Rapid synthesis of alkylaminophenols via the Petasis borono–Mannich reaction using protonated trititanate nanotubes as robust solid–acid catalysts

Abstract

An effective and rapid synthesis of alkylaminophenols using the one-pot three-component Petasis borono–Mannich (PBM) reaction was carried out using protonated trititanate (H₂Ti₃O₇) nanotubes as a heterogeneous solid–acid catalyst. Complimentary to earlier reports, Ti–O based materials with various morphologies, such as fine particles, nanospheres, nanorods, and tubes were explored for their catalytic activity in the PBM reaction. The XRD pattern revealed the layered trititanate structure of nanotubes and nanorods; anatase and biphasic anatase–rutile structures for fine particles and nanospheres, respectively. Surface area analysis and NH₃-TPD adsorption results confirmed the larger surface area and high concentration of Brønsted and Lewis acid sites present in H₂Ti₃O₇ nanotubes. The catalytic efficiency for the PBM reaction is observed in the following order: H₂Ti₃O₇ nanotubes > H₂Ti₃O₇ nanorods > TiO₂ nanospheres > TiO₂ fine particles. The remarkable catalytic performance of H₂Ti₃O₇ nanotubes was ascribed to a sufficient amount of hydroxy groups and high concentration of Brønsted and Lewis acid sites on the tubular surface, which are essential for adsorption and catalytic reactions. The recyclability of H₂Ti₃O₇ catalyst is another emphasis for the proposed methodology. For the first time, we reported novel alkylaminophenols bearing 2-(pyrimidin-2-yl)-2,5-diazabicyclo[2.2.1]-heptane.