Organic zinc compounds refer to a class of organic compounds containing carbon-zinc chemical bonds. The first discovered and prepared organic zinc compound was diethylzinc, which was found by Edward Frankland in 1849. And it is also the first compound found to have metal-carbon sigma bonds. Many organic zinc compounds are flammable and difficult to handle. Most organic zinc compounds are easily oxidized and decompose when dissolved in a protic solvent. In many reactions, organic zinc reagents need to be ready-made and cannot be separated, purified or stored for too long time. All reactions that use organozinc reagents need to be performed under inert gas protection, such as nitrogen or argon.
The most common oxidation state of organic zinc compounds is +2. It can be divided into three types: organic zinc halogen compounds (R-Zn-X, wherein X represents a halogen atom); dihydrocarbyl zinc compounds (R-Zn-R, wherein R represents an alkyl group or an aryl group); Lithium zincate or magnesium zincate (M+R3Zn-, wherein M represents lithium or magnesium).
Since the electronegativity of carbon and zinc are different (carbon is 2.55; zinc is 1.65), the polarity of the carbon-zinc chemical bond points to the carbon atom. Dihydrocarbyl zinc compounds are usually present in monomeric form, while organozinc halogen compounds form a polymeric form via a halogen bond bridge, which is similar to the Schlenk equilibrium of Grignard reagents.
The following are some methods for synthesizing organic zinc compounds:
Oxidative addition
Diethylzinc was prepared by this method, which was first discovered by Frankland. Under the “protection” of hydrogen, iodoethane and elemental zinc were subjected to an addition reaction, which is also called Frankland synthesis. The activity of metallic zinc can be enhanced by Rieke zinc reagent, which is obtained by reduction reaction of metallic potassium and zinc chloride.
Halogen-Zinc exchange reaction
The method is mainly divided into two types: exchange of zinc iodide and exchange of boron and zinc. The first step in the second reaction is the hydroboration of olefins.
Transmetalation
In a typical metal transfer reaction, diphenylmercury and elemental zinc are reacted in ether to give diphenylzinc and metallic mercury. The reaction is very slow and it will takes about two weeks.
Activation of zinc
Organic zinc compounds can also be obtained directly by the activation of metallic zinc. In this method, zinc is activated by 1,2-dibromoethane and trimethylchlorosilane (the preparation of Grignard reagents can also be activated by them). One of the key reagents is lithium chloride, which can quickly form soluble adducts with organozinc compounds and to remove the zinc reagent from the metal surface.
