Nitrogen-containing Heterocyclic Carbenes And Their Preparation Of Transition Metal Complexes And Catalysis Research

N-Heterocyclic carbenes have become universal ligands in organometallic and inorganic coordination chemistry. They not only bind to any transition metal, with low or high oxidation states, but also to main group elements such as beryllium, sulfur, and iodine. Because of their specific coordination chemistry, N-heterocyclic carbenes both stabilize and activate metal centers in quite different key catalytic steps of organic syntheses, for example, C-H activation, C-C, C-H, C-O. and C-N bond formation. There is now ample evidence that in the new generation of organometallic catalysts the established ligand class of organophosphanes will be supplemented and, in part, replaced by N-heterocyclic carbenes. Over the past few years, this chemistry has become the field of vivid scientific competition, and yielded previously unexpected successes in key areas of homogeneous catalysis. From the work in numerous academic laboratories and in industry, a revolutionary turningpoint in oraganometallic catalysis is emerging.In this thesis, Palladium(Ⅱ) acetate and N,N'-bis-(2,6-diisopropylphenyl) dihydro- imidazolium chloride 1 (2 mol%) were used to catalyze the carbonylative coupling of aryl diazonium tetrafluoroborate salts and aryl boronic acids to form aryl ketones. Optimal conditions include carbon monoxide (1 atm) in 1,4-dioxane at 100℃ for 5 h. Yields for unsymmetrical aryl ketones ranged from 76 to 90% for isolated materials with only minor amounts of biaryl coupling product observed (2-12%). THF as solvent gave mixtures of products. 1,4-Dioxane proved to be the superior solvent giving higher yields of ketone product together with less biphenyl formation. At room temperature and at 0℃ with 1 atm CO, biphenyl became the major product. Electron-rich diazonium ion substrates gave a reduced yield with increased production of biaryl product. Electron-deficient diazonium ions were even better forming ketones in higher yields with less biaryl by-product formed. 2-Naphthyldiazonium salt also proved to be an effective substrate givingketones in the excellent range. Base on above palladium NHC catalysts, aryl diazonium tetrafluoroborates have been coupled with arylboron compounds, carbon monoxide, and ammonia to give aryl amides in high yields. A saturated yV-heterocyclic carbene (NHC) ligand, H2lPr 1 was used with palladium(II) acetate to give the active catalyst. The optimal conditions with 2mol% palladium-NHC catalyst were applied with various organoboron compounds and three aryl diazonium tetrafluoroborates to give numerous aryl amides in high yield using pressurized CO in a THF solution saturated with ammonia. Factors that affect the distribution of the reaction products have been identified and a mechanism is proposed for this novel four-component coupling reaction.NHC-metal complexes are commonly formed from an imidazolium salt using strong base. Deprotonation occurs at C2 to give a stable carbene that adds to form a a-complex with the metal. Crystals were obtained from the reaction of imidazolium chloride with sodium t- butoxide, Nal and palladium(II) acetate, giving a dimeric palladium(II) iodide NHC complex . The structure adopts a flat, 4-membered ring u2 -bridged arrangement as seen in a related dehydro NHC complex formed with base. We were pleased to find that chloride, treated with palladium(II) acetate without adding base or halide in THF, also produced suitable crystals for X-ray anaysis. In contrast to the diiodide, the palladium-carbenes are now twisted out of plane, adopting a non-planar 4-ring core. The borylation of aryldiazonium tetrafluoroborates with bis(pinacolatoborane) was optimized using various NHC ligand complexes formed in situ without adding base. N,N'-Bis(2,6-diisopropylphenyl)-4,5-dihydroimidazolium 1 used with palladium acetate in THF proved optimal, giving borylated product in 79% isolated yield without forming of bi-aryl side product. With K2CO3 and ligand 1, a significant amount of biaryl product(24%) was again seen. The characterization of the palladium chloride complex by X-ray chrastallography demonstrates that adding base is not needed for carbene formation or catalyst turnover, therefore, optimal conditions exclude adding base.In view of the relative effect on ligand structures and catalytic properties,N,N'-Bis(9-phenanthryl)-4,5-dihydroimidazolium hexafluoro phosphate 2, N,N'-Bis(10-cyclohexyl-9- phenanthryl)-4,5-dihydroimidazolium hexafluoro phos phate 3 and N,N'-Bis(2,10-dicyclohexyl-9- phenanthryl)-4,5-dihydroimidazol ium hexafluoro phosphate 4 were prepared. Bulky phenanthracenyl imidazolium derived carbene ligands were investigated for copper-free Sonogashira coupling with terminal acetylenes. Aryl bromides and iodides gave coupled products in excellent yields from the Pd(PPh3)2Cl2 complex with potassium /-butoxide and 18-crown-6 in THF. A remarkable dependence on the size of the ligand was found, the