Evolution of Earth‐Abundant 3 d‐Metallaelectro‐Catalyzed C−H Activation: From Chelation‐Assistance to C−H Functionalization without Directing Groups

Abstract

Catalyzed C−H functionalizations have emerged as a transformative platform for molecular syntheses. Despite of indisputable advances, oxidative C−H activations have been largely restricted to precious transition metals and stoichiometric amounts of chemical oxidants. In contrast, we herein discuss the potential of earth‐abundant, environmentally‐benign 3d transition metals for C−H activation, which has recently gained major momentum. Thus, a strategy for full resource economy has been established in our group, with green electricity as a renewable redox agent, giving valuable hydrogen as the sole byproduct under redox mediator‐free conditions. In this account, we detail our accomplishments in 3d metallaelectrocatalysis towards green syntheses until March 2021.

Heterocyclic Electrochemistry: Renewable Electricity in the Construction of Heterocycles

“…The transformation was detectable at room temperature under incredibly moderate reaction conditions and high yields in a H 2 O-containing solvent mixture. An undivided cell with a platinum (Pt) cathode and reticulated vitreous carbon (RVC) anode is used to drive the operation at 23 °C for 16 h. As a result, a method for a full resource economy has been created, using renewable green power as a redox agent that produces only valuable hydrogen under redox mediator-free conditions. − The identical reaction conditions (Scheme -Path a) were used, but with a GF anode in place of an RVC anode, and yields of 70–76% of the product were achieved …”

“…It is possible to move on with additional developments for anodic oxidation-based transition metal recycling . Regio- and stereoselective bond functionalizations will gain a new dimension with the development of suitable catalyst designs in the future …”

Nickel‐Catalyzed Thermal Redox Functionalization of C(sp3)−H Bonds with Carbon Electrophiles**

“…While the former often involves homolysis of C(sp 3 )−H bonds, [15–17] the latter largely relies on the 2 e − coupling process wherein a directing group is generally required to trigger the C(sp 3 )−H bond activation (Figure 1a) [12–14] . Although earth‐abundant 3d transition‐metals have been exploited for CH activation, the use of noble metals (Pd, Rh, and Ir, etc) dominates the field [18–21] . Alternatively, the recent photo‐induced hydrogen‐atom transfer (HAT) from C(sp 3 )−H mediated by a catalytic excited oxidant has permitted a wide spectrum of C(sp 3 )−H bonds to couple with carbon electrophiles by merging with reductive Ni‐catalysis (Figure 1b) [22–27] .…”

“…[12][13][14] Although earth-abundant 3d transition-metals have been exploited for CH activation, the use of noble metals (Pd, Rh, and Ir, etc) dominates the field. [18][19][20][21] Alternatively, the recent photo-induced hydrogenatom transfer (HAT) from C(sp 3 )À H mediated by a catalytic excited oxidant has permitted a wide spectrum of C(sp 3 )À H bonds to couple with carbon electrophiles by merging with reductive Ni-catalysis (Figure 1b). [22][23][24][25][26][27] To our knowledge, a HAT-provoked transition-metal-catalyzed C(sp 3 )À H cou-pling with organohalides remains essentially unexplored under thermal conditions, [28] likely due to the lack of a ground-state oxidant that is compatible with catalytic reductive coupling process.…”

Hydrogen evolution-enabled rhodaelectro-catalyzed [4+2] annulations of purines and 7-deazapurines with alkynes

“…16 The concept of electrochemically enabled direct C–H activation has received significant attention as a promising strategy for precise molecular synthesis by not only avoiding the use of stoichiometric amounts of toxic and expensive oxidants, but also reducing the reaction temperature. 17 On the other hand, cationic polycyclic heteroaromatic compounds (cPHACs) are enormously valuable, largely present in natural products, and used in the chemical industry, pharmaceuticals and the medicinal industry. 18 Thus, the development of new synthetic methodologies to assemble novel cPHACs is of great importance.…”