Nucleophilic Aromatic Substitution by the Elimination-Addition Mechanism

Nucleophilic Aromatic Substitution by the Addition-Elimination Mechanism 

The addition intermediate is isoelectronic with a pentadienyl anion. 

The role of the leaving group in determining the reaction rate is somewhat different from Sjv2 and Sjyl substitution at alkyl groups. In those cases, the bond strength is 

Reviews C. F. Bernasconi, in MTP Int. Rev. Sci., Organic Series One, Vol. 3, H. Zollinger, ed., Butterworths, London, 1973 J. A. Zoltewicz, Top. Curr. Chem., 59, 33 (1975) J. Miller, Aromatic Nucleophilic Substitution, Elsevier, Amsterdam, 1968. 

The addition intermediates, which are known as Meisenheimer complexes, can often be detected spectroscopically and can sometimes be isolated. Especially in the case of adducts stabilized by nitro groups, the intermediates are often strongly colored. 

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Other aryl halides that give stabilized anions can undergo nucleophilic aromatic substitution by the addition-elimination mechanism Two exam pies are hexafluorobenzene and 2 chloropyridme... 

Cycloalkene (Section 5 1) A cyclic hydrocarbon characterized by a double bond between two of the nng carbons Cycloalkyne (Section 9 4) A cyclic hydrocarbon characterized by a tnple bond between two of the nng carbons Cyclohexadienyl anion (Section 23 6) The key intermediate in nucleophilic aromatic substitution by the addition-elimination mechanism It is represented by the general structure shown where Y is the nucleophile and X is the leaving group... 

SECTION 10.5. NUCLEOPHILIC AROMATIC SUBSTITUTION BY THE ADDITION ELIMINATION MECHANISM... 

Cyclohexadienyl anion (Section 23.6) The key intermediate in nucleophilic aromatic substitution by the addition-elimination mechanism. It is represented by the general structure shown, where Y is the nucleophile and X is the leaving group. 

The aryl halide must be one that is reactive toward nucleophilic aromatic substitution by the addition-elimination mechanism. p-Fluoronitrobenzene is far more reactive than fluorobenzene. The reaction shown yields p-nitrophenyl phenyl ether in 92% yield. 

The reason this reaction is suitable is that it involves nucleophilic aromatic substitution by the addition-elimination mechanism on a p-nitro-substituted aryl halide. Indeed, this reaction has been carried out and gives an 80-82% yield. A reasonable synthesis would therefore begin with the preparation of p -ch I oron itrobenzene. 

Large positive p values usually indicate extra electrons in the transition state delocalized into the ring itself. A classic example is nucleophilic aromatic substitution by the addition-elimination mechanism (Chapter 23). The p value is +4.9, but even this large value does not mean a complete anion on the benzene ring as the nitro group, present in all cases, takes most of the negative charge. The substituent X merely helps. 

This equation also describes the overall reaction of either an 5 2 or a nucleophilic aromatic substitution process. In some cases, the only way to distinguish an reaction from these processes is that an is inhibited by radical inhibitors. Another distinguishing feature is that the order of the relative leaving group abilities of halides are opposite that found for nucleophilic aromatic substitution by the addition-elimination mechanism (see Chapter 3). 

The presence of strong electron-withdrawing groups such as NO2 ortho or para to a potential leaving group favors nucleophilic aromatic substitution by the addition-elimination mechanism. Without such groups in these positions, the benzyne mechanism is favored. 

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