Organomercury compounds with alkenes

Other Cyclopropanation Methods. Haloalkylmercury compounds are also useful in synthesis. The addition reactions are usually carried out by heating the organomercury compound with the alkene. Two typical examples are given in Section C of Scheme 10.9. 

A number of metals salts can be used as the source of electrophiles in reactions with alkenes. One of the most interesting of these involves the attack of mercury(II) acetate in acetic acid. Reductive cleavage of the organomercury compound with sodium borohydride leads to the overall hydration of the alkene in a Markownikoff sense. There are a number of preparative advantages, such as a reduced tendency to rearrange, associated with this and similar relatively mild procedures when compared to the direct protonation of a double bond (Scheme 3.14)... 

Alkene oxymercuration is closely analogous to halohydrin formation. The reaction is initiated by electrophilic addition of (mercuric) ion to the alkene to give an intermediate mercurinium ion, whose structure resembles that of a bromonium ion (Figure 7.5). Nucleophilic attack of water, followed by loss of a proton, then yields a stable organomercury addition product. The final step, reaction of the organomercury compound with sodium boro-hydride, is not fully understood but appears to involve radicals. Note that... 

The solvomercuration reaction is thought to be a two-step process. In the first step (equation 147), electrophilic attachment of mercury ion to the alkene produces a positively charged intermediate. In the second step (equation 148), a nucleophile (generally a solvent molecule) reacts with the intermediate leading to the organomercury compound. 

Perlmutter used an oxymercuration/demercuration of a y-hydroxy alkene as the key transformation in an enantioselective synthesis of the C(8 ) epimeric smaller fragment of lb (and many more pamamycin homologs cf. Fig. 1) [36]. Preparation of substrate 164 for the crucial cyclization event commenced with silylation and reduction of hydroxy ester 158 (85-89% ee) [37] to give aldehyde 159, which was converted to alkenal 162 by (Z)-selective olefination with ylide 160 (dr=89 l 1) and another diisobutylaluminum hydride reduction (Scheme 22). An Oppolzer aldol reaction with boron enolate 163 then provided 164 as the major product. Upon successive treatment of 164 with mercury(II) acetate and sodium chloride, organomercurial compound 165 and a second minor diastereomer (dr=6 l) were formed, which could be easily separated. Reductive demercuration, hydrolytic cleavage of the chiral auxiliary, methyl ester formation, and desilylation eventually led to 166, the C(8 ) epimer of the... 

Polystyrene-bound alkenes react with alcohols or amines in the presence of mercury(II) trifluoroacetate to yield 2-alkoxy- or 2-aminoethylmercury compounds [30]. The C-Hg bond can be reduced to a C-H bond by treatment with LiBH4, or converted into a C-I bond by treatment with iodine [30]. Organomercury compounds have been immobilized with polystyrene-bound carboxylates [31]. The resulting product was used as starting material for the preparation of radiolabelled 6-iodo DOPA (Figure 4.4). 

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... 

Of the methods we have seen for Markovnikov hydration of alkenes, oxymercuration-demercuration is most commonly used in the laboratory. It gives better yields than direct acid-catalyzed hydration, it avoids the possibility of rearrangements, and it does not involve harsh conditions. There are also disadvantages, however. Organomercurial compounds are highly toxic. They must be used with great care and then must be disposed of properly. 

Mercuric acetate, Hg(02CCH3)2 Adds to alkenes in the presence of water, giving a-hydroxy organomercury compounds that can be reduced with NaBH4 to yield alcohols. The overall effect is the Markovnikov hydration of an alkene (Section 7.4). 

The two-stage process of oxymercuration-demercuration is fast and convenient, takes place under mild conditions, and gives excellent yields—often over 90%. The alkene is added at room temperature to an aqueous solution of mercuric acetate diluted with the solvent tetrahydrofuran. Reaction is generally complete within minutes. The organomercurial compound is not isolated but is simply reduced in situ by sodium borohydride, NaBH4. (The mercury is recovered as a ball of elemental mercury.)... 

In the laboratory, alkenes are often hydrated by the oxymercura-tion procedure. When an alkene is treated with mercurydl) acetate [Hg(02CCH3)2, usually abbreviated HgOAclal in aqueous tetrahydrofuran (THF) solvent, electrophilic addition to the double bond rapidly occurs. The intermediate organomercury compound is then treated with sodium boro-hydride, NaBH4, and an alcohol is produced. For example ... 

Treatment of an alkene with mercuric acetate in aqueous THF results in the electrophilic addition of mercuric ion to the double bond to form an intermediate mercuri-um ion. Nucleophilic attack by H2O at the more substituted carbon yields a stable organomercury compound, which upon addition of NaBH4 undergoes reduction. Replacement of the caiton-mercury bond by a carbon-hydrogen bond during the reduction step proceeds via a radical process. The overall reaction represents Markovnikov hydration of a double bond, which contrasts with the hydroboration-oxidation process. 

Oxidation of organomercury compounds via formation of TEMPO derivatives and cleavage with Zn-HOAc completes the functionalization of alkenes. Without TEMPO the oxidative capture of a primary radical generated from organomercurial is inefficient, and the reductive pathway (loss of functionality) becomes competitive. 

The principle underlying the use of organomercury compounds for carbene generation (entry 6, Scheme 8.1) is again the a-elimination mechanism. The carbon-mercury bond is much more covalent than the C-Li bond, however, so that the merciuy systems are generally stable at room temperature and easily isolated. They then decompose to the carbene when heated in solution with an appropriate alkene. The decomposition appears to be a reversible unimolecular reaction, and... 

Reactions of Sm(Cp )2(THF)2 Sm(Cp )2 with organomercury reagents Reaction of Sm(Cp )2(THF)2 with monodentate anionic ligands Reactions of SmCp 2 and Sm(Cp )2-(THF)2 with alkenes, alkynes, and related unsaturated compounds... 

The combined influences of polar and steric effects and of the strength of the newly formed bond was also recognized in the reaction of OE,0-unsaturated carbonyl compounds and similar electron deficient alkenes with organomercurials and NaBH4. For the addition of alkyl radicals to substituted styrenes, p assumed a... 

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