Friedel-Crafts alkylation compounds

We will show here the classification procedure with a specific dataset [28]. A reaction center, the addition of a C-H bond to a C=C double bond, was chosen that comprised a variety of different reaction types such as Michael additions, Friedel-Crafts alkylation of aromatic compounds by alkenes, or photochemical reactions. We wanted to see whether these different reaction types can be discerned by this... 

Synthesis This route has been carried out successfully (Rec. Trav. Chem., 1958, 77, 854). Note that no AICI3 is needed for Friedel-Crafts alkylation with easily formed t-alkyl compounds. 

The selectivity of an electrophile, measured by the extent to which it discriminated either between benzene and toluene, or between the meta- and ara-positions in toluene, was considered to be related to its reactivity. Thus, powerful electrophiles, of which the species operating in Friedel-Crafts alkylation reactions were considered to be examples, would be less able to distinguish between compounds and positions than a weakly electrophilic reagent. The ultimate electrophilic species would be entirely insensitive to the differences between compounds and positions, and would bring about reaction in the statistical ratio of the various sites for substitution available to it. The idea has gained wide acceptance that the electrophiles operative in reactions which have low selectivity factors Sf) or reaction constants (p+), are intrinsically more reactive than the effective electrophiles in reactions which have higher values of these parameters. However, there are several aspects of this supposed relationship which merit discussion. 

Nitration in sulphuric acid is a reaction for which the nature and concentrations of the electrophile, the nitronium ion, are well established. In these solutions compounds reacting one or two orders of magnitude faster than benzene do so at the rate of encounter of the aromatic molecules and the nitronium ion ( 2.5). If there were a connection between selectivity and reactivity in electrophilic aromatic substitutions, then electrophiles such as those operating in mercuration and Friedel-Crafts alkylation should be subject to control by encounter at a lower threshold of substrate reactivity than in nitration this does not appear to occur. 

Because acylation of an aromatic ring can be accomplished without rearrangement it is frequently used as the first step m a procedure for the alkylation of aromatic compounds by acylation-reduction As we saw m Section 12 6 Friedel-Crafts alkylation of ben zene with primary alkyl halides normally yields products having rearranged alkyl groups as substituents When a compound of the type ArCH2R is desired a two step sequence IS used m which the first step is a Friedel-Crafts acylation... 

Friedel-Crafts alkylation (Section 12 6) An electrophilic aro matic substitution in which an aromatic compound reacts with an alkyl halide in the presence of aluminum chloride An alkyl group becomes bonded to the nng... 

Tertiary, benzyl, and aHyhc nitro compounds can also be used as Friedel-Crafts alkylating agents eg, reaction of (CH2)3CN02 (2-nitro-2-methyl propane [594-70-7]) with anisole in the presence of SnCl gives 4-/-butylanisole [5396-38-3] (7). SoHd acids, such as perfluorodecanesulfonic acid [335-77-3], and perfluorooctanesulfonic acid [1763-23-1] have been used as catalysts for regio-selective alkylations (8). 

Haloall lation. Haloalkyl groups can be introduced directiy by processes similar to Friedel-Crafts alkylation into aromatic and, to some extent, ahphatic compounds. Because halo alkylations involve bi- or polyfunctional alkylating agents, they must be performed under conditions that promote the initial halo alkylation but not, to any substantial degree, subsequent further alkylations with the initially formed haloalkylated products. 

Arylation of Aromatic Compounds. In contrast to Friedel-Crafts alkylations, arylations of aromatics are not as well known, and usually require drastic conditions. They iaclude (/) dehydrogenating condensation (SchoU reaction) (2) arylation with aryl haUdes and (J) arylation with dia2onium hahdes. 

The alkylation of pyridine [110-86-1] takes place through nucleophiUc or homolytic substitution because the TT-electron-deficient pyridine nucleus does not allow electrophiUc substitution, eg, Friedel-Crafts alkylation. NucleophiUc substitution, which occurs with alkah or alkaline metal compounds, and free-radical processes are not attractive for commercial appHcations. Commercially, catalytic alkylation processes via homolytic substitution of pyridine rings are important. The catalysts effective for this reaction include boron phosphate, alumina, siHca—alurnina, and Raney nickel (122). 

FRIEDEL - CRAFTS Alkylation-Acylation Alkylation or acylation ol aromatic compounds by means of alryl halides, alcohols.alkenes, acyl halides in the presence of Lewis acids... 

