Química Orgánica 1 · Estereoquímica de las reacciones de alquenos
Hexanedial from cyclohexene
Condiciones: O3, CH2Cl2, −78 °C; then (CH3)2S
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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 8 pasos escritos
Add ozone across the double bond
Ozone is a 1,3-dipole. In one concerted [3+2] cycloaddition its two end oxygens bond to the two alkene carbons, giving the primary ozonide (molozonide) with an O–O–O chain.
Las flechas curvas
- A lone pair on the negative end oxygen bonds to the first alkene carbon.
- The alkene π pair bonds the other alkene carbon to the double-bonded end oxygen.
- The ozone π pair moves onto the central oxygen, which becomes neutral.
Split the molozonide
Two weak O–O bonds make the molozonide unstable even at −78 °C. It falls apart in a reverse [3+2]: the old C–C bond breaks, giving a carbonyl group and a carbonyl oxide (Criegee intermediate).
Las flechas curvas
- The C–C σ pair becomes a C=O π bond at the carbon bonded to the end oxygen.
- The neighbouring O–O pair moves onto the central oxygen, which becomes negative.
- A lone pair on the other end oxygen forms a C=O π bond, making that oxygen positive.
Recombine to the ozonide
The carbonyl oxide is again a 1,3-dipole and adds across the C=O in a second [3+2] with the opposite orientation, giving the ozonide (1,2,4-trioxolane) with only one O–O bond.
Las flechas curvas
- The negative oxygen bonds to the carbonyl carbon.
- The C=O π pair bonds that oxygen to the carbonyl-oxide carbon.
- The C=O⁺ π pair returns to its oxygen, which becomes neutral.
Dimethyl sulfide attacks the peroxide
In the reductive workup, dimethyl sulfide attacks one peroxide oxygen of the ozonide and breaks the weak O–O bond.
Las flechas curvas
- A sulfur lone pair bonds to a peroxide oxygen.
- The O–O pair moves onto the other oxygen, which becomes an alkoxide.
Release the first carbonyl
The alkoxide lone pair forms a C=O bond and pushes out the ring oxygen, cleaving one C–O bond.
Las flechas curvas
- The alkoxide lone pair forms the C=O π bond.
- The C–O bond to the former ring oxygen breaks; that oxygen becomes negative.
Release the second carbonyl and DMSO
The new alkoxide forms the second C=O bond and expels the sulfur-bound oxygen, which leaves as dimethyl sulfoxide.
Las flechas curvas
- The alkoxide lone pair forms the second C=O π bond.
- The C–O pair to the sulfonium oxygen leaves, giving DMSO.
Reductive or oxidative workup?
The workup decides the oxidation level of the carbonyl products.
Hexanedial
The C=C has been replaced by two C=O bonds. Dimethyl sulfide is a reducing agent and takes one oxygen as DMSO, so aldehyde carbons stay at the aldehyde level instead of being oxidised to acids.
Qué vas a practicar
Follow ozonolysis of cyclohexene through the molozonide, the tethered carbonyl oxide and the ozonide, then the dimethyl sulfide workup that opens the ring to hexanedial.
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