Research On Linkage Chemistry Of Covalent Organic Frameworks

As an emerging type of organic aggregates,covalent organic frameworks(COFs)are characteristic as crystalline porous polymers.The synthetic chemistry of COFs,that is polymeric crystallization,represents the integration between classical synthetic organic chemistry and crystallography.This thesis devotes to establish the theory of polymeric crystallization by means of developing linkage chemistry of COFs.In detail,we focused on the exploration of new strategies and methodologies for constructing highly crystalline and stable COFs based on the understanding for the reactivity and selectivity of organic reactions.The novelty of the thesis is depicted as follows:1.New Linkage:The first construction of fused-ring-linked COFs(in Chapter 3)A series of bicyclic pyrano[4,3-b]pyridine COFs have been constructed via an unprecedented cascade protocol that intramolecular[4+2]cycloaddition was first involved in and served as the key step.With a broad scope of Br?nsted or Lewis acids as the catalyst,the designed monomers,i.e.,O-propargylic salicylaldehydes and multitopic anilines,were readily converted into the fused-ring-linked frameworks in a one-pot fashion.The obtained COFs exhibited excellence in terms of crystallinity,stability,and porosity.We envisioned that the formation of fused rings would bring the fully-locked conformation that may facilitate the construction of COFs which can possess high stability and crystallinity simutaneouly.This work has advanced the linkage chemistry from the formation of solo-bond or single-ring linkages to that of fused-ring linkages.2.New Standard:High efficiency and high selectivity of COFs’ synthesis(in Chapter 3 and 4)The synthesis of COFs should also meet the "golden standard" of classical synthetic organic chemistry:high efficiency and selectivity.Through constructing fused-ring-linked COFs in a one-pot fashion,we upgraded the linkage chemistry of COFs to a new level of complexity and controllability,which represented the high efficiency of the synthesis.Besides,the highly selective formation(up to 95%)of pyrano[4,3-b]pyridine moieties was evidenced by quantitative NMR measurements combined with 13C-labeling synthesis.More importantly,in comparison with the mechanism for the synthesis of model compound,the different mechanism of COFs’synthesis was corresponded to the high selectivity of COFs’ synthesis:The preferable path in aromatization step was the aerobic oxidation rather than the transfer hydrogenation which would result the formation of side products.The high efficiency and selectivity of the synthesis of fused-ring-linked COFs exhibited the deeper integration between the synthetic organic chemistry and polymeric crystallization.3.New Chemistry:Transition-metal catalyzed dynamic covalent chemistry of amide bonds(in Chapter 5)The development of dynamic covalent chemistry of inert covalent bonds for the construction of COFs is the extremely important and challenging topic for building up the theory of polymeric crystallization and represents the frontier for the synthetic organic chemistry.In order to construct amide-linked COFs directly,we developed a new type of dynamic covalent chemistry,that is Ni(0)/NHC catalyzed dynamic transamidation and amide metathesis.The newly developed catalytic system showed high efficiency,for which the dynamic equilibrium could be reached within 30 minutes.Notably,we found an unusual role of aliphatic alcohols that can serve as co-catalysts for promoting the activation and transformation of amide bonds.Finally,we explored the direct construction of amide-linked COFs via the newly developed dynamic transamidation.