Hydrothermal formation of hydroxylated benzenes from furan derivatives

Abstract

Several furan derivatives were subjected to hydrothermolysis at 340°C and 27.5 MPa for 1 to 33 min in a flow system to study their conversion to hydroxylated benzenes. Hydrothermolysis of 0.05 M aq. 5-methyl-2-furaldehyde, 2-acetylfuran and 2-propionylfuran led to the formation of 1,4-benzenediol, 1,2-benzenediol and 3-methyl-1,2-benzenediol, respectively, with selectivities ranging from 10 to 65%. The addition of 5 mM HCl increased the rate of hydrolysis of the furan ring, but in the case of 2-acetylfuran or 2-propionylfuran it also reduced the selectivity towards the corresponding benzenediol. Hydrothermolysis of 0.01 M aq. 2-acetyl-5-(hydroxymethyl) furan and 5-(1-hydroxyethyl)-2-furaldehyde did not produce the expected benzenetriols; instead, 2-methyl-1,4-benzenediol could be identified as a product. Hydrothermolysis of 2-vinylfuran did not produce any phenol, an indication that the formation of benzene rings from furans is not accomplished by electrocyclic ring closure. 2,6-Dimethyl-γ-pyrone, a compound that is known to produce 5-methyl-1,3-benzenediol via an aldol condensation under alkaline conditions, also gave this product when subjected to hydrothermal conditions. A reaction pathway for the hydrothermal formation of hydroxylated benzenes from furan derivatives is presented which is based on carbon ring formation via an intramolecular aldol condensation.