Hydrogen Peroxide as Terminal Oxidant

The introduction of chlorinated porphyrins (10) allowed for hydrogen peroxide to be used as terminal oxidant [62], These catalysts, discovered by Mansuy and coworkers, were demonstrated to resist decomposition, and efficient epoxidations of olefins were achieved when they were used together with imidazole or imidazo-lium carboxylates as additives, (Table 6.6, Entries 1 and 2). 

The observation that addition of imidazoles and carboxylic acids significantly improved the epoxidation reaction resulted in the development of Mn-porphyrin complexes containing these groups covalently linked to the porphyrin platform as attached pendant arms (11) [63]. When these catalysts were employed in the epoxidation of simple olefins with hydrogen peroxide, enhanced oxidation rates were obtained in combination with perfect product selectivity (Table 6.6, Entry 3). In contrast with epoxidations catalyzed by other metals, the Mn-porphyrin system yields products with scrambled stereochemistry the epoxidation of cis-stilbene with Mn(TPP)Cl (TPP = tetraphenylporphyrin) and iodosylbenzene, for example, generated cis- and trans-stilbene oxide in a ratio of 35 65. The low stereospecificity was improved by use of heterocyclic additives such as pyridines or imidazoles. The epoxidation system, with hydrogen peroxide as terminal oxidant, was reported to be stereospecific for ris-olefins, whereas trans-olefins are poor substrates with these catalysts. 

The use of the ionic liquid [bmim][BF4] further improved the Burgess epoxidation system [70]. Chan and coworkers found that replacement of sodium bicarbonate for tetramethylammonium bicarbonate and performing the reaction in [hmim][BF4] allowed for efficient epoxidation of a number of different olefins, including substrates affording acid-labile epoxides (such as dihydronaphthalene (99% yield) and 1-phenylcyclohexene (80% yield)). 

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In conclusion, the above summary of oxidation methods shows that there is still room for further improvements in the field of selective olefin epoxidation. The development of active and selective catalysts capable of oxidizing a broad range of olefin substrates with aqueous hydrogen peroxide as terminal oxidant in inexpensive and environmentally benign solvents remains a continuing challenge. 

Recently we presented a new methodological approach for catalytic sulfoxidation which makes use of water as solvent in the presence of anionic surfactant, and hydrogen peroxide as terminal oxidant activated by an easily prepared chiral Pt(ll) complex, all under mild conditions (Figure 9.7). Moreover, the enantioenriched sulfoxides are isolated from the catalyst by means of simple diethyl ether extraction (Figure 9.8) which does not dissolve the catalyst. 

Shi, F., Tse, M. and Seller, M. (2007). A Novel and Convenient Process for the Selective Oxidation of Naphthalenes with Hydrogen Peroxide, Adv. Synth. Catal, 349, pp. 303-308 Shi, F., Tse, M. and Seller, M. (2007). Selective Oxidation of Naphthalene Derivatives with Ruthenium Catalysts Using Hydrogen Peroxide as Terminal Oxidant, J. Mol. Catal. A Chem., 270, pp. 68-75 Wienhofer, G., Schroder, K., Moller, K., et al. (2010). A Novel Process for Selective Ruthenium-catalyzed Oxidation of Naphthalenes and Phenols, Adv. Synth. Cataly., 352, pp. 1615-1620. 

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