Preparation by the Oxidative Addition to Zinc Metal

The oxidative addition of zinc dust to functionalized organic halides allows the preparation of a broad range of polyfunctional organozinc iodides such as 1-5 (10-14]. Several functional groups such as nitro or azide groups inhibit the radical-transfer reaction leading to the zinc reagent. On another hand, hydroxyl 

FG = CO2R, enoate, CN, halide, (RCOlsN, (TMS)2N, RNH, NH2, RCONH, (ROlsSi, (RO)2PO, RS, RSO, RSO2, PhCOS 

Under these conditions, a broad range of polyfunctional alkyl iodides are converted to the corresponding organozinc halides in high yields [6j. In the case of primary alkyl iodides, the insertion occurs at 40-50 °C, whereas secondary alkyl iodides already react at 25-30 °C. Secondary alkyl bromides also react under these conditions [18], but primary alkyl bromides are usually inert with this type of activation and much better results are obtained by using Rieke-zinc [19-21]. Thus, the reduction of zinc chloride with finely cut lithium and naphthalene produces within 1.5 h highly reactive zinc (Rieke-zinc). 

Interestingly, many electron-defident heterocychc and aryl bromides or iodides are sufRdently activated to react with commercially available zinc powder [14]. In the case of benzylic halides, bromides and even chlorides can be used [12]. Thus, for the functionalized benzylic bromide 12a, the formation of the corresponding benzylic zinc bromide (13a) by the direct insertion of zinc dust is complete within 2h at 5 °C. 

Alkylzinc bromides bearing various functional groups [31] can be readily prepared by the direct insertion of zinc metal (dust, powder or shot) to alkyl bromides by performing the reaction in the presence of iodine (1-5 mol%) in a polar solvent 

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