Aromatic-alkene alkylation

The generation of caibocations from these sources is well documented (see Section 5.4). The reaction of aromatics with alkenes in the presence of Lewis acid catalysts is the basis for the industrial production of many alkylated aromatic compounds. Styrene, for example, is prepared by dehydrogenation of ethylbenzene made from benzene and ethylene. 

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. 

The alkylation with alkenes can be catalyzed by protons. The carbon-carbon double bond of the alkene is protonated according to Markow nikojfs rule, to give a carbenium ion 10, which then reacts by the above mechanism to yield the alkylated aromatic product 11 ... 

The Friedel-Crafts allcylation reaction usually involves the interaction of an allcy-lation agent such as an alkyl halide, alcohol, or alkene with an aromatic compound, to form an alkylated aromatic compound (Scheme 5.1-44). 

This section contains dehydrogenations to form alkenes and unsaturated ketones, esters and amides. It also includes the conversion of aromatic rings to alkenes. Reduction of aryls to dienes is found in Section 377 (Alkene-Alkene). Hydrogenation of aryls to alkanes and dehydrogenations to form aryls are included in Section 74 (Alkyls from Alkenes). 

Alkenes and alcohols can also be used in place of alkyl halides for alkylating aromatic species. The presence of a proton acid is required... 

A continuous procedure for the alkylation of mesitylene and anisole with supercritical propene, or propan-2-ol in supercritical carbon dioxide, with a heterogeneous polysiloxane-supported solid acid Deloxan catalyst has been reported giving 100% selectivity for monoalkylation of mesitylene with 50% conversion at 250 °C and 150 bar by propan-2-ol in supercritical carbon dioxide. p-Toluenesulfonic acid monohydrate has been demonstrated as an efficient catalyst for the clean alkylation of aromatics using activated alkyl halides, alkenes or tosylates under mild conditions. Cyclohexene, for example, reacts with toluene to give 100% cyclohexyltoluenes (o m p-29 18 53) under these circumstances. 

Introduction of an alkyl group onto an aromatic substrate by treating the substrate with an alkylating agent such as alkyl halide, alkene, alkyne and alcohol in the presence of a Lewis acid. 

Triple bonds in side chains of aromatics can be reduced to double bonds or completely saturated. The outcome of such reductions depends on the structure of the acetylene and on the method of reduction. If the triple bond is not conjugated with the benzene ring it can be handled in the same way as in aliphatic acetylenes. In addition, electrochemical reduction in a solution of lithium chloride in methylamine has been used for partial reduction to alkenes trans isomers, where applicable) in 40-51% yields (with 2,5-dihydroaromatic alkenes as by-products) [379]. Aromatic acetylenes with triple bonds conjugated with benzene rings can be hydrogenated over Raney nickel to cis olefins [356], or to alkyl aromatics over rhenium sulfide catalyst [54]. Electroreduction in methylamine containing lithium chloride gives 80% yields of alkyl aromatics [379]. 

At higher temperatures, C—H and C—C bonds may be similarly broken. Thus, zeolite catalysts may be used for (i) alkylation of aromatic hydrocarbons (cf. the Friedel-Crafts reactions with AICI3 as the Lewis acid catalyst), (ii) cracking of hydrocarbons (i.e., loss of H2), and (Hi) isomerization of alkenes, alkanes, and alkyl aromatics. 

The Heck reaction is well developed as a method for alkylating aromatics. Frank Glorius of the Max-Planck-lnstitute, Mulheim, reports (Tetrahedron Lett. 44 5751,2003) that chloroacetamides and bromoacelonitrile can also be activated by catalytic Pd to give the coupled products. This reaction works well with enol ethers, to give highly functionalized alkenes, but it also works well with a simple cyclic alkene. 

