Alkene 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)... 

The solvomercuration of alkenes has been exhaustively studied and is now well understood. The mechanism of this reaction proceeds through two steps coordination of the mercuric ion to the alkene to form a fluxional cationic intermediate and subsequent nucleophilic anti-attack. Charge stabilization of the cationic intermediate constitutes one of the factors dictating the regioselectivity of the reaction (Scheme 11). 

Table 2 Selected examples of intramolecular solvomercuration of alkenes... 

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. 

Whereas mercuration-demercuration of alkenes in the presence of water gives alcohols, in alcohol solvents (free from H,0) ethers result. These reactions in the presence of nucleophilic solvents such as water and alcohols are examples of solvomercuration. The mercuric salts usually used are the acetate, Hg(OAc), (—OAc is an abbreviation for —OCCH,) or the trifluoracetate. Hg(OCOCF,),. ... 

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] ... 

Chiral solvomercuration was accomplished by carrying out the reaction of alkenes with Hg(OAc)2 in the presence of chiral quaternary ammonium salts synthesized from natural ephedrine.642 Chiral secondary alcohols may be isolated with ee values up to 96%. Chiral nitrogen-containing diselenides are transformed by perox-odisulfate to selenium electrophiles, which may add to alkenes to form oxyseleny-lation products. These are, however, not isolated but oxidized to induce oxidative p-hydride elimination to afford chiral allyl methyl ethers with ee values up to 75%.643... 

Intramolecular geometries, database studies, 1, 597 Intramolecular hydroalkoxylation, and etherification, 10, 672 Intramolecular pinacol coupling to cyclic 1,2-diols, 11, 51 with samarium reagents, 11, 60 Intramolecular silylformylation, alkynes and alkenes, 11, 489 Intramolecular solvomercuration alkenes, 2, 436 alkynes, 2, 439... 

Solvomercuration of Alkenes Hydration of C=C Double Bonds through Subsequent Reduction... 

Fig. 3.48. Hydration of a symmetric alkene through solvomercuration/reduction.

Fig. 3.49. Regioselective hydration of an unsymmetric alkene via solvomercuration/ reduction. The regioselectivity of the solvomercuration/reduction seguence is complementary to that of hydrobora-tion/oxidation/hydrolysis.

Besides H20, simple alcohols or acetic acid can also be added to alkenes by solvomercuration/reduction. Figure 3.50 shows MeOH addition as an example. The regioselectivities of this reaction and of the H20 addition in Figure 3.49 are identical. 

Fig. 3.50. Regioselective methanol addition to an asymmetric alkene via solvomercuration/reduction.

In order to avoid complications arising from solvomercuration of sensitive alkene substrates, care must be taken to ensure that one equivalent of mer-... 

Deoxy sugars." Branched 2-deoxy sugars can be prepared by solvomercuration of a glycal followed by the Giese reaction with an electrophilic alkene, with marked preference for formation of an equatorial substituent. 

The solvomercuration of alkenes has an important synthetic valne since it allows the asymmetric functionalization of olefins. Although a large number of organomercurials have been isolated from these reactions, more commonly, they are reduced with alkaline NaBELi to yield the corresponding metal-free organic species (see Section 4.1). In this way, olefins can be converted into a wide range of organic species such as alcohols, ethers, and amines. 

The importance of organomercurials in organic synthesis is due in large part to the valuable solvomercuration-demercuration reaction. As the reaction of proton acids with solvomercurated products derived from alkenes leads back to the alkene, protonolysis has in this procedure little synthetic utility. 

However, the protonolysis of alkenylmercurials derived from the solvomercuration of alkynes is known as a method for preparing stereoisomerically pure alkenes. Acidic deuterolysis provides also a convenient route to labelled organic substrates. For instance, in a study on the regio- and stereo-selectivity of the acetoxymercuration of alkynes, protodemercuration with acetic acid of a 3 1 mixture of the vi-... 

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. 

Radicals of the norbornyl (bicyclo[2.2. l]hcptyl) type also show high facial selectivity in addition reactions to alkenes 34,54. In solvomercuration/rcductivc alkylation reactions of norbornene34 or 2-norbornanone hydrazones54 it is found that 2-norbornyl radicals substituted at the radical center- 4 or in the / -position34 yield only e.vo-addition products. The addition of alkenes, therefore, proceeds more selectively than hydrogen, or other atom abstraction reactions, in which small amounts of endo attack are also found (see Section D.2.2.). 

Dihydropyran can be solvomercurated in methanol solution and subsequently reacted with alkenes and a reducing agent34. The intermediate tetrahydro-2-methoxypyran-3-yl radicals add to electron-deficient alkenes with similar selectivity as the 2-methoxycyclohexyl radical. The ring-oxygen atom in the y-position to the radical center seems to have only a small influence on the selectivity34. 

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