Masters Degrees (Chemistry)

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Browsing Masters Degrees (Chemistry) by Subject "Alkenes"

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    Ruthenium based homogeneous olefin metathesis
    (University of the Free State, 2005-11) Kirk, Megan Michelle; Roodt, A.; Meyer, W.
    English: The aim of this study was to investigate the decomposition of the methylidenes 4a [Ru(Cl)2(PCy3)2CH2] and 6 [Ru(Cl)2(PCy3)(NHC)CH2] and alkylidene 5 [Ru(Cl)2(PCy3)2CHR], formed in the homogeneous olefin metathesis reaction using either the first generation Grubbs catalyst 2a [Ru(Cl)2(PCy3)2CHPh] or the second generation Grubbs catalyst 3 [Ru(Cl)2(NHC)(PCy3)CHPh]. (NHC = 1,3-dimesityl-4,5-dihydroimidazole-2- ylidene). Grubbs catalyst di-bromide 2b [Ru(Br)2(PCy3)2CHPh] was found to react with chlorinated solvents with the formation of CDCl2Br and CCl3Br, indicating that there is a direct reaction with the solvent itself (CDCl3) and not HCl in the solvent. A mechanism is proposed for this reaction. The intermolecular halide exchange reaction was observed during this study and used to gain insight into the bimolecular decomposition pathway. A mechanism where Grubbs catalyst 2a acts as a ligand in a reaction with the 14 electron intermediate 7b [Ru(Br)2(PCy3)CHPh] is proposed for intermolecular halide exchange (see scheme below). It is proposed that the intermolecular halide exchange reaction occurs continuously in all solutions of Grubbs catalyst, with the formation of the dimeric intermediate 45 (see below). The decomposition of Grubbs catalyst 2a in CDCl3 results in the formation of paramagnetic species and consequently in the loss of the 31 P NMR signal. It has recently been reported that phenol and SnCl2 dramatically increase the life time of Grubbs catalyst 2a. An understanding of the formation and behaviour of dimeric intermediate 45 lead to the proposal of a mechanism for the inhibition of bimolecular decomposition in the presence of phenol and SnCl2 in CDCl3. To inhibit the formation of the dimeric intermediate 45 above, the use of solid and dissolved salts as additives was studied. 31 P and 1 H NMR studies showed that the presence of Bu4NBr dissolved in CDCl3 results in halide substitution at Grubbs catalyst 2a [Ru(Cl)2(PCy3)2CHPh] to form a mixed halide catalyst 2c [RuBrCl(PCy3)2CHPh] and finally Grubbs catalyst di- bromide 2b [Ru(Br)2(PCy3)2CHPh]. Improved conversion and selectivity were obtained when solid and dissolved salts were added to the reaction mixture in the self metathesis of 1-octene. Molecular modelling has indicated that there is a possibility of ethylene induced decomposition of the Grubbs catalyst methylidene 4a [Ru(Cl)2(PCy3)2CH2] with the formation of propene as shown below. 1 H NMR experiments monitoring the ethylene induced decomposition of methylidenes 4a and 6 in C6D6 overnight clearly showed the formation of α - olefins, supporting the proposed β -hydride decomposition pathway. This pathway includes the formation of a hydride species that is highly likely to be isomerisation active and may lead to the decreased selectivity found experimentally. This study was taken further by carrying out ethylene induced decomposition in CDCl3, where it was found that there is no propene formation but rather addition of CDCl3 across the double bond via the Kharasch reaction. This indicates that the β -hydride pathway is inhibited in chlorinated solvents, which may be part of the reason why better selectivities are obtained in the presence of chlorinated solvents.
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    Silver(I) complexes as model catalysts in olefin hydroformylation
    (University of the Free State, 2007-11) Venter, Gertruida Jacoba Susanna; Roodt, A.; Meijboom, R.
