Metal-based Oxidants

The electroactive units in the dendrimers that we are going to discuss are the metal-based moieties. An important requirement for any kind of application is the chemical redox reversibility of such moieties. The most common metal complexes able to exhibit a chemically reversible redox behavior are ferrocene and its derivatives and the iron, ruthenium and osmium complexes of polypyridine ligands. Therefore it is not surprising that most of the investigated dendrimers contain such metal-based moieties. In the electrochemical window accessible in the usual solvents (around +2/-2V) ferrocene-type complexes undergo only one redox process, whereas iron, ruthenium and osmium polypyridine complexes undergo a metal-based oxidation process and at least three ligand-based reduction processes. 

Many laboratory and even some industrial scale oxidations were historically conducted using stoichiometric, toxic, metal-based oxidants such as KMn04, K2Cr207 and 0s04 [2], However, the use of small-molecule sources of oxygen is preferable from both economic and environmental viewpoints. These oxidants include 02, H202 and NaOCl, with an additional metal catalyst if required. 

Ionic liquids based on imidazolium cations and either [BF4]- or [PF6] anions have been used to immobilize transition metal based oxidation catalysts developed... 

In this context, it was shown that polymer-supported triphenylphosphine as a ligand for metal-based oxidation is an alternative catalytic system [72]. 

The Rh(III) and Ir(III) cyclometallated units present in some of our compounds show (1) intense LC absorption bands in the UV region and weak bands, mainly of MLCT character, in the visible region of the spectrum (2) irreversible, presumably metal-based oxidation processes and (3) reversible one-electron reduction of... 

Swem oxidation of /1-amino alcohols has been shown to be a useful alternative to metal-based oxidants, which may be chelated by the substrate.115 A - VIethylpyrrolidine, A-ethylpiperidine. or triethylamine proved optimal as bases. The reaction, although successful for /1-secondary amino alcohols, gave products that readily polymerized. 

This is considerably different from the recombination reaction with, for example, typical ruthenium dyes. This slow re-reduction of the dyad is explained by the low redox potential of the osmium center, the value of 0.66 V (vs. SCE) observed, points to a small driving force for the redox process. This observation is important for the design of dyes for solar cell applications. Osmium compounds have very attractive absorption features, which cover a large part of the solar spectrum. However, their much less positive metal-based oxidation potentials will result in a less effective re-reduction of the dyes based on that metal and this will seriously affect the efficiency of solar cells. In addition, for many ruthenium-based dyes, the presence of low energy absorptions, desirable for spectral coverage, is often connected with low metal-based redox potentials. This intrinsically hinders the search for dyes which have a more complete coverage of the solar spectrum. Since electronic and electrochemical properties are very much related, a lowering of the LUMO-HOMO distance also leads to a less positive oxidation potential. 

Hydroxylation using alkali metal based oxidants (for exanqile KMn04, K2Cr20 , etc. is possible, although these somewhat harsh reagents firequently give rise to products of overoxidation and are limited with respect to substrate compatibility, particularly when one considers the complex nature of many natural products of current intoest. 

There are problems associated with the expensive disposal of toxic waste from metal-based oxidations of alcohols. Thus, the focus has been largely on catalytic reactions as typified by Ley and Griffith s tetrapropylammonium perruthenate oxidant (section 7.1.6). Completely metal-free oxidations have much potential for environment-friendly oxidations, particularly if the reagent can be recovered and recycled. The most common metal-free oxidation of alcohols are TEMPO/oxone or TEMPO/N-chlorosuccinimide oxidation, Dess-Martin periodane oxidation (section 7.1.5) and Swern oxidation (section 7.1.4) and its several variants. 

Deactivation is not only a problem for oxidation catalysts in combustion of bio-fuel, also SCR catalysts used in biomass fined boilers are deactivated. Andersson et al. [8] investigated the deactivation of SCR catalysts in a couple of different large scale bio-fuelled boilers. The SCR catalyst works at lower temperature (not above 400 C). According to the authors, no loss of surface area occurs and the deactivation is explained by adsorption of gaseous potassium on acidic site of the catalyst. The catalyst can only be partly regenerated by washing m acidic solution. The reason for this difference between precious metal based oxidation catalyst and vanadium pentaoxide... 

It has been shown that the metal-based oxidant hexachloro osmate (V) yields in some cases also decarboxylative coupling products [183c]. 

R. Mas-Balleste, L. Que, Jr., Iron catalyzed olefin epoxidation in the presence of acetic acid. Insights into the nature of metal-based oxidant, J. Am. Chem. Soc. 129 (2007) 15964. 

We previously undertook a study of hydrocarbon activation over different transition metal based oxide catalysts mainly in relation to their total oxidation [7-9]. We proposed that hydrocarbons are activated at their weakest C-H bond on high-oxidation-state transition metal cationic centers with the formation of alkoxy species [7,8]. In the case of propene and 1-butene we suggested that the primary surface intermediates are allyl-oxy species [7,8]. 

The most widely used chemical oxidants have been ammonium persulfate, (Nn4)2S208, and FeCl3, although hydrogen peroxide and a range of transition metal salts (e.g., Fe3+, Cc4+, Cu2+, Cr6+, andMn7+) have also been employed. The use of H202 (with Fe3+catalyst) is attractive environmentally, as the only by-product is water. For the metal-based oxidants considered by Chao and March,72 infrared (IR) spectroscopy confirmed that similar PPy backbones were formed in each case. 

Oxidation of the same set of substituted phenols listed in Table 17.3 was also undertaken employing the organic oxidant, cumylperoxyl radical (Cum ). Unlike the metal based oxidation. Fig. 17.11 shows that the phenol oxidation rate constants do not correlate well with the substrate army better than with substrate BDE. This behavior implicates a more synchronous HAT mechanism for the organic radical oxidant. 

Scheme 8.3. Some examples of V+5-mediated reactions of allylic alcohols with r-BuOOH. (a) A chemoselective reaction [8]. (b) Stereoselective reactions of acyclic allylic alcohols, compared to results obtained using m-CPBA [9]. Note that better selectivity is usually obtained using the metal-based oxidation system, but not always with the same relative topicity as observed using a peracid.

The (peroxo)di-iron(III) species (38b) and (38b ) " effect enantioselective sulfoxidation of aryl sulfides. In the catalytic reaction with FI2O2 as a terminal oxidant, saturation kinetics were observed with respect to both [FI2O2] and [sulfide]. The peroxo species (38b) + (38b ) generated in situ at 0°C decays in the presence of sulfide, following a saturation kinetics with respect to [sulfide]. For p-bromophenyl methyl sulfide, the ee approaches ca. 40%, providing compelling evidence for a metal-based oxidation pathway involving a (peroxo)di-iron(III)-sulfide binary complex embedded within chiral environment. 

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