Demercuration alkenes

Oxymercuration-Demercuration Alkenes react with mercuric acetate in a mixture of water and tetrahydrofuran (THF) to produce (hydroxyalkyl)mercury compounds. These can be reduced to alcohols with sodium borohydride and water (Section 8.5). 

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... 

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... 

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. 

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... 

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... 

Application of the oxymercuration-demercuration reaction176 to alkyl 3,4-dideoxy-a-DL-hex-3-enopyranosides provides177 easy access to alkyl 3-deoxyhexopyranosides (for example, 288). Interestingly, both stereoisomeric forms of the alkene are apparently attacked by mercuric acetate from the same side. It has been assumed177 that the transient, mercurinium ion 287 is stabilized by bonding to the 1-meth-oxyl group. 

Whereas mercuration-demercuration of alkenes in the presence of water gives alcohols, in alcohol solvents (free from H,0) ethers result. These reactions in the presence of nucleophilic solvents such as water and alcohols are examples of solvomercuration. The mercuric salts usually used are the acetate, Hg(OAc), (—OAc is an abbreviation for —OCCH,) or the trifluoracetate. Hg(OCOCF,),. ... 

Problem 14.6 Give the alkene and alcohol needed to prepare the following ethers by alkoxymercuration-demercuration (a) diisopropyl ether, (b) 1-methyl-l-methoxycyclopentane, (c) 1-phenyl-1-ethoxypropane, (d) di-f-butyl ether. ... 

Mechanism. The reaction is analogous to the addition of bromine molecules to an alkene. The electrophilic mercury of mercuric acetate adds to the double bond, and forms a cyclic mercurinium ion intermediate rather than a planer carbocation. In the next step, water attacks the most substituted carbon of the mercurinium ion to yield the addition product. The hydroxymercurial compound is reduced in situ using NaBH4 to give alcohol. The removal of Hg(OAc) in the second step is called demer-curation. Therefore, the reaction is also known as oxymercuration-demercuration. 

Addition of alcohol to alkenes hy alkoxymercuration-reduction produces ethers via Markovnikov addition. This addition is similar to the acid-catalysed addition of an alcohol. For example, propene reacts with mercuric acetate in aqueous THF, followed hy reduction with NaBFl4, to yield methyl propyl ether. The second step is known as demercuration, where Flg(OAc) is removed hy NaBH4. Therefore, this reaction is also called alkoxymercura-tion-demercuration. The reaction mechanism is exactly the same as the oxymercuration-reduction of alkenes. 

Solvomercuration,3111 484 187 or the addition of mercury(II) salts, is a convenient route to organomercurials. On the other hand, replacement of mercury with hydrogen allows Markovnikov functionalization of alkenes.488 A method called mercuration-demercuration, for instance, has been developed for the Markovnikov hydration of alkenes under mild conditions489 [Eq. (6.82)490] ... 

Mercury(II) trifluoroacetate is a good electrophile that is highly reactive toward carbon-carbon double bonds [52, 53, 54 When reacting with olefins in nucleophilic solvents, it usually gives exclusively mercurated solvoadducts, but never products of skeletal rearrangement Solvomercuration-demercuration of alkenes with mercury(II) trifluoroacetate is a remarkably effective procedure for the preparation of esters and alcohols with Markovnikov s regiochemistry [52, 55] (equation 24)... 

There are several methods available for the electrophilic addition of hydrogen and nitrogen to alkenes, dienes and alkynes. While the direct electrophilic addition of amines to these substrates is not feasible, aminomercuration-demercuration affords a very useful indirect approach to such amines. The addition of amides to C—C multiple bonds can be effected directly through the Ritter reaction or by the less direct, but equally useful, amidomercuration-demercuration process using either nitriles or amides. Similarly, H—N3 addition to alkenes can be carried out directly or via mercuration to produce organic azides. 

A wide variety of alkenes have been shown to undergo aminomercuration-demercuration, including alkenes bearing alcohol, ester, acid, amide, imide, urea, urethane, ether, sulfide and aryl groups.194... 

The reaction may be subject to limitations in the alkene structure similar to those of the nitrile process. Besides simple amides, one can also utilize urea and urethanes in this reaction. Demercuration is best effected by using alkaline sodium borohydride in the presence of a primary amine such as Bu"NH2. 

The formal addition of H—N3 across C—C double bonds can also be brought about via azidomercuration-demercuration, a process recently reviewed.267 In the presence of NaN3, Hg(OAc)2, Hg(02CCF3)2 or Hg(N03)2, in aqueous THF, methanol or DMF, alkenes undergo azidomercuration (equation 188).268-... 

While many different procedures have been reported for the hydroxymercuration—demercuration of alkenes, the most useful procedure uses mercury(II) acetate in aqueous THF, followed by in situ alkaline sodium borohydride reduction (equation 205).312-313 Virtually all substitution patterns about the C—C double bond are accommodated. 

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