Iodomethylzinc iodide

Subsequent reaction of (7) with the iodomethylzinc iodide reagent gives l, 2 -methylene-3 -hydroxy-A -steroids which are readily transformed to the important 3-keto-A analogs. 

What makes iodomethylzinc iodide such a useful reagent is that it reacts with alkenes to give cyclopropanes. 

This reaction is called the Simmons-Smith reaction and is one of the few methods available for the synthesis of cyclopropanes. Mechanistically, the Simmons-Smith reaction seems to proceed by a single-step cycloaddition of a methylene (CH2) unit from iodomethylzinc iodide to the alkene ... 

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 ... 

Methylene transfer from iodomethylzinc iodide is stereospecific. Substituents that were cis in the alkene remain cis in the cyclopropane. 

Iodomethylzinc iodide is often refened to as a carbenoid, meaning that it resembles a carbene in its chemical reactions. Caibenes are neutral molecules in which one of the caibon atoms has six valence electrons. Such caibons aie divalent they are directly bonded to only two other atoms and have no multiple bonds. Iodomethylzinc iodide reacts as if it were a source of the caibene H—C—H. 

Simmons-Smith reaction (Section 14.12) Reaction of an alkene with iodomethylzinc iodide to form a cyclopropane derivative. 

Understanding the importance of the zinc alkoxide, the iodomethylzinc iodide and the zinc sulfonamide allowed Denmark to propose a revised transition state structure xv (Fig. 3.22) [82]. In this picture, the complex, polymetallic aggregate invoked by Rickborn and later by Kobayashi is featured. 

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). 

Cyclopropanation with Halomethylzinc Reagents. A very effective means for conversion of alkenes to cyclopropanes by transfer of a CH2 unit involves reaction with methylene iodide and a zinc-copper couple, referred to as the Simmons-Smith reagent.169 The reactive species is iodomethylzinc iodide.170 The transfer of methylene occurs stereospecifically. Free CH2 is not an intermediate. Entries 1 to 3 in Scheme 10.9 are typical examples. 

The prepararation of a-iodoalkyltin iodides can be performed in THF by reaction of Me2SnCl2 with iodomethylzinc iodide, IZnCH2l, in molar ratio 1 1329 ... 

The landmark discovery by Simmons and Smith that iodomethylzinc iodide could convert a wide variety of alkenes into the corresponding cyclopropanes (equation 1) constitutes one of the most important methods for the generation of the simplest cycloalkane ". ... 

The reaction also displayed autocatalytic behavior, which was shown to be due to the generation of zinc iodide. This species, in combination with bis(iodomethyl)zinc, undergoes Schlenk equilibration to give iodomethylzinc iodide. This and other observations have led to the proposed transition state model in which three zinc atoms are involved in the cyclopropanation (Figure 10). 

Conversion of an alkene to a cyclopropane can be accomplished to using the Simmons-Smith reagent (iodomethylzinc iodide). 

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. 

The reaction of diazomethane with zinc halides has been studied previously by several workers 70, 326, 475), and the formation of a zinc halide—methylene complex was suggested. Wittig and Schwarzen-bach 542) demonstrated the formation of iodomethylzinc iodide (I) and bis(iodomethyl)zinc (VIII) by the reaction of zinc iodide with diazomethane. The reaction of diazomethane with zinc chloride gives bis(chloro-methyl)zinc (IX) 544). 

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