Carbon mercury cleavage

Demercuration of organomercury compounds is a critical step in synthetic procedures, which involve mercuration-initiated cyclization reactions [e.g. 41], Many of the standard procedures for demercuration result in rearrangement or ring cleavage of the system, but reductive carbon-mercury cleavage (e.g. Scheme 11.4) with an excess of the quaternary ammonium borohydride is effective under phase-transfer conditions [e.g. 42,43]. 

Carbon—Carbon Cleavage. As mentioned above, carbon—carbon cleavage usually accompanied the normal carbon-mercury cleavage reaction. While this chain-shortening process was most prominent for the n-alkylmercuric halides which were ozonated at 10°C (Reactions I and 6, Table I), it was reduced to a much lower level when the di-n-alkylmercurials were ozonated at —76°C (Reactions 5 and 10, Table I). Actually even less carbon—carbon scission occurred during the ozonation of the di-s-alkylmercurials halides (Reaction 14, Table I) and during the partial ozonation of the lerl-alkylmercuric halides (not listed). 

A study of the intermediate products from the ozonolysis of iso-proplymercuric chloride shows that the rate of build-up of isopropyl alcohol adequately accounts for the rate of acetone formation. The reaction sequence therefore appears to be one of carbon-mercury cleavage, followed by oxidation by ozone. 

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... 

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. 

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... 

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... 

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). 

Reduction of the phosphorus-carbon bond is generally achieved with the aim of preparing new phosphorus compounds, and not with the object of preparing reduced carbon compounds. It is a striking difference from mercury-carbon bond cleavage, which is usually carried out in order to prepare carbon compounds. 

Thus, the toxicity of DMM is mediated by its dealkylation. Cleavage of the carbon-mercury bond generates MMM metabolites, which can form covalent bonds with cellular ligands with amphiphilic properties. The mercury center reacts with sulfur and sulfur-containing thiol groups of enzymes and thereby inhibits them. The metal center of DMM acts as a soft acid, and binds tightly to polarizable donor atoms in soft bases. Within cells, mercury may interact with a variety of proteins, particularly microsomal and mitochondrial enzymes. This can severely impair cell function. 

---