Friedel-Crafts catalysts, boron trifluoride

One synthetic route to lavandulyl acetate starts with prenyl acetate, which dimerizes in the presence of a Friedel-Crafts catalyst, such as boron trifluoride-diacetic acid [74]. 

In carrying out the alkylation of benzene the propylene tetramer is reacted with an excess of benzene in the presence of a Friedel-Crafts catalyst such as aluminum chloride, boron trifluoride, or hydrofluoric acid. With careful control of this reaction, yields of alkylate boiling from 500° to 650° F. are of the order of 80% of theory with the losses due to slight olefin degradation and dialkylation. Inspection of commercial aromatic products, believed to be typical of this process, indicates the composition to be that shown in... 

Boron trifluoride is another important, reactive Friedel-Crafts catalyst that has been widely used. Because it is a volatile gas but forms many complexes for some applications, it is preferred and can be readily recovered for reuse. 

Thenaldehyde (thiophene-2-carbaldehyde) is readily available via the Vilsmeier-Haack reaction of DMF with thiophene catalyzed by phosphorus oxychloride. The Sommelet reaction with 2-chloromethylthiophene also gives reasonable yields (63AHC(l)l). Likewise, thiophene is readily acylated with acyl anhydrides or acid chlorides (equation 14), using mild Friedel-Crafts catalysts, such as tin(IV) chloride, zinc chloride, boron trifluoride, titanium tetrachloride, mercury(II) chloride, iodine and even silica-alumina gels or low-calcium-content montmorillonite clays (52HC(3)l). 

Sulfonation of aromatic compounds is generally carried out with sulfuric acid, halosulfuric acids, or sulfur trioxide as reagent with or without solvent.458,459 Friedel-Crafts catalysts such as aluminum chloride and boron trifluoride are effective catalysts in certain sulfonations with sulfuric acid and chlorosulfuric acid. 

A typical cationic polymerization is conducted with highly purified monomer free of moisture and residual alcohol, both of which act as inhibitors, in a suitably dry unreactive solvent such as toluene with a Friedel-Crafts catalyst, eg, boron trifluoride, aluminum trichloride, and stannic chloride. Usually low temperatures (—40 to — 70° C) are favored in order to prevent chain-transfer or sidereactions. 

There are, however, chemical reactions that make It possible to Improve the extractability of coal at lower temperatures. In particular those Involving Friedel-Crafts catalysts. Heredy and Neuworth (1) for example, used boron trifluoride and phenol for this purpose and Ouchl, Imuta and Yamashita (2) employed... 

Pure boron trifluoride is marketed as a compressed gas. It is utilized mainly in the organic industry as a Friedel-Crafts catalyst (Lewis acid) in the form of its complexes or addition compounds with, for example, ether, alcohols, carboxylic acids etc. or as a pure substance. 

It is apparent to the authors that the presence in a catalyst of a strong Brpnsted acid as such is not a required intermediate for carbonium-ion formation. It is only necessary that the catalyst has the ability to form stable carbonium ions. In reactions involving Friedel-Crafts catalysts, the aluminum halides and boron trifluoride do not form stable Brpnsted acids but they do form stable carbonium ions in the presence of certain cocatalysts and substrates. The formation of carbonium ions from ole-... 

Monomers with electron-donating groups like isobutylene form stable positive charges and are readily converted to polymers by cationic catalysts. Any strong Lewis acid like boron trifluoride (BF3) or Friedel-Crafts catalysts such as AICI3 can readily initiate cationic polymerization in the presence of a cocatalyst like water, which serves as a Lewis base or source of protons. During initiation, a proton adds to the monomer to form a carbonium ion, which forms an association with the counterion. This is illustrated for isobutylene and boron trifluoride in Equation 2.19 ... 

Obtained by the ethylene oxide ring-opening polymerization using Friedel-Crafts catalysts such as boron trifluoride, tin tetrachloride or zinc chloride, or acid or alkali catalysts. 

Cationic polymerization of unsaturated compounds proceeds through the stage of carbanion cations, called also carbocations. Typical catalysts for this reaction are strong protic acids such as sulfuric acid, perchloric and trifluoroctane or the Lewis acids, which include halides of elements III, IV and V groups of the periodic table (Friedel-Crafts catalysts), such as boron trifluoride, aluminum trichloride, tin tetrachloride and titanium tetrachloride. The activity of Friedel-Crafts catalysts increases significantly the presence of small quantities of cocatalysts, that is, ihe compounds which most often are the source of protons. 

In each case the configuration around the boron changes from trigonal planar to tetrahedral on adduct formation. Because of this ability to form additional compounds, boron trifluoride is an important catalyst and is used in many organic reactions, notably polymerisation, esterification, and Friedel-Crafts acylation and alkylations. 

