Superparamagnetic polymer composite microspheres supported Schiff base palladium complex: An efficient and reusable catalyst for the Suzuki coupling reactions
Introduction
Palladium-catalyzed Suzuki–Miyaura cross-coupling reactions have been regarded as efficient and powerful tools for the formation of carbon–carbon bond in both academic and industrial laboratories [1], [2], [3], [4], [5]. Generally, the Suzuki cross-coupling reactions are frequently carried out in the presence of palladium catalyst containing well designed phosphine-based ligands, due to which can facilitate the corresponding transformation efficiently [6], [7], [8]. However, phosphine ligands have the defects including toxicity, air-sensitivity and high cost, which will limit their application to a large extent in the field of catalysis. Thus, it is still important to develop phosphine free ligands for the Suzuki reactions. In the past decades, a number of phosphine-free ligands including N-heterocyclic carbene [9], [10], [11], carbocyclic [12], [13], thiols [14], imidazoles [15], [16], [17], anionic carbocyclic [18], pincer [19] and porphyrins ligands [20] have been developed. Among these phosphine free ligands, Schiff bases have been recognized as efficient and suitable ligands for palladium species, due to the versatility of the steric and electronic property, nontoxicity and simple preparation. Up to now, many active and well defined Schiff base palladium complexes have been developed and widely studied for the Suzuki reactions [21], [22], [23], [24], [25]. In 2009, Kantam and his co-workers have reported the successful synthesis of palladium (II) complexes of trifunctional amido/pyridyl carboxylate ligands and their catalytic activity for the Suzuki reactions [21]. Subsequently in 2012, Singh and his co-workers have reported the selenated Schiff base palladium complex, which showed high activity in the Suzuki coupling reactions of aryl halides with phenylboronic acids under mild condition [22]. Although these homogeneous Schiff base palladium catalysts have contributed to improve catalytic activity significantly as a result of electron-donating functionality, they often suffered from the separation of these expensive catalysts after the reactions have completed. Furthermore, these homogeneous catalysts often result in heavy metal contamination of the desired isolated products.
In order to overcome these problems mentioned above, immobilization of homogeneous Schiff base palladium complex onto solid materials has been extensively employed in the field of the Suzuki coupling reactions, since immobilized Schiff base palladium catalysts have the fewer of the drawbacks of homogeneous catalysts, such as the difficulties in recovery and regeneration. In recent years, many types of carriers, such as macromolecular polymers [26], [27], [28], mesoporous molecular sieve [29], [30], [31], magnetic nanoparticles (MNPS) [32], [33], [34], graphene oxide [35], [36], metal organic framework (MOF) materials [37] were applied to support Schiff base palladium catalyst. However, a substantial decrease in activity and selectivity of the immobilized catalysts is frequently observed, since there are a number of challenges and obstacles such as leaching of palladium species from the supports, easy agglomeration of palladium particles and low utilization efficiency of Pd. It is thus desirable to develop more efficient and simple supported Schiff base palladium catalysts for the Suzuki coupling reactions.
During recent years, superparamagnetic polymer composite microspheres prepared by radical polymerization technology are now emerging as new promising support materials in the field of Pd-catalyzed Suzuki coupling reactions, due to their superparamagnetic property, nano-scale structure and various kinds of functional groups on polymer shell [38], [39], [40]. In 2005, Gao and his co-workers have prepared superparamagnetic polymer composite microspheres supported NHC complexes, which were efficient for Suzuki coupling reactions [38]. In 2014, magnetic polymer microspheres prepared by miniemulsion polymerization were used to support bulky N-heterocyclic carbene ligands for the Suzuki–Miyaura coupling reactions [39]. The novel magnetic catalyst showed high catalytic activity towards the Suzuki–Miyaura reaction of phenylboronic acids with aryl bromides in ethanol–water solution. In our previous work, superparamagnetic polymer composite microspheres prepared by an emulsifier free emulsion polymerization using DPE as a free radical control agent were also used as efficient support materials to contribute Suzuki coupling reactions significantly [40]. However, superparamagnetic polymer composite microspheres, to the best of our knowledge, have not been used as supports for immobilizing Schiff base palladium catalyst for the Suzuki reactions.
In continuation of our efforts to develop efficiently immobilized catalysts for Suzuki reactions, herein we would like to report on the first application of superparamagnetic polymer composite microspheres, prepared by soap free emulsion polymerization as the support material for the immobilization of Schiff base palladium complex. The supported magnetic catalyst can provide excellent reactivity in the coupling reactions of aryl halides with phenylboronic acids under mild condition. Furthermore, the novel magnetic catalyst can be conveniently recovered by an external magnetic field and reused at least five times without significant loss of its catalytic activity or Pd leaching.
Section snippets
Characterization
All reagents were commercially available (J & K CHEMICA) and were directly used without further purification. The X-ray diffraction (XRD) patterns were collected on a Bruker D8 ADVANCE instrument using Cu-Kα radiation (λ = 1.5418 Ǻ) at 40 kV and 40 mA. The magnetization curves of the samples were measured with a vibrating sample magnetometer (VSM) (model-155, Digital Measurement System, Inc.). X-ray photoelectronic spectrograph (XPS) was collected on a PHI-5400 instrument. The elemental contents of
Results and discussion
The preparation of the supported Schiff base palladium catalyst followed the steps described in Scheme 1. First, superparamagnetic polymer composite microspheres Fe3O4/P(GMA-AA-MMA) were prepared by way of soap-free emulsion polymerization. Next, amine groups could be anchored easily onto the surface of Fe3O4/P(GMA-AA-MMA) by mixing ethylenediamine with Fe3O4/P(GMA-AA-MMA) under 80 °C in DMF for 8 h. Then Fe3O4/P(GMA-AA-MMA)-Schiff base could also be easily obtained by reacting Fe3O4
Conclusions
In summary, we have demonstrated Schiff base palladium complex immobilized on magnetic polymer microspheres prepared by soap free emulsion to be a highly active, air-stable, easily recoverable catalyst for the Suzuki reaction. A wide range of substrates were coupled successfully under optimized reaction conditions. The supported magnetic catalyst could be used at least seven times without significant loss of its catalytic activity or Pd leaching. In particular, both hot filter test and three
Acknowledgements
We gratefully acknowledge financial support by the Natural Science Foundation of Jiangxi Province (Nos. 20114BAB213011, 20151BAB203028), the Science and Technology Project of Jiangxi Provincial Department of Education (No. GJJ13447), Doctoral Scientific Research Foundation of East China Institute of Technology (No. DHBK1010) and the Opening fund Project of State Key Laboratory Breeding Base of Nuclear Resources and Environment, East China Institute of Technology (NRE1315).
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