Substitution redox catalysis

As shown in Section 2.2.7, chemical reactions may be triggered by electrons or holes from an electrode as illustrated by SrnI substitutions (Section 2.5.6). Instead of involving the electrode directly, the reaction may be induced indirectly by means of redox catalysis, as illustrated in Scheme 2.15 for an SrnI reaction. An example is given in Figure 2.30, in which cyclic voltammetry allows one to follow the succession of events involved in this redox catalysis of an electrocatalytic process. In the absence of substrate (RX) and of nucleophile (Nu-), the redox catalysis, P, gives rise to a reversible response. A typical catalytic transformation of this wave is observed upon addition of RX, as discussed in Sections 2.2.6 and 2.3.1. The direct reduction wave of RX appears at more negative potentials, followed by the reversible wave of RH, which is the reduction product of RX (see Scheme 2.21). Upon addition of the nucleophile, the radical R is transformed into the anion radical of the substituted product, RNu -. RNu -... 

The reduction of benzyl aryl ethers has been thoroughly investigated by voltammetric reduction, homogeneous redox catalysis,and currently, by convolution analysis. A family of ethers activated by proper substitution on the phenoxy side were chosen to provide a wide variation in the ET and bond cleavage properties of the molecule. ... 

Gu, Y., Lee, H., and Hudson, R.A., Bis-catechol-substituted redox-reactive analogs of hexam-ethonium and decamethonium stimulated affinity-dependent reactivity through iron peroxide catalysis, /. Med. Chem., 37, 4417, 1994. 

Initiation by electrochemical induction may have the disadvantage of low yields of substitution due to the reduction of the radicals formed near the electrode, mainly in those cases in which the radical anion of the halide compound fragments at a considerably high rate. Redox catalysis, that is activation involving a suitable ET mediator, is an important means to avoid termination steps in electrochemically induced reactions. This approach has been extensively studied by the Saveant group15. A general equation has been proposed in order to predict the yield of ET-initiated S l chain reactions and related mechanisms under preparative electrochemical conditions in the presence of a redox mediator35. 

Dithianes and gemdithioacetals could be alternatively oxidized indirectly by means of the redox catalysis method. The technique appeared to be particularly mild and mainly avoided inhibition and adsorption phenomena relative to the anode platinum interface. Thus aromatic hydrocarbons (e.g. 9,10-diphenylanthracene) [83] and judiciously substituted triphenylamines [84] afford quite stable cation radicals used homogeneously as oxidants. Their standard potential, E°x, will determine the rate of electron exchange with the concerned sulfur compound. The cleavage of a C—S bond in any dithiane can be regarded as fast enough to draw the redox catalysis process to the indirect oxidation. 

Competition experiments again feature prominently in another discussion of the possible role of transient five-coordinate [Co(NH3)5] in induced and in spontaneous aquation of pentaaminecobalt(III) derivatives. " The operation or nonoperation of the D mechanism at various cobalt(III) centers and at penta-cyanoferrate(II) still requires a few experiments providing unambiguous results. Its operation at molybdenum(O)- and tungsten(0)-penta or tetracarbonyl complexes seems more firmly based. The question of its operation at pentacyanoferrates(III) does not seem to have caused much concern. The only recent paper which mentions kinetics of such a reaction, replacement of 2-methyl imidazolate in [Fe "(CN)5(2-Meimid)] ", reports that the limiting first-order rate constant is 2.3 x 10 s at 298 K, but is more preoccupied with redox catalysis by traces of iron(II) than with simple substitution. 

Redox reactions can be catalyzed by reducible cations substituted into the framework of zeolitic systems as well as polymorphic AIPO4 systems or by cations not located in the framework but in the micropores. In Chapter 8 we will discuss more extensively catalysis by Tia,Si(i 2,)02 systems using peroxides. Here we will initiate the discussion on redox catalysis with Coa Al(i 2.)P04 oxidation catalysts where reducible ions such as Co + substitute for AP+. Catalytic oxidation carried out with oxygen provides an opportunity to discuss radical-type chemistry. A second system that we will discuss is photochemical oxidation induced by the strong electrostatic field of ion-exchanged cations. We will subsequently discuss catalysis by Fe " " and Fe " " ion exchanged zeolites with comparisons to Zn + systems and the important role of the corresponding oxycation. 

