Carbanions aromatic

Arenes cannot usually be deprotonated with LDA alone, but require mixtures of organosodium [365] or organolithium compounds and tertiary amines [181, 218, 219]. These amines, for instance TMEDA, lead to a partial dissociation of oligomeric BuLi-solvent aggregates and thereby to more powerful metalating reagents [366, 367]. Thus, although benzene cannot be deprotonated with BuLi alone, a mixture of BuLi and TMEDA leads to quantitative lithiation [181]. 

The acidity of arenes does not correlate with their electron density [374], because in a metalated arene there is no significant overlap between the aromatic it system and the Ar-M bond. More important for facile aromatic metalation is the presence of substituents able to form bonds with the metal and thereby direct the base into the proximity of an ortho Ar-H bond before metalation, and to form a stabilizing 

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If the unshared pair of electrons is involved in ring current and system becomes aromatic, carbanions become greatly stabilized. 

The ability to trap alkyl radicals during the alkylation step is suggestive of a strong balance between electron transfer and substitution reaction. Historically, naphthalene anion, in fact, has been used explicitly to generate alkyl radicals from alkyl halides (16). The presence of alkyl radicals in the alkylation of coal can be expected to complicate interpretation of reaction pathways. The observation of alkylated but unreduced aromatic products led Stock to postulate the presence of alkyl radicals during alkylation (13), although aromatic carbanions could provide similar products through nonradical pathways (1). 

When the lithium cation is unable to associate with the carbanion, as is the case for the Li (12-crown-4) complexed lithium diphenylmethane carbanion (56) or Li+ (12-crown-4) triphenylmethyl carbanion (57), the entire aromatic carbanions are relatively planar. The planarity of (56) and (57) is indicative of... 

A mechanism to explain exclusive ortAo-lithiation was first put forward by Roberts and Curtin In this mechanism, the lithiating R—Li reagent complexes first with groups like OCHj and then abstracts the ortho hydrogen, through its basic end, to generate the aromatic carbanion. A lithium atom is then introduced at this place. 

The attack being by the complexed RLi reagent, the aromatic carbanion is formed only at the sterically close position, which is ortho in the above case. 

These are systems that form aromatic carbanions (4n + 2n electrons). Such reactions are readier if HMPA is present . Indene in THF, e.g., is metallated rapidly at RT by diethylmagnesium if sufficient HMPA is added two HMPA s per dialkylmagnesium are required to maximize the effect. ... 

There are additional methods for generating carbanionic centers that are not classified in previous sections. Two of the more useful are treatment of organosilanes with fluoride and formation of resonance stabilized aromatic carbanions. These two methods will be presented in this section, with an emphasis on the more synthetically useful versions of each reaction. 

A one-pot coupling protocol of arynes with ortho-heteroatom-substituted benzoates to prepare xanthones, thioxanthones, and acridones has been reported by Zhao and Larock (Equation 12.30) [50]. The coupUng presumably proceeds by a tandem intermolecular electrophihc coupling of the aryne with the orthoheteroatom-substituted benzoates and subsequent intramolecular electrophilic cyclization. Proton abstraction by the aromatic carbanion intermediate is a major competing reaction in this cyclization. This side reaction is, however, largely suppressed by employing tetrahydrofuran (THF) as the solvent... 

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