Aryl carbanions

Rearrangement of polyhalobenzenes can also be catalyzed by very strong bases for example, 1,2,4-tribromobenzene is converted to 1,3,5-tribromobenzene by treatment with PhNHK." This reaction, which involves aryl carbanion intermediates (Sgl mechanism), has been called the halogen dance. 

It is intriguing to note that this reaction scheme for the reduction of a sulphone to a sulphide leads to the same reaction stoichiometry as proposed originally by Bordwell in 1951. Which of the three reaction pathways predominates will depend on the relative activation barriers for each process in any given molecule. All are known. Process (1) is preferred in somewhat strained cyclic sulphones (equations 22 and 24), process (2) occurs in the strained naphtho[l, 8-hc]thiete 1,1-dioxide, 2, cleavage of which leads to a reasonably stabilized aryl carbanion (equation 29) and process (3) occurs in unstrained sulphones, as outlined in equations (26) to (28). Examples of other nucleophiles attacking strained sulphones are in fact known. For instance, the very strained sulphone, 2, is cleaved by hydride from LAH, by methyllithium in ether at 20°, by sodium hydroxide in refluxing aqueous dioxane, and by lithium anilide in ether/THF at room temperature. In each case, the product resulted from a nucleophilic attack at the sulphonyl sulphur atom. Other examples of this process include the attack of hydroxide ion on highly strained thiirene S, S-dioxides , and an attack on norbornadienyl sulphone by methyllithium in ice-cold THF . ... 

This involves an aryl carbanion/enolate anion (64), and also eCQ3 derived from the action of strong bases on HCC13 (p. 267), though the latter has only a transient existence decomposing to CC12, a highly electron-deficient electrophile that attacks the aromatic nucleus ... 

Halogen exchange between an aryl carbanion and a haloarene is well established and occurs by nucleophilic substitution on the halogen substituent [154]. This pro-... 

This reaction, which involves aryl carbanion intermediates (SeI mechanism), has been called the halogen danced1... 

In the presence of an electron rich donor molecule an alternative to direct fission or reaction via an excimer is the formation of an exciplex or radical anion/radical cation ion pair (Eq. 2). The radical anion has been viewed as the key intermediate which undergoes fission to aryl radical and halide ion (Eq. 3). With polyhaloarenes there is an additional option. A polychloroarene radical anion, for example, has two possible modes for bond fission (a) fission to produce aryl radical and chloride ion or (b) fission to form an aryl carbanion and chlorine atom (Scheme 6). The options for fragmentation of a haloarene radical anion... 

The study of the photochemistry of aryl carbanions has been restricted to aryllithiums with only a limited number of studies available. Hence, a general picture of their photochemistry is not available at this time. Photolysis of phenyllithium in the presence of aromatic hydrocarbons such as naphthalene, biphenyl, phenylene, etc. in diethyl ether results in electron transfer from the phenyllithium to the aromatic hydrocarbon, with production of the corresponding hydrocarbon radical anion, as observed by ESR spectroscopy [6-8] (Eq. 1). Photolysis of phenyllithium or 2-naphthyllithium alone gave the corresponding biaryl products and metallic lithium [9-10]. For this reaction, it is possible to write a mechanism which does not require electron transfer from the anion [9,10],... 

Reduction of an aryl halide at a cadmium-modified nickel cathode in DMF containing TBABF4 leads to a formylation reaction between aryl carbanions and the solvent [186]. Two papers [187,188] have appeared in which reduction of aryl halides gives an aryl carbanion, which, by acting as a base to deprotonate a suitable nitrile, can cause coupling of the nitrile with esters, aldehydes, and ketones. Electrochemical trimethylsilylation of aryl halides can be effected at a stainless-steel or carbon-cloth cathode in THF-HMPA containing TEABF4 and trimethylchlorosilane [189]. 

As was the case for the alkynic carbanions, the crystalline aryl carbanions also offer the opportunity for observation and comparison of several different structural types for the same or closely related carb-... 

Halogen dance The rearrangement of polyhalogenobenzenes catalysed by very strong bases, which proceeds via an aryl carbanion intermediate (i.e. an SE1 mechanism). 

Preparation of Aryl Hydrazines. All methods mentioned above for the hydrazination of alkyl carbanions may also be applied to aryl carbanions. Addition of phenyllithium to a cyclic azo compound followed by in situ arylation to give a tetrasubstituted hydrazine was mentioned earlier (Eq. 19). An alternate hydrazination method, not involving aryl anions, is the reaction of electron-rich arenes with azodicarboxylic esters and aroylazocarboxylic esters under the influence of various catalysts.230,377-384... 

Preparation of Aryl Azides. Aryl azides may be prepared by reaction of aryl carbanions with tosyl azide followed by treatment of the triazene salt with sodium pyrophosphate (Eq. 67)305 or aqueous base (Eq. 72).315... 

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