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.

1. Advantages of Chemical Redox Agents 878 2. Disadvantages of Chemical Redox Agents 879 C. Potentials in Nonaqueous Solvents 879 D. Reversible vs Irreversible ET Reagents 879 E. Categorization of Reagent Strength 881 II. Oxidants 881 A. Inorganic 881 1. Metal and Metal Complex Oxidants 881 2. Main Group Oxidants 887 B. Organic 891 1. Radical Cations 891 2. Carbocations 893 3. Cyanocarbons and Related Electron-Rich Compounds 894

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Chemical Redox Agents for Organometallic Chemistry

Once considered the 'holy grail' of organometallic chemistry, synthetically useful reactions employing C-H bond activation have increasingly been developed and applied to natural product and drug synthesis over the past decade. The ubiquity and relative low cost of hydrocarbons makes C-H bond functionalization an attractive alternative to classical C-C bond forming reactions such as cross-coupling, which require organohalides and organometallic reagents. In addition to providing an atom economical alternative to standard cross - coupling strategies, C-H bond functionalization also reduces the production of toxic by-products, thereby contributing to the growing field of reactions with decreased environmental impact. In the area of C-C bond forming reactions that proceed via a C-H activation mechanism, rhodium catalysts stand out for their functional group tolerance and wide range of synthetic utility. Over the course of the last decade, many Rh-catalyzed methods for heteroatom-directed C-H bond functionalization have been reported and will be the focus of this review. Material appearing in the literature prior to 2001 has been reviewed previously and will only be introduced as background when necessary. The synthesis of complex molecules from relatively simple precursors has long been a goal for many organic chemists. The ability to selectively functionalize a molecule with minimal pre-activation can streamline syntheses and expand the opportunities to explore the utility of complex molecules in areas ranging from the pharmaceutical industry to materials science. Indeed, the issue of selectivity is paramount in the development of all C-H bond functionalization methods. Several groups have developed elegant approaches towards achieving selectivity in molecules that possess many sterically and electronically similar C-H bonds. Many of these approaches are discussed in detail in the accompanying articles in this special issue of Chemical Reviews. One approach that has seen widespread success involves the use of a proximal heteroatom that serves as a directing group for the selective functionalization of a specific C-H bond. In a survey of examples of heteroatom-directed Rh catalysis, two mechanistically distinct reaction pathways are revealed. In one case, the heteroatom acts as a chelator to bind the Rh catalyst, facilitating reactivity at a proximal site. In this case, the formation of a five-membered metallacycle provides a favorable driving force in inducing reactivity at the desired location. In the other case, the heteroatom initially coordinates the Rh catalyst and then acts to stabilize the formation of a metal-carbon bond at a proximal site. A true test of the utility of a synthetic method is in its application to the synthesis of natural products or complex molecules. Several groups have demonstrated the applicability of C-H bond functionalization reactions towards complex molecule synthesis. Target-oriented synthesis provides a platform to test the effectiveness of a method in unique chemical and steric environments. In this respect, Rh-catalyzed methods for C-H bond functionalization stand out, with several syntheses being described in the literature that utilize C-H bond functionalization in a key step. These syntheses are highlighted following the discussion of the method they employ.

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Rhodium-Catalyzed C-C Bond Formation via Heteroatom-Directed C-H Bond Activation

Over the past several years, cerium oxide and CeO2-containing materials have come under intense scrutiny as catalysts and as structural and electronic promoters of heterogeneous catalytic reactions. Recent developments regarding the characterization of ceria and CeO2-containing catalysts are critically reviewed with a special focus towards catalyst interaction with small molecules such as hydrogen, carbon monoxide, oxygen, and nitric oxide. Relevant catalytic and technological applications such as the use of ceria in automotive exhaust emission control and in the formulation of SO x reduction catalysts is described. A survey of the use of CeO2-containing materials as oxidation and reduction catalysts is also presented.

