Allene Substitution‐Controlled Switching of Dimerization to Cycloisomerization in the PdII‐Catalyzed Reaction of Terminal α‐Allenones

In this contribution, we describe the role of steric interactions in PdII‐catalyzed reactions of terminal α‐allenones, which has not previously been invoked in discussions of these versatile compounds. By adopting [PdCl2(MeCN)2] as catalyst, the cycloisomerization/dimerization ratio of α‐allenones is controlled by the substitution of the allene compound: unsubstituted allenones mainly afford dimerization, whereas allenones bearing an internal substituent favor the formation of cycloisomerization products. Thus, for the first time the mode of reaction (cycloisomerization vs. dimerization) of α‐allenones is substrate‐directable.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)