Mercurous reaction

Pd(II) compounds coordinate to alkenes to form rr-complexes. Roughly, a decrease in the electron density of alkenes by coordination to electrophilic Pd(II) permits attack by various nucleophiles on the coordinated alkenes. In contrast, electrophilic attack is commonly observed with uncomplexed alkenes. The attack of nucleophiles with concomitant formation of a carbon-palladium r-bond 1 is called the palladation of alkenes. This reaction is similar to the mercuration reaction. However, unlike the mercuration products, which are stable and isolable, the product 1 of the palladation is usually unstable and undergoes rapid decomposition. The palladation reaction is followed by two reactions. The elimination of H—Pd—Cl from 1 to form vinyl compounds 2 is one reaction path, resulting in nucleophilic substitution of the olefinic proton. When the displacement of the Pd in 1 with another nucleophile takes place, the nucleophilic addition of alkenes occurs to give 3. Depending on the reactants and conditions, either nucleophilic substitution of alkenes or nucleophilic addition to alkenes takes place. 

Similarly to mercuration reactions, Pd(OAc)2 undergoes facile palladation of aromatic compounds. On the other hand, no reaction of aromatic compounds takes place with PdClj. PdCl2 reacts only in the presence of bases. The aro-... 

The iodination reaction can also be conducted with iodine monochloride in the presence of sodium acetate (240) or iodine in the presence of water or methanolic sodium acetate (241). Under these mild conditions functionalized alkenes can be transformed into the corresponding iodides. AppHcation of B-alkyl-9-BBN derivatives in the chlorination and dark bromination reactions allows better utilization of alkyl groups (235,242). An indirect stereoselective procedure for the conversion of alkynes into (H)-1-ha1o-1-alkenes is based on the mercuration reaction of boronic acids followed by in situ bromination or iodination of the intermediate mercuric salts (243). 

Nitromersol/777-j5 , y (4) and mercurophen [52486-78-9] (5) are prepared by the same mercuration reaction as phenyhnercuric acetate, only 4-nitro-(9-cresol and o-nitrophenol are used, respectively, iastead of benzene. The second step is reaction with sodium hydroxide to form the anhydride or sodium salt, respectively. 

The synthetic utility of the mercuration reaction derives from subsequent transformations of the arylmercury compounds. As indicated in Section 7.3.3, these compounds are only weakly nucleophilic, but the carbon-mercury bond is reactive to various electrophiles. They are particularly useful for synthesis of nitroso compounds. The nitroso group can be introduced by reaction with nitrosyl chloride73 or nitrosonium tetrafluoroborate74 as the electrophile. Arylmercury compounds are also useful in certain palladium-catalyzed reactions, as discussed in Section 8.2. 

Mercuration reactions provide a convenient approach to a broad range of arylmercury derivatives.82,83 This approach has been applied to a number of substrates, including phenols, indoles, phenylpyridines, and fV-arylamides.83 The products of these reactions as well as their conditions are summarized in Table 1.84 91 Other examples of such reactions include the mercuration of 2-(2 -naphthyl)pyridine (Equation (251)92 and terephthaldehyde (Equation (26))93 with Hg(OCOCF3)2 and Hg(C104)2, which afford 72 and 73, respectively. 

Table 1 Selected examples of mercuration reactions involving aromatic substrates...

Since the regioselectivity of mercuration reactions is often hard to control, recent efforts have focused on the use of Lewis-basic substituents whose role is to direct the approach of the mercuric ion to a specific position. Reaction of the diazo derivative 74 with Hg(OAc)2 leads to mercuration of the naphthalene moiety in the 8-position rather than mercuration of the electron-rich imidazole carbon atoms. 4 Presumably, the diazo-imidazole bidentate moiety coordinates to the mercuric cation, thereby directing substitution at the naphthalene 8-position (Equation (27)). 

