Microwave-assisted C–C bond formation of diarylacetylenes and aromatic hydrocarbons on carbon beads under continuous-flow conditions

Abstract

The synthesis of polycyclic aromatic compounds generally requires stoichiometric oxidants or homogeneous metal catalysts, however, the risk of contamination of inorganic residues can affect their properties. Here we present a microwave (MW)-assisted platinum on beaded activated carbon (Pt/CB)-catalyzed C–C bond formation of diarylacetylenes and aromatic hydrocarbons under continuous-flow conditions. Various fused aromatic compounds were continuously synthesized via dehydrogenative C(sp2)–C(sp2) and C(sp2)–C(sp3) bond formation with yields of up to 87% without the use of oxidants and bases. An activated, local reaction site on Pt/CB in the flow reaction channel reaching temperatures of more than three hundred degrees Celsius was generated in the catalyst cartridge by selective microwave absorption in CB with an absorption efficiency of > 90%. Mechanistic experiments of the transformation reaction indicated that a constant hydrogen gas supply was essential for activating Pt. This is an ideal reaction with minimal input energy and no waste production.

Pt/CB-Catalyzed Chemoselective Hydrogenation Using in situ-Generated Hydrogen by Microwave Mediated Dehydrogenation of Methylcyclohexane under Continuous-Flow Conditions

“…When an MCH solution of diphenylacetylene was pumped into a 5% Pt/CB packed catalyst cartridge irradiated at 10 W MW, intramolecular cyclization (C-C bond formation reaction) to produce phenanthrene was obtained as the main product, but at the same time, conventional contact hydrogenation of diphenyl ethane was also produced in a 40% yield. This reaction was reported after optimization studies, including changing the solvent to a mixture of MCH and 2-PrOH and tuning the reaction such that intramolecularly cyclized phenanthrene was highly selective [49] (Figure 1c). In this study, optimization studies focused on the continuous-flow alkyne hydrogenation reaction via MW, and a Pt/CB-catalyzed MW-mediated continuous-flow hydrogenation reaction with MCH as the reducing agent (hydrogen carrier) was developed (Figure 1d).…”

“…We developed a hydrogen extraction reaction from methylcyclohexane (MCH) [47] and 2propanol (2-PrOH) [48] catalyzed by platinum-supported bead-shaped activated carbon (Pt/CB), which proceeded under single-mode MW irradiation conditions of only 10-20 W (Figure 1a). Furthermore, this MW-mediated selective CB heating reaction mechanism was developed into a Pt/CB-catalyzed intramolecular C-C bond formation reaction of diarylacetylene derivatives under MW-assisted continuous-flow conditions, namely, the synthesis of polycyclic aromatic compounds based on C-H activation [49] (Figure 1b). When an MCH solution of diphenylacetylene was pumped into a 5% Pt/CB packed catalyst cartridge irradiated at 10 W MW, intramolecular cyclization (C-C bond formation reaction) to produce phenanthrene was obtained as the main product, but at the same time, conventional contact hydrogenation of diphenyl ethane was also produced in a 40% yield.…”

Pt/CB-Catalyzed Chemoselective Hydrogenation Using In Situ-Generated Hydrogen by Microwave-Mediated Dehydrogenation of Methylcyclohexane under Continuous-Flow Conditions

“…We developed a hydrogen extraction reaction from methylcyclohexane (MCH) [47] and 2-propanol (2-PrOH) [48] catalyzed by platinum-supported bead-shaped activated carbon (Pt/CB), which proceeded under single-mode MW irradiation conditions of only 10-20 W (Figure 1a). Furthermore, this MW-mediated selective CB heating reaction was developed into a Pt/CB-catalyzed intramolecular C-C bond formation reaction of diarylacetylene derivatives under MW-assisted continuous-flow conditions, namely, the synthesis of polycyclic aromatic compounds based on C-H activation [49] (Figure 1b). When an MCH solution of diphenylacetylene was pumped into a 5% Pt/CB catalyst-packed cartridge irradiated at 10 W MW, intramolecular cyclization (C-C bond formation reaction) occurred to produce phenanthrene as the main product, but at the same time, the conventional contact hydrogenation of diphenyl ethane also occurred, with a product yield of 40%.…”

“…When an MCH solution of diphenylacetylene was pumped into a 5% Pt/CB catalyst-packed cartridge irradiated at 10 W MW, intramolecular cyclization (C-C bond formation reaction) occurred to produce phenanthrene as the main product, but at the same time, the conventional contact hydrogenation of diphenyl ethane also occurred, with a product yield of 40%. This reaction was reported after optimization studies, including substituting the solvent with a mixture of MCH and 2-PrOH and tuning the reaction so that it was highly selective for the intramolecular cyclization of diphenylacetylene [49] (Figure 1c). Since 2-PrOH is also known as a type of LOCH and an efficient medium for absorbing MW energy due to its high dielectric constant, we selected 2-PrOH as a co-solvent to improve the reaction efficiency and substrate solubility.…”