COMPILED AND CIRCULATED BY DR. SK MOHAMMAD AZIZ,
ASSISTANT PROFESSOR, DEPARTMENT OF CHEMISTRY, NARAJOLE RAJ COLLEGE
CHEMISTRY: SEM-IV, PAPER- C10T: ORGANIC CHEMISTRY-IV, REARRANGEMENT REACTION
Dr. SK MOHAMMAD AZIZ
1
COMPILED AND CIRCULATED BY DR. SK MOHAMMAD AZIZ,
ASSISTANT PROFESSOR, DEPARTMENT OF CHEMISTRY, NARAJOLE RAJ COLLEGE
CHEMISTRY: SEM-IV, PAPER- C10T: ORGANIC CHEMISTRY-IV, REARRANGEMENT REACTION
Rearrangement reactions are an interesting class of reactions wherein a group or an atom migration during the course of the reaction. While most of the rearrangements are designed in that fashion, it can also be undesirable in some cases. Depending on the reaction conditions, the nature of rearrangement (and the product) could also change.
In this module, various rearrangement reactions are presented. These are classified with respect the migration origin and migration terminus.
Emphasis has been placed on examples involving skeletal rearrangements that are practically used in day-to-day organic synthesis.
INTRODUCTION:
2
Molecular Rearrangements
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Molecular Rearrangements
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Molecular Rearrangements
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Molecular Rearrangements
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Molecular Rearrangements
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Molecular Rearrangements
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Molecular Rearrangements
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Molecular Rearrangements
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Molecular Rearrangements
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Molecular Rearrangements
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Molecular Rearrangements
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Molecular Rearrangements
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Molecular Rearrangements
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Molecular Rearrangements
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Molecular Rearrangements
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Molecular Rearrangements
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Molecular Rearrangements
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Molecular Rearrangements
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Molecular Rearrangements
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Molecular Rearrangements
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Molecular Rearrangements
Wolff rearrangement in Arndt-Eistert synthesis
This reaction takes place in the presence of light, heat or transition metal catalyst such as Ag
2O.
The mechanism is suggested to proceed through the involvement of a carbene in presence of heat or light. It may also proceed through a concerted
Pathway in the presence of Ag
2O with out carbene.
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Molecular Rearrangements
Wolff rearrangement in Arndt-Eistert synthesis
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Molecular Rearrangements
Under both thermolytic and photolytic conditions, there exist competing concerted and stepwise mechanisms. Many mechanistic studies have been carried out, including conformational, sensitization, kinetic, and isotopic scrambling studies.
These all point to competing mechanisms, with general trends. α-Diazo ketones that exist in the s-cis conformation generally undergo a concerted mechanism, whereas those in the s-trans conformation undergo a stepwise mechanism.
Wolff rearrangement in Arndt-Eistert synthesis
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Molecular Rearrangements
Wolff rearrangement in Arndt-Eistert synthesis
Synthetic uses:
Homologation reactions
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Molecular Rearrangements
Synthetic uses:
Wolff rearrangement in Arndt-Eistert synthesis
Ring contractions
Homologation reactions
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Molecular Rearrangements
1,2-Diketones that have no -hydrogen react with hydroxide ion to give -hydroxyacid.
The best known example is the rearrangement of benzil to benzilic acid. The driving force for the reaction lies in the removel of the product by ionization of carbonyl group.
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Molecular Rearrangements
Examples:
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Molecular Rearrangements
The Demjanov rearrangement is the chemical reaction of primary amines with nitrous acid to give rearranged alcohols.[1][2][3] It involves substitution by a hydroxyl group with a possible ring expansion. It is named after the Russian chemist Nikolai Jakovlevich Demjanov
Demjanov rearrangement
The reaction process begins with diazotization of the amine by nitrous acid. The diazonium group is a good leaving group, forming nitrogen gas when displaced from the organic structure. This displacement can occur via a rearrangement (path A), in which one of the sigma bonds adjacent to the diazo group migrates.
This migration results in an expansion of the ring. The resulting carbocation is then attacked by a molecule of water. Alternately, the diazo group can be displaced directly by a molecule of water in an SN2 reaction (path B). Both routes lead to formation of an alcohol
Mechanism:
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Molecular Rearrangements
Mechanism:
Demjanov rearrangement
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Molecular Rearrangements
Demjanov rearrangement
Synthetic Uses:
The Demjanov rearrangement is a method to produce a 1-carbon ring enlargement in four, five or six membered rings. The resulting five, six, and seven-membered rings can then be used in further synthetic reactions
It has been shown that the Demjanov reaction is susceptible to regioselectivity.
