Alkenes Simmons-Smith reaction with

TABLE 1. Yields (%) of the cyclopropanated reaction of several alkenes in the Simmons-Smith reaction with CH2X2 (X = Br, I) and several promoters... 

The Simmons-Smith Reaction with an Alkene tion i5.sc)... 

Simmons-Smith reaction (Section 14 12) Reaction of an alkene with lodomethylzmc iodide to form a cyclopropane denvative... 

A carbene, R2C , is a neutral molecule containing a divalent carbon with only six valence electrons. Carbenes are highly reactive toward alkenes, adding to give cyclopropanes. Nonlialogenated cyclopropanes are best prepared by treatment of the alkene with CH212 and zinc-copper, a process called the Simmons-Smith reaction. 

Simmons-Smith reaction (Section 7.6) The reaction of an alkene with CH2l2 and Zn—Cu to yield a cyclopropane. 

Among methods of preparing optically active cyclopropane compounds, the Simmons-Smith reaction, first reported in 1958, is of significance. This reaction refers to the cyclopropanation of alkene with a reagent prepared in situ from a zinc-copper alloy and diiodomethane. The reaction is stereospecific with respect to the geometry of the alkene and is generally free from side reactions in contrast to reactions involving free carbenes. 

The Simmons-Smith reaction " and its variants are widely used for the stereospecific synthesis of cyclopropane compounds. The methodology involves the use of copper-treated zinc metal (the zinc-copper couple) with diiodomethane to add methylene to a carbon-carbon double bond. Alternative use of diazomethane in catalytic reactions does not offer the same synthetic advantages and is usually avoided because of safety considerations. As significant as is the Simmons-Smith reaction for cyclopropane formation, its employment for organic synthesis was markedly advanced by the discovery that allylic and homoallylic hydroxyl groups accelerate and exert stereochemical control over cyclopropanation of alkenes (e.g, Eq. 21), and this acceleration has been explained by a transition state model... 

Organomercury compounds such as ICFbHgl and Hg(CH2Br)2 may be prepared by the reaction of metallic mercury with diiodomethane or by reaction of mercury(II) salts with diazoalkanes. These mercury derivatives are related to the intermediate that has been proposed in the Simmons-Smith reaction, and likewise, they have been found to convert alkenes into cyclopropanes.92... 

Even unreactive alkenes undergo the Simmons-Smith reaction (cyclopropanation) in high yield with dibromomethane and this zinc powder. Almost quantitative yields are obtained in the Reformatsky reaction of ethyl a-bromoacetate and this zinc powder. Ethyl a-chloroacetate can also be used if the reaction is conducted at higher temperatures for a longer period. 

Electrochemical cyclopropanation of alkenes occurs using dibromomethanes at a sacrificial zinc electrode in a CH2C12/DMF mixture with a one compartment cell (equation 65). Yields using more than twenty isolated and conjugated olefins were generally good (30 to 70%)98. Benzal halides give only poor yields in the same reaction, but 2,2-dibromo-propane leads to the equivalent gem-dimethylcyclopropanes in fair yields. The method represents a useful alternative to other methods of cyclopropanation of olefins such as the Simmons-Smith reaction. 

The mechanism for the formation of this carbenoid and for its reaction with alkenes need not concern us here. Just remember that it reacts as though it is methylene. The Simmons-Smith reaction is an excellent way to prepare cyclopropane derivatives from alkenes, as shown in the following examples. Note the stereochemistry in the second equation. 

Diiodomethane, CH2I2 Reacts with alkenes in the presence of zinc-copper couple to yield cyclopropanes (Simmons-Smith reaction Section 7.6). 

The Simmons-Smith reaction does not involve a free carbene. Rather, the reaction of CH2I2 with Zn(Cu) forms (iodomethyl)zinc iodide, which transfers a CH2 group to an alkene, as shown in Mechanism 26.5. 

Simmons-Smith reaction of an alkene with CHgIg and Zn(Cu) to form a cyclopropane... 

The Simmons-Smith reaction is an efficient and powerful method for synthesizing cyclopropanes from alkenes [43]. Allylic alcohols are reactive and widely used as substrates, whereas a,j8-unsaturated carbonyl compounds are unreactive. In 1988, Ambler and Davies [44] reported the electrophilic addition of methylene to a,/3-unsaturated acyl ligands attached to the chiral-at-metal iron complex. The reaction of the racemic iron complex 60 with diethylzinc and diiodomethane in the presence of ZnCl2 afforded the c/s-cyclopropane derivatives 61a and 61b in 93 % yield in 24 1 ratio (Sch. 24). 

An asymmetric Simmons-Smith reaction was reported by Kang et al. [18]. The reaction of (3-D-fructopyranoside 13 with a,(3-unsaturated aldehydes gave enrfo-acetals 14 along with exo-isomers 15 in a ratio of about 1.5 1. The enrfo-acetals afforded the best selectivity, typically giving (2/f,3/f)-hydroxymethyl cyclopropanes 17 with up to 85% ee. It should be noted that the corresponding exo-acetals 15 underwent the cyclopropanation reaction with lower stereoselectivity. In these cases, the group cannot effectively block either side of the alkene in contrast to the endo-isomer [18] (Scheme 10.3). 

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