Metal-radical interactions transformation

It is important to select stoichiometric co-reductants or co-oxidants for the reversible cycle of a catalyst. A metallic co-reductant is ultimately converted to the corresponding metal salt in a higher oxidation state, which may work as a Lewis acid. Taking these interactions into account, the requisite catalytic system can be attained through multi-component interactions. Stereoselectivity should also be controlled, from synthetic points of view. The stereoselective and/or stereospecific transformations depend on the intermediary structure. The potential interaction and structural control permit efficient and selective methods in synthetic radical reactions. This chapter describes the construction of the catalytic system for one-electron reduction reactions represented by the pinacol coupling reaction. 

Despite the radical character of these autoxidations, they may exhibit some selectivity. This can, for example, be seen in a recent report by Kim et al., which notes the selective transformation of a titaniiun(IV) diamido dimethyl complex into a mixed alkoxide alkyl (Eq. 15) and the complete lack of reactivity of the corresponding dibenzyl complex with O2 [35]. Both of these observations are probably the result of steric protection. When the metal complex becomes too hindered to allow close interaction with O2, the oxidation can no longer be initiated. 

Sulfur cycling is affected in a variety of ways, including UV photoinhibition of organisms such as bacterioplankton and zooplankton that affect sources and sinks of DMS and UV-initiated CDOM-sensitized photoreactions that oxidize DMS and produce carbonyl sulfide. Metal cycling also interacts in many ways with UVR via direct photoreactions of dissolved complexes and of metal oxides and indirect reactions that are mediated by photochemically-produced ROS. Photoreactions can affect the biological availability of essential trace nutrients such as iron and manganese, transforming the metals from complexes that are not readily assimilated into free metal ions or metal hydroxides that are available. Such photoreactions can enhance the toxicity of metals such as copper and can initiate metal redox reactions that transform non-reactive ROS such as superoxide into potent oxidants such as hydroxyl radicals. 

Studies of similar reactions under a variety of conditions had been previously reported. Thus, electroreductive, photoreductive, as well as metal-induced ketyl-olefm cyclizations had all been explored prior to our investigations. Many of these cyclization reactions using simple unsaturated ketones took place with reasonably high diastereoselectivity at two stereocenters (Eq. 3). This feature of the transformation was ascribed to favorable secondary orbital interactions between (he developing methylene radical center and the alkyl group of the ketyl, and/or to electrostatic interactions in the transition state leading to product. ... 

Even though the discussion has been mainly on homopolymerization, the same polymerization mechanism steps are valid for copolymerization with coordination catalysts. In this case, for a given catalyst/cocatalyst system, propagation and transfer rates depend not only on the type of coordinating monomer, but also on the type of the last monomer attached to the living polymer chain. It is easy to understand why the last monomer in the chain will affect the behavior of the incoming monomer as the reacting monomer coordinates with the active site, it has to be inserted into the carbon-metal bond and will interact with the last (and, less likely, next-to-last or penultimate) monomer unit inserted into the chain. This is called the terminal model for copolymerization and is also commonly used to describe free-radical copolymerization. In the next section it will be seen that, with a proper transformation, not only the same mechanism, but also the same polymerization kinetic equations for homopolymerization can be used directly to describe copolymerization. 

However, hydroperoxides can also be isomer-ized by such a reaction pathway. When they interact with free radicals (H-abstraction from -OOH group) or with heavy metal ions (cf. Reaction 3.64), they are again transformed into peroxy radicals. Thus, the 13-hydroperoxide of hnoleic acid isomerizes into the 9-isomer and vice versa ... 

Reaction of main-group organometallic compounds with a metallic oxidant is considered to proceed via transmetaUation. Another reaction path lies in the redox interaction between them, affording the radical species (Scheme 2.57). These reactions are considered to provide a new route to reactive intermediates in oxidative transformations. 

The interaction between modestly strong LAs and relatively weak Lewis bases afibrds excellent stereocontrol in many synthetic radical transformations. While weak coordinative interactions may limit the effectiveness of weak LAs for isotactic control, it is unlikely to be a limiting factor in many of the LA-mediated radical polymerizations investigated to date particularly those that employ strong LAs (such as rare earth metal triflates). 

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