N–O Bond as External Oxidant in Group 9 Cp*M(III)-Catalyzed Oxidative C–H Coupling Reactions

Group 9 Cp*M­(III) (M = Co, Rh, Ir) complexes have been extensively investigated as catalysts in a variety of C–H activation reactions. Typically, late metal-based silver or copper salt was used (while needed) as oxidant in these catalytic systems. Herein, we report our discovery of a potentially general type of N–O bond-containing oxidants, which allowed the mild and efficient syntheses of isocoumarins, isoquinolines, isoquinolinone, and styrenes via C–H activation catalyzed by group 9 Cp*M­(III) complexes. By using Cp*Rh­(III)-catalyzed isocoumarin synthesis as a model reaction, experimental and theoretical mechanistic studies were conducted. The results concluded that the Rh­(III)–Rh­(I)–Rh­(III) rather than the Rh­(III)–Rh­(V)–Rh­(III) pathway is more likely involved in the mechanism, and both the C–H activation and oxidation of the Cp*Rh­(I) species were involved in the turnover-limiting step

Mild Mn(OAc)₃‑Mediated Aerobic Oxidative Decarboxylative Coupling of Arylboronic Acids and Arylpropiolic Acids: Direct Access to Diaryl 1,2-Diketones

A simple and efficient method for the synthesis of diaryl 1,2-diketones has been developed. The reaction represents the first example of diaryl 1,2-diketones that are synthesized directly from arylboronic acids and arylpropiolic acids by a radical pathway in moderate to good yields. This reaction proceeds under mild reaction conditions and with good tolerance of a variety of functional groups. Preliminary mechanistic studies were conducted

Cp*Co(III)-Catalyzed Annulations of 2‑Alkenylphenols with CO: Mild Access to Coumarin Derivatives

Cp*Co­(III)-catalyzed annulations of 2-alkenylphenols with CO for the synthesis of coumarin derivatives have been developed. The reaction features mild reaction conditions, broad substrate scope, and good functional group tolerance. Preliminary mechanistic studies were conducted, suggesting that C–H activation is the turnover limiting step. Furthermore, the efficiency of this reaction was demonstrated by the rapid total synthesis of three natural products herniarin, xanthyletin, and seselin

Regioselective Synthesis of 5‑Aminooxazoles via Cp*Co(III)-Catalyzed Formal [3 + 2] Cycloaddition of <i>N</i>‑(Pivaloyloxy)amides with Ynamides

A simple and efficient protocol for the regioselective synthesis of 5-aminooxazoles is disclosed. The reaction, catalyzed by a cheap Cp*Co­(III) catalyst, starts from easily accessible <i>N</i>-(pivaloyloxy)­amides and ynamides. Mild reaction conditions, a broad substrate scope, good functional group tolerance, and good to excellent yields were observed

From Indoles to Carbazoles: Tandem Cp*Rh(III)-Catalyzed C–H Activation/Brønsted Acid-Catalyzed Cyclization Reactions

A tandem Cp*Rh­(III)-catalyzed C–H activation/Brønsted acid-catalyzed intramolecular cyclization allows a facile synthesis of carbazoles from readily available indoles. The reaction proceeds under rather mild reaction conditions with the generation of water and N₂ as the only byproducts. Broad substrate scope, excellent functional group tolerance, and high yields were observed. The benzannulation of pyroles for the synthesis of indoles is also feasible using the same protocol

Three-Component Catalytic Carboxygenation of Activated Alkenes Enabled by Bimetallic Rh(III)/Cu(II) Catalysis

A novel cascade Cp*Rh­(III)-catalyzed C–H alkylation/Cu­(II)-promoted α-oxygenation which enabled a three-component carboxygenation of activated alkene is reported. Mild reaction conditions, broad substrate scope, and good functional group tolerance were observed. The synthetic utility of the protocol was showcased by the facile transformations of the product to a variety of structurally diverse molecules. Preliminary mechanistic studies were conducted

Three-Component Catalytic Carboxygenation of Activated Alkenes Enabled by Bimetallic Rh(III)/Cu(II) Catalysis

A novel cascade Cp*Rh­(III)-catalyzed C–H alkylation/Cu­(II)-promoted α-oxygenation which enabled a three-component carboxygenation of activated alkene is reported. Mild reaction conditions, broad substrate scope, and good functional group tolerance were observed. The synthetic utility of the protocol was showcased by the facile transformations of the product to a variety of structurally diverse molecules. Preliminary mechanistic studies were conducted

Experimental and Theoretical Studies on Rhodium-Catalyzed Coupling of Benzamides with 2,2-Difluorovinyl Tosylate: Diverse Synthesis of Fluorinated Heterocycles

Fluorinated heterocycles play an important role in pharmaceutical and agrochemical industries. Herein, we report on the synthesis of four types of fluorinated heterocycles via rhodium­(III)-catalyzed CH activation of arenes/alkenes and versatile coupling with 2,2-difluorovinyl tosylate. With <i>N</i>-OMe benzamide being a directing group (DG), the reaction delivered a monofluorinated alkene with the retention of the tosylate functionality. Subsequent one-pot acid treatment allowed the efficient synthesis of 4-fluoroisoquinolin-1­(2<i>H</i>)-ones and 5-fluoropyridin-2­(1<i>H</i>)-ones. When <i>N</i>OPiv benzamides were used, however, [4 + 2] cyclization occurred to provide <i>gem</i>-difluorinated dihydroisoquinolin-1­(2<i>H</i>)-ones. Synthetic applications have been demonstrated and the ready availability of both the arene and the coupling partner highlighted the synthetic potentials of these protocols. Mechanistically, these two processes share a common process involving NH deprotonation, CH activation, and olefin insertion to form a 7-membered rhodacycle. Thereafter, different reaction pathways featuring β-F elimination and CN bond formation are followed on the basis of density functional theory (DFT) studies. These two pathways are DG-dependent and led to the open chain and cyclization products, respectively. The mechanistic rationale was supported by detailed DFT studies. In particular, the origins of the intriguing selectivity in the competing β-F elimination versus CN bond formation were elucidated. It was found that β-F elimination is a facile event and proceeds via a <i>syn</i>-coplanar transition state with a low energy barrier. The CN bond formation proceeds via a facile migratory insertion of the RhC­(alkyl) into the Rh­(V) amido species. In both reactions, the migratory insertion of the alkene is turnover-limiting, which stays in good agreement with the experimental studies