Alkenes reaction with mercury ions

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. 

Enol lactones with a halogen at the vinylic position have been synthesized as potential mechanism-based inactivators of serine hydrolyases <81JA5459). 5-Hexynoic acids (181) can be cyclized with mercury(II) ion catalysis to y-methylenebutyrolactones (182) (Scheme 41). Cyclization of the 6-bromo and 6-chloro analogues leads stereospecifically to the (Z)-haloenol lactones (trans addition) but is quite slow. Cyclization of unsubstituted or 6-methyl or 6-trimethylsilyl substituted 5-hexynoic acids is more rapid but alkene isomerization occurs during the reaction. Direct halolactonization of the 5-hexynoic acids with bromine or iodine in a two-phase system with phase transfer catalysis was successful in the preparation of various 5-halomethylene- or 5-haloethylidene-2-phenylbutyrolactones and 6-bromo-and iodo-methylenevalerolactones (Scheme 42). 

On the basis of theoretical and experimental results a symmetrical mercurinium ion, with most of the positive charge on mercury, has therefore been proposed in reactions of symmetrically substituted alkenes , while asymmetrical mercurinium ions or weakly bridged mercury-substituted carbocations have been proposed when there is a substituent, such as an aryl group, on the double bond -. Finally, with substituents highly capable of stabilizing carbocations, fully open intermediates have been proposed . ... 

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... 

Figure 8.3 Mechanism of the oxymercuration of an alkene to yield an alcohol. (Q) Electrophilic addition of Hg2+ gives a mercurinium ion, which (Q) reacts with water as in halohydrin formation. Loss of a proton gives an organomercury product, and (Q) reaction with NaBH4 removes the mercury. The product of the reaction is the more highly substituted alcohol, corresponding to Markovnikov regiochemistry.

As a center of high electron density, the triple bond is readily attacked by electrophiles. This section describes the resnlts of three such processes addition of hydrogen halides, reaction with halogens, and hydration. The hydration is catalyzed by mercury(II) ions. As is the case in electrophilic additions to unsymmetrical alkenes (Section 12-3), the Markovnikov rule is followed in transformations of terminal alkynes The electrophile adds to the terminal (less snbstituted) carbon atom. 

An alternative method of Markovnikov addition of water to alkenes involves an intermediate rather similar to the bromonium ion. This is a mercurinium ion, formed by the reaction of an alkene with mercury (II) acetate (check out the sizes of mercury and bromine, and you will see why these are so similar). This is often preferred to acid-catalyzed hydration, as conditions... 

Therefore mercury(II) acetate interacts as an electrophilic transition metal with the nucleophilic alkene to form the three-membered ring 52. This mercurinium ion is opened by relatively feeble nucleophiles like alcohols - or in this reaction water. Similar to a hydroboration the attack happened at the more substituted end of the mercurinium ion according to Markovnikov s rule. To get rid of the metal, solid potassium iodide is added. This means insoluble mercury(Il) iodide is formed, followed by loss of the methoxy group and formation of enol ether 54, which subsequently tautomerizes to the desired aldehyde 55. 

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... 

The reaction mechanism of alkoxymercuration/demercuration of an alkene is similar to other electrophilic additions we have studied. First, the cyclopentene n electrons attack Hg2+ with formation of a mercurinium ion. Next, the nucleophilic alcohol displaces mercury. Markovnikov addition occurs because the carbon bearing the methyl group is better able to stabilize the partial positive charge arising from cleavage of the carbon-mercury bond. The ethoxyl and mercuric groups are trans to each other. Finally, removal of mercury by NaBH4 by a mechanism that is not fully understood results in the formation of 1-ethoxy-1-methylcyclopentane. 

The mechanism of the mercurydD-catalyzed alkyne hydration reactioi is analogous to the oxymercuration reaction of alkenes (Section 7.4). Elec trophilic addition of mercury(II) ion to the alkyne gives a vinylic cation which reacts with water and loses a proton to yield a mercury-containii enol intermediate. In contrast to alkene oxymercuration, no treatment widi NaBH4 is necessary to remove the mercury the acidic reaction conditions alone are sufficient to effect replacement of mercury by hydrogen (Figure 8.3). 

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. 

Where do mercuration reactions fit into this picture A mercurinium ion has both similarities and differences, as compared with the intermediates that have been described for other electrophilic additions. The electrophile in oxymercuration reactions, +HgX or Hg +, is a soft Lewis acid and polarizes the TT-electrons of an alkene to the extent that a three-center two-electron bond is formed between mercury and the two carbons of the double bond. However, there is also back bonding from Hg +(i orbitals to the alkene tt orbital. There is weaker bridging in the mercurinium ion than in the three-center four-electron bonding of the bromonium ion. 

In general, though, it is difficult to predict whether aqueous acid will hydrate the alkene or dehydrate the alcohol. The method we are about to show you is much more reliable. The key is to use a transition metal to help you out. Alkenes are soft nucleophiles (p. 357) and interact well with soft electrophiles such as transition metal cations. In the margin, for example, is the complex formed between an alkene and mercury(II) cation. The complex should remind you of a bromonium ion, and rightly so because its reactions are similar. Even relatively feeble nucleophiles such as water and alcohols, when used as the solvent, open the mercurinium ion and give alcohols and ethers. In the next scheme, the mercury(ll) is supplied as mercury(ll) acetate, Hg(OAc)2, which we shall represent with two covalent Hg—O bonds. Unsurprisingly, water attacks at the more substituted end of the positively charged mercurinium ion. 

The stereochemistry of the reaction of mercuric trifluoroacetate with cyclopro panes has been determined formation in the rate-determining step of corner-mercurated cyclopropanes, e.g. (34) from cM.-l,2,3-trimethylcyclopropane, is proposed. For other uses of mercuric trifluoroacetate and related compounds see discussion on formation of perfluoroalkyl derivatives of mercury and carbenes (see p. 193), and on preparation of polyfluoroarylmercuric trifluoroacetates (see p. 437). An X-ray determination of the crystal structure of mercuric trifluoroacetate has been carried out and a further report has appeared on the formation of the cyclohexyl-mercurinium ion [from addition of cyclohexene-SOs to (CF3 C02)2Hg-FS03H-SbFs-S02 at -60°C via mercuric ion attack on the w-system of the alkene] (see Vol. 2, p. 128). 

With unsymmetrically substituted alkenes, the two bonds to mercury will not be equally strong. (Recall our discussion of asymmetrical bromonium ions just a few pages ago.) The bond between mercury and the more substituted carbon will be longer and weaker than the bond to the less substituted carbon of the mercurinium ion. It is at the more substituted carbon that water opens the ring. So, the first product of this reaction is the mercury-containing alcohol shown in Figure 10.22. 

DI- and Tri-anions.—Frequently, the formation of a multi-ion is necessary in order that a specific site in a molecule can be rendered active. This is especially so where that site is less easily lithiated than others within the molecule. Such a case presents itself with the dianions (36), where reaction occurs at the more reactive carbanionic centre to give access to various useful p-hydroxy-sub-stituted compounds from conventional electrophiles. " The requisite dianions can be formed from a-chloro-alcohols and a-chloro-ketones " or alternatively by lithiation of the corresponding mercurial compounds (S ). Since the mercurial compounds can in turn be obtained from an alkene by addition of Hg(OAc)a-H2X, in excellent yield, the method provides a very versatile synthesis of p-hydroxy-compounds from alkenes. The same authors have used the new trianions (38), again generated from a mercury compound by lithium-mercury... 

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