Chiral helical oligopyridine metal complexes : syntheses, characterizations and applications in asymmetric catalysis

手性螺旋多呲啶金屬的合成, 表徵及其在非對稱催化反應的應用

Student thesis: Doctoral Thesis

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Author(s)

  • Ho Lun YEUNG

Detail(s)

Awarding Institution
Supervisors/Advisors
Award date2 Oct 2008

Abstract

Chiral C2-symmetric quarter-, quinque-, and sexipyridine were prepared to study the helical complex formation with various metal ions. Quaterpyridine ligands, L1−4, were synthesized in good yields by nickel(0) catalyzed homocoupling of chiral bromo-substituted bipyridines. New quinquepyridine ligands, L5−6, were synthesized by one-step Stille coupling of chiral bromo-substituted bipyridines with 2,6-bis(trimethyltin)pyridine. New sexipyridine ligands, L7−8, were synthesized by nickel(0) catalyzed homocoupling of chiral bromo-substituted terpyridines. The palladium complexes, [Pd2(L)(3-C3H5)2](SbF6)2 (L = L1−4) were prepared by reaction of L with allylpalladium dimer, followed by counterions exchanged with AgSbF6. All complexes were characterized by NMR, ESI-MS and elemental analyses. One palladium complex, [Pd2(L4)(3-C3H5)2](SbF6)2 was characterized by X-ray crystallography and in the structure, L4 was found to coordinate to two palladium metals resulting in a single-helical structure. Its helical structure was characterized in solution by circular dichroism and NMR spectroscopies. The chiral Pd helicates were very active towards asymmetric allylic substitution of 1,3-diphenylprop-2-enyl acetate with dimethyl malonate. The fastest reaction finished within 5 h with product yields up to 99% and ee up to 85%. The rhenium(I) helicates, [Re2(L)Br2(CO)6] (L = L1−4) were prepared by reacting L with bromopentacarbonylrhenium(I) complexes. All helicates were characterized by circular dichroism, NMR and ESI-MS. NMR studies showed that the diastereoselectivities of the rhenium helicates in solution to be ranged from 15 to 60%. Optically pure rhenium helicates were obtained by recrystallization and chromatographic separation methods. Four complexes were later characterized by the X-ray crystallography and all showed single-helical structure. The rhenium(I) center adopted a distorted octahedral geometry with one bidentate ligand coordination, three monodentate carbonyl ligands in facial coordination and one bromine ligand occupied the axial position. The helical structure was established with a combination of strong interactions and hydrogen bonds. The single-stranded rhenium helicates were active catalyst in enantioselective cyclopropanation of styrene giving yields up to 85% and ee up to 25%. Copper(II) complexes, [Cu(L)(ClO4)2] (L = L1−4) were prepared by reacting L with copper(II) perchlorate. Three crystal structures of copper(II) complexes were determined by X-ray diffraction methods. The monomeric copper(II) complexes adopted distorted octahedral geometries with four pyridine rings coordinated at the equatorial position. Large steric hindrance was observed between the chiral groups on the first and last pyridine of ligands which resulted in single-helical structures of Cu(II) complex. With a combination of X-ray and circular dichroism analyses, the helical chirality of the mononuclear copper(II) complexes were found to be predetermined by the chirality of the quaterpyridine ligands. Copper(I) helicates, [Cu2(L)2](PF6)2 (L = L1−4) were prepared by reacting L with copper(I) hexafluorophosphate. One copper(I) complex, [Cu2(L3)2](PF6)2 was determined by X-ray crystallography. This tetrahedral copper(I) complex was a two-metal, two-ligand dimer which displayed double-helical structure. Strong interactions were observed within the complex. The helical structures of all copper complexes were retained in solution as characterized by circular dichroism. NMR studies showed the diastereoselectivities of the copper helicates in solution ranged from 52 to 99%. The copper(I) helicates were tested in asymmetric cyclopropanation of styrene, but all of them were inactive. Iron complexes based on chiral oliogopyridine ligands were synthesized by reacting quarter-, quinque- and sexipyridine with iron(II) chloride. L4 formed mononuclear iron complexes [Fe(L4)Cl2], while L5−6 and L7−8 formed binuclear iron complexes with a formula of [Fe2O(L)Cl4]. The binuclear iron complexes were found to contain a μ-oxo bridge between the diiron centers and it was characterized by ESI-MS, UV, IR and elemental analysis. Circular dichroism analyses indicated that the diiron(μ-oxo) complexes of quinque- and sexipyridine possessed helical structure. All iron complexes were active in asymmetric epoxidation using hydrogen peroxide as terminal oxidant. The best result was obtained from the iron-sexipyridine complexes; epoxidation of styrene was completed within 5 min at 0 °C and high selectivity for styrene oxide (97%). The highest turnover number achieved was 83 with 4-methyl and 4-methoxystyrene as substrate. The best ee was obtained with styrene oxide (43%). The relative reaction rates were measured in the competition experiments of substituted styrene versus styrene. Results revealed that electron-donating substituents on styrene accelerated the reaction and the Hammett plot exhibited a good linearity with negative + value (−1.04).

    Research areas

  • Catalysis, Synthesis, Chirality, Metal complexes, Pyridine