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5.5 FINAL REMARKS Some mechanistic considerations on the functionalization studies of the sp 3 carbon centers by hydrogen peroxide in the presence of POMs were proposed. In all these studies, no substrate oxidation took place in the presence of a radical scavenger, which suggests that these oxidations are processes involving radicals. However, taking into account all the results obtained, it is clear that even if the reaction mechanisms are radical in nature, some details must vary, depending on the catalyst and reaction conditions. This is due to the existence of several possible concurrent phenomena, namely the hydroperoxidation and the hydroxylation reactions, the decomposition of the hydroperoxide formed, and the dismutation of H 2 O 2 , all putatively catalyzed by the transition metals [43, 46, 48, 49]. Hydroperoxidation was observed with several substrates, mainly when an excess of H 2 O
was used. In the presence of the iron catalysts, it was assumed that the formation of the alkyl hydroperoxides occurred by an iron(III)-initiated generation of HO • [84]. The first step should be a reduction of iron(III) in acetonitrile (Scheme 5.1) [85], which does not occur in aqueous solution [86], and this was confirmed by cyclic voltammetry [43]. The molecular oxygen to obtain ROO • is probably originated in situ from H 2 O 2 , since some systems involving iron complexes, with excess H 2 O
, can produce their own O 2 atmosphere (Scheme 5.1) [87, 88]. This may explain the higher hydroperoxide yields when an excess of H 2 O 2 was used (H 2 O
/sub molar ratio = 9.8), and would be in good agreement with the catalytic results obtained when similar reactions were performed under a nitrogen atmosphere. The formation of the other products, such as ketones and alcohols, may be explained by considering also the Fenton reactions [87, 89]. Furthermore, as some XW 11
III + H
2 O 2 → XW 11 Fe II + HOO
• + H
+ XW 11 Fe III
+ HOO • → XW 11 Fe II + O 2 + H + XW 11 Fe II + HOOH → XW 11 Fe III + HO
• + OH
− RH + HO
• → R
• + H
2 O R • + O
2 → ROO
• ROO
• + RH
→ ROOH + R • ROO • + XW
11 Fe II + H + → XW 11 Fe III + ROOH Scheme 5.1 68 FUNCTIONALIZATION OF SP 2 AND SP
3 CARBON CENTERS CATALYZED BY POLYOXOMETALATES AND METALLOPORPHYRINS results obtained with the Keggin-type XW 12 and the lacunary-type XW 11 anions are similar to those obtained with the transition-metal-substituted anions (XW 11 M), it is likely that in the presence of XW 11 M anions the activation of H 2 O
may occur simultaneously at the W and the transition metal (M) [43]. In the case of sp 2 carbon center oxyfunctionalization with hydrogen peroxide in the presence of POMs, the possibility of autoxidation seems to be ruled out, since similar results were obtained under an argon atmosphere, in comparison with those achieved in the presence of air. Moreover, radical processes can be ruled out, as the presence of a radical scavenger in the reaction media did not inhibit the formation of any reaction products [52]. Studies dedicated to oxidation reactions catalyzed by synthetic metalloporphyrins showed that iron and manganese porphyrin complexes are exceptional catalytic models of biologically important iron- and manganese-containing enzymes [5, 11–14, 17, 18]. In fact, metalloporphyrins are known to work as biomimetic monooxygenase or as superoxide dismutase enzymes, each pathway being attained by the correct choice of the fifth ligand [90, 91]. This ligand is known to be crucial to the stabilization of the oxo– metal complex formed during the reactions and to facilitate the substrate hydrogen abstraction [91–96] and the heterocyclic cleavage in hydroperoxy-type oxidants [59, 90], similar to the cysteinate residue function in cytochrome P450 monooxygenase enzymes [92, 97, 98]. It is usually recognized that the reactions occur with the contribution of a high valent oxo–metal species that can be produced by the interaction of the metalloporphyrin with oxygen donors such as hydrogen peroxide, alkyl hydroperoxides, iodosylarenes, sodium hypochlorite, potassium monopersulfate, amine N-oxides, and peracids, among others [11, 13, 14, 17]. Hydrogen peroxide has the advantage of being a green, clean, and cheap oxidant. Hydrogen peroxide’s main problems, as an oxidant, are connected with metalloporphyrin stability under the reaction conditions and its own unproductive dismutation (catalase pathway) [90, 91, 99]. Mechanistic studies have suggested that the reactions occur in steps or involve a stepwise branch with an intermediate that generates the by-products. Two types of intermediates, namely radicals and carbocations, are usually invoked to account for the side-product formation, while the stereospecific epoxidation is thought to occur by a concerted oxygen transfer mechanism [68]. Density functional theory (DFT) studies using compound I ([Fe IV = O(protoporphyrin IX) • + ] coordinated to a thiolate residue) suggested that a multiscenario can be found for sp 2 carbon center oxygenation and that, depending on the catalyst, the substrate, and the reaction conditions, a cationic and/or a radical species can be generated thereby giving rise to the final products [82]. In conclusion, POMs and metalloporphyrins have been shown to be excellent catalysts for the in vitro biomimetic oxidative transformation of organic compounds, namely their sp 2 and sp 3 carbon centers, when hydrogen peroxide is used as the oxygen donor and acetonitrile as solvent.
The authors wish to thank FCT/FEDER for funding the Organic Chemistry Research Unit (Project PEst-C/QUI/UI0062/2011) and CICECO (Pest-C/CTM/LA0011/2011). Thanks are also due to their colleagues and students involved in the work cited here.
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are 2-coordinate and typically are ligated by bulky and strongly π-donating substituents that effectively shield the boron cation from the solvent and anion. Borenium cations LR 2 B + are 3-coordinate species that comprise two σ -bound substituents (R) and one dative interaction with a ligand (L) that serves to occupy a third coordination site as well as to reduce some of the electron deficiency at boron. The third, and the most common, class of boron cations is that of the tetrahedral, 4-coordinate boronium cations L 2 R 2 B + , with two coordination sites occupied by σ -bound substituents and the other two populated by neutral donor ligands. The coordinative saturation at the boron center in boronium cations renders these species particularly stable and some of them were proposed as novel electrolytes for rechargeable lithium batteries [11]. On the contrary, borenium cations with weakly stabilizing substituents can be classified as superelectrophiles, combining a monocationic charge with an unoccupied p orbital [12]. The unfilled p orbitals of the boron atom in borinium cations can become partially occupied as a result of π-donation from covalently bound substituents, analogous to the isoelectronic allenes. Bidentate ligation of boron generates a “chelate-restrained” borinium cation in which electrophilicity is enhanced by the nonlinear geometry at boron, resulting in an empty boron p orbital that cannot be stabilized by ligand π-donation. As result, such chelate-restrained borinium cations are able to activate C–H bonds of arenes with the formation of arylboron derivatives [13]. Advances in Organometallic Chemistry and Catalysis: The Silver/Gold Jubilee International Conference on Organometallic Chemistry Celebratory Book, First Edition. Edited by Armando J. L. Pombeiro. © 2014 John Wiley & Sons, Inc. Published 2014 by John Wiley & Sons, Inc.
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