Alkenes Alkylating agents

In eontrast, dialkylhalonium salts sueh as dimethylbromonium and dimethyliodonium fluoroantimonate, whieh we prepared from excess alkyl halides with antimony pentafluoride or fluoroantimonie acid and isolated as stable salts (the less-stable chloronium salts were obtained only in solution), are very effective alkylating agents for heteroatom eompounds (Nu = R2O, R2S, R3N, R3P, ete.) and for C-alkylation (arenes, alkenes). 

All lation of Phenols. The approach used to synthesize commercially available alkylphenols is Friedel-Crafts alkylation. The specific procedure typically uses an alkene as the alkylating agent and an acid catalyst, generally a sulfonic acid. Alkene and catalyst interact to form a carbocation and counter ion (5) which interacts with phenol to form a 7T complex (6). This complex is held together by the overlap of the filled TT-orbital of the aromatic... 

Alkenes are especially good alkylating agents. With respect to them the reaction is addition of ArH to a C=C double bond ... 

Apart from the alkyl halide-Lewis acid combination, two other sources of carbo-cations are often used in Friedel-Crafts reactions. Alcohols can serve as carbocation precursors in strong acids such as sulfuric or phosphoric acid. Alkylation can also be effected by alcohols in combination with BF3 or A1C13.37 Alkenes can serve as alkylating agents when a protic acid, especially H2S04, H3P04, and HF, or a Lewis acid, such as BF3 and A1C13, is used as a catalyst.38... 

SRN I reactions using related p-nitrophenyl or p-nitrocumyl systems41 as reductive alkylating agents have been studied by Komblum and co-workers these are well summarized in the reviews.39 At the same time, Russell discovered the S l reaction of geminal halonitroalkanes with stabilized carbanions (see Eq. 5.25).42 The products are readily converted into alkenes via elimination of nitro groups (see Section 7.3). 

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. 

The side chain alkylation of alkyl benzenes is usually performed with alkenes as alkylating agents in the presence of strong-acid catalysts (256,257). The use of highly basic catalysts such as alkali metals, their hydrides, and sodium and potassium complexes for alkylation of alkylaromatic hydrocarbons has also been reported (256,257). The reaction mechanism proposed by Pines et al. (258) involves the addition of a benzylic carboanion to the alkene (Scheme 41). 

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

Alcohols and alkenes are also useful alkylating agents. Some classify the latter reactions as Darzens-Nenitzescu Alkylation The Reaction ... 

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. 

Alkylation with Alkenes. Besides alkyl halides, alkenes,86,148-151 alkynes,152 alcohols and ethers,86,151,153 and esters86,151,154 are most frequently used for alkylation of arenes. Of all alkylating agents, alkenes are practically the most important. 

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 is a very broad reaction type and it can, depending on the nature of the alkylating agent, proceed either as a substitution or as an addition reaction. The alkylation by substitution of, for example, aromatic hydrocarbons, phenols or amines is based on the reaction with alkyl halides or alcohols. Some evidence indicates that, at least partly, the alkylation proceeds through the intermediate formation of alkenes from the alkylating agent when the reaction is conducted at atmospheric pressure and at high temperature. 

Not only benzene and its alkyl derivatives can be used as the aromatic component but also naphthalene [341], phenol [340,341] and thiophene [341,342]. Low-molecular weight alkenes, Cj to C5, cyclohexene and dodecene have served as alkylating agents. 

Support-bound alkylating agents have been used to N-alkylate pyridines and dihydropyridines (Entries 7 and 8, Table 15.21). Similarly, resin-bound pyridines can be N-alkylated by treatment with a-halo ketones (DMF, 45 °C, 1 h [267]) or other alkylating agents [246]. Polystyrene-bound l-[(alkoxycarbonyl)methyl]pyridinium salts can be prepared by N-alkylating pyridine with immobilized haloacetates (Entry 8, Table 15.21). These pyridinium salts react with acceptor-substituted alkenes to yield cyclopropanes (Section 5.1.3.6). Pyridinium salts have also been prepared by reaction of resin-bound primary amines with /V-(2,4-dinitrophenyl)pyridinium salts [268,269]. 

Geminal dihaloalkanes are very commonly used reagents for the addition of alkylidene groups to alkenes. The simpler dihalides, especially the dihalomethanes, are readily available and are certainly much less hazardous to use than the corresponding diazoalkanes. Nevertheless, any reactive alkylating agent should be regarded as a toxic substance. 

In addition to alkenes, functionalized alkenes can also be used as alkylating agents. Koltunov, Walspurger, and Sommer, have reported the alkylation of benzene with a,/3-unsaturated carboxamides in the presence of excess aluminum chloride [Eq. (5.72)]. The reaction takes place under mild conditions and gives the products in near-quantitative yields. Results with ortlro-dichlorobenzene and triflic acid are usually inferior. Triflic acid, however, can catalyze similar reactions of cyclic and open-chain unsaturated amines with benzene to give phenylalkylamines in excellent yields.180 The transformations are interpreted by invoking the involvement of dica-tionic intermediates 39 and 40. 

When olefins are used as alkylating agents, the catalytic activity of Nafion-H slowly decreases, most probably due to some polymerization on the surface, which deactivates the catalytic sites. The activity decreases faster when more reactive branched alkenes are used. The use of alcohols instead of olefins as the alkylating agents improves the lifetime of the catalyst. With alcohols, no ready polymerization takes place, since water formed as byproduct inhibits polymerization of any olefin formed (by dehydration) but does not affect the acidity of the catalyst at the reaction temperatures. 

Other catalysts which may be used in the Friedel-Crafts alkylation reaction include iron(m) chloride, antimony pentachloride, zirconium tetrachloride, boron trifluoride, zinc chloride and hydrogen fluoride but these are generally not so effective in small-scale preparations. The alkylating agents include alkyl halides, alcohols and alkenes. 

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