Hydration hydroboration-oxidation

When predicting the product of a reaction, you have to recall what you know about the kind of reaction being carried out and then apply that knowledge to the specific case you re dealing with. In the present instance, recall that the two methods of hydration—hydroboration/oxidation and oxymercura-tion—give complementary products. Hydroboration/oxidation occurs with syn stereochemi. itiy and gives the non-Markovnikov addition product oxymercuration gives the Markovnikov product. 

Carbocation intermediates are not involved m hydroboration-oxidation Hydration of double bonds takes place without rearrangement even m alkenes as highly branched as the following... 

The mechanistic complexity of hydroboration-oxidation stands m contrast to the simplicity with which these reactions are carried out experimentally Both the hydrobo ration and oxidation steps are extremely rapid reactions and are performed at room tern perature with conventional laboratory equipment Ease of operation along with the fact that hydroboration-oxidation leads to syn hydration of alkenes and occurs with a regio selectivity opposite to Markovmkov s rule makes this procedure one of great value to the synthetic chemist... 

Hydroboration-oxidation (Sections 6 11-6 13) This two step sequence achieves hydration of alkenes in a ste reospecific syn manner with a regiose lectivity opposite to Markovnikov s rule An organoborane is formed by electro philic addition of diborane to an alkene Oxidation of the organoborane inter mediate with hydrogen peroxide com pletes the process Rearrangements do not occur... 

Oxidation. The oxidation reactions of organoboranes have been reviewed (5,7,215). Hydroboration—oxidation is an anti-Markovnikov cis-hydration of carbon—carbon multiple bonds. The standard oxidation procedure employs 30% hydrogen peroxide and 3 M sodium hydroxide. The reaction proceeds with retention of configuration (216). 

Methyl-2-hexcne has a disubstituted double bond, RCH=CHR, and would probably give a mixture of two alcohols with either hydration method since Markovnikov s rule does not apply to symmetrically substituted alkenes. 3-MethyI-3-hexene, however, has a trisubstituted double bond, and would give only the desired product on non-Markovnikov hydration using the hydroboration/oxidation method. 

Hydration of an alkene—the addition of water—is carried out by either of two procedures, depending on the product desired. Oxymercuration involves electrophilic addition of Hg2+ to an alkene, followed by trapping of the cation intermediate with water and subsequent treatment with NaBH4. Hydroboration involves addition of borane (BH3) followed by oxidation of the intermediate organoborane with alkaline H202- The two hydration methods are complementary oxymercuration gives the product of Markovnikov addition, whereas hydroboration/oxidation gives the product with non-Markovnikov syn stereochemistry. 

Draw the structures of alkenes that would yield the following alcohols on hydration (red - O). Tell in each case whether you would use hydroboration/ oxidation or oxymercuration. 

Like alkenes (Sections 7.4 and 7.5), alkynes can be hydrated by either of two methods. Direct addition of water catalyzed by mercury(II) ion yields the Markovnikov product, and indirect addition of water by a hydroboration/ oxidation sequence yields the non-Markovnikov product. 

The hydroboration/oxidation sequence is complementary to the direct, mercury(ll)-catalyzed hydration reaction of a terminal alkyne because different products result. Direct hydration with aqueous acid and mercury(IJ) sulfate leads to a methyl ketone, whereas hydroboration/oxidation of the same terminal alkyne leads to an aldehyde. 

The chemistry of alkynes is dominated by electrophilic addition reactions, similar to those of alkenes. Alkynes react with HBr and HC1 to yield vinylic halides and with Br2 and Cl2 to yield 1,2-dihalides (vicinal dihalides). Alkynes can be hydrated by reaction with aqueous sulfuric acid in the presence of mercury(ll) catalyst. The reaction leads to an intermediate enol that immediately isomerizes to yield a ketone tautomer. Since the addition reaction occurs with Markovnikov regiochemistry, a methyl ketone is produced from a terminal alkyne. Alternatively, hydroboration/oxidation of a terminal alkyne yields an aldehyde. 

Formal hydration of the double bond appeared by the hydroboration-oxidation sequence. Desymmetrization reactions with catalytic asymmetric hydroboration are not restricted to norbornene or nonfunctionalized substrates and can be successfully applied to meso bicyclic hydrazines. In the case of 157, hydroxy derivative 158 is formed with only moderate enantioselectivity both using Rh or Ir precatalysts. Interestingly, a reversal of enantioselectivity is observed for the catalytic desymmetrization reaction by exchanging these two transition metals. Rh-catalyzed hydroboration involves a metal-H insertion, and a boryl migration is involved when using an Ir precatalyst (Equation 17) <2002JA12098, 2002JOC3522>. 

Hydrate to alcohol hydroborate/oxidize (THF/B2H5,H202/0H-) (syn, anti-Markovnikov)... 

The hydroboration oxidation reaction proceeds to give cis hydration from the less hindered side of the molecule. 

The r alcohol, (iii), and the 2° alcohol, (iv) can be made by either of the two remaining syntheses. However, the one-step hydration with H,0" to give (iv) is shorter than the two-step hydroboration-oxidation to give (iii). 

Hydration of alkenes can also be achieved either by oxymerciiration-reduction (Markovnikov addition of water) or hydroboration-oxidation (a ti-Markovnikov addition of water). Addition of water by oxymercura-tion-reduction or hydroboration-oxidation has two advantages over the acid-catalysed addition of water. These procedures do not require acidic condition, and carbocation rearrangements never occur. Thus, they give high yields of alcohols. 

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