Direct Synthesis of C4-Acyl Indoles via C-H Acylation.

The direct synthesis of C4-acyl indoles deprived of C2 and C3 substituents has proven to be challenging, with scarce efficient synthetic routes being reported. Herein, we disclose a highly site-selective palladium-catalyzed C-H acylation for the construction of C4-acyl indoles via a Catellani-Lautens cyclization strategy. In addition, we systematically studied the ortho C-H acylation mechanism of iodoaniline through density functional theory (DFT) calculations and combined experimental results to elucidate the principle of high chemoselectivity brought by triazine benzoate as an acylation reagent.

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.

Template Synthesis to Solve the Unreachable Ortho C-H Functionalization Reaction of Aryl Iodide.

This report describes the use of a simple Pd/NBE catalytic system to achieve ortho C-H oxylation and phosphonylation and other functionalizations of aryl iodide through templated conversion reactions. Dimethylamine is introduced in the ortho-site of aryl iodide through C-H amination, and aryl dimethylamine is quickly converted to methyl quaternary ammonium salt precipitation. Methyl quaternary ammonium salt avoids Hofmann elimination in subsequent functionalization. This method solves various ortho functionalization reactions of aryl iodide that have not been achieved for a long time in the field of Pd/NBE chemistry indirectly.

Polymer up-cycling by mangana-electrocatalytic C(sp3)-H azidation without directing groups.

The chemical up-cycling of polymers into value-added materials offers a unique opportunity to place plastic waste in a new value chain towards a circular economy. Herein, we report the selective up-cycling of polystyrenes and polyolefins to C(sp3)-H azidated materials under electrocatalytic conditions. The functionalized polymers were obtained with high retention of mass average molecular mass and high functionalization through chemo-selective mangana-electrocatalysis. Our strategy proved to be broadly applicable to a variety of homo- and copolymers. Polyethylene, polypropylene as well as post-consumer polystyrene materials were functionalized by this approach, thereby avoiding the use of hypervalent-iodine reagents in stoichiometric quantities by means of electrocatalysis. This study, hence, represents a chemical oxidant-free polymer functionalization by electro-oxidation. The electrocatalysis proved to be scalable, which highlights its unique feature for a green hydrogen economy by means of the hydrogen evolution reaction (HER).

Reusable Pd@PEG Catalyst for Aerobic Dehydrogenative C-H/C-H Arylations of 1,2,3-Triazoles.

Dehydrogenative C-H arylations of 1,2,3-triazoles were accomplished with the aid of a reusable palladium catalyst in PEG. The widely applicable oxidative palladium catalysis enabled the synthesis of fully decorated 1,2,3-triazoles with a broad functional-group tolerance and ample substrate scope. The sustainability of the aerobic C-H arylation was reflected by the use of PEG as green reaction medium and demonstrated by recycling studies of the catalyst and the reaction medium.

Micellar Catalysis for Ruthenium(II)-Catalyzed C-H Arylation: Weak-Coordination-Enabled C-H Activation in H2 O.

Chemoselective C-H arylations were accomplished through micellar catalysis by a versatile single-component ruthenium catalyst. The strategy provided expedient access to C-H-arylated ferrocenes with wide functional-group tolerance and ample scope through weak chelation assistance. The sustainability of the C-H arylation was demonstrated by outstanding atom-economy and recycling studies. Detailed computational studies provided support for a facile C-H activation through thioketone assistance.

MnCl2-Catalyzed C-H Alkylation on Azine Heterocycles.

Low-valent manganese-catalyzed C-H alkylation of pyridine derivatives with both primary and challenging secondary alkyl halides was established by amide assistance. The strategy provided expedient access to alkylated pyridines with wide functional group tolerance and ample scope through weak chelation. Mechanistic studies provided strong support for a rate-determining C-H activation and a SET-type C-X scission.

3d Transition Metals for C-H Activation.

C-H activation has surfaced as an increasingly powerful tool for molecular sciences, with notable applications to material sciences, crop protection, drug discovery, and pharmaceutical industries, among others. Despite major advances, the vast majority of these C-H functionalizations required precious 4d or 5d transition metal catalysts. Given the cost-effective and sustainable nature of earth-abundant first row transition metals, the development of less toxic, inexpensive 3d metal catalysts for C-H activation has gained considerable recent momentum as a significantly more environmentally-benign and economically-attractive alternative. Herein, we provide a comprehensive overview on first row transition metal catalysts for C-H activation until summer 2018.

Arene-Ligand-Free Ruthenium(II/III) Manifold for meta-C-H Alkylation: Remote Purine Diversification.

meta-Selective C-H alkylations of bioactive purine derivatives were accomplished by versatile ruthenium catalysis. Thus, the arene-ligand-free complex [Ru(OAc)2 (PPh3 )2 ] enabled remote C-H functionalizations with ample scope and excellent levels of chemo- and positional selectivities. Detailed experimental and computational mechanistic studies provided strong support for a facile C-H activation within a ruthenium(II/III) manifold.

Synergistic Manganese(I) C-H Activation Catalysis in Continuous Flow: Chemoselective Hydroarylation.

Chemoselective hydroarylations were accomplished by a novel synergistic Brønsted acid/manganese(I)-catalyzed C-H activation manifold. Thus, alkynes bearing O-leaving groups could, for the first time, be employed for C-H alkenylations without concurrent ß-O elimination, thereby setting the stage for versatile late-stage diversifications. Also described is the first manganese-catalyzed C-H activation in continuous flow, thus enabling efficient hydroarylations within only 20 minutes.

