By metal-halogen exchange

Most reactions have been carried out in diethyl ether or THF, but the use of dichloromethane has been advocated [33], 

Representative procedures follow, and other examples are listed in Table 3.7. 

An oi-fluorosulfonylmagnesium bromide (perftuoro-5-fluorosulfonyl-3-oxapentylmagnesium bromide) [34] 

As well as illustrating the preparation of a heterocyclic Grignard reagent by metal-halogen exchange, the following procedure is an example of the use of dichloromethane as the solvent. 

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Table 2. Perfluoroalkylmagnesium Halides (RfMgX) Prepared by Metal-Halogen Exchange...

Protection of primary aryl amines as the triazene is accomplished by diazotiza-tion of the amine followed by reaction with pyrrolidine in aq. KOH. This group is stable to metalation of the aromatic ring by metal halogen exchange. The amine is recovered by reductive cleavage with Ni-Al alloy (aq. KOH, rt, 37-68% yield). ... 

Reactivity and yields are greatly enhanced by the presence of 0.5-1% Na in the Li. The reaction is also generally available for the preparation of metal alkyls of the heavier Group 1 metals. Lithium aryls are best prepared by metal-halogen exchange using LiBu" and an aryl iodide, and transmetalation is the most convenient route to vinyl, allyl and other unsaturated derivatives ... 

Helquist and coworkers60 have developed a six-membered ring annulation via a conjugate addition of aryllithium generated by metal-halogen exchange and subsequent intramolecular alkylation. This is illustrated in equation 71. 

Retention of configuration is often observed when organolithium compounds are prepared by metal-halogen exchange. The degree of retention is low for exchange of most... 

Lithiation of 2-bromo-3,3-disubstituted-methylenecyclopropanes by metal halogen exchange reaction with EtLi in ether, followed by alkylation with epoxides, gave selectively ring-alkylated /8-alcohols, derived from attack at the epoxide primary carbon (equation 299). When R1 R2 a mixture of isomers is obtained369. 

Species IV were prepared from -bromophenoxy-substituted phosphazene trimers and high polymers, by metal-halogen exchange to yield the -lithio-derivative, followed by reaction with diphenylchlorophosphine. Both cyclic trimers and high polymers containing the pendent phosphine reacted with H2Os(CO)3q,... 

Arylsilanes are normally prepared by quenching an aryl-metal system with a chlorosilane. The aryl-metal component is formed either by deprotonation of an activated site or by metal-halogen exchange (Equations Si5.26-29). 

N. Sotomayor, E. Lete, Aryl and Heteroaryllithium Compounds by Metal-Halogen Exchange. Synthesis of Carbocyclic and Heterocyclic Systems, Curr. Org. Chem. 2003, 7, 275-300. 

The formation of a-haloenolates by metal-halogen exchange between isopropylmagnesium chloride and a,a-dichlorocarbonyl compounds such as XCl2CCOOR, where X = Cl, COR or COOR [30] may be noted. 

Primary alkyl bromides react well in this sequence except for particularly reactive compounds (e.g., methyl bromide, allyl bromide) which give the vinyl halide by metal-halogen exchange. Secondary halides, as expected, suffer from elimination as a side reaction. Other electrophiles have been used successfully including D20, aldehydes and ketones, di-methylformamide,47 chlorotrimethylsilane,4 8 1,2-dibromoethane,4 and... 

Trifluoromethyl-lithium, prepared by metal/halogen exchange, is unstable its decomposition probably involves generation of difluorocarbene, which dimerises [54] (Figure 6.38). 

Aryllithium reagents are formed by metal-halogen exchange with aryl hahdes or H-metal exchange with various aromatic compounds, and they react with alkyl hahdes. The reaction of 138 with u-butyllithium, for example, generated the... 

A single diastereoisomer was also obtained by Ireland in the same context but using a chiral lithium derivative obtained from 216 by metal-halogen exchange [155]. However, the presence of magnesium bromide is needed to achieve up to 20 1 diastereoselectivity. 

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