Rhenium-catalyzed synthesis of indenones by novel dehydrative trimerization of aryl aldehydes via C-H bond activation

By heating aryl aldehydes with catalytic amounts of a rhenium complex, ReBr(CO)₅, and N-phenylacetamide in toluene, indenone derivatives are obtained in good to excellent yields. This reaction proceeds via (1) the formation of an isobenzofuran derivative by the insertion of an aldehyde into the C-H bond of another aldehyde (C-H bond activation) and successive intramolecular nucleophilic cyclization, (2) nucleophilic addition of the formed isobenzofuran derivative to the third aldehyde, (3) isomerization, and (4) intramolecular aldol condensation.