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.

3d Transition Metals for C-H Activation.

The present review is devoted to summarizing the recent advances (2015-2017) in the field of metal-catalysed group-directed C-H functionalisation In order to clearly showcase the molecular diversity that can now be accessed by means of directed C-H functionalisation, the whole is organized following the directing groups installed on a substrate Its aim is to be a comprehensive reference work, where a specific directing group can be easily found, together with the transformations which have been carried out with it Hence, the primary format of this review is schemes accompanied with a concise explanatory text, in which the directing groups are ordered in sections according to their chemical structure The schemes feature typical substrates used, the products obtained as well as the required reaction conditions Importantly, each example is commented on with respect to the most important positive features and drawbacks, on aspects such as selectivity, substrate scope, reaction conditions, directing group removal, and greenness The targeted readership are both experts in the field of C-H functionalisation chemistry (to provide a comprehensive overview of the progress made in the last years) and, even more so, all organic chemists who want to introduce the C-H functionalisation way of thinking for a design of straightforward, efficient and step-economic synthetic routes towards molecules of interest to them Accordingly, this review should be of particular interest also for scientists from industrial R&D sector Hence, the overall goal of this review is to promote the application of C-H functionalisation reactions outside the research groups dedicated to method development and establishing it as a valuable reaction archetype in contemporary R&D, comparable to the role cross-coupling reactions play to date

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A comprehensive overview of directing groups applied in metal-catalysed C-H functionalisation chemistry.

Catalytic C–H activation has emerged as a powerful tool for sustainable syntheses. In the recent years, notable success was achieved with the development of cobalt-catalyzed C–H functionalizations with either in situ generated or single-component cobalt-complexes under mild reaction conditions. Herein, recent progress in the field of organometallic cobalt-catalyzed C–H activation is reviewed until November 2015.

Cobalt-Catalyzed C―H Activation

Transition-metal-catalyzed cross-coupling reactions have been well-established as indispensable tools in modern organic synthesis. One of the major research goals in cross-coupling area is expanding the scope of the coupling partners. In the past decade, diazo compounds (or their precursors N-tosylhydrazones) have emerged as nucleophilic cross-coupling partners in C–C single bond or C═C double bond formations in transition-metal-catalyzed reactions. This type of coupling reaction involves the following general steps. First, the organometallic species is generated by various processes, including oxidative addition, transmetalation, cyclization, C–C bond cleavage, and C–H bond activation. Subsequently, the organometallic species reacts with the diazo substrate to generate metal carbene intermediate, which undergoes rapid migratory insertion to form a C–C bond. The new organometallic species generated from migratory insertion may undergo various transformations. This type of carbene-based coupling has proven...

Transition-Metal-Catalyzed Cross-Couplings through Carbene Migratory Insertion

The development of new methods for the direct functionalization of unactivated C–H bonds is ushering in a paradigm shift in the field of retrosynthetic analysis. In particular, the catalytic enantioselective functionalization of C–H bonds represents a highly atom- and step-economic approach toward the generation of structural complexity. However, as a result of their ubiquity and low reactivity, controlling both the chemo- and stereoselectivity of such processes constitutes a significant challenge. Herein we comprehensively review all asymmetric transition-metal-catalyzed methodologies that are believed to proceed via an inner-sphere-type mechanism, with an emphasis on the nature of stereochemistry generation. Our analysis serves to document the considerable and rapid progress within in the field, while also highlighting limitations of current methods.

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Catalytic Enantioselective Transformations Involving C-H Bond Cleavage by Transition-Metal Complexes.

The first highly efficient and scalable cobalt-catalyzed directed C-H functionalization with carbene precursors is presented. This methodology provides a modular route towards a new class of conjugated polycyclic hydrocarbons with tunable emission wavelengths both in solution and in the solid state.

Cobalt(III)-Catalyzed Directed C ? H Coupling with Diazo Compounds: Straightforward Access towards Extended π-Systems

A facile route toward the synthesis of isoquinolin-3-ones through a cooperative B(C6 F5 )3 - and Cp*Co(III) -catalyzed C-H bond activation of imines with diazo compounds is presented. The inclusion of a catalytic amount of B(C6 F5 )3 results in a highly efficient reaction, thus enabling unstable NH imines to serve as substrates.

Cooperative Lewis Acid/Cp*Co(III) Catalyzed C-H Bond Activation for the Synthesis of Isoquinolin-3-ones.

The first CH bond activation with pyridotriazoles as coupling partners is presented using a RhIII catalyst. The pyridotriazoles can be used as new carbene precursors in CH activation for direct access to novel fluorescent scaffolds. These tunable fluorophores can be applied for the detection of metal ions.

RhIII‐Catalyzed CH Activation with Pyridotriazoles: Direct Access to Fluorophores for Metal‐Ion Detection

The first Rh(I)-catalyzed asymmetric approach for the intermolecular functionalization of C(sp3)–H bonds is reported. For the first time, unsymmetrical N-heterocyclic carbenes (NHCs) were used for asymmetric catalysis that is capable of achieving not only high site-selectivity but also enantioselectivity. The Rh(I)/NHC* catalytic systems were applied to asymmetric direct C(sp3)–H arylation, which provides a synthetic route toward enantioenriched triarylmethanes.

Rh(I)/NHC*-Catalyzed Site- and Enantioselective Functionalization of C(sp3)–H Bonds Toward Chiral Triarylmethanes

The first rhodium(I)-catalyzed enantioselective intermolecular Csp3 -H activation of various saturated aza-heterocycles including tetrahydroquinolines, piperidines, piperazines, azetidines, pyrrolidines, and azepanes is presented. The combination of a rhodium(I) precatalyst and a chiral monodentate phosphonite ligand is shown to be a powerful catalytic system to access a variety of important enantio-enriched heterocycles from simple starting materials. Notably, the Csp3 -H activation of tetrahydroquinolines is especially challenging due to the adjacent Csp2 -H bond. This redox-neutral methodology provides a new synthetic route to α-N-arylated heterocycles with high chemoselectivity and enantioselectivity up to 97 % ee.

Ju Hyun Kim

Papers

Cobalt(III)-Catalyzed Directed C ? H Coupling with Diazo Compounds: Straightforward Access towards Extended π-Systems

Cooperative Lewis Acid/Cp*Co(III) Catalyzed C-H Bond Activation for the Synthesis of Isoquinolin-3-ones.

RhIII‐Catalyzed CH Activation with Pyridotriazoles: Direct Access to Fluorophores for Metal‐Ion Detection

Rh(I)/NHC*-Catalyzed Site- and Enantioselective Functionalization of C(sp3)–H Bonds Toward Chiral Triarylmethanes

Ligand-Enabled Enantioselective Csp3 -H Activation of Tetrahydroquinolines and Saturated Aza-Heterocycles by RhI.