Leandros Zorba

In this study we present for the first time the smooth cycloisomerization of conjugated allenones into furans catalyzed by supported Au nanoparticles (Scheme 1).¹ This transformation has been previously reported using homogeneous ionic Au catalysts.^2-4 However, these approaches require the presence of external additives and in some occasions the formation of dimeric side-products poses a major issue. In the presence of Au/TiO₂ (1 mol%), a series of conjugated allenones are cycloisomerized into the corresponding furans in excellent yields under very mild conditions (room temperature) and typically within a few minutes. The catalyst is recyclable and reusable for up to three times without any substantial decrease of its activity. From the mechanistic point of view, we propose that Au nanoparticles interact with the terminal double bond and after attack by the carbonyl oxygen atom to form a 5-membered ring intermediate chemisorbed on Au nanoparticle, protodeauration occurs intramolecularly and not intermolecularly, as shown under homogeneous Au(I) or Au(III)-catalyzed conditions.²

Scheme 1: Cycloisomerization of conjugated allenones into furans, catalyzed by Au/TiO₂.