Other typical electrophilic aromatic substitution reactions—nitration (second entr-y), sul-fonation (fourth entry), and Friedel-Crafts alkylation and acylation (fifth and sixth entries)—take place readily and are synthetically useful. Phenols also undergo electrophilic substitution reactions that are limited to only the most active aromatic compounds these include nitrosation (third entry) and coupling with diazonium salts (seventh entry). 

Drawbacks as known from the Friedel-Crafts alkylation are not found for the Friedel-Crafts acylation. In some cases a decarbonylation may be observed as a side-reaction, e.g. if loss of CO from the acylium ion will lead to a stable carbenium species 8. The reaction product of the attempted acylation will then be rather an alkylated aromatic compound 9 ... 

The synthesis of an alkylated aromatic compound 3 by reaction of an aromatic substrate 1 with an alkyl halide 2, catalyzed by a Lewis acid, is called the Friedel-Crafts alkylation This method is closely related to the Friedel-Crafts acylation. Instead of the alkyl halide, an alcohol or alkene can be used as reactant for the aromatic substrate under Friedel-Crafts conditions. The general principle is the intermediate formation of a carbenium ion species, which is capable of reacting as the electrophile in an electrophilic aromatic substitution reaction. 

Rank the following aromatic compounds in the expected order of their reactivity toward Friedel-Crafts alkylation. Which compounds are unreactive ... 

Naphthalene and other fused ring compounds are so reactive that they react with the catalyst, and therefore tend to give poor yields in Friedel-Crafts alkylation. Heterocyclic rings are also tend to be poor substrates for the reaction. Although some furans and thiophenes have been alkylated, a true alkylation of a pyridine or a quinoline has never been described.However, alkylation of pyridine and other nitrogen heterocycles can be accomplished by a free radical (14-23) and by a nucleophilic method (13-15). 

An important use of the Friedel-Crafts alkylation reaction is to effect ring closure. The most common method is to heat with aluminum chloride an aromatic compound having a halogen, hydroxy, or alkene group in the proper position, as, for example, in the preparation of tetralin ... 

Friedel-Crafts Alkylations with Silicon Compounds... 

CHARACTERISTICS OF FRIEDEL-CRAFTS ALKYLATIONS WITH SILICON COMPOUNDS... 

In this section, the reactivities of organosilicon compounds for the Friedel-Crafts alkylation of aromatic compounds in the presence of aluminum chloride catalyst and the mechanism of the alkylation reactions will be discus.sed, along with the orientation and isomer distribution in the products and associated problems such as the decomposition of chloroalkylsilanes to chlorosilanes.. Side reactions such as transalkylation and reorientation of alkylated products will also be mentioned, and the insertion reaction of allylsilylation and other related reactions will be explained. 

Ferrocene behaves in many respects like an aromatic electron-rich organic compound which is activated toward electrophilic reactions.In Friedel-Crafts type acylation of aromatic compounds with acyl halides, ferrocene is lO times more reactive than benzene and gives yields over 80%. However, ferrocene is different from benzene in respect to reactivity and yields in the Friedel-Crafts alkylation with alkyl halides or olefins. The yields of ferrocene alkylation are often very low. and the separations of the polysubstituted byproducts are tedious. 

Vinylchlorosilanes react with aromatic compounds in the presence of Lewis acid to give the alkylation products 2-(chlorosilyl)ethylarenes. In the Friedel-Crafts alkylation of aromatic compounds, the reactivity of vinylchlorosilanes is slightly lower than that of allylchlorosilanes.Friedel-Crafts alkylation of benzene derivatives with vinylsilanes to give 2-(chlorosilyl)ethylarenes was first reported by the Andrianov group (Eq. (5))." The reactivity of vinylsilanes in the... 

The Friedel-Crafts alkylation of aromatic compounds with alkyl halides in the presence of Lewis acid is well defined in organic chemistry. However, alky-... 

Among the Friedel-Crafts alkylations of aromatic compounds with (chlorinated alkyl)silanes, the alkylation of benzene with (tt>-chloroalkyl)silanes in the presence of aluminum chloride catalyst was generally affected by two factors the spacer length between the Cl and silicon and the electronic nature of substituents on the silicon atom of (w-chloroalkyl)silanes. As the spacer length between the C—Cl and silicon increases from (chloromethyl)silane to (/i-chloroethyl)silane to (/-chloropropyl)silane, the reactivity of the silanes increases. As the number of chloro-groups on the silicon decreases from (chloromethyl)trichlorosilanes to (chloromethyl)methyldichlorosilanes to (chloromethyl)trimethylsilanes, the... 

Jung, II Nam, and Yoo, Bok Ryul, Friedel-Crafts Alkylations with Silicon Compounds. 46 145... 

---