Organomercury-oxygen compounds

The methylmercury(II) trifluoroacetate 1-methylpyridine (a-picoline) adduct self-assembles into dimers via mercury-oxygen bonds, to form an unsymmetric four-membered ring, 18 (Hg- - -O 2.668 and 2.805 A). These bonds are shorter than the van der Waals distance (3.0 A).The methylmercury-picoline cation is nearly linear (C-Hg-N 169.5°) [88]. 

Phenylmercury quinaldinate forms dimers, 19, with loose Hg- - -O bridges (Hg O 2.16 A Hg- - -0 2.79 A in the fom-membered ring) and phenylmercury oxinate forms infinite helical arrays (Hg- - -O 3.33 and 3.37 A) [92]. 

A different mode of self-assembly occurs in the 7t-complex of hexamethylbenzene with mercury(II) trifluoroacetate, 20. In this compound four trifluoroacetato 

Phenylene-bis(trifluoroacetato)dimercury, l,2-C6H4(HgOOCCF3)2, forms chains of stacked molecular tectons associated by Hg- -O bonds (2.844 and 2.878 A) further interconnected by Hg- - -O 3.057 A [97]. 

4 Supramolecular Self-Assembly by Formation of Secondary Bonds 

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The reactivity of mercury salts is a fimction of both the solvent and the counterion in the mercury salt. Mercuric chloride, for example, is unreactive, and mercuric acetate is usually used. When higher reactivity is required, salts of electronegatively substituted carboxylic acids such as mercuric trifiuoroacetate can be used. Mercuric nitrate and mercuric perchlorate are also highly reactive. Soft anions reduce the reactivity of the Hg " son by coordination, which reduces the electrophilicity of the cation. The harder oxygen anions leave the mercuric ion in a more reactive state. Organomercury compounds have a number of valuable synthetic applications, and these will be discussed in Chapter 8 of Part B. 

The R-Hg bond is chemically stable and is not split by water or weak acids or bases. This is a reflection of the low affinity of Hg for oxygen. It can, however, be readily broken biochemically. Organomercury, like other organometallic compounds, has a strong affinity for SH-groups of proteins and peptides. 

As illustrated in Section 4.1.1, the addition of nonstabihzed carbenes to the oxygen atom of a carbonyl derivative can lead to the production of carbonyl yhdes. However, these methods are not always practical for preparative scale since many side reactions can accompany the decomposition of alkyl diazo and diazirine derivatives. Landgrebe and co-worker (8) extensively studied the thermal decomposition of organomercurials in the presence of carbonyl compounds for the preparative generation of carbonyl yhdes (Scheme 4.6). 

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). ... 

Organomercury compounds have only slight reactivity. They are also completely indifferent to atmospheric oxygen and to water under normal conditions and can thus be prepared and used under conditions that are not permissible for the more reactive organometallic compounds discussed above. 

Organomercury compounds found application in organic synthesis from an early date. They are generally stable to attack by atmospheric oxygen although secondary and tertiary alkyls, allyls and benzyls do need some protection. They also resist attack by water and alcohols under neutral conditions and show little or no reactivity towards carbon electrophiles such as carbonyl compounds. 

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