Química Orgânica 1 · Estereoquímica das reações de alcenos
Replace boron by OH with retention
Condições: Second stage of hydroboration–oxidation of 1-methylcyclopentene: the alkylborane reacts with H₂O₂ and aqueous NaOH. Boron bonded to the less substituted alkene carbon, syn with hydrogen at the methyl-bearing carbon, so boron and the methyl are trans. The hydroperoxide anion HOO⁻ is already formed; Na⁺ is a spectator. R marks the other boron substituents (for example further 2-methylcyclopentyl groups); follow only the C–B bond to the drawn ring. Water is the proton source at the end.
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Hydroperoxide adds to boron
The neutral three-coordinate borane has only six valence electrons around boron, so it is a Lewis acid. The hydroperoxide anion adds to give a tetracoordinate borate in which boron carries the negative charge.
As setas curvas
- A lone pair on the negatively charged hydroperoxide oxygen bonds to the empty p orbital of boron.
Shift the alkyl group from boron to oxygen
This is the key step. The C–B bond electrons form a bond from the ring carbon to the adjacent oxygen while hydroxide departs, much like a 1,2-shift in a carbocation. That carbon never loses its bonding electrons or becomes planar: its configuration is retained, and the new C–O bond has the same orientation as the original C–B bond.
As setas curvas
- The C–B bond pair forms a C–O bond to the adjacent peroxide oxygen.
- The weak O–O bond pair leaves with hydroxide.
Hydroxide adds to the borate ester
Boron is again neutral and six-electron after the shift, so hydroxide adds to it. Every other alkyl group on boron is oxidized the same way; only the drawn ring is followed.
As setas curvas
- An oxygen lone pair on hydroxide bonds to boron.
Release the alkoxide
The borate collapses by breaking the B–O bond to the ring-bound oxygen. The C–O bond formed in the shift is not touched, so the stereocenter is unaffected.
As setas curvas
- The B–O bond pair leaves with the ring-bound oxygen, giving an alkoxide.
Protonate the alkoxide
Water protonates the alkoxide, giving the neutral alcohol and regenerating hydroxide.
As setas curvas
- An alkoxide lone pair accepts a proton from water.
- The water O–H bond pair stays on oxygen, which becomes hydroxide.
trans-2-Methylcyclopentan-1-ol
OH (wedge) sits exactly where boron was, and the neighboring methyl (hash) is on the opposite face: trans-2-methylcyclopentan-1-ol. The hydrogen and boron were delivered syn in hydroboration, and oxidation replaced C–B by C–O with retention, so net addition of H and OH is syn and anti-Markovnikov (OH on the less substituted alkene carbon). Boron leaves as a borinic acid (R₂BOH) that is oxidized further to borate; the product is racemic because the borane formed on both alkene faces.
O que você vai praticar
Carry an alkylborane from 1-methylcyclopentene through hydroperoxide addition, the 1,2-alkyl shift from boron to oxygen, and borate hydrolysis, keeping the C–O bond where the C–B bond was to give trans-2-methylcyclopentanol.
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