Iodomethylzinc iodide preparation

Iodomethylzinc iodide is known as the Simmons-Smith reagent, after Howard E. Simmons and Ronald D. Smith of Du Pont who first described its use in the preparation of cyclopropanes. 

What alkenes would you choose as starting materials in order to prepare each of the following cyclopropane derivatives by reaction with iodomethylzinc iodide ... 

An improved route to iodomethylzinc iodide has made the iodometh-yltin compounds, R3SnCH2l, more accessible (280). Bimetallic compounds of the types MejSnCBrjMgCl (281), MejSnCClBrLi (282), and MesSnCHIZnI (280, 283), have been prepared in solution at low temperature, and thence, the ditin compounds (MeaSnljCClBr, (MegSnljCBri, (MejSnlaCHI (284). 

Allylcopper/zinc reagents can be prepared directly by reaction of vinyl copper reagents with (iodomethyl)zinc iodide, the Simmons-Smith reagent. These allylcopper/zinc reagents do not couple with an alkyl iodide or benzyl bromide, but react readily with electrophiles such as aldehydes, ketones, or imines.3 This approach to organodimetallic reagents is apparently limited (see Iodomethylzinc iodide, this volume). 

The nature of the zinc (dust, foil, or shots) is important for the success of the formation of an organozinc halide by oxidative addition. Usually, the use of zinc dust (-325 mesh) from Aldrich or Riedel de Haen gives the best results. In some cases, such as for the preparation of allylic zinc halide reagents,22,28 iodomethylzinc iodide,65 or benzylic zinc halides,66 the use of zinc foil (0.1 mm or 0.25 mm thick, Aldrich or Merck) may be advantageous or even essential. 

An organozinc compound that occupies a special niche in organic synthesis is iodomethylzinc iodide (lCH2Znl), prepared by the reaction of zinc-copper couple [Zn(Cu), zinc that has had its surface activated with a little copper] with diiodomethane in ether. 

An authentic sample of iodomethylzinc iodide was prepared from zinc metal and diiodomethane. The formation of ethylene is interesting. It likely arises from an Elcb reaction from an a-iodoethylzinc compound. This, in turn, could arise from a 1,2-migration or metathesis pathway (Scheme 20.13). Adding diiodomethane to diethylzinc at -60°C resulted in an initial exotherm. We used the energy of this exotherm to warm the internal temperature to -40°C. 

Although methylene prepared from diazomethane itself is not synthetically useful, addition of methylene to an alkene can be accomplished using a reaction first reported by the American chemists Howard Simmons and Ronald Smith. The Simmons-Smith reaction uses diiodomethane and zinc dust activated by a small amount of copper (a so-called "zinc-copper couple") to produce iodomethylzinc iodide, in a reaction reminiscent of a Grignard reaction. Even though we show the Sinunons-Smith reagent here as ICH2ZnI, its structure is considerably more complex and not fully imderstocxl. 

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