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

A century after the publication of Max Weber's The Protestant Ethic and the "Spirit" of Capitalism, a major new work examines network-based organization, employee autonomy and post-Fordist horizontal work structures.

The New Spirit of Capitalism

Gold salts and complexes have emerged in the past few years as the most powerful catalysts for electrophilic activation of alkynes toward a variety of nucleophiles under homogeneous conditions. In a simplified form, nucleophilic attack on the [AuL]-activated alkyne proceeds via π complexes 1 to give trans-alkenyl gold complexes of type 2 as intermediates (Scheme 1). This type of coordination is also a common theme in gold-catalyzed cycloisomerizations of enynes, in which the alkene function acts as the nucleophile. In the reaction of enynes with complexes of other transition metals, an Alder-ene cycloisomerization can take place by simultaneous coordination of the alkyne and the alkene to the metal followed by an oxidative cyclometalation. In contrast, this process does not occur for gold(I) since oxidative addition processes are not facile for this metal. 6 In addition, the [AuL] fragment, which is isolobal to H and HgL, adopts a linear coordination and binds to either the alkene or the alkyne. Thus, cycloisomerizations of enynes catalyzed by gold proceed by an initial coordination of the metal to the alkyne, and as illustrated in Scheme 2, the resulting complex 3 reacts with the alkene by either the 5-exo-dig or 6-endo-dig pathway to form the exoor endocyclopropyl gold carbene 4 or 5, respectively, as has been established with other electrophilic transition-metal complexes or halides MXn as catalysts. The proposed involvement of cyclopropyl metal carbenes of type 4 in the electrophilic activation of enynes by transition metals was first substantiated in reactions catalyzed by Pd(II), in which the initially formed cyclopropyl palladium carbenes undergo [4 + 2] cycloaddition with the double bond of the conjugate enyne. Strong evidence for the existence of cyclopropyl metal carbenes as intermediates was also obtained in the reaction of enynes bearing additional double bonds at the alkenyl chain with Ru(II) and Pt(II) catalysts. In these reactions, the cyclopropyl metal carbenes are trapped intramolecularly by the terminal alkene to give tetracycles containing two cyclopropanes. Gold(I) complexes usually surpass the reactivity shown by Pt(II) and other electrophilic metal salts and complexes for the activation of enynes. They are highly reactive yet uniquely selective Lewis acids that have a high affinity for π bonds. This high π-acidity is linked to relativistic effects, which reach a maximum in the periodic table with gold. However, on occasion, the stronger Lewis acidity of gold complexes can be detrimental in terms of selectivity and because of their low tolerance to certain functional groups. In these instances, the less-strongly Lewis acidic Pt(II) complexes could be the catalysts of choice. * To whom correspondence should be addressed. E-mail: aechavarren@ iciq.es. † Additional affiliation: Departamento de Quı́mica Orgánica, Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid, Spain. Scheme 1 Chem. Rev. 2008, 108, 3326–3350 3326

Gold-catalyzed cycloisomerizations of enynes: a mechanistic perspective.

The Golden Age of Transfer Hydrogenation

Palladium-catalyzed carbonylation reactions of aromatic halides in the presence of various nucleophiles have undergone rapid development since the pioneering work of Heck and co-workers in 1974, such that nowadays a plethora of palladium catalysts are available for different carbonylative transformations. The carboxylic acid derivatives, aldehydes, and ketones prepared in this way are important intermediates in the manufacture of dyes, pharmaceuticals, agrochemicals, and other industrial products. In this Review, the recent academic developments in this area and the first industrial processes are summarized.

Palladium-catalyzed carbonylation reactions of aryl halides and related compounds.

Ruthenium(II) Complexes Containing Optically Active Hemilabile P,N,O-Tridentate Ligands. Synthesis and Evaluation in Catalytic Asymmetric Transfer Hydrogenation of Acetophenone by Propan-2-ol

Ethyl-, n-propyl- and [(trimethylsilyl)methyl]niobium (4e)-alkyne complexes Tp*Nb(Cl)(CH2R)(PhC⋮CR‘) (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate; R = Me, R‘ = Me (2a), Et (2b), n-Pr (2c); R = Et, R‘ = Me (3a), Et (3b); R = SiMe3, R‘ = Me (4a)) exhibit α-agostic structures in solution (1H and 13C NMR) and in the solid state (X-ray structure at 173 K for 4a). The phenylpropyne complexes 2a, 3a, and 4a undergo a thermally induced rearrangement which yields niobium methyl complexes Tp*Nb(Cl)(Me)(PhC⋮CEt), Tp*Nb(Cl)(Me)(PhC⋮C−n-Pr), and Tp*Nb(Cl)(Me)(PhC⋮CCH2SiMe3), respectively. This reaction follows a first-order rate law. The rate decreases with increased steric bulk of the alkyl group in the starting complex. For 2a, a full kinetic analysis yields a high enthalpic barrier and an entropy of activation close to zero. Heating a toluene solution of Tp*Nb(Cl)(n-Pr)(PhC⋮CEt) (3b) establishes a 1:1 equilibrium between 3b itself and Tp*Nb(Cl)(Et)(PhC⋮C−n-Pr) (2c). Heating a solution of Tp*Nb(Cl)(Et)(PhC⋮CEt) (2b...

Reversible Migratory Insertion/β-Alkyl Elimination in α-Agostic Alkylniobium Alkyne Complexes

Protonation of the metal-metal bond in [.mu.-(SCH3)Fe(CO)2L]2 complexes (L = P (CH3)3-x (C6H5)x). Experimental evidence of the variation of nucleophilicity of the metal-metal bond with donor properties of phosphorus ligands

Study of the Bonding Properties of the New Ligands C5H3N(2-R‘)(6-CH2PPhR) toward Rhodium(I). Evidence for a Dynamic Competition for Bonding between O- and N-Donor Centers When R = o-Anisyl, R‘ = Me

Formation of iron-osmium, iron-ruthenium, and iron-cobalt bimetallic particles by thermal decomposition of heteropolynuclear clusters supported on partially dehydroxylated magnesia

René Mathieu

Papers

Ruthenium(II) Complexes Containing Optically Active Hemilabile P,N,O-Tridentate Ligands. Synthesis and Evaluation in Catalytic Asymmetric Transfer Hydrogenation of Acetophenone by Propan-2-ol

Reversible Migratory Insertion/β-Alkyl Elimination in α-Agostic Alkylniobium Alkyne Complexes

Protonation of the metal-metal bond in [.mu.-(SCH3)Fe(CO)2L]2 complexes (L = P (CH3)3-x (C6H5)x). Experimental evidence of the variation of nucleophilicity of the metal-metal bond with donor properties of phosphorus ligands

Study of the Bonding Properties of the New Ligands C5H3N(2-R‘)(6-CH2PPhR) toward Rhodium(I). Evidence for a Dynamic Competition for Bonding between O- and N-Donor Centers When R = o-Anisyl, R‘ = Me

Formation of iron-osmium, iron-ruthenium, and iron-cobalt bimetallic particles by thermal decomposition of heteropolynuclear clusters supported on partially dehydroxylated magnesia