Química Orgánica 2 · Herramientas de oxidación y reducción
OsO4 dihydroxylation of cyclohexene to cis-cyclohexane-1,2-diol
Condiciones: OsO4 (cat.), NMO, acetone/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
[3+2] cycloaddition of OsO4
Osmium tetroxide adds across the C=C in one concerted step: two oxo oxygens bond to the two alkene carbons on the same face, and osmium is reduced from Os(VIII) to Os(VI) in a cyclic osmate ester.
Las flechas curvas
- The alkene π pair forms a C–O bond to one oxo oxygen.
- An Os=O π pair forms the second C–O bond on the same face.
- The other Os=O π pair moves onto osmium, reducing it.
Why is the diol cis?
Both new C–O bonds form in one concerted step from the same osmium atom.
Water opens the first Os–O bond
A water molecule adds to the electrophilic osmium while one Os–O bond of the ester breaks. The ring oxygen never becomes a free alkoxide: as it leaves osmium it takes a proton from the attacking water, giving the first OH group and a neutral Os–OH.
Las flechas curvas
- A water lone pair forms a bond to osmium.
- The Os–O bond pair moves onto the ring oxygen.
- The ring oxygen takes a proton from the water bonding to osmium.
- The O–H pair stays on the oxygen now bonded to osmium.
A second water releases the diol
A second water adds to osmium and the remaining Os–O bond to the ring breaks; the leaving oxygen takes a proton from that water, completing the diol and leaving osmic acid, the Os(VI) species that NMO turns back into OsO4.
Las flechas curvas
- A water lone pair forms a bond to osmium.
- The Os–O bond pair moves onto the ring oxygen.
- The ring oxygen takes a proton from the water bonding to osmium.
- The O–H pair stays on the oxygen now bonded to osmium.
cis-Cyclohexane-1,2-diol
Both OH groups sit on the same face of the ring: cyclohexene has been converted to cis-cyclohexane-1,2-diol (a meso compound). NMO reoxidises the Os(VI) so only a catalytic amount of OsO4 is needed.
Qué vas a practicar
Draw the concerted [3+2] cycloaddition of OsO4 to cyclohexene and the hydrolysis of both Os–O bonds of the cyclic osmate ester, giving cis-cyclohexane-1,2-diol.
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