Oxymercuration of alkenes

Thomsonl Click Organic Interactive to use a web-based palette to predict products of the oxymercuration of alkenes. 

Diastereoselectivity has been observed in oxymercuration of alkenes having nearby oxygen substituents. Terminal allylic alcohols show a preference for formation of the anti 2,3-diols. 

Alkenes participate readily in oxymercuration with increasing alkyl-substitution resulting in decreasing reactivity.489,491,492 The oxymercuration of alkenes is usually an anti process.493 195 Exceptions are strained bicyclic olefins, such as norbomene495,496 [Eq. (6.83)],497 and frans-cyclooctene and frans-cyclononene.494... 

Oxymercuration of alkenes is an electrophilic reaction involving the addition of mercuric salt and of a protic solvent (alcohols) according to reaction 9. UV/VIS spectrophotometric investigation of the olefin/mercuric salt reaction mixtures in methylene chloride provides evidence of the presence of an electron donor-acceptor complex between olefin and mercuric salt110 which is considered to be on the reaction pathway of the oxymercuration. 

The carboxylate anion may neutralize a carbocation formed by the attack of an electrophilic metal on an alkene, as in the oxymercuration of alkenes. 

There are distinct patterns of reactivities in brominations and oxymercurations of alkenes. However, the reactivities of the alkenes appear identical when the differences between the steric effects in the transition states for brominations and oxymercuration are taken into account . 

The mercuric ion-catalyzed hydration of alkynes probably proceeds in a similar manner to the oxymercuration of alkenes (see Section 5.1). Electrophilic addition of Hg to the triple bond leads to a vinylic cation, which is trapped by water to give an vinylic organomercury intermediate. Unlike the alkene oxymercuration, which requires reductive removal of the mercury by NaBH4, the vinylic mercury intermediate is cleaved under the acidic reaction conditions to give the enol, which tautomerizes to the ketone. Hydration of terminal alkynes follows the Mai kovnikov rule to furnish methyl ketones. ° ... 

Table 2. Equilibrium Constants for the Oxymercuration of Alkenes with Mercury(II) T rifluoroacetate...

A potentially important appUcation of micelles or similar species is the extent to which they can control product composition. The abiUty of cationic micelles to speed E2 eliminations and inhibit S l reactions has been cited [53-55]. But in addition micelles can control isomer distribution to some extent, as in the halogenation of phenolic ethers [179]. In another example Sukenik and coworkers have used micelles to mediate the isomeric composition of the products of oxymercuration of alkenes [180],... 

Oxymercuration of alkenes probably involves formation of a cationic alkene complex, which undergoes nucleophilic attack by solvent (Eq. 14.31) and gives the Markownikov product and so complements Eq. 14.30. 

Like alkenes, alkynes can be hydrated. The reaction is generally catalyzed hy mercuric ions in an oxymercuration process (Fig. 10.71), although simple acid catalysis is also known. In contrast to the oxymercuration of alkenes, no second, reduction step is required in this alkyne hydration. By strict analogy to the oxymercuration of alkenes, the product should be a hydroxy mercury compound, hut the second double bond exerts its influence and further reaction takes place. The double hond is protonated and mercury is lost to generate a species called an enol. An enol is part alkene and part alcoho/, hence the name. 

FIGURE 10.71 The oxymercuration of alkynes resembles the oxymercuration of alkenes. In the alkyne case, the product is an enol, which can react further. 

At the same oxidation level, alcohols can be prepared by substitution reactions and addition reactions. Alkali hydroxides will convert appropriate alkyl chlorides, bromides, iodides, and sulfonates to alcohols. Oxymercuration of alkenes gives Markovnikov alcohols, and hydrobora-tion followed by oxidation gives anti-Markovnikov alcohols. Hindered boranes such as 9-BBN are used when selectivity toward one double bond or higher regioselectivity is needed (Eq. 6.41) [66]. 

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