Alkenes oxymercuration-reduction

Moreover, because of the involvement of cationic intermediates, rearrangements can occur in systems in which a more stable cation can result by aryl, alkyl, or hydrogen migration. Oxymercuration-reduction, a much milder and more general procedure for alkene hydration, is discussed in the next section. 

Oxymercuration-reduction of alkenes preparation of alcohols Addition of water to alkenes by oxymercuration-reduction produces alcohols via Markovnikov addition. This addition is similar to the acid-catalysed addition of water. Oxymercuration is regiospecific and auft -stereospecific. In the addition reaction, Hg(OAc) bonds to the less substituted carbon, and the OH to the more substituted carbon of the double bond. For example, propene reacts with mercuric acetate in the presence of an aqueous THF to give a hydroxy-mercurial compound, followed by reduction with sodium borohydride (NaBH4) to yield 2-propanol. 

Addition of alcohol to alkenes hy alkoxymercuration-reduction produces ethers via Markovnikov addition. This addition is similar to the acid-catalysed addition of an alcohol. For example, propene reacts with mercuric acetate in aqueous THF, followed hy reduction with NaBFl4, to yield methyl propyl ether. The second step is known as demercuration, where Flg(OAc) is removed hy NaBH4. Therefore, this reaction is also called alkoxymercura-tion-demercuration. The reaction mechanism is exactly the same as the oxymercuration-reduction of alkenes. 

In addition to the hydration reaction described in Section 11.3, the oxymercuration-reduction reaction can be used to add the elements of water to a carbon-carbon double bond in a two-step process. First the alkene is reacted with mercuric acetate, Hg(02CCH3)2, in water, followed by treatment with sodium borohydride in sodium hydroxide solution ... 

The hydroxy group must be located on one of the doubly bonded carbons of the original alkene, so first draw all of the alkenes that meet this criterion. Examine the alkenes to determine whether it is possible to selectively add the OH group to the desired carbon. Remember that we can add the OH with either Markovnikov orientation (acid-catalyzed hydration or oxymercuration-reduction) or anti-Markovnikov orientation (hydroboration-oxidation), but we will have difficulty selecting between two carbons that are similarly substituted. 

Oxidation of an Alcohol- Section 10.14 Figure 10.8 Oxymercuration-Reduction Section 11.6 Figure 11.5 Ozonolysis of an Alkene Section 11.11... 

The overall result of the hydroboration-oxidation sequence is addition of water to an alkene with the opposite regiochemistry to that expected for a conventional acid-catalysed hydration. The usual way to do such a hydration is by oxymercuration-reduction. 

Only oxymercuration/reduction can be used to produce an alcohol that has -OH bonded to the more substituted carbon. A third alkene, 2,3-dimethyl-2-pentene, gives a mixture of tertiary alcohols when treated with either BH3 or Hg(OAc)2. 

Alcohols can be prepared by hydration of alkenes. Because the dii hydration of alkenes with aqueous acid is generally a poor ruacuoo in the laboratury. tvw> indirect methods are commonly uaed Hydro-borationfmtidation yields the product of sy , r on-Morkovnikov hydr, tion iSection 7.5). whereas oxymercuration/reduction yields the p od-1 urt of Markovnikow hydration (Section 7,4). Both reactions are generally applicable to most alkenes. 

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. ° ... 

Addition of Water to Alkenes Oxymercuration 239 Addition of Water to Alkenes Hydroboration 242 Addition of Carbenes to Alkenes Cyclopropane Synthesis Reduction of Alkenes Hydrogenation 249... 

After the addition step is complete, the mercury is usually replaced by hydrogen, using a reducing agent. The net result is the addition of hydrogen and the nucleophile to the alkene, and the reaction is known as oxymercuration reduction. The regioselectivity is in the same sense as is observed for proton-initiated additions. ... 

In Section 4.5, you learned that water adds to an alkene if an acid catalyst is present. This is the way aUcenes are converted into alcohols industrially. However, under normal laboratory conditions, water is added to an alkene by a procedure known as oxymercuration-reduction. The addition of water by oxymercuration-reduction has two advantages over acid-catalyzed addition It does not require acidic conditions that are harmful to many organic molecules, and because carbocation intermediates are not formed, carbocation rearrangements do not occur. 

You might compare the product of oxymercuration-reduction of 3,3-methyl-l-butene with the product formed by acid-catalyzed hydration of the same alkene (Section 6.3C). In the former, no rearrangement occurs. In the latter, the major product is 2,3-dimethyl-2-butanol, a compound formed by rearrangement. The fact that no rearrangement occurs during oxymercuration-reduction indicates that at no time is a free carbocation intermediate formed. 

Oxymercuration-Reduction (Section 6.3F) Oxymercuration-reduction is used to convert alkenes to alcohols. The mechanism involves reaction of the alkene tt bond with an HgOAc+ to give a bridged mercurinium ion intermediate (a cation) that is, in turn, attacked by HjO from the backside to give a new intermediate, which loses a proton. In a second step, NaBH is added to replace the Hg atom with H.The first step is anti stereoselective because HgOAc and OH add from opposite faces of the alkene. However, the NaBH reduction step scrambles the stereochemistry as H replaces Hg, so the overall process is scrambled... 

Oxymercuration-reduction (Section 6.3F) A method for converting an alkene to an alcohol. The alkene is treated with mercury(II) acetate followed by reduction with sodium borohydride. 

Perlmutter used an oxymercuration/demercuration of a y-hydroxy alkene as the key transformation in an enantioselective synthesis of the C(8 ) epimeric smaller fragment of lb (and many more pamamycin homologs cf. Fig. 1) [36]. Preparation of substrate 164 for the crucial cyclization event commenced with silylation and reduction of hydroxy ester 158 (85-89% ee) [37] to give aldehyde 159, which was converted to alkenal 162 by (Z)-selective olefination with ylide 160 (dr=89 l 1) and another diisobutylaluminum hydride reduction (Scheme 22). An Oppolzer aldol reaction with boron enolate 163 then provided 164 as the major product. Upon successive treatment of 164 with mercury(II) acetate and sodium chloride, organomercurial compound 165 and a second minor diastereomer (dr=6 l) were formed, which could be easily separated. Reductive demercuration, hydrolytic cleavage of the chiral auxiliary, methyl ester formation, and desilylation eventually led to 166, the C(8 ) epimer of the... 

The stereochemical outcome would also be different as the hydroboration adds syn to the alkene, whereas oxymercuration gives the anti product though in this case the stereochemistry is lost in the reduction step. 

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