Mercury salts oxymercuration reactions

Monoalkylthallium(III) compounds can be prepared easily and rapidly by treatment of olefins with thallium(III) salts, i.e., oxythallation (66). In marked contrast to the analogous oxymercuration reaction (66), however, where treatment of olefins with mercury(II) salts results in formation of stable organomercurials, the monoalkylthallium(III) derivatives obtained from oxythallation are in the vast majority of cases spontaneously unstable, and cannot be isolated under the reaction conditions employed. Oxythallation adducts have been isolated on a number of occasions (61, 71,104,128), but the predominant reaction pathway which has been observed in oxythallation reactions is initial formation of an alkylthallium(III) derivative and subsequent rapid decomposition of this intermediate to give products derived by oxidation of the organic substrate and simultaneous reduction of the thallium from thallium(III) to thallium(I). The ease and rapidity with which these reactions occur have stimulated interest not only in the preparation and properties of monoalkylthallium(III) derivatives, but in the mechanism and stereochemistry of oxythallation, and in the development of specific synthetic organic transformations based on oxidation of unsaturated systems by thallium(III) salts. 

There are several useful means for preparation of organomercury compounds. The general metal-metal exchange reaction between mercury(II) salts and organolithium or magnesium compounds is applicable. The oxymercuration reaction discussed in Section... 

The most common oxymercuration reaction involves alkene, an alcohol such as methanol, and a mercury salt such as the acetate. The intermediate (alkoxy mercuri-acetate) is easily reduced by alkaline NaBH4 to the alkoxy derivative (Scheme 11). When applied to methyl oleate, this reaction gives a mixture of methyl 9- and 10-methoxystearates in high yield. 

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... 

It has therefore been established170 from the product distributions that, while the oxymercuration is reversible, unless a base (e.g. sodium acetate) is added to the reaction medium, and gives almost exclusively the more stable compound 199, the aminomercu-ration takes place to give the kinetically controlled adduct 200, or under thermodynamic control the aminomercurial 201. Reactions are kinetically controlled when the mercurating species is a mercury(II) salt deriving from a weak acid such as mercury(II) acetate. Conversely, they are thermodynamically controlled with the covalent mercury(II) chloride. In the latter case, the presence of a strong acid in the medium allows the thermodynamically controlled product to be obtained. 

Oxymercuration,3 Commercial mercury(II) acetate is the usual reagent for this reaction, although higher yields and greater selectivity obtain with the more expensive mercury(II) trifluoroacetate. A variety of mercury(II) salts can be prepared by sonication of a mixture of HgO with a carboxylic acid (2 equiv.) in solvents such as CH2C12, hexane, and aqueous THF. They can also be prepared in situ by oxymercuration, in which case only 1 equiv. of the acid is required since 1 equiv. of the acid is formed on oxymercuration. And this oxymercuration can result in increased selectivity in monooxymercuration of a diunsaturated substrate such as limonene (1). 

The ratio of the two products is primarily affected by the nature of the mercury(II) salt and also by the reaction conditions. Since the formation of these compounds could result from either a kinetically or a thermodynamically controlled mercuration process, a study of the mercuration of 3 in the presence of aromatic amines using various mer-cury(II) salts has been more recently carried out in order to determine the conditions under which aminomercuration is reversible, and the results have been compared to those of the oxymercuration . 

The best characterized of these reactions involve the mercuric ion, Hg, as the cation. The usual nucleophile is the solvent. The term oxymercuration is used to refer to reactions in which water or an alcohol acts as the nucleophile. The adducts can be isolated as halide salts, but in synthetic applications the mercury is often replaced by hydrogen (oxymercuration reduction see below). 

The problems that follow explore various synthetic aspects of oxymercuration-demercuration. Experimental procedures sometimes vary depending on the particular transformation. The source of the electrophile may be a mercury(II) salt other than Hg(OAc)2, the nucleophile may be other than H2O, and the reaction may be intramolecular rather than intermolecular. 

The proposed mechanism of the Ferrier carbocyclization reaction is oudined in Scheme 12.13. First, oxymercuration of the exo-olefin in 48 affords mercurial-hemiacetal 49, whose aglycon moiety (-OMe) eliminates to give mercurial-aldehyde derivative 50. This mercurial intermediate 50 was isolable when a stoichiometric amount of Hg salt was employed at low temperature. Intramolecular aldol-type cyclization of 50 provides product 51. 

The last example in Table 12-2 is an electrophilic addition of a mercuric salt to an alkene. The reaction is called mercuration, and the resulting compound is an alkyl-mercury derivative, from which the mercury can be removed in a subsequent step. One particularly useful reaction sequence is oxymercuration-demercuration, in which mercuric acetate acts as the reagent. In the first step (oxymercuration), treatment of an alkene with this species in the presence of water leads to the corresponding addition product. 

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