Lewis acids titanium-based catalysts

The relatively weaker Lewis acidic titanium complexes require the use of a stronger nucleophile than allylsilanes, and tributylallyltin (6.81) is the most common aUylating agent employed when using titanium-based catalyst systems. In 1993, Umani-Ronchi and Keck published related results using BINOL/titanium derived catalysts. In the Umani-Ronchi system, BINOL is employed, in combination with TiCl2 (0 Pr)2 and shown to work weU with aliphatic... 

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... 

Beller and coworkers [312] have recently published a very useful domino reaction using a Ti-based Lewis acid as catalyst to prepare indoles. The new procedure consists of a titanium-catalyzed amination of a chloroalkylalkyne to give an aryl hydra-... 

Dipolarophiles D5. Electron-deficient alkenes based on acrolein and its analogs are widely used as dipolarophiles. To carry out asymmetrical 1,3-dipolar cycloadditions between various nitrones and acrolein, the bis-titanium catalyst (543) (Fig. 2.37) was used as the chiral Lewis acid (Table 2.22) (754a). 

Snapper and Hoveyda reported a catalytic enantioselective Strecker reaction of aldimines using peptide-based chiral titanium complex [Eq. (13.11)]. Rapid and combinatorial tuning of the catalyst structure is possible in their approach. Based on kinetic studies, bifunctional transition state model 24 was proposed, in which titanium acts as a Lewis acid to activate an imine and an amide carbonyl oxygen acts as a Bronsted base to deprotonate HCN. Related catalyst is also effective in an enantioselective epoxide opening by cyanide "... 

Dialkylzincs are much less reactive than phenyl or alkynylzincs. In 2002, Kozlowski et al. developed a chiral salen-based catalyst 62 that can promote the diethylzinc addition to a-ketoesters in high yield, [Eq. (13.38)]. In their catalysis, titanium acts as a Lewis acid, and amine nitrogen acts as a Lewis base (63). The enantioselectivity was up to 78% ee ... 

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. 

Predominantly cis-1,4-polybutadiene is produced by coordination polymerization with mixed catalysts.187,487,488 Three catalyst systems based on titanium, cobalt, or nickel are used in industrial practice. Iodine is an inevitable component in titanium-alkylaluminum sytems to get high cis content. Numerous different technologies are used 490,491 A unique process was developed by Snamprogetti employing a (Tr-allyl)uranium halide catalyst with a Lewis acid cocatalyst.492-494 This catalyst system produces poly butadiene with 1,4-ris content up to 99%. 

Our proposed transition state model for this catalytic enantioselective cyanosilylation of ketone is shown as 35.30a The titanium acts as a Lewis acid to activate the substrate ketone, while the phosphine oxide acts as a Lewis base to activate TMSCN. The intramolecular transfer of the activated cyanide to the activated ketone should give the ( )-cyanohydrin in high selectivity. The successful results described above clearly demonstrate the practicality of our asymmetric catalyst for cyanosilylation of ketones. 

Balan and Adolfsson [28] reported a direct catalytic enantioselective three-component aza Baylis-Hillman reaction between arylaldehydes, tosylamides, and Michael acceptors using the quinidine-based Hatekayama catalyst 96 [29] together with titanium isopropoxide as a Lewis acid cocatalyst (Scheme 9.18). High chemical yields and stereoselectivity ranging between 49 and 74% ee were obtained using various substituted arylaldehydes. 

Amorphous Ti/SiCL oxides and crystalline Ti zeolites are two classes of well-studied solid Ti catalysts (11-14). In both classes, a Lewis-acidic Ti atom is anchored to the surrounding siliceous matrix by Si-O-Ti bonds. The oxidant of choice for Ti zeolites such as titanium silicalite 1 (TS-1) and 11-/1 is H2O2, whereas the amorphous, silica-based materials function optimally with organic peroxides such as /-butyl hydroperoxide (/-BuOOH) or ethyl benzene hydroperoxide. However, there are strictly no homogeneous analogues of these materials, and they therefore do not fit within the context of anchoring of homogeneous catalysts. 

Practical and efficient asymmetric allylation of aldehydes is successfully promoted by Lewis acid catalysts bearing chiral auxiliaries to afford high levels of enantioselectivity.165 The effective catalysts for asymmetric allylation to benzaldehyde are shown below (Scheme j) 166-176 The catalytic asymmetric allylation of ketones has proved to be a more challenging transformation owing to the significantly low reactivity compared to aldehydes. In 2002, a catalyst based on titanium complex was developed (Equation (51)).A ... 

Other important titanium alkoxide-based Lewis acids are Ti-TADDOLate (a,a,a, a -tetraaryl-l,3-dioxolane-4,5-dimethanol)ates, among the most effective chiral catalysts for several important asymmetric reactions. These will be discussed in the sections on polymer-supported Diels-Alder reactions (Section 21.10) and alkylations (Section 21.9). 

Introduction of an alkylthio group on the allene system increased the reactivity of the allene moiety in [2 + 2] cycloaddition reactions. It proved possible to conduct reactions of this allene at much lower temperatures. By adding Lewis acids, the reaction temperature could be decreased even more, as was illustrated by the Lewis acid catalyzed [2-1-2] cycioadditions of l-trimethylsilyl-l-methylthio-l,2-propadiene with a variety of electron-poor alkenes, including cyclic and non-cyclic enones, acrylates, methyl fumarate and acrylonitrile. When a chiral diol 21 based titanium catalyst was employed, the [2-1-2] cycloaddition reactions of /-acryloyl-l,3-oxazolidin-2-ones 17a and 17b with allenyl sulfides 18 yielded methylenecyclobutanes 19 and 20 with high optical purities (equation The highest yields were obtained with electron-poor allenophile 17b. 

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