Mercuric acetate reaction with alkenes

The structure of 71 indicates that the Hg is at the less hindered carbon atom, and experiments show that there is a carbocation intermediate. The reaction with mercuric acetate can be related to the reaction of HBr in that the more stable carbocation is formed as a new C-Hg or C-H bond is formed. It is known that alkenes react with an acid catalyst and water to produce an alcohol (see Section 10.3). Using this as an analogy, the OH unit in 71 is derived from the reaction of the nucleophilic water with an intermediate carbocation. 

The addition reactions discussed in Sections 4.1.1 and 4.1.2 are initiated by the interaction of a proton with the alkene. Electron density is drawn toward the proton and this causes nucleophilic attack on the double bond. The role of the electrophile can also be played by metal cations, and the mercuric ion is the electrophile in several synthetically valuable procedures.13 The most commonly used reagent is mercuric acetate, but the trifluoroacetate, trifluoromethanesulfonate, or nitrate salts are more reactive and preferable in some applications. A general mechanism depicts a mercurinium ion as an intermediate.14 Such species can be detected by physical measurements when alkenes react with mercuric ions in nonnucleophilic solvents.15 The cation may be predominantly bridged or open, depending on the structure of the particular alkene. The addition is completed by attack of a nucleophile at the more-substituted carbon. The nucleophilic capture is usually the rate- and product-controlling step.13,16... 

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. 

Sometimes the reaction conditions used in this reaction are too harsh since heating is involved and rearrangement reactions can occur. A milder method that gives better results is to treat the alkene with mercuric acetate [Hg(OAc)2] then sodium borohydride. The reaction involves electrophilic addition of the mercury reagent to form an intermediate mercuronium ion. This reacts with water to give an organomercury intermediate. Reduction with sodium... 

In addition to the hydration reaction described in Section 11.3, the oxymercuration-reduction reaction can be used to add the elements of water to a carbon-carbon double bond in a two-step process. First the alkene is reacted with mercuric acetate, Hg(02CCH3)2, in water, followed by treatment with sodium borohydride in sodium hydroxide solution ... 

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). 

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. 

In oxymercuration, the alkene is treated with mercuric acetate in aqueous tetrahy-drofuran (THF). When reaction with that reagent is complete, sodium borohydride is added to the reaction mixture. (The numbers 1 and 2 in front of the reagents above and below the arrow in the chemical equation indicate two sequential reactions the second reagent is not added until reaction with the first reagent is completely over.)... 

Cycloaddition approaches to pyrazole derivatives were employed extensively in 1997. Thus, aldhydrazones combine with mercuric acetate to form nitrileimines which react in situ with olefins to give 1,3,5-trisubstituted 2-pyrazolines <97SC3737>. Oxidation of the latter to pyrazoles with iodobenzene diacetate was reported <97SC2683>. The most common cycloaddition pathway, however, is from the reaction of alkenes or alkynes with diazo compounds. The facial selectivity of the regiospecific reactions of diazomethane (and other... 

One of the steps in the indirect hydration of alkenes (Section 16.8) is the electrophilic addition of mercuric acetate, Hg(OAc)j —a covalent compound—according to the following equation. What is the electrophile Predict the structure of the product of the reaction of mercuric acetate with 2-methyl-l-propene,... 

We saw in Section 7.4 that alkenes react with water in the presence of mercuric acetate to yield a hvdroxymercuration product. Subsequent treatment with NaBH4 breaks the C-Hg bond and yields the alcohol. A similar alkoxymercuration reaction occurs when an alkenc is treated with an alcohol in the presence of mercuric acetate or, even better, mercuric trifluoroacetate, (Cl CCtitiHg. Demercura-tion by reaction with NaBH4 then yields an ether. The net result is Markovnikov addition of the alcohol to the alkene. 

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.)... 

We have seen that alkenes react with alcohols in the presence of an acid catalyst to form ethers (Section 4.5). Just as the addition of water works better in the presence of mercuric acetate than in the presence of a strong acid, the addition of an alcohol works better in the presence of mercuric acetate. [Mercuric trifluoroacetate, Hg(02CCp3)2, works even better.] This reaction is called alkoxymercuration-reduction. 

