Bromine-zinc exchange reactions

The above-mentioned method is applied to the selective carbon-carbon bond formation between sp carbons [150]. A bromine-zinc exchange reaction of the gcm-dibromo-cyclopropane 96 selectively occurs at the position cis to the phenyl group by treatment with Me4ZnLi2 [151], The reaction of the thus-obtained zincate 97 with VO(OEt)Cl2 leads to the stereoselective formation of l-bromo-l-methyl-2-phenylcyclopropane (98). On the other hand, when the reaction mixture is wanned up to 0 °C, followed by treatment with VO(OEt)Cl2, dimelhylation at the em-positions takes place to give the dimethylcyclopropane (100) via the organozinc 99 as shown in Scheme 2.79)... 

Harada T, Katsuhira T, Oku A (1992) Stereochemistry in carbenoid formation by bromine/ lithium and bromine/zinc exchange reactions of 1,1-dibromoalkenes higher reactivity of the sterically more hindered bromine atom. J Org Chem 57 5805-5807. doi 10.1021/jo00048a002... 

Numerous modifications of the direct zinc insertion procedure can be found in the hterature. For example, simple diaUcylzincs can be used as reagents instead of metallic Zn, bnt in this case the reaction is accelerated by catalytic qnantities of zinc salts or transition metal see Transition Metals) salts. Whereas the Cu -catalyzed iodine-zinc exchange reaction provides diorganozincs, the Pd, Mn and nF catalyzed iodine- or bromine-zinc exchange leads to organozinc halides. 

The bromine-metal exchange in a-bromo carbonyl compounds is a common method for the formation of zinc enolates - a procedure whereupon the well-known Reformatsky reaction is based [159]. The question whether zinc enolates feature a carbon-zinc or an oxygen-zinc bond seems to depend on the individual... 

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