Research On Synthesis Of Imidazolium For The Hydroxyl-Group-Modified N-heterocyclic Carbenes And Catalysis

The heterocyclic carbene imidazolium salts and their derived metal complexes have been a hotspot in the field of chemistry,and some palladium complexes have broad application prospects in constructing C-C bonds.In recent years,some N-heterocyclic carbenes containing functional groups(N,O,NO,etc.)have been reported,and these oxygen-functionalized heterocyclic carbenes are formed by the functional side arms were more stable,and its catalytic activity were also more excellent.In this paper,two kinds of hydroxyl-group-modified imidazolium precursors were synthesized,and their in situ generated carbene ligands were used in Suzuki-Miyaura coupling reactions,Heck coupling reactions and Sonogashira coupling reactions,the reactivity in these coupling reactions was explored.First of all,four imidazolium precursors 1a-1d for the N-heterocyclic carbenes(NHCs)with a hydroxyl functionalized four-membered heterocyclic side arm and the corresponding transition metal complexes were synthesized and characterized by IR,XRD and NMR spectroscopies.The corresponding NHC ligands thus generated in situ from these imidazolium precursors in the presence of base such as KOH,together with the palladium source PdCl2(CH3CN)2,exhibited excellent catalytic activity in Suzuki-Miyaura cross-coupling reactions for the synthesis of a range of biaryl compounds.The reactions could be conducted in the mixed solvent of tert-butyl alcohol/water(1:1)at room temperature with the advantages of mild conditions,high efficiency as well as environmental friendliness.In addition,although in minor amount,the mixed catalyst system containing imidazolium salts,PdCl2(CH3CN)2(0.1%)and Cu I(1%)exhibited excellent catalytic activity in Sonogashira coupling reaction.In particular,the combination system of PdCl2/imidazolium salt 1b has been shown to be higher catalytically active for both coupling reactions.And the imidazolium salts 2a-2b,which do not contain hydroxyl functional groups,exhibit poor reactivity in both of the palladium-catalyzed coupling reactions.Secondly,two imidazolium precursors 3a-3b for the substituted N-heterocyclic carbenes(NHCs)with a hydroxyl functionalized heterocyclic side arm were synthesized and characterized by NMR spectroscopy.The results show that the mixed catalyst system containing imidazolium salts and PdCl2(CH3CN)2 exhibited excellent catalytic activity in the Heck coupling reaction.The reactions could be conducted in the presence of KOH as the base in the DMF solvent at 150 °C.In particular,the combination system of PdCl2/imidazolium salt 3b has been shown to be higher catalytically active in the Heck cross-coupling reactions.At last,under the same conditions,the reactivity of the imidazolium salts 1a-2b and 3a-3b in these three types of cross-coupling reactions was compared.The results showed that the imidazolium salts,which contain hydroxyl functional groups,had a higher reactivity in the palladium-catalyzed coupling reactions.In particular,the imidazole salts 3a-3b has been shown to be higher catalytically active as compared with the imidazolium salst 1a-2b.