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

Agostic zirconium.cntdot..cntdot..cntdot.carbon(.beta.)-silicon interactions in Cp'2Zr(CH2CH2SiMe3)(L)+ complexes (L = THF, CH3CN, PMe3). Evidence for an organometallic .gamma.-silicon effect

Sequential one-pot addition of alkylpyrazines, alkynes, and a proton source to a solution of Cp 2 Zr(Me)(THF) + (1) in CH 2 Cl 2 at room temperature affords (E)-alkenyl-substituted alkypyrazines 2-10 in excellent yiels. The regio- and stereolectively observed in these reactions is similar to the observed previously for related early transition metal-mediated reactions and is ascribed to steric and Si electronic effets

Zirconium-mediated reactions of alkylpyrazines and alkynes. Synthesis of highly substituted alkylpyrazines

The tetraaza macrocycle complexes (Me4taen)Zr(OtBu)2 (4) and (Me4taen)Hf(NMe2)2 (5) have been prepared and characterized by X-ray crystallography. Both species adopt cis structures with distorted octahedral metal geometries. These structures are similar to that observed earlier for (Me4taen)Zr(NMe2)2 (3) but quite different from the trigonal prismatic structures observed for (Me4taen)ZrCl2 (1) and (Me4taen)Zr(CH2Ph)2 (2). The structures of these d0 (Me4taen)MX2 species are rationalized on electronic grounds. The conformational properties of the Me4taen2- macrocycle and the covalent character of and absence of a strong π-component in the M−X bonds favor trigonal prismatic structures for 1 and 2, whereas M−X π-bonding in 3−5 promotes distortion toward octahedral structures. Compound 4, C22H40N4O2Zr, crystallizes in the triclinic space group P1 with a = 9.6080(8) A, b = 9.7407(8) A, c = 14.5916(12) A, α = 92.714(1)°, β = 107.808(1)° γ = 99.810(1)°, and Z = 2. Compound 5, C18H34N6Hf·0.5(C7H8), crystallizes i...

Structural Trends in Group 4 Metal Tetraaza Macrocycle Complexes. Molecular Structures of (Me4taen)Zr(OtBu)2 and (Me4taen)Hf(NMe2)2

Cationic-zirconium catalysts for C-C bond forming chemistry

The reaction of the chiral chelated bis-amide complex Zr{(2R,4R)-PhNCHMeCH2CHMeNPh}Cl2(THF)2 (R,R-7) with lithium ansa-bis-indenyl reagents Li2[SBI](Et2O) (8a, SBI = (1-indenyl)2SiMe2) or Li2[EBI](Et2O) (8b, EBI = 1,2-(1-indenyl)2ethane) in THF affords the corresponding ansa-zirconocenes S,S-(SBI)Zr{(2R,4R)-PhNCHMeCH2CHMeNPh} (S,S,R,R-9a) or S,S-(EBI)Zr{(2R,4R)-PhNCHMeCH2CHMeNPh} (S,S,R,R-9b) in >95% isolated yield and >99% enantiomeric excess. Compound 9b was converted to the corresponding enantiomerically pure dichloride S,S-(EBI)ZrCl2 (S,S-10b) in 91% isolated yield by reaction with HCl in Et2O. The chiral diamine (2R,4R)-HPhNCHMeCH2CHMeNHPh (R,R-5) was recovered from this reaction.

Richard F. Jordan

Papers

Agostic zirconium.cntdot..cntdot..cntdot.carbon(.beta.)-silicon interactions in Cp'2Zr(CH2CH2SiMe3)(L)+ complexes (L = THF, CH3CN, PMe3). Evidence for an organometallic .gamma.-silicon effect

Zirconium-mediated reactions of alkylpyrazines and alkynes. Synthesis of highly substituted alkylpyrazines

Structural Trends in Group 4 Metal Tetraaza Macrocycle Complexes. Molecular Structures of (Me4taen)Zr(OtBu)2 and (Me4taen)Hf(NMe2)2

Cationic-zirconium catalysts for C-C bond forming chemistry

Enantioselective Synthesis of ansa-Zirconocenes