Copper coordination chemistry with Pyridyl-Imine ligands and their application in aerobic oxidation reactions

Hu, Zhenzhong (2012) Copper coordination chemistry with Pyridyl-Imine ligands and their application in aerobic oxidation reactions. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Recyclable catalysts and oxidation chemistry are two important areas in the field of Green Chemistry. In this thesis, research efforts towards the development of a re-usable aerobic oxidation catalyst arc described. The background to this field and a review of relevant literature are presented in Chapter 1. -- In Chapter 2, the preparation and characterization of a series of six potentially tetradentate di(pyridyl-imine) ligands presented. In each case the pyridyl-imine groups were separated by a different linking unit (in L1, CH₂CH₂CH₂(SiMe₂O)₂₀SiMe₂Ch₂CH₂CH₂ ; in L2, CH₂CH₂CH₂SiMe₂OSiMe₂CH₂Ch₂CH₂ ; in L3, CH₂CH₂ ; in L4, CH₂(CH₂)₄CH₂ ; in L5, CH₂(CH₂)₇CH₂ ; in L6, CH₂CH₂CH₂OCH₂CH₂OCH₂CH₂ OCH₂CH₂CH₂). The solubilities of L1 and L2 in supercriticaI carbon dioxide were determined. The coordination chemistry of L1-L2 with Cu(I) was studied by UV-Vis, multinuclear NMR and IR spectroscopies, MALDI -TOF and ESI mass spectrometries and elemental analysis. Dicopper complexes of L3-L6 were prepared for comparison. PGSE (pulsed-gradient spin-echo) NMR spectroscopy was used to determine the hydrodynamic radii of the species in solution and comparison of these data with computational models for the complexes was made. There is no evidence to support linear metallopolymer formation but data suggest that [2+2] and [1+1] metallomacrocyles were formed, with siloxane linking groups encouraging the formation of [1+1] species. Solid-state NMR data on [Cu(Li)]PF₆) indicate the presence of two different environments for the PF₆⁻ anions and this could be due to the location of PF₆⁻ anions both inside and outside the metallomacrocyclic species. -- In Chapter 3, an investigation into the ability of L1-L6 in catalytic oxidations of alcohols when combined with copper bromide (CUBT2) and 2,2,6,6-tetramethylpiperidyl-1-oxy (TEMPO) is described. Analogous bidentate ligand (L 7) showed poorer catalytic activity than L 1- IA Also, the ratio of Cu:ligand was of crucial importance in maintaining high yields. The polydimethylsiloxane (PDMS) derived pyridyl-imine terminated ligand L1 combined with copper (II) ions afforded an effective and selective catalyst for aerobic oxidations of primary and secondary alcohols under aqueous conditions. Preliminary mechanistic studies suggested that bimetallic complexes may be playing a role in the catalytic transformation. -- In Chapter 4, oxidation of amines using a copper-containing catalytic system is described. CuBr₂ with TEMPO was successfully employed for the aerobic oxidation of primary and secondary benzyl amines in aqueous acetonitrile. The same catalyst system was also used for oxidative cross-couplings of benzylamines with anilines. The electronic and steric properties of the aniline partner were found to be of crucial importance in determining reactivity or lack thereof. A mechanism for these reactions is proposed based on the data obtained to date. In the absence of benzyl amines, electron-rich anilines were found to undergo dehydrogenative coupling and yields of the resulting azo products could be increased by replacing CuBr₂ with CuBr. No ligand (e.g. pyridine) is required for either reaction to proceed and presumably water and acetonitrile solvate the copper-containing intermediates. -- Chapter 5 contains a summary of the thesis and suggestions for future research in this area.

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