Catalytic activity of titanium compounds

TABLE 2.3 Catalytic Activities of Titanium Compounds with Cyclopentadienyl Ligands... 

In this correlation, the catalytic activities of titanium compounds with bulky substituents at the cyclopentadienyl ligand show lower activities than expected from the chemical shifts of Ti-NMR. The electron densities of titanium compounds can also be observed when the titanium complexes possess the same... 

Figure 2.4 Catalytic activities of titanium compounds with different ring sizes and substituents. (The numbers are the catalytic activity (kg/[gTi x h]) and the Mw of SPS polymerization conditions styrene/MAO/TIBA/Ti (molar ratios) = 26300/200/200/1 at 60°Cforlh.)...

Many references relate to polyesterifications catalyzed by tetraalkoxy - particularly te-trabutoxy - titaniums, and the catalytic activity of these compounds is daimed in many... 

Conventionally, HAS are blended with PO during processing. 2-(Diethy-lamino)-4,6-bis[butyl(l,2,2,6,6-pentamethyl-4-piperidyl) amino]-l,3,5-triazine may be fed with an olefin directly into the low pressure polymerization process catalyzed with a modified MgCl2 supported Ziegler-Natta catalyst [142]. The catalytic activity was not impaired [143], Tetramethylpiperidine was reported to be a useful component in MgC -supported Ziegler-Natta catalysts as well. Very high stereospecificity of the synthesised PO was achieved. A complex of HAS with the alkyl aluminium activator was envisaged without interaction with the catalytically active alkyl titanium compound [144],... 

Na-, La-, and Re-exchanged zeolites have also been used as catalysts of the Michael reaction between silyl enol ethers and a, 6-unsaturated carbonyl compounds. This study, performed by Sasidharan et al. [87], focused mainly on the catalytic activity of titanium silicalite molecular sieves (TS-1 and TS-2). They found that TS-1 and TS-2 catalyze 1,4-Michael addition of silyl enol ethers and a,y5-unsatu-rated carbonyl compounds under anhydrous conditions. The zeolites tested as catalysts of this reaction, e. g. ReY, LaY, steamed zeolite Y, and cation-exchanged ZnZSM-5, were less active (or inactive). 

Catalytic use of these compounds revealed an activity comparable to the titanium species. The most important points of this work are that under the experimental conditions there is no metal loss in solution (a rare cases in heterogeneous catalysis with such reactive media) and that the reaction is completely heterogeneous (Figure 3.29). 

Abstract The term Lewis acid catalysts generally refers to metal salts like aluminium chloride, titanium chloride and zinc chloride. Their application in asymmetric catalysis can be achieved by the addition of enantiopure ligands to these salts. However, not only metal centers can function as Lewis acids. Compounds containing carbenium, silyl or phosphonium cations display Lewis acid catalytic activity. In addition, hypervalent compounds based on phosphorus and silicon, inherit Lewis acidity. Furthermore, ionic liquids, organic salts with a melting point below 100 °C, have revealed the ability to catalyze a range of reactions either in substoichiometric amount or, if used as the reaction medium, in stoichiometric or even larger quantities. The ionic liquids can often be efficiently recovered. The catalytic activity of the ionic liquid is explained by the Lewis acidic nature of then-cations. This review covers the survey of known classes of metal-free Lewis acids and their application in catalysis. 

Mechanistic information is difficult to obtain when the catalytically active titanium centers are present in a dilute matrix of silica. Only few techniques can be applied, and the available information does not allow discrimination between possible mechanisms. Consequently, it is necessary in this discussion to rely on analogies with the known chemistry of titanium compounds. 

The polymerization activities of bis-cyclopentadienyl titanium compounds are lower than those of bridged bis-cyclopentadienyl titanium compounds. Miya-shita et al. reported the polymerization activities of several bridged bis-cyclopentadienyl titanium compounds [15]. They found that the catalytic activity of CH2(Cp)2TiCl2 is the highest among bis-cyclopentadienyl titanocene compounds. The data indicate that the polymerization activities and also syndiospecificity increase with a decreasing angle between the Cp centroid-Ti-Cp centroid in bis-cyclopentadienyl titanocene compounds. 

