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

The halogen bond occurs when there is evidence of a net attractive interaction between an electrophilic region associated with a halogen atom in a molecular entity and a nucleophilic region in another, or the same, molecular entity. In this fairly extensive review, after a brief history of the interaction, we will provide the reader with a snapshot of where the research on the halogen bond is now, and, perhaps, where it is going. The specific advantages brought up by a design based on the use of the halogen bond will be demonstrated in quite different fields spanning from material sciences to biomolecular recognition and drug design.

The Halogen Bond

The chemistry of heterocyclic carbenes has experienced a rapid development over the last years. In addition to the imidazolin-2-ylidenes, a large number of cyclic diaminocarbenes with different ring sizes have been described. Aside from diaminocarbenes, P-heterocyclic carbenes, and derivatives with only one, or even no heteroatom within the carbene ring are known. New methods for the synthesis of complexes with N-heterocyclic carbene ligands such as the oxidative addition or the metal atom template controlled cyclization of β-functionalized isocyanides have been developed recently. This review summarizes the new developments regarding the synthesis of N-heterocyclic carbenes and their metal complexes.

Heterocyclic Carbenes: Synthesis and Coordination Chemistry

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Metal–Organic Frameworks and Self-Assembled Supramolecular Coordination Complexes: Comparing and Contrasting the Design, Synthesis, and Functionality of Metal–Organic Materials

Within the field of nanotechnology, nanoparticles are one of the most prominent and promising candidates for technological applications. Self-assembly of nanoparticles has been identified as an important process where the building blocks spontaneously organize into ordered structures by thermodynamic and other constraints. However, in order to successfully exploit nanoparticle self-assembly in technological applications and to ensure efficient scale-up, a high level of direction and control is required. The present review critically investigates to what extent self-assembly can be directed, enhanced, or controlled by either changing the energy or entropy landscapes, using templates or applying external fields.

Directed Self-Assembly of Nanoparticles

Cyclometalated phosphine-based pincer complexes: Mechanistic insight in catalysis, coordination, and bond activation

https://onlinelibrary.wiley.com/doi/pdf/10.1002/anie.200906382

Inside Cover: Sequential Logic Operations with Surface‐Confined Polypyridyl Complexes Displaying Molecular Random Access Memory Features (Angew. Chem. Int. Ed. 1/2010)

“Smart” patternable polymer-based materials that can be designed from various molecular building blocks show great potential, as they may be used in many fields, including nanotechnology, biochemistry, organic and physical chemistry, and materials science. The focus of this highlight will be on the basic design characteristics of practical Stimuli Responsive Materials (SRMs), the wide range of potential applications and the challenges to be accomplished in this rapidly expanding area. In particular, recent developments are described which are related to two of the many fundamental aspects of stimuli triggered responses: those that are photo-triggered and those that are solvent triggered. These selected state-of-the-art examples demonstrate the large scope and diversity in terms of activation mechanism, response time and property control.

/pdf/stimuli-responsive-materials-new-avenues-toward-smart-4r9mmvht9l.pdf

Stimuli responsive materials: new avenues toward smart organic devices

Stimuli responsive materials are capable of mimicking the operation characteristics of logic gates such as AND, OR, NOR, and even flip-flops. Since the development of molecular sensors and the introduction of the first AND gate in solution by de Silva in 1993, Molecular (Boolean) Logic and Computing (MBLC) has become increasingly popular. In this Account, we present recent research activities that focus on MBLC with electrochromic polymers and metal polypyridyl complexes on a solid support.Metal polypyridyl complexes act as useful sensors to a variety of analytes in solution (i.e., H2O, Fe2+/3+, Cr6+, NO+) and in the gas phase (NOx in air). This information transfer, whether the analyte is present, is based on the reversible redox chemistry of the metal complexes, which are stable up to 200 °C in air. The concurrent changes in the optical properties are nondestructive and fast. In such a setup, the input is directly related to the output and, therefore, can be represented by one-input logic gates. These i...

Surface-confined assemblies and polymers for molecular logic.

Reaction of [RhCl(C8H14)2]2 (C8H14 = cyclooctene) with 2 equiv of the aryl methyl ether phosphine 1 in C6D6 results in an unprecedented metal insertion into the strong sp2−sp3 aryl−O bond. This remarkable reaction proceeds even at room temperature and occurs directly, with no intermediacy of C−H activation or insertion into the adjacent weaker ArO−CH3 bond. Two new phenoxy complexes (8 and 9), which are analogous to the product of insertion into the ArO−CH3 bond (had it taken place) were prepared and shown not to be intermediates in the Ar−OCH3 bond cleavage process. Thus, aryl−O bond activation by the nucleophilic Rh(I) is kinetically preferred over activation of the alkyl−O bond. The phenoxy Rh(I)−η1-N2 complex (8) is in equilibrium with the crystallographically characterized Rh(I)−μ-N2−Rh(I) dimer (12). Reaction of [RhCl(C8H14)2]2 with 2 equiv of the aryl methyl ether phosphine 2, PPh3, and excess HSiR3 (R = OCH2CH3, CH2CH3) results also in selective metal insertion into the aryl−O bond and formation o...

Milko E. van der Boom

Papers

Cyclometalated phosphine-based pincer complexes: Mechanistic insight in catalysis, coordination, and bond activation

Inside Cover: Sequential Logic Operations with Surface‐Confined Polypyridyl Complexes Displaying Molecular Random Access Memory Features (Angew. Chem. Int. Ed. 1/2010)

Stimuli responsive materials: new avenues toward smart organic devices

Surface-confined assemblies and polymers for molecular logic.

Alkyl− and Aryl−Oxygen Bond Activation in Solution by Rhodium(I), Palladium(II), and Nickel(II). Transition-Metal-Based Selectivity