Hydroboration
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
ص446
2025-06-03
655
These methods for adding water across a double or triple bond involve cationic intermediates, and always end up putting the new hydroxyl group at the position best able to stabilize a positive charge. By what about addition of water the other way round? How would you do the reaction in the margin for example? The answer is to make use of yet another element: boron. Boranes, including both BH3 itself and analogues with one or two alkyl groups, HBR2 (an important example is shown in the margin), add to alkenes to make a new C–H bond and a new C–B bond by a mechanism we can write like this. The alkene pushes electrons into the boron’s empty p orbital, while the hydrogen transfers onto the alkene.
Importantly, if the alkene is unsymmetrical, the boron tends to end up on the less substituted carbon atom. This reaction can happen several times so, for example, if you start with an alkene and BH3, you will typically end up with a trialkylborane:
So far so good, if you want to make boranes, but we started out this section posing ourselves the problem of adding water across a double bond. This is where a quirk of boron chemistry helps us out. The C–B bond(s) we have just created can be oxidized to C–O bonds by using a mixture of NaOH and H2O2. The mixture generates the hydroperoxide anion HO–O−, which adds to that important empty p orbital on boron. The product is a negatively charged structure, shown below.
We now have the C–O bond where we want it, and all that has to happen is for the hydroxide anion to come back in and displace B from the alcohol product. The product, on protonation, is our alcohol. How can we be sure the correct R group will migrate? Well, if we use BH3 we will get a trialkyborane, where all three groups on boron are the same, and all three C–B bonds can be oxidized one after another. If we use the HBR2 reagent 9-BBN, then only the non-cyclic substituent formed in the hydroboration reaction will migrate, selectively giving us the product we want.
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