Schotten-Baumann Reaction¶
Overview¶
The Schotten-Baumann reaction is a classical method for the acylation of amines and alcohols with acid chlorides or other activated electrophiles under biphasic aqueous-organic conditions. In peptide chemistry, the Schotten-Baumann reaction is primarily used for the installation of carbamate protecting groups (e.g., Boc, Fmoc, Cbz) onto amino acids.
The reaction is performed in a mixture of water and an organic solvent (typically dioxane, THF, or DCM) with an aqueous base (NaOH, NaHCO3, or Na2CO3) to neutralize the acid generated during the acylation.
Carbamate Protecting Group Installation¶
The Schotten-Baumann conditions are used to install carbamate protecting groups from three classes of electrophile:
| Electrophile Type | Example | Reactivity | Notes |
|---|---|---|---|
| Chloroformates | Fmoc-Cl, Cbz-Cl, Alloc-Cl | Most reactive | Fast reactions, may require cooling. Can acylate water if added too quickly. |
| Mixed Carbonates (OSu) | Fmoc-OSu, Boc-OSu | Moderate | Selective for amines over water. Preferred for Fmoc installation to avoid dibenzofulvene side products. |
| Dicarbonates | Boc2O (di-tert-butyl dicarbonate) | Least reactive | Very selective for amines. Standard reagent for Boc installation. Releases tBuOH and CO2 as byproducts. |
Chloroformates¶
Chloroformates are the most reactive electrophiles. The reaction is typically performed at 0 C with slow addition of the chloroformate to a solution of the amino acid in aqueous base. The major side reaction is hydrolysis of the chloroformate by water, which can be minimized by using a slight excess of reagent and keeping the temperature low. Chloroformates are inexpensive and widely available.
Mixed Carbonates (OSu Esters)¶
The OSu esters (e.g., Fmoc-OSu) react selectively with amines over water, making them more convenient for installations where hydrolysis of the electrophile would be problematic. Fmoc-OSu is preferred over Fmoc-Cl for Fmoc installation because Fmoc-Cl can generate dibenzofulvene (DBF) under basic conditions, which can alkylate the product.
Dicarbonates¶
Di-tert-butyl dicarbonate (Boc2O) is the standard reagent for Boc installation. It is very selective for amines and releases only tBuOH and CO2 as byproducts. The reaction can be performed at room temperature in aqueous dioxane or THF with NaOH or NaHCO3 as base.
Melting Point Considerations
Boc2O has a low melting point (~22 C) and can be a liquid at room temperature. This is normal and does not affect reactivity. If it has solidified, gentle warming in a water bath will liquefy it.