Other catalysts which may be used in the Friedel - Crafts alkylation reaction include ferric chloride, antimony pentachloride, zirconium tetrachloride, boron trifluoride, zinc chloride and hydrogen fluoride but these are generally not so effective in academic laboratories. The alkylating agents include alkyl halides, alcohols and olefines. 

Cationic polymerization of coal-tar fractions has been commercially achieved through the use of strong protic acids, as well as various Lewis acids. Sulfuric acid was the first polymerization catalyst (11). More recent technology has focused on the Friedel-Crafts polymerization of coal fractions to yield resins with higher softening points and better color. Typical Lewis acid catalysts used in these processes are aluminum chloride, boron trifluoride, and various boron trifluoride complexes (12). Cmde feedstocks typically contain 25—75% reactive components and may be refined prior to polymerization (eg, acid or alkali treatment) to remove sulfur and other undesired components. Table 1 illustrates the typical components found in coal-tar fractions and their corresponding properties. 

Catalysts used in the polymerization of C-5 diolefins and olefins, and monovinyl aromatic monomers, foUow closely with the systems used in the synthesis of aHphatic resins. Typical catalyst systems are AlCl, AIBr., AlCl —HCl—o-xylene complexes and sludges obtained from the Friedel-Crafts alkylation of benzene. Boron trifluoride and its complexes, as weU as TiCl and SnCl, have been found to result in lower yields and higher oligomer content in C-5 and aromatic modified C-5 polymerizations. 

Although all four tocopherols have been synthesized as their all-rac forms, the commercially significant form of tocopherol is i7//-n7i a-tocopheryl acetate. The commercial processes ia use are based on the work reported by several groups ia 1938 (15—17). These processes utilize a Friedel-Crafts-type condensation of 2,3,5-trimethylhydroquinone with either phytol (16), a phytyl haUde (7,16,17), or phytadiene (7). The principal synthesis (Fig. 3) ia current commercial use iavolves condensation of 2,3,5-trimethylhydroquiQone (13) with synthetic isophytol (14) ia an iaert solvent, such as benzene or hexane, with an acid catalyst, such as ziac chloride, boron trifluoride, or orthoboric acid/oxaUc acid (7,8,18) to give the all-rac-acetate ester (15b) by reaction with acetic anhydride. Purification of tocopheryl acetate is readily accompHshed by high vacuum molecular distillation and rectification (<1 mm Hg) to achieve the required USP standard. 

Alkylation of furan and thiophene has been effected with alkenes and catalysts such as phosphoric acid and boron trifluoride. In general, Friedel-Crafts alkylation of furans or thiophenes is not preparatively useful, partly because of polymerization by the catalyst and partly because of polyalkylation. 

Epoxides can also serve as effective carbocyclization promotors, either through a polyene cyclization, as in the biomimetic epoxy-olefin cyclization of 100 in the presence of boron trifluoride etherate <99CC325>, or by a Friedel-Crafts approach, as exemplified by the cycli-alkylation of arylalkyl epoxides 102 under the influence of solid acid catalysts <99EJOC837>. 

The exceedingly high reactivity of ferrocene to Friedel-Crafts acylation is exemplified by the fact that mild catalysts such as stannic chloride (63), boron trifluoride (32), zinc chloride (86), and phosphoric acid (29), can be used with considerable success. When ferrocene and anisole were allowed to compete for limited amounts of acetyl chloride and aluminum chloride, acetylferrocene was the sole product isolated, again illustrating the high reactivity of ferrocene toward electrophilic reagents (6). 

The polymerization of isobutylene using Friedel-Crafts type catalysts, including BF3 and related complexes, is a well-known procedure. The early reports on the polymerization use still boron trifluoride at subambient temperature as polymerization catalyst (1,9-11). 

Friedel-Crafts acylation of benzo[6]thiophene using a variety of catalysts gives a mixture of isomers, in which the 3-isomer predominates in a ratio of about 4 1. The milder catalysts, such as boron trifluoride etherate or iodine, give the best yields, since there is less destruction of the ring. While aluminum chloride gives the 3-isomer in about 9 1 ratio over the 2-isomer, the total yield is only 38%, as contrasted to yields over 70% with milder catalysts (70AHC(11)177). 

The Friedel-Crafts acetylation of 3-phenylsydnone was accomplished with boron trifluoride etherate as catalyst. Formylation at C-4 by the Vilsmeier procedure occurred with 3-phenylsydnone. Mercuration is easily afforded with mercury(II) acetate or mer-cury(13) chloride and thioethers can be made directly with DMSO in acetyl chloride (74T409). At least one fused ring as in compound (35) has been made by a coupling reaction on the sydnone (34) at C-4 (79JCS(P2)175l). 

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