for the Claisen rearrangement. The variant was discovered when it was noticed that thermolysis of 1-diazo-3,3,3-triarylpropan-2-ones gave unexpected isomeric products. Copper (II) and rhodium (II) salts tend to give vinylogous Wolff rearranged products, and CuSO4 and Rh2(OAc)4 are the most commonly used catalysts. This is because they promote metal carbene formation, which can add to the olefin to form a cyclopropane, which can reopen via a retro [2 + 2] to form a formally 1,3-shifted ketene (vis-à-vis a normal Wolff rearranged ketene), which can be trapped by a nucleophile to give the vinylogous Wolff product. See also Diazo Ketene Arndt-Eistert reaction Curtius rearrangement Schmidt reaction References Rearrangement reactions Name reactions Ring contraction reactions