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.

This Review summarizes the advancements in Pd-catalyzed C(sp3)–H activation via various redox manifolds, including Pd(0)/Pd(II), Pd(II)/Pd(IV), and Pd(II)/Pd(0). While few examples have been reported in the activation of alkane C–H bonds, many C(sp3)–H activation/C–C and C–heteroatom bond forming reactions have been developed by the use of directing group strategies to control regioselectivity and build structural patterns for synthetic chemistry. A number of mono- and bidentate ligands have also proven to be effective for accelerating C(sp3)–H activation directed by weakly coordinating auxiliaries, which provides great opportunities to control reactivity and selectivity (including enantioselectivity) in Pd-catalyzed C–H functionalization reactions.

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Palladium-Catalyzed Transformations of Alkyl C-H Bonds.

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.

During the past decades, synthetic organic chemistry discovered that directing group assisted C–H activation is a key tool for the expedient and siteselective construction of C–C bonds. Among the v...

Bidentate Directing Groups: An Efficient Tool in C-H Bond Functionalization Chemistry for the Expedient Construction of C-C Bonds.

The conservation of our element resources is a fundamental challenge of mankind. The development of alcohol refunctionalization reactions is a possible fossil carbon conservation strategy since alcohols can be obtained from indigestible and abundantly available biomass. The conservation of our rare noble metals, frequently used in key technologies such as catalysis, might be feasible by replacing them with highly abundant metals. The alkylation of amines by alcohols and related C–C coupling reactions are early examples of alcohol refunctionalization reactions. These reactions follow mostly the borrowing hydrogen or hydrogen autotransfer catalysis concept, and many 3d-metal catalysts have been disclosed in recent years. In this review, we summarize the progress made in developing Cu, Ni, Co, Fe, and Mn catalysts for C–N and C–C bond formation reactions with alcohols and amines using the borrowing hydrogen or hydrogen autotransfer concept. We expect that the findings in this field will inspire others to dev...

3d-Metal Catalyzed N- and C-Alkylation Reactions via Borrowing Hydrogen or Hydrogen Autotransfer

The ability of hydrogen-transfer transition-metal catalysts, which enable increasingly rapid access to important structural scaffolds from simple starting materials, has led to a plethora of research efforts on the construction of heterocyclic scaffolds. Transition-metal-catalyzed hydrogen-transfer annulations are environmentally benign and highly atom-economical as they release of water and hydrogen as by-product and utilize renewable feedstock alcohols as starting materials. Recent advances in this field with respect to the annulations of alcohols with various nucleophilic partners, thus leading to the formation of heterocyclic scaffolds, are highlighted herein.

Transition-metal-catalyzed hydrogen-transfer annulations: access to heterocyclic scaffolds.

Cobalt-catalyzed acceptorless dehydrogenative coupling of aminoalcohols with alcohols: direct access to pyrrole, pyridine and pyrazine derivatives

Development of sustainable catalytic systems for fundamentally important synthetic transformations and energy storage applications is an intellectually stimulating challenge. Catalytic dehydrogenation of feedstock chemicals, such as alcohols and amines to value-added products with the concomitant generation of dihydrogen is of much interest in the context of hydrogen economy and is an effective alternative to the classical oxidation reactions. Despite a number of homogeneous catalysts being identified for the acceptorless dehydrogenation, the use of high price and limited availability of precious metals and poor recovery of the catalyst have spurred interest in catalysis with more earth-abundant alternatives, especially iron. However, no report has described a reusable iron-based heterogeneous catalyst for oxidant-free and acceptorless dehydrogenation reactions. Here we replace expensive noble metal catalysts with an inexpensive, benign, and sustainable nanoscale iron catalyst for the efficient acceptorless dehydrogenation of N-heterocycles and alcohols with liberation of hydrogen gas.

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Iron-based nanocatalyst for the acceptorless dehydrogenation reactions.

Cobalt-Catalyzed Bis-alkynylation of Amides via Double C–H Bond Activation

Development of multicatalytic approach consisting of two or more mechanistically distinct catalytic steps using a single-site catalyst for rapid and straightforward access of structurally complex molecules under eco-benign conditions has significance in contemporary science. We have developed herein a rhodium-catalysed C-H activation strategy which uses an unprotected anilines and an electron-deficient alkynes to C-C bonded products as a potential intermediate in contrast to the archetypical C-N bonded products with high levels of regioselectivity. This is followed by carbonylation of C-H bond activated intermediate and subsequent annulation into quinolines has been described. This rhodium-catalysed auto-tandem reaction operates under mild, environmentally benign conditions using water as the solvent and CO surrogates as the carbonyl source with the concomitant generation of hydrogen gas. The strategy may facilitate the development of new synthetic protocols for the efficient and sustainable production of chemicals in an atom-economic way from simple, abundant starting materials.

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Vinod G. Landge

Papers

Transition-metal-catalyzed hydrogen-transfer annulations: access to heterocyclic scaffolds.

Cobalt-catalyzed acceptorless dehydrogenative coupling of aminoalcohols with alcohols: direct access to pyrrole, pyridine and pyrazine derivatives

Iron-based nanocatalyst for the acceptorless dehydrogenation reactions.

Cobalt-Catalyzed Bis-alkynylation of Amides via Double C–H Bond Activation

Reversed reactivity of anilines with alkynes in the rhodium-catalysed C–H activation/carbonylation tandem