Side-Chain Alkylation of Arylalkanes

The acid-catalyzed reaction of aromatics with monoolefins results exclusively in addition of alkyl groups to the aromatic ring. In contrast, the base-catalyzed reaction of aromatics with monoolefins results in alkylation 

Catalysts and reaction conditions used are generally similar to those used for olefin isomerization. Catalysts reported are sodium-organosodium catalysts prepared in situ by reaction of a promoter such as o-chloro-toluene or anthracene with sodium 19-24), alkali metal hydrides 20,21), alkali metals 22), benzylsodium 26), and potassium-graphite 26). These catalysts are strong bases that can react with alkylaromatics to replace a benzylic hydrogen [Reaction (2)]. 

Sodium is the metal which has been studied most for base-catalyzed alkylations. The use of it results in the most selective reactions. 

Of the olefins, ethylene has been most extensively studied (19, 21, 23-26, 36) it reacts most readily in base-catalyzed alkylations. In general temperatures of 150-200 are used with relatively low ethylene pressures (0-70 atm.). Benzylic hydrogens are replaced by ethyl groups i.e., toluene yields n-propylbenzene. Additional substitution on the a-carbon may yield 3-phenylpentane and 3-ethyl-3-phenylpentane [Reaction (3)]. 

Thus ethyl groups may be added to a-carbons as long as benzylic hydrogens are available for replacement. The mechanism which has been proposed by Pines et al. 19) for the reaction consists of the addition of the benzylic carbanion formed by reaction of the aromatic and the catalyst with the olefin followed by a transmetalation reaction with more of the aromatic [Reaction (4)]. 

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