Synthesis And Catalysis Applications Of Functionalized N-heterocyclic Carbene Metal Complexes

It's well known that different steric properties of the N-heterocyclic carbene ligands can result in drastically different consequences on the catalytic efficiency of their bound metal catalysts. Compared with phosphine ligands, in order to achieve a high selectivity, the design and synthesis of N-heterocyclic carbene ligands that have different topological structures have become one of the biggest challenges. Therefore, we selected N-heterocyclic carbenes as ligands to synthesize and explore different types of N-heterocyclic carbene metal complexes and their catalytic applcaiton in the different reactions. This dissertation is focused on the design and synthesis of functionalized N-heterocyclic carbene ligands and application in metal coordination chemistry and organometallic catalytic reactions, and it consists of four parts of research work as follows:Part 1. The functionalized N-heterocyclic carbene ligands [HL1]PF6 (2-5a, L1= 3-(1-ethyl-1H-benzimidazol-2-yl)methyl)-1-((6-methylpyridin-2-yl)methyl)imidazoly-ide ne) and [HL2]PF6 (2-5b, L2= 3-(1-ethyl-1H-benzimidazol-2-yl)methyl)-1-((6-methylpyridin-2-yl)-methyl)benzimidazolylidene) with both containing benzimidazole and pyridyl groups were prepared by the reaction of 2-chloromethyl-l-ethyl-benzimidazole with N-(6-methylpyridyl-methyl)imidazole or N-(6-methylpyridylmethyl)benzimidazole. The deprotonation reaction of [HL](PF6) (2-5a and 2-5b) with Ag2O proceed smoothly to generate corresponding silver-carbene compounds, subsequently by metal exchange with Ni(PPh3)2Cl2 resulting in Ni(II)-NHCs (2-6a and 2-6b). And the deprotonation reaction of [HL](PF6) (2-5a) with Ag2O proceed smoothly to generate corresponding silver-carbene compounds, subsequently by metal exchange with PdCl2(CH3CN)2 resulting in Pd(Ⅱ)-NHC complex (2-7a). In addition, Hg(II)-NHC complex (2-8a) was prepared by refluxing [HL](PF6) (2-5a) with Hg(OAc)2. The structures of N-heterocyclic carbene metal complexes were fully characterized by 'HNMR,13C NMR, H,H-COSY, ESI-MS spectroscopies, and X-ray diffraction analysis.Part 2. Two benzimidazole-functionalized Ni(Ⅱ)-NHCs (2-6a and 2-6b) were applied in Friedel-Crafts alkylation and Suzuki coupling reactions. Under the optimal condition, high isolated yield (up to 96%) was obtained in the F-C alkylation reaction of indoles andβ-nitrostyrenes. A variety of functional groups were well-tolerated on the substrates, and the substrates with either the electron-deficient or electron-rich aromatic rings were active in these two reactions and yields were uniformly moderate. Furthermore, under the optimal condition, the yields were up to 90%in the Suzuki coupling reactions of aryl boronic acids and aryl chlorides or bromides.Part 3. Much attention has been attracted for the structural modification of 2,2'-dihydroxy-1,1'-binaphthalene (BINOL) and the molecular scaffold for organocatalysts in asymmetric catalysis. Therefore, in its 3-position of structural modification for BINOL skeleton, the introduction of various types of ancillary ligands containing N-functionalized groups imidazole were attempted, and finally two types of N-heterocyclic carbenes ligands based on BINOL were obtained. The bidentate N-heterocyclic carbenes ligands based on BINOL are divided into two categories:(1) binaphthol hydroxyl groups protected by methyl containing the weak coordination ability of pyridine and pyrimidine groups, and three types of anions (Br-, Cl-and PF6-). (2) binaphthol structure of the phenol hydroxyl unprotected. In addition, we also obtained two types of monodentate N-heterocyclic carbene ligands based on BINOL. One is the phenolic hydroxyl protected by methoxymethyl, and the other is unprotected, in which anions are chloride ions. The different structures of B-heterocyclic carbene ligands based on BINOL have been obtained by a viable synthetic route from chiral binaphthol as a synthetic substrate via multi-step reactions. Part 4. An effective, regioselective C-2 arylation of imidazo[2,1-b]thiazoles catalyzed by Cu(I) was developed firstly without the promotion of the ligands. Under the optimal condition, high isolated yield (up to 98%) was obtained in the arylation of imidazo[2,1-b]thiazoles and aryl iodides. Various functional (22 samples) groups were well tolerated, and the electron-deficient or electron-rich aryl iodides were active and yields were uniformly moderate-excellent. Preliminary mechanistic studies of the arylation supported the addition between Cu(Ⅰ) and heterocycle gave the unstable cationic intermediates, then C2-H proton was removed by t-BuOLi, and subsequently, a reductive elimination of Cu(Ⅲ) intermediate to afford the desired arylated product.