Organometallics metal-halogen exchange

Reaction of 2-bromobenzocyclopropene(202) with BuLi leads to metal-halogen exchange and affords the organometallic derivative 297, which has been converted to 298. In turn, 1,1-difluorobenzocyclopropene (21) is deprotonated with BuLi at C2,5, and the resulting lithium derivative 299 may be intercepted with electrophiles such as Ij, COj, McjSiCl, and (PhS)2 to afford 2-mono or 2,5-disubstituted derivatives 300, 301. ... 

Thus, from the series of reactions presented In this paper it appears that a major reaction pathway for chloror-cyclophosphazenes with certain organometallic reagents, involves a metal-halogen exchange process. Subsequent chemistry however appears to be extremely dependent on the metal involved In the organometallic reagent. 

Si-M bond. As generally found in organometallic chemistry, the alkali salt method may be unsatisfactory in certain cases because of side reactions involving alkali metal-halogen exchange. 

Lithiation Reactions. One of the earliest reactions of this type made use of metal-halogen exchange reactions carried out on poly[bis(p-bromophenoxy)phosphazene]. Polyphosphazenes that bear p-bromophenoxy side oups are normally unreactive. However, they can be lithiated, as shown in Scheme III, and the lithio derivatives react with a wide variety of electrophiles that range from chlorophosphines (19) to organometallic halides (42-45), This provides an access route to polymer-bound transition metal catalysts and other metallated or silylated polymers. 

Direct deprotonation of quinolines or isoquinolines can only be succeeded via directed ort/io-metalation. Metal-halogen exchange can be used to produce organometallic nucleophiles from the halides located either on the benzene or the pyridine ring. ... 

Among the extensive studies for the preparation of heteroaryl compounds, organometallic reagents are frequently used and mainly prepared from metal-halogen exchange reactions or the metathesis of the corresponding organolithium... 

Bercier et al. reported the domino reactions of iodofuranoses and various organometallic reagents [55]. The sequence involves an initial ring-opening by metal-halogen exchange and p-elimination, followed by in situ nucleophilic addition to the intermediate aldehyde. Thus, upon treatment with 3.5 equivalents of alkyl lithium in THF at 0°C, iodofuranose 62 was converted into diols 63 in excellent yields (Scheme 3.20). 

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