Cyanogen Iodide (ICN) has been used extensively for the cyanation of alkenes and aromatic compounds [12], iodination of aromatic compounds [13], formation of disulfide bonds in peptides [14], conversion of dithioacetals to cyanothioacetals [15], formation of trans-olefins from dialkylvinylboranes [16], lactonization of alkene esters [17], formation of guanidines [18], lactamization [19], formation of a-thioethter nitriles [20], iodocyanation of alkenes [21], conversion of alkynes to alkyl-iodo alkenes [22], cyanation/iodination of P-diketones [23], and formation of alkynyl iodides [24]. The products obtained from the reaction of ICN with MFA in refluxing chloroform were rrans-16-iodo-17-cyanomarcfortine A (14)... 

Alkylation with Alkanes. Alkylation of aromatic hydrocarbons with alkanes, although possible, is more difficult than with other alkylating agents (alkyl halides, alkenes, alcohols, etc.).178 This is due to the unfavorable thermodynamics of the reaction in which hydrogen must be oxidatively removed. 

Alkylation. Friedel-Crafts alkylation (qv) of benzene with ethylene or propylene to produce ethylbenzene [100-41 -4], CgH10, or isopropylbenzene [98-82-8], C9H12 (cumene) is readily accomplished in the liquid or vapor phase with various catalysts such as BF3 (22), aluminum chloride, or supported polyphosphoric acid. The oldest method of alkylation employs the liquid-phase reaction of benzene with anhydrous aluminum chloride and ethylene (23). Ethylbenzene is produced commercially almost entirely for styrene manufacture. Cumene [98-82-8] is catalytically oxidized to cumene hydroperoxide, which is used to manufacture phenol and acetone. Benzene is also alkylated with C1Q—C20 linear alkenes to produce linear alkyl aromatics. Sulfonation of these compounds produces linear alkane sulfonates (LAS) which are used as biodegradable deteigents. 

ALKYLATION OF AROMATIC HYDROCARBONS 5.2.1. Alkylation with Alkenes... 

Many authors note the fact that adsorption of alkenes and alkyl aromatics upon the alumosilicate surfaces leads to a marked increase in their proton acidity (e.g., Fame et al. 1972). Formation of cation radicals and their further deprotonation explain many features of the hydrocarbon catalytic transformations (Vishnetskaya et al. 1997). Scheme 1-44 (see below) represents this phenomenon. 

Aromatic C and double-bonded alkyl C C=C Protonated and alkylated aromatic C Carbonyl substituted aryl C Alkene C 1. V—71 284.9-285.5M-kJ-m -rt-v... 

Ainmoxidation, sometimes also termed oxidative ammonolysis, describes the process of catalytic oxidation of hydrocarbons (particularly alkenes, alkanes, alkyl-aromatics and alkyl-pyridines) to organic nitriles in the presence of ammonia, typically using mixed oxide catalysts ... 

The procedure described here is typical for the catalytic alkylation of aromatic ketones at the ortho position by alkenes. Aromatic ketones are readily available by Friedel-Crafts acylation and many other methods, and many of these ketones are suitable substrates for the present catalytic alkylation with alkenes affording the corresponding ortho-alkylated ketones. The present method provides a direct way to alkylate aromatics with olefins. Moreover, the C-C bond formation takes place with exclusive ortho selectivity, while mixtures of 0-, m-, p-isomers are usually obtained in the conventional Friedel-Crafts alkylation of aromatic compounds. 

Why is the prohibition against secondary alkylzirconium relaxed for phenyl substituents Buchwald et al. suggest that the flat phenyl group is less sterically demanding than an alkyl, while others have proposed an electronic effect favoring nzylic zirconium compounds. Evidence supports the latter for internal aromatic alkenes hydrozirconation initially gives mostly the benzylic isomer (25) (based on the alcohol products of oxidation), which slowly (48-96 h at 40 C) converts to the terminal isomer (26 equation 30). ... 

The alkylation of aromatic rings, called Friedel-Crafts alkylation, is a reaction of very broad scope. The most important reagents are alkyl halides, alkenes, and... 

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