8.3c Hydroboration Oxidation
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|Two Groups Added|| || |
|Reagents Added|| 1. BH3.THF|
2. H2O2, OH-, H2O
|Other Characteristics||Intermediate is a trialkylborane.|
Hydroboration-oxidation results in the anti-Markovnikov addition of a hydrogen (more substituted side) and a hydroxyl group (less substituted side) across an alkene forming an alcohol. The reaction exhibits syn stereospecificity and is not subject to rearrangements as it does not involve a carbocation intermediate. The reaction occurs in two steps with the first involving the addition of borane which exists as a dimer but is often complexed with tetrahydrofuran (1. BH3.THF or 1. B2H6). The resulting intermediate is then oxidized with hydrogen peroxide and sodium hydroxide (2. H2O2, NaOH).
Undergraduate students are typically only responsible for the mechanism of the 1st step, hydroboration (1. BH3.THF) as the mechanism for the 2nd oxidation step is somewhat complicated, and therefore many textbooks and professors choose to omit the mechanism for the oxidation step from the curriculum. If you're taking a Ochem 1 you should pay particular attention to see if you're responsible for the oxidation mechanism.
Hydroboration - 1. BH3.THF
Boron does not have a filled octet making it electron deficient and a highly reactive electrophile. The pi electrons of the alkene attack the boron atom attaching it to the less substituted side of the alkene. Simultaneously, one of the B-H bonds breaks and this hydrogen attaches to the more substituted carbon of the alkene. The result is an alkylborane.
This single mechanistic step is essentially the entire mechanism for hydroboration though it does repeat twice to form a trialkylborane intermediate. Students are often shown and are responsible for the transition state for this step and it is included in the mechanism below. We can see that the boron and the hydrogen attach to both sides of the alkene at the same time and originate from the same BH3 molecule. This explains why they must add to the same face of the alkene making this a syn addition.
Oxidation - 2. H2O2, NaOH
The net result of this 2nd step is the conversion of the trialkylborane intermediate into three equivalents of the product alcohol.
Once again, most undergraduate students will not need to know the mechanism for this second step but for the few of you that do (and my apologies to you!) the mechanism for a generic trialkylborane is shown below.
Step 1: Proton Transfer - A hydroxide ion deprotonates hydrogen peroxide to form a hydroperoxide ion.
Step 2: Nucleophilic Attack - The hydroperoxide ion attacks the boron of the trialkylborane.
Step 3: Anionic Rearrangement - An uncommon anionic rearrangement takes place with the loss of hydroxide.
Steps 1-3 are repeated two more times.
Step 4: Nucleophilic Attack - A hydroxide ion attacks the boron of the intermediate.
Step 5: Loss of Leaving Group - The bond between boron and oxygen breaks with an alkoxide ion serving as the leaving group.
Step 6: Proton Transfer - The alkoxide ion is protonated by a water molecule to yield the alcohol product.
Steps 4-6 are repeated two more times two yield two more equivalents of the alcohol product.