Mercuric salts induce oxidative rearrangement of cyclic alkenes and give cycloalkane carboxaldehydes. Cyclohexene, for example, gave cyclopentane carboxaldehyde (404) in 53% yield. Acyclic alkenes such as 2-butene can also be oxidized with mercuric sulfate and sulfuric acid, giving 2-butanone in this case.566 Thallium nitrate [T1(N03)3] is an important reagent for the oxidative rearrangement of cyclic alkenes.567 The reaction is not restricted to carbocyclic compounds, but can also be applied to heterocyclic compounds as shown by the oxidation of 3,4-dihydro-277-pyran (405) to the dimethyl acetal (406) in 65% yield.568... 

In the oxymercuration process, the electrophilic addition of the mercuric species occurs resulting in a mercurinium ion which is a three-membered ring. This is followed by the nucleophilic attack of water and as the proton leaves, an organomercuric alcohol (addition product) is formed. The next step, demercuration, occurs when sodium borohydride (NaBH ) substitutes the mercuric acetate substituent with hydrogen. If an alkene is unsymmetric, Oxymercuration-demercuration results in Markovnikov addition. The addition of mercuric species and water follows an anti (opposite side) addition pattern. This reaction has good yield, since there is no possibility of rearrangement unlike acid-catalyzed hydration of alkenes. 

The reaction of an alkene with an alcohol in the presence of a mercury salt such as mercuric acetate or trifluoroacetate leads to an alkoxymercury intermediate, which on reaction with sodium borohydride yields an ether. 

Mercuric salts (HgXg) are Lewis acids that react readily with alkenes, and experiments show that the reaction proceeds by formation of a carboca-tion intermediate. When mercuric acetate—HgCOAcla see the structure and Chapter 20, Section 20.5—reacts with 3-methyl-l-hexene in a THF-water mixture, the isolated product is a mercuric compound identified as 71 (with a C-Hg bond) and it is formed by reaction of water with a carbocation intermediate. The initial acid-base reaction is the donation of the 7i-electrons to form a new C-Hg covalent bond, leaving behind a carbocation at the other carbon of the C=C unit. 

Oxymercuration occurs with an alkyne as with an alkene, but differences in reactivity lead to a modification in the procedure. For reasons that will not be discussed, a mixture of mercuric sulfate (HgS04) and mercuric acetate [Hg(OAc)2] is used. When 1-heptyne is treated with this mixture in aqueous solvent, the initially formed enol (107) tautomerizes to 2-heptanone (108), which is isolated in 80% yield. Note that the ketone product mentioned in connection with vinyl chloride 92 in Section 10.4.5 results from formation of an enol. There is an important difference in the oxymercuration of alkynes and alkenes that is notable in this transformation. The mercury reacts with the alkyne, but the mercury is lost when the enol is formed and there is no need to add NaBH in a second step. This observation is general for oxymercuration of alkynes under these conditions. The more stable secondary vinyl carbocation is an intermediate, but the vinyl-mercury compound formed by reaction with the carbocation is unstable in the presence of water, so the enol is the product. 

Oxymercuration (Section 8.4) A method for double-bond hydration by reaction of an alkene with aqueous mercuric acetate followed by treatment with NaBH4. 

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. 

The isoflavone 406 is prepared by the indirect a-phenylation of a ketone by reaction of phenylmercury(II) chloride with the enol acetate 405, prepared from 4-chromanone[371]. A simple synthesis of pterocarpin (409) has been achieved based on the oxypalladation of the oriho-mercurated phenol derivative 408 with the cyclic alkene 407[372,373]. 

The iodination reaction can also be conducted with iodine monochloride in the presence of sodium acetate (240) or iodine in the presence of water or methanolic sodium acetate (241). Under these mild conditions functionalized alkenes can be transformed into the corresponding iodides. AppHcation of B-alkyl-9-BBN derivatives in the chlorination and dark bromination reactions allows better utilization of alkyl groups (235,242). An indirect stereoselective procedure for the conversion of alkynes into (H)-1-ha1o-1-alkenes is based on the mercuration reaction of boronic acids followed by in situ bromination or iodination of the intermediate mercuric salts (243). 

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