What Is Neber Rearrangement?
The Neber rearrangement is defined as the base‑promoted conversion of an O‑acylated ketoxime (most commonly the O‑tosylate) into the corresponding α‑amino ketone, proceeding through a transient azirine (2H‑azirine) intermediate. The reaction provides an atom‑economical route to α‑amino ketones, which serve as versatile building blocks for the synthesis of nitrogen‑containing heterocycles and other complex molecules. The Neber rearrangement was first reported in 1926 by P. W. Neber and A. Friedolsheim.
The Neber rearrangement has an intimate historical connection to the Beckmann rearrangement. The latter involves the acid‑catalyzed conversion of oximes to amides via O‑protonation followed by an alkyl migration. In contrast, the Neber rearrangement requires O‑sulfonylation (activation) and base catalysis, leading to quite different products. Notably, the two rearrangements are mechanistically distinct and can be controlled by the choice of reaction conditions.
- Reagents: Base (KOEt, NaOEt, KOH, or K2CO3); activating agent for oxime (p‑toluenesulfonyl chloride, mesyl chloride, or acetyl chloride); often a phase‑transfer catalyst (chiral quaternary ammonium salt) or organocatalyst (bifunctional thiourea) for asymmetric variants; acidic work‑up (HCl) to isolate product as hydrochloride salt.
- Reactants: O‑Sulfonyl (or O‑acyl) ketoximes (R1R2C=N–OSO2R3) that contain at least one α‑methylene (CH2) group adjacent to the C=N bond. Aldoximes do not react.
- Products: α‑Amino ketones (R1–C(=O)–CH(NH2)–R2) (usually isolated as hydrochloride salts or protected derivatives). In cases where the intermediate 2H‑azirine is trapped: indoles (from α‑aryl systems) or 2H‑azirine carboxylic esters.
- Reaction Type: Base‑induced rearrangement proceeding via an anionic [2+1] cyclization.
- Related Reactions:
– Beckmann rearrangement (acid‑catalyzed oxime → amide; different conditions and products).
– Schmidt rearrangement (ketone + HN3 → amide or nitrile).
– Staudinger aza‑Wittig (forms iminophosphoranes from azides, not directly analogous but complementary for nitrene chemistry). - Experimental Tips:
a) Freshly prepare the oxime tosylate: The O‑tosylate intermediate should be used immediately after preparation, as it can decompose upon storage.
b) Control temperature carefully: Lower temperatures (0 °C or below) favor the rearrangement and suppress side reactions, particularly substrate‑dependent processes such as competitive Beckmann rearrangement.
Fig 1. Neber rearrangement reaction and its mechanism. [1]
Mechanism of Neber Rearrangement
The generally accepted mechanism begins with formation of an O-activated oxime, usually an O-sulfonate. Under basic conditions, deprotonation at the α-position generates a carbanion or equivalent nucleophilic center, which then undergoes intramolecular displacement of the O-sulfonate to form a 2H-azirine. In many cases, this azirine is not isolated because it is readily hydrolyzed under the reaction or workup conditions to the α-aminoketone.
A useful mechanistic nuance is that the azirine stage can sometimes be intercepted or isolated. The literature notes that the Neber reaction of oxime O-sulfonates with base normally results in hydrolysis of the 2H-azirine to an α-aminoketone, but there are examples in which the azirine itself is obtained in good yield. That behavior is one reason the reaction is closely connected to 2H-azirine chemistry as well as amino-ketone synthesis.
Application Examples of Neber Rearrangement
Heterocycle Synthesis: The main synthetic value of the Neber rearrangement is the direct access it provides to α-aminoketones, which are highly useful handles for heterocycle formation. These intermediates can be elaborated into imines, enamines, pyrroles, pyridines, imidazoles, and other nitrogen-containing ring systems. The reaction also has a notable role in indole synthesis. One overview notes that α-aryl azirines obtained through the Neber reaction can undergo thermolytic rearrangement to give substituted indoles in high yields, making the chemistry complementary to more classical indole-forming methods. [2]
Cathinone Derivatives: The Neber rearrangement and its modified variants have been explored for the synthesis of cathinone derivatives. A 2024 study evaluated both the classical Neber rearrangement and the Sato modification for the synthesis of cathinone from commercially available phenylpropanones. The classical Neber rearrangement of phenylpropanones gave non‑viable results for cathinone synthesis, instead yielding amides. The modified Neber rearrangement (via stable aziridine salts followed by acidic work‑up) provided cathinone derivatives in low yields of 2–17%. Although not efficient for clandestine synthesis, the mass‑spectral fragmentation patterns of by‑products provided valuable route‑specific markers for forensic profiling. [3]
Fig 2. Synthetic examples via Neber rearrangement reaction.
Related Products
References
- Jie Jack Li. Name Reactions-A Collection of Detailed Mechanisms and Synthetic Applications, Sixth Edition, 2021, 383-385.
- Taber, Douglass F., et al. Journal of the American Chemical Society 128.4 (2006): 1058-1059.
- Huxley, Cohan, et al. Forensic Chemistry 38 (2024): 100558.
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