Oxidatively added transition state

Once the classics of C—H activation mechanisms have been described, our interest will focus to more up-to-date examples were the choice of the mechanism is more ambiguous. Lin, Lau, and Eisenstein [4c] studied the C—H activation of methane by [M(T -Tp)(PH3)(CH3)] (M = Fe, Ru, and Os Tp = hydridotris(3,5-dimethylp3Tazolyl)borate). As seen in Figure 25.12, depending on the nature of the metal center, the computed activation barriers increase, following the trend Os < Ru < Fe. An oxidative addition path was computed for the respective Os catalyst. The two-step mechanism identifies an Os" intermediate, which is typical for an oxidative addition mechanism. In the case of the Fe system, no intermediate is located, and the transition state features a very short Fe-"H distance (1.53A). Finally, the Ru system exhibits an intermediate behavior, as a very shallow Ru" intermediate was identified. After a structural study of each of the transition states, the authors concluded that the Fe system undergoes reaction through a transition state that resembles an oxidative addition mechanism. Therefore, transition states which feature short M---H contacts have been named oxidatively added transition state (OATS) by Lin and coworkCTs [22]. 

The reaction is catalysed by adding phosphoric acid. Presumably two V(V) entities are bound to the arsenite in the transition state and effect concerted one-equivalent oxidations to avoid formation of the energetic As(IV). 

Fig. 12.4. Successive models of the transition state for Sharpless epoxidation. (a) the hexacoordinate Ti core with uncoordinated alkene (b) Ti with methylhydroperoxide, allyl alcohol, and ethanediol as ligands (c) monomeric catalytic center incorporating t-butylhydroperoxide as oxidant (d) monomeric catalytic center with formyl groups added (e) dimeric transition state with chiral tartrate model (E = CH = O). Reproduced from J. Am. Chem. Soc., 117, 11327 (1995), by permission of the American Chemical Society. 

A variety of monomers, including styrene, acrylonitrile, (meth) acrylates, (meth) acrylamides, 1,3-dienes, and 4-vinylpyridine, undergo ATRP. ATRP involves a multicomponent system of initiator, an activator catalyst (a transition metal in its lower oxidation state), a deactivator (the transition state metal in its higher oxidation state) either formed spontaneously or deliberately added, ligands, and solvent. Successful ATRP of a specific monomer requires matching the various components so that the dormant species concentration exceeds the propagating radical concentration by a factor of 106. 

Stabilizing the Support Oxide. Promoter elements can be added to the support oxide resulting in a decreased Co compound formation with the support oxide. This is illustrated in Figure 3A. More specifically, strategies should be followed to avoid the formation of either cobalt titanate, cobalt silicate or cobalt aluminate as a result of Co solid-state diffusion under reducing or regeneration conditions in the subsurface of these support oxides. Some transition metals, for example Zr or La, could act in such a way. 

X-ray absorption near edge structure (XANES) is useful in determining the coordination number and the oxidation state of metal ions (Sankar et al, 1983). In Figs. 2.16 and 2.17 we show the XANES of Co and Cu in some compounds as well as catalysts. The ls-3[Pg.99]

These redox reactions, in which oxygen transfer occurs, involve changes of two units in the oxidation numbers of reactant and product. One-electron redox reactions may occur with the transfer of a halogen. The reaction between Cr+2 and Fe 3, for example, is strongly catalyzed by added chloride ion and when chloride is added to the reaction mixture, the resultant Cr(III) is present as (BUO CrC 2. It might be suggested that Cl becomes attached to Cr+a after the redox has occurred, but this cannot be. In the first place, independent experiments show that under these conditions the reaction between Cr+3 and Cl is very slow second, chloride attachment after the redox has occurred would not explain the catalytic role of chloride. It is more likely that the reaction occurs via a chloride-bridge transition state, and that the redox is accomplished by a chlorine transfer ... 

The [W(OR )(S2C2R2)2r (R = Ph, 2-Ad, -Pr, or p-C6H4X, where X = CN, Br, Me, OMe, or NH2 R = Me or Ph) complexes react with an N-, P-, As-, S-, or Se- oxide (QO) to form the corresponding [WO(OR )(S2C2R2)2] complex (85). As for the reactions described above, the direct nature of the OAT reaction has been demonstrated by the transfer of lsO to the W center from Ph2Se180. These reactions are second order and involve a large negative entropy of activation (Table III), indicative of an associative transition state. The relative rates of the reactions vary with the substrate as ... 

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