Methyllead compounds

B. Methyllead Compounds Found in the Natural Environment and Arising from Biomethylation... 

As for tin, two potential routes to methyllead compounds might appear to exist (1) Me carbanion attack on lead(II) (e.g. by Me- from MeCoBn) followed by dismutation and (2) oxidative addition by a Me+ carbonium ion from e.g. S adenosylmethionine or Mel etc. 

The major organolead compounds found in the environment are the tetraalkyllead compounds and their di- and trialkyl decomposition products. Elevated levels of tetraalkyl-leads have two possible sources either (i) anthropogenic leaded petroleum inputs or (ii) environmental methylation of natural lead compounds. While the former is well established, the latter is the subject of some controversy in the literature. Interest in the environmental methylation process derives from the increased toxicity of methyllead compounds compared to their inorganic analogs. 

A blank and also a sterile solution containing Pb2+ of methyllead compounds showed no Pb content in the methanolic solution after the same treatment. The author concluded that Me4Pb was produced in the biomethylation of Pb2+ by bacteria123. Thompson and Crerar122 also observed that about 0.03% of lead as Pb(NC>3)2 underwent methylation and trimethyllead acetate, (CH3)3PbOAc, was methylated nearly quantitatively in incubation experiments with marine sediments from the British Columbia coastline. 

TABLE 22. Experimental design and yields of methyllead compounds (modified from Reference 74)... 

It is now well established that organometallic compounds are formed in the environment from mercury, arsenic, selenium, tellurium and tin and hence were also deduced on the basis of analytical evidence for lead, germanium, antimony and thallium. Biological methylation of tin has been demonstrated by the use of experimental organisms. Methylgermanium and methyllead were widely found in the environment but it is debatable whether germanium and lead are directly methylated by biological activity in natural environment. 

Outside of the antiknock field, the commercial applications of organolead compounds are relatively few and quite small. The largest of these applications is in the manufacture of organomercury fungicides by alkylation. This application is old and is relatively static. Other alkylations consume a little tetraalkyllead compound. A small amount of tetra-methyllead has long been used to fill Geiger counters. 

The restricted versatility can, to a certain degree, be overcome by replacing NaBH4 by alkyl or arylborates [1]. The most common derivatization procedure relies on ethylation with sodium tetraethylborate, which was initiated for determining methyllead ionic compounds [9] ... 

In mammals, inhalation or adsorption of ICrPb compounds results in the formation of trialkyllead in tissues and body fluids. With the exception of methylleads found in the blood of petrol workers, these are usually below the detection limits in blood and urine samples." ... 

Other organometallic initiators which have been mentioned include tetra-methyllead (or tetramethyltin) in ter/-butanol dispersed in an aqueous solution of ammonium persulfate and borax [64] and silver compounds such as silver nitrate with organic lead or tin compounds and organic promoters such as acetone, dimethyl sulfoxide, ethanol, or fert-butanol [65]. 

In Britain almost all of the tetraaUcyllead petrol additive is used in the form of tetramethyUead or tetraethyllead with little usage of the mixed alkyl compounds, whilst in the US rather more of the mixed alkyls are in use. Tetra-methyllead is more thermally stable, and generally less chemically reactive than tetraethyllead and European observations indicate that the former compound predominates in the atmosphere [17]. 

An additional feature of such transformations is the more effective use of the RX compoimd or the use of a lesser quantity of the RLi or RMgX compound. Incidentally, overheating of the Me3Tl induces violent detonation. As mentioned elsewhere in connection with free alkyl radicals and with thermal stabilities, other methyl metallic compounds such as tetra-methyllead, methylcopper, and dimethylcadmium decompose explosively if overheated. 

Other experiments indicated that conversion of lead(ll) compounds to Pb(CH3)4 was inconsistent and time-independent [130]. Lead contained in mine tailings is not mobilized in detectable amounts by biomethylation [125, 130]. A volatile organolead compound, presumably the methyllead derivative, was observed upon aerobic, not upon anaerobic, incubation of marine sediment with lead(ll) acetate [131]. Marine sediments inoculated with (CH3)3PbOOCCH3 produced Pb(CH3)4 in experiments employing flow-through conditions both under anaerobic and aerobic conditions. Production under anaerobic conditions was approximately threefold greater [130]. 

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