Mercury Solvomercuration

Mercury(II) trifluoroacetate is a good electrophile that is highly reactive toward carbon-carbon double bonds [52, 53, 54] When reacting with olefins in nucleophilic solvents, it usually gives exclusively mercurated solvoadducts, but never products of skeletal rearrangement Solvomercuration-demercuratton of alkenes with mercury(II) trifluoroacetate is a remarkably effective procedure for the preparation of esters and alcohols with Markovnikov s regiochemistry [52, 5J] (equation 24)... 

Addition of electrophilic mercury(II) salts to carbon-carbon double bonds in nucleophilic solvents (i.e. oxymercuration, solvomercuration etc.) is a well documented methodology in organic synthesis146. In these reactions a mercuric salt, usually the chloride or... 

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. 

Subsequent isolation of solvomercuration products has supplied information about the stereoselectivity of the mercury addition and at the same time has shown that these reactions can give 1,2- and/or 1,4-addition products. In particular, the identification by XH NMR spectroscopy of a 1,4-adduct from 1,3-pentadiene and mercury(II)nitrate in methanol has provided155 the first direct evidence that oxymercuration of conjugated dienes can proceed by 1,4-addition. Furthermore, the observed Z,E-isomerization of the diene has shown that the 1,4-oxymercuration is a reversible process. 

Solvomercuration,3111 484 187 or the addition of mercury(II) salts, is a convenient route to organomercurials. On the other hand, replacement of mercury with hydrogen allows Markovnikov functionalization of alkenes.488 A method called mercuration-demercuration, for instance, has been developed for the Markovnikov hydration of alkenes under mild conditions489 [Eq. (6.82)490] ... 

Several examples of intramolecular solvomercuration of arylalkynes have been reported (equations 81 to 83).200 The mercury moiety can be removed by protiodemetallation or converted to a variety of other functional groups. The use of a methyl ether as the nucleophilic functionality is noteworthy, as is the change in the regioselectivity (equation 82 versus equation 83). A bromocyclization related to equation (83) using a phenylethynylbenzoate ester also gives an isocoumarin derivative via 6-endo cyclization.201 Palladium-catalyzed cyclizations of 3,y-alkynic ketones or 2-methyl-3-alkyn-l-ols to form substituted furans are discussed in Chapter 1.4, Section 3.1.3 in this volume. 

The Giese reaction has been used for reductive cyclization of a,/l-enones containing an isolated terminal double bond. Solvomercuration can be carried out selectively on the latter group. On hydride reduction, the mercurial group is reduced preferentially and undergoes addition to the enone group. Experimentally, no products resulting from prior reduction of the enone are observed. Yields of cyclized products are usually satisfactory.4... 

Mercury(II) salts react with alkynes in two different ways. As discnssed in Section 2.2, terminal alkynes react with mercnry nnder basic conditions to yield species with general formula Hg(C=CR)2 or Hg(X)(C=CR). Alternatively, under neutral or acidic conditions, alkynes undergo solvomercuration reactions to yield anti addition products (for example, see equation 17). ... 

Amides.1 The solvomercuration-demercuration of terminal olefins or cyclic olefins with acetonitrile and mercury 11) nitrate followed by reduction of the intermediate organomercury compound affords amides, hydrolyzable to amines. Neither mercuric acetate nor mercuric trifiuoroacetate is satisfactory in this reaction. Preliminary attempts to use a tertiary olefin failed. 

Cyclopentyl radicals substituted in the /1-position relative to the radical center are formed during the solvomercuration/reductive alkylation reaction of cyclopentene34. The organomer-curial produced in the first solvomercuration step is reduced by sodium borohydride and yields free cyclopentyl radicals in a radical chain mechanism. Addition of alkenes can then occur tram or cis to the / -alkoxy substituent introduced during the solvomercuration step. The adduct radical is finally trapped by hydrogen transfer from mercury hydrides to yield the tram- and ris-addition products, The transicis ratio depends markedly on the alkene employed and it appears that the addition of less reactive alkenes occurs with higher trans selectivity. In reactions of highly substituted alkenes, this reactivity control is compensated for by steric effects. Therefore, only the fnms-addition product is observed in reactions of tetraethyl ethenetetracarboxylate. The choice of alcohol employed in the solvomercuration step has, however, only a small influence on the stereoselectivity. 

Exclusive axial attack on bridged cyclohexyl radicals is observed in the solvomercuration/re-duction reaction of carvone57. The solvomercuration step leads to the bridged mercury(II) acetate precursor, which is reduced with sodium borohydride to yield the bicyclic ketone with equatorial /Tmethyl substituent. 

The oxjnnercuration reaction is thought to be a two-step process. In the first step, electrophilic attachment of the mercury ion to the alkene produces a positively charged intermediate (equation 9.39). In the second step of oxy-mercuration, a nucleophile (most likely a solvent molecule, SOH) reacts with the intermediate to produce the organomercury compoimd (equation 9.40). For reactions in water, both the organomercurial and the final product are alcohols. The reaction produces an ether if the hydroxylic solvent is an alcohol, and the reaction is called solvomercuration or alkoxymercuration. Better yields are obtained if the anion of the mercuric salt is a weaker nucleophile than is the solvent. For this reason, mercuric... 

In contrast, the mercurial enones (89) (obtained by specific solvomercuration of the corresponding pendant terminal olefins) are isolated, and on treatment with sodium trimethoxyborohydride are reductively cyclized to (90). Alkynes... 

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