Rhenium-Catalyzed Formation of Indene Frameworks via C−H Bond Activation:  [3+2] Annulation of Aromatic Aldimines and Acetylenes

Abstract

A rhenium complex, [ReBr(CO)3(thf)]2, catalyzes the reaction of an aromatic aldimine with an acetylene to give an indene derivative in a quantitative yield. The reaction proceeds via C-H bond activation, insertion of the acetylene, intramolecular nucleophilic cyclization, and reductive elimination. In contrast to ruthenium and rhodium catalysts, which are usually employed in this type of reaction, the rhenium catalyst promotes the intramolecular nucleophilic cyclization of the alkenylmetal species generated by insertion of the acetylene.

Reactions and Mechanistic Studies of Rhenium-Catalyzed Insertion of α,β-Unsaturated Carbonyl Compounds into a C–H Bond

“…rhenium-catalyzed C-H activation. However, neither N,Ndimethylbenzylamine nor 2,5-diphenylfuran gave the desired adduct upon treatment with acrylate 2a in the presence of a catalytic amount of ReBr(CO) 5 . In contrast, a pyridyl group was a good directing group for the transformation; for example, 2-phenylpyridine gave a mixture of mono-and di-adducts 3a and 4a in quantitative yield (Table 2, Entry 1).…”

“…Once rhenium inserts into a C-H bond at the ortho position, an ,-unsaturated carbonyl compound can insert into the Re-H bond (step [3] or [7]) or the Re-C bond (step [5] or [9]) of an aryl-rhenium intermediate ii. (Table 3).…”

“…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 ,-unsaturated carbonyl compounds into a Re-C bond, and intramolecular nucleophilic cyclization followed by reductive elimination and the elimination of an amine.Transformations via the insertion of unsaturated molecules into an inactivated C-H bond are efficient and useful methods to synthesize more complex molecules.1 Previously, ruthenium and rhodium complexes have usually been employed as catalysts in such transformations.1 These reactions proceed via C-H bond activation, insertion of unsaturated molecules into the metal-hydrogen bond, and reductive elimination.2-4 We have recently reported on rhenium-catalyzed C-H bond activation followed by the insertion of unsaturated molecules, such as acetylenes, 5,6 isocyanates, 6 aldehydes, 7 and acrylates. 8 However, the reaction style is quite different from that with ruthenium and rhodium catalysts.…”

“…2-4 We have recently reported on rhenium-catalyzed C-H bond activation followed by the insertion of unsaturated molecules, such as acetylenes, 5,6 isocyanates, 6 aldehydes, 7 and acrylates. 8 However, the reaction style is quite different from that with ruthenium and rhodium catalysts.…”

“…1 These reactions proceed via C-H bond activation, insertion of unsaturated molecules into the metal-hydrogen bond, and reductive elimination. [2][3][4] We have recently reported on rhenium-catalyzed C-H bond activation followed by the insertion of unsaturated molecules, such as acetylenes, 5,6 isocyanates, 6 aldehydes, 7 and acrylates. 8 However, the reaction style is quite different from that with ruthenium and rhodium catalysts.…”