Demercuration

Benzene and some of its derivatives react with solutions of mercuric nitrate in concentrated nitric acid to give nitrophenols. These reactions, known as oxynitrations may proceed by mercuration followed by nitroso-demercuration the resulting nitroso compound becomes a diazonium compound and then a phenol, which is nitrated. ... 

W. Kitching, Organomet. Chem. Rev. 3 61 (1968) R. C. Larock, Solvomercuration/Demercuration Reactions in Organic Synthesis, Springer-Verlag, New bik, 1986. 

The mercuration-demercuration sequence was applied to the stereospecific synthesis of 2 deoxy-o-hexopyranoside derivative via a mercury(ll) trifluoroace-tate-promoted cyclization [54] (equation 25)... 

Ammonia can be added to double bonds (even ordinary double bonds) in an indirect manner by the use of hydroboration (15-16) followed by treatment with NH2CI or NH2OSO2OH (12-29). This produces a primary amine with anti-Markovnikov orientation. An indirect way of adding a primary or secondary amine to a double bond consists of aminomercuration followed by reduction (see 15-3 for the analogous oxymercuration-demercuration procedure), for example. 

Hydrazoic acid can be added to certain Michael-type substrates (Z is as defined on p. 975) to give 3-azido compounds. The reaction apparently fails if R is phenyl. Ammonia also adds to enol ethers CH2=CHOR to give CH3—CH(OR)N3, and to silyl enol ethers, but it does not add to ordinary alkenes unless a Lewis acid catalyst, such as TiCU, is used, in which case good yields of azide can be obtained. Ammonia can also be added indirectly to ordinary alkenes by azidomercuration, followed by demercuration, analogous to the similar procedures mentioned in... 

Benzoselenadiazole (bsd) has been used as a ligand in the ruthenium complex [RuClH-(CO)(bsd)(PPh3)2]. This complex was used to catalyse the transalkynylation and catalytic demercuration of bis(alkynyl)mercurials <96CC1059>. 

Tri-n-butyltin hydride can also be used for reductive demercuration.20 An alternative reagent for demercuration is sodium amalgam in a protic solvent. Here the evidence is that free radicals are not involved and the mercury is replaced with retention of configuration.21... 

The trapping of the radical intermediate in demercuration by oxygen can be exploited as a method for introduction of a hydroxy substituent (see p. 295). The example below and Entries 3 and 4 in Scheme 4.6 illustrate this reaction. 

On the basis of the mechanistic pattern for oxymercuration-demercuration, predict the structure and stereochemistry of the alcohol(s) to be expected by application of the reaction to each of the following substituted cyclohexenes. 

The particular example shown also has a special mechanism for stabilization of the cyclized carbocation. The adjacent acetoxy group is captured to form a stabilized dioxanylium cation. After reductive demercuration (see Section 4.1.3) and hydrolysis,... 

The threo stereoisomer was the major product obtained by the synthesis in Scheme 13.14. This stereochemistry was established by the conjugate addition in Step A, where a significant (4-6 1) diastereoselectivity was observed. The C(4)-C(7) stereochemical relationship was retained through the remainder of the synthesis. The other special features of this synthesis are in Steps B and C. The mercuric acetate-mediated cyclopropane ring opening was facilitated by the alkoxy substituent.19 The reduction by NaBH4 accomplished both demercuration and reduction of the aldehyde group. 

Oxymercuration/demercuration provides a milder alternative for the conventional acid-catalyzed hydration of alkenes. The reaction also provides the Markovnikov regiochemistry for unsymmetrical alkenes.33 Interestingly, an enantioselective/inverse phase-transfer catalysis (IPTC) reaction for the Markovnikov hydration of double bonds by an oxymercuration-demercuration reaction with cyclodextrins as catalysts was recently reported.34 Relative to the more common phase-transfer... 

Eq. 34). The resultant p-mercurioalkyl peroxides can often be demercurated with sodium borohydride (Eq. 35), or by brominolysis (Eq. 36) without substantial cleavage of the 0-0 bond. Both peroxymercuration and demercurations occur rapidly under mild conditions 48). 

The reductive demercuration was marred by the loss of about half of the peroxide due to competing deoxymercuration which afforded 4-cycloocten-l-ol. An additional complication was the formation of a small amount of trans-1,2-epoxy-cw-cyclooct-5-ene. The bicyclic peroxide 50 was readily separated from the unsaturated alcohol by silica chromatography, but complete removal of the epoxide was more difficult. Preservation of the peroxide linkage was markedly higher in the bromodemercuration. The diastereoisomeric dibromoperoxides 51 were separated by HPLC, although only one isomer was fully characterised. 

Use of mild conditions was crucial and the development of diimide reduction of singlet oxygenates, silver-salt-assisted displacement of halide by peroxide nucleophiles, peroxymercuration and demercuration, peroxide transfer from organotin to alkyl triflates, and oxygen trapping of azoalkane-derived diradicals have all played a part in providing the rich harvest of new bicyclic peroxides described herein. 

In 1978, Corey reported a general synthetic route for the conversion of alkenes to conjugated nitroalkenes via nitro-mercuration and demercuration.74 Since then, many chemists have used this method for the preparation of cyclic nitroalkenes such as 1-nitrocyclohexene. However, the use of mercury salts is not recommended even for the small-scale preparation of nitroalkenes. This reaction is not as clean as expected, and formidable efforts are required to remove the mercury in the waste. 

The irreproducibility (98,100) of the Pesci decarboxylations [Eqs. (84) and (86)] (97,99) has further implications since the decarboxylation products were claimed to undergo novel nucleophilic displacements of mercury (96,97,99). Some reported nucleophilic demercurations of an authentic mercurial (96) could not be repeated (98). A Pesci type hemidecarboxylation of 5-norbornene-2,3-dicarboxylic acid has been reported by Takahashi (101), but this has also been found to be irreproduci-ble (101a). 

Oxymercuration-demercuration allows the Markovnikov addition of H-and -OH without rearrangements. 

Oxymercuration-demercuration gives Markovnikov addition of H- and -OH to an alkene, yet it is not complicated by rearrangement. 

Alcohols from Alkenes through Oxymercuration-Demercuration... 

Demercuration normally requires less than an hour. 

Rearrangements of the carbon skeleton seldom occur in oxymercuration-demercuration. 

Two complementary orientations for the addition of water to a double bond 1) Acid-catalyzed hydration (or oxymercuration-demercuration) of 1-hexene ... 

Dimethyl-l,3-butadiene underwent reaction to give the expected product 2,3-di-methyl-3-buten-2-ol besides a product containing a rearranged carbon structure, whose formation has been attributed to a radical process occurring during the demercuration step (equation 151). 

Finally, although mercuration -demercuration of dienes is a suitable method for synthesis of unsaturated alcohols and amines, 1,5-dienes cannot be used for this purpose... 

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