Organozinc compound

Organozinc compounds in organic chemistry contain carbon to zinc chemical bonds. Organozinc chemistry is the science of organozinc compounds describing their physical properties, synthesis and reactions.

The first organozinc ever prepared, diethylzinc by Edward Frankland in 1849, was also the first ever compound with a metal to carbon sigma bond. Many organozinc compounds are pyrophoric and therefore difficult to handle. Organozinc compounds in general are sensitive to oxidation, dissolve in a wide variety of solvents where protic solvents cause decomposition. In many reactions they are prepared in situ, not isolated and reacted further. All reactions require a protective gas (nitrogen or argon) blanket.

The three main classes of organozincs are: organozinc halides R-Zn-X with X a halogen atom, diorganozincs R-Zn-R with R an alkyl or aryl group and lithium zincates or magnesium zincates M+R3Zn- with M lithium or magnesium.

The carbon zinc bond is polarized towards carbon due to the differences in electronegativity (carbon:2.55 and zinc: 1.65). Diorganozincs are always monomeric, the organozinc halides form aggregates through halogen bridges very much like Grignard reagents and also like Grignards they display a Schlenk equilibrium.

Synthesis
Several general methods exist for the generation of organozinc compounds.
 * Oxidative addition. The original diethylzinc synthesis by Frankland was an oxidative addition of iodoethane to zinc metal with hydrogen gas as a "protective" blanket (this reaction is called the Frankland synthesis). The reactivity of zinc metal is increased in so-called Rieke zinc obtained by reduction of zinc chloride by potassium metal.
 * Halogen zinc exchange. Two main halogen zinc exchange reactions are iodine zinc exchange and boron zinc exchange. The first step in the second procedure is hydroboration of an alkene:
 * [[Image:HalogenZincExchange.png|400px|Halogen zinc exchange]]


 * Transmetalation. In a typical transmetalation, diphenylmercury reacts with zinc metal to diphenylzinc and metallic mercury in diethyl ether. Reaction time: two weeks. This reaction is driven by the formation of the organic compound with the most electropositive element.
 * Organozinc compounds can be obtained directly from zinc metal :
 * [[Image:OrganozincSynthesisbyDirectInsertion.png|400px|Organozinc Synthesis by Direct Insertion]]
 * In this method zinc is activated by the action of 1,2-dibromoethane and trimethylsilyl chloride. A key ingredient is lithium chloride which quickly forms a soluble adduct with the organozinc compound thus removing it from the metal surface.

Reactions
In many of their reactions organozincs appear as intermediates.
 * In the Frankland-Duppa Reaction (1863) an oxalate ester (ROCOCOOR) reacts with an alkyl halide R'X, zinc and hydrochloric acid to the α-hydroxycarboxylic acid ROR'COHCOOR
 * The related Barbier reaction (1899) is the zinc equivalent of the magnesium Grignard reaction and actually the older and the more tolerant of the two. In presence of just about any water the formation of the organomagnesium halide will fail whereas the Barbier reaction can even take place in water. On the downside organozincs are much less nucleophilic than Grignards. Among the Group 12 elements zinc is the most reactive.
 * The Reformatskii reaction converts α-halo-esters and aldehydes to β-hydroxy-esters also through an intermediate organozinc halide.
 * In the Simmons-Smith reaction the carbenoid (iodomethyl)zinc iodide reacts with alkens to cyclopropanes
 * Reactions of zinc metal acetylides
 * Addition reaction of organozinc compounds to carbonyl compounds. Commercially available diorganozinc compounds are dimethylzinc, diethylzinc and diphenylzinc. These reagents are expensive and difficult to handle. In one study the active organozinc compound is obtained from much cheaper organobromine precursors:
 * [[Image:DiphenylzincCarbonylAddition.png|400px|Addition of diphenylzinc to an aldehyde]]


 * The Negishi coupling is an important reaction for the formation of new carbon carbon bonds between unsaturated carbon atoms in alkenes, arenes and alkynes. The catalysts are nickel and palladium. A key step in the catalytic cycle is a transmetalation in which a zinc halide exchanges its organic substituent for another halogen with the palladium (nickel) metal center.

Organozincates
The first ever organozinc ate complex (organozincate) was discovered in 1858 by an assistant to Frankland and concerned the reaction of elemental sodium with diethylzinc:


 * 2Na + 3 Et2Zn -> 2Et3Zn-Na+ + Zn

In 2007 it was reported that by tweaking the reaction conditions the zincate proceeds to react to sodium hydridoethylzincate(II) (with hydrogen atoms as bridging ligands) as a result of beta-hydride elimination of one of the ethyl groups :


 * [[Image:Organozincatereactions.png|500px|Organozincate reaction Lennartson 2007]]