Química Orgânica 2 · Rotas de síntese: reagentes, grupos protetores e planejamento em várias etapas
TFA removes the Boc group: benzylamine
Condições: TFA, CH2Cl2
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TFA protonates the carbonyl oxygen
Trifluoroacetic acid gives a proton to the carbamate C=O oxygen, the most basic site because the cation is shared by both oxygens and the nitrogen.
As setas curvas
- A carbonyl oxygen lone pair takes the acidic proton.
- The O–H electrons stay on trifluoroacetate.
The tert-butyl cation leaves
The O–tert-butyl bond breaks heterolytically: its electrons become a new C=O π bond to the carbamate carbon while the protonated C=O pair returns to its oxygen. The departing group is a stable tertiary cation, which is why only tert-butyl carbamates fall apart this easily in acid.
As setas curvas
- The C–O bond pair to the tert-butyl carbon becomes the new carbamate C=O π bond.
- The protonated C=O π pair moves onto its oxygen, which keeps the proton as an O–H.
The tert-butyl cation loses a proton
Trifluoroacetate removes a proton from a methyl group next to the cation (E1), giving isobutylene gas and regenerating TFA.
As setas curvas
- A trifluoroacetate oxygen lone pair takes a β-proton.
- The C–H pair forms the C=C π bond of isobutylene.
Why does the carbamic acid fall apart?
Carbamic acids are unstable.
Decarboxylation releases benzylamine
The carbamic acid OH oxygen pushes a lone pair toward carbon to form a C=O+ bond, the C–N bond breaks, and its electrons take a proton from TFA so nitrogen leaves as the neutral amine, never as an amide ion.
As setas curvas
- The OH oxygen lone pair forms a second C=O bond.
- The C–N bond pair takes the TFA proton, freeing the amine.
- The O–H electrons stay on trifluoroacetate.
Protonated CO2 loses its proton
Trifluoroacetate removes the proton from protonated carbon dioxide, giving CO2 gas and regenerating TFA.
As setas curvas
- The trifluoroacetate lone pair takes the proton.
- The O–H electrons stay on oxygen as CO2 forms.
Benzylamine is free
The Boc group leaves as two gases, isobutylene and CO2, which drive the reaction to completion. In excess TFA the benzylamine is isolated as its trifluoroacetate salt and freed with base.
O que você vai praticar
Remove a Boc group with TFA by drawing tert-butyl cation loss, its E1 to isobutylene and the decarboxylation of the carbamic acid.
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