Novel Routes For Synthesizing Porous Organic Catalysts

The search for green, safe, and environmentally friendly technologies is one of the priorities for current scientific research, especially for chemical and pharmaceutical industries. Particularly, making chemical synthesis more sustainable requires green reaction systems that can maximize the desired products and simplify separation process. With these objectives, the use of recyclable catalysts and green solvents for organic synthesis to minimize waste production and optimize catalyst efficiency has been paid much attention in the past decades.Homogeneous organometallic catalysts dominate the organic synthesis such as coupling reactions and hydroformylation, but the difficulty in recycling restrict their more extensive applications. Heterogenization of metal complexes has been expected to solve this problem, where polymers and porous silicas are the most commonly used materials for such supports. Mesoporous silicas have robust porous structure and high surface areas, but their chemical nature limits their potential for chemical modification processes. On the other hand, diffusion limitation remains as drawbacks for conventional polymer supports, due to their low surface areas. More recently, another approach emerged as a powerful alternative, in which porous cross-linked polymers (PCPs) are served as a versatile platform for deployment of highly stable and recyclable heterogeneous catalysts by taking advantage of their permanent porosity and the ability to tune their compositions and properties at the molecular level. On this background, we developed various alternative heterogeneous catalysts based on pours polymers, which showed excellent catalytic performance in a series of important reactions.In the second chapter, I have successfully prepared heterogeneous palladium catalyst by coordination of Pd2+ species with Schiff base functionalized porous polymer through the post-modification strategy, which shows very high activities and excellent recyclability in a series of coupling reactions including Suzuki, Heck, and Sonogashira reactions.In the third chapter, I have provided an alternative and universal route for preparation of functional porous polymers by copolymerization of divinylbenzene (DVB) with vinyl modified organic ligand under solverthermal conditions. Many important organic ligands including chiral ligands such as phenanthroline (phen), BINAP [2,2’-bis(diphenylphosphino)-1,1’-binaphthyl] and TsDPEN [N-(p-toluenesulfonyl)-1,2-diphenylethylenediamine] have been successfully incorporated into porous polymer via this strategy (PCP-Phen, PCP-BINAP, and PCP-TsDPEN). After metalation with metal species the obtained heterogeneous chiral catalysts (PCP-Phen-Cu, PCP-BINAP-Ru, and PCP-TsDPEN-Ru) exhibit superior catalytic properties in corresponding coupling reactions and asymmetric reactions. It is worth mentioning that the superhydrophobic PCP-TsDPEN-Ru catalyst has significantly enhanced catalytic performance in ATH of ketones compared with the corresponding homogeneous chiral catalyst. This phenomenon is strongly related to the unique features of good enrichment of the reactants and easy product transfer from the catalyst into water phase due to the catalyst superhydrophobicity.In the fouth chapter, I have demonstrated a synthetic methodology of porous organic ligands (POLs) via solvothermal polymerization of corresponding vinyl-functionalized organic ligands. The POLs were obtained quantitatively, and show high surface area, large pore volume, hierarchical porosity, superior thermal stability, and high concentration of organic ligands. Notably, the achieved POLs can be used as both supports and ligands. On the other hand, a large amount of solid supports or other monomer should be introduced as we descripted above for heterogenization of organometallic catalysts, which results in the concentration of organic ligands in the materials are relatively low and they are separated by the supports or other copolymerized monomers. Thus these materials disfavor for their interaction, which is usually required for the formation and stabilization of the highly active species in the heterogeneous catalysts. The development of novel catalytic systems that combine the advantages of homogeneous and heterogeneous catalysis is therefore still a major aim of modern chemistry. After treating with metal precursors, the obtained materials as heterogeneous catalysts (M/POLs) exhibited unprecedentedly high activities and excellent recyclabilities in their classic reactions. Particularly, the heterogeneous M/POL-PPh3 catalysts show even higher activities than their conventional heterogeneous catalysts and corresponding homogeneous analogues. Furthermore, this strategy also present a novel platform for the generation of a high density of catalytically active centers within the confined nanospace for the development of new classes of highly efficient heterogeneous catalytic systems, especially for these catalytic processes involving cooperative, bimetallic or multi-metallic catalysis. For example, Co(Ⅲ)-salen complex catalyzes hydration of alkylene oxide to monoalkylene glycols is a representative important example, which involving a bimetallic catalysis process. The Co(Ⅲ)/POL-salen affords superior catalytic performance than homogeneous counterpart and Co(Ⅲ) species supported on conventional porous polymer incorporated salen ligand, especially at high substrate to catalyst ratio. These phenomena are reasonable attribute to the boosted synergistic effect of Co(Ⅲ)/salen complexes confined in nanospace. In addition, it is well known that the special structure of diphosphine ligands (e.g. dppe) has an enormous impact on stability, reactivity, and selectivity of various transition metal-catalyzed reactions, thus I have synthesized porous diphosphine ligands from the polymerization of the monomer. When Rh species are supported on porous diphosphine ligand of dppe (POL-dppe), the heterogeneous catalysts (Rh/POL-dppe) exhibit much better catalytic performance than corresponding homogeneous analogues and Rh/POL-PPh3 systems in catalytic hydroformylation reactions.It is strongly desirable to synthesize porous polymerized ionic liquid solids due to a combination of the advantages for both porous polymers and ionic liquids, In the fifth chapter, I have successfully synthesized a hierarchically porous cationic polymer solid formed by polymerization of vinyl-functionalized P-containing ionic liquid monomer under solvothermal conditions. The solid shows extraordinary stabilities and excellent amphiphilicity. After anion-exchange of peroxotungstates, the sample as a phase-transfer catalyst shows unprecedentedly high activities and excellent recyclabilities in catalytic epoxidation of olefin and oxidation of dibenzothiophene.In the sixth chapter, I have presented a synthetic methodology of promising replacements of these wildly used but difficulty separation solvents such as imidazolium type ionic liquid, 1-methyl-2-pyrrolidinone (NMP) and dimethyl sulfoxide (DMSO) by construction of vinyl functionalized solvent themselves into insoluble porous polymers via free radical polymerization. The obtained POSs offer a potential platform for deployment of highly efficient heterogeneous catalytic system by taking advantage of their permanent porosity and the ability to introduce catalytic active sites at the molecular level through co-polymerization with other functionalized monomer, for example after incorporating sulfonic acids groups, POSS-SO3H can direct catalytic dehydration of fructose to HMF in ca.95.3% isolated yields, using THF as readily separable solvent. More importantly, the co-localized catalyst and solvent analogues provide a more favorable catalytic environment for mutual promotion, giving the highest reported HMF yields to date in a readily separable, monophasic solvent. These features should be applied to design and develop a wide variety of efficiently and environmentally benign catalytic systems.