Alkenes mercury salts

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. 

Nonconjugated dienes or polyenes can be monomercurated if the diene or polyene is employed in excess, but more frequently these compounds are allowed to react with an excess of the mercury salt and di- or poly-mercurated products are obtained. The relative reactivity of isolated double bonds is basically that expected from studies on simple alkenes. 

The acyloxymercuration-demercuration of alkenes provides an alternative route to esters which is probably less prone to caibon skeleton rearrangements than the direct addition of carboxylic acids to alkenes (equation 282). This reaction has recently been reviewed.477 The reaction is most commonly run using mercury(II) acetate in acetic acid, though other mercury salts may be used and aprotic solvents can also be employed. Equilibria have been measured for the reaction of mercury(II) trifluoroacetate and alkenes in tetrahydrofuran, and were found to be solvent dependent.478... 

The mercury-promoted amination of double bonds proceeded in an anti mode with >99% diastereoselectivity. By treating (Z)-2-butene with dimethylamine in the presence of mer-cury(II) chloride, the syn-adduct of 1 was obtained, while ( )-2-butene under the same conditions afforded the anti-adduct. However, by treating alkenes with ammonia and mercury salts, only ammonia-mercury complexes were formed8-14,171. 

Isohypsic reactions of alkenes, like electrophilic additions of H2O or HX, represent a conventional pathway for the preparation of alcohols and alkyl halides from alkenes. The scope of their application was originally limited as unsymmetrical alkenes (e.g. 125) gave product mixtures composed of both Markovnikov (M) adducts and anti-Markovnikov (aM) adducts. As was already mentioned above (see Scheme 2.10), an efficient and general method for the conversion of alkenes into alcohols or ethers 126 (Scheme 2.47), with a nearly complete M selectivity, was elaborated using mercury salts as electrophiles in conjunction with the reduction of the formed adducts. It is also... 

Mercury salts have previously been shown to be capable of cleaving cyclopropyl bonds. Phenyl-substituted cyclopropanes react with mercury(II) acetate to give 3-mercurio-l-methoxy-l-phenylpropane derivatives, which can undergo further reactions such as reduction with sodium borohydride or addition to alkenes. For example, treatment of phenyl-cyclopropane (1) with mercury(II) acetate in methanol, followed by the addition of sodium chloride, produced 3-(chloromercurio)-l-methoxy-l-phenylpropane(2) in good yield.When the primary mercury adduct was reacted with alkenes without isolation, the yields were lower than those in the two-step transformation. ... 

By Addition to Unsaturated Substrates. Bromo- and iodo-functionalization of alkenes using a combination of the halogen and a mercury(II) salt has enabled the preparation of a series of 1,2-disubstituted products in which the second substituent, derived from the mercury salt, may be another halide or a series of other groups including esters, nitrate, thiocyanide, or a sulphone. Stereoselective bromination of acetylenes to furnish... 

The most common oxymercuration reaction involves alkene, an alcohol such as methanol, and a mercury salt such as the acetate. The intermediate (alkoxy mercuri-acetate) is easily reduced by alkaline NaBH4 to the alkoxy derivative (Scheme 11). When applied to methyl oleate, this reaction gives a mixture of methyl 9- and 10-methoxystearates in high yield. 

Alkenes react with mercuric acetate (or other mercury salts) and methanol (or other nucleophiles) to give a mercury-containing adduct in a trans addition process. This intermediate furnishes the alkene in its original stereoisomeric form by treatment with hydrochloric acid. Alternatively, the mercury group is replaceable by bromine or hydrogen thus ... 

The reaction of an alkene with an alcohol in the presence of a mercury salt such as mercuric acetate or trifluoroacetate leads to an alkoxymercury intermediate, which on reaction with sodium borohydride yields an ether. 

For small-scale hydrations, it is convenient to use mercury salts, such as mercuric acetate, Hg(OAc)2, to carry out the addition of H—OH across a double bond. This process is called oxymercuration. In this two-step process, an alkene is first treated with mercuric acetate, then the initial alkylmercury compound formed is reduced with sodium borohydride (Na BH4). 

Oxymercuration of simple alkenes is a stereospecific anti addition. This result is in agreement with the involvement of a mercurinium intermediate which is opened by nucleophilic attack. The reactivity of the mercury salt is a function both of the... 

The ability of mercury salts to effect nucleophilic alkene addition is apparent in Lin and coworkers synthesis of the phorboxazole D ring (Scheme 26) [39]. Initial attempts at cyclization proceeded by addition to a halo-activated alkene. The use of iodine under basic conditions afforded the desired THP in 46 % yield as a 2.6 1 ratio of cisitrans isomers. The use of bulkier NIS increased the diastereoselectivity to 7.7 1 with no increase in yield. A more efficient cyclization was realized by using mercury(II) acetate. Installation of the iodide was accomplished by the addition of iodine after cyclization was complete. These conditions gave 2,6-cis THP 77 in 86 % yield and moderate diastereoselectivity (5 1). This method also benefits firom the displacement of the organomercurial intermediate in a single-pot procedure, thereby mitigating the isolation of potentially toxic mercury-containing substrates. 

