Iodine-zinc exchange

The utility of the iodine-zinc exchange for the synthesis of diorganozincs having functionalized organic groups is demonstrated by the synthesis of bis(5-pivaloxypentyl)zinc 2 from diethylzinc and 5-iodopentylpivalate.25... 

Scheme 2.30. Preparation of a diorganozinc compound by iodine-zinc exchange.

It was reported that the iodine-zinc exchange process induced by treatment of alkyl iodides with EbZn could be catalyzed by Cul, leading to shorter reaction times and reduction of the amount of Et2Zn38. The use of palladium or nickel catalysts turned out to be also extremely efficient but produced an organozinc iodide instead of a dialkylzinc, with evolution of ethane and ethylene34 (equation 20). 

Preparation of dioctylzinc by iodine-zinc exchange and its copper-mediated coupling with an electrophile preparation of ethyl 2-nonylacrylate... 

Besides the iodine-zinc exchange reaction, the boron-zinc exchange reaction proves to be an excellent method for preparing a broad range of polyfunctional diorganozincs (Scheme 5.8). The exchange reaction for primary diethylalkyl boranes proceeds at room temperature and is complete within a few minutes. 

Preparation of b/s(5-acetoxypentyl)zinc by iodine-zinc exchange and its enantioselective addition to an aldehyde preparation of (6S)-5-bromo-6-hydroxy-l 1 -acetoxyundec-4-ene9... 

The extension of this method to heterocyclic products took advantage of nickel catalysis for the iodine-zinc exchange, for example, of iodoacetals 22, and the organozinc intermediates 23 formed by radical cyclization were again transmetallated to the corresponding copper species which could be reacted with the usual Michael acceptors to afford the products 24 with high diastereoselectivity (Scheme 6).33>33a... 

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. 

Recently, Knochel and co-workers [51] have reported that primary organic iodides undergo an iodine-zinc exchange reaction when treated with an excess of Et2Zn (5 equiv, neat, 50-55 °C, 10-14 h) and that several functional groups can be present in the organic iodides. In order to obtain further insight into the carbocyclization of dialkylzinc instead of... 

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