Fast Catalytic Hydroxylation of Hydrocarbons with Ruthenium Porphyrins

Abstract

Ruthenium porphyrin complexes such as carbonylruthenium(II) tetrakispentafluorophenylporphyrin [Ru(II)(TPFPP)(CO)] were found to be efficient catalysts for the hydroxylation of alkanes in the presence of 2,6-dichloropyridine N-oxide as the oxidant under mild, nonacidic conditions. Up to 14 800 turnovers (TO) and rates of 800 TO/min were obtained for the hydroxylation of adamantane. The hydroxylation of cis-decalin afforded cis-9-decalol and cis-decalin-9,10-diol, exclusively, thus, excluding a long-lived radicals mechanism. The kinetics of product evolution in a typical catalytic oxygenation showed an initial induction period followed by a fast, apparently zero-order phase with maximum rates and high efficiencies. Deuterium isotope effects (kH/kD) in the range of 4.2-6.4 were found for the hydroxylation of alkanes. A Hammett treatment of the data for the oxidation of para-substituted toluene derivatives showed a linear correlation with a highly negative rho+ value of -2.0. On the basis of kinetic and spectroscopic evidence, Ru(VI)(TPFPP)(O)2, Ru(II)(TPFPP)(CO), and Ru(IV)(TPFPP)Cl2 observed during catalysis were ruled out as candidates for the active catalyst responsible for the high efficiencies and turnover rates in the oxidation reactions. The fastest rates of adamantane hydroxylation with 2,6-dichloropyridine N-oxide were achieved by the reductive activation of Ru(IV)(TPFPP)Cl2 with a zinc amalgam. This redox activation is consistent with the formation of an active Ru(III) intermediate in situ by a one-electron reduction of the Ru(IV) porphyrin. EPR spectra characteristic of Ru(III) have been observed upon the reduction of Ru(IV)(TPFPP)Cl2 with a zinc amalgam. In the adamantane oxidation mediated with Ru(III)(TPFPP)(OEt), it was found that, during this process, the Ru(III) porphyrin was gradually converted to a dioxoRu(VI) porphyrin. Concomitant with this conversion, the reaction rates decreased. Catalyst activation was also stimulated by autoxidation of the solvent CH2Cl2. On the basis of these data, a mechanism is proposed that incorporates a "fast" cycle involving metastable Ru(III) and oxoRu(V) intermediates and a "slow" oxidation cycle, mediated by oxoRu(IV) and trans-dioxoRu(VI) porphyrin complexes.

Probing the C–O Bond-Formation Step in Metalloporphyrin-Catalyzed C–H Oxygenation Reactions

“…Less than 5% 18 O was incorporated in xanthone in the absence of hydroxide or fluoride, consistent with an oxygen rebound pathway involving xanthyl radical. In the presence of 20 equiv.…”

“…of TBAH (the trans-dioxo species is formed under these conditions) or 30 equiv. TBAF, we observed 48% and 84% of 18 O, respectively, into the xanthone carbonyl. The lower 18 O incorporation in the TBAH case was due to the exchange of xanthone- 18 O with 16 O-hydroxide in TBAH.…”

Oxygenation by Ruthenium Monosubstituted Polyoxotungstates in Aqueous Solution: Experimental and Computational Dissection of a Ru(III)–Ru(V) Catalytic Cycle

“…The Ru V -oxene generated within the highly electron withdrawing environment of a perfluorinated porphyrin displays some unique reactivity and selectivity features. [96,100] In this perspective the POM alternative offers the unprecedented opportunity to replicate bioinspired pathways on inorganic molecular metal-oxides, thus bridging the gap with heterogeneous catalysis by active surfaces. [105] Experimental Section [ 43] In …”

“…This observation is again consistent with a high-valent Ru V =O species, responsible for substrate oxygenation. [95][96][97][98][99][100] While in heme-type complexes oxygen exchange entails an oxo-hydroxo tautomerism, [101] in the present case where trans coordination is prevented by the POM ligand, a cis-dihydroxo intermediate can be postulated to trigger oxygen exchange within the coordination sphere of the ruthenium site. [99] Electrocatalytic oxidation of DMSO Cyclic Voltammetry (CV) experiments have been performed to show the impact of added DMSO on the electrocatalytic current registered at high applied voltage, for Ru II (DMSO)PW 11 in aqueous solution (Figure 9).…”

Efficient Selective Oxidation of Cyclohexane Catalyzed by Metallodeuteroporphyrins with Air in the Presence of l-Cysteine

“…Simulation of the function of cytochrome P-450 with transition metal complexes has led to the discovery of metalloporphyrins-catalyzed oxidation of hydrocarbons with various oxygen donors such as iodosylbenzene (PhIO) [1,2], ROOH [3,4], pyridine N-oxide [5], KHSO 5 [6] and hydrogen peroxide [7,8]. From the view of potential practical use of the metalloporphyrin-based systems for oxidation, the research emphasis should be directed to the use of cheap, easily accessible and environmentally friendly oxidants such as molecular oxygen.…”