Organic Chemistry 1 · Alkene addition and regiochemistry
Disiamylborane hydroboration of (Z)-4-methylpent-2-ene
Conditions: Sia2BH, THF; then H2O2, NaOH, H2O
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The mechanism, step by step
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Concerted syn hydroboration
In one concerted four-centre step the π electrons of (Z)-4-methylpent-2-ene bond C2, the alkene carbon next to the methyl group to the empty p orbital of boron in disiamylborane, while the B–H hydrogen moves to C3, the alkene carbon next to the isopropyl group. Boron and hydrogen add to the same face (syn), and no carbocation forms. Both alkene carbons are CH, so the choice is purely steric: the bulky sec-isoamyl groups keep boron away from the carbon next to the isopropyl group.
The curved arrows
- The C=C π pair forms the C–B bond at C2, the alkene carbon next to the methyl group, the less hindered alkene carbon.
- The B–H bond pair delivers hydrogen to C3, the alkene carbon next to the isopropyl group on the same face.
Hydroperoxide adds to boron
In the oxidation stage, hydroperoxide ion (HOO⁻, formed from H2O2 and NaOH) bonds to the empty p orbital of boron in the trialkylborane, giving a negatively charged borate.
The curved arrows
- A lone pair on the negatively charged peroxide oxygen bonds to boron.
Alkyl shift from boron to oxygen
The alkyl group bonded to boron migrates to the adjacent oxygen with its bonding pair as the weak O–O bond breaks and hydroxide leaves. The migrating carbon keeps its configuration, so oxygen replaces boron at exactly the carbon boron occupied.
The curved arrows
- The C–B bond pair moves the carbon onto the neighbouring oxygen, forming the C–O bond.
- The O–O bond pair leaves with the far oxygen as hydroxide.
Hydroxide adds to boron
Boron is electron-deficient again in the borate ester, so a hydroxide ion bonds to it. This sets up cleavage of the B–O bond to the new alkoxy oxygen.
The curved arrows
- A hydroxide lone pair bonds to boron.
Release the alkoxide
The B–O bond to the alkoxy oxygen breaks with both electrons going to oxygen, releasing the alkoxide and a hydroxyborane by-product.
The curved arrows
- The B–O bond pair stays on the alkoxy oxygen, giving the alkoxide.
Protonate the alkoxide
Water in the basic oxidation mixture gives a proton to the alkoxide, forming 4-methylpentan-2-ol and regenerating hydroxide.
The curved arrows
- The alkoxide lone pair takes a proton from water.
- The water O–H bond pair stays on oxygen as hydroxide.
4-methylpentan-2-ol
4-methylpentan-2-ol carries the OH on C2, the alkene carbon next to the methyl group, the carbon that boron bonded to: the anti-Markovnikov orientation. C2 becomes a stereocentre; the borane adds to both faces of the alkene, so the alcohol is racemic. With BH3 the two regioisomers form in nearly equal amounts, so the bulky borane is what makes this selective.
What you'll practice
Draw hydroboration of (Z)-4-methylpent-2-ene by disiamylborane with boron on C2, away from the isopropyl group, then the oxidation to 4-methylpentan-2-ol.
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