The toxicity of organomercury compounds[1][2] presents both dangers and benefits. Dimethylmercury in particular is notoriously toxic, but found use as an antifungal agent and insecticide. Merbromin and phenylmercuric borate are used as topical antiseptics, while thimerosal is safely used as a preservative for vaccines and antitoxins.[3]
Organomercury compounds are generated by many methods, including the direct reaction of hydrocarbons and mercury(II) salts. In this regard, organomercury chemistry more closely resembles organopalladium chemistry and contrasts with organocadmium compounds.
Electron-rich arenes, such as phenol, undergo mercuration upon treatment with Hg(O2CCH3)2. The one acetate group that remains on the mercury atom can be displaced by chloride:[4]
The Hg2+ center binds to alkenes, inducing the addition of hydroxide and alkoxide. For example, treatment of methyl acrylate with mercuric acetate in methanol gives an α--mercuri ester:[8]
This reaction is called the Hofmann–Sand reaction.[9]
Reaction of Hg(II) compounds with carbanion equivalents[edit]
A general synthetic route to organomercury compounds entails alkylation with Grignard reagents and organolithium compounds. Diethylmercury results from the reaction of mercury chloride with two equivalents of ethylmagnesium bromide, a conversion that would typically be conducted in diethyl ether solution.[10] The resulting (CH3CH2)2Hg is a dense liquid (2.466 g/cm3) that boils at 57 °C at 16 torr. The compound is slightly soluble in ethanol and soluble in ether.
Similarly, diphenylmercury (melting point 121–123 °C) can be prepared by reaction of mercury chloride and phenylmagnesium bromide. A related preparation entails formation of phenylsodium in the presence of mercury(II) salts.[11]
Hg(II) can be alkylated by treatment with diazonium salts in the presence of copper metal. In this way 2-chloromercuri-naphthalene has been prepared.[12]
Organomercury compounds are versatile synthetic intermediates due to the well controlled conditions under which they undergo cleavage of the Hg-C bonds. Diphenylmercury is a source of the phenyl radical in certain syntheses. Treatment with aluminium gives triphenyl aluminium:
3 Ph2Hg + 2 Al → (AlPh3)2 + 3 Hg
As indicated above, organomercury compounds react with halogens to give the corresponding organic halide.
Organomercurials are commonly used in transmetalation reactions with lanthanides and alkaline-earth metals.
Cross coupling of organomercurials with organic halides is catalyzed by palladium, which provides a method for C-C bond formation.
Usually of low selectivity, but if done in the presence of halides, selectivity increases. Carbonylation of lactones has been shown to employ Hg(II) reagents under palladium catalyzed conditions. (C-C bond formation and Cis ester formation).[14]
Due to their toxicity and low nucleophilicity, organomercury compounds find limited use. The oxymercuration reaction of alkenes to alcohols using mercuric acetate proceeds via organomercury intermediates. A related reaction forming phenols is the Wolffenstein–Böters reaction. The toxicity is useful in antiseptics such as thiomersal and merbromin, and fungicides such as ethylmercury chloride and phenylmercury acetate.
Thiols are also known as mercaptans due to their propensity for mercury capture. Thiolates (R-S−) and thioketones (R2C=S), being softnucleophiles, form strong coordination complexes with mercury(II), a soft electrophile.[15] This mode of action makes them useful for affinity chromatography to separate thiol-containing compounds from complex mixtures. For example, organomercurial agarose gel or gel beads are used to isolate thiolated compounds (such as thiouridine) in a biological sample.[16]
^Hintermann, H. (2010). Organomercurials. Their Formation and Pathways in the Environment. Metal Ions in Life Sciences. Vol. 7. Cambridge: RSC publishing. pp. 365–401. ISBN978-1-84755-177-1.
^"Reactivity control in palladium-catalyzed reactions: a personal account" Pavel Kocovsky J. Organometallic Chemistry 687 (2003) 256-268. doi:10.1016/j.jorganchem.2003.07.008
^Masao Ono & Masaya Kawakami (1977). "Separation of Newly-Synthesized RNA by Organomercurial Agarose Affinity Chromatography". J. Biochem.81 (5): 1247–1252. PMID19428.