Subshtuted 3-alken-l-ynes can be hydroaminated with primary or secondary aliphahc or aromatic amines at the alkynyl sites or at the alkynyl and at the alkenyl sites in the presence of Hg(ll) salts. However, the reachon is essentially stoichiometric in nature, even if the mercury compound can be recycled without apparent loss of achvity [262-264]. 

In a process known as peroxymercuration, hydrogen peroxide and alkyl hydroperoxides can be alkylated by alkenes in the presence of a suitable mercury(II) salt... 

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] ... 

That the mercurial species released during the ring-closure step can, at least with some mercury(ll) salts, recycle and act catalytically is shown by the finding that mercury (II) trifluoroacetate, used in 0.1 M proportions (and even lower), in aqueous acetone at room temperature catalyzes the reaction of alkene 43 (R1 = Me, R2 = R3 = Bn, R4 = Ac a-isomer), and gives the cyclized products in 96% yield as a mixture of the alcohol 46 (R2 = R3 = Bn, R4 = Ac) and its epimer at the alcohol center, in the ratio 8 1 [24]. 

Organomercury compounds such as ICFbHgl and Hg(CH2Br)2 may be prepared by the reaction of metallic mercury with diiodomethane or by reaction of mercury(II) salts with diazoalkanes. These mercury derivatives are related to the intermediate that has been proposed in the Simmons-Smith reaction, and likewise, they have been found to convert alkenes into cyclopropanes.92... 

The electrochemical generation of hydrogen in aqueous acid or alkaline solutions reduces unactivated alkynes 46 a b). This process is similar to catalytic hydrogenation, however, and does not involve electron transfer to the substrate. The electrochemical generation of solvated electrons in amine solvents or HMPA has also been effective in reducing these compounds 29). The focus of this section, however, is the electrolysis of these difficult to reduce alkynes and alkenes at mercury cathodes with tetraalkyl-ammonium salts as electrolytes. Specific attention is also given to competitive reductions of benzenoid aromatics and alkynes or alkenes. 

Indeed only alkynes could be reduced in (CH3)4N+ solutions while alkenes were inactive. Reduction of 1-hexyne, propargyl alcohol and 1,4-butyne diol were performed 13 at a mercury cathode with (CH3)4NC1 as the electrolyte. The corresponding olefins were formed and the respective yields were 45 %, 62 % and 82 %. The diacetate of 15 behaved similarly. However only the trans isomer 16 was formed from 15 while a mixture of trans and cis (6 4) isomers resulted from the reduction of the diacetate. Polarography of several alkynes in methanol with (C4H9)4N+ electrolytes showed, 3) that they react close to background decomposition. It was therefore proposed 14) that (CH3)4N+-mercury may be involved in the cathodic reduction of alkynes when (CH3)4N+ salts serve as the electrolytes. 

Other transition metal salts mediate in similar oxidations. For example, mercury(II) acetate, a milder reagent than LTA, effects a-acetoxylation through a comparable mechanism. However the corresponding yields for these processes are poor. 3,3-Dimethylcyclohexanone, for example, is oxidized to the a-acetoxy derivative in only 14% yield.The, 7-unsaturated ketone, isopugelone, exhibits no oxidation at the a- or a -positions, but affords a product derived from isomerization of the alkene and allylic oxidation. Not surprisingly therefore the reagent has found little synthetic application for this transformation. 

Gold(l) complexes of alkenes and alkynes appear to play an important role as intermediates of the gold(l)-catalyzed addition of water, alcohols, carboxylic acids, or amines to these substrates. The elfect of this 7r-complexation (22) is superior to the performance of mercury(ll) in this type of reactions. " In a similar way, gold(III) salts have found applications in homogeneous catalysis. ... 

The reaction between mercury(ll) salts and alkenes is the most important general method for the synthesis of organomercurials. Although there are some examples where mercury reacts directly with olefins to prodnce vinyl mercurials, the most coimnon reaction is the addition of mercury to the alkene s double bond with the participation of a nucleophile (equation 9). 

Another synthetic route to monoorganothallium compounds is the reaction of aryl or vinyl derivatives of mercury(II) or tin(IV) with TIX3 (X = halide, carboxy-late). Monoalkylthallium derivatives are intermediates m the oxidation of alkenes and alkynes by thallium(III) salts (oxythallation) (see Section 7). 

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