Lewis titanium-based

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... 

The dipolar cycloaddition of nitronates has been applied to the synthesis of several natural products in the context of the tandem [4+2] / [3 + 2] nitroalkene cycloaddition process. All of these syntheses have focused on the construction of pyrrolidine, pyrrolizidine, and indolizidine alkaloids. For example, the synthesis of ( )-hastanecine (316), a necine alkaloid, involves the elaboration of a p-benzoy-loxynitroalkene 311 via [4 + 2] cycloaddition with a chiral vinyl ether (312) in the presence of a titanium based Lewis acid, to provide the nitronate 313 with high diastereo- and facial selectivity (Scheme 2.30) (69). The dipolar cycloaddition of... 

The most frequently used Lewis acids based on boron, aluminum, tin, and titanium halides are discussed in more detail in the next section. These acids are used most frequently because of their availability and relatively low cost. Strong Lewis acids such as GaCl3 and the halides of antimony and arsenic are used less often. 

Lewis acids based on titanium tend to aggregate and form dimers which are usually more reactive than their monomeric precursors (cf., Chapter 2). The degree of aggregation depends on the solvent, temperature, and the ligands attached to titanium no dimerization was detected by cryoscopy at -95° C in CH2CI2 [174], However, kinetic measurements of isobutene and styrene polymerizations indicate that polymerization is second order in titanium chloride [175,176], perhaps due to formation of a low concentration of the more reactive dimer or more stable Ti2Cl9-anions. However, polymerizations performed at lower [TiCl4] were reported to be first order in titanium chloride [105]. 

Anions formed from group 6 and manganese Fischer carbene complexes undergo aldol condensations with aldehydes and ketones. Allylic carbenes exclusively react in the y position with aldehydes affording dienyl-substituted carbenes. For alkoxy-substituted carbenes, the presence of an excess Lewis acid see Lewis Acids Bases), such as boron trifluoride etherate, titanium tetrachloride, or tin tetrachloride is required for the reaction to proceed in reasonable yield. The initial aldol product can be isolated without elimination (Scheme 12). ... 

Titanium tetrachloride and amine yield an equilibrium mixture of Lewis acid-base complexes. At least one of these complexes has an increased reactivity towards the carbonyl oxygen and forms a ternary ketone—amine—TiCl,j complex [1] which in the presence of excess amine yields a titanium enolate. The reaction then proceeds as... 

Chiral Lewis acids based on titanium [33, 36, 37] or scandium [38] have been used to catalyze the Diels-Alder reactions, and a high asymmetric amplification has been described by Narasaka et al. [36]. This occurs presumably because of... 

Similar acid mediated cyclizations have been reported on a variety of substrates. For example, as shown in Scheme 7.3.2, Casiraghi, etal.,14 treated 2,3,4,6-tetra-O-benzyl-D-glucopyranose with a pyrrole-derived Grignard reagent and a titanium based Lewis acid. As shown, the addition proceeded in 64% yield with the formation of a single diasteriomer. Subsequent exposure to an acidic DOWEX resin provided a 50% yield of the p-C-glycoside and an additional 21% isolated as the corresponding a anomer. 

Neutral compounds such as boron trifluoride and aluminum trichloride form Lewis acid-base complexes by accepting an electron pair from the donor molecule. The same functional groups that act as electron pair donors to metal cations can form complexes with boron trifluoride, aluminum trichloride, titanium tetrachloride, and related compounds. In this case the complex is formed between two neutral species, it too is neutral, but there is a formal positive charge on the donor atom and a formal negative charge on the acceptor atom. 

Diels-Alder reactions. Many excellent Lewis acid catalysts for the Diels-Alder reaction are formed from enantiomeric binaphthols and substituted binaph-thols. Ytterbium-, aluminum-, and titanium-based species have been developed. A chiral iron catalyst" is also quite effective, but a zirconocene-based catalyst is disappointing in terms of asymmetric induction. 

As for the chiral ytterbium and scandium catalysts, the following structures were postulated. The unique structure shown in scheme 13 was indicated by 13C NMR and IR spectra. The most characteristic point of the catalysts was the existence of hydrogen bonds between the phenolic hydrogens of (R)-binaphthol and the nitrogens of the tertiary amines. The 13 C NMR spectra indicated these interactions, and the existence of the hydrogen bonds was confirmed by the IR spectra (Fritsch and Zundel 1981). The coordination form of these catalysts may be similar to that of the lanthanide(III)-water or -alcohol complex (for a review see Hart 1987). It is noted that the structure is quite different from those of conventional chiral Lewis acids based on aluminum (Maruoka and Yamamoto 1989, Bao et al. 1993), boron (Hattori and Yamamoto 1992), or titanium... 