One example of this is a study conducted by D. Fattori looking at the regioselectivity of the Demjanov rearrangement in one-carbon enlargements of naked sugars. It showed that when an exo methylamine underwent Demjanov nitrous acid deamination, ring enlargement was not produced.
However, when the endo methylamine underwent the same conditions, a mixture of rearranged alcohols were produced.
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Molecular Rearrangements
The Tiffeneau–Demjanov rearrangement (TDR) is the chemical reaction of a 1- aminomethyl-cycloalkanol with nitrous acid to form an enlarged cycloketone.
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Molecular Rearrangements
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Molecular Rearrangements
Hofmann Rearrangement
This rearrangement provides an effective method for the synthesis of primary aliphatic and aromatic amines from primary amides
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Molecular Rearrangements
Mechanism
Treatment of amide with sodium hypobromite gives N-bromo-amide which reacts with base to afford a conjugate base within which rearrangement takes place to give isocyanate. The formed isocyanate may be isolated in anhydrous conditions or it can be converted into amine by aqueous workup
The workup can also be with alcohol or amine to give urethane or urea, respectively
Hofmann Rearrangement
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Molecular Rearrangements
Hofmann Rearrangement
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Molecular Rearrangements
Curtius Rearrangement
This rearrangement describes the transformation of acyl azide into isocyanate by decomposition on heating and its application for the synthesis of primary amines, urethanes and ureas as presented in Hofmann rearrangement.
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Molecular Rearrangements
Preparation of acyl azide:
The acyl azide is usually made from the reaction of acid chlorides or anydrides with sodium azide or trimethylsilyl azide. Acyl azides are also obtained from treating acylhydrazines with nitrous acid. Alternatively, the acyl azide can be formed by the direct reaction of a carboxylic acid with diphenylphosphoryl azide
Curtius Rearrangement
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Molecular Rearrangements
Curtius Rearrangement
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Molecular Rearrangements
Lossen Rearrangement
Ester of hydroxamic acid reacts with base to give isocyanate that could be converted into amine as shown in Hofmann rearrangement
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Molecular Rearrangements
Lossen Rearrangement
Mechanism :
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Molecular Rearrangements
Lossen Rearrangement
Example:
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Molecular Rearrangements
Schmidt Rearrangement
Carboxylic acid reacts with hydrazoic acid in the presence of conc. H2SO4 to give acid azide which is present in the form of conjugate acid eliminates nitrogen to afford isocyanate that could be converted into amine as reported in Hofmann rearrangement
The reaction is also effective with aldehydes, ketones, tertiary alcohols and substituted alkenes.
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Molecular Rearrangements
Mechanism:
Schmidt Rearrangement
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Molecular Rearrangements
Schmidt Rearrangement
Example:
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Molecular Rearrangements
Schmidt Rearrangement
Example:
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Molecular Rearrangements
Schmidt Rearrangement
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Molecular Rearrangements
Oximes rearranges in acidic conditions to give amides. The reaction is intramolecular and stereospecific: the substituent trans to the leaving groups migrates
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Molecular Rearrangements
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Molecular Rearrangements
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Molecular Rearrangements
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Molecular Rearrangements
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Molecular Rearrangements
Treatment of ketones with peroxyacid gives ester. The reaction is effective with acid or base and the mechanism is closely related to pinacol rearrangement:
nucleophilic attack by the peroxyacid on the carbonyl group gives an intermediate that rearranges with the expulsion of the anion of the acid.
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Molecular Rearrangements
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Molecular Rearrangements
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Molecular Rearrangements
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Molecular Rearrangements
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Molecular Rearrangements
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Molecular Rearrangements
Hydroperoxide Rearrangement
Tertiary hydroperoxide with acid undergoes rearrangement to give ketone and alcohol or phenol. The mechanism is similar to that of Baeyer-Villiger reaction.
For example, cumene forms hydroperoxide by autoxidation which rearranges in the presence of an acid to give phenol and acetone
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Molecular Rearrangements
Dakin Reaction
Benzaldehyde or acetophenone bearing hydroxyl substituent in the ortho or para position proceed rearrangement to give catechol or quinol, respectively.
The reaction is performed in the presence of alkaline hydrogen peroxide and the mechanism is similar to that of Baeyer-Villiger reaction.
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Molecular Rearrangements
Dakin Reaction
Example:
62
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