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

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The reaction of IrX(CO)(dppe) (X=Br, I; dppe=1,2-bis(diphenylphosphino)ethane) with Cp 2 TaH 3 (Cp=η 5 -cyclopentadienyl) is extremely rapid and leads to clean formation of fac-IrH 3 (CO)(dppe) and Cp 2 TaX, or Cp 2 TaXL (L=CO, C 2 H 4 , C 3 H 7 C≡CC 3 H 7 ) in the presence of added ligand. Trapping and isotope-labeling experiments indicate that the reaction does not proceed through production of free IrH 2 X(CO)(dppe), [IrH(CO)(dppe)], or [Cp 2 TaH]. The results are consistent with formation of unstable hybride and halide-bridged binuclear intermediates, in which transfer of all hybride and halide ligands occurs before fragmentation into mononuclear species

Hydride abstraction. The reaction of [bis(diphenylphosphino)ethane]carbonylhaloiridium with bis(cyclopentadienyl)trihydridotantalum

: The oxidative addition of H2 to IrBr(CO) (doppe), 2, (dppe - 1,2-bis(diphenyphosphino)ethane) yields a kinetic dihydride species, 3, which then isomerizes to a more stable isomer 4. This isomerizatoin of 3 to 4 has been studied kinetically as a function of initial H2 pressure. Two pathways are operative at ambient temperature. The first is a reductive elimation/oxidative addition sequence which is first order in complex, while the second is a bimolecular pathway involving dihydride transfer from 3 to 20 to produce 4 and regenerate 2. The dihydride transfer pathway is second order in complex and becomes the dominant isomerizatoin mechanism when less than one equivalent of H2 relative to 2 has been added to the system. All of the kinetic data have been fit to a complete rate law which leads to a bimolecular rate constant for dihydride transfer of 0.21/M.min. Below -20C, the dhydride transfer pathway for isomerization is the only one operating.

Dihydride transfer: a bimolecular mechanism in the isomerization of cis-dihydridobromo(carbonyl)[bis(diphenylphosphino)ethane]iridium, IrH2Br(CO)(dppe)

The photochemical activation of the C(sp)-C(sp2) bond in Pt(0)-η2-aryl-phosphaalkyne complexes leads selectively to coordination compounds of the type LnPt(aryl)(C≡P). The oxidative addition reaction is a novel, clean, and atom-economic route for the synthesis of reactive terminal Pt(II)-cyaphido complexes, which can undergo [3 + 2] cycloaddition reactions with organic azides, yielding the corresponding Pt(II)-triazaphospholato complexes. The C-C bond cleavage reaction is thermodynamically uphill. Upon heating, the reverse and quantitative reductive elimination toward the Pt(0)-phosphaalkyne-π-complex is observed.

Photochemical C(sp)-C(sp2) Bond Activation in Phosphaalkynes: A New Route to Reactive Terminal Cyaphido Complexes LnM-C≡P.

Selective carbon-carbon bond formation plays a key role in organic synthesis. Existing methods are not perfect, however, and elaborate synthesis routes may be needed for a seemingly simple addition. In his Perspective,
 Jones
 highlights the report by
 Cho
 et al ., who describe an important new route to carbon-carbon bond formation between unsaturated carbon centers. The reactions are quite general and selective and are likely to find widespread application in organic synthesis.

William D. Jones

Papers

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Hydride abstraction. The reaction of [bis(diphenylphosphino)ethane]carbonylhaloiridium with bis(cyclopentadienyl)trihydridotantalum

Dihydride transfer: a bimolecular mechanism in the isomerization of cis-dihydridobromo(carbonyl)[bis(diphenylphosphino)ethane]iridium, IrH2Br(CO)(dppe)

Photochemical C(sp)-C(sp2) Bond Activation in Phosphaalkynes: A New Route to Reactive Terminal Cyaphido Complexes LnM-C≡P.

The Key to Successful Organic Synthesis Is...