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J. Appl. Cryst.の発刊に際して

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.

N-Heterocyclic carbenes have become universal ligands in organometallic and inorganic coordination chemistry. They not only bind to any transition metal, be it in low or high oxidation states, but also to main group elements such as beryllium, sulfur, and iodine. Because of their specific coordination chemistry, N-heterocyclic carbenes both stabilize and activate metal centers in quite different key catalytic steps of organic syntheses, for example, C-H activation, C-C, C-H, C-O, and C-N bond formation. There is now ample evidence that in the new generation of organometallic catalysts the established ligand class of organophosphanes will be supplemented and, in part, replaced by N-heterocyclic carbenes. Over the past few years, this chemistry has been the field of vivid scientific competition, and yielded previously unexpected successes in key areas of homogeneous catalysis. From the work in numerous academic laboratories and in industry, a revolutionary turning point in oraganometallic catalysis is emerging.

http://aether.cmi.ua.ac.be/artikels/Artikels%20Gitte/Artikels/Reviews/NHC-%20liganden%20Herrmann.pdf

N-heterocyclic carbenes: a new concept in organometallic catalysis.

The structure of 1,2-bis(2,4,6-trimethylbenzylideneamino)ethanedichloropalladium(II) was determined by X-ray diffraction methods The structure derived from NMR and IR spectra and elemental analysis is consistent with it The complex is orthorhombic, space group Pcab, with a = 148662(12), b = 150861(13), c = 206255(14) A, V = 46257(6) A3 and Z = 8 In the lattice, there exists a weak C–H ··· Cl hydrogen bond

Synthesis, structure and spectroscopic characterization of 1,2-bis-(2,4,6-trimethylbenzylideneamino)ethanedichloropalladium(II)

A series of unsymmetrical 1,3-disubstituted benzimidazolium chlorides 2a-f having two nitrogen atoms substituted by various alkyl groups were synthesized as N-heterocyclic carbene (NHC) precursors in high yields. The benzimidazolium salts readily converted into the corresponding PEPPSI-type palladium–NHC complexes 3a-f (PEPPSI = pyridine-enhanced precatalyst preparation, stabilization, and initiation). The structures of all compounds have been characterized by 1 H NMR, 13 C NMR, and IR spectroscopy, as well as X-rays ( 3a, 3d and 3e ) techniques, which support the proposed structures.  Next, the palladium-NHC-PEPPSI complexes were used as catalysts in the direct C(5)-arylation of 2-acetyl furan and 2-acetylthiophene with various aryl bromides. These complexes exhibited moderate to high catalytic activities and gave C-H activation selectively at the C(5)-position of  2-acetylfuran and  2-acetylthiophene.

Synthesis of [PdBr2(Benzimidazole-2-Ylidene)(Pyridine)] Complexes and Their Catalytic Activity in the Direct C-H Bond Activation of 2-Substituted Heterocycles

This article explores the possibility of using imidazoline and benzimidazole derivatives as interesting ligands due to the fact that they are structurally simple, readily available, inexpensive, and they allow easy introduction of various substituents into their structure. Therefore, Nsubstituted imidazoline and benzimidazole ligands and their new ruthenium complexes [RuCl2(η-p-cymene)(L)] (L = N-substituted imidazoline/benzimidazole) were synthesized and characterized. The catalytic performances of new complexes are investigated for the transfer hydrogenation of ketones. Furthermore, corresponding secondary alcohols were also obtained in good yields.

/pdf/transfer-hydrogenation-of-ketones-in-the-presence-of-half-58cz2u0m01.pdf

Transfer hydrogenation of ketones in the presence of half sandwich ruthenium (II) complexes bearing imidazoline and benzimidazole ligand

https://www.heterocycles.jp/newlibrary/downloads/PDF/27646/104/7

Synthesis and Enzyme Inhibitory Properties of Quinoxaline Bridged Bis(imidazolium) Salts

The title compound was prepared by treatment of N,N’-di(4-picolyl¬amino)ethane with N,N-dimethylformamide dimethylacetal in toluene and it was characterized by elemental analysis, 1H NMR and 13C NMR.

https://www.mdpi.com/1422-8599/2010/1/M649/pdf

İsmail Özdemir

Papers

Synthesis, structure and spectroscopic characterization of 1,2-bis-(2,4,6-trimethylbenzylideneamino)ethanedichloropalladium(II)

Synthesis of [PdBr2(Benzimidazole-2-Ylidene)(Pyridine)] Complexes and Their Catalytic Activity in the Direct C-H Bond Activation of 2-Substituted Heterocycles

Transfer hydrogenation of ketones in the presence of half sandwich ruthenium (II) complexes bearing imidazoline and benzimidazole ligand

Synthesis and Enzyme Inhibitory Properties of Quinoxaline Bridged Bis(imidazolium) Salts

2-(4-Pyridyl)-1,3-di(4-picolyl)imidazolidine