Carbene diiodomethane

The diiodomethane and zinc react to produce a carbene-like species called a carbenoid. 

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-type cyclopropanation of olefins is one of the most well-known reactions of carbenes and carbenoids. However, cyclopropanation of simple olefins with magnesium carbenoids is usually very difficult and only cyclopropanation of allylic alcohols was reported. Thus, treatment of allylic alcohols (23) in CH2CI2 at —70°C with i -PrMgCl and diiodomethane for 48 to 60 h afforded cyclopropanes in up to 82% yield as a mixture of syn- and and-isomers. In this reaction, 5yn-isomers were mainly or exclusively obtained (synianti = 5 1-400 1) (equation 10). 

The cyclopropanation requires a methylene carbene that can add to the double bond. Under which of the following sets of conditions can methylene be prepared in situ from diiodomethane a) CH.I2, Cu/Zn b) CH2I2, KMn04 c) CH2I2, ZnEt2 ... 

Another reaction that builds a cyclopropane ring onto a carbon-carbon double bond is called the Simmons-Smith reaction. This reaction does not actually involve a car-bene, but rather a carbenoid, an organometallic species that reacts like a carbene. This species is generated by reaction of diiodomethane with zinc metal from a special alloy of zinc and copper ... 

Because of these and other useful molecules containing three-membered rings, methods to make them are important as well as interesting. Most chemical syntheses of compounds containing cyclopropyl groups make use of the addition of a carbene, or carbene equivalent, to an aikene. What do we mean by carbene equivalent Usually, this is a molecule that has the potential to form a carbene, though it may not actually react via a carbene intermediate. One such example is a zinc carbenoid formed when diiodomethane is reacted with zinc metal it reacts with alkenes just as a carbene would—it undergoes addition to the 7t bond and produces a cyclopropane. 

The fact that the Simmons-Smith reaction is regio- and stereoselective enabled the preparation of tritium containing cyclopropane derivatives useful for biological studies. The tritiated diiodomethane was prepared by reduction of iodoform with sodium arsenite in the presence of tritiated water. The carbene generated from tritiodiiodomethane and triisobutylaluminum in chloroform underwent regioselective addition to the unhindered double bond in perillyl alcohol (34). ... 

I.2.I.I. Carbenes Generated from Diazo Compounds and Diiodomethane... 

While many examples are available demonstrating the mechanism-based stereoselectivity in carbene transfer reactions, knowledge of the simple diastereoselectivity exhibited by alkylcarbenoids is rather limited. A few typical reactions (Table 1) arc sufficient to demonstrate the basic principles (for further examples, see Vol. E 19h, pp 165-390). Thus, photolysis of diiodomethane in the presence of (E)- or (Z)-3-hexene stereospecifically provides /r ns-l,2-diethylcyclopropane and cM-l,2-diethylcyclopropane, respectively1. For a general procedure for this photocyclopropa-nation method, which proceeds via the iodomethyl cation, see Vol. E 19b, p 190. 

The rhodium chemistry just presented actually involves C=Rh species rather than a free carbene. In this section, we will look at a general class of compounds called carbenoids. A carbenoid is a reactive intermediate that reacts similarly to a carbene but does not actually involve formation of a carbene. The most commonly used carbenoid is generated by reaction of diiodomethane and a Zn/Cu couple this reaction, when it adds to alkenes, is called the Simmons-Smith reaction.316 A simple example is the conversion of cyclohexene to bicyclo[4.1.0]heptane (norcarane, 336). Initial reaction of diiodomethane with zinc gave an iodozinc compound (399-see sec. 9.8.B for a brief discussion of organozinc compounds), which added to the alkene to give 400. Loss of zinc iodide (Znl2) gave the cyclopropane derivative (343 in this case).3ll A one-step... 

Although the Simmons-Smith reaction has found considerable use in organic synthesis, it is not readily applicable to the formation of highly substituted cyclopropanes, since 1,1 -diiodoalkanes (other than diiodomethane) are not readily available. Substituted zinc carbenoids can be prepared from aryl or a,p-unsaturated aldehydes (or ketones) with zinc metal, and these species can be trapped with an alkene to give substituted cyclopropanes.The addition of chromium carbenes (see Section 1.2.2) to alkenes can be used to effect cyclopropanation to give substituted cyclopropanes. Thus, addition of excess 1-hexene to the chromium carbene 113 gave the cyclopropane 114 as a mixture of diastereomers, with the isomer 114 predominating (4.92). ... 

Non functionalized alkenes undergo cyclopropanation with diazomethane/Pd(n) if they are strainedt or tenninal substituted. Consequently, carbene transfer to alkenyl substituted cyclohexenes occurs highly regioselective, contrasting the reactivity of such derivatives towards cyclopropanation using diiodomethane/zinc or metal carbenoids (Table 5). 

A useful cyclopropane synthesis was developed by H. E. Simmons and R. D. Smith of the DuPont Company. In this synthesis diiodomethane and a zinc-copper couple are stirred together with an alkene. The diiodomethane and zinc react to produce a carbene-like species called a carbenoid ... 

Carbenoid (Section 8.14C) A carbene-like species. A species such as the reagent formed when diiodomethane reacts with a zinc-copper couple. This reagent, called the Simmons—Smith reagent, reacts with alkenes to add methylene to the double bond in a stereospecific way. 

Chapter 7. On the other hand, methylene (rCH ) and alkyl substituted methylenes (carbenes, CHR CR2 R H) can be readily prepared either by the thermal (or photochemical) decomposition of the corresponding diazo compounds (Equation 6.51) themselves prepared from an amine (Chapter 10) or ketone (or aldehyde) (Chapter 9) precursor. Alternatively, for a species that behaves as if it were singlet methylene (rCH ), the treatment of methylene iodide (diiodomethane, I2CH2) with copper (Cu)-activated zinc (Zn) metal (Equation 6.52) can be used (the Simmons-Smith reaction ). 

In another route to cyclopropanes, diiodomethane is treated with zinc powder (usually activated with copper) to generate ICH2ZnI, called the Simmons-Smith reagent This species is an example of a carbenoid, or carbene-like substance, because, like carbenes, it also converts alkenes into cyclopropanes stereospecifically. Use of the Simmons-Smith reagent in cyclopropane synthesis avoids the hazards associated with diazomethane preparation. 

In Summary Diazomethane is a useful synthetic intermediate as a methylene source for forming cyclopropanes from alkenes. Halogenated carbenes, which are formed by dehydro-halogenation of halomethanes, and the Simmons-Smith reagent, a carbenoid arising from the reaction of diiodomethane with zinc, also convert alkenes into cyclopropanes. Additions of carbenes to alkenes differ from other addition processes because a single carbon atom becomes bonded to both alkene carbons. 

Methylene ( CH2), the simplest carbene, can be prepared by decomposition of the highly toxic and explosive reagent diazomethane (CH Nj). However, more easily used reagents have been developed that, while they do not produce methylene directly, function as methylene transfer agents. They are called carbenoid species because they react like carbenes. lodomethylzinc iodide, known as the Simmons-Smith reagent, is a carbenoid. In the Simmons-Smith method, diiodomethane reacts with a zinc-copper alloy to produce an intermediate I—CH Zn—I compound. 

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