J. Appl. Cryst.の発刊に際して

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

Late-metal catalysts for ethylene homo- and copolymerization.

N-Heterocyclic Carbenes in Late Transition Metal Catalysis

Publisher Summary This chapter discusses the chemistry of cationic dicyclopentadienyl group 4 metal–alkyl complexes. Neutral, d 0 group 4 metal alkyl complexes of general form Cp 2 M(R) 2 (R = H, hydrocarbyl) or Cp 2 M(R)(X) (X = anionic, two–electron donor) comprise the most extensively studied class of group 4 organometallics (I). For many years, it has been suspected that cationic d 0 metal–alkyl species Cp 2 M(R) + are involved in metallocene-based Ziegler–Natta olefin polymerization catalyst systems of the general type Cp 2 MX 2 /AIR n X 3 – n . Several key advances in the early 1980s generated renewed interest in the proposal that Cp 2 M(R) + ions are active species in the soluble catalyst systems. A simple but key point in the development of this chemistry was the realization that noncoordinating, nonreactive counterions, such as BPh 4 – are required for the isolation of stable salts. In 1976, Kaminsky reported the isolation of [(C 5 H 5 ) 2 Zr(CH 2 C(H)– (AIEt 2 ) 2 )][C 5 H 5 ] from the reaction of (C 5 H 5 ) 4 Zr and AIEt 3 . Cationic Cp 2 M(R)(L) + complexes exhibit a variety of interesting structural features, which result from the high Lewis acidity of the cationic d 0 metal centers. The tetrahydrofuran (THF) ligands in these complexes are labile and generally undergo rapid (nuclear magnetic resonance (NMR) time scale) exchange with free THF by dissociative or associative mechanisms at ambient temperature. Cationic, d 0 Cp 2 M(R)(L) + and base-free Cp 2 M(R) + complexes are easily prepared from readily available Cp 2 M(R) 2 compounds. There is strong evidence that Cp 2 M(R) + ions are the active species in Cp 2 MX 2 -based Ziegler–Natta olefin polymerization catalysts.

Chemistry of Cationic Dicyclopentadienyl Group 4 Metal-Alky I Complexes

Ligands, Lewis bases that coordinate to the metal center in a complex, can completely change the catalytic behavior of the metal center. In this Account, we summarize new reactions enabled by a single class of ligands, phosphine–sulfonates (ortho-phosphinobenzenesulfonates). Using their palladium complexes, we have developed four unusual reactions, and three of these have produced novel types of polymers.In one case, we have produced linear high-molecular weight polyethylene, a type of polymer that group 10 metal catalysts do not typically produce. Secondly, complexes using these ligands catalyzed the formation of linear poly(ethylene-co-polar vinyl monomers). Before the use of phosphine–sulfonate catalysts, researchers could only produce ethylene/polar monomer copolymers that have different branched structures rather than linear ones, depending on whether the polymers were produced by a radical polymerization or a group 10 metal catalyzed coordination polymerization. Thirdly, these phosphine–sulfonate ca...

Richard F. Jordan

Papers

Chemistry of Cationic Dicyclopentadienyl Group 4 Metal-Alky I Complexes

Ortho-phosphinobenzenesulfonate: a superb ligand for palladium-catalyzed coordination-insertion copolymerization of polar vinyl monomers.

Ethylene polymerization by a cationic dicyclopentadienyl zirconium(IV) alkyl complex

Cationic Aluminum Alkyl Complexes Incorporating Amidinate Ligands. Transition-Metal-Free Ethylene Polymerization Catalysts

Synthesis and Structures of Mono- and Bis(amidinate) Complexes of Aluminum