Cyclohexadienyl anions

Cyclohexadienyl radical Sodium Cyclohexadienyl anion Sodium ion... 

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 rate determining intermediate is a cyclohexadienyl anion and is sta bilized by electron withdrawing substituents... 

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... 

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

Biphenylbis(tricarbonyt)chromium (l).1 Lithium anthracenide converts biphenyl complexed by two Cr(CO), groups into a fairly stable dianion of structure 2, in which two V-cyclohexadienyl anions are coordinated with Cr(CO)3. This dianion is alkylated by an alkyl halide to form 3, in which one benzene ring is... 

The first (reversible) electron transfer generates the ketyl anion-radical. The ketyl moiety then attacks the aryl group in the ortho position. The resulting cyclohexadienyl radical is reduced to a cyclohexadienyl anion by a second electron transfer, and the anion is finally protonated. HMPA as... 

Fluorine stabilizes cyclohexadienyl anion by withdrawing electrons. Chlorine is less electronegative than fluorine and does not stabilize cyclohexadienyl anion to as great an extent. 

Diethyl sodiomalonate is an example of type (i). Reaction with [(fluorobenzene)Cr(CO)3] proceeds to completion after 20 h at 50 C in HMPA to give the diethyl phenylmalonate complex in over 95% yield. Monitoring the reaction by NMR gave no evidence for an intermediate (e.g. the cyclohexadienyl anion complex) interruption of the reaction by addition of iodine at less than 20 h gave significant amounts of unreacted fluorobenzene. A satisfactory picture is the simple one, that the anion adds reversibly and unfavorably (k < k-i, as in Scheme 3), slowly finding itself at the ipso position then irreversible loss of fluoride gives the substitution product.5152... 

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). 

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