Resonance cyclohexadienyl anions

The negative charge of the cyclohexadienyl anion 5 is delocalized over several carbon centers, as is illustrated by the following resonance structures ... 

The most stable resonance structure for the cyclohexadienyl anion formed by reaction of methox-ide ion with o-fluoron i t n >ben/ene involves the nitro group and has the negative charge on oxygen. 

Thus, Hine (1966a) used PLNM successfully to rationalise the sites of attack on conjugated reactive intermediates (cations, radicals and anions). The data is puzzling since the thermodynamically less stable non-conjugated isomers predominate protonation of the cyclohexadienyl anion, for example, yields predominantly cyclohexa-1,4-diene. The PLNM rationalisation of this result is set out in Scheme 14 in terms of the resonance structures of the pentadienyl anion fragment. 

Show the intermediates, including all the resonance hybrid structures for the cyclohexadienyl anion, that are formed in the following aromatic nucleophilic substifution reactions ... 

The mechanism is similar to the addition-elimination mechanism of acyl derivatives in which a tetrahedral intermediate forms. The addition step is a nucleophilic attack that yields a resonance-stabilized cyclohexadienyl anion. The anion can then eject the nucleophile or a leaving group in the elimination step. 

FIGURE 13.68 The radical anion is protonated to give a resonance-stabilized cyclohexadienyl radical that goes on to produce the 1,4-cyclohexadiene through another reduction- rotonation sequence (Et = CH2CH3). 

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