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

The increasing demand to produce enantiomerically pure pharmaceuticals, agrochemicals, flavors, and other fine chemicals has advanced the field of asymmetric catalytic technologies.1,2 Among all asymmetric catalytic methods, asymmetric hydrogenation utilizing molecular hydrogen to reduce prochiral olefins, ketones, and imines, have become one of the most efficient methods for constructing chiral compounds.3 The development of homogeneous asymmetric hydrogenation was initiated by Knowles4a and Horner4b in the late 1960s, after the discovery of Wilkinson’s homogeneous hydrogenation catalyst [RhCl(PPh3)3]. By replacing triphenylphosphine of the Wilkinson’s catalystwithresolvedchiralmonophosphines,6Knowles and Horner reported the earliest examples of enantioselective hydrogenation, albeit with poor enantioselectivity. Further exploration by Knowles with an improved monophosphine CAMP provided 88% ee in hydrogenation of dehydroamino acids.7 Later, two breakthroughs were made in asymmetric hydrogenation by Kagan and Knowles, respectively. Kagan reported the first bisphosphine ligand, DIOP, for Rhcatalyzed asymmetric hydrogenation.8 The successful application of DIOP resulted in several significant directions for ligand design in asymmetric hydrogenation. Chelating bisphosphorus ligands could lead to superior enantioselectivity compared to monodentate phosphines. Additionally, P-chiral phosphorus ligands were not necessary for achieving high enantioselectivity, and ligands with backbone chirality could also provide excellent ee’s in asymmetric hydrogenation. Furthermore, C2 symmetry was an important structural feature for developing new efficient chiral ligands. Kagan’s seminal work immediately led to the rapid development of chiral bisphosphorus ligands. Knowles made his significant discovery of a C2-symmetric chelating bisphosphine ligand, DIPAMP.9 Due to its high catalytic efficiency in Rh-catalyzed asymmetric hydrogenation of dehydroamino acids, DIPAMP was quickly employed in the industrial production of L-DOPA.10 The success of practical synthesis of L-DOPA via asymmetric hydrogenation constituted a milestone work and for this work Knowles was awarded the Nobel Prize in 2001.3k This work has enlightened chemists to realize * Corresponding author. 3029 Chem. Rev. 2003, 103, 3029−3069

New Chiral Phosphorus Ligands for Enantioselective Hydrogenation

The Golden Age of Transfer Hydrogenation

This review describes a multidimensional treatment of molecular recognition phenomena involving aromatic rings in chemical and biological systems. It summarizes new results reported since the appearance of an earlier review in 2003 in host-guest chemistry, biological affinity assays and biostructural analysis, data base mining in the Cambridge Structural Database (CSD) and the Protein Data Bank (PDB), and advanced computational studies. Topics addressed are arene-arene, perfluoroarene-arene, S⋅⋅⋅aromatic, cation-π, and anion-π interactions, as well as hydrogen bonding to π systems. The generated knowledge benefits, in particular, structure-based hit-to-lead development and lead optimization both in the pharmaceutical and in the crop protection industry. It equally facilitates the development of new advanced materials and supramolecular systems, and should inspire further utilization of interactions with aromatic rings to control the stereochemical outcome of synthetic transformations.

Aromatic rings in chemical and biological recognition: energetics and structures.

Fundamentals of H2 Binding and Reactivity on Transition Metals Underlying Hydrogenase Function and H2 Production and Storage

19F and 31P NMR evidence for silver hexafluorophosphate hydrolysis in solution. New palladium difluorophosphate complexes and x-ray structure determination of [Pd(.eta.3-2-Me-C3H4)(PO2F2)(PCy3)]

The ligands 2-pyridin-2-yl-1H-benzimidazole (HL1), 1-methyl-2-pyridin-2-ylbenzimidazole (HL2), and 2-(1H-imidazol-2-yl)pyridine (HL3) and the proligand 2-phenyl-1H-benzimidazole (HL4) have been used to prepare five different types of new ruthenium(II) arene compounds: (i) monocationic complexes with the general formula [(η6-arene)RuCl(κ2-N,N-HL)]Y [HL = HL1, HL2, or HL3; Y = Cl or BF4; arene = 2-phenoxyethanol (phoxet), benzene (bz), or p-cymene (p-cym)]; (ii) dicationic aqua complexes of the formula [(η6-arene)Ru(OH2)(κ2-N,N-HL1)](Y)2 (Y = Cl or TfO; arene = phoxet, bz, or p-cym); (iii) the nucleobase derivative [(η6-arene)Ru(9-MeG)(κ2-N,N-HL1)](PF6)2 (9-MeG = 9-methylguanine); (iv) neutral complexes consistent with the formulation [(η6-arene)RuCl(κ2-N,N-L1)] (arene = bz or p-cym); (v) the neutral cyclometalated complex [(η6-p-cym)RuCl(κ2-N,C-L4)]. The cytototoxic activity of the new ruthenium(II) arene compounds has been evaluated in several cell lines (MCR-5, MCF-7, A2780, and A2780cis) in order to est...

