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

Isotope effects in arene carbon-hydrogen bond activation by [(C5Me5)Rh(PMe3)]

Catalytic C−C Bond Activation in Biphenylene and Cyclotrimerization of Alkynes: Increased Reactivity of P,N- versus P,P-Substituted Nickel Complexes

Cp*2ZrH2 (1) (Cp* = pentamethylcyclopentadienyl) reacts with vinylic carbon−fluorine bonds of CF2CH2 and 1,1-difluoromethylenecyclohexane (CF2C6H10) to afford Cp*2ZrHF (2) and hydrodefluorinated products. Experimental evidence suggests that an insertion/β-fluoride elimination mechanism is occurring. Complex 1 reacts with allylic C−F bonds of the olefins, CH2CHCF3, CH2CHCF2CF2CF2CF3, and CH2C(CF3)2 to give preferentially 2 and CH3−CHCF2, CH3−CHCF−CF2CF2CF3, and CF2C(CF3)(CH3), respectively, by insertion/β-fluoride elimination. In the reactions of 1 with CH2CHCF3 and CH2CHCF2CF2CF2CF3, both primary and secondary alkylzirconium olefin insertion intermediates were observed in the 1H and 19F NMR spectra at low temperature. A deuterium labeling study revealed that more than one olefin-dihydride complex is likely to exist prior to olefin insertion. In the presence of excess 1 and H2, CH2CHCF3 and CH2CHCF2CF2CF2CF3 are reduced to propane and (E)-CH3CH2CFCFCF2CF3, respectively.

Mechanism of Vinylic and Allylic Carbon−Fluorine Bond Activation of Non-Perfluorinated Olefins Using Cp*2ZrH2

C−C Activation in Biphenylene. Synthesis, Structure, and Reactivity of (C5Me5)2M2(2,2‘-biphenyl) (M = Rh, Co)

Here, we report an additive-free catalytic system for hydrogenation of carboxylic acid esters to alcohols with a well-defined cobalt pincer catalyst precursor. Various substrates, including methyl, ethyl, and benzyl esters, have been evaluated under hydrogenation conditions; however, methyl esters have low reactivity compared to those of the corresponding ethyl and benzyl esters. The biomass-derived γ-valerolactone successfully formed 1,4-pentanediol with a turnover number of 3890 with this system. Metal–ligand cooperativity is probed with the related [PN(Me)P] derivative of the cobalt catalyst, and the results suggest a nonbifunctional hydrogenation mechanism.

William D. Jones

Papers

Isotope effects in arene carbon-hydrogen bond activation by [(C5Me5)Rh(PMe3)]

Catalytic C−C Bond Activation in Biphenylene and Cyclotrimerization of Alkynes: Increased Reactivity of P,N- versus P,P-Substituted Nickel Complexes

Mechanism of Vinylic and Allylic Carbon−Fluorine Bond Activation of Non-Perfluorinated Olefins Using Cp*2ZrH2

C−C Activation in Biphenylene. Synthesis, Structure, and Reactivity of (C5Me5)2M2(2,2‘-biphenyl) (M = Rh, Co)

Additive-Free Cobalt-Catalyzed Hydrogenation of Esters to Alcohols