3.2 Organozinc Reagents
Organozinc reagents are one of the most important of organometallic compounds. The first instance of an organozinc compound goes back to 1849 when Edward Frankland discovered that heating a mixture of Zn and ethyl iodide gives highly pyroporric Et2Zn. Organozinc compounds in general are sensitive to oxidation, dissolve in a wide variety of solvents whereas protic solvents cause decomposition. Organozinc compounds also exhibit the Schlenck equilibrium like Grignard reagents (Scheme 1).
Scheme 1
In terms of reactivity, organozinc compounds are less reactive than Grignard reagents. This can be explained on the basis of relative position of Mg and Zn in the periodic table. Since Zn is more electropositive than Mg thus the Zn-C bonds have a higher degree of covalency compared to the Mg-C bond. In a typical case, the electrons forming the C-Zn bond reside in two sp hybridized molecular orbitals resulting in linear geometry about the Zn centre.
Organozinc reagents are less reactive than organomagnesium and organolithium reagents thereby allowing a higher functional group tolerance. However, this low reactivity means that they need to be often aided by additives or catalysts.
3.2.1 Reformatsky Reaction
The reaction of Zn, α-haloester and a carbonyl compound gives β-hydroxyester. It involves the formation of a Zn enolate which attacks the carbonyl group (Scheme 2).
Scheme 2
In case of α, β--unsaturated carbonyl compounds, 1,2-addition occurs regioselectively (Scheme 3).
Scheme 3
Examples