Derivation of Structure–Activity Relationships from the Anticancer Properties of Ruthenium(II) Arene Complexes with 2-Aryldiazole Ligands

The reaction of [NBun4][AuR2](R = C6F5 or C6F3H2-2,4,6) with Ag[ClO4] leads to complexes [{AuAgR2}n], which react with neutral ligands to give complexes [{AuAgR2L}n](L = neutral O-, N-, S- or P-donor ligand, alkene, or alkyne). For R = C6F5 and L = diphenylacetylene, the product is [{AuAgR2·0.5L}n]; the 0.5L can be displaced by other ligands, such as acetone, arenes, or alkenes, to reform [{AuAgR2L}n]. An X-ray diffraction study of [{AuAgR2L}n](R = C6F5, L = tetrahydrothiophene) reveals (AuAg)2 rings with Au–Ag 2.726 and 2.718 A(the first reported Au–Ag bond lengths), linked by Au ⋯ Au short contacts (2.889 A) to form infinite metal-atom chains. This complex crystallizes in space group Pccn, with a= 11.185(3), b= 22.475(6), c= 14.802(4)A, Z= 8, R= 0.041 for 2 005 reflections. The complex [{AuAgR2L}n](R = C6F5, L = benzene) crystallizes in space group C2/c, with a= 24.231(5), b= 7.570(1), c= 22.613(5)A, β= 117.49(2)°, Z= 8, and R= 0.035 for 3 008 reflections; it shows the same type of metal-atom chain (Au ⋯ Au 3.013; Au–Ag 2.702 and 2.792 A). The benzene ring is co-ordinated by one edge to silver. In both structures the gold atoms lie on a crystallographic two-fold axis.

Synthesis and reactivity of bimetallic Au–Ag polyfluorophenyl complexes; crystal and molecular structures of [{AuAg(C6F5)2(SC4H8)}n] and [{AuAg(C6F5)2(C6H6)}n]

A new and safer methodology has been developed for the synthesis of bis(pyrazol-1-yl)methane ligands (NN). Several ligands containing different phenyl groups on the central carbon atom have been obtained. Ruthenium derivatives of the type [Ru(arene)Cl(NN)]BPh4 (arene = benzene, p-cymene) have been synthesised using these ligands. One or two isomers that differ regarding the axial or equatorial disposition of the phenyl group on the metallacycle have been obtained. Their formation is rationalised by considering steric effects. The structures of five derivatives were determined by X-ray diffraction. In four complexes the phenyl substituent is in the axial disposition of the metallacycle and in one case in the equatorial orientation. The dihedral angle formed by the planes of the two pyrazole rings is always bigger for the complexes containing unsubstituted pyrazolyl heterocycles. The behaviour of the new derivatives in the transfer hydrogenation of benzophenone in the presence of KOH was studied. The benzene derivatives showed higher activity than the p-cymene complexes. A marked and positive effect of the methyl groups on the pyrazolyl rings was observed. The effect of the substituents on the benzyl carbon atom was also important. It has been observed that the benzophenone hydrogenation was possible without the addition of complexes. The effect of the KOH concentration was evaluated and a concentration that leads to negligible conversion in a base-only process was chosen. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

(Arene)ruthenium(II) Complexes Containing Substituted Bis(pyrazolyl)methane Ligands – Catalytic Behaviour in Transfer Hydrogenation of Ketones

The synthesis and full characterization of the new aqua-complex [(η6-p-cymene)Ru(OH2)(κ2-N,N-2-pydaT)](BF4)2, [2](BF4)2, and the nucleobase derivative [(η6-p-cymene)Ru(9-MeG)(κ2-N,N-2-pydaT)](BF4)2, [4](PF6)2, where 2-pydaT = 2,4-diamino-6-(2-pyridyl)-1,3,5-triazine and 9-MeG = 9-methylguanine, are reported here. The crystal structures of both [4](PF6)2 and the chloro complex [(η6-p-cymene)RuCl(κ2-N,N-2-pydaT)](PF6), [1](PF6), have been elucidated by X-ray diffraction. The former provided relevant information regarding the interaction of the metallic fragment [(η6-p-cymene)Ru(κ2-N,N-2-pydaT)]2+ and a simple model of DNA. NMR and kinetic absorbance studies have proven that the aqua-complex [2](BF4)2 binds to the N7 site of guanine in nucleobases, nucleotides, or DNA. A stable bifunctional interaction (covalent and partially intercalated) between the [(η6-p-cymene)Ru(κ2-N,N-2-pydaT)]2+ fragment and CT-DNA has been corroborated by kinetic, circular dichroism, viscometry, and thermal denaturation experiments....

Blanca R. Manzano

Papers

19F and 31P NMR evidence for silver hexafluorophosphate hydrolysis in solution. New palladium difluorophosphate complexes and x-ray structure determination of [Pd(.eta.3-2-Me-C3H4)(PO2F2)(PCy3)]

Derivation of Structure–Activity Relationships from the Anticancer Properties of Ruthenium(II) Arene Complexes with 2-Aryldiazole Ligands

Synthesis and reactivity of bimetallic Au–Ag polyfluorophenyl complexes; crystal and molecular structures of [{AuAg(C6F5)2(SC4H8)}n] and [{AuAg(C6F5)2(C6H6)}n]

(Arene)ruthenium(II) Complexes Containing Substituted Bis(pyrazolyl)methane Ligands – Catalytic Behaviour in Transfer Hydrogenation of Ketones

Anticancer activity and DNA binding of a bifunctional Ru(II) arene aqua-complex with the 2,4-diamino-6-(2-pyridyl)-1,3,5-triazine ligand.