Química Orgânica 2 · Rotas de síntese: reagentes, grupos protetores e planejamento em várias etapas
Protected Grignard reagent plus acetone gives 5-hydroxy-5-methylhexanal
Condições: acetone, THF; then H3O+ (workup removes the acetal)
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The Grignard carbon adds to acetone
4-Bromobutanal cannot form a Grignard reagent directly because the C–Mg carbon would attack its own aldehyde. With the aldehyde masked as a 1,3-dioxolane, the C–Mg bond adds to the acetone carbonyl carbon and the C=O π pair moves onto oxygen as an alkoxide.
As setas curvas
- The nucleophilic C–Mg bond pair forms the new C–C bond to the carbonyl carbon.
- The C=O π pair moves onto oxygen, giving the magnesium alkoxide.
Why protect the aldehyde?
Grignard reagents attack carbonyls.
Hydronium protonates the alkoxide
The aqueous acid workup first protonates the alkoxide oxygen, giving the tertiary alcohol.
As setas curvas
- The alkoxide lone pair takes a proton from hydronium.
- The O–H electrons stay on water.
Protonate an acetal oxygen
Excess hydronium protonates one of the two acetal oxygens, turning it into a leaving group.
As setas curvas
- A ring oxygen lone pair takes a proton.
- The O–H electrons stay on water.
The ring opens to an oxocarbenium ion
The other acetal oxygen pushes a lone pair into a C=O+ bond as the bond to the protonated oxygen breaks, opening the ring.
As setas curvas
- The ring oxygen lone pair forms the C=O+ π bond.
- The C–O bond to the protonated oxygen breaks, giving a hydroxyethyl group.
Water adds to the oxocarbenium carbon
A water molecule attacks the electrophilic carbon and the C=O+ π pair returns to the ether oxygen.
As setas curvas
- A water lone pair bonds to the oxocarbenium carbon.
- The C=O+ π pair moves onto the ether oxygen.
Water removes a proton
A second water molecule removes a proton from the added water, giving a neutral hemiacetal.
As setas curvas
- The water lone pair takes the proton.
- The O–H electrons stay on the hemiacetal oxygen.
Protonate the alkoxy oxygen
Hydronium protonates the hydroxyethoxy oxygen of the hemiacetal so ethylene glycol can leave.
As setas curvas
- The ether oxygen lone pair takes a proton.
- The O–H electrons stay on water.
Ethylene glycol leaves
The hemiacetal OH pushes a lone pair into a C=O+ bond and ethylene glycol departs, giving the protonated aldehyde.
As setas curvas
- The OH lone pair forms the C=O+ π bond.
- The C–O bond breaks, releasing ethylene glycol.
Water deprotonates the aldehyde
Water removes the proton from the protonated carbonyl, revealing the free aldehyde and regenerating hydronium.
As setas curvas
- The water lone pair takes the proton.
- The O–H electrons stay on the carbonyl oxygen.
5-Hydroxy-5-methylhexanal
The aldehyde returns intact and the new tertiary alcohol sits four carbons away; three carbons were added to acetone without the Grignard reagent ever seeing a free aldehyde. Ethylene glycol is the by-product of deprotection.
O que você vai praticar
Add a dioxolane-protected Grignard reagent to acetone, then unmask the aldehyde by acid hydrolysis of the acetal.
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