Organomercury compound, formation from

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. 

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. 

Carbon-carbon bond formation via free radicals formed from organotin or organomercury compounds. 

Oxidation of organomercury compounds via formation of TEMPO derivatives and cleavage with Zn-HOAc completes the functionalization of alkenes. Without TEMPO the oxidative capture of a primary radical generated from organomercurial is inefficient, and the reductive pathway (loss of functionality) becomes competitive. 

Studies on the electrochemical reduction of anhydro-2-azacephams 26 at a mercury electrode gave azetidine-type products 27 and 28 resulting from exclusive S-N bond cleavage (Equation 5). Formation of an organomercury compound was postulated as an intermediate in the reduction process <2001MI229>. 

The organomercury compound is reduced with sodium borohydride, and the HgOAc group is replaced by a hydrogen atom. The mechanism is not well established, but is thought to involve free radicals. Thus, the reaction is not necessarily stereospecific. Only the location of the hydroxyl group can be predicted from knowledge of the formation of the mercurinium ion and the ditection of attack of water on that ion. 

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. 

Table 4. Formations of Organomercurials from Mercury(II) Salts with 0--B0NDED Transition-Metal Compounds... 

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