Mercury—carbon bonds reactions with

The occurrence of a hydrogen isotope effect in an electrophilic substitution will certainly render nugatory any attempt to relate the reactivity of the electrophile with the effects of substituents. Such a situation occurs in mercuration in which the large isotope effect = 6) has been attributed to the weakness of the carbon-mercury bond relative to the carbon-hydrogen bond. The following scheme has been formulated for the reaction, and the occurrence of the isotope effect indicates that the magnitudes of A j and are comparable ... 

The synthetic utility of the mercuration reaction derives from subsequent transformations of the arylmercury compounds. As indicated in Section 7.3.3, these compounds are only weakly nucleophilic, but the carbon-mercury bond is reactive to various electrophiles. They are particularly useful for synthesis of nitroso compounds. The nitroso group can be introduced by reaction with nitrosyl chloride73 or nitrosonium tetrafluoroborate74 as the electrophile. Arylmercury compounds are also useful in certain palladium-catalyzed reactions, as discussed in Section 8.2. 

Nesmeyanova and Perevalova in related studies involving diferrocenylmercury and palladium black obtained similar result (58). Their yields of biferrocenyl amounted to only 1 to 6%, however. It is postulated that the reactions proceed via homolytic scission of the carbon-mercury bond with the formation of ferrocenyl radicals. 

This photocyclization proceeds in a one pot reaction with photolytic cleavage of the intermediate carbone-mercury bond and subsequent hydrogen abstraction through a probable hypoiodite intermediate 6 [15 a]. With secondary alcohols (R = H), the reaction is less efficient than with primary ones (R = H), and mainly undergoes a retention of configuration at the anomeric carbon [15 b] by abstraction of the a-anomeric hydrogen. 

Acetyl hypofluorite also cleaves the carbon-mercury bond which provides an easy entry to many fluoroethers. Since the electrophilic fluorine attacks the electrons of the C—Hg bond, the reaction proceeds with a full retention of configuration. Several l-fluoro-2-methoxy derivatives were prepared from the corresponding olefins271, formally accomplishing the addition of the elements of MeOF across a double bond (equation 153)272. Such reactions were also used for the fluorination of very activated aromatic compounds (equation 154)273. 

Organomercurials react rapidly with a variety of electrophilic species, and a recent book by Jensen and Rickborn fully describes the scope of these reactions (I). Foremost among the electrophilic reagents are the halogens which cleave the carbon-mercury bond to produce alkylmercuric halides and/or alkyl halides, depending on the type of organo-mercurial involved. 

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. 

Drouin, Conia et al. have found that these reactions can be carried out under even milder conditions by cyclization of silyl enol ethers of alkynones. Treatment of (194), as an (E)-(Z) mixture, with HgCb in CH2CI2 in the presence of hexamethyldisilazene for 30 min at 30 C gives vinylmercurial (195 R = HgCl) in quantitative yield. Cleavage of the carbon-mercury bond can be carried out to give (195 R - H, D, CQzMe, Br or COMe). 

However, intramolecular nucleophilic participation by the conjugate base during protonolysis of a C—Hg bond is questionable. A study of the acidolysis of the carbon-mercury bond in unsymmetrical di-alkylmercurials rather suggests that the reaction proceeds via a three-center transition state.In any case, substantial kinetic and stereochemical evidence has led to the idea that reaction occurs by a concerted, front side attack with a transition state that involves a pentacoordinate carbon center. In some cases unimolecular mechanisms, SeI, also have been observed. 

The intramolecular version of the oxymercuration reaction affords cyclic ethers. Furthermore, treatment of organomercury compounds (R-HgX) with NaBH4 in DMF in the presence of O2 replaces the carbon-mercury bond by a carbon-oxygen bond and yields the corresponding alcohol (R-OH). ... 

Elementary uni- and bimolecular reactions will necessarily show first- and second-order kinetic behaviour, but the reverse is not necessarily true a first-order reaction may not be unimolecular and a second-order reaction may not be bimolecular. For example, we considered the decomposition of dibenzylmercury in Chapter 1, in which the mechanism could either be elementary, giving a mercury atom and a 1,2-diphenylethane molecule directly (reaction 2.13a), or the reaction could be complex, with a slow initial homolysis of a carbon-mercury bond, followed by rapid further reactions to give the products (reaction 2.13b). Similarly for the Cope rearrangement of diene 2 to diene 4, the reaction could be elementary, with a concerted cyclic movement of electrons (reaction 2.14a), or might involve a di-radical intermediate 3 which rapidly reacted further to give the observed product 4 (reaction 2.14b). Both these mechanisms would lead to first-order kinetics, so the establishment of first-order kinetic behaviour for both these reaction schemes does not establish the... 

Other Carbon-Heteroatom Bonds. Carbon-mercury bonds are readily formed by treating the metalated species with mercuric chloride (Reaction 21) (27). The resulting chloromercury derivative in one case is a useful intermediate in the preparation of 2-iododimethylaminomethyl-ferrocene (27). 

There have been several reports of transition metal-carborane complexes with covalent two-electron bonds. An early report indicates that carbon-mercury bonds were formed by the reaction of C-lithiocarboranes with mercuric halides (6, 7). 

The principle underlying the use of organomercury compounds for carbene generation (entry 6, Scheme 8.1) is again the a-elimination mechanism. The carbon-mercury bond is much more covalent than the C-Li bond, however, so that the merciuy systems are generally stable at room temperature and easily isolated. They then decompose to the carbene when heated in solution with an appropriate alkene. The decomposition appears to be a reversible unimolecular reaction, and... 

The behavior of Hg(CN)2 toward the dinuclear gold(I) amidinate complexes requires comment. In the case of the dinuclear gold(I) ylide, oxidation of the Au(I) to Au(II) resulted in the formation of a reduced mercury(O) product. Figure 1.19(a) [36]. In the mercury(II) cyanide reaction with the dinuclear gold(I) dithiophosphinate. Figure 1.19(b), the stability of the gold(I)-carbon bond compared... 

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