Category: Molecular Rearrangements

Stevens Rearrangement In the Stevens rearrangement, keto-quaternary ammonium or tertiary sulphonium salts rearrange to amino ketones or mercapto ketones under the influence of strong base. Quaternary ammonium ions, which contain ß-hydrogen atoms undergo E2 (Hofmann) elimination with base: If, however, none of the alkyl groups possesses a ß-hydrogen atom, but one has a ß-carbonyl group,…

Wolff rearrangement is an example of nucleophilic rearrangement involving carbene. It is a rearrangement of α-diazoketones leading to carboxylic acid derivatives via ketene intermediates, which is similar to the Curtius rearrangement. It can be achieved with metal catalysis or photochemically. There has been much debate about the timing of the mechanism and many attempts to distinguish between…

There is a group of closely related rearrangements in which carbon migrates from carbon to nitrogen that may be represented generally as, where R is an alkyl or aryl group and −X is a leaving group, which may be – Br (Hofmann rearrangement),  (Curtius and Schmidt rearrangement) and – OCOR (Lossen rearrangement). In each case, if the migrating…

Beckmann Rearrangement In the Beckmann rearrangement, an oxime is converted to an amide. An oxime is easily obtained by treatment of aldehyde or ketone with hydroxylamine. The OH group of ketoximes R1R2C(=NOH) can become a leaving group. Tosylation is one way to convert this hydroxyl group into a leaving group. The oxime OH group also…

Transformation of a diketone to phenol in the presence of acid is known as the Dienone–Phenol rearrangement. As the name implies, this reaction results in the transformation of a quinoid structure to a benzenoid ring. It may be considered as reverse pinacol rearrangement, since pinacol and semipinacol rearrangements are driven by the formation of a carbonyl…

When phenols are treated with acid chlorides or acid anhydrides, esters are obtained. Conversion of phenolic esters, on warming with a Lewis acid (Friedel-Crafts catalyst), to hydroxy ketones is known as Fries rearrangement It is an acid-catalyzed rearrangement from side chains onto the aromatic ring. Acylation of phenols by Friedel-Crafts reaction also forms the same product…

The Baeyer–Villiger oxidation (discovered in 1899 by the Germans Adolf van Baeyer, who won the Nobel Prize in 1905, and V. Villiger) involves the reaction of a ketone with H2O2 or a peroxy acid (RCO3H) to give an ester. Here, the starting material is a ketone, in which an oxygen atom is inserted between the carbonyl group…

When a positive charge is formed on an alicyclic carbon, migration of an alkyl group may occur to give ring contraction by one carbon smaller than the original. This abnormal change involves conversion of more stable secondary carbenium ion to a less stable primary carbenium ion. Similarly, when a positive charge is formed on a…

This is a variant of pinacol reaction. By converting a 1,2-diol into its monotosylate, base-catalyzed rearrangement can be brought about. Since the ease of formation of tosylates decreases in the order primary > secondary > tertiary, this provides a means of effecting a rearrangement in the opposite direction from the acid-catalyzed reaction. For example, in…

This is an acid catalyzed 1,2–migration of a diol to an oxo derivative. When vic–diols (glycols) are treated with acids, they can be rearranged to give aldehydes or ketones. The reaction gets its name from the typical compound pinacol Me2C(OH)C(OH)Me2, which is rearranged to pinacolone Me3CCOCH3. The rearrangement has been carried out with alkyl, aryl, hydrogen…