Halogen-lithium exchange reactions functionalized compounds

Dianions 126, alkenyllithium compounds of type VIII, have been obtained by halogen-lithium exchange from the corresponding chlorinated or brominated precursors 125 using lithium naphthalenide it was necessary to carry out a previous deprotonation with PhLi. The reaction of 126 with electrophiles gave regio-selective functionalized allyl amines 127 (Scheme 2.17) [106]. 

The only way to introduce two different electrophilic fragments in compounds such as 508 is to have a starting material with different halogens. This is the case with 510, which could be lithiated (bromide-lithium exchange) with t-butyllithium in THF at — 100°C giving intermediates 511, which reacted with a carbonyl compound R R CO and, after naphthalene-catalyzed lithiation, gave the new functionalized organolithium intermediate 512. Final reaction with 3-pentanone followed by hydrolysis yielded mixed products 513 (Scheme 142) °. 

Metal-halogen exchange is a rapid reaction and is usually carried out at -60 to -120°C. This makes it possible to prepare aryllithium compounds containing functional groups, such as cyano and nitro, which would react under the conditions required for preparation from lithium metal. Entries 5 and 6 in Scheme 7.2 are examples. 

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