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Aerobic alcohol and amine oxidations catalyzed by Trans-dioxo(porphyrinato)ruthenium(VI) complexes Dodson, Heather K.


The stoichiometric and aerobic catalytic oxidations of amines and benzhydrols by Trans-Ru(porp)(O)2 complexes (porp = TMP, OCP, OCP-Cl8, see Figure) in benzene under mild conditions are described. [Figure with caption: Trans-dioxo(porphyrinato)ruthenium(VI) complexes. TMP, OCP and OCP-Cl8 are the dianions of meso-tetramesitylporphyrin, meso-tetra(2,6- dichlorophenyl)porphyrin and meso-tetra(2,6-dichlorophenyl)-β- octachloroporphyrin, respectively.] Kinetic data, determined by UV-VIS and ¹H-NMR spectroscopic analyses, for the stoichiometric oxidation of p-substituted benzhydrols indicate that the oxidation mechanism proceeds through the formation of a {Ru-alcohol} adduct (governed by an equilibrium constant K) which subsequently decomposes in a slower step (with rate constant k1) leading to ketone formation. Isotope effects of KH/KD ~ 0.6 and k1H/k1D ~ 15 for α-deuteration of the alcohol indicate that the adduct is formed through hydrogen-bonding interactions probably between the Ru=0 and the α-CH with cleavage of this C-H bond occurring in the rate-determining step. Electron-donating substituents favour alcohol oxidation, as illustrated by a limited linear Hammett relationship of k1 with 2σp for di-p-substituted benzhydrols. The {Ru-alcohol} adduct formation is essentially isenthalpic, while the k1 activation parameters are ΔH1‡ = 58 ± 10 kJ/mol and ΔS1‡ = -120 ± 30 J/(mol K). Trans-RuIV(TMP)(alkoxo)2 complexes have been isolated from the stoichiometric oxidation reactions and characterized by ¹H-NMR, UV-VIS and IR spectroscopies. Aerobic oxidation of benzhydrols is catalyzed by trans-Ru(porp)(O)2 at 50°C in benzene; ketones are the only organic products of the reaction. Water is essential for higher catalytic activity and turnovers of up to 24 (98% conversion) are seen at 50°C after 19 h. Under these conditions, the catalyst activities follow the trend: trans-Ru(OCP-C18)(O)2 > trans-Ru(TMP)(O)2 > trans-Ru(OCP)(O)2. Limited linear Hammett relationships, based on % conversion after 45 h at 50°C, indicate that catalytic oxidation is also favoured by the p-substitution of electron-donating substituents on the benzhydrols. Catalytic activity is limited by the rate of Ru(VI)-dioxo regeneration. Stoichiometric amine oxidation by trans-Ru(OCP)(O)2 and trans-Ru(TMP)(O)2 studied by UV-VIS spectroscopy gives irreproducible results. The source of the irreproducibility is not determined, though it is not from trace oxygen, the amine, the catalyst, trace acid or water in the system. The reactions, however, are light-sensitive. Aerobic oxidation of amines is catalyzed by trans-Ru(porp)(O)2 at 50°C under 1 atm of air; however, turnovers greater than 3 are only seen after 90 h in biphasic benzene/water systems, a result that contradicts a previous report. [Certain scientific formulae used in this abstract could not be reproduced.]

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