The arene groups in (r/ -arcnc)tricarbonylchromium complexes are typically electron poor and display poor reactivity toward electrophiles. In the case of mercuration reactions, this lack of reactivity can be overcome by attachment of Lewis-basic substituents to the arene ring. For example, in the case of 75a-c, the presence of a pyridyl, oxazolyl, or methyl-A,A-dimcthylami no group promotes ortho-mercuration, leading to the formation of the bimetallic complexes 76a-c (Equation (28)). 07... 

Generally, mercuration reactions are overall second order, first order in the alkene and first order in the mercuric salt (equation 149)... 

Mercury(II) acetate is the starting material to prepare many organomer-cury compounds for paints and medicinal use. Such mercuration reactions of organics are discussed briefly below. 

Alternatively, the compound can be prepared by the reaction of mercury with methyl iodide in the presence of sunlight or by mercuration reaction of organics. 

The use of mercury has the merit of easy preparation of the aryl mercury compounds directly from the aromatic compound and an inorganic mercury salt, sometimes in complete regioselective manner. Preparation of the other metallated species generally involves more synthetic steps. An example of a radiofluorode-mercuration reaction (Scheme 29) is the synthesis of 6-p F]fluorometaraminol... 

The only exception to this rule concerns the mercuration reaction, but there are good reasons to believe that mercuration of thiophene derivatives occurs by a mechanism different from that in benzene derivatives (see Section II, F, 2). 

T n the search for new synthetic uses of organomercurials and the olefin A mercuration reactions in general, attention was drawn to ozone as a possible reactant. 

Table X lists the trifluoroacetato complexes of zinc, cadmium, and mercury that have been prepared. Mercury(II) trifluoroacetates in trifluoroacetic acid have been used widely in organic syntheses since they effect fast mercuration reactions of aromatic rings 35, 241),...

Cyclopropanol is significantly (a factor of 10 ) more reactive toward mercuric acetate than is phenyl cyclopropane. A cyclopropyl methyl ether is less reactive than the corresponding cyclopropanol by a factor of 10—20, and the cyclopropyl acetate is decreased in reactivity by a factor of nearly 10. These results are fully consistent with the large q value for the mercuration reaction reported by Oullette (vide supra). 

The usefulness of catalysts other than aluminum chloride, e.g., anhydrous hydrogen fluoride 202, 223), boron trifluoride 100), and phosphoric acid 94), has been examined. The mercuration reaction which is characteristic of aromatic systems takes place also for ferrocene, as has been described by Nesmeyanov and his co-workers 143,147) Through the mercury compounds these workers have also obtained the halogeno-ferrocenes. 

This mercuration reaction has been adapted by Stevens and Kenneyto perform elegant stereospe-cific syntheses of the Aristotelia indole alkaloids (+)-makomakine (97) and (+)-aristoteline (98) from (—)-3-pinene and indol-3-ylacetonitrile (96) in only two and three steps, respectively (Scheme 51). A similar procedure from (+)-a-pinene yielded racemic hobartine (99) in 11% overall yield. Enantiospecificity is not observed in this reaction for the same reasons encountered during the preparations of (93) and (94). 94... 

The simplest reaction is the mercuration reaction of aromatic compounds, which is commonly achieved by the action of mercuric acetate in methanol, e.g.,... 

Where do mercuration reactions fit into this picture A mercurinium ion has both similarities and differences, as compared with the intermediates that have been described for other electrophilic additions. The electrophile in oxymercuration reactions, +HgX or Hg +, is a soft Lewis acid and polarizes the TT-electrons of an alkene to the extent that a three-center two-electron bond is formed between mercury and the two carbons of the double bond. However, there is also back bonding from Hg +(i orbitals to the alkene tt orbital. There is weaker bridging in the mercurinium ion than in the three-center four-electron bonding of the bromonium ion. 

M. Smith and K. C. Langry, Mercuration reactions of porphyrins New efficient syntheses of harderoporphyrin and isoharderoporphyrin, J. Chem. Soc., Chem. Commun., 217 (1980). 

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