Ru-nanoparticles supported on zeolite-Y for ortho-benzylation of phenols and activation of H2O2 for selective synthesis of BINOLs

Abstract

Benzylation of phenols with primary aromatic alcohols and the oxidative C–C coupling of naphthols to biaryl derivatives with hydrogen peroxide (H₂O₂) have long posed challenges for organic chemists. The present work addresses the limitations associated with these reactions using a ruthenium (Ru)-based catalyst supported on a zeolite-NaY matrix. A detailed investigation of three different types of Ru-catalysts provided the information that creation of strong Lewis acid sites with high valent Ru species endorsed high productivity coupled with high ortho-selectivity in the benzylation of phenols. The presence of low-valent Ru(0) species, however, had a negative impact on the catalytic process. The oxidative C–C coupling of 2-naphthol in the presence of H₂O₂ proceeded rapidly, yielding only BINOL as the product. The benzylation of phenol with the same Ru catalyst occurred at 100 °C while C–C coupling of naphthols proceeded very well at room temperature. Density functional theory (DFT) calculation on the stability of benzyl carbocation and the reaction mechanism clearly highlighted the role of the supported catalyst and its impact on the catalytic reaction. Fee radical trapping experiment supported by mass spectroscopy analysis suggested the involvement of 2-naphthyloxy radicals in selective [1,1′-binaphthalene]-2,2′-diol (BINOL) formation.