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 CH/NO 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 CH activation, alkyne insertion, reductive elimination, and NO 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.
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.
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
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