Ethylene-bridged bis(indenyl) titanium complexes in which the Ind ligands are attached at the 2-position and containing alkyl substituents in 1-position have been obtained as a mixtures of meso- and racemo-isomers (Scheme 649). The assignment of the stereochemistry for these compounds has been established on the basis of spectroscopic data and crystal structure determinations. These compounds act as catalysts for the epoxidation of unfunctionalized alkenes. The catalytic activity of this new class of complexes in epoxidation reactions was in some cases found to exceed that of known bis-Cp titanium complexes.1670... 

Mechanistically related to the Mukaiyama aldol reaction, the carbonyl ene reaction is the reaction between an alkene bearing an allylic hydrogen and a carbonyl compound, to afford homoallylic alcohols. This reaction is potentially 100% atom efficient, and should be a valuable alternative to the addition of organometallic species to carbonyl substrates. However, the carbonyl ene reaction is of limited substrate scope and works generally well in an intermolecular manner only with activated substrates, typically 1,1-disubstituted alkenes and electron-deficient aldehydes (glyoxylate esters, fluoral, a,p-unsaturated aldehydes, etc.), in the presence of Lewis acids. The first use of chiral catalyst for asymmetric carbonyl ene was presented by Mikami et al. in 1989. ° By using a catalytic amount of titanium complexes prepared in situ from a 1 1 ratio of (rPrO)2titaniumX2 (X = Cl or Br) and optically pure BINOL, the homoallylic alcohols 70a,b were obtained in... 

The metal acetylacetonates and their pyridine complexes were used as catalysts in the epoxy-novolac systems [310-320]. These compounds improved the mechanical strength of the polymers [310,312], but the titanium and zirconium chelates also advanced the adhesive strength and the water resistance [319]. The investigation of the effect of the acetylacetonates on the interaction of the epoxy oligomers with the phenolformaldehyde resins showed that the catalytic activity of the metal cations diminishes in the series [Eq. (2)]... 

An unoccupied coordination site and titanium alkyl bond in the titanium complex are fundamental requirements for its catalytic activity in olefin oligomerization. The titanium-alkyl bond is formed in the reaction of a titanium compound with alkyl or alkylchloroaluminum compounds when, for example, halide atoms or alkoxy groups of titanium compounds are replaced by alkyl groups of aluminum derivatives. There are many proposals concerning the structure of the Ziegler-Natta catalyst active centers. These are presented in structural formulas (16)-(21) [291. 

In case of borate as cocatalyst, the catalytic activity of the titanium complex with a pentamethylcyclopentadienyl hgand is high, but a titanium complex with a cyclopentadienyl ligand without any substituents is not active for the syndiospecific styrene polymerization. The reason is that the reaction product of the borate and the cycopentadienyltitanium compound is unstable. The stability of the active site with the borate compound is lower in comparison to that with MAO. The reaction of CH2(Cp)2Ti(Me)2 with dimethylanilinium tetrakis(pentafiuorophenyl)borate or tris(pentafluorophenyl)borane in an equimolar mixture has been examined by Miyashita, Nabika, and Suzuki [11]. Two types of methylene bis(cyclopentadienyl)titanium ion complexes were isolated (see Fig. 3.6). These complexes were active in the polymerization of styrene, but only atactic polystyrene was formed. 

PeUecchia et al. [15] also examined the catalytic activity of several organo-metaUic derivatives with similar boranes. They observed that the catalytic activity of Cp TiMe3 with dimethylaniliniumtetrakis(pentafluorophenyl) borate was lower than that with tris(pentafluorophenyl)borane. It was proposed that the free amine coordinates with the active site and interferes with the polymerization reaction. The reaction of the titanium compounds and the borate forms the active site. The equations are as follows ... 

The catalyst for the styrene polymerization is prepared by the mixing of a compound with MAO. The catalytic activity of Cp Ti(OMe)3 is increased by the mixing time under room temperature, and the intensity of the electron spin resonance (ESR) spectrum (g=1.998) also showed an increase of the trivalent titanium species (Fig. 4.6). The polymerization activity of a titanium catalyst increases with an increase in the molar ratio of MAO to Ti. 

Several aluminum- and titanium-based compounds have been supported on silica and alumina [53]. Although silica and alumina themselves catalyze cycloaddition reactions, their catalytic activity is greatly increased when they complex a Lewis acid. Some of these catalysts are among the most active described to date for heterogeneous catalysis of the Diels-Alder reactions of carbonyl-containing dienophiles. The Si02-Et2AlCl catalyst is the most efficient and can be... 

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