Química Orgânica 2 · Ferramentas de oxidação e redução
Ozonolysis of cyclohexene to hexanedial
Condições: O3, CH2Cl2, −78 °C; then Me2S
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O mecanismo, passo a passo
Próximo mostra as setas curvas de cada passo e as explica. Os passos escritos estão abaixo.
Ler todos os 8 passos escritos
Ozone adds across the C=C
Ozone is a 1,3-dipole. Its terminal O⁻ and the electrophilic terminal oxygen bond to the two alkene carbons in one concerted cycloaddition, giving the unstable primary ozonide (molozonide).
As setas curvas
- The negatively charged oxygen of ozone forms a C–O bond.
- The alkene π pair forms the other C–O bond.
- The O=O π pair moves onto the central oxygen, neutralising it.
The molozonide falls apart
The molozonide has two weak O–O bonds. It cleaves in a retro-[3+2] step: the C–C bond breaks, giving an aldehyde at one end and a carbonyl oxide (Criegee intermediate) at the other, still joined by the ring chain.
As setas curvas
- The weak O–O pair moves onto the terminal oxygen, giving O⁻.
- The C–C pair becomes the aldehyde C=O π bond.
- An oxygen lone pair forms the C=O⁺ π bond of the carbonyl oxide.
Recombination to the ozonide
The carbonyl oxide and the aldehyde recombine in a second 1,3-dipolar cycloaddition with the opposite orientation, giving the more stable bicyclic ozonide (a 1,2,4-trioxolane).
As setas curvas
- The O⁻ lone pair attacks the aldehyde carbon.
- The aldehyde C=O π pair forms a bond from its oxygen to the carbonyl-oxide carbon.
- The C=O⁺ π pair moves onto the oxygen, neutralising it.
Dimethyl sulfide attacks the peroxide
In the workup the sulfur lone pair of dimethyl sulfide attacks one peroxide oxygen of the ozonide, and the weak O–O bond breaks onto the other oxygen.
As setas curvas
- A sulfur lone pair forms the S–O bond.
- The O–O pair moves onto the other peroxide oxygen.
The first aldehyde is released
The new alkoxide re-forms a C=O bond and pushes out the bridging ether oxygen, which becomes an alkoxide on the other carbon.
As setas curvas
- The alkoxide lone pair forms the C=O π bond.
- The C–O bond to the bridging oxygen breaks onto that oxygen.
The second aldehyde forms; DMSO leaves
The second alkoxide forms a C=O bond and expels the sulfonium-bound oxygen, which leaves as dimethyl sulfoxide.
As setas curvas
- The alkoxide lone pair forms the C=O π bond.
- The C–O pair leaves with the oxygen bonded to sulfur.
Why add Me2S?
The ozonide is a peroxide; the workup reagent decides the oxidation level of the products.
Hexanedial
The C=C bond has been cut and each carbon became an aldehyde: cyclohexene has been converted into hexanedial, and dimethyl sulfide into dimethyl sulfoxide.
O que você vai praticar
Draw the cycloaddition of ozone to cyclohexene, the fragmentation of the molozonide to a carbonyl oxide and an aldehyde, their recombination to the ozonide, and the reduction of the ozonide by dimethyl sulfide to hexanedial.
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