1.1 Introduction to Pd-catalyzed directed C–H functionalization
The development of methods for the direct conversion of carbon–hydrogen bonds into carbon-oxygen, carbon-halogen, carbon-nitrogen, carbon-sulfur, and carbon-carbon bonds remains a critical challenge in organic chemistry. Mild and selective transformations of this type will undoubtedly find widespread application across the chemical field, including in the synthesis of pharmaceuticals, natural products, agrochemicals, polymers, and feedstock commodity chemicals. Traditional approaches for the formation of such functional groups rely on pre-functionalized starting materials for both reactivity and selectivity. However, the requirement for installing a functional group prior to the desired C–O, C–X, C–N, C–S, or C–C bond adds costly chemical steps to the overall construction of a molecule. As such, circumventing this issue will not only improve atom economy but also increase the overall efficiency of multi-step synthetic sequences.

Direct C–H bond functionalization reactions are limited by two fundamental challenges: (i) the inert nature of most carbon-hydrogen bonds and (ii) the requirement to control site selectivity in molecules that contain diverse C–H groups. A multitude of studies have addressed the first challenge by demonstrating that transition metals can react with C–H bonds to produce C–M bonds in a process known as “C–H activation”.1 The resulting C–M bonds are far more reactive than their C–H counterparts, and in many cases they can be converted to new functional groups under mild conditions.

The second major challenge is achieving selective functionalization of a single C–H bond within a complex molecule. While several different strategies have been employed to address this issue, the most common (and the subject of the current review) involves the use of substrates that contain coordinating ligands. These ligands (often termed “directing groups”) bind to the metal center and selectively deliver the catalyst to a proximal C–H bond. Many different transition metals, including Ru, Rh, Pt, and Pd, undergo stoichiometric ligand-directed C–H activation reactions (also known as cyclometalation).2,3 Furthermore, over the past 15 years, a variety of catalytic carbon-carbon bond-forming processes have been developed that involve cyclometalation as a key step.1b–d,4

The current review will focus specifically on ligand-directed C–H functionalization reactions catalyzed by palladium. Palladium complexes are particularly attractive catalysts for such transformations for several reasons. First, ligand-directed C–H functionalization at Pd centers can be used to install many different types of bonds, including carbon-oxygen, carbon-halogen, carbon-nitrogen, carbon-sulfur, and carbon-carbon linkages. Few other catalysts allow such diverse bond constructions,5,6,7 and this versatility is predominantly the result of two key features: (i) the compatibility of many PdII catalysts with oxidants and (ii) the ability to selectively functionalize cyclopalladated intermediates. Second, palladium participates in cyclometalation with a wide variety of directing groups, and, unlike many other transition metals, promotes C–H activation at both sp2 and sp3 C–H sites. Finally, the vast majority of Pd-catalyzed directed C–H functionalization reactions can be performed in the presence of ambient air and moisture, making them exceptionally practical for applications in organic synthesis.

While several accounts have described recent advances, this is the first comprehensive review encompassing the large body of work in this field over the past 5 years (2004–2009). Both synthetic applications and mechanistic aspects of these transformations are discussed where appropriate, and the review is organized on the basis of the type of bond being formed.

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Palladium-Catalyzed Ligand-Directed C−H Functionalization Reactions

C-H bonds are ubiquitous in organic compounds. It would, therefore, appear that direct functionalization of substrates by activation of C-H bonds would eliminate the multiple steps and limitations associated with the preparation of functionalized starting materials. Regioselectivity is an important issue because organic molecules can contain a wide variety of C-H bonds. The use of a directing group can largely overcome the issue of regiocontrol by allowing the catalyst to come into proximity with the targeted C-H bonds. A wide variety of functional groups have been evaluated for use as directing groups in the transformation of C-H bonds. In 2005, Daugulis reported the arylation of unactivated C(sp(3))-H bonds by using 8-aminoquinoline and picolinamide as bidentate directing groups, with Pd(OAc)2 as the catalyst. Encouraged by these promising results, a number of transformations of C-H bonds have since been developed by using systems based on bidentate directing groups. In this Review, recent advances in this area are discussed.

