J. Appl. Cryst.の発刊に際して

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.

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

Green Chemistry is a relatively new emerging field that strives to work at the molecular level to achieve sustainability. The field has received widespread interest in the past decade due to its ability to harness chemical innovation to meet environmental and economic goals simultaneously. Green Chemistry has a framework of a cohesive set of Twelve Principles, which have been systematically surveyed in this critical review. This article covers the concepts of design and the scientific philosophy of Green Chemistry with a set of illustrative examples. Future trends in Green Chemistry are discussed with the challenge of using the Principles as a cohesive design system (93 references).

Green Chemistry: Principles and Practice

The site-selective formation of carbon-carbon bonds through direct functionalizations of otherwise unreactive carbon-hydrogen bonds constitutes an economically attractive strategy for an overall streamlining of sustainable syntheses. In recent decades, intensive research efforts have led to the development of various reaction conditions for challenging C-H bond functionalizations, among which transition-metal-catalyzed transformations arguably constitute thus far the most valuable tool. For instance, the use of inter alia palladium, ruthenium, rhodium, copper, or iron complexes set the stage for chemo-, site-, diastereo-, and/or enantioselective C-H bond functionalizations. Key to success was generally a detailed mechanistic understanding of the elementary C-H bond metalation step, which depending on the nature of the metal fragment can proceed via several distinct reaction pathways. Traditionally, three different modes of action were primarily considered for CH bond metalations, namely, (i) oxidative addition with electronrich late transition metals, (ii) σ-bond metathesis with early transition metals, and (iii) electrophilic activation with electrondeficient late transition metals (Scheme 1). However, more recent mechanistic studies indicated the existence of a continuum of electrophilic, ambiphilic, and nucleophilic interactions. Within this continuum, detailed experimental and computational analysis provided strong evidence for novel C-H bond metalationmechanisms relying on the assistance of a bifunctional ligand bearing an additional Lewis-basic heteroatom, such as that found in (heteroatom-substituted) secondary phosphine oxides or most prominently carboxylates (Scheme 1, iv). This novel insight into the nature of stoichiometric metalations has served as stimulus for the development of novel transformations based on cocatalytic amounts of carboxylates, which significantly broadened the scope of C-H bond functionalizations in recent years, with most remarkable progress being made in palladiumor ruthenium-catalyzed direct arylations and direct alkylations. These carboxylate-assisted C-H bond transformations were mostly proposed to proceed via a mechanism in which metalation takes place via a concerted base-assisted deprotonation. To mechanistically differentiate these intramolecular metalations new acronyms have recently been introduced into the literature, such as CMD (concerted metalationdeprotonation), IES (internal electrophilic substitution), or AMLA (ambiphilic metal ligand activation), which describe related mechanisms and will be used below where appropriate. This review summarizes the development and scope of carboxylates as cocatalysts in transition-metal-catalyzed C-H functionalizations until autumn 2010. Moreover, experimental and computational studies on stoichiometric metalation reactions being of relevance to the mechanism of these catalytic processes are discussed as well. Mechanistically related C-H bond cleavage reactions with ruthenium or iridium complexes bearing monodentate ligands are, however, only covered with respect to their working mode, and transformations with stoichiometric amounts of simple acetate bases are solely included when their mechanism was suggested to proceed by acetate-assisted metalation.

Carboxylate-assisted transition-metal-catalyzed C-H bond functionalizations: mechanism and scope.

N-Heterocyclic Carbenes in Late Transition Metal Catalysis

The activation of alkane C−H bonds by oxidative addition and its reverse reaction, reductive elimination, are believed to occur via transient σ-alkane complexes. This Account summarizes how isotope effects can be used to probe the nature of these intermediates and points out some pitfalls in interpreting kinetic data. Comparisons are made with arene C−H activation and other activation systems.

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Isotope effects in C-H bond activation reactions by transition metals.

Alkanes are among the most abundant and unreactive of all organic compounds. Industrial use of these resources often relies upon free-radical activation of carbon-hydrogen bonds, often at high temperatures, thereby limiting the selectivities that can be achieved in any functionalization reaction. Consequently, the selective activation of C-H bonds by homogeneous transition-metal compounds has been a topic that has been of great interest to the organometallic community for many years. In this Account, mechanistic studies with a series of homogeneous rhodium organometallic complexes are summarized that provide for the first time a comparative evaluation of the relative equilibrium constants and rates of reaction for both alkane and arene hydrocarbon activation.

Comparative reactivities of hydrocarbon C-H bonds with a transition-metal complex

Bi-, tri-, and tetracyclic isoquinoline salts were readily synthesized in excellent yields at room temperature from readily available starting materials after three reaction steps. Aromatic C−H activation was first promoted by sodium acetate with [Cp*MCl2]2 (M = Rh, Ir) at room temperature to form cyclometalated compounds. Dimethylacetylenedicarboxylate was then found to insert into the metal−carbon bonds of the cyclometalated compounds. Finally, the insertion compounds underwent oxidative coupling to form the desired isoquinoline salts and regenerate [Cp*MCl2]2. All of the intermediate compounds following C−H activation, alkyne insertion, and oxidative coupling were fully characterized, including the determination of X-ray structures in several cases, and the results shed light on the overall mechanism. Moreover, it was possible to synthesize the isoquinoline salts from readily available starting materials using one-pot procedures; thus, this work provides a novel, efficient method for metal-mediated syn...

An efficient low-temperature route to polycyclic isoquinoline salt synthesis via C-H activation with [Cp*MCl2]2 (M = Rh, Ir).

A well-defined iron complex (3) supported by a bis(phosphino)amine pincer ligand efficiently catalyzes both acceptorless dehydrogenation and hydrogenation of N-heterocycles. The products from these reactions are isolated in good yields. Complex 3, the active catalytic species in the dehydrogenation reaction, is independently synthesized and characterized, and its structure is confirmed by X-ray crystallography. A trans-dihydride intermediate (4) is proposed to be involved in the hydrogenation reaction, and its existence is verified by NMR and trapping experiments.

William D. Jones

Papers

Isotope effects in C-H bond activation reactions by transition metals.

Comparative reactivities of hydrocarbon C-H bonds with a transition-metal complex

An efficient low-temperature route to polycyclic isoquinoline salt synthesis via C-H activation with [Cp*MCl2]2 (M = Rh, Ir).

A Molecular Iron Catalyst for the Acceptorless Dehydrogenation and Hydrogenation of N‑Heterocycles

C−H Activation of Phenyl Imines and 2-Phenylpyridines with [Cp*MCl2]2 (M = Ir, Rh): Regioselectivity, Kinetics, and Mechanism