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of the Hofmann Rearrangement. O O. RCNH RCNHBr RNCO RNH. The Hofmann rearrangement involves 6 steps in 3 stages. 1. formation of an N‐bromo amide (2 steps) 2. conversion of the N‐bromo amide to an isocyanate (2 steps) 3. hydrolysis of the isocyanate (2 steps) O H.
May 1, 2009 · Hoffmann Rearrangement Dima Berbasov May, 1, 2009 MSU. Presentation Outline • Biography • Similar reactions • Recent appli ilications. August Wilhelm von Hofmann
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NH. H. (100%) N. ". . . after ca. 10 seconds, a relatively violent reaction occurred which was accompanied by a dense cloud of white smoke and change in color from the characteristic yellow-green of the starting material to a dark brown."
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- 8.1 Types of Rearrangements
- 8.2 Rearrangement to Electron Deficient Carbon
- Features of this migration
- Isobornyl chloride
- Examples:
- 8.3.2 Curtius Rearrangement
- 8.3.4 Beckmann Rearrangement
- 8.4.1 Baeyer Villiger Reaction
- 8.5 Rearrangement to Electron-Rich Carbon
- A number of rearrangements occur in aromatic compounds of the type:
- 8.6.1 Intermolecular Migration from Nitrogen to Carbon
- 8.6.1.2 N-Alkyl-N-nitrosoanilines (Fisher-Hepp Rearrangement)
- 8.6.2 Fries Rearrangement
Rearrangements are divided into intramolecular and intermolecular processes. In intramolecular process, the group that migrates is not completely detached from the 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.
These reactions are classified according to the nature of group that migrates.
The carbocation may be produced by a variety of ways. Hydrogen can also migrate in this system. X Me Me Me Me heat Me + HX Aryl groups have a greater migratory aptitude than alkyl group or hydrogen due to the formation of lower-energy bridged phenonium ion. Me Me Cl Me H Me H Phenonium ion Me Me -H Me Me Rearrangements in bicyclic systems are com...
The rearrangement is stereosepecific Two or more rearrangements may take place simultaneously. Me Me Me H H H Me H Me
Me Me Me H Me Me OAc Me Me Me Me Me Me HO H Me H Me Me H Me Me Me Me HO Me Me -Amyrin
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. Cl
Oximes rearranges in acidic conditions to give amides. The reaction is intramolecular and stereospecific: the substituent trans to the leaving groups migrates. R' R O
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.
This group of reaction has been less explored, and is less of synthetic importance compared to the rearrangements to electron deficient carbons. The rearrangements to electron deficient hetero atom may be generally explained as:
Y X XH Y XH + Y X is usually nitrogen or oxygen. Both intermolecular and intramolecular migrations are known.
Aniline derivatives readily proceed rearrangement on treatment with acid. First, the formation of conjugate acid of the amine takes place which then eliminates the electrophilic species that reacts at the activated ortho or para position of the aromatic ring. Treatment of N-chloroacetanilide with hydrochloric acid affords a mixture of ortho and par...
The conjugate acid of the amine releases nitrosonium ion which reacts at para-position to give the p-nitroso product. Me N H Me N Me H N O N -NO Me H N NO -H N=O : NO H R H N NO R H -H ON ON intramolecular transfer of NO NHR R N .. H + NO
Aryl esters with Lewis acid undergo rearrangement to give phenols having keto substituent at ortho and para positions. The complex between the ester and Lewis acid gives an acylium ion which reacts at the ortho and para positions as in Friedel-Crafts acylation. O Lewis acid Protic workup R OH or OH R O In general, low temperature favors the forma...
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- Edward C. Franklin
- 1934
Hofmann rearrangement Upon treatment of primary amides with hypohalites, primary amines with one less carbon are obtained via the intermediacy of isocyanate. Also know as the Hof-mann degradation reaction. RNH2 O RNCO H2O RNH2 CO2n Br2 NaOH RNH O OH Br Br H RNH O RN O H Br OH RN O Br RNH2 CO2n NCO R H NO O H R OH OH isocyanate intermediate ...
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rearrangement is called antarafacial, and this is an orbital symmetry allowed pathway when the total number of electrons in the process is 4n, where n is a positive integer. The topological properties of the interacting orbitals dictate the facility of most of the sigmatropic rearrangements and their stereochemistry.
