Química Orgánica 1 · Estereoquímica de las reacciones de alquenos
cis-1-Methylcyclopentane-1,2-diol from 1-methylcyclopentene
Condiciones: OsO4; then NaHSO3, H2O
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Arrastra desde los electrones que se mueven hasta donde van, o toca uno y luego el otro. Cada flecha se revisa; Pista te ayuda cuando te atascas. No necesitas cuenta.
El mecanismo, paso a paso
Siguiente reproduce las flechas curvas de cada paso y las explica. Los pasos escritos están abajo.
Leer los 5 pasos escritos
Form the cyclic osmate ester
Three electron pairs move around a five-atom ring in one concerted cycloaddition. Both C–O bonds form on the same face of the alkene, and osmium is reduced from +8 to +6.
Las flechas curvas
- The alkene π pair forms a bond to one osmium-bound oxygen.
- That Os=O π pair moves onto osmium.
- A second Os=O π pair forms the C–O bond to the other alkene carbon.
Hydrolyse the first Os–O bond
In the aqueous workup a water molecule adds to osmium while its proton is passed to the ring oxygen on the methyl-bearing carbon, cutting that Os–O bond. Only O–Os bonds break, so the C–O bonds and their face stay intact.
Las flechas curvas
- A water lone pair bonds to osmium.
- The Os–O pair moves to oxygen and bonds the water proton, giving the first OH.
- The water O–H pair stays on its oxygen, now bonded to osmium as Os–OH.
Hydrolyse the second Os–O bond
A second water does the same to the remaining Os–O bond, releasing the diol and osmium(VI) as osmic acid.
Las flechas curvas
- A water lone pair bonds to osmium.
- The Os–O pair moves to the ring oxygen and bonds the water proton.
- The water O–H pair stays on its oxygen as a second Os–OH.
cis or trans?
The workup only cuts O–Os bonds.
cis-1-Methylcyclopentane-1,2-diol
Both OH groups are on the same face of the ring (cis) and the methyl is on the opposite face to the OH of its neighbour. Attack on either face gives the two enantiomers equally.
Qué vas a practicar
Draw the concerted [3+2] addition of OsO4 to 1-methylcyclopentene and hydrolysis of the cyclic osmate ester, keeping both C–O bonds to give cis-1-methylcyclopentane-1,2-diol.
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