Orientation in hydroboration reactions

The hydroboration reaction is also very predictable with regard to the stereochemistry of addition. The addition occurs stereospecifically syn through a four-center transition state with essentially simultaneous bonding to boron and hydrogen. Both the new C-B and C-H bonds are therefore formed from the same side of the multiple bond. In molecular orbital terms, the addition reaction is viewed as taking place by interaction of the olefin rr-orbital with the empty p-orbital on trivalent boron. Formation of the carbon-boron bond is accompanied by concerted rupture of a B-H bond  

Hydroboration is thermally reversible. At 160° and above, B-H moieties are eliminated from alkylboranes, but in this temperature range the equilibrium is still in favor of the addition. These reversible additions lead to migration of boron to the 

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. 

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