Rearrangement Reactions

The term of “rearrangements” is used to describe organic reactions which involve the migration of an H atom or of a larger molecular fragment. Nucleophilic Rearrangements Electrophilic rearrangements Radical rearrangements

1. Nucleophilic Rearrangements

Wagner-Meerwein Rearrangement Wagner-Meerwein rearrangements are associated with 1° carbenium ions rearranging to more stable 2° and 3° carbenium ion via the 1,2-shift of an adjacent alkyl or aryl function. Wagner-Meerwein rearrangements can be used to (i) contract or expand non-aromatic rings or (ii) make sterically crowded 4° alkyl centres. Rearrangements are stereospecific.

Wagner-Meerwein Rearrangements are [1,2]-rearrangements of H atoms or alkyl groups in carbenium ions that do not contain any heteroatoms attached to the valence-unsaturated center C-1 or to the valence-saturated center C-2.

Pinacol Rearrangements The pinacol rearrangement or pinacol–pinacolone rearrangement is a method for converting a 1,2-diol to a carbonyl compound in organic chemistry. This 1,2-rearrangement takes place under acidic conditions. The name of the reaction comes from the rearrangement of pinacol to pinacolone. Mechanism of the pinacol rearrangement of a symmetrical glycol

• Which hydroxyl group is lost as water • Which hydroxyl group is lost as water? or Which carbrnium ion forms first? • What is the inherent shifting tendency (migratory apptitude) of different substituent groups? or Which group migrates? • What is the influence of steric hindrance and other strain factors on the rearrangement? (the steric chemistry) • Are epoxides formed as intermediates in the pinacol rearrangement? • Do the reaction conditions (i.e. type of acid, concentration, solvent and temperature) influence the course of rearrangement?

Which carbenium ion forms first?

The stable cation formed superior + Which intermediate carbocation is more stable?

Which group migrates? Phenyl group move first + P h C H C H 2 O H Phenyl group move first Group moved as the following order: tertiary alkyl＞aryl＞H＞secondary alkyl ＞primary alkyl＞methyl

Tiffeneau-Demjanov Rearrangements ----Semipinacol Rearrangements Primary amine Nitrosonium ion Protonated N-nitrosoamine Diazonium ion N-nitrosoamine

Reaction Mechanism

Beckmann rearrangement, The Beckmann rearrangement, named after the German chemist Ernst Otto Beckmann (1853–1923), is an acid-catalyzed rearrangement of an oxime to an amide. Cyclic oximes yield lactams. An acid-induced rearrangement of oximes to give amides. This reaction is proceed by protonation of the oxime hydroxyl, followed by migration of the alkyl substituent "trans" to nitrogen. The N-O bond is simultaneously cleaved with the expulsion of water.

Beckmann Rearrangement (insert N) inserts in anti bond

Beckmann Rearrangement (insert N) The reaction mechanism of the Beckmann rearrangement is in general believed to consist of an alkyl migration with expulsion of the hydroxyl group to form a nitrilium ion followed by hydrolysis: R- migration in cation