Wolff Rearrangement

Abstract

The thermal, photochemical or catalytic transformation of α‐diazoketones into ketenes is generally known as the Wolff rearrangement. The readiness of Wolff rearrangement is probably due to the evolution of a nitrogen molecule, forming a carbenic intermediate. Several transition metal catalysts have been used and it has been reported that the copper salt can facilitates the generation of a carbene intermediate and lower the energy barrier for the Wolff rearrangement, however, a silver salt is even better than the copper salt for such purpose. It has been reported that cyclic α‐diazoketones undergo the Wolff rearrangement to give the ring‐contracted cyclic carboxylic acids. The study finds that in the case of β‐oxy‐α‐diazoketones, such as β‐hydroxy‐α‐diazoketones, the regular alkyl migration in the normal Wolff rearrangement is depressed by the competitive 1,2‐H shift from hydroxyl to carbene, leading to 100% of vinyl ketones. In terms of the structures of α‐diazoketones, α‐diazo malonates often form vinyl alkyl ethers due to the loss of carbon monoxide. Furthermore, both carbon monoxide and oxygen are reported to suppress the Wolff rearrangement. It has also been reported that 2‐diazodibenzyl malonate undergoes the Wolff rearrangement more easily than 2‐diazodimethyl malonate in the presence of a transition metal catalyst. The reversal of the Wolff rearrangement, known as the retro ‐Wolff rearrangement, has also been reported. This reaction has very broad application in organic synthesis as well as in industrial imaging.