Hydroboration of Alkenes | Chemistry Optional Notes for UPSC PDF Download

Description

Hydroboration-oxidation transforms alkenes into alcohols. It performs the net addition of water across an alkene.

Notes

• Note that the oxygen is always attached at the less substituted carbon (anti-Markovnikoff). Furthermore the stereochemistry is always syn (H and OH add to same side of the alkene).
• The boron byproduct will depend on the # of equivalents of BH₃ used reative to the alkene. Here their molar ratio is 1:1. One equivalent of BH₃ can hydroborate up to 3 equivalents of alkene.
• BH₃-THF is the same as BH₃. Tetrahydrofuran (THF) is merely a solvent. Sometimes B₂H₆ is written, which is another form of BH₃. It behaves in exactly the same way as BH₃.
• You might also see 9-BBN or (Sia)₂BH. These are hydroboration reagents in which two of the H atoms in BH₃ have been replaced by carbon atoms. They will do the exact same reaction as BH₃.

Examples

Notes: Example 1 just shows a simple anti-Markovnikov addition of BH₃ to an alkene. Examples 2 and 3 show the hydroboration of a cyclic alkene. Note the syn addition; the C-H bond and C-OH bond are formed on the same side of the ring (this results in a mixture of enantiomers in this case). Example 4 has an existing chiral center (the methyl group) which is unaffected by hydroboration, so the result will be a mixture of diastereomers.

Mechanism

The reaction begins with the concerted syn addition of B and H across the double bond, with the boron adding to the less substituted carbon (Step 1, arrows A and B). In the second step, hydrogen peroxide and a base such as NaOH are added. the NaOH deprotonates the hydrogen peroxide (Step 2, arrows C and D) which makes the conjugate base of hydrogen peroxide (a better nucleophile than H₂O₂ itself). The resulting NaOOH then attacks the boron (Step 3, arrow E). This sets up the key migration step, where the carbon-boron bond migrates to the oxygen bound to boron, breaking the weak oxygen-oxygen bond (Step 4, arrows F and G). The OH expelled then comes back to form a bond on the boron (Step 5, arrows H and I) resulting in the deprotonated alcohol (alkoxide). The alkoxide is then protonated by water or some other comparably acidic species (Step 6, arrows J and K).

Notes

• In step 1 the addition is syn and the reaction is concerted.
• If excess alkene is present the two remaining B-H bonds can do subsequent hydroborations.
• Note that only one enantiomer is shown here (but the product will be racemic)
• The migration step (Step 4, arrows F and G) occurs with retention of stereochemistry at the carbon.
• For the last step it’s reasonable to use water, HOOH or any other comparably acidic species as the acid.