Química Orgánica 2 · Herramientas de oxidación y reducción
Cold KMnO4 dihydroxylation of cyclopentene to cis-cyclopentane-1,2-diol
Condiciones: KMnO4, NaOH, H2O, 0 °C
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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.
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[3+2] cycloaddition of permanganate
Permanganate adds across the C=C in one concerted step: two oxo oxygens bond to the two alkene carbons on the same face, and manganese is reduced from Mn(VII) to Mn(V) in a cyclic manganate ester.
Las flechas curvas
- The alkene π pair forms a C–O bond to one oxo oxygen.
- A Mn=O π pair forms the second C–O bond on the same face.
- The other Mn=O π pair moves onto manganese, reducing it.
Hydroxide attacks manganese; one Mn–O bond breaks
Hydroxide adds to manganese and one Mn–O bond of the cyclic ester breaks, leaving an alkoxide on the ring.
Las flechas curvas
- A hydroxide lone pair forms a bond to manganese.
- The Mn–O pair moves onto the ring oxygen.
Water protonates the alkoxide
Water gives a proton to the alkoxide, forming the first OH group.
Las flechas curvas
- The alkoxide lone pair takes a proton from water.
- The O–H pair stays on oxygen as hydroxide.
Hydroxide attacks manganese again
A second hydroxide adds to manganese and the remaining Mn–O bond to the ring breaks, freeing the second alkoxide.
Las flechas curvas
- A hydroxide lone pair forms a bond to manganese.
- The Mn–O pair moves onto the ring oxygen.
Why must the solution stay cold and basic?
Cold, dilute, basic permanganate stops at the diol; warm or acidic permanganate keeps oxidising.
Water protonates the second alkoxide
Water protonates the second alkoxide, completing the diol and leaving a manganese(V) oxo-anion.
Las flechas curvas
- The alkoxide lone pair takes a proton from water.
- The O–H pair stays on oxygen as hydroxide.
cis-Cyclopentane-1,2-diol
Both OH groups are on the same face: cyclopentene has been dihydroxylated to cis-cyclopentane-1,2-diol (meso), and permanganate has been reduced to a brown manganese species.
Qué vas a practicar
Draw the concerted [3+2] cycloaddition of permanganate to cyclopentene and the base hydrolysis of both Mn–O bonds of the cyclic manganate ester, giving cis-cyclopentane-1,2-diol.
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