Lewis acids Titanium chloride Zinc

The use of trifluoroborane etherate185 and titanium tetrachloride166 186 in thioa-cetalisation reactions is common but milder Lewis acids such as zinc chloride will suffice in some circumstances as shown by the large scale transprotection reaction depicted in Scheme 2.93.150... 

But-3-enyltrimethylsilanes undergo cyclodesilylation to give cyclopropanes 7 on reaction with acid chlorides activated by a Lewis acid. Titanium(IV) chloride was found to be the most effective activator of the Lewis acids studied. No reaction was observed for boron trifluoride-diethyl ether complex, zinc(II) chloride and iron(III) chloride. A variety of aliphatic, aromatic and alkenoyl chlorides were successfully utilized affording the corresponding cyclo-propylmethyl ketones in fairly good yields. It has been verified that the j8-chloro ketones 9 are secondary reaction products derived from the cyclopropyl ketones 7. The formation of these chloro ketones can be avoided by performing the reaction at low temperature. 

Investigations on the trimerization of phenylcyanamide have shown that either triphenylmelamine (12) or triphenylisomelamine (13) is formed, depending on the reaction conditions.1 19,230,231 Under mild conditions (20-80°C) and in the presence of basic catalysts (e.g triethylamine, pyridine) the iso form 13 is obtained, whereas with Lewis acids [e.g., zinc(IT) chloride, titanium(IV) chloride] at 200 °C, triphenylmelamine (12) is formed. 

Further refinements to the alkylation of silyl enol ethers have appeared. Ketones can be alkylated at the a-position by treatment of their silyl enol ethers with tertiary alkyl halides in the presence of Lewis acids such as titanium tetrachloride or tin tetrachloride.a-Cumylation of ketones is possible using catalytic quantities of mild Lewis acids such as zinc chloride. The method can be used to couple two tertiary centres. 

Related Reagents. See entries for other Lewis acids, e.g. Zinc Chloride, Aluminum Chloride, Titanium(IV) Chloride also see entries for Boron Trifluoride (and combination reagents), and combination reagents employing boron trifluoride etherate, e.g. n-Butyllithium-Boron Trifluoride Etherate Cerium(III) Acetate-Boron Trifluoride Etherate Lithium Aluminum Hydride-Boron Trifluoride Etherate Methylcopper-Boron Trifluoride Etherate. 

Zinc chloride is a Lewis acid catalyst that promotes cellulose esterification. However, because of the large quantities required, this type of catalyst would be uneconomical for commercial use. Other compounds such as titanium alkoxides, eg, tetrabutoxytitanium (80), sulfate salts containing cadmium, aluminum, and ammonium ions (81), sulfamic acid, and ammonium sulfate (82) have been reported as catalysts for cellulose acetate production. In general, they require reaction temperatures above 50°C for complete esterification. Relatively small amounts (<0.5%) of sulfuric acid combined with phosphoric acid (83), sulfonic acids, eg, methanesulfonic, or alkyl phosphites (84) have been reported as good acetylation catalysts, especially at reaction temperatures above 90°C. 

Allylsilanes or allylstannanes in the presence of a bidentate Lewis acid such as tin(IV) chloride, titanium(IV) chloride, zinc chloride, and magnesium bromide as well as diallylzinc, are promising choices (Table 1). 

Several modifications of the Simmons-Smith procedure have been developed in which an electrophile or Lewis acid is included. Inclusion of acetyl chloride accelerates the reaction and permits the use of dibromomethane.174 Titanium tetrachloride has similar effects in the reactions of unfunctionalized alkenes.175 Reactivity can be enhanced by inclusion of a small amount of trimethylsilyl chloride.176 The Simmons-Smith reaction has also been found to be sensitive to the purity of the zinc used. Electrolytically prepared zinc is much more reactive than zinc prepared by metallurgic smelting, and this has been traced to small amounts of lead in the latter material. 

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. 

Cationic mechanisms are much more characteristic of the polymerization of oxygen heterocycles, both ethers and acetals. A wide variety of catalysts has been used, including protonic acids, such Lewis acids as boron trifluoride, phosphorus pentafluoride, stannic chloride, antimony pentachloride, titanium tetrachloride, zinc chloride, and ferric chloride, and salts of carbocations or tri-alkyloxonium ions having anions derived from Lewis acids. Some complex, coordination catalysts that appear to operate by a mechanism... 

The use of a Lewis acid (e.g., friethylfluoroborate, zinc chloride, stannous chloride, titanium chloride, iron(m)chloride) and other reagents (e.g., iodine, trimethylsilane, trifluoiomethane-sulfonylsilane) have also been recommended. Exhaustive lists of catalysts and conditions can be found in reviews devoted to carbohydrates [5-7], or to general organic chemistry [27,28], However, one can add the new catalyst, which has been introduced for the smooth formation of p-methoxybenzylidene acetals and p-methaxy-phenylmethyl methyl ether [29], namely 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ), and has been applied very recently [30] to the synthesis of isopropylidene mixed acetals. 

