Products of hydroboration-oxidation

The less substituted carbon of the double bond in 3-ethyl-l-pentene is at the end of the chain. It is this carbon that bears the hydroxyl group in the product of hydroboration-oxidation. 

Problem 15.9 Predict the products of hydroboration-oxidation of (a) as 2 phcnyl-2-butene (b) /ro/fj-2-phenyl-2-butene (c) 1-methylcyclohexene. 

The initial product of hydroboration-oxidation of an alkyne is an enol, a compound containing a hydroxyl group bonded to a carbon of a carbon-carbon double bond. The name enol is derived from the fact that it is both an alkene -en-) and an alcohol (-0I). To this point, hydroboration-oxidation of all[Pg.318]

When drawing the products of hydroboration-oxidation, it is essential to consider the number of chirality centers that are created during the process. If no chirality centers are formed, then the stereospecificity of the reaction is not relevant. 

A carbonyl compound will be the product of hydroboration-oxidation only if a second molecule of BH3 or R2BH does not add to the ir-bond of the boron-substituted alkene. In the case of internal alkynes, the substituents on the boron-substituted alkene prevent the approach of the second boron-containing molecule. In the case of terminal alkynes, however, there is an H instead of a bulky alkyl group on the carbon that the second molecule adds to, so there is less steric hindrance toward the second addition reaction. Therefore, either BH3 or R2BH can be used with internal alkenes, but the more sterically hindered R2BH should be used with terminal alkynes. 

Radicals, lacking a closed outer shell of electrons, are capable of reacting with double bonds. However, a radical requires only one electron for bond formation, unlike the electrophiles presented in this chapter so far, which consume both electrons of the tt bond upon addition. The product of radical addition to an alkene is an alkyl radical, and the final products exhibit anti-Markovnikov regiochemistry, similar to the products of hydroboration-oxidation (Section 12-8). 

Write the structure, showing the stereochemistry, of the product of hydroboration—oxidation of 2,6,6-trimethyl-2-bicyclo[3.1.l]heptene (a-pinene). 

Draw the final product of hydroboration-oxidation of each of the compounds in Exercise 15.56. 

Figure 13.21 (a) The 1H NMR spectrum of cyclohexylmethanol, the product from hydroboration/oxidation of methylenecyclohexane, and (b) the 1H NMR spectrum of 1-methylcyclohexanol, the possible alternative reaction product. 

RCH=CHZ —> RCHJCHODialkylchloroboranes, obtained as the major products of hydroboration of 1-alkenes with monochloroborane complexed with dimethyl sulfide, are oxidized by PCC to aldehydes (66-68% yield). Similar oxidation of dialkylchloroboranes derived from cyclic alkenes with PCC gives ketones in 70-85% yield. 

J.46 ( )-Kaur-16-en-19-oic acid (23) was synthesized from the minor product ( )-K of hydroboration-oxidation via ( )-L.46... 

The reaction in equation 9.52 does not provide a convincing demonstration of the stereochemistry of hydroboration-oxidation, since 52 is thermodynamically more stable than the corresponding cis isomer. Therefore, Brown and Zweifel also carried out the procedure on 1,2-dimethylcyclopentene (53, equation 9.53). The product of that reaction was the less stable stereoisomer... 

The products of hydroboration—organoboranes—can be elaborated to a variety of organic functional groups, making hydroboration one of the most powerful synthetic reactions. The strategy in all such transformations involves adding a nucleophile to the Lewis-acidic boron, which makes it into an anionic center. In hydroboration-oxidation, the nucleophile is typically alkaline hydrogen peroxide ... 

Below are several examples of hydroboration-oxidation. In each case, consider the expected regioselectivity, and then draw the product ... 

Which alcohols can be prepared as a single product by hydroboration-oxidation of an alkene Which alcohols can be prepared as a single product by the acid-catalyzed addition of H2O to an alkene ... 

Preparation.—Variations continue to appear on the theme of alcohol production by hydroboration-oxidation of olefins. 5-Methoxydialkylboranes react with olefins in the presence of lithium aluminium hydride to afford a new route to trialkylboranes and thence, by carbonylation-oxidation, to trialkylcarbinols. Carbonylation with carbon monoxide is avoided in a new procedure in the presence of an excess of trifluoroacetic anhydride, trialkyl cyanoborates undergo a triple alkyl migration from boron to carbon to give, on oxidation, high yields of trialkylcarbinols (Scheme 126). Tri-... 

Asymmetric Hydroboration. Hydroboration—oxidation of (Z)-2-butene with diisopinocampheylborane was the first highly enantioselective asymmetric synthesis (496) the product was R(—)2-butanol in 87% ee. Since then several asymmetric hydroborating agents have been developed. Enantioselectivity in the hydroboration of significant classes of prochiral alkenes with representative asymmetric hydroborating agents is shown in Table 3. 

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