SUBSTITUTIONS IN ORGANOMERCURY COMPOUNDS

The reactivity of mercury salts is a fimction of both the solvent and the counterion in the mercury salt. Mercuric chloride, for example, is unreactive, and mercuric acetate is usually used. When higher reactivity is required, salts of electronegatively substituted carboxylic acids such as mercuric trifiuoroacetate can be used. Mercuric nitrate and mercuric perchlorate are also highly reactive. Soft anions reduce the reactivity of the Hg " son by coordination, which reduces the electrophilicity of the cation. The harder oxygen anions leave the mercuric ion in a more reactive state. Organomercury compounds have a number of valuable synthetic applications, and these will be discussed in Chapter 8 of Part B. 

A variety of organomercury compounds that can serve as precursors of substituted carbenes have been synthesized. For example, carbenes with carbomethoxy or trifluoromethyl substituents can be generated in this way.166... 

In the gas phase, divalent mercury has been shown to be linear and therefore to be sp hybridized. However, in solution the X—R—X, R—Hg—X, or R—Hg—R bond angle in divalent mercury compounds varies from 130 to 180°. The variation in geometry is not yet entirely understood, so we shall follow Jensen s example and assume that, even in solution, divalent mercury is sp hybridized and that if a divalent mercury compound donates one empty orbital to coordinate with a Lewis base it rehybridizes to sp2 (F. R. Jensen and B. Rickborn, Electrophilic Substitution of Organomercurials, pp. 35, 36). 

Treatment of 1-organylsilatranes with mercury(II) salts in protic or aprotic media gives the corresponding organomercury compound and the substituted silatrane (equation 108)132,330. 

Another approach for the preparation of either symmetrical or unsymmetrical iodonium salts used organolithium or organomercury compounds and (dichloroiodo)arenes [12]. The problem of the formation of unwanted isomers during reactions involving aromatic electrophilic substitution may also be overcome by the condensation of iodosylarenes with iodylarenes [12]. Several iodonium triflates were prepared in high yield from activated or mildly deactivated arenes with iodosylbenzene and triflic anhydride or triflic acid [13,14] or sulphur trioxide [15]. Some of these compounds are shown in Table 8.2. 

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. 

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. ... 

F. R. Jensen, Electrophilic Substitution of Organomercurials, McGraw Hill, New York (1968) Tetr 38 1713 (1982) R. C. Larock, Organomercury Compounds in Organic Synthesis, Springer, New York (1985), Chpt 3... 

It was demonstrated by Heck in the late 1960s that arylpaUadium salts, prepared by trans-metallation of organomercury compounds, constitute useful reactants in various vinylic substitution reactions.f t Independently, Moritani, Fujiwara, and colleagues conducted similar vinylic substitutions, but generated the organopalladium intermediates by direct electrophilic palladation of arenes.t ° In these reactions the palladium(II) salt employed is reduced to paUadium(O) (Scheme ). ... 

Organomercury compounds, in particular, have already been largely substituted and are no longer of much importance as microbicides for material protection. In the truest sense of the word they may be termed biocides, since they are effective not only against microbes, but also against all forms of life. 

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