Reagents Simmons-Smith

Simmons-Smith reagent Named after the duPont chemists who discovered that diiodo-mechane would react with an active zinc-copper couple in ether to give a reagent with molecular formula ICHiZnl. The reagent adds stereospecifically cis- to alkenes to give cyclopropanes in high yields. 

The first substance examined in the steroid field was 3j6-hydroxycholest-4-ene (1) and the epimeric 3a-alcohol (3). These compounds react stereospecifically in dry ether with the Simmons-Smith reagent to yield the isomeric cyclopropyl carbinols (2) and (4) in 90 % and 67 % yields, respectively. The rate of this reaction is about one fifth of that observed with simple cyclic car-binols. ... 

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. 

Aminocyclopropanes were prepared from enamines by the addition of Simmons-Smith reagent (688) or best through the cuprous-chloride-promoted decomposition of diazomethane (689). The reaction of an enamine with chloroform and base and opening of the resultant aminocyclopropane to an ynamine was reported (690). 

These early studies on zinc carbenoids provide an excellent foundation for the development of an asymmetric process. The subsequent appearance of chiral auxiliary and reagent-based methods for the selective formation of cyclopropanes was an outgrowth of a clear understanding of the achiral process. However, the next important stage in the development of catalytic enantioselective cyclopropanations was elucidation of the structure of the Simmons-Smith reagent. 

Simmons-Smith reagent that contradicts path B, and path A has therefore been widely believed to represent the experimental reality. For lithium carbenoids, on the other hand, the alternative carbometalation/cyclisation pathway has received experimental support. Actually, the factors that determine the... 

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 hydroxy group in Zi-cycloocten-3-ol determines the stereochemistry of the reaction with the Simmons-Smith reagent. By examining a model, predict the stereochemistry of the product. 

Diazomethane in the presence of palladium acetate gave with allenes 613 a similar mixture of methylenecyclopropanes 614 and spiropentanes 615. In contrast to Simmons-Smith reagent, diazomethane prefers to add to the less substituted allenic double bond (Scheme 87) [162]. 

The use of an excess (3 equivalents) of the Simmons-Smith reagent gave exclusively the spiropentane derivatives 618 in 60-70% yield [163a], In contrast, not even trace amounts of these products were observed by using samar-ium-dihalomethane as the carbene source [4b],... 

Gem-dibromospiropentane (620), obtained by the reaction of MCP with dibromocarbene, was used in the synthesis of BCP (3) (Scheme 90) [6a]. The reaction of the intermediate vinylidenecyclopropane 621 with a large excess of Simmons-Smith reagent gave exclusively BCP (3), but using modified Simmons-Smith procedures the [3]-triangulane (247) was also obtained in small yield (Scheme 91) [165],... 

The Simmons-Smith Reagent and Related Carbenoid Compounds 337... 

The organization of the material in this chapter is naturally subjective, and certain topics could equally well have been discussed in another section or in a different order. For example, the Simmons-Smith reagent is both an alkylzinc iodide and a carbenoid, and because both sections exist in this chapter, it is discussed under the more specific heading of zinc carbenoids. 

In contrast to the Simmons-Smith reagent and similar carbenoids, which are reactive and therefore difficult to characterize, adducts of the fV-heterocyclic l,3-diorganylimidazol-2-ylidenes are remarkably stable. The first iV-heterocyclic carbene complex of zinc, namely the l,3-di(l-adamantyl)imidazol-2-ylidene diethylzinc complex 48 (Figure 22), was reported by Arduengo et al. in 1993.98 Because of the general utility of these iV-heterocyclic... 

A simpler example of this kind of stereochemical control is the cyclopropanation of cyclopenten-2-ol (5) by the Simmons-Smith reagent in which the cij-biciclo[3.1.0]hexan-2-ol (7) is the diastereomer exclusively formed. As in the case of the hydrogen in strychnine, the methylene enters stereoselectively by intramolecular transference from an intermediate complex ( . 

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