Catalytic Properties of Ceria and CeO2-Containing Materials

Low Molecular Mass Gelators of Organic Liquids and the Properties of Their Gels

Pentamethylcyclopentadienylrhodium and -iridium halides. I. Synthesis and properties

Metallomesogens, metal complexes of organic ligands which exhibit liquid crystalline (mesomorphic) character, combine the variety and range of metal-based coordination chemistry with the extraordinary physical properties exhibited by liquid crystals Thermotropic metallomesogens have been made incorporating many metals, including representatives of s-, p-, d-and even f-block elements Both rodlike (calamitic) and disklike (discotic) thermotropic metallomesogens are known, and examples of all the main mesophase types are found Many different varieties of ligand can be used: monodentate (4-substituted pyridines), bidentate (β-diketonates, dithiolenes, carboxylates, cyclometalated aromatic amines), or polydentate (phthalocyanines, porphyrins) As with organic mesogens, molecular shape and intermolecular forces play an important role, ie the ligands are important in determining mesophase character The chief requirement for a metallomesogen is a rigid core, usually unsaturated and either rod- or disklike in shape, bearing several long hydrocarbon tails The metal atom is usually at or near the center of gravity of the molecule In some cases the ligands are themselves mesogenic, but this is not a requirement The presence of one or more metals opens many exciting possibilities: new shapes, not easily generated by organic compounds, and hence new properties are then accessible The incorporation of d-block metals brings with it features such as color and paramagnetism Profound effects arise from the large and polarizable concentration of electron density that every metal atom possesses, since the molecular polarizability is a key factor in determining whether a molecule will form liquid crystals Enhanced physical properties (eg high birefringencies), as well as new and unexpected ones, will result A major requirement for metallomesogens to find applications in new device technology is that the metal–ligand bonds are strong and inert and the complexes stable; this can be accomplished with, for example, chelating ligands and the 5d metals

Metallomesogens: Metal Complexes in Organized Fluid Phases

Calamitic metallomesogens : metal-containing liquid crystals with rodlike shapes

(η5-Pentamethylcyclopentadienyl)Rhodium and -Iridium Compounds

Monsanto initiated development of its rhodium- and iodide-catalysed process for the carbonylation of methanol to acetic acid in 1966. Ownership of the technology was acquired in 1986 by BP Chemicals who have further extended it. The work of the Sheffield group, in developing a deeper understanding of the mechanism of the process, is reviewed. The rate-determining step in the rhodium–iodide catalysed reaction is the oxidative addition of methyl iodide to [Rh(CO)2I2]–1a: the product from this reaction, the reactive intermediate [MeRh(CO)2I3]–2a has been detected and fully characterised spectroscopically. The rates of the reversible reactions linking 1a, 2a and the acetyl complex [(MeCO)Rh(CO)I3]–3a, as well as activation parameters for several of the processes involved, have been measured. The efficiency of methanol carbonylation arises primarily from rapid conversion of 2a into 3a, leading to a low standing concentration of 2a, and minimising side reactions such as methane formation. By contrast, in the iridium-catalysed carbonylation, for which similar cycles can be written, the reaction of [MeIr(CO)2I3]–2b with CO to give [(MeCO)Ir(CO)2I3]–4b is rate determining. Model studies show that while kRh/kIr is ca. 1 : 150 for the oxidative addition, it is ca. 105–106 : 1 for migratory CO insertion. The migratory insertion for iridium can be substantially accelerated by adding either methanol or a Lewis acid (SnI2); both appear to facilitate substitution of an iodide ligand by CO, resulting in easier methyl migration. The carbonylation of higher alcohols (ROH) has also been successfully modelled; the corresponding alkyl iodides (RI) react much more slowly than MeI with [M(CO)2I2]–, but again give the acyls [(RCO)M(CO)I3]– for M = Rh and the alkyls [RM(CO)2I3]– for M = Ir. The greater stability of [MeIr(CO)2I3]– compared with [MeRh(CO)2I3]– accounts for the very different characters of the reactions catalysed by the two metals. It is suggested that the broad features of the Rh/Ir reactivities can be rationalised since the M–C bond to a 5d metal is generally stronger than that to the corresponding 4d metal; thus if metal–ligand bond making plays a key role in a step, then the 5d metal is more likely to react faster (e.g. in the oxidative addition), but if a metal–ligand bond-weakening or -breaking step plays a key role in a process (e.g. in the migration), it is likely that the 4d metal will be faster.

Peter M. Maitlis

Papers

Pentamethylcyclopentadienylrhodium and -iridium halides. I. Synthesis and properties

Metallomesogens: Metal Complexes in Organized Fluid Phases

Calamitic metallomesogens : metal-containing liquid crystals with rodlike shapes

(η5-Pentamethylcyclopentadienyl)Rhodium and -Iridium Compounds

Methanol carbonylation revisited: thirty years on.