Catalytic transformation of ubiquitous C–H bonds into valuable C–N bonds offers an efficient synthetic approach to construct N-functionalized molecules. Over the last few decades, transition metal catalysis has been repeatedly proven to be a powerful tool for the direct conversion of cheap hydrocarbons to synthetically versatile amino-containing compounds. This Review comprehensively highlights recent advances in intra- and intermolecular C–H amination reactions utilizing late transition metal-based catalysts. Initial discovery, mechanistic study, and additional applications were categorized on the basis of the mechanistic scaffolds and types of reactions. Reactivity and selectivity of novel systems are discussed in three sections, with each being defined by a proposed working mode.

Transition Metal-Catalyzed C–H Amination: Scope, Mechanism, and Applications

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.

Organic reactions that involve the direct functionalization of non-activated C–H bonds represent an attractive class of transformations which maximize atom- and step-economy, and simplify chemical synthesis. Due to the high stability of C–H bonds, these processes, however, have most often required harsh reaction conditions, which has drastically limited their use as tools for the synthesis of complex organic molecules. Following the increased understanding of mechanistic aspects of C–H activation gained over recent years, great strides have been taken to design and develop new protocols that proceed efficiently under mild conditions and duly benefit from improved functional group tolerance and selectivity. In this review, we present the current state of the art in this field and detail C–H activation transformations reported since 2011 that proceed either at or below ambient temperature, in the absence of strongly acidic or basic additives or without strong oxidants. Furthermore, by identifying and discussing the major strategies that have led to these improvements, we hope that this review will serve as a useful conceptual overview and inspire the next generation of mild C–H transformations.

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Mild metal-catalyzed C–H activation: examples and concepts

Transition metal-catalyzed direct functionalization of C–H bonds is one of the key emerging strategies that is currently attracting tremendous attention with the aim to provide alternative environmentally friendly and efficient ways for the construction of C–C and C–hetero bonds. In particular, the strategy involving regioselective C–H activation assisted by various functional groups shows high potential, and significant achievements have been made in both the development of novel reactions and the mechanistic study. In this review, we attempt to give an overview of the development of utilizing the functionalities as directing groups. The discussion is directed towards the use of different functional groups as directing groups for the construction of C–C and C–hetero bonds via C–H activation using various transition metal catalysts. The synthetic applications and mechanistic features of these transformations will be discussed, and the review is organized on the basis of the type of directing groups and the type of bond being formed or the catalyst.

Transition metal-catalyzed C–H bond functionalizations by the use of diverse directing groups

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.

A Cu(OAc)2-mediated C-H amidation and amination of arenes and heteroarenes has been developed using a readily removable directing group. A wide range of sulfonamides, amides, and anilines function as amine donors in this reaction. Heterocycles present in both reactants are tolerated, making this a broadly applicable method for the synthesis of a family of inhibitors including 2-benzamidobenzoic acids and N-phenylaminobenzoates.

Cu(II)-mediated C-H amidation and amination of arenes: exceptional compatibility with heterocycles.

Cu(II)-promoted ortho alkynylation of arenes and heteroarenes with terminal alkynes has been developed to prepare aryl alkynes. A variety of arenes and terminal alkynes bearing different substituents are compatible with this reaction, thus providing an alternative disconnection to Sonogashira coupling.

Cu(II)-mediated ortho C-H alkynylation of (hetero)arenes with terminal alkynes.

A mild, chemo- and site-selective catalytic protocol that allows for incorporating an alkylboron fragment into unactivated olefins is described. The use of internal olefins enables C–C bond-formation at remote sp3 C–H sites, constituting a complementary and conceptually different approach to existing borylation techniques that are currently available at sp3 centers.

Site-Selective Ni-Catalyzed Reductive Coupling of α-Haloboranes with Unactivated Olefins.

The direct ortho-trifluoromethylation of arenes, including heteroarenes, with TMSCF3 has been accomplished by a copper (II)-promoted C-H activation reaction which completes within 30 minutes. Mechanistic investigations are consistent with the involvement of C-H activation, rather than a simple electrophilic aromatic substitution (SEAr), as the key step.

Exceedingly Fast Copper(II)‐Promoted ortho C ? H Trifluoromethylation of Arenes using TMSCF3

A catalytic deaminative alkylation of unactivated olefins is described. The protocol is characterized by its mild conditions, wide scope, including the use of ethylene as substrate, and exquisite site-selectivity pattern for both α-olefins and internal olefins, thus unlocking a new catalytic platform to forge sp3-sp3 linkages, even in the context of late-stage functionalization.

Shang-Zheng Sun

Papers

Cu(II)-mediated C-H amidation and amination of arenes: exceptional compatibility with heterocycles.

Cu(II)-mediated ortho C-H alkynylation of (hetero)arenes with terminal alkynes.

Site-Selective Ni-Catalyzed Reductive Coupling of α-Haloboranes with Unactivated Olefins.

Exceedingly Fast Copper(II)‐Promoted ortho C ? H Trifluoromethylation of Arenes using TMSCF3

Site-Selective Catalytic Deaminative Alkylation of Unactivated Olefins