Zinc Iodide organozinc reagents

When the Pd-catalyzed exchange protocol was applied to the 5-alkenyl iodides of type 51, the cyclized organozinc reagents 52 were directly obtained and functionalized by allylation with ethyl 2-bromomethylacrylate after transmetallation with CuCN 2LiCl. The five-membered ring products 53 were obtained with high trans diastereoselectivity, irrespective of the nature of the allylic substituent (R = Ph or OBz) at the allylic position (equation 21)34,39. By contrast, the zinc-induced carbocychzations were found to be moderately cis stereoselective when an acyloxy group was present at the allylic position (see Section II.B.2)29. 

Imidazole zinc reagent 372 was made from 2-iodoimidazole 371 with activated zinc dust. The organozinc reagent was cross-coupled with aryl iodides and alkenyl iodides, in good yields (Scheme 88), <1997T7237>. 

Other coupling partners to organozinc reagents include heterocyles such as 2-methylthiobenzothiazole, alkenyl aryl iodonium triflates (alkenyl group transfer for synthesis of trisubstituted alkenes), and aryl heteroaryl ethers. " Improved nickel-catalyzed cross-coupling conditions between oz-rto-substituted aryl iodides-nonaflates and alkyl-zinc iodides in. solution and in the solid phase have been defined. ... 

Organozinc reagents. Zn-Cu couple often shows higher reactivity than zinc dust towards alkyl halides. Thus perfluoroalkylzinc iodides are rapidly formed in the presence of AIBN, and quenching such reagents in situ by DMF gives perfluoroalkyl aldehydes. (Note that Zn-Ag on graphite is well suited for the preparation of aryl-zinc species). ... 

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... 

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