Química Orgânica 2 · Rotas de síntese: reagentes, grupos protetores e planejamento em várias etapas
DMAP-catalysed acetylation of 2-phenylethanol
Condições: 2-Phenylethanol, acetic anhydride (1.5 equiv), Et₃N (1.5 equiv), DMAP (0.05 equiv), CH₂Cl₂, 25 °C
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O mecanismo, passo a passo
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DMAP adds to acetic anhydride
The ring nitrogen of DMAP uses its lone pair to attack one carbonyl carbon of acetic anhydride; the C=O π pair moves onto oxygen.
As setas curvas
- The ring nitrogen lone pair forms the N–C bond.
- The C=O π pair moves onto oxygen.
Acetate leaves: the acetylpyridinium ion
The oxyanion re-forms C=O and acetate is expelled, leaving the N-acetylpyridinium ion, a far more reactive acylating agent than the anhydride.
As setas curvas
- The oxygen lone pair rebuilds the C=O π bond.
- The C–O bond breaks and acetate leaves.
Why does a little DMAP speed things up?
DMAP is a nucleophilic catalyst: it is acylated faster than the alcohol and hands the acyl group on.
The alcohol adds to the acetylpyridinium ion
The hydroxyl oxygen of 2-phenylethanol attacks the acyl carbon and the C=O π pair moves onto oxygen.
As setas curvas
- The alcohol oxygen lone pair forms the new O–C bond.
- The C=O π pair moves onto oxygen.
DMAP is released
The oxyanion re-forms C=O and the bond to the ring nitrogen breaks. Neutral DMAP leaves and is free to start another cycle.
As setas curvas
- The oxygen lone pair rebuilds the C=O π bond.
- The C–N bond breaks and the pair returns to the ring nitrogen.
Acetate removes the oxonium proton
Acetate takes the proton from the positively charged ester oxygen, giving the neutral ester and acetic acid (neutralised by triethylamine).
As setas curvas
- The acetate oxygen lone pair takes the proton.
- The O–H pair stays on the ester oxygen.
2-Phenylethyl acetate
The alcohol is protected as its acetate ester and DMAP is regenerated. An acetate survives mild acid, oxidants such as PCC and acid chlorides, but not Grignard reagents, LiAlH₄ or hydroxide; it is removed with K₂CO₃ or NaOMe in methanol.
O que você vai praticar
Draw how 4-(dimethylamino)pyridine turns acetic anhydride into an N-acetylpyridinium ion, how 2-phenylethanol is acetylated by that ion, and how the catalyst is released to protect the alcohol as its acetate ester.
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