Xinju Zhu

High-Valent-Cobalt-Catalyzed C−H Functionalization Based on Concerted Metalation–Deprotonation and Single-Electron-Transfer Mechanisms

C−H functionalization has been established as a powerful strategy for the commercial construction of organic molecules. This review encompasses the most recent advances in C−H activation catalyzed by earth‐abundant cobalt complexes, involving two types of fundamental reaction paths, that is, the concerted metalation–deprotonation (CMD) path associated with non‐oxidative C−H activation and the intermolecular single‐electron‐transfer (SET) path. Transformations catalyzed by high‐valent cobalt based on the two mechanisms and used in organic syntheses, including those involving C−C and C−X (X=O and N) bond formation, are herein presented.

Cobalt(II)-Catalyzed C-H Amination of Arenes with Simple Alkylamines.

A new method of cobalt-catalyzed amination of arylamides with simple alkylamines is reported through C(sp(2))-H bond functionalization. For the first time, inexpensive cobalt is exploited as the catalyst in the amination of C(sp(2))-H bond using simple alkylamines.

Cobalt(II)-Catalyzed Decarboxylative C–H Activation/Annulation Cascades: Regioselective Access to Isoquinolones and Isoindolinones

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.

Mixed Directing-Group Strategy: Oxidative C-H/C-H Bond Arylation of Unactivated Arenes by Cobalt Catalysis

A mixed directing-group strategy for inexpensive [Co(acac)3 ]-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.

Cobalt(II)-Catalyzed Oxidative C–H Arylation of Indoles and Boronic Acids

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.

Ni(II)-Catalyzed C(sp2)–H Alkynylation/Annulation with Terminal Alkynes under an Oxygen Atmosphere: A One-Pot Approach to 3-Methyleneisoindolin-1-one

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.

Rhodium-Catalyzed Direct Bis-cyanation of Arylimidazo[1,2-α]pyridine via Double C–H Activation

An efficient Cp*Rh(III)-catalyzed selective bis-cyanation of arylimidazo[1,2-α]pyridines with N-cyano-N-phenyl-p-methylbenzenesulfonamide via N-directed ortho double C-H activation has been developed. The reaction proceeds with broad functional group tolerance to furnish various cyanated imidazopyridines in high yields. The current methodology exhibits unique characteristics, including high bis-cyanation selectivity, operational convenience, and gram-scale production.

Reactivity of p-Toluenesulfonylmethyl Isocyanide: Iron-Involved C-H Tosylmethylation of Imidazopyridines in Nontoxic Media.

A novel iron-involved tosylmethylation of imidazo[1,2-α]pyridines with p-toluenesulfonylmethyl isocyanide in a solvent mixture of H2O and PEG400 under an Ar atmosphere has been developed. This protocol provides a facile synthetic route for the functionalization of the imidazo[1,2-α]pyridine scaffold with broad substrate compatibility, which is less expensive and environmentally friendly. The current methodology could further enable regioselective C-H tosylmethylation of indole at the C3 position. Also, p-toluenesulfonylmethyl isocyanide was utilized as the tosylmethylating reagent for the first time.

Highly efficient synthesis of primary amides via aldoximes rearrangement in water under air atmosphere catalyzed by an ionic ruthenium pincer complex

The transformation of aldoximes to primary amides has been evaluated using pincer ruthenium complexes a–c, among which the ionic Ru catalyst a proved to be the most efficient in water under air atmosphere. A variety of (hetero)arene aldoximes proceeded smoothly to afford amides in high yields with good functional group compatibilities. Furthermore, a direct synthetic route of amides from aldehydes, hydroxylamine hydrochloride and sodium carbonate was also described with broad substrates including conjugated and aliphatic aldehydes. This protocol is operationally simple and proceeds with a low catalyst loading (0.5 mol%).

Iodine-Mediated Difunctionalization of Imidazopyridines with Sodium Sulfinates: Synthesis of Sulfones and Sulfides

Novel iodine-induced sulfonylation and sulfenylation of imidazopyridines have been described using sodium sulfinates as the sulfur source. This strategy enables highly selective difunctionalization of imidazo[1,2-a]pyridine to access sulfones and sulfides in good yields. A wide range of substrates and functional groups were well-tolerated under optimized conditions. Moreover, control experiments have been conducted, indicating a radical pathway involved in the reaction mechanisms.