Stereochemistry of hydroboration-oxidation

Organoborane interiflediates can also be used to synthesize alkyl halides. Replacement of boron by iodine is rapid in the presence of base. ° Only two of the alkyl groups are efficiently used for primary alkyl groups, and only one with secondary alkyl groups. A similar process using bromine and sodium hydroxide 

The discovery that carbon monoxide reacts with organoboranes under mild 

It should be noted that the latter synthesis utilizes only one third of the alkyl groups in the starting organoborane, a considerable limitation in the case of valuable olefins. The remaining alkyl groups are converted to the unhomologated alcohol and present a separation problem. 

A route to aldehydes from olefins via carbonylation has also been developed. The 9-BBN reagent formed on hydroboration of 1,5-cyclooctadiene is used to effect hydroboration of the olefin. Carbonylation of the resulting trialkylborane proceeds 

A second aspect of hydroboration-oxidation concerns its stereochemistry. As illustrated for the case of 1-methylcyclopentene, H and OH add to the same face of the double bond. 

Borane (BH3) does not exist as such under normal conditions of temperature and atmospheric pressure. Two molecules of BH3 combine to give diborane (B2H6), which is the more stable form. 

Overall, the reaction leads to syn addition of the elements of water to the double bond. This fact has an important bearing on the mechanism of the process. 

PROBLEM 6.13 Hydroboration-oxidation of a-pinene (page 213), like catalytic hydrogenation, is stereoselective. Addition takes place at the less hindered face of the double bond, and a single alcohol is produced in high yield (89%). Suggest a reasonable structure for this alcohol. 

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The regioselectivity and syn stereochemistry of hydroboration-oxidation coupled with a knowledge of the chemical properties of alkenes and boranes contribute to our under standing of the reaction mechanism... 

Problem 15.10 The stereochemistry of hydroboration-oxidation is the net result of the stereochemistry of the two steps, and is consistent with either of two combinations of stereochemistry for the individual steps. What aic these two combinations ... 

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... 

In the following sections we shall consider details of the mechanism that lead to the anti-Markovnikov regiochemistry and syn stereochemistry of hydroboration-oxidation. 

NEW Regiochemistry of Hydroboration/Oxidation of an Acyclic Alkene, p. 394 Regio- and Stereochemistry of Hydroboration/Oxidation of a Cyclopentene, p. 394... 

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