Oxygen reaction with transition metals

Table 1. Typical Reactions of Tin-Nitrogen and Tin-Oxygen Bonds with Transition-Metal Compounds" ...

Reactions of free radicals with molecules (or ions) can occur via an addition reaction, for instance the addition of a C-centered radical to oxygen (4.2), hydrogen atom abstraction (4.3), or electron transfer mechanism (e.g., oxidation of CO " by SO" ). Since the total number of electrons is odd, one of the products is a new radical (e.g., (4.2) and (4.3)), except for the reactions with transition metal ions, such as the oxidation of superoxide radicals by Fe3+ ions ... 

Another possibility for the formation of free radical species from hypochlorite is through its reactions with transition metal ions. Thus, Guilmet and Meunier (1980) reported a manganese-promoted epoxidation of olefins such as styrene (Equation 5.13) and cyclohexene in a two-phase dichloromethane-water solvent mixture. The epoxide oxygen was derived from HOCl, not from air, but no mechanistic details were speculated upon. Further evidence needs to be obtained on the possibility of free-radical reactions in water and wastewater chlorination. 

Articles by Sweigart and coworkers have shown that manganese tricarbonyl complexes of r -hydroquinone deprotonate to give ii -semiquinone and q -quinone complexes. " " Solution NMR analysis and crystal structures of the q -semiquinone complex indicated that this complex was polymeric due to strong intermolecular hydrogen bonding, as shown in scheme 29 Reaction of the q -semiquinone and the q" -quinone complexes allowed the production of one-, two- and three-dimensional polymers. These complexes have also been shown to self-assemble by complexation of the oxygen atoms with transition metal ions. 

Lenoir, D. (2006) Selective Oxidation of Organic Compounds - Sustainable Catalytic Reactions with Oxygen and without Transition Metals Angewandte Chemie International Edition, 45, 3206-3210. 

The S02 molecule has unshared pairs of electrons on both the sulfur and oxygen atoms. As a result, it forms numerous complexes with transitions metals in which it is known to attach in several ways. These include bonding through the sulfur atom, through an oxygen atom, by both oxygen atoms, and various bridging schemes. In most cases, the complexes involve soft metals in low oxidation states. Another important reaction of sulfur dioxide is known as the insertion reaction, in which it is placed... 

In the examples above, one or both of the reaction centers are already attached to the metal center. In many cases, the reactants are free before reaction occurs. If a metal ion or complex is to promote reaction between A and B, it is obvious that at least one species must coordinate to the metal for an effect. It is far from obvious whether both A and B enter the coordination sphere of the metal in a particular instance. A number of metal-oxygen complexes can oxygenate a variety of substrates (SOj, CO, NO, NO2, phosphines) in mild conditions. Probably the substrate and O2 are present in the coordination sphere of the metal during these so-called autoxidations. In the reaction of oxygen with transition metal phosphine complexes, oxidation of metal, of phosphine or of both, may result. The initial rate of reaction of O2 with Co(Et3P)2Cl2 in tertiary butylbenzene. 

A fundamental problem in characterizing metal surfaces in oxidation catalysis is that, as with transition metal oxides, the chemistry of the surface is shaped by the reaction conditions. Margolis has taken the plausible position that most metal surfaces in oxygen are covered with oxygen and behave like metal oxides (13, 14). This is true even of platinum, a classical example of a metal catalyst, and here again predictions from bulk thermodynamics are unreliable with respect to the surface. 

There are many other reactions devised for oxygen transfer such as various oxidants combined with transition metals with or without macrocyclic hosts and industrial specific epoxidations using oxygen, to name just a few. 

Thiols are susceptible to oxidation by peroxides, molecular oxygen, and other oxidizing processes (e.g., radical-catalyzed oxidation) (Fig. 67). Because thiols easily complex with transition metals, it is believed that most thiol autoxidation reactions are metal-catalyzed (108). Autoxidation of thiols is enhanced by deprotonation of the thiol to the thiolate anion. Thiol oxidation commonly leads to disulfides, although further autoxidation to the sulfinic and, ultimately, sulfonic acid can be accomplished under basic conditions. Disulfides can be reduced back to the thiol (e.g., upon addition of a reducing agent such as dithiothreitol). Thiols are nucleophilic and will readily react with available electrophilic sites. For a more thorough discussion, see Hovorka and Schoneich (108) and Luo et al. (200). 

The stereochemical outcome of these electrophilic additions is consistent with a transition state in which the metal chelates the oxazolidinone carbonyl and the enolate oxygen. Reaction with an electrophile would, therefore, occur at the less hindered diastereotopic face of the (Z)-enolate, away from the shielding methyl groups of the auxiliary (Figure 24.6). Because both enantiomers of oxazolidinone 108 are equally available, the direction of the asymmetric induction can be controlled by proper choice of the absolute stereochemistry of the chiral auxiliary.106... 

The most common pathway for catalysis of autoxidations by transition metal complexes involves the decomposition of alkyl hydroperoxides. Another route that may be possible for chain initiation involves direct oxygen activation, whereby the complexation of molecular oxygen by a transition metal would lower the energy of activation for direct reaction with the substrate [reaction (9)]. For example, oxygen coordinated to a metal might be expected to possess properties similar to alkylperoxy radicals and undergo hydrogen transfer with a hydrocarbon ... 

During a cell s normal life cycle under aerobic conditions, some of the consumed oxygen is reduced to highly reactive molecules called reactive oxygen species (ROS). Transition metal ions such as iron, with their frequently unpaired electrons, act as excellent catalysts for the creation of ROS. The body s inability to modulate free iron availability creates an environment prone to the formation of ROS and free-radical induced cellular damage in the event of iron overload. The classical reaction between Fe3+ and superoxide (02 ) is known as the Haber-Weiss reaction ... 

Cytochrome c, a small heme protein (mol wt 12,400) is an important member of the mitochondrial respiratory chain. In this chain it assists in the transport of electrons from organic substrates to oxygen. In the course of this electron transport the iron atom of the cytochrome is alternately oxidized and reduced. Oxidation-reduction reactions are thus intimately related to the function of cytochrome c, and its electron transfer reactions have therefore been extensively studied. The reagents used to probe its redox activity range from hydrated electrons (I, 2, 3) and hydrogen atoms (4) to the complicated oxidase (5, 6, 7, 8) and reductase (9, 10, 11) systems. This chapter is concerned with the reactions of cytochrome c with transition metal complexes and metalloproteins and with the electron transfer mechanisms implicated by these studies. 

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