Chelation-Assisted Iron-Catalyzed C-H Activations: Scope and Mechanism

ConspectusTo improve the resource economy of molecular syntheses, researchers have developed strategies to directly activate otherwise inert C-H bonds, thus avoiding cumbersome and costly substrate prefunctionalizations. During the past two decades, remarkable progress in coordination chemistry has set the stage for developing increasingly viable metal catalysts for C-H activations. Despite remarkable advances, C-H activations are largely dominated by precious 4d and 5d transition metal catalysts based primarily on palladium, ruthenium, iridium, and rhodium, thus decreasing the inherent sustainable nature of the C-H activation approach. Therefore, advancing catalytic reactions based on Earth-abundant and less toxic 3d transition metals, especially nontoxic and inexpensive iron, represents a desirable and attractive alternative. While research had previously focused on 8-aminoquinoline directing groups in C-H activations, we have devised easily accessible, tunable, and clickable triazoles, which feature widespread applications in bioactive compounds and drugs, among others, as peptide isosteres. Thus, in contrast to other directing groups, the triazole group is a highly desirable structural motif and functions as a bioisostere in medicine and biology, where it is exploited to mimic amide bonds.This Account summarizes the evolution of chelation-assisted iron-catalyzed C-H activations via C-H, C-H/N-H, and C-H/N-H/C-C bond cleavages, with a topical focus on the most recent contributions of our team. Thus, the triazole-enabled iron catalysis has surfaced as a transformative platform for a large variety of C-H transformations, including arylations, alkylations, alkenylations, allylations, annulations, and alkynylations, achieved through C-H activations with organometallic reagents, organohalides, alkynes, alkenes, allenes, and bicyclopropylidenes among others. Consequently, we developed widely applicable methods for the versatile preparation of decorated arenes and heteroarenes, providing access to benzamides, isoquinolones, pyrrolones, pyridones, phenones, indoles, and isoindolinones, among others. Most of these reactions employed 1,2-dichloroisobutane (DCIB) as an oxidant. Notably, chemical-oxidant-free strategies were also developed, with the major breakthroughs being the use of internal oxidants in oxidative annulations, the use of resource-economic electrocatalysis, and the development of well-defined iron(0)-mediated catalysis. In addition, a highly enantioselective inner-sphere C-H alkylation of (aza)indoles was developed by designing novel remotely decorated N-heterocyclic carbene ligands with dispersion energy donors. In addition, detailed mechanistic experiments including kinetic analyses, intermediate isolation, Mößbauer spectroscopy, and computation provided strong support for the mode of catalysis operation, enabling unprecedented C-H activations. Thereby, low-valent iron catalysts paved the way toward weakly coordinating ketones and enantioselective iron-catalyzed C-H activations through organometallic intermediates.