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

Pick your Pd partners: A number of catalytic systems have been developed for palladium-catalyzed CH activation/CC bond formation. Recent studies concerning the palladium(II)-catalyzed coupling of CH bonds with organometallic reagents through a PdII/Pd0 catalytic cycle are discussed (see scheme), and the versatility and practicality of this new mode of catalysis are presented. Unaddressed questions and the potential for development in the field are also addressed.

In the past decade, palladium-catalyzed CH activation/CC bond-forming reactions have emerged as promising new catalytic transformations; however, development in this field is still at an early stage compared to the state of the art in cross-coupling reactions using aryl and alkyl halides. This Review begins with a brief introduction of four extensively investigated modes of catalysis for forming CC bonds from CH bonds: PdII/Pd0, PdII/PdIV, Pd0/PdII/PdIV, and Pd0/PdII catalysis. A more detailed discussion is then directed towards the recent development of palladium(II)-catalyzed coupling of CH bonds with organometallic reagents through a PdII/Pd0 catalytic cycle. Despite the progress made to date, improving the versatility and practicality of this new reaction remains a tremendous challenge.

/pdf/palladium-ii-catalyzed-c-h-activation-c-c-cross-coupling-2546yox1wp.pdf

Palladium(II)-catalyzed C-H activation/C-C cross-coupling reactions: versatility and practicality.

The area of transition-metal-catalyzed direct arylation through cleavage of CH bonds has undergone rapid development in recent years, and is becoming an increasingly viable alternative to traditional cross-coupling reactions with organometallic reagents In particular, palladium and ruthenium catalysts have been described that enable the direct arylation of (hetero)arenes with challenging coupling partners—including electrophilic aryl chlorides and tosylates as well as simple arenes in cross-dehydrogenative arylations Furthermore, less expensive copper, iron, and nickel complexes were recently shown to be effective for economically attractive direct arylations

Transition-metal-catalyzed direct arylation of (hetero)arenes by C-H bond cleavage.

The transition-metal-catalyzed functionalization of C-H bonds is a powerful method for generating carbon-carbon bonds. Although significant advances to this field have been reported during the past decade, many challenges remain. First, most of the methods are substrate-specific and thus cannot be generalized. Second, conversions of unactivated (i.e., not benzylic or alpha to heteroatom) sp(3) C-H bonds to C-C bonds are rare, with most examples limited to t-butyl groups, a conversion that is inherently simple because there are no beta-hydrogens that can be eliminated. Finally, the palladium, rhodium, and ruthenium catalysts routinely used for the conversion of C-H bonds to C-C bonds are expensive. Catalytically active metals that are cheaper and less exotic (e.g., copper, iron, and manganese) are rarely used. This Account describes our attempts to provide solutions to these three problems. We have developed a general method for directing-group-containing arene arylation by aryl iodides. Using palladium acetate as the catalyst, we arylated anilides, benzamides, benzoic acids, benzylamines, and 2-substituted pyridine derivatives under nearly identical conditions. We have also developed a method for the palladium-catalyzed auxiliary-assisted arylation of unactivated sp(3) C-H bonds. This procedure allows for the beta-arylation of carboxylic acid derivatives and the gamma-arylation of amine derivatives. Furthermore, copper catalysis can be used to mediate the arylation of acidic arene C-H bonds (i.e., those with pK(a) values <35 in DMSO). Using a copper iodide catalyst in combination with a base and a phenanthroline ligand, we successfully arylated electron-rich and electron-deficient heterocycles and electron-poor arenes possessing at least two electron-withdrawing groups. The reaction exhibits unusual regioselectivity: arylation occurs at the most hindered position. This copper-catalyzed method supplements the well-known C-H activation/borylation methodology, in which functionalization usually occurs at the least hindered position. We also describe preliminary investigations to determine the mechanisms of these transformations. We anticipate that other transition metals, including iron, nickel, cobalt, and silver, will also be able to facilitate deprotonation/arylation reaction sequences.

