Mercurinium ions

A mercurinium ion has both similarities and differences as compared with the intermediates that have been described for other electrophilic additions. The proton that initiates acid-catalyzed addition processes is a hard acid and has no imshared electrons. It can form either a carbocation or a hydrogen-bridged cation. Either species is electron-deficient and highly reactive. 

Reactions of alkynes with electrophiles are generally similar to those of alkenes. Because the HOMO of alkynes (acetylenes) is also of n type, it is not surprising that there IS a good deal of similarity between alkenes and alkynes in their reactivity toward electrophilic reagents. The fundamental questions about additions to alkynes include the following. How reactive are alkynes in comparison with alkenes What is the stereochemistry of additions to alkynes And what is the regiochemistry of additions to alkynes The important role of halonium ions and mercurinium ions in addition reactions of alkenes raises the question of whether similar species can be involved with alkynes, where the ring would have to include a double bond ... 

Figure 7.3 Mechanism of the oxymercuration of an alkene to yield an alcohol. The reaction involves a mercurinium ion intermediate and proceeds by a mechanism similar to that of halohydrin formation. The product of the reaction is the more highly substituted alcohol, corresponding to Markovnikov regiochemistry.

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... 

The stereochemistry of oxymercuration has been examined in a number of systems. Conformationally biased cyclic alkenes such as 4-r-butylcyclohexene and 4-f-butyl-l-methycyclohexene give exclusively the product of anti addition, which is consistent with a mercurinium ion intermediate.17,22... 

Scheme 4.1 includes examples of oxymercuration reactions. Entries 1 and 2 illustrate the Markovnikov orientation under typical reaction conditions. The high exo selectivity in Entry 3 is consistent with steric approach control on a weakly bridged (or open) mercurinium ion. There is no rearrangement, indicating that the intermediate is a localized cation. 

Entries 4 and 5 involve formation of ethers using alcohols as solvents, whereas the reaction in Entry 6 forms an amide in acetonitrile. Entries 7 and 8 show use of other nucleophiles to capture the mercurinium ion. 

The most synthetically valuable method for converting alkynes to ketones is by mercuric ion-catalyzed hydration. Terminal alkynes give methyl ketones, in accordance with the Markovnikov rule. Internal alkynes give mixtures of ketones unless some structural feature promotes regioselectivity. Reactions with Hg(OAc)2 in other nucleophilic solvents such as acetic acid or methanol proceed to (3-acetoxy- or (3-methoxyalkenylmercury intermediates,152 which can be reduced or solvolyzed to ketones. The regiochemistry is indicative of a mercurinium ion intermediate that is opened by nucleophilic attack at the more positive carbon, that is, the additions follow the Markovnikov rule. Scheme 4.8 gives some examples of alkyne hydration reactions. 

Hg(OAc)2 easily reacts with allene to yield methoxymercuration products, i.e. vinylmercury compound 41, with the trans isomer being the major product. The chirality of allene was transferred into the final products, indicating the intermediacy of a cr-bridged mercurinium ion [23-26], The stereoselectivity of this reaction was determined by the relative stability of intermediates 40 and 42 and steric hindrance for the incoming methoxy group. 

On the basis of the mechanistic picture of oxymercuration involving a mercurinium ion, predict the structure and stereochemistry of the major alcohols to be expected by application of the oxymercuration-demercuration sequence to each of the following substituted cyclohexenes. 

Application of the oxymercuration-demercuration reaction176 to alkyl 3,4-dideoxy-a-DL-hex-3-enopyranosides provides177 easy access to alkyl 3-deoxyhexopyranosides (for example, 288). Interestingly, both stereoisomeric forms of the alkene are apparently attacked by mercuric acetate from the same side. It has been assumed177 that the transient, mercurinium ion 287 is stabilized by bonding to the 1-meth-oxyl group. 

Mechanism. The reaction is analogous to the addition of bromine molecules to an alkene. The electrophilic mercury of mercuric acetate adds to the double bond, and forms a cyclic mercurinium ion intermediate rather than a planer carbocation. In the next step, water attacks the most substituted carbon of the mercurinium ion to yield the addition product. The hydroxymercurial compound is reduced in situ using NaBH4 to give alcohol. The removal of Hg(OAc) in the second step is called demer-curation. Therefore, the reaction is also known as oxymercuration-demercuration. 

Mechanism. Addition of HgS04 generates a cyclic mercurinium ion, which is attacked by a nucleophilic water molecule on the more substituted carbon. Oxygen loses a proton to form a mercuric enol, which under work-up produces enol (vinyl alcohol). The enol is rapidly converted to 2- butanone. 

The anti addition and the lack of rearrangements are compatible with a mechanism with the involvement of the cyclic mercurinium ion.493,495 Mercurinium ions are known to exist, for example, in superacidic media.498... 

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. 

Stable mercurinium ions, often postulated as intermediates in solvomercuration reactions, have been observed for the first time.170 They may be prepared in media of low nucleophilicity either by direct mercuration or by ionization of a p-substituted organomercurial. For example, the ethylene mercurinium ion may be prepared from 2-methoxyethylmercuric chloride in an ionizing medium such as HS03F-SbF5-S02... 

Other ions studied include the cyclohexene- and norbomylene-mercurinium ions. N.m.r. studies show deshielding of both protons and carbons, indicating charge transfer from mercury to the carbon skeleton both mercury-proton and mercury-carbon spin-spin couplings are observed. Some of the spectra are best interpreted in terms of an equilibrium between the mercurinium ion and the free olefin. Quenching experiments are consistent with the presence of mercurinium ions. 

Mercury(II) salts add to C=C double bonds (Figure 3.48) in nucleophilic solvents via the the onium mechanism of Figure 3.42. However, the heterocyclic primary product is not called an onium, but rather a mercurinium ion. Its ring opening in an H20-containing solvent gives a... 

The preparation involves an oxymercuration (Section 3.5.3) of the C=C double bond of the ethyl vinyl ether. The Hg(OAc) ion is the electrophile as expected, but it forms an open-chain cation A as an intermediate rather than a cyclic mercurinium ion. The open-chain cation A is more stable than the mercurinium ion because it can be stabilized by way of oxocarbe-nium ion resonance. Next, cation A reacts with the allyl alcohol, and a protonated mixed acetal B is formed. Compound B eliminates EtOH and Hg(OAc) in an El process, and the desired enol ether D results. The enol ether D is in equilibrium with the substrate alcohol and ethyl vinyl ether. The equilibrium constant is about 1. However, the use of a large excess of the ethyl vinyl ether shifts the equilibrium to the side of the enol ether D so that the latter can be isolated in high yield. 

Figure 11.5 shows a mechanism that has been postulated for this reaction. First, an electrophilic mercury species adds to the double bond to form a cyclic mercurinium ion. Note how similar this mechanism is, including its stereochemistry and regiochemistry, to that shown in Figure 11.4 for the formation of a halohydrin. The initial product results from anti addition of Fig and OH to the double bond. In the second step, sodium borohydride replaces the mercury with a hydrogen with random stereochemistry. (The mechanism for this step is complex and not important to us at this time.) The overall result is the addition of H and OH with Markovnikov orientation. 

Some additional examples are provided by the following equations. Note the excellent yields in all of the examples. Also note that the last example proceeds without rearrangement because the intermediate is a mercurinium ion rather than a carbocation. Attempts to prepare this alcohol by acid-catalyzed addition of water result in completely rearranged product. 

The mercury electrophile adds to the double bond in a process very similar to the formation of a bromonium ion. A species called a mercurinium ion is formed. 

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