Electrophilic substitution phenyllithium

In contrast to benzene compounds, which undergo ready electrophilic substitution, the highly n-deficient pyrazine and quinoxaline nuclei are susceptible towards nucleophilic attack by a variety of reagents. The direct nucleophilic substitution with organolithium or Grignard reagents to form alkyl or aryl heteroaromatics, however, is rather ineffective, and the dihydro and/or tetrahydro compounds are formed preferentially. For example, reaction of pyrazine with phenyllithium affords... 

Through a significant rc-backbonding interaction, the coordination of benzene to [Os] (1) serves both to activate the arene toward the electrophilic addition of dimethoxymethane (Table 1, entries 1-4) or 3-penten-2-one (entry 5) and to stabilize the resulting benzenium intermediate 8. If manipulated at low temperature (—40 °C), 8 can be trapped with either a silyl ketene acetal (entries 1, 4, and 5), 2-trimethylsiloxypropene (entry 2), or phenyllithium (entry 3) to yield the substituted 1,4-cyclohexadiene complex 9 [15]. This species can be oxi-... 

As described in Section II.A, the tandem sequence reaction of organolithium compounds with carbon monoxide followed by reaction with suitable electrophiles provides an useful tool for the preparation of diphenyldialkyl carbinols, and the reaction could be easily extended to produce substituted cyclic ethers in a one-pot synthesis. Thus, by carrying out the carbonylation of phenyllithium in the presence of conveniently substituted chloroalkyl bromides, Br(CH2)3+ Cl, at -78 °C, the oxo-lithiated intermediates 243 are obtained and cyclized to 244 by warming up the reaction mixture (Scheme 74)21. 

The high electrophilicity of the 4-position in quinazoline makes this position very reactive towards nucleophilic additions such as in alkylation by acetophenone, acetone, 2-butanone, or cyclohexanone, which all have been reported to add across the 3,4-double bond (283) (Scheme 45). Alkyl-and arylmagnesium halides and phenyllithium add similarly to give the corresponding 4-substituted 3,4-dihydroquinazolines (284) . 

The equilibrium between a-silyl alkoxides and silojq carbanions can be shifted toward the carbanion side by introduction of a conjugating group into either or both the acylsilane and the nucleophile. In 1980, Reich et al. reported that treatment of all l-substituted acylsilanes with vinyllithium followed by a variety of electrophiles affords a-substituted enol silyl ethers 22 via a siloxy allyllithium intermediate 21 fScheme 8.1Similar reactions using phenyllithium give products in which electrophilic quenching occurred at the benzylic position. 

In this context, one may also pay attention to the so-called "betaine-ylides" that act as key intermediates in stereocontrolled Wittig olefination reactions. They are generated from the ordinary adducts obtained by the combination of a phosphine ylide and an aldehyde in the presence of lithium bromide (or another soluble lithium salt). When the P-betaines are a-deprotonated with phenyllithium, the stereocenter at the phosphorus-adjacent carbon atom becomes configurationally mobile. In this way, erythro/threo mixtures can spontaneously convert into virtually pure /Areo-betaine ylides (p-lithiooxy ylides, P-oxido ylides). Reprotonation and subsequent elimination of triphenylphosphine oxide affords trans olefins, whereas a-substitution by electrophiles other than acids leads to branched alkenes exhibiting a well-defined stereochemistry "("three-dimensional"" Wittig reaction or SCOOPY method). ... 

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