The Design,Preparation And Characterization Of PEG-functionalized Azole Salts And Their Applications In Heck Reaction In Water

At present,the problem of the environment is more and more serious.The ideas of reducing emissions of ‘three wastes',saving energy,and sustainable development are the starting points when scientists design new chemical reaction systems.Each year there are a large amount of organic solvents discharged into the nature.Thus,performance organic reactions in a green reaction medium can fundamentally solve this problem.Among the green solvents which had been developed,water is the most preferable due to its abundance,low volatility and safety.Up to now,there have been a lot of organic reactions being performed in water.Stilbene derivatives is a kind of important compounds which are widely used in fields of pharmaceuticals,agrochemicals and material science.The palladium-catalyzed Heck reaction is an effective method for the synthesis of these compounds.So far,several articles using aqueous medium were reported,which are normally suffered from the requirement of large amount of Pd catalysts.Most of them need organic co-solvents and/or phase transfer catalysts.In this paper,we designed and prepared six different kinds of PEG functionalized azole salts,and employed them as water soluble NHC precursors for in-situ generation of corresponding Pd-NHC catalysts for Heck reaction in water.The contentsare as follows:Part 1:Six different kinds of PEG functionalized azole salts were synthesized from commercially available materials by three steps.Firstly,MeO-PEG1900-OH was reacted with MsCl to form MeO-PEG1900-OMs,which was then treated with imidazole and benzoimidazole to form MeO-PEG1900-Im and MeO-PEG1900-BIm.Finally,the resulted MeO-PEG1900-Im and Me O-PEG1900-BIm were heated with various organic bromides(2-(bromomethyl)pyridine,benzyl bromide and 1-(bromomethyl)naphthalene)to generate the corresponding imidazolium salts L1-L3(CH3O-PEG1900-Im-Py,CH3O-PEG1900-Im-Ph and CH3O-PEG1900-Im-Naph)and benzoimidazolium salts L4-L6(CH3O-PEG1900-BIm-Py,CH3O-PEG1900-BIm-Ph,CH3O-PEG1900-BIm-Naph)under solvent-free conditions.The structures of L1-L6 have been characterized by 1H NMR,13 C NMR,and MALDI-TOF-MS.Part 2: The obtained PEG functionalized azole salts L1-L6 were used as NHC precursorfor in-situ generation of corresponding Pd-NHC catalysts for Heck reaction in water.Firstly,with the Heck coupling reaction of 4-bromoacetophenone and styrene to form stilbene as model reaction,the catalytic performance of PEG functionalized azole salts L1-L6 were compared.The results revealed that L1 and L4 exhibited better catalytic performance than their analogues L2-L3 and L5-L6.Moreover,L1 was better than L4.It showed that the catalysts with pyridinyl group were more stable and thus resulted the better catalytic performance than benzimidazoles.Other reaction conditions,including palladium sources,the ratios of palladium and ligands and base were optimized.Finally,with the 1:1 of Na2PdCl4 and L1,0.05-0.1 mol% of Na2PdCl4 as catalyst,2 equivalences of NaOEt as base,under N2 atmosphereand at 100-120 oC for 12 h in water,a wide range of substituted aryl bromides and heterocyclic aromatic bromides were reacted with alkenes to achieve corresponding olefin products in good to excellent yields(51-98%)and the catalyst TONs value were in the range of 510-10000.Noteworthy,the selectivity of E-isomer products were generally more than 99%.This paper established a novelcatalytic system for Heck reaction in water.The catalyst loading was much lower than previous reported counterparts.Additionally,the protocol did not require the addition of organic co-solvents and phase transfer catalysts.It provided a green,facile and efficient method for the synthesis of stilbene compounds.