Methylating process

Methylmercury in the marine environment may originate from industrial discharges or be synthesised by natural methylation processes. Fish do not themselves methylate inorganic mercury [62,64], but can accumulate methyl mercury from sea water [63]. Methylmercury has been detected in sea water only from Minamata Bay, Japan, an area with a history of gross mercury pollution from industrial discharge. It has been found in some sediments but at very low concentrations, mainly from areas of known mercury pollution. It represents usually less than 1% of the total mercury in the sediment, and frequently less than 0.1% [65-67]. Microorganisms within the sediments are considered to be responsible for the methylation [65,68], and it has been suggested that methylmercury may be released by the sediments to the sea water, either in... 

The work of the Harrison group on alkylleads in air is perhaps the most persuasive of the reports as to the advocation of an environmental methylating process. Some of the sites whose atmospheric lead was sampled were very remote, such as Harris Island, Outer Hebrides, UK. Air reaching this site should not have any anthropogenic content but concentrations of alkylleads were found to be in the 3-7 ngm-3 range61. There was a higher than normal ratio of alkyl to total lead present here also (from 10-30%). A maritime source of volatile alkyllead was proposed61,62. 

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. 

Dimethylarsinic acid is the major metabolite of orally administered arsenic trioxide, and is excreted rapidly in the urine (Yamauchi and Yamamura 1985). The methylation process is true detoxification, since methanearsonates and cacodylates are about 200 times less toxic than sodium arsenite (NAS 1977). The marmoset monkey (Callithrix jacchus), unlike all other animal species studied to date, was not able (for unknown reasons) to metabolize administered As+5 to demethylarsinic acid most was reduced to As+3. Only 20% of the total dose was excreted in urine as unchanged As+5, and another 20% as As+3. The rest was bound to tissues, giving distribution patterns similar to arsenite (Vahter and Marafante 1985). Accordingly, the marmoset, like the rat, may be unsuitable for research with arsenicals. 

Ghosh, A.K. and Harvey, P. (2005) Toluene methylation process. WO Patent 2005/033071A2. 

Plant sterols such as stigmasterol typically contain an extra ethyl group when compared with cholesterol. Now this is not introduced by an electrophilic ethylation process instead, two successive electrophilic methylation processes occur, both involving SAM as methyl donor. Indeed, it is a methylene derivative like that just seen in ergosterol formation that can act as the alkene for further electrophilic alkylation. After proton loss, the product has a side-chain with an ethylidene substituent the side-chains of the common plant sterols stigmasterol and sitosterol are then related by repeats of the reduction and dehydrogenation processes already seen in ergosterol formation. 

In a similar manner, the methylation of cyclophosphines with MeOTf in the absence of solvent produces dications of the type [Mc2(PR)4]. For example, the treatment of the cyclic tetramer cyclo-(CyP)4 with an excess of MeOTf produces the dication [Mc2(PCy)4] in excellent yield (Scheme 11.3). The observation of two 1 2 1 triplets in the P NMR spectrum (an A2X2 spin system) suggests a symmetrical structure for this dication and this is confirmed by the X-ray structure, which shows that dimethylation occurs at the 1,3-positions of the four-membered ring (Figure 11.9b). Monitoring of the methylation process by P NMR spectroscopy reveals the intermediate formation of the monomethylated cation [Me(PCy)4] (an A2MX spin system) (Scheme 11.3)... 

The rate of methylation of mercury in anaerobically incubated estuarine sediments proved to be inversely related to salinity (267) this is consistent with results reported in Section II,A. Methylmercuric ion forms in sediments upon addition of HgCl2, with a lag phase of 1 month (268). Biomethylation by lake water columns and by sediments coincided, apparently being related to overall microbial activity, and showed periodic fluctuations (269). Topping and Davies have demonstrated that mercury can be methylated in the water column of a sea loch (270). As has previously been noted, tin compounds can be methylated by sediments (121-124), and this is also true for lead (134-136, 271). The relative proportions of biotic and abiotic methylation processes for such systems still remain to be determined. 

In an effort to understand better the catalytic chemistry associated with this reaction, the reactivity of dodecylnitrile or dodecylamine was measured under the same experimental conditions. The results listed in Table 1 show that the nitrile and the primary amine are much more easily transformed into N-dimethylalkylamine than the ester or the acid (Table 2). The rate determining step in the methylation process is directly related to one of the first reactions converting the ester or the acid into nitrile. It can be assumed that ... 

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