Química Orgánica 1 · Reacciones de alquenos y alquinos: repaso para el examen
OsO4 dihydroxylation of hex-1-ene to hexane-1,2-diol
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
Concerted cycloaddition of OsO4
Two Os=O oxygens add across the alkene in one step: the π pair bonds C1 to one oxygen, an Os=O π pair moves onto osmium, and a second Os=O π pair bonds its oxygen to C2. Both C–O bonds form on the same face.
Las flechas curvas
- The alkene π pair forms the C1–O bond.
- An Os=O π pair moves onto osmium, reducing it.
- A second Os=O π pair forms the C2–O bond.
Why syn?
The two oxygens arrive together.
Water opens the first Os–O bond
In the aqueous workup a water lone pair bonds to osmium while the Os–O bond to C1's oxygen breaks. That oxygen never becomes a free alkoxide: as it leaves osmium it takes a proton from the attacking water, so the C1 OH and a neutral Os–OH form together.
Las flechas curvas
- A water lone pair forms a bond to osmium.
- The Os–O bond pair moves onto the C1 oxygen.
- The C1 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 bonds to osmium and the remaining Os–O bond breaks; the C2 oxygen takes a proton from that water as it leaves, releasing the neutral diol.
Las flechas curvas
- A water lone pair forms a bond to osmium.
- The Os–O bond pair moves onto the C2 oxygen.
- The C2 oxygen takes a proton from the water bonding to osmium.
- The O–H pair stays on the oxygen now bonded to osmium.
Hexane-1,2-diol
Both OH groups come from osmium and were added to one face (syn). C2 is a new stereocentre formed as a racemate because OsO4 adds to either face of the terminal alkene. NaHSO3 reduces the osmium(VI) by-product.
Qué vas a practicar
Draw the concerted cycloaddition of OsO4 to hex-1-ene giving the cyclic osmate ester, then its hydrolysis by water to hexane-1,2-diol.
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