Oxidation of Polynuclear Aromatic Hydrocarbons using Ruthenium‐Ion‐Catalyzed Oxidation: The Role of Aromatic Ring Number in Reaction Kinetics and Product Distribution

Abstract

Oxidation of aromatic hydrocarbons with differing numbers of fused aromatic rings (2-5), have been studied in two solvent environments (monophasic and biphasic) using ruthenium-ion-catalyzed oxidation (RICO). RICO reduces the aromaticity of the polyaromatic core of the molecule in a controlled manner by selective oxidative ring opening. Moreover, the nature of the solvent system determines the product type and distribution, for molecules with more than two aromatic rings. Competitive oxidation between substrates with different numbers of aromatic rings has been studied in detail. It was found that the rate of polyaromatic hydrocarbon oxidation increases with the number of fused aromatic rings. A similar trend was also identified for alkylated aromatic hydrocarbons. The proof-of-concept investigation provides new insight into selective oxidation chemistry for upgrading of polyaromatic molecules.

Mechanistic Insights into Selective Oxidation of Polyaromatic Compounds using RICO Chemistry

“…The initial coordination of RuO 4 at the 9,10‐position is also consistent with the experimentally observed oxidation products previously reported in which oxidation was found to give diketones, dialdehydes or acids at these positions both in pyrene and phenanthrene . To see how this [3+2] complex may lead to the observed products, we have mapped out the reaction steps for oxidation of the PAHs starting with the [3+2] adduct in the 9,10‐positions using the same DFT approach.…”

“…Analysis by 1 H NMR spectroscopy of the products from the pyrene oxidation reaction carried out in the monophasic solvent system showed two major products: pyrene‐4,5‐dione, 6 , and pyrene‐4,5‐dialdehyde, 4 , as expected from Scheme . Similarly, analysis of the oxidation product mixture when using phenanthrene, 1 , as substrate suggested that the main product in the monophasic solvent system is the dione (9,10‐phenanthrenequinone, 9 ) along with a trace amount of biphenyl‐2,2′‐dicarbaldehyde, 10 (Scheme S2, Supporting Information) . These products are consistent with the two primary products expected from the mechanism discussed in the DFT section above.…”

“…The initial coordinationo fR uO 4 at the 9,10-position is also consistentw ith the experimentally observed oxidation products previously reportedi nw hich oxidation was found to give diketones, dialdehydes or acids at these positions both in pyrene and phenanthrene. [16] To see how this [3+ +2] complex may lead to the observed products,w eh ave mapped out the reactions teps for oxidation of the PAHs startingw itht he [3+ +2] adduct in the 9,10-positionsu sing the same DFT approach.S cheme 1s hows the mechanistic steps considered in calculations carriedo ut for this work. As we have seen, the formation of the [3+ +2] adduct, 3,l eads to ar eduction of the Ru centre from an oxidation state of + 8t o+ 6.…”

“…We have previously reported how RICO chemistry can be used in a more controlled manner such that partial oxidation of the PAH takes place to generate aldehydes, ketones, and acid functional groups with low levels of oxidation of any alkyl substituents . We further explored this subject by studying the role of aromatic ring number in reaction kinetics and product distribution using RICO chemistry . In the present contribution, we aim to use a combination of density functional theory (DFT) calculations and isotopic labelling experiments to identify the mechanistic steps that lead to these products in the initial stages of the oxidation reaction using pyrene and phenanthrene as exemplar PAH molecules.…”

“…[15] We further explored this subjectb ys tudying the role of aromatic ring number in reactionkinetics and product distribution using RICO chemistry. [16] In the present contribution, we aim to use ac ombination of density functional theory (DFT) calculations and isotopic labellinge xperiments to identify the mechanistic steps that lead to these products in the initial stages of the oxidation reactionu sing pyrene and phenanthrene as ex-emplarP AH molecules.…”