Mechanistic Insights into Selective Oxidation of Polyaromatic Compounds using RICO Chemistry

Abstract

Ruthenium-ion-catalyzed oxidation (RICO) of polyaromatic hydrocarbons (PAHs) has been studied in detail using experimental and computational approaches to explore the reaction mechanism. DFT calculations show that regioselectivity in these reactions can be understood in terms of the preservation of aromaticity in the initial formation of a [3+2] metallocycle intermediate at the most-isolated double bond. We identify two competing pathways: C-C bond cleavage leading to a dialdehyde and C-H activation followed by H migration to the RuO complex to give diketones. Experimentally, the oxidation of pyrene and phenanthrene has been carried out in monophasic and biphasic solvent systems. Our results show that diketones are the major product for both phenanthrene and pyrene substrates. These diketone products are shown to be stable under our reaction conditions so that higher oxidation products (acids and their derivatives) are assigned to the competing pathway through the dialdehyde. Experiments using O-labelled water do show incorporation of oxygen from the solvents into products, but this may take place during the formation of the reactive RuO species rather than directly during PAH oxidation. When the oxidation of pyrene is carried out using D O, a kinetic isotope effect (KIE) is observed implying that water is involved in the rate-determining step leading to the diketone products.

Capture and immobilization of gaseous ruthenium tetroxide RuO4 in the UiO-66-NH2 metal–organic framework

“…8). This type of reaction has been studied for OsO 4 53,54 or RuO 4 55 on simple aromatic systems or double bonds. DFT calculations confirmed the good affinity between the ditopic ligand of UiO-66-NH 2 and RuO 4 , through strong hydrogen bonding between the porous framework (from NH 2 groups connected to neighboring ligands) and the gaseous species in intermediate 1.…”

Controlled reduction of aromaticity of alkylated polyaromatic compounds by selective oxidation using H2WO4, H3PO4 and H2O2: a route for upgrading heavy oil fractions

“…Previously, using RICO chemistry, we found that the number of aromatic rings in a substituted PAH determines the reaction rate, with larger aromatic systems reacting faster than smaller ones. 13 In the case of the tungsten/peroxide catalyst system, when naphthalene, (12), phenanthrene, (13), and pyrene, (14), were used as substrates, it was also found that the reaction rate increased with the increasing number of fused aromatic rings (Fig. 2).…”

“…11 In our RICO work, selectivity was not affected by the choice of solvent for 2-ethylnaphtahlene oxidation (CH 3 CN, H 2 O or CH 3 CN, DCM, H 2 O). 11,14 However, it was shown that, using RICO, the solvent affects the selectivity for oxidation of molecules with three or more aromatic rings, giving a lower degree of aliphatic carbon oxidation in the presence of acetonitrile. 13 A possible explanation for the change of selectivity with solvent for 2-ethylnaphtalene using the H 2 WO 4 /H 2 O 2 system could be the distribution of the tungsten complex in the reaction medium.…”

“…It has been found from DFT calculations for the RICO oxidation system, that positions 9 and 10 in the phenanthrene ring system are the most energetically favourable positions for oxidation as the removal of these carbons from the aromatic system has only a limited effect on the resonance structures available. 14 The a-carbon of an alkyl chain substituted at these positions is likely to be placed close to the oxidant making it more susceptible to oxidation. The equivalent positions in pyrene are 4, 5, 9 and 10 so that the high level of preserved aliphatic H for 9 can be understood in a similar way.…”

“…regioselectivity favours the formation of a [3+2] intermediate in positions that lead to minimal loss of aromaticity. 14 This provides a route to selectively oxidise aromatic carbon producing predominantly aldehyde and carboxylic acid functionality. However, this ruthenium-based catalytic system relies on an expensive metal and requires a strong oxidant which produces a stoichiometric equivalent of salt waste, therefore a cheaper catalyst and more environmentally benign oxidant would be preferred.…”