Boc protection: practice and mechanism
Boc₂O reacts with the nucleophilic amino group to give the carbamate (N–CO–O–tBu). The carboxylic acid stays free; in base it is a carboxylate and does not attack.
Example
The amino group attacks Boc anhydride and becomes a carbamate (tert-butoxycarbonyl, Boc). The N is then no longer nucleophilic and cannot take part in peptide coupling. The carboxylic acid stays free.
Mechanism step by step
- 1
NaOH keeps the amino group unprotonated (nucleophilic) and the acid as the carboxylate.
- 2
The amine attacks one C=O group of Boc₂O; a tetrahedral intermediate forms.
- 3
tert-Butyl carbonate leaves and decomposes into CO₂ and tert-butanol; the Boc carbamate remains.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Background
Typical conditions
The amine is treated with Boc₂O (di-tert-butyl dicarbonate), usually in dichloromethane, THF or water/dioxane with a base such as triethylamine or NaHCO₃; sluggish amines need catalytic DMAP. The only by-products are CO₂ and tert-butanol, so workup is simple.
Removing Boc again
With acid: classically trifluoroacetic acid (TFA, often 20–50 % in dichloromethane) or HCl in dioxane or ethyl acetate. The free amine (as a salt), CO₂ and isobutene form via the tert-butyl cation. Electron-rich groups in the molecule are protected with scavengers such as anisole or triethylsilane so the cation does not alkylate them.
Boc, Cbz and Fmoc compared
All three protect amines as carbamates but come off differently: Boc with acid, Cbz (also Z) by catalytic hydrogenation with H₂ and Pd/C, Fmoc with base (piperidine). Because no removal touches the others, they can be used side by side and removed one after another, for example in peptide synthesis.
Typical mistakes
- The acid is not esterified. The amine is the stronger nucleophile and the carboxylate does not form a tert-butyl ester under these conditions.
- Boc₂O transfers a tert-butoxycarbonyl group (with C=O), not a tert-butyl group. A carbamate forms on N, not a tert-butylamine.
- Once it carries one Boc group, the N (as a carbamate) is hardly nucleophilic. A second Boc group is not added under these conditions.
Often confused with: Acylation of amines · Amide from acyl chloride
Frequently asked questions
Why are protecting groups needed in peptide synthesis?
Every amino acid has NH₂ and COOH. Without protection each would react with every other (including itself). Protecting the N of one and the COOH of the other gives exactly one dipeptide.
Boc or Fmoc?
Boc is removed with acid (TFA), Fmoc with base (piperidine). Together they allow "orthogonal" protection: one group can be removed selectively without touching the other.
What is the Boc group used for?
It protects amines as a carbamate. Boc is stable to bases, nucleophiles and hydrogenation and comes off with acid (TFA or HCl).
Which reagent introduces Boc?
Di-tert-butyl dicarbonate (Boc₂O), usually with a base such as triethylamine, NaHCO₃ or catalytic DMAP.
What is the Cbz protecting group?
The benzyloxycarbonyl group (Cbz or Z) protects amines as a carbamate. It is put on with Cbz-Cl and base and removed by catalytic hydrogenation (H₂, Pd/C), giving toluene and CO₂. It is more stable to acid than Boc.
How is a Boc group removed?
With acid, usually trifluoroacetic acid in dichloromethane or HCl in dioxane at room temperature. What remains is the amine as its salt, CO₂ and isobutene.