An open metal site metal–organic framework Cu(BDC) as a promising heterogeneous catalyst for the modified Friedländer reaction

Highlights

• Cu(BDC) was used as catalyst for the modified Friedländer.

• High conversions were achieved using catalytic amounts of the Cu(BDC).

• The catalyst could be recovered and reused.

Abstract

A crystalline porous metal–organic framework Cu(BDC) was synthesized, and characterized by several techniques, including X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), Fourier transform infrared (FT-IR), atomic absorption spectrophotometry (AAS), hydrogen temperature-programmed reduction (H₂-TPR), and nitrogen physisorption measurements. The Cu(BDC) exhibited high catalytic activity for the modified Friedländer transformation using 2-aminobenzyl alcohol as the starting material, thus offering advantages over the conventional Friedländer reaction in terms of avoiding the problems associated with the storage of the highly unstable 2-aminobenzaldehyde. Moreover, the Cu(BDC) could offer significantly higher catalytic activity than that of other Cu-MOFs such as Cu₃(BTC)₂, Cu(BPDC), and Cu₂(BDC)₂(DABCO). The catalyst could be recovered and reused several times without a significant degradation in catalytic activity. The modified Friedländer reaction could only occur in the presence of the solid Cu(BDC) with no contribution from leached active species.

Introduction

Quinoline derivatives have attracted significant attention as a major class of heterocycles possessing a wide variety of pharmacological and biological activities [1], [2], [3], [4]. Conventionally, these structures are synthesized by the Friedländer condensation between a 2-aminoaryl carbonyl and an another carbonyl compound containing a reactive α-methylene group followed by cyclodehydration [1], [5], [6], [7]. Although several homogeneous [8], [9], [10], [11] or heterogeneous [12], [13], [14], [15], [16] Lewis acid catalysts have been employed for the reaction, the protocol would suffer a number of serious drawbacks, especially problems related to the storage of the highly unstable 2-aminobenzaldehyde [17] as well as the formation of undesired products as a result of the self-aldol condensation of the 2-aminoaryl carbonyls [18]. Several efforts have been made to achieve a more efficient approach for the formation of quinoline derivatives, in which the modified Friedländer reaction using 2-aminobenzyl alcohols instead of 2-aminoaryl carbonyls have emerged as a promising alternative, offering several advantages as compared to the conventional Friedländer condensation [19], [20], [21]. A number of catalysts were previously employed for the modified Friedländer reaction, including CuCl₂ [19], [22], [IrCl(cod)]₂ and triphenylphosphine [23], Pd(OAc)₂ [24], Ag–Pd alloy nanoparticles supported on carbon [21], o-benzenedisulfonimide [1], and ruthenium-grafted hydrotalcite [25]. Moreover, base-mediated modified Friedländer transformations using more than one equivalents of a strong base were also investigated [2]. Unfortunately, difficulties in the cost of catalysts, drastic reaction conditions, low yields, and tedious workup procedures have restricted the application of these methods. For the development of more environmentally benign processes, a greener catalyst that could offer the ease of handling, simple workup, recyclability and reusability should be targeted for the reaction.

Metal–organic frameworks (MOFs) are hybrid crystalline porous materials assembled with metal ions and organic linkers, emerging as promising materials with potential applications in several fields [26], [27], [28], [29], [30], [31]. Although the application of MOFs in catalysis is a young research area, MOFs have been employed as solid catalysts or catalyst supports for a variety of organic transformations [32], [33], including the three-component (aldehyde–alkyne–amine) coupling reaction [34], the Biginelli reaction [35], asymmetric α-alkylation of aldehydes [36], NO generation from biologically occurring substrates [37], the conventional Friedländer condensation [38], [39], [40], the arylation of aldehydes with arylboronic acids [41], the Paal–Knorr reaction [42], Friedel–Crafts reactions between pyrroles and nitroalkenes [43], Friedel–Crafts alkylation and acylation [44], [45], [46], oxidation [47], [48], [49], [50], [51], [52], [53], [54], alkene epoxidation [55], [56], [57], cycloaddition of CO₂ with epoxides [58], cyanosilylation [59], [60], hydrogenation [61], Suzuki cross-coupling [62], [63], Sonogashira reaction [64], transesterification reaction [65], Knoevenagel condensation [66], [67], [68], aldol condensation [69], [70], aza-Michael condensation [71], [72], 1,3-dipolar cycloaddition reactions [73], N-methylation of aromatic primary amines [74], epoxide ring-opening reaction [75], [76], [77], hydrolysis of ammonia borane [78], and cyclopropanation of alkene [79]. However, it is apparent that the application of MOFs in the field of catalysis is still an immature phase and will attract further research in the near future [80]. In this work, we wish to report the use of Cu(BDC) as an efficiently heterogeneous catalyst for the modified Friedländer transformation using 2-aminobenzyl alcohol as the starting material. The Cu(BDC)-catalyzed protocol offers several advantages as compared to the conventional approach in the formation of quinoline derivatives.