Bonds carbon-mercury

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. 

Voltammetric peaks, especially those of UDP and UTP, were sharper than those of the third compound, as they were adsorbed at the electrode surface. Coulometric studies (at — 1.0 V, electroreduction) at mercury pool electrode (also glassy carbon) and analysis of the obtained products have shown that for all three compounds, the two-electron reductive cleavage of the carbon-mercury bond occurs. The electrode... 

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. 

Organic mercury compounds, especially phenyl and methoxyethyl mercury may also be biotransformed into inorganic mercury by cleavage of the carbon-mercury bond. Although such compounds are more readily absorbed than inorganic mercury compounds, the toxicity is similar. 

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. 

At C-2 position, carbohydrate radicals were formed starting from unsaturated sugars by acetoxymercuration followed by reduction of the carbon-mercury bond... 

Kreevoy et al., 1966a). A comparison with the values in Table 7 suggests some progress away from covalent carbon-mercury bonds in the transition states, but certainly does not permit any formulation which requires... 

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. 

By comparison of the n-alkylmercurial results alone, it is possible to generalize that both higher ozonation temperatures and the presence of a halogen ligand on mercury promote carbon-carbon cleavage during the ozonolysis of the carbon-mercury bond. 

The need for further investigation into the dialkylmercurial intermediates is also indicated. Rough rate profiles for these reactions suggest that the majority of the initial organic product from cleavage of the first carbon-mercury bond was a carbonyl function. 

Reaction Stoichiometry. The results of the stoichiometry study (see Table II) were varied but did suggest a 1 1 ozone mercurial ratio for the cleavage of a carbon-mercury bond. Several problems were evident in such a study. Since mostly only the highest oxidation states for carbon were observed in the products of partial ozonation of 1°, 2°, and 3° organomercurials, it could be assumed that ozonolysis of the C—Hg bond involved the slowest step in the total reaction sequence. Hence, some... 

Jensen and Rickbom (2) discussed the possibility of transition states in which the electrophilic species was partially bound to the o- orbital of carbon-mercury bonds. Thus the fourth "center of the four-center bromination mechanism was pictured as the carbon-mercury bond itself, or more specifically, the carbon sps orbital (2). 

Mechanism. In consideration of a possible mechanism for cleavage of the carbon-mercury bond by ozone, it should be possible to rule out the SE1 mechanism (Reaction 3) altogether. This mechanism clearly demands the formation of a carbanion in the rate-determining step. Likewise, the SEi mechanism for electrophilic cleavage seems improbable... 

Such a five-membered cyclic transition state has been proposed for the triiodide cleavage of carbon-mercury bonds (2). While such a structure is unlikely for the linear I3 ion, it should easily accommodate the ozone molecule whose bonding angle is 116° (12). Of course the geometrical plausibility of Structure 27 depends on the degree of carbon-mercury... 

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. 

Perfluoroalkyl anions, which form carbenes upon subsequent elimination of a-fluorine, may be generated by cleavage of the carbon-tin and carbon-mercury bonds in, for example, (trifluoromethyl)trimethyltin [56] and phenyl(trifluoromethyl)mercury [57] (Figure 6.40) under very mild conditions. Carbenes may be generated from... 

This approach is especially useful for the synthesis of organopalladium and -platinum compounds. These oxidation-reduction reactions proceed via initial insertion of the transition metal into the carbon-mercury bond and subsequent loss of mercury ... 

Insertion into the carbon-mercury bond proceeds with retention of the carbon stereochemistry - . 

One further reaction of this type studied by Kreevoy and his group [50] is the acid catalysed cleavage of unsaturated compounds containing a carbon—mercury bond, for example, allyl mercuric iodide (29). The important conclusion is that rate-determining proton transfer to unsaturated carbon occurs as a first step (A—SE2 mechanism) and as expected the reactions are catalysed by general acids. 

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. 

An organometallic compound is a compound that contains a bond between a carbon atom and a metal atom. Many such compounds are known, and organometallic chemistry is a very large area, occupying a borderline region between organic and inorganic chemistry. Many carbon-metal bonds (e.g., carbon-mercury bonds)... 

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