Insertion of Polar and Nonpolar Unsaturated Molecules into Carbon−Rhenium Bonds Generated by C−H Bond Activation:  Synthesis of Phthalimidine and Indene Derivatives

Abstract

A rhenium complex, [ReBr(CO)(3)(thf)](2), catalyzes the reaction of an aromatic aldimine with an isocyanate and an acetylene to give a phthalimidine and an indene derivative in a quantitative yield, respectively. The reactions proceed via C-H bond activation, insertion of the isocyanate or the acetylene, intramolecular nucleophilic cyclization to the aldimine of the generated amido- or alkenyl-rhenium species, and reductive elimination. In contrast to ruthenium and rhodium catalysts, which are usually employed in this type of reaction, the rhenium catalyst promotes the insertion of a polar unsaturated molecule. This occurs more easily than the insertion of a nonpolar unsaturated molecule.

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

“…The cyclization to ix should proceed from vi (step [15]), and so, if insertion of an acrylate into the Re-H bond of iii occurs (step [6]), indene derivative xi should be produced from vi via viii (step [14]) because reductive elimination of v generates viii (step [12]). Thus, we prepared the key intermediate viii, which would be formed via the C-H activation followed by the insertion of an acrylate into the Re-H bond (steps [6] and [12]), 20 and examined the reaction (eq 3). Treatment of 11 with a catalytic amount of [ReBr(CO) 3 (thf)] 2 under the same reaction conditions gave indene 6a in only 9% yield and most of 11 remained unchanged.…”

“…In addition, we did not observe 11 in the reaction mixture between 5a and ethyl acrylate (2a) (eq 2). These results suggest that the main pathway does not go through viii (via steps [6] and [12]), and the pre-cyclization intermediate vi should be generated directly by the insertion of an acrylate into the Re-C bond (step [8] …”

“…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.…”