Catalytic Functionalization of C(sp2) ? H and C(sp3) ? H Bonds by Using Bidentate Directing Groups

The use of coordinating moieties as directing groups for the functionalization of aromatic CH bonds has become an established tool to enhance reactivity and induce regioselectivity. Nevertheless, with regard to the synthetic applicability of CH activation, there is a growing interest in transformations in which the directing group can be fully abandoned, thus allowing the direct functionalization of simple benzene derivatives. However, this approach requires the disclosure of new strategies to achieve reactivity and to control selectivity. In this review, recent advances in the emerging field of non-chelate-assisted CH activation are discussed, highlighting some of the most intriguing and inspiring examples of induction of reactivity and selectivity.

Beyond directing groups: transition-metal-catalyzed C-H activation of simple arenes.

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.

Metal-catalysed C–H bond functionalisation has had a significant impact on how chemists make molecules. Translating the methodological developments to their use in the assembly of complex natural products is an important challenge for the continued advancement of chemical synthesis. In this tutorial review, we describe selected recent examples of how the metal-catalysed C–H bond functionalisation has been able to positively affect the synthesis of natural products.

Recent developments in natural product synthesis using metal-catalysed C–H bond functionalisation

The catalytic reactions that involve the cleavage of C–CN bonds and carbon–carbon triple bonds are described in this tutorial review. Regarding the cleavage of a C–CN bond, the catalytic reaction can proceed by two different mechanisms: oxidative addition and deinsertion of silyl isocyanide. A carbon–carbon triple bond can be cleaved in the absence of an organic promoter via the formation of unique organometallic species, such as allenylidene and cyclopropyl carbenoid complexes.

Catalytic reactions involving the cleavage of carbon–cyano and carbon–carbon triple bonds

Where HAS you been? A manganese-mediated annulation of 2-isocyanobiaryls with organoboronic acids is developed for the synthesis of a broad range of phenanthridine derivatives. Mechanistic studies indicate that the reaction proceeds by the intramolecular homolytic aromatic substitution (HAS) of an imidoyl radical intermediate.

Modular Synthesis of Phenanthridine Derivatives by Oxidative Cyclization of 2-Isocyanobiphenyls with Organoboron Reagents†

The reaction of electron-deficient N-heteroaromatic compounds, such as pyridines and quinolines, with arylzinc reagents in the presence of a catalytic amount of a nickel complex affords the arylated products. The reaction is likely to proceed through a formal nucleophilic 1,2-addition, thus exhibiting a reactivity complementary to conventional direct arylation through electrophilic substitution.

Nickel-catalyzed reaction of arylzinc reagents with N-aromatic heterocycles: a straightforward approach to C-H bond arylation of electron-deficient heteroaromatic compounds.

Catalytic amination: The title reaction demonstrates the use of aryl carboxylates as suitable electrophilic coupling substrates in catalytic amination reactions. N-heterocyclic carbene ligands and NaOtBu promote the amination of aryl pivalates through the cleavage of normally unreactive aryl carbon-oxygen bonds (see scheme; cod=cyclooctadiene).

Mamoru Tobisu

Papers

Catalytic reactions involving the cleavage of carbon–cyano and carbon–carbon triple bonds

Modular Synthesis of Phenanthridine Derivatives by Oxidative Cyclization of 2-Isocyanobiphenyls with Organoboron Reagents†

Nickel-catalyzed reaction of arylzinc reagents with N-aromatic heterocycles: a straightforward approach to C-H bond arylation of electron-deficient heteroaromatic compounds.

Nickel-catalyzed amination of aryl pivalates by the cleavage of aryl C-O bonds.

Palladium-catalyzed direct alkynylation of C-H bonds in benzenes.