TEMPO-immobilized metal-organic frameworks for efficient oxidative coupling of 2-aminophenols and aldehydes to benzoxazoles

Abstract

This study presents the utilization of 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) radical-immobilized metal-organic frameworks (MOFs) for the oxidative cyclization of aldehydes and aminophenols, leading to the heterogeneous synthesis of 2-substituted benzoxazole molecules. The TEMPO-immobilized MOFs are successfully employed in one-pot sequential aerobic oxidation and oxidative cyclization reactions that enable the synthesis of 2-phenyl benzoxazoles from readily available benzyl alcohols and ortho-aminophenol precursors. The modified MOFs exhibit exceptional reactivity towards benzoxazole synthesis, demonstrating a wide range of substrate compatibilities and efficient performance under simple reaction conditions. Through the implementation of a mixed-ligand approach, precise control over the molar fraction and density of TEMPO radicals is achieved. Among the synthesized catalysts with radical density control, UiO-68-(TEMPO)1.0, which features the complete immobilization of TEMPO, displays the highest catalytic efficiency in the oxidative cyclization process. This enhanced performance can be attributed to the dual involvement of TEMPO species in the proposed reaction mechanism for oxidative cyclization. Notably, the MOF-TEMPO catalyst can be recycled up to five times without any loss of catalytic activity or crystallinity. This research is significant because the sequential approach proposed in this study bridges the TEMPO-catalyzed aerobic oxidation of alcohols and TEMPO-catalyzed oxidative cyclization of aldehydes with aminophenols under the same TEMPO catalyst and O2 atmosphere. The utilization of TEMPO radical-immobilized metal-organic frameworks for the oxidative cyclization of aldehydes and aminophenols to benzoxazoles has been achieved.