Curtius Rearrangement

What Is Curtius Rearrangement?

The Curtius rearrangement, discovered by Theodor Curtius in 1890, stands as a cornerstone transformation in organic synthesis. The reaction describes the conversion of an acyl azide to an isocyanate upon heating. It is worth mentioning that the Curtius rearrangement can generally maintain the stereochemical configuration of the original chiral carbon without causing configuration inversion.

The highly reactive isocyanate intermediate can then be readily trapped by various nucleophiles (water, alcohols, amines) to afford primary amines (via unstable carbamic acid), carbamates (urethanes), or ureas, respectively. The net transformation effectively converts a carboxylic acid (RCOOH) into an amine (RNH₂), an amine with one less carbon atom (if R is alkyl), or into a protected amine derivative. This reaction is extremely effective for the synthesis of amines and their derivatives such as urea and carbamate.

Curtius rearrangement is widely used in drug synthesis and natural product synthesis, and is often used to prepare tertiary amine derivatives. Classic synthetic examples include the synthesis of Himandrine and the synthesis of 2,6-diamino-4-bremopyridine drug intermediates.

• Reagents:
  a) Azide Source: Sodium azide (NaN₃), diphenylphosphoryl azide (DPPA), or trimethylsilyl azide (TMSA).
  b) Carboxylic Acid Activator: Thionyl chloride (SOCl₂), oxalyl chloride.
  c) Nucleophiles (for trapping): Water, alcohols, or amines.
• Reactants: Acyl azide; Carboxylic acid (activated as acid chloride or anhydride).
• Products: Primary amine; carbamate/urethane; urea.
• Reaction Type: Rearrangement reaction.
• Related Reactions: Hofmann rearrangement, Schmidt reaction, Lossen rearrangement, Beckmann rearrangement.

Experimental Tips:

• How to prepare acyl azide? Acyl azide is an important intermediate in organic synthesis, which is generally prepared by reacting activated carboxylic acid derivatives such as acyl chloride or mixed anhydride with NaN₃. Improved methods for obtaining acyl azide include reacting acyl chloride with TMSA and reacting hydrazide with nitrous acid. For compounds with functional groups sensitive to water or strong acids in the molecule, carboxylic acid can be directly reacted with DPPA to prepare.
• Both protic acids and Lewis acids can be used to catalyze the reaction. In addition, photochemical conditions can also induce Curtius rearrangement, but several by-products will be produced.

Fig 1. Curtius rearrangement and its mechanism. [1]

Mechanism of Curtius Rearrangement

• Rearrangement Step: Upon heating (usually 60-100°C) or photolysis, the acyl azide undergoes loss of N₂. This generates a highly electron-deficient nitrene species. However, concerted collapse occurs immediately where the R group migrates from carbon to nitrogen simultaneously with nitrogen loss, forming the isocyanate directly. This concerted pathway avoids the formation of a discrete, high-energy nitrene intermediate.
• Electron Demand: The migration aptitude of the R group follows the order: Tertiary alkyl > Secondary alkyl ≈ Aryl > Primary alkyl > Methyl. Stereochemistry at chiral α-carbons is typically retained due to the intramolecular nature of the migration.
• Trapping of Isocyanate: The electrophilic isocyanate is highly reactive and rarely isolated. It is trapped in situ with water, alcohols, or amines.

Application Examples of Curtius Rearrangement

• Example 1: Daisuke Sawada et al. developed a stereospecific synthesis method for the synthesis of various carbamate-linked oligosaccharides and even dendrimers from sugar carboxylic acids and sugar alcohols via a modified Curtius rearrangement reaction. [2]
• Example 2: Tomoyuki Tada et al. reported a simple strategy for preparing 2,2-[60]fullerenoalkanoyl azides from 2,2-[60]fullerenoalkanoyl chlorides via Curtius rearrangement in the presence of tert-butyl alcohol, which was then further converted into 2,2-[60]fullerenoalkylamines. [3]

Fig 2. Synthetic examples via Curtius rearrangement.

Related Products

CAS No.	Structure	Product
358780-13-9		6-(Trifluoromethyl)nicotinoyl chloride
337508-58-4		1H-Benzimidazole-2-carbonyl chloride hydrochloride
27914-73-4		4-Acetoxy-benzoylchloride
17213-57-9		3,5-Dimethoxybenzoyl chloride
774-74-3		(2,4-DICHLORO-PHENYL)-ACETYL CHLORIDE
15893-42-2		3-(4-Methoxyphenyl)propionyl chloride
88958-33-2		5-(2-Furyl)isoxazole-3-carbonyl chloride
54745-92-5		2-QUINOXALINECARBONYL CHLORIDE
2243-83-6		2-Naphthoyl chloride
58881-45-1		1H-INDOLE-2-CARBONYL CHLORIDE
59496-25-2		1H-INDOLE-3-CARBONYL CHLORIDE
67748-61-2		2-Benzothiazolecarbonylchloride
39827-11-7		Benzo[b]thiophene-2-carbonyl chloride
41717-28-6		BENZOFURAN-2-CARBONYL CHLORIDE
4755-50-4		4-Dimethylaminobenzoyl chloride
59084-16-1		4-Piperidinecarbonylchloride,1-acetyl-
18496-54-3		4-Phenylbutyryl chloride
39098-87-8		3-(3-FLUOROPHENYL)-2-PROPENOYLCHLORIDE
79538-29-7		2,4,6-Trifluorobenzoyl chloride
19810-31-2		Benzyloxyacetyl chloride
1711-11-1		3-Cyanobenzoyl chloride
2972-52-3		2,4-Dichloro-5-pyrimidinecarbonyl chloride
10375-34-5		2,5-Furandicarbonyl Dichloride
151982-51-3		4-Bromo-2-fluorobenzoyl chloride
672-75-3		3-Bromo-4-fluorobenzoyl chloride

References

1. Jie Jack Li. Name Reactions-A Collection of Detailed Mechanisms and Synthetic Applications, Sixth Edition, 2021, 134-136.
2. Sawada, Daisuke, et al. Eur. J. Org. Chem. 2007, 1064-1068.
3. Tada, Tomoyuki, et al. Synlett. 2007, 02, 0235-0238.

Please kindly note that our products are for research use only.