Química Orgânica 2 · Deslocamentos sigmatrópicos, cicloadições e reações eletrocíclicas
Ozonolysis of 2-methylbut-2-ene with dimethyl sulfide workup
Condições: O3, CH2Cl2, −78 °C (until blue); then Me2S (2 equiv), −78 °C to rt
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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.
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Ozone adds across the alkene
Ozone is a 1,3-dipole. Its anionic terminal oxygen bonds to one alkene carbon, the alkene π pair bonds the other carbon to the far terminal oxygen, and the O=O π pair moves onto the central oxygen. The concerted [3+2] cycloaddition gives the five-membered primary ozonide, which has an unstable O–O–O chain.
As setas curvas
- The anionic ozone oxygen forms an O–C bond to the CH carbon.
- The alkene π pair bonds the dimethyl carbon to the other terminal oxygen.
- The O=O π pair becomes a lone pair on the central oxygen, removing its positive charge.
Retro-[3+2]: the primary ozonide falls apart
The weak O–O bond and the C–C bond break together. The C–C pair becomes the C=O π bond of acetone, the O–O pair goes to the central oxygen, and the oxygen on the CH carbon donates a lone pair to make a C=O⁺ bond: a carbonyl oxide (Criegee intermediate) forms alongside acetone.
As setas curvas
- The C–C bonding pair becomes the π bond of the acetone C=O.
- The O–O bonding pair leaves with the central oxygen, which becomes the anionic end of the carbonyl oxide.
- The oxygen on the CH carbon donates a lone pair to form the C=O⁺ bond of the carbonyl oxide.
What are the fragments?
The C=C carbons are now separated; each carries oxygen. Acetone is a finished carbonyl, while the acetaldehyde unit is a carbonyl oxide that still holds two oxygens.
Recombination to the ozonide
The carbonyl oxide is itself a 1,3-dipole and acetone is its dipolarophile. The anionic oxygen bonds to the acetone carbon, the acetone C=O π pair bonds its oxygen to the carbonyl oxide carbon, and the C=O⁺ π pair moves onto the oxonium oxygen. The product is the more stable ozonide (1,2,4-trioxolane) with only one O–O bond.
As setas curvas
- The anionic oxygen of the carbonyl oxide bonds to the acetone carbonyl carbon.
- The acetone C=O π pair bonds the acetone oxygen to the carbonyl oxide carbon.
- The C=O⁺ π pair becomes a lone pair on that oxygen, neutralising it.
Dimethyl sulfide attacks the peroxide
On workup, the sulfur lone pair of dimethyl sulfide attacks one peroxide oxygen, and the weak O–O bond breaks with the pair going to the other oxygen. A sulfonium-substituted ether and an alkoxide result.
As setas curvas
- The sulfur lone pair forms an S–O bond to the peroxide oxygen on the CH side.
- The O–O pair leaves with the other peroxide oxygen as an alkoxide.
Acetone is released
The alkoxide lone pair forms a C=O bond and the ring C–O bond breaks, expelling the other ring oxygen as a new alkoxide. Acetone leaves.
As setas curvas
- The alkoxide lone pair forms the acetone C=O π bond.
- The C–O bond breaks, leaving the negative charge on the oxygen attached to the CH carbon.
Acetaldehyde and DMSO are released
The new alkoxide forms a C=O bond, and the C–O bond to the sulfonium oxygen breaks with the pair moving onto that oxygen. This gives acetaldehyde and dimethyl sulfoxide.
As setas curvas
- The alkoxide lone pair forms the aldehyde C=O π bond.
- The C–O pair leaves onto the sulfonium oxygen, giving DMSO.
Acetone, acetaldehyde and DMSO
The alkene carbons end up as the carbonyl carbons of acetone and acetaldehyde; dimethyl sulfide has been oxidised to DMSO, which is why no peroxide or carboxylic acid survives the workup.
O que você vai praticar
Draw the Criegee mechanism for ozonolysis of 2-methylbut-2-ene — primary ozonide, carbonyl oxide, ozonide — and its reduction by dimethyl sulfide to acetone and acetaldehyde.
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