The surface catalyzed racemization of a simple organic reaction - the racemization of 1,1'-binaphthyl - has been studied using carbon, Raney Nickel, and platinum (Adams' Catalyst) as solid catalysts. In each case the rate of the catalyzed reaction was studied using polarimetric methods. The dependence of the rate on various parameters was then used to determine features of the catalyzed reactions. In the case of the carbon catalyzed reaction, 4,4'-disub-stituted-1,1'-binaphthyls were synthesized and their catalyzed reaction rates determined. All the reactions followed smooth first order kinetics. Using a Hammett (σ[sup P]) plot for the catalyzed and uncatalyzed reactions, catalysis was found to proceed by way of electron donation into the binaphthyl ring system. Oxidation or reduction of the catalyst, which would modify disorganized areas on the surface, had no effect on its activity. Halogenation of the catalyst, which is believed to occur on the organized basal planes of the carbon, dramatically increased its activity and led to new solvent effects. The effect of halogenation was explained by considering chemisorbed solvent molecules to be a type of inhibitor whose desorption rate is increased when the sites of adsorption (the basal planes) are disrupted by bound halogen. A potassium-graphite intercalate was used as a model for the carbon surface and led to the conclusion that carbon catalysis may involve the formation of the radical anion of 1,1' binaphthyl on the basal planes of the carbon surface. Raney Nickel catalyzed both the racemization and the re-, duction of 1,1'-binaphthyl. In addition, an extensive adsorption process occurred. These three interactions could be controlled independently by careful poisoning with elemental sulfur or dodecanethiol. This suggested that there were three different types of surface sites on Raney Nickel, each responsible for a single type of interaction with 1,1'-binaphthyl. Also, it allowed the three processes to be studied independently. The reduction proceeded through three intermediates to give 5,6,7,8,5',6',71,81-octahydro-1,1'-binaphthyl as the final product. NMR studies showed this molecule to exist as two enantiomers which could interconvert at a slow rate (t[sub 1/2] = 17 hr) The racemization process showed good first order kinetic plots after an initial curved portion. The curved portion of the plots is due to the concurrent adsorption process. Problems of reproducibility hampered further kinetic studies. The adsorption process showed a Langmuir-type adsorption isotherm, as well as an adsorption isobar and kinetics which are typical of other slow chemisorptions. The platinum (Adams' catalyst) catalyzed racemization of 1,1'-binaphthyl also showed good first order kinetics. The observed rate constant, k[sub obs] was inversely proportional to the concentration of substrate. The reaction rate was also independent of the concentration of the catalyst. This peculiar kinetic effect could not be accounted for by the intervention of diffusion-controlled processes and remains unexplained. The poisoning effect of air and the inhibitory effect of cyclohexene and cyclohexane indicate that reduction and racemization occur on the same site. Continued studies were not possible because of an uncontrollable decrease in catalyst activity. / Science, Faculty of / Chemistry, Department of / Graduate