    English: The aim of this study was to synthesize Ag(I) complexes of the type [AgXLn] (L = tertiary phosphine; n = 1-4; X = coordinating or non-coordinating anion) and explore the olefin hydroformylation activity and ligand exchange rates of these complexes. Tertiary phosphine complexes of Ag(I) of the type [AgXLn] (L = P(p-tol)3; n = 1-4; X = Br-, ClO4 -, PF6 -) were synthesized and characterized through X-ray crystallography. Selected crystal data is shown in Table 1. Table 1 Selected crystal data as obtained for the three Ag(I) crystal structures solved in this study. These complexes are comparable to similar complexes containing transition metals, other phosphine ligands or different counterions. Occurrences of similar structures in literature, however, were limited, indicating a field open to study. The behavior of Ag/PX3 complexes in solution are not yet explored, due to, amongst others, rapid and complex kinetics, and could be expanded on in future. The coordination of CO to these complexes for application as hydroformylation catalysts were investigated through high-pressure infrared spectroscopy. No evidence could be obtained through high-pressure infrared of coordination of CO to Ag(I) complexes. The aversion of the silver molecule to coordinate the CO molecule could be attributed to the coordination of bulky phosphine ligands, which could prevent the coordination of CO ligand to the metal centre, as well as the absence of a strong electron-accepting ligand, for example boron- or nitrogen-containing ligands. Another explanation is the high electron density surrounding the silver atom, which prevents -back bonding from the silver atom to the CO molecule. Kinetics of the exchange rate between coordinated phosphine ligands in these complexes and free phosphine is important, as this exchange rate could have an influence on the coordination of other ligands on the silver atom. The exchange rate was investigated using a NMR technique called magnetic spin transfer, or spin saturation transfer. In this method, the sample is saturated at a specific frequency, and through the relaxation of the peak at that frequency the exchange between free and coordinated phosphine could be established. The sample was investigated for different concentrations, shown in Table 2 with the calculated values of the rate of exchange. a) No e.s.d.’s were obtained from the fitting program, but are estimated to be ca. 10%. The rate of exchange at different concentrations is shown in Figure 1.The observed rate of exchange of free phosphine with coordinated phosphine is fast, in ca. 7 s-1 at -80 °C, with an average value of 29±60 mM-1.s-1 for k1 and 6.9±0.7 s-1 for k-1. The rate of exchange between coordinated and free phosphine has been found to be independent of the concentration of phosphine, indicating a dissociative mechanism. Since no evidence could be obtained of coordination of CO to the metal centre, application of complexes of Ag(I) of the type [AgXLn] (L = tertiary phosphine; n = 1-4; X = coordinating or non-coordinating anion) as hydroformylation catalysts does not seem feasible.
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    Synthesis of novel polymer-bound morpholine-N-oxides as possible oxidants in alkene oxidation
    (University of the Free State, 2008-03) Buitendach, Blenerhassitt Edward; Swarts, J. C.; Bezuidenhoudt, B. C. B.
    English: In this dissertation is reported the syntheses and characterisation of polysuccinimide- and polyepichlorohydrin-bound morpholine-N-oxide as possible polymeric oxidants. The use of 1H NMR spectroscopy to determine polymer chain length and degree of fuctionalisation is described in detail. The synthesised polymers were used as potential oxidants in catalytic oxidation of alkenes. However, none of the preliminary trials on the epoxidation and dihydroxylation of trans-stilbene were successful. Two metal-containing phthalocyanines; one coordinated to Mn(III), the other to Zn(II), were synthesized by metal insertion into the metal-free non-peripheral octa substituted phthalocyanine (2HPc-(C13H27)8). The initial complex, 2HPc-(C13H27)8, was synthesized by tetramerization of 3,6-tridecylphthalonitrile, which was prepared by a three-step synthesis from thiophene. Characterization of the phthalocyanines included electrochemical and thermal analysis. The cyclic voltammograms of the 2H and Zn phthalocyanines showed two ringbased oxidations (0.116 V; 0.487 V and 0.044 V; 0.558V respectively) as well as two ringbased reductions (-1.791 V; -1.456 V and -2.054 V; -1.663 V respectively), vs Fc/Fc+ at 100 mV/s. The Mn derivative showed two ring-based oxidations (0.373 V and 0.864 V) while only one ring-based reduction was observed (-1.732 V). The Mn(II) oxidation was observed at 0.641 V while Mn(III) reductions was observed at -0.742 V and -0.660 V (for Cl- and CH3Oaxial ligands) giving large Ep values of 1.566 V and 1.484 V.The newly synthesized tridecyl-substituted metal-free and zinc phthalocyanines exhibited liquid crystalline mesophase behavior when subjected to differential scanning calorimetric studies of between 40 oC – 120 oC and 40 oC – 280 oC respectively, giving mesophase temperature ranges of 12.4 oC and 83.0 oC respectively. The manganese phthalocyanine, xii however, did not show any liquid crystal behavior. The manganese tridecyl-substituted phthalocyanine was used as catalyst in the molecular oxygen based epoxidation of transstilbene and gave low yields of the desired epoxide, trans-stilbene oxide. Epoxidation of trans-stilbene using N-methylmorpholine-N-oxide as oxidant and Mn(III)salen as co-catalyst gave trans-stilbene oxide in moderate yields.