In addition to the wide range of metal oxide catalysts that can cany out oxidation via redox catalysis, there are a host of other materials that can carry out oxidation over non-reducible metal oxides. The oxidation mechanisms over non-reducible metal oxides are quite different and typically involve the production of free radical intermediates. The mechanisms tend to contain both heterogeneous and homogeneous activation and functionality. The oxide is used to activate a free radical process that can then proceed in the gas phase or at the surface. Li-substituted MgO and the rare earth metal oxides are two classes of materials that are considered non-reducible oxidation catalysts. Here we wiU specifically focus on the activation of alkanes over non-reducible metal oxides. 

Considerable effort has been expended in studies of the interaction of metal ions with catechols with a view to understanding oxygenase activity. In aprotic media, the electrochemical properties of substituted catechols have been examined. Reactions of 3,5-di-tert-butyl-o-quinone with manganese(II) result in stable tris-Mn(IV) or bis-Mn(III) complexes of the corresponding catecholate dianion, Bu C , depending on whether the initial ratio of reactants is 1 3 or 1 2. This flexible redox chemistry may be important for redox catalysis. The O2 oxidation of the iron-catechol complex [Fe(salen)(Bu2CH)] has also been examined in aprotic media. 

Redox catalysis of substitution is well established for chromium(ii) catalysis of substitution at chromium(m), and for platinum(ii) catalysis of substitution at platinum(iv) it has also recently been demonstrated for rhodium(i) catalysis of substitution in a particular series of rhodium(ra) complexes (see later. Section 4). Redox catalysis of substitution at cobalt(iii) is less common, cobalt(n) being generally ineffective, but chromium(ii) is an effective catalyst for aquation of [Co(NHg)5(NCO)] +. ... 

Redox catalysis of substitution occurs in the aquation of [Cr(CN)6l in the presence of chromium(n) an outer-sphere mechanism is proposed. ... 

Halide substitution at [Ru(OH2) ] is catalysed by [Ru(OHa)6]. This behaviour is similar to redox catalysis of substitution at, for example, chromium(m) by chromium(n) or at platinum(iv) by platinum(n). In this ruthenium(m)-ruthenium(n) system it appears that the rate-determining step is in fact substitution at the [Ru(OH2) ] + cation, for which the activation enthalpy is about 20 kcal mol ... 

Finally, 8-CF3 substituted purines can also be synthesised by an electrochemical approach. In this case, reduction of CFsBr in the presence of an adenine anion 115 was performed under redox catalysis [143] (Scheme 41). It was postulated that this reaction occurs via an SrnI mechanism to afford the corresponding 8-CF3 substituted purines (116-119) however, no yields have been reported for these transformations. 

Rh(NH3)5X] +, where X = Cl, Br, or I, are also included in Table 13. The suggested mechanism involves slow dissociation of an adduct [Rh(NH3)s-XHg] + which is in equilibrium with the starting reactants. Both Hg +- and HgCl+-catalysed aquations of rhodium(ni)-halide complexes can be incorporated in the general kinetic versus stability-constant correlation for metalion catalysis of aquation of halogeno-complexes mentioned in Section 1 of this chapter. Silver(i) is not an efficient catalyst for the aquation of [Rh(NH3)5-Q]2+ 240 Redox catalysis of substitution at rhodium(m) has been reviewed. ... 

Amatore, C., Oturan, M. A, Pinson, J., Saveant, J. M., and Thiebault, A., Electron-transfer-induced reactions a novel approach based on electrochemical redox catalysis. Apphcation to aromatic nucleophilic substitutions, /. Am. Chem. Soc., 106, 6318, 1984. 

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