Synthesis of heteroaromatic compounds by oxidative aromatization using an activated carbon/molecular oxygen system

Abstract

A variety of heteroaromatic compounds, such as substituted pyridines, pyrazoles, indoles, 2-substituted imidazoles, 2-substituted imidazoles, 2-arylbenzazoles and pyrimidin-2(1H)-ones are synthesized by oxidative aromatization using the activated carbon and molecular oxygen system. Mechanistic study focused on the role of activated carbon in the synthesis of 2-arylbenzazoles is also discussed. In the final section, we will disclose the efficient synthesis of substituted 9,10-anthracenes via oxidative aromatization.

Scheme 1

Hydrogen transfer reaction using Pd/C–ethylene system.

Scheme 2

Oxidation of some glycols to enones.

Scheme 3

Synthesis of chiral Schiff bases.

Scheme 4

Preparation of substituted salicylketones.

Figure 1

Time course of 2-phenylbenzoxazole, Schiff base, and aldehyde using activated carbon as catalysts which have different properties as shown in Table 9, were employed. Reactions were performed using 2-aminophenol (0.5 mmol), benzaldehyde (0.5 mmol), and activated carbon (50 mg) in p-xylene-d₁₀ (2 mL) at 120 °C under O₂. Yields were determined by ¹H-NMR analyses. Anthracene (0.1 mmol) was used as internal standard.

Figure 2

Reaction yields using washed and unwashed activated carbons.

Figure 3

Effect of properties of activated carbons listed in Table 2 on reaction yields. (a) Specific surface area, (b) pore volume, (c) mean pore diameter, (d) amount of oxygen evolved as [CO + CO₂], (e) amount of oxygen evolved as CO, and (f) amount of oxygen evolved as CO₂. ● Chemical activation (wood), ■ chemical activation (coconut shell), ○ steam activation (wood), □steam activation (coconut shell), ∆ steam activation (coal).