Transition-metal-catalyzed C–H bond functionalizations have had an enormous influence on organic synthesis in recent times. However, the use of low-abundance 4d and 5d metals is almost inevitable, and they are in high demand. This will be a cause of concern, and hence, it is important to develop methods based on 3d metals, which are widely present in the Earth’s crust. In this regard, the use of 3d metal catalysts or their precursors for catalysis, in general, and C–H bond functionalizations, in particular, has gained significant momentum in the recent times. The major development in catalytic C–H bond functionalizations with 3d metals has been achieved predominantly with strongly coordinating directing groups such as pyridyl, pyrimidinyl, pyrazolyl, and 8-amino-quinolinyl groups. Thus, prefunctionalization of substrates with these directing groups is necessary, which contradicts the step- and atom-economy of C–H bond activation. However, commonly available functional groups such as aldehyde, ketone, carboxylic acid, amide, hydroxy, and N-oxides loosely bind to metals through weak-coordination. These weakly coordinating directing groups orient the metal to activate C–H bond regioselectively without the need for preinstalled strongly coordinating directing groups. Although it is challenging, this contemporary topic has been actively pursued by many researchers in recent times. Through this article, we provide a comprehensive overview of 3d metal-catalyzed, weakly coordinating, directing-group-enabled C–H bond functionalizations reported until March 2021.

A cross-dehydrogenative coupling of arene carboxylic acids with olefins is reported with ruthenium(II) catalyst employing air and water as green oxidant and solvent, respectively. It offers a robust synthesis of valuable phthalide molecules. A one-pot sequential strategy is also disclosed to access Heck-type products that are apparently difficult to make directly from arene carboxylic acids. With the increasing awareness of green and sustainable chemistry principles, devising straightforward catalytic protocols to access high-value products with enriched molecular complexity is highly significant in the organic chemistry community. [1] In this scenario, the transition-metal-catalyzed CÀ H bond activation concept that accounts direct utilization of otherwise inert CÀ H bond of organic molecules as a synthetic handle turned out very promising. [2] It renovates the synthetic policies as a greener alternative to traditional cross-coupling reactions by avoiding the need of pre-functionalized substrates and thereby improving overall step-and atom-economy. [3] Specifically, the cross-coupling reaction between two different C(sp 2)À H bonds represents a very powerful CÀ C bond-forming technology as it constitutes a twofold CÀ H functionalization manifold. [4] However, such oxidative cross-dehydrogenative couplings very often demand the employment of stoichiometric amounts of metal-based terminal oxidants such as Cu(II) or Ag (I) salts, generating significant amount of metallic wastes in conflict of the green chemistry principles. [5] Employment of abundant molecular oxygen in lieu of these metal-based oxidants would be a green asset, where water is the sole byproduct. [6] Further, a simple and cost-effective setup is expected if air can be directly engaged for the same purpose. Another critical environmental issue arises from the fact that these transformations, in general, consider a huge amount of organic solvent as compared to the other reagents and thus produce a bulk quantity of chemical waste. On the other hand, solvents play crucial roles in most of the organic transformations by controlling the reaction equilibrium and rate of the reaction. [7] Given the environmentally benign portfolio of water, a prompt [a] A

An efficient protocol for the synthesis of biologically essential pyrroloquinolinones has been developed under Cp*CoIII catalysis, which involves a cascade reaction of C(7)–H alkenylation with alkynes followed by nucleophilic addition. A wide variety of internal alkynes including enyne, diyne, and ynamide and more challenging terminal alkynes were successfully employed for the annulation in good to excellent yield with high regioselectivity.

A new protocol is developed for the mono and bis-ortho-C-H alkynylation of easily accessible benzamide derivatives using alkynyl bromides at room temperature merging cobalt and photo redox catalysis. The diverse...

The C(8)-selective nucleophilic cascade cyclization of quinoline N-oxide with easily derived 1,6-enyne from phenol derivatives is demonstrated. A variety of quinoline N-oxide and alkynes are discovered to be suitable for producing a library of quinoline N-oxide tethered cis-hydrobenzofurans with high yields and excellent functional group tolerance. The utility of the protocol has been accomplished by post-synthetic modification of the cyclized product. The mechanistic studies indicate a base-assisted internal electrophilic-type substitution (BIES)-type pathway for C–H bond activation, and electrospray ionization mass spectrometry (ESI–MS) analysis of the stoichiometric reaction confirmed the formation of a key five-membered cobaltacycle.