Diborane Hydroboration-oxidation

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... 

With this as introduction, let us now look at the individual steps in more detail for the case of hydroboration-oxidation of 1-decene. A boron hydride that is often used is dibomne (B2H6). Diborane adds to 1-decene to give tridecylborane according to the balanced equation ... 

The stereochemical outcome of the hydroboration-oxidation of 1, V-bicyclohexenyl depends on the amount of diborane used. When 1.1 equivalent... 

The stereochemical outcome of the hydroboration-oxidation of l,l -bicyclohexenyl depends on the amount of diborane used in the hydroboration. When 1.1 equiv is used, the product is a 3 1 mixture of C and D. When 2.1 equiv is used, C is formed nearly exclusively. Offer an explanation of these results. 

From A17(20)-Olefins The addition of an oxygen function to C-20 is best accomplished by the hydroboration-oxidation reaction on a A17(20) olefin. The ready preparation of such olefins has been discussed earlier predominantly m-olefins are obtained via the Wittig reaction on 17-keto steroids, and the /ra/w-olefins via dehydration reactions. Hydroboration-oxidation of m-A17(20)-olefins gives the 20a-alcohol in high yield, presumably by attack of diborane on the a-side of the double bond 137... 

It is a fairly simple matter to carry out hydroboration-oxidation with optically active boranes. The readily available optically active terpene (H-)-2-pinene is converted by diborane to a dialkylborane." This reagent can then add to alkenes, and, on oxidation, optically active alcohols are obtained." Table 10.1 gives some data on the percent enantiomeric excess observed in such reactions. 

The hydroboration-oxidation procedure is a valuable method to hydrate an alkene with anti-Markovnikov orientation and with syn addition of the H and OH groups. ° Addition of BH3 (which may be added to the reaction mixture as diborane, B2Hg) to an alkene occurs readily in diethyl ether, THF, or similar solvent. The hydroboration is strongly exothermic, with a AH of -33kcal/mol per B-H bond that reacts. If stoichiometry and the steric requirements of the alkyl substituents on the boron atom permit, the reaction proceeds until three alkyl groups are attached to each boron atom. The trialkylborane can then be oxidized with hydrogen peroxide in aqueous base to produce the alcohol. 

While the four-center transition structure for BH3 addition is a widely used model, other reaction pathways have also been considered. In a synthesis of optically active (-)-l-butanol-l-d, Streitwieser and co-workers used the optically active borane formed from diborane and (-l- )-a-pinene (R2BH) to carry out the hydroboration-oxidation of (Z)-l-butene-l-d. To explain the observed stereochemistry of the reduction, they proposed that hydroboration involves a n complex between R2BH and the alkene (Figure 9.41). Note the close resemblance of such a complex to the cyclic structures... 

Hydroboration-Oxidation An alkene reacts with BHsTHF or diborane to produce an alkylborane. Oxidation and hydrolysis of the alkylborane with hydrogen peroxide and base yield an alcohol (Section 8.6). 

The regiochemistry of acid-catalyzed hydration of alkenes to give alcohols is that predicted by Markovnikov s rule (Sec. 10.4) because the more stable intermediate carbocation is preferentially formed. Sometimes it is desirable to add the elements of water across a carbon-carbon Ti-bond in the opposite regiochemical sense to provide the anti-Markovnikov product (see the Historical Highlight at the end of this chapter). In order to accomplish this goal, a process termed hydroboration-oxidation was developed that involves the reaction of an alkene sequentially with diborane, B2H, and basic hydrogen peroxide (Eq. 10.26). 

A study of the hydroboration-oxidation of unsaturated arylsulphones by 9-BBN or diborane followed by hydrogen peroxide has appeared as has a report concerning the high regioselectivity of 9-BBN, disiamylborane, and dicyclohexylborane in the hydroboration of vinyltrimethylsilane. ... 

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