The kinetics and mechanism of the heterogeneous catalysis of the mutarotation of glucose by alumina have been investigated. Various types of aluminas held in suspension, in dimethyl sulfoxide, were used. At 25.0°C, the first order kinetic plots for mutarotation by alumina neutral (for thin layer chromatography; y-form) were curved due, first, to relatively slow adsorption of glucose on alumina and, second, to progressive deactivation of the catalyst. Partially deactivated catalysts produced linear first order plots over three half lives and hence, glucose mutarotation by alumina is a first order reaction. Further, there were <1% side products formed during the surface reaction. Dehydration of the catalyst at low temperatures (i.e. upto 600°C) decreased the catalytic activity, unlike the other reactions studied on alumina surfaces. On further dehydration at higher temperatures the catalytic activity increased, and the activity per unit area of α-alumina (= 3.6 x 10⁻⁵ sec⁻¹ m⁻² ) formed at 1250°C was about 26 times that of the standard alumina neutral. High catalytic activity for the ct-form of alumina compared with the y-form was previously virtually unknown. Further, this a-alumina did not deactivate during catalysis and produced linear first order plots over three half lives. Adsorption studies showed the presence of (0.70 ± 0.02) x 10⁻⁴ moles of irreversible adsorption sites on the surface of a gram of alumina neutral. The isotherm for adsorption of glucose on alumina neutral showed only monolayer adsorption. The Langmuir plot for reversible adsorption of glucose sites; (1.0 ± 0.1) x 10⁻⁴ moles of strong adsorption sites with an equilibrium constant for adsorption = (8.2 ± 1.2) x 10² litre mole⁻¹ , and (1.4 ± 0.2) x 10⁻⁴ moles of weak adsorption sites with an equilibrium constant for adsorption = 44 ± 3 litre mole The study of the variation of initial rate with concentration of a-D-glucose showed that only the weak adsorption sites are catalytically active. Hence the active site density on alumina neutral was obtained as (5.4 ± 0.6) x 10⁻¹³ sites/cm². The turnover number of a catalytic site was determined to be 2 x 10⁻³ molecules/site/sec. This is one of the highest turnover numbers for a reaction catalyzed by an alumina surface. The observed first order rate constant for the surface reaction, [Figure 1] was shown to be k[sub=obs] = {(k₃ + k₄) [catalystic sites]}/{(k₂/k₁) + [Glucose]}, where k₁/k₂ = k₂ . The catalytic constant (k₃ + k₄) for the interconversion of α-D-glucose (G[sub=α]) and β-D-glucose (G[sub=β]) on the surface was determined to be 5 x 10⁻³ sec⁻¹. Comparison with the catalytic constant for mutarotation in pure water (= 4 x 10⁻⁴ sec⁻¹) showed that the alumina surface offers a better medium for mutarotation than water. Further, the activity of the catalytic sites on alumina neutral is about 9 times that of strong acids in water. The inhibitory effects of 'neutral' molecules (water, methanol, methyl glucoside, inositol etc.) indicate that the glucose adsorption sites on alumina neutral are relatively specific for adsorption of polyhydroxy compounds. In addition, aldehyde (e.g. hexanal) groups seem to interact preferentially with the catalytic sites on the alumina surface. Studies with acidic (carbon dioxide) and basic (e.g. pyridine, n-butylamine) inhibitor molecules suggest that the catalytic activity of alumina towards glucose mutarotation is due to the presence of basic oxide ions and weak Bronsted acid sites on the surface. About 85% of the activity of α-alumina formed at 1250°C is due to these basic sites, while the weak Bronsted acid sites give rise to about 90% of the activity of alumina neutral. The observed high catalytic activity of these weak Bronsted acid sites is probably due to the stabilization of the transition state leading to acyclic intermediate by the polar alumina surface. Normal deuterium isotope effects were observed with alumina neutral (k[sub=H]/K[sub=D] = 1.3) and the other aluminas prepared by dehydration of alumina neutral (e.g. k[sub=H]/K[sub=D] = 1.9) with α-alumina formed at 1250°C). There was no isotope effect on the adsorption-desorption process and hence, the observed isotope effect is due to the catalytic reaction on the surface. Therefore, the observed normal isotope effects indicate that glucose mutarotation on alumina surface is a general acid-base catalyzed reaction, and occurs by a consecutive mechanism, via the acyclic intermediate. There is probably no bifunctional catalysis of the glucose mutarotation on the alumina surface. Further, the acid sites seem to show an isotope effect of 1.2 and the basic sites an isotope effect of 2.1. Hence, these studies have shown that glucose mutarotation differs (e.g. higher catalytic activity of the hydrated surface, high catalytic activity of a-alumina) from many other reactions studied on alumina surfaces. This difference in behaviour under mild conditions is probably due to the high sensitivity of the mutarotation reaction to the weak acidic and basic sites on the alumina surface. / Science, Faculty of / Chemistry, Department of / Graduate

A cooperative ligand is defined as one which actively participates in substrate activation to facilitate catalysis with a metal ion in a synergistic fashion. This dissertation focuses on the synthesis and catalytic activity of base-metal complexes with cooperative SNS pincer ligands to explore unconventional reaction pathways that are a consequence of diverging from traditional phosphine-based ligands. Two new NHC–Cu(I)-[κ2-SNS] complexes were synthesized to directly compare the bifunctional catalytic activity between the two SNS ligands. The Cu thiolate complex catalyzed ketone hydroboration but not hydrosilylation, while the Cu amido complex is a high-performing carbonyl reduction catalyst boranes and silanes, through a conventional outer-sphere mechanism. The bifunctional reactivity of three M[SMeNSMe]2 complexes was computationally assessed by comparing the nucleophilicity of the M–Namido donor (M = Mn, Fe, Co), and the Mn analogue was identified as the most promising catalyst candidate. A combined experimental and mechanistic study of the chemoselective hydroboration of carbonyls by Mn[SMeNSMe]2 follows. The catalyst allows for room temperature hydroboration of carbonyls at low catalyst loadings (0.1 mol%) and reaction times (<30 min). Mechanistic studies highlight the significance of bifunctional amido bis(thioether) ligand to the success of the reaction. DFT calculations showed that thioether hemilability is crucial during catalysis for providing the active coordinating site. A non-traditional inner-sphere reaction pathway with carbonyl coordination to the metal center and amido-promoted B–H reactivity is proposed to be operative, as opposed to the traditional metal-hydride pathway enabled by phosphine-based bifunctional ligands. The manganese(I)-thiolate complex Mn(κ3-SMeNS)(CO)3 is an active precatalyst for the photo-catalyzed dihydroboration of nitriles. Reaction optimization studies revealed that catalysis requires the presence of UV light to enter and remain in the catalytic cycle. Stoichiometric mechanistic studies showed that HBpin borylates the imine N=C of the ligand backbone in the absence of nitrile, forming an inactive off-cycle by-product. Isotopic labeling studies with 13CO revealed that the catalyst resting state features a single CO ligand coordinated to the Mn center. DFT calculations showed that the bifunctional thiolate donor, coordinative flexibility of the SMeNS ligand, and access to an open-shell intermediate are all crucial to accessing low-energy intermediates during catalysis. The electronic structure of a Fe[N2S3] complex was investigated in detail. Cyclic voltammetry and spectroelectrochemistry studies show a reversible oxidation and reduction to stable species. The anionic redox partner of Fe[N2S3] was synthesized and characterized by X-ray diffraction, Mössbauer spectroscopy, and X-ray absorption techniques along with Fe[N2S3]. Both Mössbauer and X-ray absorption near edge structure (XANES) data indicated a ligand-based reduction and DFT studies of the Zn analogs allowed us to propose a new bonding scheme for the reduced ligand. Redox interconversions in these complexes are dominated by changes in electron population in the N2S3 ligand, and intimate mixing with the Fe dxy orbital due to a high degree of covalency.