Aluminum trichloride is the most commonly used catalyst, although aluminum tribromide is more efficient.1 For the rearrangement of l-broino-2-chloro-1,L2-lrifluoroethane (3) to 2-bromo-2-chloro-l,l,l-trifhioroethane (4). none of the following Lewis acids are effective iron(III) chloride. iron(III) bromide, antimony(III) chloride, antimony(V) chloride. tin(IV) chloride, titanium(IV) chloride, zinc(II) chloride, and boron trifluoride-diethyl ether complex.1" ... 

The addition of thiols to C—C multiple bonds may proceed via an electrophilic pathway involving ionic processes or a free radical chain pathway. The main emphasis in the literature has been on the free radical pathway, and little work exists on electrophilic processes.534-537 The normal mode of addition of the relatively weakly acidic thiols is by the electrophilic pathway in accordance with Markovnikov s rule (equation 299). However, it is established that even the smallest traces of peroxide impurities, oxygen or the presence of light will initiate the free radical mode of addition leading to anti-Markovnikov products. Fortunately, the electrophilic addition of thiols is catalyzed by protic acids, such as sulfuric acid538 and p-toluenesulfonic acid,539 and Lewis acids, such as aluminum chloride,540 boron trifluoride,536 titanium tetrachloride,540 tin(IV) chloride,536 540 zinc chloride536 and sulfur dioxide.541... 

This reaction has been extended to a similar alkylation with more reactive primary and secondary alkyl halides, such as benzylic and allylic halides. For this purpose the milder Lewis acid zinc bromide is generally preferable to titanium(IV) chloride as catalyst. ... 

Phenylthioalkylation of silyl enol ethers. Silyl enol ethers of ketones, aldehydes, esters, and lactones can be alkylated regiospecifically by a -chloroalkyl phenyl sulfides in fhe presence of a Lewis acid. Zinc bromide and titanium(IV) chloride are the most effective catalysts. The former is more satisfactory for enol ethers derived from esters and lactongs. ZnBr2 and TiCL are about equally satisfactory for enol ethers of ketones. The combination of TiCL and Ti(0-f-Pr)4 is more satisfactory for enol ethers of aldehydes. Since the products can be desulfurized by Raney nickel, this reaction also provides a method for alkylation of carbonyl compounds. Of more interest, sulfoxide elimination provides a useful route to a,B-unsaturated carbonyl compounds. 

Friedrich et al. [45] discovered that a catalytic amount of titanium(IV) chloride as a Lewis acid greatly facilitates cyclopropanation reactions of alkenes by the system CFl2Br2-Zn-CuCl. The Lewis acid catalyst might bind to the oxygen atom of the allylic alcohol present as the (iodomethyl)zinc alkoxide, and thus increase the electro-philicity of the methylene group [46]. 

Various Lewis acids have been employed, most commonly BF3-etherate [34,35], but also zinc chloride, stannic chloride, ferric chloride [36], zirconium chloride [37] and titanium chlo-ride/indium chloride [38], with similar results. Recently 18 different oxometallic species were investigated as catalysts, and of these M0O2CI2 (3 mol%) was found to be the most efficient affording thioglycosides in 75-94% yield [39]. 

The same reaction products are obtained from epoxides and TMS-CN in the presence of aluminum al-koxides, samarium, cerium or lanthanum chlorides as well as titanium tetraisopropanoate, which may all be considered comparatively hard Lewis acids. On the other hand various groups demonstrated that softer Lewis acids like zinc, tin and palladium dichloride may give rise to isocyanides (Scheme 15). 

Preferably the reaction in accordance with the invention is performed in the presence of a suitable catalyst, proton acids such as for instance haloid acids, sulfuric acid, phosphoric acid, perchloric acid, organic sulfonic acids, such as for instance methanesulfonic acid and p-toluenesulfonic acid, carboxylic acids, such as for instance oxalic acid, trifluoroacetic acid and other Lewis acids, such for instance boron trifluoride, ferric chloride, zinc chloride, zinc bromide, stannic chloride, titanium chloride or iodine having proved to be suitable. Furthermore mixtures of the individual catalysts may be used in certain cases. 

As a catalyst for the ring condensation Lewis acids such as for instance zinc chloride, zinc bromide, boron trifluoride, ferric chloride, stannic chloride, titanium chloride or iodine are used, zinc chloride and zinc bromide having proved to be more particularly suitable. Water binding substances such as neutral substances as for instance magnesium sulfate, sodium sulfate, calcium sulfate or molecular sieves may be used, the last-named having proved more particularly suitable. 

Other methods used to improve the cyclopropanation in Simmons Smith reactions are ultrasonic cavitation and the use of catalytic amounts of titanium(IV) chloride to promote the reaction. A much better method is to use 1 mol% of acetyl chloride (based on zinc) and dibromo-methane in the presence of zinc dust and copper(I) chloride in diethyl ether. This system not only strongly accelerates alkene cyclopropanation, but also causes no special problems with Lewis acid sensitive substrates. Acetyl chloride works as a promoter by reacting with... 

BTF is known to react with strong Lewis-Acids such as AICI3. [42] However, milder Lewis-Acids do not readily react with BTF. Zinc chloride catalyzed Friedel-Crafts acylation (8.1) leads to better yields in refluxing BTF compared to sym-tetrachloroethane. [43] The deactivating trifluoromethyl group is presumably responsible for the inertness of BTF towards aromatic substitution under these conditions. Titanium tetrachloride has successfully been used for Sakurai [44]... 

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