Catalysts TiCl4-Lewis acid

The catalysts for cationic polymerization are either protonic acids or Lewis acids, such as H2SO4 and HCIO4 or BF3, AICI3, and TiCl4 ... 

Scheme 2.25 shows some examples of additions of enolate equivalents. A range of Lewis acid catalysts has been used in addition to TiCl4 and SnCl4. Entry 1 shows uses of a lanthanide catalyst. Entry 2 employs LiC104 as the catalyst. The reaction in Entry 3 includes a chiral auxiliary that controls the stereoselectivity the chiral auxiliary is released by a cyclization using (V-methylhydroxylamine. Entries 4 and 5 use the triphenylmethyl cation as a catalyst and Entries 6 and 7 use trimethylsilyl triflate and an enantioselective catalyst, respectively. 

A variety of electrophilic catalysts promote the addition of allylic silanes to carbonyl compounds.86 The original catalysts included typical Lewis acids such as TiCl4 or BFj.87 This reaction is often referred to as the Sakurai reaction. 

It was first observed that reactions of imines with ketene silyl acetals proceeded smoothly in the presence of 5mol.% Yb(OTf)3 to afford the corresponding /3-amino ester derivatives in moderate yields.50 However, Sc(OTf)3 was found to be a more active catalyst in this reaction. Benzoylhy-drazones also react with ketene silyl acetals in the presence of a catalytic amount of Sc(OTf)3 to afford the corresponding adducts in high yields (Scheme 11).51 In contrast, catalytic activation of benzoylhydrazones by use of a typical Lewis acid such as TiCl4, SnCl4, or BF3-OEt2, etc. is not effective. 

In many ways, TiCl4 behaves as a covalent compound of a nonmetal. It is a strong Lewis acid that forms complexes with many types of Lewis bases, and it hydrolyzes in water. It also reacts with alcohols to yield compounds having the formula Ti(OR)4. However, it is the behavior of TiCl4 (reacting with [A1(C2H5)3]2) as a catalyst in the Ziegler-Natta polymerization of ethylene that is the most important use of the compound (see Chapter 22). 

Propargylic ethers. These ethers can be prepared by reaction of acetals with 1-trimethylsilylalkynes in the presence of at least 1 equiv. of a Lewis acid such as TiCl4 or SnCl4 (12,375). However a combination of SnCl4 and ZnCl2 (10 mole % each) can effect this reaction (equation I). The actual catalyst may be +SnCl3--ZnCl,. 

First developments in the Friedel-Crafts alkylation were concentrated on the use of stoichiometric amounts of Lewis acids, such as A1C13, BF3 or TiCl4, to produce stoichiometric amounts of salt by-products [5-9]. However, in recent years more and more catalytic methods have been developed. In particular, rare earth metal triflates, including Sc(OTf)3, La(OTf)3 and Yb(OTf)3, have been extensively used as Lewis acid catalysts in various C-C and C-X bond forming reactions [10-13], Despite the benefit of their versatility for organic synthesis, these Lewis acids possess major drawbacks. They are expensive, rather toxic [14], and air- and moisture-sensitive. 

Hydrazoic acid can be added to certain Michael-type substrates (Z is as defined on p. 741) to give (3-azido compounds.214 The reaction apparently fails if R is phenyl. HNj also adds to enol ethers CH2=CHOR to give CH,—CH(OR)N3, and to silyl enol ethers,215 but it does not add to ordinary alkenes unless a Lewis acid catalyst, such as TiCl4, is used, in which case good yields of azide can be obtained.215 HN3 can also be added indirectly to ordinary olefins by azidomercuration, followed by demercuration,216 analogous to the similar... 

Inorganic Lewis acids, specifically, Friedel-Crafts catalysts, such as SnCU, TiCl4, AICI3, AlBr3, and BF3, usually require a cocatalyst (e.g., H20 always considered to be present), and in this case, initiation is also induced by the proton formed [Eq. (13.18)]. Pioneering work on the role of proton acids as catalysts and cocatalysts in acid-catalyzed polymerization was carried out by Polanyi and... 

Aldol reactions.1 The anion generated (BuLi) from chromium carbene complexes undergoes aldol reactions with aldehydes or ketones activated by a Lewis acid. Best results are obtained with ketones in the presence of BF3 etherate, whereas TiCl4 is the preferred catalyst for aldehydes and acetals. 

Mukaiyama aldol condensation (6, 590-591).8 This reaction can be effected in the absence of a Lewis acid catalyst under high pressure (10 kbar). Surprisingly the stereoselectivity is the reverse of that of the TiCl4-catalyzed reaction (equation I). The reaction can also be effected in water with the same stereoselectivity, but the yield is low because of hydrolysis of the silyl enol ether. Yields are improved by use of water-oxolane (1 1) and by sonication.9... 

R)-( - )-3-Benzyloxy-2-methylpropionaldehyde (2), obtained from N-methyl-ephedrine, reacts with the ketene f-butylthio trialkylsilyl acetal 3 in CH2C12 in the presence of TiCl4 to give 4 in >99% de. Use of BF3 etherate as the Lewis acid catalyst results in three stereoisomers.13... 

Diels-Alder catalyst.2 The acrylates (4) of three chiral alcohols (1-3) have been found to undergo asymmetric Diels-Alder reactions with cyclopentadiene (equation I) in the presence of a Lewis acid catalyst. For this purpose, catalysts of the type TiCl2(X2) are superior to TiCl4 because they do not promote polymerization of the acrylate. The final products (6) all have the enrfo-orientation the configuration (R) or (S) depends upon the chiral alcohols. Those derived from 1 all have (S)-configuration ... 

Diels-Alder catalyst. Unlike other Lewis acids, TiCl4 favors formation of the exo-isomer in the Diels-Alder reaction of cyclopentadiene and mesityl oxide (equation... 

Organogallium reagents, RsGa, have been added enantioselectively to aldehydes, using TiCl4 as a Lewis acid catalyst, with a chiral salan ligand.258... 

Silica-supported Lewis acids are useful catalysts with microwave irradiation for conjugate additions. The silica-supported catalysts are obtained by treatment of silica with ZnCh [Si(Zn)], Et2AlCl [Si(Al)] or TiCl4 [Si(Ti)] [ 150-152], The Michael addition of methyl a-acetamidoacrylate (196) with indole (2) under Si(M) heterogeneous catalysis assisted by microwave irradiation afforded the alanine derivative 197 within 15 min and/or bis-indolyl 198, depending on the reaction conditions (Scheme 45) [153]. While the bis-indolyl product 198 is only formed when Si(Zn) was used as catalyst, the alanine derivative 197, as a single product is formed under thermal heating in a yield of 12%. The best yields were observed with Si(Al) (Table 5). The product 198 was obtained by elimination of acetamide followed by a-Michael addition between intermediate 199 with a second mole of indole. 

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