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

Abstract

A rhenium complex, [ReBr(CO) 3 (thf)] 2 , catalyzes the insertion of ,-unsaturated carbonyl compounds into a C-H bond of aromatic compounds having nitrogen-containing directing groups. In this reaction, Re 2 (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 ,-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. Although only the insertion of unsaturated molecules into a C-H bond proceeds in the case of the ruthenium-and rhodium-catalyzed transformations, the rhenium catalyst can promote both the insertion of unsaturated molecules and intramolecular nucleophilic cyclization.9 Thus, we have been interested in the reaction mechanism of the rhenium-catalyzed transformations. Here, we focus on the reactions of aromatic compounds having a directing group with acrylates. When aromatic ketimines were used as the aromatic compounds, indene derivatives were synthesized in one operation. Results and DiscussionIn the first half of the paper, we discuss the rhenium-catalyzed insertion of ,-unsaturated carbonyl compounds into an aromatic C-H bond of 2-phenylpyridine. Several mechanistic studies using H/D scrambling between an aromatic compound and an acrylate are also described. In the second half of the paper, we employ aromatic ketimines as the substrate, and the reaction and its mechanistic studies are discussed. We show that when an imine moiety is used as the directing group, intramolecular cyclization occurs after insertion of ,-unsaturated carbonyl compounds. Insertion of Ethyl Acrylate into an Aromatic C-H Bond at the Ortho Position of a Directing Group. Survey of Catalysts:Using the reaction between 2-phenylpyridine (1a) with ethyl acrylate (2a) as a probe, the catalytic activity of several transition-metal complexes was surveyed. Treatment of 1a with acrylate 2a in the p...

Cobalt‐Catalyzed, Imine‐Directed Olefin Hydroarylation under Grignard‐Free Conditions

“…The transition metal‐catalyzed hydroarylation reaction of olefins through chelation‐assisted C−H bond activation offers a step‐ and atom‐economical solution to this task, which complements conventional alkylation reactions such as the Friedel–Crafts reaction and cross‐coupling between aryl (or alkyl) halides and alkyl (or aryl) metal reagents. Since the seminal work of Lewis and Smith on ruthenium‐catalyzed ortho ‐ethylation of phenol with ethylene and the ground breaking discovery of ruthenium‐catalyzed ortho ‐alkylation of aryl ketones with olefins by Murai and co‐workers, various catalytic systems using late transition metals such as ruthenium, rhodium, rhenium, iridium, cobalt, and iron have been developed for the hydroarylation of olefins assisted by oxygen‐ and nitrogen‐based directing groups. In pursuit of C−H activation with earth‐abundant transition metal catalysts, our group has been committed to the development of cobalt‐catalyzed hydroarylation of olefins using sp 2 nitrogen directing groups ,.…”

Transition-Metal-Catalyzed C–H Bond Addition to Carbonyls, Imines, and Related Polarized π Bonds

“…Takai and Kuninobu later found that preformed imines 297 were also effective for the preparation of indenes 298 (Scheme 135). 169 Both enoate and enone coupling partners provided the indene products 298 in good yield. Interestingly, while Nphenyl imines coupled efficiently to give the desired indene products, N-benzyl imines gave high yields of the aminoindane intermediates instead of the expected indenes, highlighting that benzylamine eliminates more slowly than aniline under the reaction conditions.…”

Diastereodivergent [3 + 2] Annulation of Aromatic Aldimines with Alkenes via C–H Activation by Half-Sandwich Rare-Earth Catalysts

“…The catalytic tandem [3 + 2] annulation of aromatic imines with alkenes, which involves the insertion of an alkene unit into an ortho-C−H bond of the aromatic substituent in the imine substrate followed by the intramolecular addition (cyclization) to the imine group, is among the most atom-efficient routes for the synthesis of multisubstituted 1-aminoindanes. 3,4 Obviously, the diastereodivergent catalysis of this transformation would be an ideal route for the efficient synthesis of different diastereomers of the 1-aminoindane products from the same set of imine and alkene starting materials. 5,6 However, such catalyst-controlled diastereodivergent synthesis of 1-aminoindane compounds has not been reported previously despite great interest and extensive studies in this area.…”