Wolfgang Kirmse

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.

Die Steuerung der Selektivität reaktiver Zwischenstufen ist ein wichtiges Ziel in der organischen Chemie. Photochemisch oder thermisch aus Diazoverbindungen erzeugte Carbene lassen sich nicht von außen beeinflussen und bilden Produktgemische. Dagegen führt die katalytische Zersetzung von Diazoverbindungen zu selektiven Carbenoiden, die häufig in der Synthese verwendet werden – bei geringer Kenntnis ihrer Eigenschaften. Andererseits sind zahlreiche Carben‐Metall‐Komplexe bekannt, denen die carbenoide Reaktivität fehlt. Das Zusammenwirken von Experiment und Theorie liefert Informationen über die labilen Zwischenstufen der katalytischen Zersetzung von Diazoverbindungen. Im Mittelpunkt dieses Kurzaufsatzes stehen Kupfer‐Komplexe, doch werden auch andere Metalle berücksichtigt.

Alkenylidenes R₂CC: (= alkylidene carbenes) undergo regio‐ and stereoselective intramolecular CH insertion reactions that are excellently suited for the synthesis of cyclopentenes. The 1, 2‐shifts occurring with RH and RAr are useful for the preparation of alkynes. Alkenylidenes are efficiently generated from carbonyl compounds by diazomethylation, from vinyl halides by α‐elimination, from alkynyliodonium salts by addition of nucleophiles, and from alkynes by retro‐1, 2‐shifts. Specific applications of the various methods, particularly in the synthesis of natural products, are discussed.