Copper-promoted reductive Claisen rearrangement of allyloxyanthraquinones in the presence of 1,10-phenanthroline in ionic liquid: Antibacterial and antioxidant properties of products, and DFT-D mechanistic studies

An efficient method is described for rapid reductive Claisen rearrangement of 1-prop-2′-enyloxy(allyloxy)anthraquinones to 1-hydroxy-2-(prop-2′-enyl)anthraquinones using copper powder in the presence of 1,10-phenanthroline in ionic liquid 1-methylimidazolium tetrafluoroborate ([Hmim]BF₄) in excellent yields. Some other functional groups tolerate this [3,3] sigmatropic reaction so that it can be operated in an excellent chemoselective manner. Also in continuation, the results of dispersion-corrected density functional theory (DFT-D) mechanistic studies for non-reductive and reductive Claisen rearrangements of 1-allyloxyanthraquinones with various substituents are compared. In consistency with the experimental observations, it was found that the reductive [3,3] sigmatropic transformation is kinetically and thermodynamically more favorable. Also, the energy barrier for the keto-enol tautomerization of the resulting intermediates is significantly reduced in the presence of ionic liquid. In addition, from the obtained minimum inhibitory and bactericidal concentrations (MIC and MBC) results in antibacterial study, it was found that between the evaluated rearranged products, 1,4-dihydroxy-2,3-bis(prop-2′-enyl)anthraquinone 19 exhibits antibacterial activity against both kinds of bacteria S. aureus and especially E. coli. Finally, in the study of antioxidant property of the rearranged products, 1,8-dihydroxy-2,7-bis(prop-2′-enyl)anthraquinone 29 showed the highest activity.