Metal-Mediated Synthesis of Allenes

Modem AUene Chemistry. Edited by N. Krause and A.S.K. Hashmi Copyright 2004 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 3-527-30671-4 

In these transformations, organocopper compounds are often the reagents of choice, although recently other metals such as aluminum, titanium, samarium and indium have also proven to be highly useful. Since earlier contributions to this field have already been summarized extensively [1], we shall concentrate on more recent contributions published after 1980. Rather than trying to be comprehendsive, representative examples with references to the most important reaction types will be given. 

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Scheme 2.1 Important reaction types in the metal-mediated synthesis of allenes. X= leaving group Acc = acceptor substituent.

One of the most versatile methodologies of EPC synthesis of allenes is the chirality transfer reaction which involves highly stereoselective, mechanism-controlled, metal-mediated propyn-yl-allenyl transpositions. Of these, the organocopper(l)-mediated reactions are especially useful and provide a relatively straightforward route to a broad variety of allenes of high enantiomeric purity. 

Palladium chemistry has been used in the synthesis of tetrahydroisoquinolines. Different combinations of iodoaryl-amine-alkene can be used in these multicomponent reactions. For example, the metal-mediated o-alkylated/alkenyl-ation and intramolecular aza-Michael reaction (Scheme 109) give moderate yields of heterocycle <2004TL6903>, whereas the palladium-catalyzed allene insertion-nucleophilic incorporation-Michael addition cascade (Equation 172) produces good yields of tetrahydroisoquinolines in 15 examples <2003TL7445> with further examples producing tetrahydroquinolines (Scheme 110) <2000TL7125>. 

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