Hofmann rearrangement

Hofmann rearrangement
Named after	August Wilhelm von Hofmann
Reaction type	Rearrangement reaction
Identifiers
RSC ontology ID	RXNO:0000410

The Hofmann rearrangement is the organic reaction of a primary amide to a primary amine with one fewer carbon atoms.^[1][2][3]

The Hofmann rearrangement.

The reaction is named after its discoverer - August Wilhelm von Hofmann. This reaction is also sometimes called the Hofmann degradation or the Harmon Process, and should not be confused with the Hofmann elimination.

Mechanism

The reaction of bromine with sodium hydroxide forms sodium hypobromite in situ, which transforms the primary amide into an intermediate isocyanate via a formation of a nitrene. The intermediate isocyanate is hydrolyzed to a primary amine, giving off carbon dioxide.^[2]

1. Base abstracts an acidic N-H proton, yielding an anion.
2. The anion reacts with bromine in an α-substitution reaction to give an N-bromoamide.
3. Base abstraction of the remaining amide proton gives a bromoamide anion.
4. The bromoamide anion rearranges as the R group attached to the carbonyl carbon migrates to nitrogen at the same time the bromide ion leaves, giving an isocyanate.
5. The isocyanate adds water in a nucleophilic addition step to yield a carbamic acid (aka urethane).
6. The carbamic acid spontaneously loses CO₂, yielding the amine product.

Variations

Several reagents can substitute for bromine. Sodium hypochlorite,^[4] Lead tetraacetate,^[5] N-bromosuccinimide, (bis(trifluoroacetoxy)iodo)benzene,^[6] and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) can effect a Hofmann rearrangement. In the following example, the intermediate isocyanate is trapped by methanol, forming a carbamate.^[7]

The Hofmann rearrangement using NBS.

In a similar fashion, the intermediate isocyanate can be trapped by tert-butyl alcohol, yielding the tert-butoxycarbonyl (Boc)-protected amine.

The Hofmann Rearrangement also can be used to yield carbamates from α,β-unsaturated or α-hydroxy amides^[2][8] or nitriles from α,β-Acetylenic amides^[2][9] in good yields (≈70%).

For Amiloride, hypobromous acid was used to effect Hofmann rearrangement.

Applications

• Aliphatic & Aromatic amides are converted into aliphatic and aromatic amines, respectively
• In the preparations of anthranilic acid from phthalimide
• Nicotinic acid is converted into 3-Amino pyridine
• The Symmetrical structure of α-phenyl propanamide does not change after Hofmann reaction.
• Gabapentin from mono-amidation 1,1-cyclohexane diacetic acid anhydride with ammonia to 1,1-cyclohexane diacetic acid mono-amide; followed by ‘Hoffmann’ rearrangement: U.S. Patent 20,080,103,334

See also

• Beckmann rearrangement
• Curtius rearrangement
• Iodoform reaction
• Lossen rearrangement
• Schmidt reaction
• Weerman degradation

References

1. Hofmann, A. W. (1881). "Ueber die Einwirkung des Broms in alkalischer Lösung auf Amide" [On the action of bromine in alkaline solution on amides]. Berichte der deutschen chemischen Gesellschaft 14 (2): 2725–2736. doi:10.1002/cber.188101402242.
2. Everett, Wallis; Lane, John (1946). "The Hofmann Reaction". Organic Reactions 3 (7): 267–306. doi:10.1002/0471264180.or003.07. ISBN 9780471005285.
3. Shioiri, Takayuki (1991). "Degradation Reactions". Comprehensive Organic Synthesis 6: 795–828. doi:10.1016/B978-0-08-052349-1.00172-4. ISBN 9780080359298.
4. Mohan, Ram S.; Monk, Keith A. (1999). "The Hofmann Rearrangement Using Household Bleach: Synthesis of 3-Nitroaniline". Journal of Chemical Education 76 (12): 1717. doi:10.1021/ed076p1717.
5. Baumgarten, Henry; Smith, Howard; and Staklis, Andris (1975). "Reactions of amines. XVIII. Oxidative rearrangement of amides with lead tetraacetate". The Journal of Organic Chemistry 40 (24): 3554–3561. doi:10.1021/jo00912a019.
6. Almond, Merrick R.; Stimmel, Julie B.; Thompson, Alan; Loudon, Marc (1988). ""Hofmann Rearrangement under Mildly Acidic Conditions using [I,I-Bis(Trifluoroacetoxy)]iodobenzene: Cyclobutylamine Hydrochloride from Cyclobutanecarboxamide". Organic Syntheses 66: 132. doi:10.15227/orgsyn.066.0132.
7. Keillor, Jeffrey W.; Huang, Xicai (2002). "Methyl Carbamate Formation via Modified Hofmann Rearrangement Reactions: Methyl N-(p-Methoxyphenyl)carbamate". Organic Syntheses 78: 234. doi:10.15227/orgsyn.078.0234.
8. Weerman, R.A. (1913). "Einwirkung von Natriumhypochlorit auf Amide ungesättigter Säuren". Justus Liebigs Annalen der Chemie 401 (1): 1–20. doi:10.1002/jlac.19134010102.
9. Rinkes, I. J. (1920). "De l'action de l'Hypochlorite de Sodium sur les Amides D'Acides". Recueil des Travaux Chimiques des Pays-Bas 39 (12): 704–710. doi:10.1002/recl.19200391204.

External links

• "Mechanism In Motion: Hofmann rearrangement".

Bibliography

• Clayden, Jonathan (2007). Organic Chemistry. Oxford University Press Inc. p. 1073. ISBN 978-0-19-850346-0.
• Fieser, Louis F. (1962). Advanced Organic Chemistry. Reinhold Publishing Corporation, Chapman & Hall, Ltd. pp. 499–501.