A halogen-metal interconversion

Many organic halides do not react satisfactorily with lithium to form RLi ecMnpounds or with metallic magnesium to form Grignard reagents. The desired organolithium compound can often be prepared by a halogen-metal interconversion reaction ... 

The Gihnan-Swiss test, or Color Test II [151], is specific to alkyllithium reagents. Here, a halogen-metal interconversion in presence of an alkyllithium compound occurs with 4-bromo-iV,iV-dimethylaniline 70. After addition of benzophenone and, a few minutes later, concentrated hydrochloric acid, a bright red color for the triaryhnethyl cation 71 appears (Scheme 26.20). 

Thiophenedithiol (170) has been prepared by halogen-metal interconversion between the lithium salt of 4-bromo-3-thiophenethiol and n-butyllithium at —70°C, followed by reaction with sulfur/ IR, NMR, and UV spectra showed that this compound exists in the dithiol form (170). The compound obtained as a by-product in the... 

Halogen-Metal Interconversions with Halo-geneted Anilines. A highly explosive compound which appears to be p-N,N-Trilithioani-line is formed as a by-product in the reaction of BuLi and p-Br-aniline but not o-Br-aniline (Ref 1). More recently this reaction was repeated but no by-product is mentioned (Ref... 

Halogen-metal interconversion between bromothiophenes and n -butyllithium occurs almost instantaneously and in very high yield. The reaction with 3-bromothiophene is usually carried out at -70 °C to avoid any (rans-lithiation. When bromine-lithium exchange takes place, steric crowding in the molecule decreases. As a consequence the percentage of 2-lithio derivative in relation to the 5-lithio isomer formed from 3-alkyl-2,5-... 

Another important feature of the halogen/metal interconversion is that it proceeds almost exclusively on reactants RX where X is iodine or bromine. In the case of haloarenes, a few examples of chlorine/metal exchange have been reported in very particular cases—strong electron depletion in the arene [31-33] or buttressing effect [34] (vide infra)—but fluorides are unreactive. Iodides are the most reactive toward exchange and are several orders of magnitude faster than bromides. When... 

The halogen-metal interconversion, using organolithium compounds , also presents a problem of classification. One representation of its progress , shown below, makes it in one sense an electrophilic substitution, and with a clear recognition of the ambiguity, we shall for classification purposes regard it as such. 

Halogen/metal permutation and hydrogen/metal permutation (usually apostrophed as "metalation") dominate the interconversion methods. They employ organoalkali reagents such as phenyllithium, methyllithium, -butyllithium, iec-butyllithium, or the superbasic LIC-KOR mixture. However, even if commercial, these reagents have to be made beforehand. The reaction of a chloro- or bromo-substituted hydrocarbon with lithium, sodium, or magnesium offers a standard entry to them. Thus, ultimately one always has to revert to the metal. 

The use of lithionaphthalene and lithio 8-methylnaphthalene as functionally substituted anionic initiators is very disappointing (21, 22). The deactivation of living polyanions onto a-bromonaphthalene or bromo-8-methylnaphthalene is another unsatisfactory approach due to metal halogen interconversion (eqs 9, 10) (23). 

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