The conversion of α‐diazo ketones into ketenes, and products derived therefrom, was discovered by Wolff in 1902. Major applications of the Wolff rearrangement, such as the Arndt−Eistert reaction (homologation of carboxylic acids) and the ring contraction of cyclic ketones, have been with us for some while. Nevertheless, substantial progress has been made recently. The reaction mechanism has been explored by means of matrix isolation, time‐resolved spectroscopy, and computation. Full retention of configuration of the migrating group has been established by using enantioselective chromatography. The Arndt−Eistert reaction has witnessed a renaissance in the field of natural products, in particular β‐amino acids and β‐peptides. Ring‐contracting Wolff rearrangements and ketene cycloadditions have been applied in syntheses of increasingly complex, biologically active target molecules. These accomplishments, as well as unsolved problems, are addressed in the present review. (© Wiley‐VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

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Controlling the selectivity of reactive intermediates is a major goal of organic chemists. Carbenes generated by thermal or photochemical extrusion of nitrogen from diazo compounds defy external control and tend to give complex product mixtures. However, the catalyzed decomposition of diazo compounds gives rise to highly selective "carbenoids" which have found extensive application in synthesis although little is known of their precise nature. On the other hand, a large variety of carbene metal complexes have been prepared and characterized which more or less lack carbenoid reactivity. Structural evidence for the transients of catalyzed diazo decomposition is becoming available through experiment and computation. Although the present report focuses on copper complexes, outstanding results with other metals are also covered.