Transition‐Metal Catalysis of the Michael Reaction of 1,3‐Dicarbonyl Compounds and Acceptor‐Activated Alkenes

Transition‐metal catalysis of the Michael reaction of 1,3‐dicarbonyl compounds with acceptor‐activated alkenes is introduced as a valuable alternative to the classic base catalysis of this reaction. Owing to the mild, neutral reaction conditions, the chemoselectivity of these processes is superior to that offered by the base catalysis, since the latter suffers from various unwanted side‐ and subsequent reactions, such as aldol cyclizations and ester solvolyses. The most efficient transition‐metal catalysts do not require inert or anhydrous conditions, even solvents are unnecessary in some cases, and quantitative conversions can be achieved at room temperature. Furthermore, the development of transition‐metal catalysts on solid supports has allowed very simple work‐up procedures. Despite the extraordinary chemoselectivity, in terms of diastereoselectivity transition‐metal catalysis gives better results than base catalysis only in a very few special cases. Also, in terms of enantioselectivity, results recently achieved with basic rare‐earth metal catalysts cannot be reproduced by transition metals. Nevertheless, with transition‐metal catalysis, even new reactivities can be observed, which have hitherto been unknown for base‐catalyzed systems. For example, Michael donor molecules have been found to react in an unprecedented vinylogous fashion.