Química Orgânica 1 · Revisão de estereoquímica para a prova
Cold permanganate turns (E)-stilbene into (1R,2R)-hydrobenzoin
Condições: KMnO4, NaOH, H2O, 0 °C
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Cycloaddition of permanganate
In one concerted step the alkene π pair bonds to one oxo oxygen while a second Mn=O oxygen bonds to the other alkene carbon, giving a five-membered cyclic manganate ester. Both oxygens are delivered to the same face of the alkene, which is why the addition is syn.
As setas curvas
- The alkene π pair forms a C–O bond to one oxo oxygen.
- That Mn=O π pair moves onto manganese.
- The π pair of a second Mn=O forms the bond from its oxygen to the other alkene carbon.
What does syn addition give here?
Both C–O bonds form on the same face; the phenyl groups keep the trans relationship they had in the alkene.
Hydroxide attacks manganese and frees one oxygen
Hydroxide from the basic solution adds to manganese, and one Mn–O bond of the ester breaks so that oxygen becomes an alkoxide on the carbon chain.
As setas curvas
- A hydroxide lone pair bonds to manganese.
- The Mn–O bond pair stays on the carbinol oxygen, giving an alkoxide.
Water protonates the alkoxide
The alkoxide is a strong base and takes a proton from water, the solvent.
As setas curvas
- The alkoxide lone pair takes a proton from water.
- The O–H pair stays on the water oxygen as hydroxide.
Hydroxide releases the second oxygen
A second hydroxide adds to manganese and breaks the remaining ester bond, freeing the second alkoxide.
As setas curvas
- A hydroxide lone pair bonds to manganese.
- The Mn–O bond pair stays on oxygen as an alkoxide.
Water protonates the second alkoxide
Water gives a proton to the second alkoxide, completing the diol.
As setas curvas
- The alkoxide lone pair takes a proton from water.
- The O–H pair stays on oxygen as hydroxide.
(1R,2R)-1,2-Diphenylethane-1,2-diol (with its enantiomer)
Both hydroxyls came from the same face, so the trans alkene gives the chiral (1R,2R) diol drawn here; reaction on the other face gives (1S,2S) equally, so the product is racemic. Manganese leaves as a reduced Mn(V) oxo species.
O que você vai praticar
Draw the cycloaddition of permanganate to (E)-stilbene and the hydrolysis of the cyclic manganate ester, and explain why syn addition gives racemic (1R,2R)/(1S,2S)-1,2-diphenylethane-1,2-diol.
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