Organomercury compounds reduction

The reductive decomposition of alkylmercury compounds is also a useful source of radicals.300 The organomercury compounds are available by oxymercuration (see Section 4.1.3) or from organometallic compounds as a result of metal-metal exchange (see Section 7.3.3). 

Alkyl radicals generated by reduction of organomercury compounds can also add to alkenes having EWG groups. Radicals are generated by reduction of the organomercurial by NaBH4 or a similar reductant. These techniques have been... 

Demercuration of organomercury compounds is a critical step in synthetic procedures, which involve mercuration-initiated cyclization reactions [e.g. 41], Many of the standard procedures for demercuration result in rearrangement or ring cleavage of the system, but reductive carbon-mercury cleavage (e.g. Scheme 11.4) with an excess of the quaternary ammonium borohydride is effective under phase-transfer conditions [e.g. 42,43]. 

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... 

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. 

Complicating side reactions may occasionally occur - as in the oxymercuration-demercuration of styrene to 1-phenylethanol for which experimental details are also given. In this case evidently some organomercurial compounds survive the reductive stage, and their subsequent decomposition during final distillation complicates the isolation of the pure product. 

On the other hand, why does a single diastereoisomer of this organomercury compound give a mixture of dia stereoisomers (68 32) on reduction with borohydride in the presence of acrylonitrile ... 

A number of metals salts can be used as the source of electrophiles in reactions with alkenes. One of the most interesting of these involves the attack of mercury(II) acetate in acetic acid. Reductive cleavage of the organomercury compound with sodium borohydride leads to the overall hydration of the alkene in a Markownikoff sense. There are a number of preparative advantages, such as a reduced tendency to rearrange, associated with this and similar relatively mild procedures when compared to the direct protonation of a double bond (Scheme 3.14)... 

The former cannot be considered a good synthetic method because a mixture of reaction products is usually obtained. The protonolysis of organomercury compounds has been extensively studied from a mechanistic standpoint but has found little synthetic utility except for isotopic labelling of organic molecules. Reduction is the most useful method for the demercuration of organomercury compounds, particularly of solvomercurials in the solvomercuration-demercuration reaction, and a lot of reductants are used for this purpose. 

Organometallic compounds of less active metals and metalloids (e.g., silicon," antimony, and bismuth, are quite inert to water. Organomercury compounds (RHgX or R2Hg) can be reduced to RH by H2, NaBITj, or other reducing agents." The reduction with NaBH4 takes place by a free-radical mechanism." Alkyl-Si... 

Ethers are prepared either by a Williamson synthesis or by alkoxymercuration/demercuration sequence. The Williamson ether sy thesis involves S -2 attack of an alkoxide ion on a primary alkyl halide. The alkoxymercuration reaction involves the formation of an intermediate organomercury compound, followed by NaBH4 reduction of the C-Hg bond. The net result is Markovnikov addition of an alcohol to an alkene. 

Treatment of an alkene with mercuric acetate in aqueous THF results in the electrophilic addition of mercuric ion to the double bond to form an intermediate mercuri-um ion. Nucleophilic attack by H2O at the more substituted carbon yields a stable organomercury compound, which upon addition of NaBH4 undergoes reduction. Replacement of the caiton-mercury bond by a carbon-hydrogen bond during the reduction step proceeds via a radical process. The overall reaction represents Markovnikov hydration of a double bond, which contrasts with the hydroboration-oxidation process. 

Evans 0, McKee GD. 1988. Determination of mercury(II) and organomercury compounds by reversed-phase liquid chromatography with reductive electrochemical detection. Analyst 113(2) 243-246. 

Oxymercuration of simple alkyl- and acyl-substituted cyclopropenes generally results in ring opening.Addition of mercury(II) acetate to 3-methyl-3-phenylcyclopropene, however, gave a low yield of a cyclopropane containing organomercury compound (15-20%), which was converted into an isomeric mixture of 1 -methoxy-2-methyl-2-phenylcyclopropanes by reduction with lithium aluminum hydride. Reaction of 5 with mercury trifluoroacetate in methanol and then sodium hydroxide led predominantly to one cylopropane. ... 

This element is a chalcophile, and in unweathered rocks is most commonly found as the mineral cinnabar (HgS). In soil environments, the cationic form, is most common, as the reduced oxidation state (+1) has a limited stability range. Reduction to the metallic elemental form, H, is easily achieved in soils by both biological and chemical reactions. Elemental mercury is somewhat volatile, and the vapor is extremely toxic to organisms. Under anaerobic conditions at least, soil microbes methylate mercury, forming volatile organomercury compounds that are bioavail-able and present a health hazard. At the same time, however, anaerobic conditions can convert Hg into the exceedingly insoluble sulhde, HgS. Some of the more important transformations possible for mercury in soil are summarized in Figure 9.9. 

Amides.1 The solvomercuration-demercuration of terminal olefins or cyclic olefins with acetonitrile and mercury 11) nitrate followed by reduction of the intermediate organomercury compound affords amides, hydrolyzable to amines. Neither mercuric acetate nor mercuric trifiuoroacetate is satisfactory in this reaction. Preliminary attempts to use a tertiary olefin failed. 

Organomercury compounds can also be obtained in preparatively useful quantitites by the action of radicals on metallic mercury. For example, aryl-mercuric compounds are formed on decomposition of arenediazonium salts in the presence of metallic mercury 28 and when 3-iodopropionitrile is reduced electrolytically at a mercury cathode, the radicals produced react with the cathode material forming bis-(2-cyanoethyl)mercury.202 Organomercury compounds are also obtained analogously on reduction of ketones at a mercury cathode.203... 

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. 

Until recently the main disadvantage of this technique was the poor sensitivity of the detectors. Development of more sensitive detectors such as a reductive amperometric electrochemical detection, ultraviolet detection, ICP-AES, ICPMS, AFS, and AAS has resulted in wider applications in environmental studies. The main advantage of liquid chromatography is the possibility to separate a great variability of organomercury compounds. Applications of HPLC for Hg speciation studies have been reviewed by Harrington. ... 

HPLC has also gained increased interest in recent years, due to the development of more sensitive detection systems such as reductive amperometric electrochemical detection, UV detection, ICP-AES, AFS, and AAS. The main advantage of HPLC over other methods is the possibility of separating a great variety of organomercury compounds (Hempel et al. 1992, Falter and Scholer 1994). 

---