Ligand‐Free Copper‐Catalyzed Coupling of Phenols with Nitroarenes by using a Metal–Organic Framework as a Robust and Recoverable Catalyst

Abstract

A highly porous metal–organic framework Cu2(BDC)2(DABCO) (H2BDC=1,4‐benzenedicarboxylic acid, DABCO=1,4‐diazabicyclo[2.2.2]octane) was synthesized and used as an efficient recyclable heterogeneous catalyst for the coupling reaction of phenols with nitroarenes to form diaryl ethers without using a ligand. Physical characterization of the MOF was obtained by using XRD, SEM, TEM, thermogravimetric analysis (TGA), FTIR spectroscopy, atomic absorption spectrophotometry (AAS), H2 temperature‐programmed reduction (H2‐TPR), and N2 physisorption measurements. The Cu2(BDC)2(DABCO)‐catalyzed coupling reaction offers several advantages compared to the conventional Ullmann reaction for the synthesis of unsymmetrical diaryl ethers. To the best of our knowledge, the ligand‐free Cu‐catalyzed O‐arylation reaction of phenols with nitroarenes that uses a heterogeneous catalyst has not been mentioned previously in the literature.

Recyclable Cu-MOF-74 Catalyst for Ligand-Free O-Arylation Reaction of 4-Nitrobenzaldehyde and Phenol

“…until NB/Ph = 1/2 produced a significant enhancement of the NB conversion. It must be noted that similar results were reported using other copper-based MOF materials, like Cu2(BDC)2(DABCO), being the best NB/Ph molar ratio 1/1.5 [20]. This result can be explained by the role of phenol as a ligand in the catalytic mechanism, which accelerates the coupling with aryl halides, as observed in homogeneous copper catalytic systems [26].…”

“…60%. At the reaction temperature of 100 °C, the Cu-MOF-74 showed a catalytic activity similar to another Cu-based MOF material (Cu2(BDC)2(DABCO)) reported in the literature [20]. However, Cu2(BDC)2(DABCO) evidenced a stronger loss of activity at lower reaction temperatures as compared to Cu-MOF-74.…”

“…These reactions were carried out in DMF with a NB/Ph molar ratio of 1/2, 2 equivalents of K2CO3 as a base and 5 mol % of catalyst, conditions that are similar to those previously published (Phan et al [20]). A remarkable 88% conversion of NB was achieved after 6 minutes and 100% conversion after 1 hour for the highest reaction temperature (140 °C).…”

“…The pore size diameter of Cu-MOF-74 (10.1 Å) is larger than that reported for Cu2(BDC)2(DABCO) (6.4 Å), a difference that enables an easier diffusion of reactants within the porous structure of Cu-MOF-74, favoring its catalytic activity. On the other hand, the Cu2(BDC)2(DABCO) material was not tested at temperatures above 100 °C, where the catalytic performance of Cu-MOF 74 is significantly enhanced [20]. Therefore, the Cu-MOF-74 is active at mild reaction temperatures (60 °C) and becoming extremely active at 120 °C, with a NB conversion close to 80% in few minutes.…”

“…Although MOF materials have been originally used for gas separation and storage, their potential as active and selective catalysts in a wide range of chemical 2 of 13 reactions are being lately demonstrated [15][16][17][18][19]. Phan and co-workers have paid attention on ligand-free copper-catalyzed coupling reactions of phenols with aryl iodines or nitroarenes [20] using MOF-199 and Cu2(BDC)2(DABCO) (BDC = 1,4-bencene dicarboxylate and DABCO = 1,4-diazabicyclo[2,2,2]octane), respectively. T. Maity and coworkers have also investigated the O-arylation of aryl alcohols with aryl bromides using a Zn-based isoreticular metal organic framework (IRMOF-3) in which copper(II) was anchored following a post-synthetic modification method [21].…”