Weak-Coordination in C–H Bond Functionalizations Catalyzed by 3d Metals

Abstract

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.

C–H Activation by Isolable Cationic Bis(phosphine) Cobalt(III) Metallacycles

“…Developments in transition metal-catalyzed C–H activation have led to important advances in synthetic disconnection strategies by enabling the direct transformation of ubiquitous C–H bonds into C–C or C–heteroatom bonds. Directed arene C­(sp 2 )–H activation has been widely exploited in catalytic reactions and a diverse array of mechanisms have been proposed. , Among earth-abundant metals that promote directed C­(sp 2 )–H functionalization, cobalt has proven to be particularly versatile due to its ability to promote C–H activation through a number of distinct pathways depending on the oxidation state, overall charge, and ligand environment …”

Highly Chemoselective Synthesis of Azaarene-Equipped CF3-Tertiary Alcohols under Metal-Free Conditions and Their Fungicidal Activities

“…On the other hand, the direct functionalization of ubiquitous inert C–H bonds has been rapidly developed in past decades, owing to the advantages in terms of atom- and step-economy . In this regard, C­(sp 3 )–H functionalization of azaarenes has emerged as a powerful method for the rapid assembly of functional N -heterocycle molecules, which are pervasive motifs found in a series of pharmaceuticals, natural products, and materials .…”

“…On the other hand, the direct functionalization of ubiquitous inert C–H bonds has been rapidly developed in past decades, owing to the advantages in terms of atom- and step-economy. 9 In this regard, C(sp 3 )–H functionalization of azaarenes has emerged as a powerful method for the rapid assembly of functional N -heterocycle molecules, which are pervasive motifs found in a series of pharmaceuticals, natural products, and materials. 10 However, C(sp 3 )–H functionalization of methylquinoline and its derivatives coupled with trifluoromethyl ketones to construct CF 3 -tertiary alcohols have been less explored, and Lewis acid is often required ( Scheme 2 a).…”

Meta-Selective C–H Functionalization of Arylsilanes Using a Silicon Tethered Directing Group

“…The functionalization of C–H bond in aromatic compounds has been extensively studied over the past two decades . Different types of directing groups have played a significant role in delivering reactive transition metals to ortho -, meta -, or para -positions depending on their respective structure.…”