Chloride Lewis acids, catalyst preparation

For preparative reactions, Lewis acid catalysts are used. Zinc chloride or ferric chloride can be used in chlorination, and metallic iron, which generates ferric bromide, is often used in bromination. The Lewis acid facilitates cleavage of the halogen-halogen bond. 

Aiyl chlorides and bromides can be easily prepared by electrophilic substitution of arenes with chlorine and bromine respectively in the presence of Lewis acid catalysts like iron or Iron(III) chloride. 

The epoxidalion is generally conducted in iwo steps (11 the polyol is added to cpichlorohydrin in the presence of a Lewis acid catalyst (stannic chloride, boron irifluoride) to pruduce the chlorollydrin intermediate, and (2] the intermediate is dchydrohalogenaled with sodium hydroxide iu yield the aliphatic glycidyl ether. Solid epoxy resins are prepared by the Taffy or Advancement processes. 

Although it is perhaps not used as extensively as aluminum chloride, aluminum bromide is also widely used as a Lewis acid catalyst. Aluminum fluoride is used in the preparation of cryolite, Na3AlF6, which is added to alumina to reduce its melting point and increase its electrical conductivity in the electrolytic production of aluminum. One reaction that can be employed to produce the fluoride is... 

Aluminum chloride and its derivatives are the most familiar Lewis acids and are routinely employed in many Lewis acid-promoted synthetic transformations. The first polymer-supported metal Lewis acids to be studied were polymers attached by weak chemical or physical interactions to a Lewis acid. In the 1970s Neckers and coworkers reported the use of styrene-divinylbenzene copolymer-supported AlCl,- or BF3 as catalyst in condensations, esterifications, and acetalization of alcohols [11,12]. This type of polymer-supported AICI3 (1) is readily prepared by impregnation of a polystyrene resin with AICI3 in a suitable solvent. Subsequent removal of the solvent leaves a tightly bound complex of the resin and AICI3. The hydrophobic nature of polystyrene protects the moisture-sensitive Lewis acid from hydrolysis, and in this form the Lewis acid is considerably less sensitive to deactivation by hydrolysis. This polymer complex could be used as a mild Lewis acid catalyst for condensation of relatively acid-sensitive dicyclopropylcarbinol to an ether (Eq. 1) [13],... 

Important reactions of rosin are summarized in Figure 8. Dimerization is generally carried out by a Lewis acid catalyst such as sulfuric acid, boron fluoride, or zinc chloride (14-16). A rosin dimer finds uses in resin preparation because it allows quick buildup of molecular weight through a polyfunctional alcohol or amine. 

According to a patent of the Sumitomo Co. (1965), tris-phenol phosphate is prepared from phenol by esterification with phosphorus oxychloride, which is then chlorinated, yielding after hydrolysis a chlorophenol mixture with an ortho para ratio of 8 1. Using sulfuryl chloride (SO2CI2) as chlorinating agent, a chlorophenol mixture of ortho para ratio 4.1 5.7 is formed in the presence of Lewis acid catalysts. 

For example both SbFa and SbFs can be prepared from the respective chlorides (equations 5 and 6). In many reactions, much more rapid conversion of halides to the fluorides in HF is achieved by adding a Lewis acid catalyst such as SbFs or SbFa/CL (SbFaCL). The pentahalide becomes the fluorination reagent and the resulting Sb-Cl bonds are converted to Sb-F bonds by the HF. 

Different methods for the preparation of Novel Lewis-Acid Catalysts (NLACs) consisting of ionic liquids immobilised on mesoporous support materials are presented. The focus will be placed on materials bound to the carrier via the organic cation of the ionic liquid, either by grafting or by the preparation of organically modified HMS. After addition of aluminium(III)chloride the materials were used as catalysts e.g. in Friedel-Crafts alkylations, in which they displayed high activities and selectivities. 

A-homoallylic alcohols were prepared in good yields by allylation of dialdehydes or their acetals with allyl bromide, tin(II) chloride, and potassium iodide in water or water/THF (Eq. 8.46). ° Under ultrason-ication, it was found that SnCU could efficiently mediate the aqueous Barbier reactions between carbonyl compounds and allyl bromide to give the corresponding homoallylic alcohols in high yields without using any Lewis-acid catalyst. 

The acylation of arenes with alcohols has been shown to be possible using a palladium chloride catalyst in the presence of f-butylhydroperoxide. In 2-arylpyridines, substitution is directed to the ortho-position and, after initial paUadation, the formation of intermediate (59) is likely before reductive elimination yields the acylated product. The regioselective acetoxylation of indoles, at the 3-position, has been achieved using the palladium-catalysed reaction with phenyliodonium acetate. 3-Acyl indoles may also be prepared using acetyl chlorides with zirconium tetrachloride as a Lewis acid catalyst. 

Preparation of phenolic aldehydes, phenol ethers or heterocyclic compounds by treatment of the aromatic substrate with hydrogen cyanide and hydrogen chloride in the presence of Lewis acid catalysts ... 

Vinyl-substituted cyanoacrylates have been used from time to time, either as modifiers for alkyl cyanoacrylates or on their own as adhesives. These cyanopentadienoates (2) are less reactive toward anionic polymerization than are the cyanoacrylates. This feature allows their preparation from acrolein and a cyanoacetate ester using a Lewis acid catalyst such as zinc chloride. " The monomer can be isolated and purified without going through a depolymerization step. 

Darabi et al. introduced a different, facile, and economic approach to the preparation of quinoxalines and pyrido[2,3-l)]pyrazines with aryl substituents at the 2- and/or 3-positions by employing recyclable sihca-supported bismuth(III) chloride, providing the desired products in good-to-excellent yields (Scheme 59) (13JOMC170). A minor decrease in the activity of this new heterogeneous Lewis acid catalyst upon repeated usage was noted. A variety of substituents were tolerated under these reaction conditions. 

Among a number of metal chlorides used in organic synthesis, anhydrous aluminum chloride is undoubtedly one of the most effective of Lewis acid catalysts. During the chloromethylation of polystyrene, using aluminum chloride as catalyst, it was observed that all the aluminum chloride could not be removed, even after repeated washing. This was attributed to the formation of a tightly bound polystyrene-aluminum chloride complex. Complex formation was demonstrated by the increase in color (yellow) intensity of the polymer, and by the development of a new characteristic ir band at 1650 cm This complex could act as a mild Lewis acid catalyst for certain organic preparations. 

In the case of metals and transition metals, condensation occurs around 1(X)°C, bringing out higher activation energies than hydrolytic processes. However, silicon is less reactive tertiary alkyl or benzylic R groups are needed (Corriu, 1994) otherwise, Lewis acids, such as iron or aluminum chloride, are required to catalyze the condensation (Bourget, 1998). As a consequence the reactivities of silicon and metal precursors are expected to be leveled in the preparation of silicates, since the metal species are expected to act as Lewis acid catalysts (see below). 

Preparative Methods lithiation of 1,3-dichloropropene followed by treatment with a silyl chloride reaction of 1,3-dichloropropene with Mg° and a silyl chloride electrophile treatment of allylsilanes with BuLi/TMEDA and a silyl electrophile reaction of allyl alcohol with HMPA, Mg°, and a Lewis acid catalyst cross-metathesis of allylsilanes reductive lithiation of allylthioethers. ... 

---