Ruthenium(II)-catalysed remote C-H alkylations as a versatile platform to meta-decorated arenes.

The full control of positional selectivity is of prime importance in C-H activation technology. Chelation assistance served as the stimulus for the development of a plethora of ortho-selective arene functionalizations. In sharp contrast, meta-selective C-H functionalizations continue to be scarce, with all ruthenium-catalysed transformations currently requiring difficult to remove or modify nitrogen-containing heterocycles. Herein, we describe a unifying concept to access a wealth of meta-decorated arenes by a unique arene ligand effect in proximity-induced ruthenium(II) C-H activation catalysis. The transformative nature of our strategy is mirrored by providing a step-economical entry to a range of meta-substituted arenes, including ketones, acids, amines and phenols-key structural motifs in crop protection, material sciences, medicinal chemistry and pharmaceutical industries.

meta-C-H Bromination on Purine Bases by Heterogeneous Ruthenium Catalysis.

Methods for positionally selective remote C-H functionalizations are in high demand. Herein, we disclose the first heterogeneous ruthenium catalyst for meta-selective C-H functionalizations, which enabled remote halogenations with excellent site selectivity and ample scope. The versatile heterogeneous Ru@SiO2 catalyst was broadly applicable and could be easily recovered and reused, which set the stage for the direct fluorescent labeling of purines. In contrast to palladium, rhodium, iridium, or cobalt complexes, solely the ruthenium catalysis manifold provided access to meta-halogenated purine derivatives, illustrating the unique power of ruthenium C-H activation catalysis.

C-H arylations of 1,2,3-triazoles by reusable heterogeneous palladium catalysts in biomass-derived γ-valerolactone.

C-H arylations were accomplished with a user-friendly heterogeneous palladium catalyst in the biomass-derived γ-valerolactone (GVL) as an environmentally-benign reaction medium. The user-friendly protocol was characterized by ample substrate scope and high functional group tolerance in the C-H arylation of 1,2,3-triazoles, and the palladium catalyst could be recycled and reused in the C-H activation process.

Cobalt-Catalyzed Oxidase C-H/N-H Alkyne Annulation: Mechanistic Insights and Access to Anticancer Agents.

Cp*-free cobalt-catalyzed alkyne annulations by C-H/N-H functionalizations were accomplished with molecular O2 as the sole oxidant. The user-friendly oxidase strategy proved viable with various internal and terminal alkynes through kinetically relevant C-H cobaltation, providing among others step-economical access to the anticancer topoisomerase-I inhibitor 21,22-dimethoxyrosettacin. DFT calculations suggest that electronic effects control the regioselectivity of the alkyne insertion step.

Ruthenium Oxidase Catalysis for Site-Selective C-H Alkenylations with Ambient O2 as the Sole Oxidant.

Ruthenium(II) oxidase catalysis by direct dioxygen-coupled turnover enabled step-economical oxidative C-H alkenylation reactions at ambient pressure. Versatile ruthenium(II) biscarboxylate catalysts displayed ample substrate scope and proved applicable to weakly coordinating and removable directing groups. The twofold C-H functionalization strategy was characterized by exceedingly mild reaction conditions as well as excellent positional selectivity.

Single-Component Phosphinous Acid Ruthenium(II) Catalysts for Versatile C-H Activation by Metal-Ligand Cooperation.

Well-defined ruthenium(II) phosphinous acid (PA) complexes enabled chemo-, site-, and diastereoselective C-H functionalization of arenes and alkenes with ample scope. The outstanding catalytic activity was reflected by catalyst loadings as low as 0.75 mol %, and the most step-economical access reported to date to angiotensin II receptor antagonist blockbuster drugs. Mechanistic studies indicated a kinetically relevant C-X cleavage by a single-electron transfer (SET)-type elementary process, and provided evidence for a PA-assisted C-H ruthenation step.

N-Acyl Amino Acid Ligands for Ruthenium(II)-Catalyzed meta-C-H tert-Alkylation with Removable Auxiliaries.

Acylated amino acid ligands enabled ruthenium(II)-catalyzed C-H functionalizations with excellent levels of meta-selectivity. The outstanding catalytic activity of the ruthenium(II) complexes derived from monoprotected amino acids (MPAA) set the stage for the first ruthenium-catalyzed meta-functionalizations with removable directing groups. Thereby, meta-alkylated anilines could be accessed, which are difficult to prepare by other means of direct aniline functionalizations. The robust nature of the versatile ruthenium(II)-MPAA was reflected by challenging remote C-H transformations with tertiary alkyl halides on aniline derivatives as well as on pyridyl-, pyrimidyl-, and pyrazolyl-substituted arenes. Detailed mechanistic studies provided strong support for an initial reversible C-H ruthenation, followed by a SET-type C-Hal activation through homolytic bond cleavage. Kinetic analyses confirmed this hypothesis through an unusual second-order dependence of the reaction rate on the ruthenium catalyst concentration. Overall, this report highlights the exceptional catalytic activity of ruthenium complexes derived from acylated amino acids, which should prove instrumental for C-H activation chemistry beyond remote functionalization.

Ruthenium(II)-catalyzed C-H activation/alkyne annulation by weak coordination with O2 as the sole oxidant.

Aerobic oxidative CH functionalizations of weakly coordinating benzoic acids have been accomplished with versatile ruthenium(II) biscarboxylates under ambient oxygen or air. Mechanistic studies identified the key factors controlling the elementary step of the oxidation of the ruthenium(0) complex.