (40 °C), k2 = 2.5 xlO-5, fc3 = 3.3 xlO-5 toluene (2.5 °C), k2 = 0.75 xlO"4, k3 = 3.83 xlO-4 o-xylene (0 °C), k2 = 1.83 x 10-3, k3 = 4.50 x 10-3. They suggest the equilibrium... 

The oligomerization of the ethylene proceeds as a ligand reaction in the coordination sphere of the catalyst complex, as the following reaction scheme shows. The reaction course corresponds with the Ziegler process for ethylene polymerization. 

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