rearrangement reaction

[علي زيد الحسني]

A rearrangement reaction
is a broad class of organic reactions where the carbon skeleton of a molecule is rearranged to give a structural isomer of the original molecule [1] . Often a substituent moves from one atom to another atom in the same molecule. In the example below the substituent R moves from carbon atom 1 to carbon atom 2:
Intermolecular rearrangements also take place.
A rearrangement is not well represented by simple and discrete electron transfers (represented by curly arrows in organic chemistry texts). The actual mechanism of alkyl groups moving, as in Wagner-Meerwein rearrangement, probably involves transfer of the moving alkyl group fluidly along a bond, not ionic bond-breaking and forming. In pericyclic reactions, explanation by orbital interactions give a better picture than simple discrete electron transfers. It is, nevertheless, possible to draw the curved arrows for a sequence of discrete electron transfers that give the same result as a rearrangement reaction, although these are not necessarily realistic.
Three key rearrangement reactions are 1,2-rearrangements, pericyclic reactions and olefin metathesis.
1,2-rearrangement

A 1,2-rearrangement is an organic reaction where a substituent moves from one atom to another atom in a chemical compound. In a 1,2 shift the movement involves two adjacent atoms but moves over larger distances are possible. Examples are the Wagner-Meerwein rearrangement:
and the Beckmann rearrangement:

pericyclic reactions
A pericyclic reaction is a type of reaction with multiple carbon-carbon bond making and breaking wherein the transition state of the molecule has a cyclic geometry, and the reaction progresses in a concerted fashion. Examples are hydride shifts

and the Claisen 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.[1][2][3] Cyclic oximes yield lactams.

his example reaction[4] starting with cyclohexanone, forming the reaction intermediate cyclohexanonoxime (in the image, the ending 'ono' in the name is missing) and resulting in caprolactam is one of the most important applications of the Beckmann rearrangement, as caprolactam is the feedstock in the production of Nylon 6.
The Beckmann solution consists of acetic acid, hydrochloric acid and acetic anhydride, and was widely used to catalyze the rearrangement. Other acids, such as sulfuric acid or polyphosphoric acid, can also be used. sulfuric acid is the most commonly used acid for commercial lactam production due to its formation of an ammonium sulfate by-product when neutralized with ammonia. Ammonium sulfate is a common agricultural fertilizer providing nitrogen and sulfur.
The reaction mechanism of the Beckmann rearrangement is generally believed to consist of an alkyl migration with expulsion of the hydroxyl group to form a nitrilium ion followed by hydrolysis:

The Curtius rearrangement (or Curtius reaction or Curtius degradation),
as first defined by Theodor Curtius, is a chemical reaction that involves the rearrangement of an acyl azide to an isocyanate.[1][2] Several reviews have been published.[3][4]

The isocyanate can be trapped by a variety of nucleophiles. Often water is added to hydrolyze the isocyanate to an amine.[5] When done in the presence of tert-butanol, the reaction generates Boc-protected amines, useful intermediates in organic synthesis.[6][7]
Carboxylic acids 1 can be easily converted to acyl azides 3 using diphenylphosphoryl azide 2.[8][9][10]

The Hofmann rearrangement
is the organic reaction of a primary amide to a primary amine with one fewer carbon atom.[1][2][3]

Mechanism
The reaction of bromine with sodium hydroxide forms sodium hypobromite in situ, which transforms the primary amide into an intermediate isocyanate. The intermediate isocyanate is hydrolyzed to a primary amine giving off carbon dioxide

Lossen rearrangement
is the conversion of a hydroxamic acid (1) to an isocyanate (3) via the formation of an O-acyl, sulfonyl, or phosphoryl intermediate hydroxamic acid O-derivative (2) and then conversion to its conjugate base. Here, 4-Toluenesulfonyl chloride is used to form a sulfonyl O-derivative of hydroxamic acid. [1] [2]

The isocyanate can be used further to generate ureas in the presence of amines (4) or generate amines in the presence of H2O (5

Reaction Mechanism
The mechanism below begins with an O-acylated hydroxamic acid derivative that is treated with base to form an isocyanate that generates an amine and CO2 gas in the presence of H2O. The hydroxamic acid acid derivative is first converted to its conjugate base by abstraction of a hydrogen by a base. Spontaneous rearrangement kicks off a carboxylate anion to produce the isocyanate intermediate. The isocyanate in the presence H2O hydrolyzes and then decarboxylation via abastraction of a hydrogen by a base generates an amine and CO2 gas.

The Schmidt reaction
is an organic reaction involving alkyl migration over the carbon-nitrogen chemical bond in an azide with expulsion of nitrogen.[1][2] A key reagent introducing this azide group is hydrazoic acid and the reaction product depends on the type of reactant: carboxylic acids form amines through an isocyanate intermediate (1) and ketones form amides (2):

A catalyst is required which can be a protic acid usually sulfuric acid or a Lewis acid. The reaction was discovered in 1924 by Karl Friedrich Schmidt
(1887-1971),[3] who successfully converted benzophenone and hydrazoic acid to benzanilide. It is a tool regularly used in organic chemistry for the synthesis of new organic compounds for example in that of the unusual 2-quinuclidone.
An oxime is one in a class of chemical compounds with the general formula R1R2C=NOH, where R1 is an organic side chain and R2 is either hydrogen, forming an aldoxime, or another organic group, forming a ketoxime. O-substituted oximes form a closely related family of compounds. Amidoximes are oximes of hemiaminals with general structure RC(=NOH)(NRR'). Certain amidoximes react with benzenesulfonyl chloride to substituted ureas in the Tiemann rearrangement [1][2]
Oximes are usually generated by the reaction of hydroxylamine and aldehydes or ketones. The term oxime dates back to the 19th century, a portmanteau of the words oxygen and imide[citation needed].
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The Wolff rearrangement
is a rearrangement reaction converting a α-diazo-ketone into a ketene.[1][2][3] This reaction was first reported by Ludwig Wolff in 1912.[4]

The rearrangement is catalyzed by light[5], heat[6], or a transition metal catalyst such as silver oxide. Nitrogen gas is expelled forming a carbenic intermediate which rearranges. This 1,2-rearrangement is the key step in the Arndt-Eistert synthesis.[7][8]
The reaction may or may not proceed in a concerted mechanism. A carbene intermediate is avoided if the reaction proceeds through the concerted mechanism. Mechanistic studies have been aimed at determining if migration is concerted with loss of nitrogen. The conclusion that has emerged is that a carbene is generated in photochemical reactions but the reaction can be concerted under the thermal conditions.
In one application,[9] the Wolff rearrangement is performed in an electrochemical setup in which the catalyst silver oxide is reduced to elemental silver in the shape of monodisperse nanoparticles (2-4 nm diameter) which trigger the decomposition of the diazoketone by the formation of a radical cation.