Zinc Bromide organozinc reagents

Organozinc reagent prepared from the 2-(trimethylsilyl)allyl bromide 156 and zinc react with alkynes or carbonyl compounds as well as nitriles to give the corresponding coupling products in moderate to good yields (equation 130)225. 

Another annellation reaction starts from a,a -dibromo-o-xylene, an activated olefin and zinc.63 This reaction, according to the authors proceeds by a Diels-Alder cycloaddition via the o-xylylene (Scheme 15.10), and was employed in carbohydrate chemistry to prepare anthracyclinones analogues.64 No reaction occurs without sonication. Instead of a cycloaddition, another explanation can be postulated, consisting first of addition of a mono-organozinc reagent to the activated olefin, followed by alkylation of the resulting enolate by the second benzylic bromide. 

The most common method for the formation of an organozinc reagent involves the insertion of zinc metal into the carbon-iodine bond of an alkyl iodide. For allylic substrates the corresponding bromide or even chloride can be used. The zinc metal normally needs to be activated by washing first with 1,2-dibromoethane and... 

---