New Supported N-Heterocyclic Carbene Palladium Complex In The Suzuki-Miyaura Coupling Of Aryl Chlorides

The Pd-catalyzed Suzuki-Miyaura coupling is currently a fundamental reaction for the synthesis of polymers, agrochemical and pharmaceutical compounds. The use of aryl chlorides as substrates has proven difficult but is highly desirable in view of the wide diversity, ready availability and low cost of aryl chlorides in contrast with aryl bromides and iodides. N-heterocyclic carbine complex Pd-IPr [IPr=N,N'-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene] was recently found to be the most efficient catalyst for Suzuki-Miyaura coupling of aryl chlorides. Compared with the well-established phosphine systems, Pd-IPr complex is more appealing because it is air-stable and low toxic. Such advantages make Pd-IPr complex a potential catalyst for practical applications if this complex can be immobilized in a proper fashion for easy reuse. However, there is no report on immobilization of the bulky bis(2,6-diisopropylphenyl)-substituted carbene Pd complex for Suzuki-Miyaura coupling although the grafting of small methylsubstituted carbene complexes on insoluble supports has been reported, which exhibited low activity toward aryl chlorides due to lacking bulky electron-donating groups like 2,6-diisopropylphenyl group on the imidazole ring.New cubic cage-like mesoporous materials with a bulky N-heterocyclic carbene [IPr,1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene] precursor in the framework were synthesized by a co-condensation of IPr precursor-bridged organosilane and TEOS in the presence of template. N2 sorption, XRD and TEM characterizations revealed that the mesostructural orderings of the synthesized materials depended on the molar fraction of the bridged organosilane in the initial gel mixture. With the increase in the molar fraction of the organosilane from 2.5% to 15%, the mesostructure of the synthesized material changed from a well-ordered 3D ordered structure to a amorphous structure. FT-IR and solid-state NMR characterizations confirmed that IPr carbene precursor was covalently integrated with the solid materials.Such hybrid materials were able to coordinate Pd(acac)2, leading to active solid catalysts for Suzuki-Miyaura couplings of less reactive aryl chlorides. In the Suzuki-Miyaura couplings of chlorobenzene and phenyl-boronic acid, using iso-propyl alcohol as solvent and potassium tert-butoxide as base, the catalyst SBA-16-IPr(10%)-Pd can afford an isolated yield of 78% within 24 h, for electron-rich aryl chlorides even with steric hindrances over 70% yield was obtained. Under the same conditions, the developed solid catalyst has much higher activity than commercialized catalyst Pd/C. The solid catalyst could also be reused 8 times without a significant decrease in activity. The excellent recyclability of this catalyst can be explained by fact that the unique three-dimensional structure of SB A-16 can efficiently prevent the Pd nanoparticles from growth during the catalytic reaction.