What Is Beckmann Rearrangement?
The Beckmann rearrangement formally exchanges the hydroxyl group of an oxime (R1R2C=NOH) with the group (R1 or R2) located anti to it on the carbon atom, resulting in a substituted amide (R2C(=O)NHR1 or R1C(=O)NHR2). The reaction requires activation of the oxime hydroxyl group, typically achieved using strong Brønsted acids or Lewis acids, or alternatively via the formation of activated intermediates like mesylates or tosylates. The rearrangement step itself is concerted and stereospecific, proceeding with retention of configuration at the migrating group.
With profound industrial significance, the Beckmann rearrangement is widely used in the synthesis of cyclic amides. The most famous is the Beckmann rearrangement of cyclohexanone oxime to obtain ε-caprolactam, which is a monomer for the synthesis of nylon-6, with an annual output of 3×109kg.
- Reagents (Catalysts/Activators): Brønsted acids (e.g., H2SO4, PPA, HCl); Lewis acids (e.g., SOCl2, PCl5, BF3); sulfonylating agents [e.g., methanesulfonyl chloride (MsCl), p-toluenesulfonyl chloride (TsCl)].
- Reactants: Oximes (Ketoximes).
- Products: N-substituted amides; cyclic amides (from cyclic ketoximes).
- Reaction Type: Rearrangement reaction.
- Related Reactions: Nebber rearrangement reaction, Pinacol rearrangement reaction, Bayer-Viliger rearrangement reaction, Wagner-Meerwein rearrangement reaction, Hoffmann rearrangement reaction, Scmidt rearrangement reaction.
Fig 1. Beckmann rearrangement and its mechanism in protic acid or with PCl5. [1]
Mechanism of Beckmann Rearrangement
Activation
The oxime oxygen is protonated (in acid catalysis) or converted into a good leaving group (e.g., -OMs, -OTs). This generates a highly electrophilic iminium species.
Ionization (Rate-Limiting Step)
Loss of water (from protonated oxime) or the sulfonate leaving group generates a highly reactive nitrilium ion intermediate. This step is typically rate-limiting.
Migration and Capture (Concerted Step)
The nitrilium ion is highly electrophilic at nitrogen. The R group anti periplanar to the departing leaving group migrates from carbon to nitrogen in a concerted [1,2]-shift. This migration occurs with complete retention of configuration at the migrating carbon center. The migrating group moves with its bonding electrons to form a new C-N bond, resulting in a carbocation at the original oxime carbon.
Solvolysis
The transient carbocation is rapidly attacked by a nucleophile (Nu), typically the solvent (e.g., H2O, ROH) or the conjugate base of the acid used (e.g., HSO4-). This yields the final substituted amide product.
Experimental Tips for Beckmann Rearrangement
- Migratory Aptitude: The relative ease of migration follows the general order: Tertiary Alkyl > Secondary Alkyl, Aryl > Primary Alkyl > Methyl > H. Aryl groups generally migrate faster than alkyl groups. Electron-donating substituents on an aryl ring enhance its migratory aptitude.
- Usually, the reaction requires high temperature conditions (>130℃) and a large amount of strong acid (Brønsted or Lewis acid), and sensitive substrates should be avoided.
- It is necessary to confirm that the oxime will not isomerize under the reaction conditions, otherwise a mixture of two possible amides will be obtained. p-Toluenesulfonyl chloride and phosphorus pentachloride can help avoid this situation.
- Pre-forming oxime sulfonates (e.g., tosylates, mesylates) allows for milder rearrangement conditions (e.g., heating in inert solvent or using base). This minimizes side reactions like hydrolysis.
- Solvent choice depends heavily on the acid catalyst. H2SO4/PPA often used neat or with minimal cosolvent. For sulfonate esters, aprotic solvents (DCM, toluene, acetonitrile) are common.
Application Examples of Beckmann Rearrangement
- Example 1: Wang Wang et al. introduced an amide structure on the A ring through the Beckmann rearrangement reaction and synthesized two series of andrographolide derivatives. It was found that compound 8h had a significant inhibitory effect on HK2 enzyme activity and exhibited anti-inflammatory effects. [2]
- Example 2: Yuna Zhang et al. reported the total synthesis route of galanthamine and lycoramine, in which the nitrogen atom insertion reaction was carried out by aqueous Beckmann rearrangement reaction. [3]
Fig 2. Synthetic examples via Beckmann rearrangement.
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References
- Jie Jack Li. Name Reactions-A Collection of Detailed Mechanisms and Synthetic Applications, Sixth Edition, 2021, 25-27.
- Wang, Wang, et al. European Journal of Medicinal Chemistry, 2019, 173, 282-293.
- Zhang, Yuna, et al. Organic Letters, 2020, 22(4), 1244-1248.
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