Reactivity alkyl organozinc reagents

Organozinc reagents aie not nearly as reactive towaid aldehydes and ketones as Grig-nai d reagents and organolithium compounds but are intermediates in certain reactions of alkyl halides. 

The presence of Cu(i) or Cu(n) salts in the aforementioned reactions is critical. It is believed that organozinc reagents undergo transmetallation with copper species to yield more reactive complexes.301 A proposed301 catalytic cycle (Scheme 118) suggests that the alkyl group transferred to the enone from the copper metal in a bimetallic intermediate 207. 

Crucially, this allows organozinc reagents to be prepared from less reactive aryl bromides and secondary or tertiary alkyl bromides. Alternatively, organozinc iodides can be prepared by means of a palladium(0)-catalyzed reaction between alkyl iodides and Et2Zn (Scheme 2.25) [53-56]. 

A prototypical study for this section has been obtained as early as in 1983 for carbonylative cross-coupling of the mixture of aryl iodide and alkyl iodide in the presence of Zn metal and palladium catalyst. This system apparently works due to differences of reactivity of aryl versus alkyl iodide toward metallation by Zn. Further studies were rather scarce to involve only preformed functionalized alkylzincs. Carbonylative cross-coupling of functionalized organozinc reagents with allylic esters and GO (1 atm) can be carried out in THF in the presence of HMPA, which suppresses side-reactions (Scheme 4). ... 

Compared to oxabenzonorbomenes, oxabicyclo[2.2.1]heptenes or oxabicyclo[3.2.1] octenes are less reactive substrates, and the Pd-catalyzed alkylative ring-opening process with organozinc reagents generally required heating in 1,2-dichloroethane at reflux. The addition of Zn(OTf)2 dramatically improved the rate of the reaction which could be carried out in CH2C12 at room temperature, as illustrated for the conversion of 83 to the... 

Due to the low reactivity of alkyl and arylorganozinc reagents towards alkenes and alkynes, it appears clear that the carbozincation chemistry for this class of reagents is intimately associated with transition metal catalysts. Some of the metal-catalyzed/promoted reactions do indeed produce organozinc reagents as the final organometallic species that can further react with an appropriate electrophile, whereas other processes lead to highly functionalized products by an entirely different pathway. 

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