A green atom‐economical method for the synthesis of highly functionalized 1‐amino and 1‐carbon substituted isoquinolines from the reaction of N′‐hydroxybenzimidamides and aryl ketoximes, respectively, with alkynes via pentamethylcyclopentadienylcobalt(III)‐catalyzed CH/NO bond activation is described. The external oxidant‐free annulation reaction uses the =NOH moiety in N′‐hydroxybenzimidamides or N‐aromatic ketone oximes as the directing group and internal oxidant. This first row transition metal‐catalyzed annulation serves as an efficient alternative for the synthesis of isoquinolines, as water is the only by‐product and expensive noble metals such as rhodium(III), iridium(III), palladium(II), and ruthenium(II) are not required. The reaction proceeds via CH activation, alkyne insertion, reductive elimination, and NO activation.magnified image

Four cobalt-catalyzed oxidative annulation reactions of nitrogen-containing arenes with alkynes proceeds by C-H activation, thus leading to biologically useful quaternary ammonium salts, including pyridoisoquinolinium, cinnolinium, isoquinolinium, and quinolizinium salts, in high yields. The results are comparable to those reactions catalyzed by rhodium and ruthenium complexes. The transformation of the salts into various N-heterocycles has also been demonstrated.

The combination of nucleophilic nitrenoids and π-acid catalysis has emerged as a powerful tool in heterocycle synthesis. Accessing more varied heterocycle-substitution patterns by maintaining the same reaction pathways across different alkynes remains a challenge. Here we show that Au(I) catalysis of isoxazole-based nitrenoids with alkynyl thioethers provides controlled access to (3 + 2) annulation by a regioselective addition β to the sulfenyl group. The reaction with isoxazole-containing nitrenoids delivers sulfenylated pyrroles and indoles as single regioisomers bearing useful functional groups and structural variety.

A mild approach for a Cp*Co(III)‐catalyzed C−H naphthylation of arenes by 7‐oxabicyclic alkenes has been developed. In some cases, intermediate products with a 1,2‐dihydronaphthalen‐1‐ol group have been isolated at room temperature in good yield. The catalytic reaction proceeds via C−H activation, insertion, β‐oxygen elimination, protonation, and dehydration, respectively. These simple protocols, featuring mild conditions and tolerance of functional groups, exhibit great potential for a variety of synthetic applications.magnified image

Considerable research attention has been directed towards the use of first‐row transition metals in organic synthesis. The more abundant, and less expensive cobalt is considered to be an effective alternative for expensive second and third‐row noble metals, especially in C−H bond functionalization reactions. In this Minireview, we will summarize the features, and recent achievements of the Co‐catalyzed directing group assisted C−H activation/cyclization reactions and their mechanistic insights.

We report a one-step procedure to directly reduce unactivated aryl esters into their corresponding tolyl-derivatives. This is achieved by the action of a Ni/NHC catalyst and an organosilane reducing agent that is activated in situ by stoichiometric KOtBu. The resulting conditions provide a direct and efficient alternative to multi-step procedures for this transformation that often require use of hazardous metal hydrides. Applications in the synthesis of-CD3 containing products, derivatization of bioactive molecules, and chemoselective reduction in the presence of other CO bonds is demonstrated. File list (2) download file view on ChemRxiv Manuscript.pdf (1.92 MiB) download file view on ChemRxiv Supporting information.pdf (4.97 MiB)

Four cobalt-catalyzed oxidative annulation reactions of nitrogen-containing arenes with alkynes proceeds by CÀH activation, thus leading to biologically useful quaternary ammonium salts,i ncluding pyridoisoquinolinium, cinnolinium, isoquinolinium, and quinolizinium salts,i nh igh yields. The results are comparable to those reactions catalyzedb y rhodium and ruthenium complexes.The transformation of the salts into various N-heterocycles has also been demonstrated.Transition metal catalyzed oxidative annulations of arenes through CÀHbond activation has drawn substantial attention because of their broad synthetic utility in the synthesis of natural products,d rugs,a nd materials. [1] Recently,m any nitrogen-containing,g roup-directed oxidative annulation reactions catalyzed by noble metals such as rhodium, ruthenium, palladium, and iridium, through CÀHa ctivation have been developed. [2] However,s imilar reactions using more abundant, less expensive,a nd less toxic first-row transition metals as the catalysts are still rare.Recently,f irst-row transition metal catalyzed C À Hb ond activation of arenes has become very attractive because these metals are less expensive,m ore abundant, and less toxic. [3] Thus,c obalt catalysts have been employed as alternative active C À Ha ctivation catalysts for rhodium and ruthenium complexes.I nt his context, the groups of Kanai, [4a-c] Ackermann, [4d,e] Glorius, [4f,g] Ellman, [4h-i] and Chang [4j,k] reported using high-valent cobalt catalysts for CÀHf unctionalization of arenes by various electrophiles.R ecently,M atsunaga, Kanai, and co-workers demonstrated the catalytic activity of [Co III Cp*] and [Rh III Cp*] in the synthesis of pyrroloindolones from N-carbamoyl indoles and alkynes. [4l] Very recently,t he groups of Kanai and Matsunaga, [4m] Ackermann, [4n] and Sundararaju [4o] individually reported isoquinoline synthesis by aredox-neutral strategy in the presence of [CoCp*(CO)I 2 ] as the catalyst. However,there is still no report on the cobaltcatalyzed CÀHo xidative annulation of arene with alkyne in the presence of an external oxidant.N-heterocyclic quaternary ammonium salts such as pyridoisoquinolinium, cinnolinium, pyridinium, and quinolizinium salts are found in the core structures of many bioactive compounds,p harmaceuticals,a nd organic materials. [5] How-

Hydroarylation of bicyclic alkenes has been developed using a low-valent Re -catalyzed, directing group-assisted C-H bond activation strategy. The addition of sodium acetate significantly improves the reaction efficiency; moreover, bicyclic alkenes such as 7-oxa and aza benzonorbornadienes worked efficiently under this reaction condition. Preliminary mechanistic studies suggest that, after the alkene insertion, the rhenacycle preferentially undergoes protonolysis rather than reductive elimination.