Alcohols by hydroboration

Conversion of alkenes to alcohols by hydroboration is a synthetically-valuable reaction as it leads to the anti-Markovnikov product. 

One of royal jelly acids (10-hydroxy-2-decenoic acid) (141) was prepared from the telomer of acetoacetate, 142 (128). The terminal double bond was converted to terminal alcohol by hydroboration. The internal double bond was reduced and then reintroduced at the conjugated position by the addition of phenylselenyl group, and its oxidative removal completed the synthesis ... 

This is attributed to the unfavourable steric interactions which arise in the transition state that is required for antiperiplanar migration of the exocyclic substituent.143 Some examples of synthesis of alcohols by hydroboration-oxidation are included in Scheme 4.8. More vigorous oxidizing agents such as Cr(VT) reagents effect replacement of boron... 

Problem 7.10 What alkenes might be used to prepare the following alcohols by hydroboration/... 

This group is a pivalate ester equivalent that still has the steric advantage associated with pivalic acid but can be removed after the olefin is converted to an alcohol by hydroboration. 

When alkenes are converted to alcohols by hydroboration-oxidation, the hydroxyl group is introduced at the less substituted carbon of the double bond. 

Specify the appropriate alkene and reagents for synthesis of each of the following alcohols by hydroboration-oxidation. 

The methylenation product was transformed into the alcohol by hydroboration-oxidation without isolation. The yield for the two-step transformation is given. 

Some examples of synthesis of alcohols by hydroboration-oxidation are included in Scheme 4.9. 

The following cycloalkene gives a mixture of two alcohols on hydroboration loi-lowed by oxidation. Draw the structures of both, and explain the result. 

With ring G in place, the construction of key intermediate 105 requires only a few functional group manipulations. To this end, benzylation of the free secondary hydroxyl group in 136, followed sequentially by hydroboration/oxidation and benzylation reactions, affords compound 137 in 75% overall yield. Acid-induced solvolysis of the benzylidene acetal in 137 in methanol furnishes a diol (138) the hydroxy groups of which can be easily differentiated. Although the action of 2.5 equivalents of tert-butyldimethylsilyl chloride on compound 138 produces a bis(silyl ether), it was found that the primary TBS ether can be cleaved selectively on treatment with a catalytic amount of CSA in MeOH at 0 °C. Finally, oxidation of the resulting primary alcohol using the Swem procedure furnishes key intermediate 105 (81 % yield from 138). 

Several alkylboranes are available in enantiomerically enriched or pure form and can be used to prepare enantiomerically enriched alcohols and other compounds available via organoborane intermediates.196 One route to enantiopure boranes is by hydroboration of readily available terpenes that occur naturally in enantiomerically enriched or pure form. The most thoroughly investigated of these is bis-(isopinocampheyl)borane (Ipc)2BH), which can be prepared in 100% enantiomeric purity from the readily available terpene a-pinene.197 Both enantiomers are available. 

Catechol allenylboranes have also been used to synthesize homopropargylic alcohols [25], These reagents are prepared by hydroboration of an enyne with catechol-borane in the presence of a Pd(0) catalyst possessing monodentate phosphine ligands. Dienylboranes were formed as minor products. Optimum results were obtained by treatment of the catecholborane with molar equivalents of triphenylpho-sphine and the palladium catalyst. Although several allenylboranes were prepared, only the dimethyl reagent was further examined. Treatment of that borane with benzaldehyde afforded the homopropargylic alcohol in 62% yield (Eq. 9.21). 

Hydroboration-oxidation of alkenes preparation of alcohols Addition of water to alkenes by hydroboration-oxidation gives alcohols via anti-Markovnikov addition. This addition is opposite to the acid-catalysed addition of water. Hydrohoration is regioselective and syn stereospecific. In the addition reaction, borane bonds to the less substituted carbon, and hydrogen to the more substituted carbon of the double bond. For example, propene reacts with borane and THF complex, followed by oxidation with basic hydrogen peroxide (H2O2), to yield propanol. 

Albert S.C. Chun of the Hong Kong Polytechnic University reports (J. Org. Chem. 68 1589, 2003) two important transformations. The three-component (Mannich) condensation of 10 with 11 and 12 proceeds with high diastereoselectivity, to give the amino alcohol 13. Hydroboration of the alkyne 14 followed by transmetalation of the intermediate vinyl borane gives a zinc species, which under catalysis by the easily-prepared 3-naphthol 13 adds to aromatic and branched aldehydes with high . The product allylic alcohols are useful intermediates for organic synthesis. 

Asymmetric hydroboration.2 Extensive studies indicate that 1 is the reagent of choice for chiral hydroboration of rrans-disubstituted alkenes and trisubstituted alkenes. The corresponding alcohols are obtained in 72-100% ee and all have the same absolute configuration. Surprisingly, this configuration is the opposite to that obtained by hydroboration with diisopinocampheylborane. 

Hydroboration-oxidation converts 1-methylcyclopentene to the desired alcohol by anti-Markovnikov syn hydration of the double bond. The resulting alcohol is then converted to its p-toluenesulfonate ester and treated with acetate ion as in part (a) to give m-2-methyl-cyclopentyl acetate. 

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