bigger the ligand, the higher the yield. The highest yield was obtained with the bulky 4 ligand. On the other hand, above novel bis-phenanthryl N-heterocyclic carbene (NHC) based palladium acetate catalyst has also been shown to be effective for the coupling of various aryl and vinyl chlorides with organoboron compounds. N,N'-Bis(2,10-dicyclohexyl-9-phenanthryl)-4,5-di-hydroimidazol ium chloride 5 (H2ICP-HC1) was used with Pd(OAc)2 with KF/ 18-C-6 in THF at room temperature for the Suzuki-Miyaura coupling of aryl and vinyl chorides, including unactivated and di-ortho substituted substrates, and organoboronic acids in high yields. Less hindered carbene ligands were far less effective. Hindered tri-and tetra-substituted products were efficiently produced. Benzyl chloride was also found to be a useful coupling partner. Trimethylboroxine was used to give methylated products. Remarkably, an electron rich arylfluoride also proved to be an effective substrate under these conditions. The effect of ligand, base, temperature, solvent, and reaction time are reported along with the various substrates including halides and triflates.There are two factors that may contribute to the success of the more bulky ligands in this case: dimmer inhibition and enhancement of the rate of the reductive elimination step. The more bulky aryl groups may disfavor palladium complex dimerization, allowing for a higher concentration of the reactive palladium carbene monomer. Known crystal structures of palladium-NHC complexes are (^-bridged dimmers. Herein, a dramatic dependence on the steric bulky of extended aromatic phenanthryl imidazolium carbene ligands for palladium-catalyzed Sonogashira andSuzuki-Miyaura coupling has been demonstrated. Hindered NHC ligands of this type may be found to possess enhanced activity with related catalytic processes.Dimeric chiral [2.2]paracyclophanes are rare and their use as catalysts has not been reported so far. In this thesis, the new planar chiral ligands began with the p-Xylene, 5p-N,N'-Bis(4-[2,2]paracyclophanyl)-4,5-di-hydroimidazolium tetra fluoroborate 6, 5p-N,N'-Bis(12-phenyl-4-[2,2]paracyclophanyl)-4,5-dihydroimida zolium tetrafluoroborate 7, Sp-N,N'-Bis(12-cyclohexyl-4-[2,2]paracyclophanyl)-4,5-dihydroimidazolium tetrafluoroborate 8, Sp-tyN'-BisCn-o-methoxyphenyl-4-[2,2]paracyclophanyl)-4,5-dihydroimidazolium tetrafluoroborate 9 were prepared. These new ligands and Rh(acac)(C2H4)2 were performed with phenyl boronic acid (1.5 equivalent) and cyclohexanone under standard conditions in THF/water, excellent yields and asymmetric selectivities are obtained. Numerous variations of the reaction conditions were explored using o-toluylboronic acid added to cyclohexanone using the rhodium-NHC 9 complex. With the amount of rhodium held constant at 2 mol%, the amount of imidazolium 9 was varied from 0 to 4 mol%. Without adding ligand, no product was obtained after 3 hours. The yield was very low, 27% when 1 mol% 9 was used , but the selectivity remained high at 89%ee. Three mol% of 9 proved optimal, 2:1 ligand to Rh, giving a 91% yield and 95%ee at 60° after three hours. When the reaction was stopped after two hours, a 56% yield was obtained with 96%ee. At 35°C a lower yield was obtained with no improvement in selectivity. At 80 °C, the selectivity dropped off to 66%ee. When THF alone was used without water, the reaction was much slower and gave a lower yield and selectivity. The rate was faster when a greater proportion of water was used, however the selectivities were lowered to a greater extent. Use of 10:1 dioxane/water mixture as solvent, typical conditions for the Rh-BINAP conditions, gave an 89% yield with 95%ee after 5 hours. Methanol and methanol/water combinations gave good reaction rates, but with lowered selectivities. Effect of bases was investigated, Na2CO3, NaHCO3, CS2CO3, CsF, and K2CO3, all increased the rate of the reaction but gave poor selectivity. When the amount of boronic acid was lowered to 1.2 equivalents, the high selectivitywas maintained and the yield was lowered to 82%. A similar effect was noted when the counter anion on the imidazolium salt 9 was changed from tetrafluoroborate to chloride. The optimal reaction conditions with ligand 9 were applied to cyclic enones and various aryl boronic acids and potassium trifluoroborates. Phenyl boronic acid added to all entries with high yield and selectivity. Potassium trifluoroborate reacted at a faster rate with lower selectivity under these conditions. The p- methoxyphenyl and o-toluyl boron reagents gave similar results. The electron deficient reagents, /?-acetyl and trifluoromethyl boronic acid also gave excellent yields and selectivities. Acyclic enones were also explored using the phenyl boron reagents. In general, the yields were again excellent, however, the selectivities were reduced significantly. The substrate showing the highest selectivity in the case was the isopropylmethylvinyl ketone at 91%ee. Trifluoroborate again reacted at a faster rate with lower selectivity.