Aerobic Dehydrogenative Coupling of Naphthols and Phenols with a Ru(OH)(x)/Al2O3 Catalyst under Continuous-Flow Conditions

Dehydrogenative coupling reactions of naphthols and phenols were achieved with a heterogeneous catalyst, alumina-supported ruthenium hydroxide (Ru(OH)(x)/Al2O3), using continuous-flow conditions and aerobic oxygen as the sole oxidant. The reactions were operated continuously for more than 16 h with good reactivity and selectivity. The Ru catalyst could be recycled at least three times by a simple online washing/activation procedure without depackaging the catalyst cartridge. The reaction provided binaphthols and biphenols as valuable products for synthetic purpose and functional materials, while an over-oxidized quinone was obtained with 2,6-Di-tert-butylphenol; combining this reaction with catalytic hydrogenation under continuous-flow conditions enabled smooth conversion of quinone to the 4,4 '-biphenol derivative, which is a member of an important class of compounds for liquid crystals, engineering plastics, and other functional materials.