Water alkenes + mercuric salts

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). 

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,),. ... 

Alkynes, unlike alkenes, are not hydrated readily in aqueous acid unless a mercuric salt is present as a catalyst. Also, the products that are isolated are either aldehydes or ketones instead of alcohols. Even though the addition of one molecule of water to ethyne probably gives ethenol (vinyl alcohol) initially, this compound is unstable relative to its structural isomer (ethanal) and rapidly rearranges ... 

Neutral cyclodextrins have been used as chiral phase-transfer catalysts for an interesting inverse phase-transfer catalysis reaction [50]. The Markovnikovhydration of the double bond by an oxymercuration-demercuration reaction has been demonstrated in the presence of cyclodextrins as chiral phase-transfer catalysts to obtain products in low to moderate enantioselectivity (Scheme 7.16). The mercuric salts are water-soluble, and remain in the aqueous phase, whereas the neutral alkenes prefer an organic phase. A neutral cyclodextrin helps to bring the alkenes into the aqueous phase in a biphasic reaction, and also provides the necessary asymmetric environment. 

The oxjnnercuration reaction is thought to be a two-step process. In the first step, electrophilic attachment of the mercury ion to the alkene produces a positively charged intermediate (equation 9.39). In the second step of oxy-mercuration, a nucleophile (most likely a solvent molecule, SOH) reacts with the intermediate to produce the organomercury compoimd (equation 9.40). For reactions in water, both the organomercurial and the final product are alcohols. The reaction produces an ether if the hydroxylic solvent is an alcohol, and the reaction is called solvomercuration or alkoxymercuration. Better yields are obtained if the anion of the mercuric salt is a weaker nucleophile than is the solvent. For this reason, mercuric... 

Mercuric salts (HgXg) are Lewis acids that react readily with alkenes, and experiments show that the reaction proceeds by formation of a carboca-tion intermediate. When mercuric acetate—HgCOAcla see the structure and Chapter 20, Section 20.5—reacts with 3-methyl-l-hexene in a THF-water mixture, the isolated product is a mercuric compound identified as 71 (with a C-Hg bond) and it is formed by reaction of water with a carbocation intermediate. The initial acid-base reaction is the donation of the 7i-electrons to form a new C-Hg covalent bond, leaving behind a carbocation at the other carbon of the C=C unit. 

The last example in Table 12-2 is an electrophilic addition of a mercuric salt to an alkene. The reaction is called mercuration, and the resulting compound is an alkyl-mercury derivative, from which the mercury can be removed in a subsequent step. One particularly useful reaction sequence is oxymercuration-demercuration, in which mercuric acetate acts as the reagent. In the first step (oxymercuration), treatment of an alkene with this species in the presence of water leads to the corresponding addition product. 

The acid-catalyzed hydration of alkynes (Table 6.7, example 2) is commonly carried out using mercury (11) salts, such as mercuric sulfate (HgS04), as catalysts. The addition (Scheme 6.67) appears to involve a bridged mercurinium ion, which, for unsymmetrical cases such as 1-alkynes other than ethyne (acetylene [HC CH]), is subsequently attacked by water (FI2O) at the carbon that best supports a positive charge. The regiochemistry of Markownikoff addition, seen with alkenes, is followed. 

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