Palladium Complexes Derived From Nitrogen-and Phosphorus-containing Heterocycles: Synthesis And Catalytic Application In C-C Coupling Reactions

The synthesis of palladium complexes is a very active field in organometallics chemistry.Because of their good thermal stability and easy storage,palladium complexes with different ligands are used as effective catalysts for construction of C-C and C-N bond.Palladium complexes-catalyzed coupling reactions are widely used in organic synthesis,drug development,material chemistry,natural products and so on.In this paper,we mainly study the synthesis of palladium complexes derived from nitrogen-and phosphorus-containing heterocycles and their application in C-C coupling reactions.This paper mainly includes three parts:Part 1 IntroductionIn this chapter,the recent progress on the synthesis and catalytic C-C coupling reactions of phosphorus-coordinated palladium complexes and N,N'-coordinated palladium complexes were mainly introduced.Part 2 Synthesis,characterization and catalytic application in C-C coupling reactions of monodentate phosphorus-coordinated palladium complexes1.3,5-disubstituted-1,2,4-diazaphospholes(H[3,5-R2dp](L1: R = H,L2: R = i-Pr,L3: R = Ph,L4: R = t-Bu)as monodentate ligands reacted with Pd(CH3CN)2Cl2 to obtain four novel monodentate phosphorus-coordinated palladium complexes,namely Pd{H[3,5-H2dp]}2Cl2(I),Pd{H[3,5-i-Pr2dp]}2Cl2(II),Pd{H[3,5-Ph2dp]}2Cl2(III),Pd{H[3,5-t-Bu2dp]}2Cl2(IV),which were characterized by melting point,NMR,IR and elemental analysis.The crystal structure of the complex(IV)was determined by X-ray structural analysis,which displayed that the central Pd ions took a precisely planar geometry and coordinated with two phosphorus atoms from both H[3,5-t-Bu2dp] and two chlorine atoms.2.The Suzuki-Miyaura reaction of aryl halides with arylboronic acids were carried out with monodentate phosphorus-coordinated palladium complexes(I-IV)as catalysts.By studying the effects of catalysts loading,catalyst,temperature,base and solvent on the model reaction,the optimal reaction conditions were obtained as follows: palladium complex(IV)(8 mol%),Cs2CO3(2 equiv),4-bromoacetophenone(1a)/ phenylboronic acid(2a)= 1:1.5,DMF(1 m L),24 h,110 °C.Then we further studied the substrate scope of this reaction.The results showed that the reaction had a wide range of substrates and obtained a series of different biphenyl derivatives in good yields(up to 99%).To a certain extent,the electron effect and steric hindrance of the substituents had an obvious influence on the substrates' reactivity.The aryl halides bearing electron-withdrawing groups(EWG)were more favorable for the reaction.3.Monodentate phosphorus-coordinated palladium complexes(I-IV)were catalyzed by the Mizoroki-Heck reaction of aryl halides with olefin derivatives.The optimal conditions were obtained by screening different parameters as follows: complexe(II)(5 mol %),K2CO3(2 equiv),4-bromoacetophenone(1b)/ methyl acrylate(2a)= 1:1.5,DMF(1 m L),24 h,110 °C.We performed the substrate extension of this reaction and the corresponding products were obtained in good yields(up to 98%).These experiment demonstrated that aryl halides bearing electron-withdrawing groups(EWG)(R = CHO,COCH3)gave higher yields than those bearing electron-donating group(EDG)(R = H,OCH3).Beyond that,the activity of Mizoroki–Heck reaction was also influenced by the steric hindrance and electronic effect of substituents on olefins.At the same time,we had also studied the Mizoroki-Heck reaction of aryl halides with olefins under ligand L2(10%)and Pd(OAc)2 catalytic system and better yields were obtained.Part 3 Study on the C-C coupling reactions catalyzed by N,N '-bidentate palladium complexesThe Suzuki-Miyaura reaction of aryl halides with arylboronic acids were carried out with the N,N'-bidentate palladium complexes(V-VIII)as catalysts.By screening the type of catalysts,the loading of catalysts,the base,the solvent and the temperature,optimal conditions were established as follows: complex(VI)(5 mol%),NEt3(2 equiv),4-bromoacetophenone(1a)/ phenylboronic acid(2a)= 1:1.5,DMF(1 mL),24 h,110 °C.Under the optimal conditions,the reactions of different aryl halides with arylboronic acids were performed,giving the coupling products in good yields(up to 98%).Different groups of aryl halides had obvious effect on the Suzuki-Miyaura reaction and the reactivity of substrates with electron-withdrawing groups was higher than those with electron-donating groups.The steric hindrance of the substituents also had different effects on the reaction.Finally,the Mizoroki-Heck reaction of aryl halides with arylboronic acids were achieved in the presence of ligands L6 and Pd(OAc)2,affording the products in good yields.