Palladium catalyzed cross-couplings have revolutionized the field of organic synthesis. Using this family of reactions, many different bonds can be formed in a selective manner, include C-C, C-N, C-S and C-O bonds. One reaction included in this family is Negishi cross-coupling, which uses an organozinc reagent as the nucleophilic coupling partner. One aspect of these reactions that has gone largely overlooked is their dependence on inorganic salt additives. In this work, the beneficial, and sometimes harmful, effects of salt additives are investigated. This work realized two new roles for salt additives on Negishi-coupling, the prevention of product inhibition and catalyst deactivation. Throughout these studies, the choice of catalyst is shown to have significant impacts on the yield and selectivity of Negishi coupling reactions. In particular, the use of a chlorine functionalized N-heterocyclic carbene (NHC) ligand is shown to be a critical choice that must be carefully considered. In alkyl-alkyl couplings, this modification was shown to erode the selectivity under certain circumstances. This thesis comes around full circle in the final chapter where insights from mechanistic studies are applied to a commercially relevant cross-coupling. A variety of unnatural amino acids were synthesized using a Negishi coupling as the critical, diversification step. A chlorine functionalized catalyst and salt additive were critical for these couplings, highlighting the importance of studying mechanistic intricacies.

Novel H+ -sulfori"ated poly( styrene-co-divinylbenzene) ion-exchange resins have been prepared with· higher acid strength and higher thermal stabilities than conventionai ion-exchange resins. An objective of the study has been to increase the understanding of the relationships between resins structures, acidities and acid catalytic activities in aqueous and non-aqueous media. Some related materials, supported sulfonic acid catalysts, have been prepared using HMS (hexagonal mesoporous silicas) and SBA 15 as supports. These catalysts were characterised in terms of surface area and pore size distribution from nitrogen adsorption, powder X-ray diffraction, cation exchange capacity, thermogravimetric analysis and FT-Raman spectroscopy. These functionalised inorganic materials have been compared with their organic counterparts to improve the understanding of these systems. Both of these ranges of catalysts, with varying level of sulfonation, have been characterised by ammonia adsorption microcalorimetry in the absence of solvent. The resulting molar enthalpies of ammonia adsorption have been interpreted in terms of the relative strength of acid sites. The same catalysts have also been neutralised in a titration microcalorimeter (TITRYS) in which the materials have been held under a solvent (water, acetonitrile, cyclohexane) and a base probe compound titrated into the sample cell in the same solvent. Molar enthalpies of neutralisation measured for thes'e solid acid catalysts in this way have been compared with molar enthalpies of neutralisc:!ion for similar acid groups (sulfonic acids) in homogeneous solutions. The resulting molar enthalpies of neutralisation in non-aqueous solvents have been interpreted in terms of the relative strength of acid sites. In addition, FT-Raman spectroscopy has been used to study the sulfonic acid groups in the resin gels and comparison made with similar acid groups in homogeneous aqueous solutions. These in turn have been compared with catalytic data measured for the sulfonated catalysts in three liquid phase reactions. Variation in measured acidity (with and without) solvent, catalytic activities have been interpreted in terms of structural features of the catalysts and their interaction with solvents.

No description available.

No description available.

No description available.

No description available.

No description available.