Friedel-Crafts catalysts halides

Acid Halides (Lewis Acids). AH metal haUde-type Lewis catalysts, generally known as Friedel-Crafts catalysts, have an electron-deficient central metal atom capable of electron acceptance from the basic reagents. The most frequendy used are aluminum chloride and bromide, followed by... 

Solid Superacids. Most large-scale petrochemical and chemical industrial processes ate preferably done, whenever possible, over soHd catalysts. SoHd acid systems have been developed with considerably higher acidity than those of acidic oxides. Graphite-intercalated AlCl is an effective sohd Friedel-Crafts catalyst but loses catalytic activity because of partial hydrolysis and leaching of the Lewis acid halide from the graphite. Aluminum chloride can also be complexed to sulfonate polystyrene resins but again the stabiUty of the catalyst is limited. 

All lation. Thiophenes can be alkylated in the 2-position using alkyl halides, alcohols, and olefins. Choice of catalyst is important the weaker Friedel-Crafts catalysts, eg, ZnCl2 and SnCl, are preferred. It is often preferable to use the more readily accompHshed acylation reactions of thiophene to give the required alkyl derivatives on reduction. Alternatively, metalation or Grignard reactions, on halothiophenes or halomethylthiophenes, can be utilized. 

Furan can also be acylated by the Vilsmeier-Haack method. Acylation of furans can also be carried out with acid anhydrides and acyl halides in the presence of Friedel-Crafts catalysts (BF3-Et20, SnCU or H3PO4). Reactive anhydrides such as trifluoroacetic anhydride, however, require no catalyst. Acetylation with acetyl p-toluenesulfonate gives high yields. 

The Phillips approach involved the reaction of aryl halides with aromatic nuclei in the presence of Friedel-Crafts catalysts. Whilst many variations in the process existed three main cases were distinguishable. 

The Friedel-Crafts reaction is a very important method for introducing alkyl substituents on an aromatic ring. It involves generation of a carbocation or related electrophilic species. The most common method of generating these electrophiles involves reaction between an alkyl halide and a Lewis acid. The usual Friedel-Crafts catalyst for preparative work is AICI3, but other Lewis acids such as SbFj, TiC, SnCl4, and BF3 can also promote reaction. Alternative routes to alkylating ecies include protonation of alcohols and alkenes. 

A study of alkylations with a group of substituted benzyl halides and a range of Friedel-Crafts catalysts has provided insight into the trends in selectivity and orientation that accompany changes in both the alkyl group and the catalysts. There is a marked increase in substrate selectivity on going from / -nitrobenzyl chloride to /i-methoxybenzyl chloride. For example, with titanium tetrachloride as the catalyst, Aitoi Abenz increases from 2.5 to 97. This increase in substrate selectivity is accompanied by an increasing preference for para substitution. With /i-nitrobenzyl chloride, the ortho para ratio is 2 1 (the... 

Diaryl sulfones can be formed by treatment of aromatic compounds with aryl sulfonyl chlorides and a Friedel-Crafts catalyst. This reaction is analogous to Friedel-Crafts acylation with carboxylic acid halides (11-14). In a better procedure, the aromatic compound is treated with an aryl sulfonic acid and P2O5 in polypho-sphoric acid. Still another method uses an arylsulfonic trifluoromethanesulfonic anhydride (ArS020S02CF3) (generated in situ from ArS02Br and CF3S03Ag) without a catalyst. ... 

Aryl halides can be dehalogenated by Friedel-Crafts catalysts. Iodine is the most easily cleaved. Dechlorination is seldom performed and defluorination apparently never. The reaction is most successful when a reducing agent, say, Br or 1 is present to combine with the I" or Br coming off." Except for deiodination, the reaction is seldom used for preparative purposes. Migration of halogen is also found," both intramolecular and intermolecular." The mechanism is probably the reverse of that of 11-11." ... 

The relative reactivity of Friedel-Crafts catalysts has not been described in a quantitative way, but comparative studies using a series of benzyl halides has resulted in the qualitative groupings shown in Table 11.1. Proper choice of catalyst can minimize subsequent product equilibrations. 

The metal halide catalysts include aluminum chloride, aluminum bromide, ferric chloride, zinc chloride, stannic chloride, titanium tetrachloride and other halides of the group known as the Friedel-Crafts catalysts. Boron fluoride, a nonmetal halide, has an activity similar to that of aluminum chloride. 

Catalyst systems with improved heat transfer capability for the production of PIB polymers in continuous slurry polymerization processes have been described. The catalyst consists of a Lewis acid, including Friedel-Crafts catalysts such as AICI3, and a tertiary halide containing compound, e.g., 2-chloro-2,4,4-trimethylpentane or ethylaluminum dichloride (12,13). 

This powerful halide-abstracting ability results in SbFs being the strongest Lewis add with which HS03F forms the strongest superacid , the so-called magic add .70 The Sbv halides are excellent Friedel-Crafts catalysts.269... 

Niobium and tantalum compounds form adducts with virtually all types of neutral ant anionic donors. The coordination chemistry of the higher halides is widely developed, and thei activity as Friedel-Crafts catalysts is another manifestation of their Lewis acidity. The stron acceptor capacity of the high valent metal compounds tends to favor the formation of dimers and sometimes of higher condensation products, which competes with coordination with othe donor molecules. Numerous simple anionic or heteropolyanionic species, but little cationi chemistry, and no simple metal salts, are known. 

Besides aluminum chloride, the most often used and studied Friedel-Crafts catalyst, many other acid catalysts are effective in alkylation. Although Friedel-Crafts alkylation was discovered and explored mainly with alkyl halides, from a practical point of view, alkenes are the most important alkylating agents. Others include alcohols, ethers, and esters. 

Friedel-Crafts catalysts and hydrogen halides, protic acids, silica-alumina-type catalysts, or other protic catalysts are effective. 

Inflates of aluminum, gallium and boron, which are readily available by the reaction of the corresponding chlorides with triflic acid, are effective Friedel-Crafts catalysts for alkylation and acylation of aromatic compounds [119, 120] Thus alkylation of toluene with various alkyl halides in the presence of these catalysts proceeds rapidly at room temperature 111 methylene chloride or ni-tromethane Favorable properties of the Inflates in comparison with the correspond 1 ng fluorides or chlorides are considerably decreased volatility and higher catalytic activity [120]... 

Friedel-Crafts acylation of aromatics is of considerable practical value owing to the importance of aryl ketones and aldehydes as chemical intermediates.346 Whereas alkylation of aromatics with alkyl halides requires only catalytic amounts of catalysts, acylation to ketones generally necessitates equimolar or even some excess of the Friedel-Crafts catalysts. Usually one molar equivalent of catalyst combines with an acyl halide, giving a 1 1 addition compound, which then acts as the active acylating agent [Eq. (5.137)]. 

Through the interaction of an alkylating agent, a hydrogen atom of the aromatic core, in the case of alkylation of arenes, is replaced by an alkyl group driven by a Friedel-Crafts catalyst. A variety of alkylating agents (e.g. alkyl halides, alcohols,... 

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