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The Beckmann Rearrangement Alkyl group that migrates does so with retention of configuration, and is always anti to the oxime leaving group N OH N OLVG NH2O H NO Versatile reaction to make lactams and amides Prepared starting from hydroxime, with many leaving groups possible E. Beckmann, Ber. Dtsch. Chem. Ges. 1886, 19, 988.
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- Significance of the Beckmann Rearrangement
- Intramolecular or Intermolecular?
- Stereospecificity
- Further Support
- Classic BKR
- Organocatalyzed BKR
- Proposed Mechanism: TsCl
- Organocatalyzed Cyclohexanone Oxime BKR
- Rearrangement Step
Industrial Significance Academic Significance Mechanism still uncertain Illustrated in numerous natural product syntheses Half of all produced cyclohexanone is converted to the oxime for this process. Precursor to nylon-6 Beckmann Rearrangement Publications
Is the migrating carbon group kinetically “free”? Entry of the migrating group into a separate molecule has never been observed Large ring expansion Retention of stereochemistry Unlikely that the ring opened as closure would be prohibitive
FMO Addition of EWG on the oxime oxygen polarizes the LUMO contributions
Carbenium Ion Trapping and Observation R = EWG, fragmentation less favorable (stability of benzylic cation) o-methylacetophenone oxime “More similar to anilinium than phenonium, and concluded to be nitrilium ion..”
Summary Rearrangement is a stereospecific trans shift Rearrangement occurs through an ester-like oxime intermediate (either EWG reagent or autocatalysis) The migrating carbon group is never kinetically free Migration of chiral group does not decrease ee The migration step involves an isomerization followed by ionization to a nitrilium ion intermedi...
First example, advertised as a milder method Proposed mechanism Propose Grob type isomerization but not ionization 5a requires addition of acid to act as a catalyst Other Organocatalysts Lead To Mechanistic Questions Cyclopropenium Promoted Stoichiometric Catalytic In situ generation prior to addition of oxime
Cycle I the more favorable pathway Proposed Mechanism: HCl
Sequestration of nitrilium ion leads to poor conversion Generality of Self-Propagation Mechanism Organocatalyzed BKR
Concerted or Stepwise? Acetophenone: Three step process
Jan 23, 2023 · The Beckmann rearrangement is a reaction discovered in the mid-1880’s by the chemist Ernst Otto Beckmann. The reaction converts oximes into their corresponding amides 1 allowing the insertion of the nitrogen atom from the C=N bond into the carbon chain forming a C–N bond. Depending on the starting material, it could also produce nitriles ...
system in which rearrangement is taking place. In contrast, in intermolecular process, the migrating group is first detached and later re-attached at another site. 8.2 Rearrangement to Electron Deficient Carbon These reactions are classified according to the nature of group that migrates. 8.2.1 Carbon Migration 8.2.1.1 Wagner-Meerwein Rearrangement
Prof. Kevin Shea (Smith College) 3.3: Rearrangements is shared under a CC BY 4.0 license and was authored, remixed, and/or curated by LibreTexts. This chapter focuses on synthetically useful rearrangements including the pinacol, Payne, benzilic acid, Favorskii, Tiffeneau-Demjanov, Wolff, Curtius, Baeyer-Villager, and Beckmann rearrangements.
1 RearrangementReactions. 1. Rearrangement ReactionsA rearrangement reaction is a board class of organic reactions in which an atom, ion, group of atoms, or chemical unit migrates from one atom to another atom in the same or different species, resulting in a structural isomer o.
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Note that in some instances, for example the Beckmann Rearrangement, only one group ever migrates regardless of the aptitude, simply because of stereoelectronic requirements for the reaction. Likewise, in for example the pinacol rearrangement, the most stable cation controls the reaction pathway, as opposed to the migrating group.
