Late-Stage Peptide Diversification through Cobalt-Catalyzed C-H Activation: Sequential Multicatalysis for Stapled Peptides

Bioorthogonal late-stage diversification of structurally complex peptides has enormous potential for drug discovery and molecular imaging. In recent years, transition-metal-catalyzed C-H activation has emerged as an increasingly viable tool for peptide modification. Despite major accomplishments, these strategies largely rely on expensive palladium catalysts. We herein report an unprecedented cobalt(III)-catalyzed peptide C-H activation, which enables the direct C-H functionalization of structurally complex peptides, and sets the stage for a multicatalytic C-H activation/alkene metathesis/hydrogenation strategy for the assembly of novel cyclic peptides.

A noncovalent interaction insight onto the concerted metallation deprotonation mechanism

The CMD/AMLA mechanisms of cyclopalladation and the parent fictitious cyclonickelation of N,N-dimethylbenzylamine have been investigated by joint DFT-D and DLPNO-CCSD(T) methods assisted by QTAIM.

Mössbauer and mass spectrometry support for iron(ii) catalysts in enantioselective C–H activation

A combination of electrospray-ionization mass spectrometry and Mössbauer spectroscopy was used to investigate the species generated in situ in highly enantioselective Fe/NHC-catalyzed C–H alkylations.

A computational study of cobalt-catalyzed C–H iodination reactions using a bidentate directing group with molecular iodine

A computational methodology was used to collect detailed mechanistic information on the cobalt-catalyzed C–H iodination of aromatic amides with molecular iodine using an N,N′-bidentate directing group.

Metal-catalyzed regiodivergent organic reactions

This review discusses metal-catalysed regiodivergent additions, allylic substitutions, CH-activation, cross-couplings and intra- or intermolecular cyclisations.

Controllable chemoselectivity in the coupling of bromoalkynes with alcohols under visible-light irradiation without additives: synthesis of propargyl alcohols and α-ketoesters

The chemoselectivity of visible-light-induced coupling reactions of bromoalkynes with alcohols can be controlled by simple changes to the reaction atmosphere.

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.

Cobalt-Catalyzed Secondary Alkylation of Arenes and Olefins with Alkyl Ethers through the Cleavage of C(sp2)-H and C(sp3)-O Bonds

A novel cobalt-catalyzed C-H alkylation of arenes and olefins is achieved with (pyridin-2-yl)isopropyl amine as an N, N-bidentate directing group. Different linear, branched, and cyclic alkyl ethers were used as practical secondary alkylating reagents through cleavage of C(sp³)-O bond, providing an efficient approach to the synthesis of verstile o-alkylated arylamides and tetrasubstituted acrylamides. Mechanistic studies indicate that cleavage of the inert C(sp³)-O bond involves a cobalt-promoted radical process and that cleavage of the inert C(sp²)-H bond by a cobalt catalyst is a rate-limiting step.

Recent Advances on Mechanistic Studies on C–H Activation Catalyzed by Base Metals

During the last ten years, base metals have become very attractive to the organometallic and catalytic community on activation of C-H bonds for their catalytic functionalization. In contrast to the statement that base metals differ on their mode of action most of the manuscripts mistakenly rely on well-studied mechanisms for precious metals while proposing plausible mechanisms. Consequently, few literature examples are found where a thorough mechanistic investigation have been conducted with strong support either by theoretical calculations or experimentation. Therefore, we consider of highly scientific interest reviewing the last advances on mechanistic studies on Fe, Co and Mn on C-H functionalization in order to get a deep insight on how these systems could be handle to either enhance their catalytic activity or to study their own systems in a similar systematic fashion. Thus, in this review we try to cover the most insightful articles for mechanistic studies on C-H activation catalyzed by Fe, Co and Mn based on kinetic and competition experiments, stoichiometric reactions, isolation of intermediates and theoretical calculations.

Iridium-Catalyzed Electrooxidative C-H Activation by Chemoselective Redox-Catalyst Cooperation

Iridium-catalyzed electrochemical C-H activation was accomplished within a cooperative catalysis manifold, setting the stage for electrooxidative C-H alkenylations through weak O-coordination. The iridium-electrocatalyzed C-H activation featured high functional-group tolerance through assistance of a metal-free redox mediator through indirect electrolysis. Detailed mechanistic insights provided strong support for an organometallic C-H cleavage and a synergistic iridium(III/I)/redox catalyst regime, enabling the use of sustainable electricity as the terminal oxidant with improved selectivity features.

Hydroarylations by cobalt-catalyzed C-H activation

As an earth-abundant first-row transition metal, cobalt catalysts offer a broad range of economical methods for organic transformations via C-H activation. One of the transformations is the addition of C-H to C-X multiple bonds to afford alkylation, alkenation, amidation, and cyclization products using low- or high-valent cobalt catalysts. This hydroarylation is an efficient approach to build new C-C bonds in a 100% atom-economical manner. In this review, the recent developments of Co-catalyzed hydroarylation reactions and their mechanistic studies are summarized.

Delineating Physical Organic Parameters in Site-Selective C-H Functionalization of Indoles

Site-selective C-H functionalization is a great challenge in homogeneous transition-metal catalysis. Herein, we present a physical organic approach to delineate the origin of regioselective amidation of N-acylindoles through Ir(III) catalysis. Bulkiness of N-directing groups of indole substrates and electronics of carboxylate additives were identified as two major factors in controlling C2 and C7 selectivity, and their microscopic mechanisms were studied with DFT-based transition state analysis. Computational insights led us to interrogate a linear free energy relationship, and parametrization of molecular determinants enabled the establishment of an intuitive yet robust statistical model that correlates an extensive number of validation data points in high accuracy. This mechanistic investigation eventually allowed the development of a new C2 amidation and alkenylation protocol of indoles, which affords the exclusive functionalization at the C2 position with up to >70:1 selectivity.

Thiocarbonyl-enabled ferrocene C-H nitrogenation by cobalt(III) catalysis: thermal and mechanochemical

Versatile C–H amidations of synthetically useful ferrocenes were accomplished by weakly-coordinating thiocarbonyl-assisted cobalt catalysis. Thus, carboxylates enabled ferrocene C–H nitrogenations with dioxazolones, featuring ample substrate scope and robust functional group tolerance. Mechanistic studies provided strong support for a facile organometallic C–H activation manifold.

Donor-acceptor interactions between cyclic trinuclear pyridinate gold(i)-complexes and electron-poor guests: nature and energetics of guest-binding and templating on graphite

Aromatic stacking interactions of π-basic Au(i) complexes with π-acids were analyzed experimentally, theoretically and at the solid/liquid interface using STM.

Donor–acceptor-type interactions between π-electron systems are of high relevance in the design of chemical sensors. Due to their electron-rich nature, cyclic trinuclear complexes (CTCs) of gold(i) are ideal receptor sites for electron-deficient aromatic analytes. Scanning tunneling microscopy provided insight into the structures of two-dimensional crystals of pyridinate gold CTCs that form on a graphite template at the solid/liquid interface. One polymorph thereof – in turn – templated the on-top co-adsorption of π-acidic pyrazolate CTCs as electron-poor guests up to a certain threshold. From NMR titration experiments, we quantified free energies of –6.1 to –7.5 kcal mol^–1 for the binding between pyridinate gold(i) CTCs and π-acidic pyrazolate CTCs. Quantum chemical calculations revealed that these interactions are largely dominated by London dispersion. These results give a more detailed insight into a rational design of sensitive CNT- or graphene-based sensors for π-acidic analytes, such as electron-deficient aromatics.