/pdf/palladium-and-copper-catalyzed-arylation-of-carbon-hydrogen-53ohvhqdgn.pdf

Palladium- and Copper-Catalyzed Arylation of Carbon-Hydrogen Bonds

3.1. C-M and X-H as Nucleophiles 1806 3.2. C-H and X-M as Nucleophiles 1809 3.2.1. C-Halogen Bond Formations 1809 3.2.2. C-O Bond Formations 1812 3.3. C-H and X-H as Nucleophiles 1812 3.3.1. C-O Bond Formations 1812 3.3.2. C-N Bond Formations 1815 4. Oxidative X-X Bond Formations between Two Nucleophiles 1819 5. Conclusions 1819 Author Information 1819 Biographies 1819 Acknowledgment 1820 References 1820

Bond Formations between Two Nucleophiles: Transition Metal Catalyzed Oxidative Cross-Coupling Reactions

https://onlinelibrary.wiley.com/doi/pdf/10.1002/anie.200790209

Suzuki-Miyaura coupling reaction by PdII-catalyzed aromatic C-H bond activation directed by an N-alkyl acetamino group.

New switchable hydrogels are developed. Under acidic conditions, hydrogels undergo self-cyclization and can catch and kill bacteria. Under neutral/basic conditions, hydrogels undergo ring-opening and can release killed bacterial cells and resist protein adsorption and bacterial attachment. Smart hydrogels also show a dramatically improved mechanical property, which is highly desired for biomedical applications.

Switchable antimicrobial and antifouling hydrogels with enhanced mechanical properties.

Oxidative stress acts as a trigger in the course of neurodegenerative diseases and neural injuries. An antioxidant-based therapy can be effective to ameliorate the deleterious effects of oxidative stress. Resveratrol (RSV) has been shown to be effective at removing excess reactive oxygen species (ROS) or reactive nitrogen species generation in the central nervous system (CNS), but the delivery of RSV into the brain through systemic administration is inefficient. Here, we have developed a RSV delivery vehicle based on polylactic acid (PLA)-coated mesoporous silica nanoparticles (MSNPs), conjugated with a ligand peptide of low-density lipoprotein receptor (LDLR) to enhance their transcytosis across the blood–brain barrier (BBB). Resveratrol was loaded into MSNPs (average diameter 200 nm, pore size 4 nm) at 16 μg/mg (w/w). As a gatekeeper, the PLA coating prevented the RSV burst release, while ROS was shown to trigger the drug release by accelerating PLA degradation. An in vitro BBB model with a co-culture of rat brain microvascular endothelial cells (RBECs) and microglia cells using Transwell chambers was established to assess the RSV delivery across BBB. The conjugation of LDLR ligand peptides markedly enhanced the migration of MSNPs across the RBECs monolayer. RSV could be released and effectively reduce the activation of the microglia cells stimulated by phorbol-myristate-acetate or lipopolysaccharide. These ROS responsive LDLR peptides conjugated PLA-coated MSNPs have great potential for oxidative stress therapy in CNS.

/pdf/ros-responsive-resveratrol-delivery-from-ldlr-peptide-gps1tyrj4w.pdf

ROS responsive resveratrol delivery from LDLR peptide conjugated PLA-coated mesoporous silica nanoparticles across the blood-brain barrier.

Zwitterionic polycarboxybetaine (pCB)-based materials have drawn special attention due to their outstanding properties of resisting proteins adsorption, biofilm formation and cell attachment on a variety of substrates and surfaces, reducing the uptake of pCB-coated nanosized materials, as well as providing the capability of further functionalization. Zwitterionic pCBs have been proven to be excellent choices for many applications, not only limited to the field of antifouling. This review will summarize the recent progress on the development of pCB materials and the understanding of the structure–function–property relationships of carboxybetaine materials.

Recent advances of zwitterionic carboxybetaine materials and their derivatives.

This paper reports a multifunctional zwitterionic poly(sulfobetaine-3,4-ethylenedioxythiophene) (PSBEDOT), which could be facilely synthesized by electropolymerization in aqueous solution. The PSBEDOT surface exhibits excellent conductivity, stability and switchable antifouling/antimicrobial properties. It shows great promise for applications in organic bioelectronics. This new material may significantly increase the performance and service life, minimize the foreign body reaction, improve the biocompatibility and reduce the infection of bio-electronic devices.

/pdf/electroactive-poly-sulfobetaine-3-4-ethylenedioxythiophene-3ea0219hgn.pdf

Electroactive poly(sulfobetaine-3,4-ethylenedioxythiophene) (PSBEDOT) with controllable antifouling and antimicrobial properties

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