A atom-efficient cross-coupling reaction of aryl iodides with triarylbismuths catalyzed by immobilization of palladium(II)-Schiff base complex in MCM-41
Graphical abstract
Introduction
Biaryl compounds are widely used as chiral phosphine ligands,1 monomers of functional polymers,2 and as important intermediates in the preparation of natural products, agrochemicals, and pharmaceutics.3 Among numerous methods accessing to biaryl structures,4 the palladium-catalyzed cross-coupling reaction of organometallic compounds with aryl halides or triflates such as the Suzuki-Miyaura,5 Stille,6 and Hiyama7 coupling reactions is one of the most important synthetic methods for the construction of unsymmetrical biaryls. Although various organometallic compounds such as organoboron, -tin, -silicon, -zinc, and -magnesium compounds have been successfully used for the cross-coupling reactions, organobismuth compounds have not been well explored.8 Organobismuth compounds are potentially useful candidates for environmentally benign reagents since bismuth is known to be a low-level toxic element.9 Importantly, organobismuth compounds can be easily prepared through standard procedures.10 Triarylbismuths, unlike organoboron, organotin, and other organometallic reagents, can react with three equivalents of an electrophilic reagent.11 Recently, palladium-catalyzed cross-coupling reactions of organobismuth compounds with electrophilic reagents for C–C bond formation have been reported.12 It is noteworthy that only para- or meta-substituted triarylbismuths are used as the coupling partners due to the steric requirement for this type of transformation. Among the limited successful examples for the construction of C–C bonds, homogeneous palladium complexes such as Pd(PPh3)4, Pd(OAc)2, or PdCl2 are usually used as catalysts for these coupling reactions. However, these homogeneous palladium catalysts are expensive, cannot be recycled, and difficult to separate from the product mixture, which may have a very serious negative impact on their possible industrial applications, especially the pharmaceutical industry. In contrast, heterogeneous catalysts can be easily separated from the reaction mixture by simple filtration and reused in successive reactions provided that the active sites have not become deactivated. Heterogeneous catalysis also helps to minimize wastes derived from reaction workup, contributing to the development of green chemical processes.13 Thus, the development of supported palladium catalysts showing a much higher catalytic activity without leaching is highly desirable. So far, supported palladium catalysts have successfully been used for the Heck reaction, the Suzuki-Miyaura reaction, the Sonogashira reaction, Hiyama reaction, and the Stille reaction, etc.14 In spite of the significant advances in this area, there are very few reports that employ the heterogeneous palladium complexes as catalysts for the cross-coupling reactions of triarylbismuths with electrophiles.15
The discovery of mesoporous material MCM-41 has given an enormous stimulus to research in heterogeneous catalysis and provided a new possible candidate for a solid support for immobilizing homogeneous catalysts.16 MCM-41 has a regular pore diameter of ca. 5 nm and a specific surface area >700 m2 g−1 and rich silanol groups in the inner walls.17 To date, functionalized MCM-41-immobilized palladium, rhodium, molybdenum, and copper complexes have been prepared and successfully used in organic reactions.18 In continuation of our efforts to develop greener synthetic pathways for organic transformations,18(h), 18(i), 18, 18(f), 18(g) we herein report the synthesis of an MCM-41-immobilized palladium(II)-Schiff base complex [MCM-41-N,N-Pd(OAc)2] and its successful application to the atom-efficient cross-coupling of triarylbismuths with aryl iodides. The new heterogeneous palladium catalyst exhibits high catalytic activity in the reaction and can easily be separated from the reaction mixture by a simple filtration of the reaction solution, and its catalytic efficiency remains unaltered even after recycling ten times.
Section snippets
Results and discussion
Although phosphine ligands can stabilize palladium and influence its reactivity, the simplest and cheapest palladium catalysts are of course phosphine-free systems, specifically when used in low loading. It is well known that the catalysts containing phosphine ligands at higher temperatures are unstable.19 Furthermore, the procedure for preparing the supported phosphine palladium complexes is rather complicated since the synthesis of the phosphine ligands requires multi-step sequences.
Conclusion
In summary, we have developed a novel, phosphine-free, practical and economic catalyst system for the atom-efficient cross-coupling reaction of triarylbismuths with aryl iodides by using an MCM-41-immobilized palladium(II)-Schiff base complex [MCM-41-N,N-Pd(OAc)2] as catalyst and K2CO3 as base in NMP. This new heterogeneous palladium catalyst can be conveniently prepared by a simple procedure from commercially available and inexpensive reagents and exhibits high catalytic activity. More
Physical measurements and materials
All chemicals were reagent grade and used as purchased. All solvents were dried and distilled before use. Triarylbismuths were prepared by standard protocols.10(b), 10, 10(a) The products were purified by flash chromatography on silica gel. Mixture of EtOAc and hexane was generally used as eluent. All cross-coupling products were characterized by comparison of their spectra and physical data with authentic samples. IR spectra were determined on a Perkin-Elmer 683 instrument. 1H NMR spectra were
Acknowledgments
We thank the National Natural Science Foundation of China (No. 21462021) and Scientific Research Fund of Education Department of Jiangxi Province (KJLD13022) for financial support.
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