Mercuration, of alkenes

Halo-, carbo-, and nitro-mercurations of alkenes (and alkynes) have been previously described. However, these reactions are not as general and have not found as many applications in organic synthesis as the ones described in the previous two sections. Consequently, they will not be discussed any further in this review and the interested readers are directed to previous reviews and some recent references. ... 

The mercuration of alkenes in the presence of nitrogen-containing species such as amines, amides, nitriles, azides, or nitro groups has also been studied, although not as... 

Pd(II) compounds coordinate to alkenes to form rr-complexes. Roughly, a decrease in the electron density of alkenes by coordination to electrophilic Pd(II) permits attack by various nucleophiles on the coordinated alkenes. In contrast, electrophilic attack is commonly observed with uncomplexed alkenes. The attack of nucleophiles with concomitant formation of a carbon-palladium r-bond 1 is called the palladation of alkenes. This reaction is similar to the mercuration reaction. However, unlike the mercuration products, which are stable and isolable, the product 1 of the palladation is usually unstable and undergoes rapid decomposition. The palladation reaction is followed by two reactions. The elimination of H—Pd—Cl from 1 to form vinyl compounds 2 is one reaction path, resulting in nucleophilic substitution of the olefinic proton. When the displacement of the Pd in 1 with another nucleophile takes place, the nucleophilic addition of alkenes occurs to give 3. Depending on the reactants and conditions, either nucleophilic substitution of alkenes or nucleophilic addition to alkenes takes place. 

Thallation of aromatic compounds with thallium tris(trifluoroacetate) proceeds more easily than mercuration. Transmetallation of organothallium compounds with Pd(II) is used for synthetic purposes. The reaction of alkenes with arylthallium compounds in the presence of Pd(Il) salt gives styrene derivatives (433). The reaction can be made catalytic by use of CuCl7[393,394], The aryla-tion of methyl vinyl ketone was carried out with the arylthallium compound 434[395]. The /9-alkoxythallium compound 435, obtained by oxythallation of styrene, is converted into acetophenone by the treatment with PdCh[396]. 

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)... 

In 1978, Corey reported a general synthetic route for the conversion of alkenes to conjugated nitroalkenes via nitro-mercuration and demercuration.74 Since then, many chemists have used this method for the preparation of cyclic nitroalkenes such as 1-nitrocyclohexene. However, the use of mercury salts is not recommended even for the small-scale preparation of nitroalkenes. This reaction is not as clean as expected, and formidable efforts are required to remove the mercury in the waste. 

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). 

Mercuration usually occurs without rearrangement of the carbon skeleton and gives products arising from an almost complete Markonikov addition, with only a few exceptions. The product stereochemistry depends widely upon the structure of the alkene generally anti addition is obtained although mercuration of strained alkenes can occur by syn addition. 

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,),. ... 

Oxymercuration-reduction of alkenes preparation of alcohols Addition of water to alkenes by oxymercuration-reduction produces alcohols via Markovnikov addition. This addition is similar to the acid-catalysed addition of water. Oxymercuration is regiospecific and auft -stereospecific. In the addition reaction, Hg(OAc) bonds to the less substituted carbon, and the OH to the more substituted carbon of the double bond. For example, propene reacts with mercuric acetate in the presence of an aqueous THF to give a hydroxy-mercurial compound, followed by reduction with sodium borohydride (NaBH4) to yield 2-propanol. 

Addition of alcohol to alkenes hy alkoxymercuration-reduction produces ethers via Markovnikov addition. This addition is similar to the acid-catalysed addition of an alcohol. For example, propene reacts with mercuric acetate in aqueous THF, followed hy reduction with NaBFl4, to yield methyl propyl ether. The second step is known as demercuration, where Flg(OAc) is removed hy NaBH4. Therefore, this reaction is also called alkoxymercura-tion-demercuration. The reaction mechanism is exactly the same as the oxymercuration-reduction of alkenes. 

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] ... 

The reaction of alkenes or dienes, mercuric salts, and either nitriles or amides generates (3-amidomer-curials, which can be easily reduced to the corresponding amides (equation 174). This reaction effects H—NHCOR addition to alkenes and dienes, and has been thoroughly reviewed recently.242... 

Oxymercuration of alkenes is an electrophilic reaction involving the addition of mercuric salt and of a protic solvent (alcohols) according to reaction 9. UV/VIS spectrophotometric investigation of the olefin/mercuric salt reaction mixtures in methylene chloride provides evidence of the presence of an electron donor-acceptor complex between olefin and mercuric salt110 which is considered to be on the reaction pathway of the oxymercuration. 

Fig. Synthesis of an alcohol from an alkene using mercuric acetate. Alkenes to Ethers...

A similar reaction to the mercuric acetate/sodium borohydride synthesis of alcohols allows the conversion of alkenes to ethers. In this case, mercuric trifluoracetate is used ... 

Mercuric Ion-Catalyzed Hydration Alkynes undergo acid-catalyzed addition of water across the triple bond in the presence of mercuric ion as a catalyst. A mixture of mercuric sulfate in aqueous sulfuric acid is commonly used as the reagent. The hydration of alkynes is similar to the hydration of alkenes, and it also goes with Markovnikov orientation. The products are not the alcohols we might expect, however. 

Allylic esters result from the reaction of alkenes with itri-butyl per-oxyacetate [233], or the more reactive peroxybenzoate [233, 234, 1113], in the presence of cuprous bromide with mercuric acetate [404, or with lead tetraacetate [1114] (equation 127). 

Mechanism of the o] mercuration of an alkene to yield an alcohol. TTiis electrophilic addition reaction involves a mercurinium ion intermediate, and its mechanism is similar to that of halohydrin formation. The product of the reaction is the more highly substituted alcohol, corresponding to Markovnikov regiochemistry. 

Mercuration of 1-alkenes in the presence of nucleophilic solvents such as alcohols, amines, and nitriles or in the presence of sodium azide provides convenient access to the corresponding ethers, amines, amides, " and azides. ... 

The mercuric ion-catalyzed hydration of alkynes probably proceeds in a similar manner to the oxymercuration of alkenes (see Section 5.1). Electrophilic addition of Hg to the triple bond leads to a vinylic cation, which is trapped by water to give an vinylic organomercury intermediate. Unlike the alkene oxymercuration, which requires reductive removal of the mercury by NaBH4, the vinylic mercury intermediate is cleaved under the acidic reaction conditions to give the enol, which tautomerizes to the ketone. Hydration of terminal alkynes follows the Mai kovnikov rule to furnish methyl ketones. ° ... 

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