Nuclear Alkylation of Aromatics

Acid-catalyzed reactions of aromatics with monoolefins result in nuclear alkylation. But the base-catalyzed reactions of aromatics with olefins do not result in nuclear alkylation as long as benzylic hydrogens are available. This is true even with aromatics, such as cumene, which have deactivated benzylic hydrogens resulting in facile metalation of the ring. Apparently phenyl carbanions do not readily add to olefins. Pines and Mark (20) found that in the presence of sodium and promoters only small yields of alkylate were produced at 300° in reactions of benzene with ethylene and isobutylene and of t-butylbenzene with ethylene. With potassium, larger yields may be obtained at 190° (24)- 

The nuclear-alkylation reaction of aromatics is accompanied by formation of biphenyls (20, 24). They may form by an addition of phenyl carb-anion to the aromatic ring, followed by an elimination of a hydride ion [Reaction (28)]. 

In a study of reaction of phenylsodium with benzene, Morton and Lanpher (4S) found that at lower temperatures the adduct was stabilized by trans-metalation with another molecule of phenylsodium, yielding a dianion [Reaction (29)]. 

Two anionic mechanisms may be proposed for base-catalyzed nuclear alkylation. One, which is analogous to side-chain alkylation, is Reaction (30a, b). 

The other, which is similar to the biphenyl-forming reaction, may be initiated by hydride ions produced by biphenyl formation [Reaction (31a, b, c)]. 

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