Química Orgânica 2 · Aminoácidos e conectividade dos peptídeos
H-Gly-Ala-OMe trifluoroacetate from Boc-Gly-Ala-OMe and TFA
Condições: TFA, CH2Cl2
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TFA protonates the carbamate carbonyl
Trifluoroacetic acid is a strong acid; it protonates the carbamate carbonyl oxygen, the most basic site of the Boc group.
As setas curvas
- The carbonyl oxygen lone pair takes the TFA proton.
- The O–H pair stays on oxygen as trifluoroacetate.
The tert-butyl cation leaves
The O–tert-butyl bond breaks heterolytically because it gives a stable tertiary carbocation; its electron pair becomes a C=O bond while the protonated carbonyl becomes an O–H group, giving the carbamic acid. This is AAL1 cleavage (alkyl–oxygen bond, unimolecular).
As setas curvas
- The C–O σ pair stays on oxygen and becomes the new C=O π bond.
- The C=O+ π pair moves onto the protonated oxygen, giving an O–H group.
The cation loses a proton
Trifluoroacetate removes a proton from a methyl group next to the cation, giving isobutylene gas (E1).
As setas curvas
- A trifluoroacetate oxygen lone pair takes a C–H proton next to the cation.
- The C–H pair forms the C=C π bond.
Fate of the tert-butyl group
A tertiary cation forms.
Protonate the carbamic acid nitrogen
A second TFA protonates the carbamic acid nitrogen, which sets up loss of CO2.
As setas curvas
- The nitrogen lone pair takes the TFA proton.
- The O–H pair stays on oxygen.
Decarboxylation
Trifluoroacetate removes the O–H proton; that O–H pair forms a C=O bond and the C–N bond breaks, releasing CO2 and the neutral amine.
As setas curvas
- A trifluoroacetate lone pair takes the O–H proton.
- The O–H pair becomes the second C=O π bond.
- The C–N pair stays on nitrogen.
TFA protonates the amine
In neat TFA the free amine is immediately protonated, so the product is the ammonium trifluoroacetate salt.
As setas curvas
- The amine lone pair takes the TFA proton.
- The O–H pair stays on oxygen.
H-Gly-Ala-OMe trifluoroacetate
The products are the dipeptide ester as its ammonium trifluoroacetate, isobutylene and CO2. The alanine stereocentre is untouched; a base wash would free the amine for the next coupling.
O que você vai praticar
Draw the TFA cleavage of the Boc group: carbonyl protonation, loss of the tert-butyl cation, its elimination to isobutylene, and decarboxylation of the carbamic acid.
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