Regioselective methanolysis of epoxides by urea-functionalized metal-organic frameworks as heterogeneous organocatalysts

Hydrogen-bonding organocatalysis has emerged as a promising biomimetic alternative to Lewis acid catalysis. Urea moieties represent the most common hydrogen-bond donors used for the preparation of these catalysts. However, their significant tendency to undergo self-quenching often decreases their solubility and reactivity. Recently, scientists have found a promising way around this problem by immobilizing the hydrogen-bonding organocatalysts on Metal–organic frameworks (MOFs). 1,2 Compared to other porous materials, MOFs have given chemists the opportunity to tune the topology, pore size and functionality by rational selection of organic linkers and inorganic metal centers. Ring opening of epoxides is one of the most important reactions producing vital intermediates in organic synthesis. There are some reports for this reaction catalyzed by MOFs containing Lewis acid sites, especially with Cu, Fe and Hf metal nodes.3 In this regard, there is no report of urea-containing MOFs as hydrogen-bond catalysts in this reaction. In this paper, we report an extension of these studies aimed at investigating the organocatalytic role of urea-containing MOFs in the activation of epoxides. A novel Zn-MOF was synthesized by combining the ditopic urea strut, 4,4ʹ-oxybisbenzoic acid and Zn(NO3)2.6H2O using the solvothermal method at 90°C for 120 h to give suitable X-ray quality crystals, Fig.1. The productivity of the catalyst was evaluated by determination of the reaction selectivity for the conversion of styrene oxide to 2-methoxy-2-phenylethanol as the major product in our catalytic system. The prepared urea MOF catalyst was suspended in a MeOH (3mL) solution of epoxide (25 mg) and stirred at 60ºC. After 40 h reaction time, 95% selectivity was calculated for the above mentioned major product. When the reaction prolonged to 48 h, quantitative conversion of styrene oxide was obtained with 98% selectivity for 2-methoxy-2-phenylethanol that was characterized by GC and NMR.