Oxymercu ration

Markovnikov s rule. 191-193 alkene additions and, 191-193 alkyne additions and. 263 carbocation stability and. 192-193 Hammond postulate and, 198-199 hydroboration and. 224-225 oxymercu ration and, 222 Mass number (A), 4 Mass spectrometer, double-focusing, 411... 

Many electrophilic additions to alkenes in solution proceed via an intermediate, which reacts subsequently with nucleophiles. Experimental evidence indicates that protonation of unstrained double bonds leads to an acyclic intermediate, whereas cyclic ionic intermediates are found in oxymercu-rations, brominations, and similar electrophilic additions. In general, the... 

Of the methods we have seen for Markovnikov hydration of alkenes, oxymercu-ration-demercuration is most commonly used in the laboratory. It gives better yields than direct acid-catalyzed hydration, it avoids the possibility of reanangements, and it does not involve harsh conditions. There are also disadvantages, however. Organomer-curial compounds are highly toxic. They must be used with great care and then must be disposed of properly. 

Methyl-l-hexene is a terminal alkene, and formation of the more stable secondary cation leads to a single major product. However, oxymercu-ration of an unsymmetrical internal alkene leads to a mixture of products. Oxymercuration of 3,3-dimethylcyclopentene, for example, gives a mixture of 74 and 75. Although rearrangement did not occur, two regioisomers are formed because both of the possible carbocation intermediates are secondary. The two mercury-stabilized carbocations are essentially equal in stability, so both are formed and subsequent reaction with water leads to the mixture of alcohols shown after reduction with sodium borohydride. 

Attraction between this acidic hydrogen and an oxygen of the hydrate (see 38) leads to proton transfer to oxygen and loss of water from the hydrate to form a new species (39), which is called an enol. An enol is formally defined as a molecule with an OH group attached directly to a C=C unit. As first observed in Chapter 10 (Section 10.4.5), where enols are formed by oxymercu-ration of alkynes, enols are very unstable and exist in an equilibrium with the corresponding carbonyl compound. Experiments show that this equilibrium between the enol form and the carbonyl form lies toward the carbonyl. 

Preparative Methods three methods of preparation have been reported (a) from the treatment of ethyl bromoacetate with zinc followed by the reaction with chlorotrimethylsUane and subsequent reduction of the resultant ethyl trimethylsUylacetate with lithium aluminum hydride or borane-tetrahydrofuran (eq 1) (b) from the hydroboration/oxidation or oxymercu-ration/ demercuration of vinyltrimethylsUane (eq 2) and (c) most conveniently, by the reaction of the Grignard reagent formed from (chloromethyl)trimethylsilane with paraformaldehyde (eq 3). ... 

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