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

This chapter presents a wide variety of tridentate pincer ligands, the chemistry that they enable to take place at a metal center, and the catalytic reactions they can perform Tridentate pincer ligands have shown a long period of development and an explosive period of exploitation The future of further developments of the usage of these ligand scaffolds also is bright and lucrative While predictions of specific applications are speculative, it is clear that pincer ligands will have a strong impact on future research The ability to produce such a broad spectrum of ligand properties is unparalleled compared to other ligand systems The chapter concludes that pincer ligands will act as anchors for the future of organometallic chemistry

Perspective and prospects for pincer ligand chemistry

Substituted bisindolines were synthesized from the reaction of 3,5-dimethoxyaniline and vicinal diones (2,3-butanedione, benzil, and 4-methylbenzil). The reaction of 3,5-dimethoxyaniline with 2,3-butanedione under polar acidic conditions proceeded with an unanticipated double cyclization reaction forming 2,2′-dimethyl-5,5′,7,7′-tetramethoxybisindole, (+/−)-1. The reaction of 3,5-dimethoxyaniline with benzil under similar conditions did not form a bisindoline, but instead formed cis-1,2-diphenyl-2-[(3,5-dimethoxyphenyl)imino]ethanone, 2. When the solvent, acid, and reaction temperature were changed for the reaction of 3,5-dimethoxyaniline with benzil, 3,3-diphenyl-4,6-dimethoxyindolin-2-one, 3 (an isomer of 2), and 2,2′-diphenyl-5,5′,7,7′-tetramethoxybisindole, (+/−)-4, were isolated as major and minor products, respectively. Similarly, the reaction of 3,5-dimethoxyaniline with 4-methylbenzil produced a 1 : 1 mixture of cis-1-(4-methylphenyl)-2-phenyl-2-[(3,5-dimethoxyphenyl)-imino]ethanone, 6a, and cis-1-(phenyl)-2-(4-methylphenyl)-2-[(3,5-dimethoxyphenyl)imino]ethanone, 6b, under polar acidic conditions and 3-(4-methylphenyl)-3-phenyl-4,6-dimethoxyindolin-2-one, (+/−)-7 and 2-(4-methylphenyl)-2′-phenyl-5,5′,7,7′-tetramethoxybisindole, (+/−)-8, under apolar acidic conditions at elevated temperature. We propose the mechanism of bisindoline and indolinone formation to include a Friedel–Crafts type electrophilic attack on the electron rich aryl ring at the ring closing C–C bond forming steps, and further propose a common carbocationic monocyclized intermediate.

Bisindolines from the Reaction of 3,5‐Dimethoxyaniline with Vicinal Diones.

The reaction of (C 5 Me 5 )Rh(PMe 3 )(Ph)H with thiophene leads to the elimination of benzene and oxidative addition of the thiophene C-S bond across the Rh(I) center, giving (C 5 Me 5 )Rh(PMe 3 )(SCH=CHCH=CH). Similar reactions occur with 2-methylthiophene, 3-methylthiophene, 2,5-dimethylthiophene, benzothiophene, and dibenzothiophene. Selectivity studies performed with these complexes are consistent with the coordination of sulfur to rhodium prior to C-S bond cleavage. Reversible reductive elimination of thiophene occurs at ∼80°C. The diene portion of the C-S insertion ligand undergoes a Diels-Alder reaction with dimethyl acetylenedicarboxylate to give dimethyl phthalate as a major product

Insertion of Rhodium into the Carbon‐Sulfur Bond of Thiophene. Mechanism of a Model for the Hydrodesulfurization Reaction.

Chemical and Electrochemical Oxidation of CpRe(PAr3)2H2 Complexes to Give Stable 17-Electron Radical Cations. Disproportionation to Diamagnetic Species via Electron-Transfer Catalysis.

VV2Wlo0405and VV,W90406spectra until or unless individual isomers can be separated and identified by 2-D W NMR. We can however extrapolate from our assignments for the 1,4and 1,4,9-isomers to estimate the chemical shifts of the currently unknown VVWlIO.,,,": V(tetrahedral), -564 ppm; V(octahedral), -530 ppm. The 1,4,9-isomer of PV3Wg040G should have 6 near -500 ppm, i.e. -25 ppm downfield from 1,4-PV2W,o0405-,"7 and this has recently been confirmed by Domaille and W a t ~ n y a . ~ * Chemical Shifts and ESR g Values. Some years ago Kazanskii@ reported a linear correlation between 6(s'V) of four tungstovanadates, v w 5 O l g 3 , PVMo110404-, PVW110404-, and H2VW I 10407-, and ( g ) for the corresponding anions reduced by one electron to contain vanadium(1V). Both paramagnetic nuclear shielding of Vv and the g value of VIv are dependent upon excitation energies and orbital angular momentum, and changes in (g) and 6 might be expected to be roughly parallel. A plot of all currently available data for polyanions that contain a single vanadium (Figure 12) shows that there is some correlation but with considerable ~cat ter .~ ' . There is no correlation for ions containing two or more vanadiums, nor is there a correlation between 6(IE3W) for dodecatungstates and (g) for the corresponding one-electron heteropoly blues.49

William D. Jones

Papers

Perspective and prospects for pincer ligand chemistry

Bisindolines from the Reaction of 3,5‐Dimethoxyaniline with Vicinal Diones.

Insertion of Rhodium into the Carbon‐Sulfur Bond of Thiophene. Mechanism of a Model for the Hydrodesulfurization Reaction.

Chemical and Electrochemical Oxidation of CpRe(PAr3)2H2 Complexes to Give Stable 17-Electron Radical Cations. Disproportionation to Diamagnetic Species via Electron-Transfer Catalysis.

Preparation and Structural Examination of a Series of New, Low-Valent Iron Phosphine Isocyanide Complexes with Bent C-N-C Linkages.