Organomercurials formation

The pollutant (xenobiotic) forms a stable covalent bond with its target. Examples include the phosphorylation of cholinesterases by the oxon forms of OPs, the formation of DNA adducts by the reactive epoxides of benzo[a] pyrene and other PAHs, and the binding of organomercury compounds to... 

Monoalkylthallium(III) compounds can be prepared easily and rapidly by treatment of olefins with thallium(III) salts, i.e., oxythallation (66). In marked contrast to the analogous oxymercuration reaction (66), however, where treatment of olefins with mercury(II) salts results in formation of stable organomercurials, the monoalkylthallium(III) derivatives obtained from oxythallation are in the vast majority of cases spontaneously unstable, and cannot be isolated under the reaction conditions employed. Oxythallation adducts have been isolated on a number of occasions (61, 71,104,128), but the predominant reaction pathway which has been observed in oxythallation reactions is initial formation of an alkylthallium(III) derivative and subsequent rapid decomposition of this intermediate to give products derived by oxidation of the organic substrate and simultaneous reduction of the thallium from thallium(III) to thallium(I). The ease and rapidity with which these reactions occur have stimulated interest not only in the preparation and properties of monoalkylthallium(III) derivatives, but in the mechanism and stereochemistry of oxythallation, and in the development of specific synthetic organic transformations based on oxidation of unsaturated systems by thallium(III) salts. 

Formation of labeled molecules has been studied in a few cases, but has not been exploited usefully. Various radioactive organomercury compounds have been prepared diphenylmercury (33, 90), fluorescein (53), and chloromeredrin (43). A number of other potentially useful syntheses could doubtless be developed with a wide variety of nuclides with easily detectable y-rays—pharmaceuticals, pesticides, physiological tracers, oil-soluble markers for labeling oil shipments, and so on—if it could be established what molecules are of interest to the various consumers ... 

Formation of the intermediate organomercury peroxide 56 was rationalised in terms of an allylic mercuration providing an unsaturated hydroperoxide 55 that can cyclise by the favoured 54) 5-exo mode (equation 42 X = 02CCF3). However, this was not the main reaction pathway and the yields (2.7% and 0.6% of 2,m-10-dibromo-8,9-dioxabicyclo[5.2.1]decanes 54 were an order of magnitude lower than those of the 2,6-dibromides 53 obtained from 1,4-cyclooctadiene. 

Peroxides. See also Inorganic peroxides Organic peroxides acid hydrolysis of, 23 459 diacyl, 24 282-284 explosive, 20 569-573 formation of, 20 577 as free-radical initiators, 24 279-293 organomercury-containing, 23 445 potassium salts of, 18 478 silylation and, 22 703 stereoisomers of, 28 459 as vulcanizing agents, 22 795 ... 

The Hg cathode plays the role of a reducing catalyst. Electrochemical reduction of 8-bromoboman-2-one in an HMPA-Bu4NBr-(Pt/Hg) system also suggests the formation of an organomercurial intermediate that leads to the formation of dihydrocarvone as the major product [553]. 

Perlmutter used an oxymercuration/demercuration of a y-hydroxy alkene as the key transformation in an enantioselective synthesis of the C(8 ) epimeric smaller fragment of lb (and many more pamamycin homologs cf. Fig. 1) [36]. Preparation of substrate 164 for the crucial cyclization event commenced with silylation and reduction of hydroxy ester 158 (85-89% ee) [37] to give aldehyde 159, which was converted to alkenal 162 by (Z)-selective olefination with ylide 160 (dr=89 l 1) and another diisobutylaluminum hydride reduction (Scheme 22). An Oppolzer aldol reaction with boron enolate 163 then provided 164 as the major product. Upon successive treatment of 164 with mercury(II) acetate and sodium chloride, organomercurial compound 165 and a second minor diastereomer (dr=6 l) were formed, which could be easily separated. Reductive demercuration, hydrolytic cleavage of the chiral auxiliary, methyl ester formation, and desilylation eventually led to 166, the C(8 ) epimer of the... 

The approach to polyketide synthesis described in Scheme 5.2 requires the relatively nontrivial synthesis of acid-sensitive enol acetals 1. An alternative can be envisioned wherein hemiacetals derived from homoallylic alcohols and aldehydes undergo dia-stereoselective oxymercuration. Transmetallation to rhodium could then intercept the hydroformylation pathway and lead to formylation to produce aldehydes 2. This proposal has been reduced to practice as shown in Scheme 5.6. For example, Yb(OTf)3-cata-lyzed oxymercuration of the illustrated homoallyhc alcohol provided organomercurial 14 [6]. Rhodium(l)-catalyzed hydroformylation of 14 proved successful, giving aldehyde 15, but was highly dependent on the use of exactly 0.5 equiv of DABCO as an additive [7]. Several other amines and diamines were examined with variation of the stoichiometry and none proved nearly as effective in promoting the reaction. This remarkable effect has been ascribed to the facilitation of transmetallation by formation of a 2 1 R-HgCl DABCO complex and the unique properties of DABCO when both amines are complexed/protonated. 

As in other electrophilic substitutions, mercuration of thienothiophenes (7) and (3) leads to a-substituted derivatives (102) and (103). When the a-positions are blocked as in (73), /3-mercuration occurs leading to the formation of the mercury derivative (104). In the early investigations on the classical thienothiophenes, organomercury derivatives were used for characterization purposes. Although the mercury function in (102)-(104) can be replaced by other electrophiles, at the present time the procedure has no practical significance <76AHC(19)123). 

Examples of radical-mediated C-alkylations are listed in Table 5.4. In these examples, radicals are formed by halogen abstraction with tin radicals (Entries 1 and 2), by photolysis of Barton esters (Entry 3), and by the reduction of organomercury compounds (Entry 4). Carbohydrate-derived, polystyrene-bound a-haloesters undergo radical allylation with allyltributyltin with high diastereoselectivity (97% de [41]). Cleavage from supports by homolytic bond fission with simultaneous formation of C-H or C-C bonds is considered in Section 3.16. 

Irradiation of phenyliodonium salts lead to the formation of phenyl radicals. In the presence of C60 these radicals are efficiently trapped under formation of pheny-lated C6o derivatives, mainly the monoadduct. In reaction mixtures of C6o, phenyliodonium salts and spin traps like nitroso-tert-butane ( BuNO) or nitroso-durene (ND) no phenyl adducts with the spin traps could be observed after irradiation. This suggests that C6o is a more efficient scavenger for phenyl radicals than the spin traps [177], Other investigations yielded similar results, e.g., the photolysis of organomercury compounds in the presence of fullerenes leads to fullerene-derived radical adducts. These radical adducts can combine to form dimers that are thermally stable and accumulate in the samples [Eq. (7)] [178],... 

Another approach for the preparation of either symmetrical or unsymmetrical iodonium salts used organolithium or organomercury compounds and (dichloroiodo)arenes [12]. The problem of the formation of unwanted isomers during reactions involving aromatic electrophilic substitution may also be overcome by the condensation of iodosylarenes with iodylarenes [12]. Several iodonium triflates were prepared in high yield from activated or mildly deactivated arenes with iodosylbenzene and triflic anhydride or triflic acid [13,14] or sulphur trioxide [15]. Some of these compounds are shown in Table 8.2. 

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