A second type of oxygen-chelated complex that can be formed with acetylacetone is the simple Lewis acid-base adduct. In these complexes acetylacetone does not lose its acidic proton to form an enolate anion, but rather as the neutral molecule in the keto tautomer donates electrons from the oxygens of each carbonyl to an acceptor or acidic species. Examples of this type of complex are the six-coordinate adducts formed between typically strong Lewis adds as tin tetrachloride or titanium... 

Even more efficient catalysis of the Mukaiyama aldol reaction is possible with complexes of transition metals. A number of titanium-based Lewis acids with binaphthyl hgands have been reported to give high enantioselectivities. For example, only 2 mol% of the Lewis acid 80 is required to effect the aldol reaction of... 

The asymmetric fluorination of 3-ketoesters has been achieved in 62-90% ee using F-TEDA (Selectfluor) as fluorine source in the presence of 0.5 mol% of the chiral nonracemic titanium-based Lewis acid (5.108). ° A greater range of p-ketoesters are fluorinated with higher ee using catalytic quantities of the palladium-BINAP complex (5.109) and N-fluorobenzenesulfonamide (NFSI). ° In both cases the reaction proceeds through the intermediacy of a chiral enolate. 

The addition of nucleophiles to carbonyl groups is a fundamental process in organic synthesis. The addition of diethylzinc to aldehydes occurs with high ee in the presence of a wide range of aminoalcohol ligands and also titanium-based Lewis acids. This methodology has recently been extended to the enantioselective addition of alkenyl, alkynyl and arylzincs and also to the more challenging addition to ketones. 

An alternate approach to the asymmetric alkylation of aldehydes with dialkylzinc compounds uses chiral nonracemic titanium-based Lewis add catalysts. This has been primarily achieved using complexes prepared from stoichiometric amounts of Ti(0 Pr)4 and enantiomerically enriched ligands. A variety of BINOL deriva-... 

The Lewis acid-catalysed addition of trimethylsilyl cyanide to aldehydes has been reported using several different catalysts. Titanium-based Lewis adds have proved to be particularly popular. In a typical reaction, benzaldehyde (6.01) is converted into the cyanohydrin (6.57) usually after removal of the trimethylsilyl group by hydrolysis. A wide variety of ligands have been used for this reaction and... 

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... 

There are several common Lewis acids based around titanium, including titanium tetrachloride and titanium tetraisopropoxide. Enantiomerically pure variants of these Lewis acids provide the basis for catalytic asymmetric reactions. 

Cyanohydrins are usually prepared from carbonyl compounds and a cyanide source. Initially performed with volatile and very toxic hydrogen cyanide, the reaction is now carried out with safer cyanide agents, such as acetone cyanohydrin, acyl cyanides, cyanoformates or the most used trimethylsilyl cyanide. In terms of atom economy, this reaction is 100% atom efficient and is widely used despite the toxicity of the reagents. The asymmetric reaction can now be efficiently catalysed by a variety of chiral Lewis acids, and a recent review presents in detail the work realised in this field, with a large description of titanium-based catal)dic systems. 

Titanium-based Lewis acids are widely used in the Mukaiyama reaction, i. e. the condensation of an enol silane and an aldehyde, giving 3-hydroxycarbonyl derivatives. In this context, BINOL-derived chiral titanium complexes were used in the condensation of thioester-derived ketene silyl acetals with a large range of aldehydes, in high yields and enantioselectivities (Scheme 7.39). ... 

Many ot-olefins were polymerized by the Ziegler-Natta catalysts to yield high polymers and many such polymers were found to be stereospecific and crystalline. Polymerizations of a-olefins of the general structure of CH2 = CH — (CH2) — R, where x is 0-3 and R denotes CH3, CH-(CH3)2, C(CH3)3, or CsHs, can be catalyzed by vanadium trichloride/triethyl aluminum [80]. The conversions are fairly high, though higher crystallinity can be obtained with titanium-based catalysts [81]. Addition of Lewis bases, such as ( 4119)20, (C4H9)3N, or ( 4119)3 , to the catalyst system further increases crystallinity [82]. 

Since the previous titanium-based catalyst was not able to perform a-olefin polymerization, it was obviously interesting to investigate other soluble active systems to further study the problem of stereoregulation. That was done by Zambelli [Makromol Chemie, 112, 160 (1968)] with the complex obtained from VCI4 and AIR2CI (Fig. 21) in the presence of either an excess of A1 derivative or a Lewis base ligand, e.g. anisole. 

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