Química Orgânica 2 · Ferramentas de oxidação e redução
Swern oxidation of cyclohexanol to cyclohexanone
Condições: (COCl)2 (1.1 equiv), DMSO (2.2 equiv), CH2Cl2, −78 °C; then cyclohexanol; then Et3N (5 equiv), −78 °C → 25 °C
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O mecanismo, passo a passo
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Ler todos os 9 passos escritos
DMSO oxygen adds to oxalyl chloride
The negatively charged oxygen of DMSO attacks one acyl chloride carbon of oxalyl chloride, and the C=O π pair moves onto oxygen.
As setas curvas
- A DMSO oxygen lone pair forms the new O–C bond.
- The C=O π pair moves onto oxygen.
Chloride is expelled
The tetrahedral intermediate collapses: the oxygen lone pair re-forms C=O and chloride leaves, giving an acyloxysulfonium ion.
As setas curvas
- An oxygen lone pair rebuilds the C=O π bond.
- The C–Cl pair leaves with chloride.
Fragmentation to the chlorosulfonium ion
Chloride attacks the positively charged sulfur and the acyl fragment falls apart: the S–O pair becomes a C=O bond of carbon dioxide, the C–C bond breaks, carbon monoxide forms and the second chloride leaves.
As setas curvas
- A chloride lone pair forms the S–Cl bond.
- The S–O pair becomes a second C=O bond, making carbon dioxide.
- The C–C pair stays on the other carbonyl carbon.
- The C–Cl pair leaves as chloride.
- An oxygen lone pair forms the third C–O bond of carbon monoxide.
Cyclohexanol attacks sulfur
The alcohol oxygen bonds to the electrophilic sulfur of the chlorodimethylsulfonium ion as chloride leaves.
As setas curvas
- An alcohol oxygen lone pair forms the O–S bond.
- The S–Cl pair leaves with chloride.
Loss of the O–H proton
The oxonium proton is lost (drawn going to chloride as HCl), leaving the alkoxysulfonium ion.
As setas curvas
- A chloride lone pair takes the proton.
- The O–H pair stays on oxygen.
Triethylamine forms the sulfur ylide
Triethylamine removes a proton from a methyl group on the positively charged sulfur, giving a sulfur ylide whose carbanion sits next to S⁺.
As setas curvas
- The amine nitrogen lone pair takes a methyl hydrogen.
- The C–H pair stays on carbon as a carbanion.
Intramolecular hydrogen transfer
Through a five-atom cyclic transition state the ylide carbanion takes the hydrogen on the carbinol carbon; that C–H pair becomes the C=O π bond and the O–S pair returns to sulfur, releasing dimethyl sulfide.
As setas curvas
- The carbanion lone pair takes the carbinol hydrogen.
- The C–H pair becomes the C=O π bond.
- The O–S pair returns to sulfur, giving dimethyl sulfide.
Who removes the carbinol hydrogen?
Triethylamine only makes the ylide; the hydrogen on the carbinol carbon is removed intramolecularly by the ylide carbanion.
Cyclohexanone
Cyclohexanol has been oxidised to cyclohexanone without any metal or water, so no over-oxidation is possible. Dimethyl sulfide (the smell of a Swern), CO2 and CO are the by-products.
O que você vai praticar
Draw the Swern oxidation of cyclohexanol: activation of DMSO by oxalyl chloride to the chlorodimethylsulfonium ion, formation of the alkoxysulfonium ion, ylide formation with triethylamine and the intramolecular hydrogen transfer that gives cyclohexanone and dimethyl sulfide.
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