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Syllabus:General mechanistic considerations – nature of migration, migratory aptitude, memory effects. A detailed study of the following rearrangements-Pinacolpinacolone, Wagner-Meerwein, Demjanov, Benzil-Benzilic acid, Favorskii, ArndtEistert synthesis, Neber, Beckmann, Hofman, Curtius, Schimdt, Baeyer-Villigr, Shapiro reaction.
1. Nature of Migration: Migration in organic rearrangements refers to the movement of an atom or a group from one center to another within a molecule under the influence of electronic factors.
1. Nature of Migration: Migration in organic rearrangements refers to the movement of an atom or a group from one center to another within a molecule under the influence of electronic factors.
Key Features:
Key Features:
Generally occurs in carbocationic, carbanionic, or radical intermediates.
Follows a concerted or stepwise pathway.
Driven by electronic, steric, and orbital interactions.
Stereoselective and Stereospecific in Nature
Types of Migration:
Types of Migration:
Intramolecular Migration – Within the same molecule.
Intermolecular Migration – Between two different molecules(Rare)
Mechanistic Pathways of Migration:
Mechanistic Pathways of Migration:
Concerted Mechanism: Direct shift of migrating group to a new position.
Stepwise Mechanism: Involves intermediate formation before migration.
Electrophilic or Nucleophilic Migration: Depends on whether an electron-deficient or electron-rich species initiates the process.
2. Migratory Aptitude
2. Migratory Aptitude
Migratory aptitude refers to the tendency of a substituent or group to migrate during a rearrangement reaction. It is influenced by:
Electronic Factors – Groups with strong donating or withdrawing effects influence the migration.
Steric Factors – Less sterically hindered groups migrate more easily.
Resonance Stabilization – Groups that stabilize the intermediate migrate preferentially.
General Order of Migratory Aptitude in Carbocationic Rearrangements:
General Order of Migratory Aptitude in Carbocationic Rearrangements:
H>Ph>3°alkyl>2°alkyl>1°alkyl>Methyl
Examples:
Examples:
Pinacol-Pinacolone Rearrangement: Phenyl (Ph) migrates preferentially over alkyl groups due to resonance stabilization.
Beckmann Rearrangement: Migration of the group anti to the leaving group (–OH in oximes).
Baeyer-Villiger Oxidation: Migration order: H > 3° alkyl > 2° alkyl > Ph > 1° alkyl > Methyl
3. Memory Effects in Organic Rearrangements:
Memory effect refers to the retention of stereochemical and electronic information from the precursor molecule in the rearranged product.
Key Features:
Key Features:
Arises due to non-random or specific migration of groups.
Strongly influences stereochemical outcomes in rearrangement reactions.
Memory effects are seen in systems where intermediates preserve the initial molecular framework.
Examples:
Examples:
Carbocationic Rearrangements:
In Norbornyl Cation Rearrangements, the bicyclic framework dictates the migration.
Stereospecificity in Beckmann Rearrangement:
The group anti to the hydroxyl migrates preferentially, preserving stereochemical information.
Retention of Configuration in Sigmatropic Rearrangements:
Cope and Claisen rearrangements show a predictable shift based on orbital symmetry considerations.
Memory effects
Memory effects
Solvolysis of the endo bicyclic compound 1 (X= ONs or Br) gave mostly the bicyclic allylic alcohol 4, along with smaller amount of the tricyclic alcohol 7, while solvolysis of the exo isomers 5 gave mostly 7, with smaller amount of 4. Thus the two isomers gave entirely different ratios of products, though the carbocation initially formed seems to be the same 2.
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