Reactions and mechanistic studies of rhenium-catalyzed insertion of α,β-unsaturated carbonyl compounds into a C-H bond

A rhenium complex, [ReBr(CO)₃(thf)]₂, catalyzes the insertion of α, β-unsaturated carbonyl compounds into a C-H bond of aromatic compounds having nitrogen-containing directing groups. In this reaction, Re₂(CO)₁₀ can also be used as a catalyst. When imines are employed as the aromatic substrates, sequential cyclization proceeds and indene derivatives are obtained in good to excellent yields. This reactivity is in contrast to those of ruthenium and rhodium complexes, which are usually used as catalysts in the insertion reactions of unsaturated molecules into a C-H bond. Investigations on the reaction mechanism indicate that the rhenium catalyst promotes C-H bond activation of aromatic compounds, the insertion of α, β-unsaturated carbonyl compounds into a Re-C bond, and intramolecular nucleophilic cyclization followed by reductive elimination and the elimination of an amine.