Transition metal ions addition reactions

Most of the free-radical mechanisms discussed thus far have involved some combination of homolytic bond dissociation, atom abstraction, and addition steps. In this section, we will discuss reactions that include discrete electron-transfer steps. Addition to or removal of one electron fi om a diamagnetic organic molecule generates a radical. Organic reactions that involve electron-transfer steps are often mediated by transition-metal ions. Many transition-metal ions have two or more relatively stable oxidation states differing by one electron. Transition-metal ions therefore firequently participate in electron-transfer processes. 

The specific feature of polymerization as a catalytic reaction is that the composition and structure of the polymer molecule formed show traces of the mechanism of the processes proceeding in the coordination sphere of the transition metal ion to which a growing polymer chain is bound. It offers additional possibilities for studying the intimate mechanism of this heterogeneous catalytic reaction. 

Organic hydroperoxides have also been used for the oxidation of sulphoxides to sulphones. The reaction in neutral solution occurs at a reasonable rate in the presence of transition metal ion catalysts such as vanadium, molybdenum and titanium - , but does not occur in aqueous media . The usual reaction conditions involve dissolution of the sulphoxide in alcohols, ethers or benzene followed by dropwise addition of the hydroperoxide at temperatures of 50-80 °C. By this method dimethyl sulphoxide and methyl phenyl sulphoxide have been oxidized to the corresponding sulphone in greater than 90% yields . A similar method for the oxidation of sulphoxides has been patented . Unsaturated sulphoxides are oxidized to the sulphone without affecting the carbon-carbon double bonds. A further patent has also been obtained for the reaction of dimethyl sulphoxide with an organic hydroperoxide as shown in equation (19). 

In real systems (hydrocarbon-02-catalyst), various oxidation products, such as alcohols, aldehydes, ketones, bifunctional compounds, are formed in the course of oxidation. Many of them readily react with ion-oxidants in oxidative reactions. Therefore, radicals are generated via several routes in the developed oxidative process, and the ratio of rates of these processes changes with the development of the process [5], The products of hydrocarbon oxidation interact with the catalyst and change the ligand sphere around the transition metal ion. This phenomenon was studied for the decomposition of sec-decyl hydroperoxide to free radicals catalyzed by cupric stearate in the presence of alcohol, ketone, and carbon acid [70-74], The addition of all these compounds was found to lower the effective rate constant of catalytic hydroperoxide decomposition. The experimental data are in agreement with the following scheme of the parallel equilibrium reactions with the formation of Cu-hydroperoxide complexes with a lower activity. 

An extensive study of the reactions of the bare transition metal ions with phosphine (PH3) has been undertaken (111). Although a few metals formed addition complexes [M(PH3) ]+, most transition metal ions first reacted by a dehydrogenation reaction. As an example of addition reactions, Cu+ and Ag+ reacted very slowly with PH3 (incomplete reactions after 50 s with PH3 at a pressure of 1 x 10 5 Pa in a FT-ICR) sequentially forming [M(PH3)]+ and [M(PH3)2]+. 

The reactions of trialkyl and triaryl phosphines (R3P) with a wide variety of bare transition metal ions have not yet been studied but it is possible to get a phosphine such as Ph3P into the gas phase (145). Figure 6 shows the reaction of Eu+ with Ph3P. The addition of only two phosphine molecules was observed under the conditions used (113) (Ph3P at an uncorrected pressure of 2 x 10-6 Pa). The spectrum also shows the addition of only one Ph3P to [EuO]+. 

Examination of the reaction kinetics of the M+ + H2S reactions show that these reactions are not simple first-order reactions, that is, nonlinear slope for the rate of disappearance of M+ shown in Fig. 7 for Pt+. The non-first-order rate of disappearance of M+ suggests that there is more than one intermediate, possibly due to the presence of electronic excited states of the metal ions or intermediates with different interactions between the metal and H2S. The addition of H2S to Au+ is similar to the reaction of H2S with Ag+ and Cu+ (M+ — [MH2S]+ — [M(H2S)2]+), but is dissimilar to most of the second- and third-row transition metal ions. 

Both PH3 and S i 114 react similarly with bare transition metal ions except that no simple addition ions [MSiELJ have so far been observed. The Cu+ ion reacts by dehydrogenation and the most unreactive ion, Mn+, does not react in its ground state. The excited-state ion forms [MnH]+. The reactions of several first-row transition metal ions with silane (117-119) have been studied by the GIB method. The major product was [MSiH2]+ for the ground-state ions Ti+, V+, Cr+, Fe+, Co+, Ni+, Cu+, and Zn+. The group 3 (IIIB) ions Sc+, Y+, La+, and Lu+ (49) all reacted with silane in a similar manner to the first-row transition metal ions. 

Laser ablation of many metallic compounds will produce not only the bare metal ion M+ but also ions such as [MX]+, where X = O, S, Cl. The early bare transition metals ions react vigorously with background water in the mass spectrometers and the [MO]+ ion is always present when metals such as Ti are ablated. The [MX]+ ions can undergo several types of reaction and three types will be considered here substitution, addition, and polymerization reactions. Table II gives examples of the reactions of [MX]+ and [ML]+ ions. 

TCP and its metal-substituted derivatives can be made to dimerize quantitatively by the addition of cations (NU, K, Cs, Ba ) to solutions of these porphyrins ( ). The stoichiometry of the dimerization reaction establishes that two porphyrins are linked by four crown-cation-crown bridges ( ). This forces the porphyrins in a parallel configuration. For steric reasons, one ring system must be rotated relative to the other. EPR ( ) and ENDOR (IS) studies of the ground state triplet [CuTCP]2 and [VOTCP12 dimers fit a model of two parallel porphyrins with the transition metal ions positioned on a common axis perpendicular to the planes, with a center-to-center distance of 0.43 nm. 

The importance of the electron transfer reaction between RS" and an electron acceptor (Reactions 2 and 3) has been amply confirmed by the observation that the least acidic thiols are least resistant to oxidation (2), and by the enormously enhanced rate of reaction in the presence of redox catalysts, such as transition metal ions (13) or organic redox additives (14). In these latter cases, reactions of the type below become important,... 

The reduction and oxidation of radicals are discussed in Chapter. 6.3-6.5. That in the case of radicals derived from charged polymers the special effect of repulsion can play a dramatic role was mentioned above, when the reduction of poly(U)-derived base radicals by thiols was discussed. Beyond the common oxidation and reduction of radicals by transition metal ions, an unexpected effect of very low concentrations of iron ions was observed in the case of poly(acrylic acid) (Ulanski et al. 1996c). Radical-induced chain scission yields were poorly reproducible, but when the glass ware had been washed with EDTA to eliminate traces of transition metal ions, notably iron, from its surface, results became reproducible. In fact, the addition of 1 x 10 6 mol dm3 Fe2+ reduces in a pulse radiolysis experiment the amplitude of conductivity increase (a measure of the yield of chain scission Chap. 13.3) more than tenfold and also causes a significant increase in the rate of the chain-breaking process. In further experiments, this dramatic effect of low iron concentrations was confirmed by measuring the chain scission yields by a different method. At present, the underlying reactions are not yet understood. These data are, however, of some potential relevance to DNA free-radical chemistry, since the presence of adventitious transition metal ions is difficult to avoid. 

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