Halogen/metal interconversion reagents

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 ... 

Several 2-lithio-3-bromofurans (206), prepared either by halogen-metal interconversion from the 2,3-dibromofurans (207) or by metallation of the 3-bromofurans (208), show typical properties of aryllithium reagents and give no evidence for the intermediacy of 2,3-didehydrofuran (180). 

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). 

An important feature of the halogen/metal interconversion is that it is an equilibrium-controlled process, which is therefore shifted toward the most stable organolithium or organomagnesium reagent or, to put it in other words, toward the weakest base [18-20],... 

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. 

Metal-halogen interconversions are primarily used to prepare organolithium compounds from alkyl and aryl hahdes. Particular Grignard reagents can be prepared from this methodology. They often give better yields than reactions between halides and magnesium... 

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