A new cobalt(II)-catalyzed decarboxylative C-H activation/annulation of benzamides and alkynyl carboxylic acids has been described. Alkynyl carboxylic acids were first employed as the coupling partners using inexpensive Co(OAc)2·4H2O as the catalyst. This method enables a switchable cyclization to isoquinolones and isoindolinones with excellent selectivity. Moreover, a catalytic amount of Ag2O was adopted as co-catalyst and O2 (from air) as a terminal oxidant for the preparation of isoquinolones.

Co(II)-catalyzed C-H C2 selective arylation of indoles with boronic acids through monodentate chelation assistance has been achieved for the first time. The unique features of this methodology include mild reaction conditions, highly C2 regioselectivity, and employment of a Grignard reagent-free catalytic system. A wide range of substrates, including unreactive arenes, are well tolerated, which enables the construction of the coupling products efficiently. This new strategy provides an alternative and versatile approach to construct biaryls using inexpensive cobalt catalyst.

As a powerful synthetic tool for the formation of aromatic C−N bonds, the reported transition-metalcatalyzed direct C−H amination has been ineffective for the synthesis of triarylamines for a long time. Herein, an elegant strategy for the preparation of triarylamines was disclosed using inexpensive Co(OAc) 2 •4H 2 O as the catalyst. It is noteworthy that unactivated arenes and simple anilines were employed as the starting materials with good functional group tolerance. In addition, an organometallic Co(III) species was isolated and identified by X-ray crystallography, thus providing some in-depth insights into the mechanism.

A mixed directing-group strategy for inexpensive [Co(acac) ]-catalyzed oxidative C-H/C-H bond arylation of unactivated arenes has been disclosed. This strategy enables the arylation of a wide range of benzamide and arylpyridines effectively to afford novel bifunctionalized biaryls, which are difficult to achieve by common synthetic routes. Two different pathways, namely, a single-electron-transmetalation process (8-aminoquinoline-directed) and a concerted metalation-deprotonation process (pyridine-directed), were involved to activate two different inert aromatic C-H bonds. Moreover, the aryl radicals have been trapped by 2,6-di-tert-butyl-4-methylphenol to form benzylated products. This unique strategy should be useful in the design of other arene C-H/C-H cross-couplings as well.

A nickel(II)-catalyzed alkynylation/annulation cascade via double C-H cleavage has been successfully achieved. This methodology adopted a removable N,O-bidentate directing group with a broad range of amide substrates and terminal alkynes being well tolerated. The catalytic system allowed for atom-economical and environmentally benign one-pot construction of the corresponding 3-methyleneisoindolin-1-one derivatives using O2 as the external oxidant.

The diversity-oriented synthesis of acyclic nucleosides has been achieved via ring-opening of vinyl cyclopropanes with purines. With Pd2(dba)3·CHCl3 as a catalyst, the 1,5-ring-opening reaction proceeded well and afforded N9 adducts as the major form, in which the C=C bonds in the side chain were exclusively E-form. In the presence of AlCl3, the 1,3-ring-opening reaction occurred smoothly, giving N9 adducts as the dominate products. Meanwhile, when MgI2 was used as the catalyst, the 1,3-ring-opening reaction also worked well to form N7 adducts.

The multiple possible pathways of high‐valent cobalt catalyzed C−H activation/annulation of 2‐benzamidopyridine 1‐oxide with terminal alkyne were investigated in this combined theoretical and experimental study. The calculated results indicated that CoIII catalyzed C−H activation occurs via a concerted‐metalation deprotonation (CMD) process and that the subsequent annulation involves the CoIV intermediate generated through a proton‐coupled electron transfer (PCET) process in DMSO solvent or through an intermolecular single electron transfer (SET) process in trifluoroethanol (TFE) solvent. Moreover, the theoretical calculations also revealed that CoIII is the actual catalyst, and the solvent controls the chemoselectivity in the annulation stage, triggering a switch between five‐ and six‐membered ring products. Kinetic isotope effect (KIE), electron paramagnetic resonance (EPR), and TEMPO inhibition experiments were performed to confirm the computational results. The mechanistic insights should be valuable for understanding the PCET and SET processes involved in transition metal‐catalyzed C−H functionalizations.magnified image

N,O-Bidentate directing-enabled, traceless heterocycle synthesis is described via Cp*-free cobalt-catalyzed C–H activation/annulation. The weakly coordinating nature of the carboxylic acid was employed for the preparation of isoquinolines. Meanwhile, the N–O bond of the α-imino-oxy acid can serve as an internal oxidant. Terminal as well as internal alkynes can be efficiently applied to the catalytic system. This operationally simple approach shows a broad substrate scope with the products obtained in good to excellent yields.