Cyclohexadienyl anion, intermediate

A nitro group is a strongly activating substituent in nucleophilic aromatic substitution where it stabilizes the key cyclohexadienyl anion intermediate... 

Cyclohexadienyl anion intermediate nitro group IS stabilizing... 

Here it is the combined electron attrachng effects of the six fluorine substituents that stabi hze the cyclohexadienyl anion intermediate and permit the reaction to proceed so readily... 

The processes depend on the formation of the cyclohexadienyl anion intermediates in a favorable equilibrium (carbon nucleophiles from carbon acids with pKt > 22 or so), protonation (which can occur at low temperature with even weak acids, such as acetic acid) and hydrogen shifts in the proposed diene-chromium intermediates (25) and (26). Hydrogen shifts lead to an isomer (26), which allows elimination of HX and regeneration of an arene-chromium complex (27), now with the carbanion unit indirectly substituted for X (Scheme 9). 

The efficient trapping of the cyclohexadienyl anionic intermediates with protons raises the possibility of quenching with carbon electrophiles. The process is not as general as the proton quench early expert-... 

Nucleophilic addition occurs in the rate-determining step at one of the six equivalent carbons of hexafluorobenzene to give the cyclohexadienyl anion intermediate. 

Elimination of fluoride ion from the cyclohexadienyl anion intermediate restores the aromaticity of the ring and completes the reaction. 

Nucleophilic addition of hydroxide ion gives a cyclohexadienyl anion intermediate. 

This pathway is favorable because the cyclohexadienyl anion intermediate formed in the ratedetermining step is stabilized by the electron-withdrawing inductive effect of its fluorine substituents. 

The efficient trapping of the cyclohexadienyl anionic intermediates with protons raises the possibility of qnenching with carbon electrophiles. The process is not as general as the proton quench. However, when the nucleophile adds essentially irreversibly, quenching with a limited set of carbon electrophiles is successful. For example, addition of 2-lithio-l,3-dithiane to benzene-Cr(CO)2T, followed by addition of ethyl iodide and then oxidation or addition of a donor ligand (CO, PhsP), produces a cyclohexa-l,3-diene substituted by both acetyl (Me + CO) and the nucleophile (Scheme 47).134,209 insertion of CO occurs, without... 

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