Wolff rearrangement with mechanism and its Application

There are several reactions that are conceptually related to carbene reactions but do not involve carbene, or even carbenoid, intermediates. Usually, these are reactions in which the generation of a carbene is circumvented by a concerted rearrangement process. Important examples of this type are the thermal and photochemical reactions of alpha-diazo ketones.

When alpha diazo ketones are decomposed thermally or photochemically, they usually rearrange to ketenes, in a reaction known as the Wolff rearrangement

If this reaction proceeds in a concerted fashion, a carbene intermediate is avoided. Mechanistic studies have been aimed at determining whether migration is concerted with the loss of nitrogen. The conclusion that has emerged is that a carbene is generated in photochemical reactions but that the reaction can be concerted under thermal conditions.

The rearrangement may also occur on irradiation or on heating. When the reaction Is carried out In the piesence of water, alcohol, ammonia or amine, the highly reactive ketene readily reacts with the nucleophiles present, e.g., H20 .. AOH,’etc., to give respectively acids, esters, amides or substituted amides of the next higher homologue of the acid from which the a-cliazoketone is prepared

Mechanism

It has been shown (Huggett et a/.) with isotopically labelled carbon ( 13 C) in a series of transformations• that the carbonyl carbon of a-diazoketone is present in the resulting acid as the carboxyl carbon when the reaction is carried out in the presence of water. Obviously, migration must have occurred during the rearrangement. On the basis of this, the following mechanism has been suggested

Splitting of nitrogen and migration of R group may be concerted. In some cases ketenes have been isolated. The group R migrates with retention of configuration. This has been confirmed by the following observation. A higher homologue of an optically active acid (I) obtained by Arndt-Eistert-Wolff rearrangement on degradation by Barbier-Weiland method gave the original acid with the same configuration (Lane and Wallis)

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