Novel polymer supported iminopyridylphosphine palladium (Ⅱ) complexes: An efficient catalyst for Suzuki–Miyaura and Heck cross-coupling reactions
Graphical abstract
A new cross-linked polyallylamine polymer supported iminopyridylphosphine palladiumⅡ complexes was designed and synthesized, which was used as outstanding palladium catalyst for the Suzuki–Miyaura and Heck cross-coupling reactions. The supported catalyst could be recycled and reused by the simple filtration, while keeping similar catalytic activity for five successive reactions.
Introduction
Palladium-catalyzed cross coupling reactions for carbon–carbon bond formation are extremely useful transformations in chemical industry and in research, especially Suzuki–Miyaura coupling and Heck reaction [1], [2], [3], [4]. In traditional protocols, catalysts used in the Suzuki reaction or Heck reaction are based on homogeneous palladium complexes such as Pd(PPh3)4, PdCl2(dppf), which are already known to show catalytic activity [5], [6]. Over the past few decades, significant progress in this filed have been achieved with a variety of homogeneous catalysts being developed [7], [8], [9]. Although homogeneous catalysts have many advantages, they have some obvious problems in terms of the separation and recycling. Additionally, the residual Pd metal along with the products could induce serious problems in API (Active Pharmaceutical Ingredients) program, which is always a terrible thing in pharmaceutical industry. In contrast, heterogeneous catalysts can be easily separated from the reaction mixture through simple filtration and reused in successive reactions, which has recently attracted much interest due to the increasing environmental concern [10], [11], [12]. Among them, the immobilization of active homogeneous catalysts onto insoluble supports has become a popular strategy to facilitate the recovery and recycling of potentially expensive ligands and complexes [13], [14], [15]. For decades, a variety of solid-supported palladium catalysts has been developed by immobilizing metal Pd on various supports, whose applications are more environmental friendly. Although various supports including silica [16], zeolites [17], activated charcoal [18], and polymers have been used to immobilize palladium metal complexes, polymer supports performed better in controlling the catalytic activities, because of their excellent stability (chemical and thermal), high surface area, good accessibility, and functionalization [19], [20], [21]. These heterogenized catalysts combine the advantage of homogeneous and heterogeneous catalysts and are capable of minimizing the drawbacks of both types of catalyst.
Schiff-bases have played a key role as privileged chelating multidentate ligands in coordination chemistry due to their high stability under a variety of oxidative and reductive conditions and the ease availability by which modified variations [22], [23], [24], [25]. Currently, the functionalized polymer-supported Schiff base complexes have been considered as versatile catalytic reagents for a wide range of organic reactions [26], [27], [28]. The catalytic activities of polymer-supported Schiff base complexes are stable in the presence of moisture and during their applications in high temperature reactions. Among the various ligands, phosphine-based ligands are still considered to be the most preferred one for Suzuki reaction. Phosphine-Schiff base type complexes (sometimes they are also called iminophosphine type complexes), which having P and N donor atoms, have shown an exponential increase for various organic transformations [29], [30], [31], [32]. Metal complexes with N and P donor atoms display a variety of coordination well beyond those of P–P or N–N ligands, which have emerged as an important class of ligands. Furthermore, these ligands show a particular behavior in binding to soft metal centers such as palladium(Ⅱ) that make their complexes good precursors in catalytic process [33]. Despite the general use of polymer-supported Schiff base for the Suzuki–Miyaura coupling and Heck reaction, polymer-supported iminophosphine complexes of palladium catalysts have not been widely used for this reaction yet. Study of new types of polymer-supported iminophosphine palladium catalysts which might be suitable for the C–C coupling has theoretical and practical significance. In continuing our efforts to develop greener synthetic pathways for organic transformations [34], our new approach, described in this paper, was to design and synthesize a new cross-linked polyallylamine polymer supported iminopyridylphosphine palladiumⅡ complexes showed in Scheme 1, which could be used as outstanding palladium catalyst for the Suzuki–Miyaura and Heck cross-coupling reactions.
Section snippets
Results and discussion
Cross-linked polyallylamine polymer was obtained by copolymerization of polyallyamine hydrochloride salt with epichlorohydrin under basic condition, and converted to carbonate form thereof by ion exchange with sodium hydrogen carbonate. According to the previous reported, the idealized structure of cross-linked polyallylamine polymer was shown in Fig. 1.
It was proved that cross-linked polyallylamine polymer was insoluble in most common organic solvents including THF, EtOH, MeOH, acetone and
Conclusion
In summary, a novel cross-linked polyallylamine polymer supported iminopyridylphosphine palladiumⅡ complexes has been successfully prepared. This new heterogeneous palladium catalyst presented good activity and high selectivity in Suzuki–Miyaura and Heck cross-coupling reaction without the need of addition of other sources of ligands. In addition, the supported catalyst developed in this study also has the advantage to be completely recoverable with simple filtration. The catalyst could be
Experimental section
All chemicals were of reagent grade. 2-(diphenylphosphino)benzaldehyde was purchased from Sigma–Aldrich. Cross-linked polyallylamine polymer and 6-(diphenylphosphino)picolinaldehyde were prepared according to reported procedures [36]. IR spectra were recorded in KBr disks with an SHIMADZU IRPrestige-21 FT-IR spectrometer. TG was performed on DSC 204. 1HNMR spectra were measured with a Bruker Avance III 500 analyzer. GC–MS analyses were performed on a Saturn 2000GC/MS instrument. Palladium
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