A series of cationic Rh(I) diphosphine and phosphine-alkene complexes have been isolated and fully characterised. The reactivity of these species towards hydrogenation, dehydrogenation and dehydrocoupling reactions has been investigated. The use of potentially hemilabile ligands DPEphos and XANTphos in the intramolecular dehydrogenation chemistry of tricyclopentylphosphine is reported. The comparison in reactivity of these isolated diphosphine phosphine-alkene complexes towards hydrogenation and with acetonitrile is discussed along with their ability to dehydrocouple secondary silane, Ph₂SiH₂, and amine-borane H₃B·NMe₂H. The acceptorless dehydrogenation of a tethered cyclopentane with cationic Rh(I) diphosphine complexes has also been extended to include thioethers. Isolated cationic Rh(I) phosphine-alkene complexes with labile fluorobenzene ligands are found to act as a source of the reactive 12-electron [Rh{PR₂(ƞ²-C₅H₇)}]+ (R = cyclopentyl (Cyp)/ iPr) fragment in solution and can coordinate two amine-borane ligands (either H₃B·NMe₃, H₃B·NMe₂H or H₃B·NMeH₂) in a novel and unique bis-σ-binding mode. The catalytic activity of some of these isolated complexes in the dehydrocoupling of H₃B·NMe₂H and H₃B·NMeH₂ has been determined. With a view to further understanding the mechanism of catalytic transition metal assisted amine-borane dehydrogenation and dehydrocoupling, known B-N intermediates H₃B·NMe₂BH₂·NMe₂H and [H₂B·NMeH]₃ were also coordinated to the [Rh{PCyp₂(ƞ²-C₅H₇)}]+ fragment and investigated with regard to their role in the catalytic cycle. Structure activity relationships determined from stoichiometric reactions of cationic Rh(I) diphosphine fluorobenzene complexes with amine-boranes enabled the design of a highly efficient homogeneous catalyst capable of dehydrogenating H₃B·NMe₂H to [H₂BNMe₂]₂ at 0.2 mol% loading in 30 minutes at 298 K. Rapid dehydrogenation and dehydrocoupling of H₃B·NMeH₂ to form high molecular weight poly(N-methylaminoborane) with a low PDI has also been achieved. Investigations using model aminoborane H₂B=NiPr₂ and intermediate B-N species H₃B·NMe₂BH₂·NMe₂H and [H₂B·NMeH]₃ has helped establish an overall mechanistic rationale for this process.

We report here the development of a Lewis acid catalyzed method for the dehydrative coupling of cyclic alcohols and nitriles to form amides with retention of configuration. By contrast, the formation of amides by nitrile trapping of carbocations (Ritter reaction) usually affords racemic product. The present reaction was accomplished by first converting alcohol starting materials to their corresponding chlorosulfites in situ. Even after an extensive search, only copper (II) salts were able to produce the desired conversion of these chlorosulfites to amides though with low catalytic turnover. Improving the turnover without deteriorating the stereochemical outcome was eventually accomplished by a careful selection of the reagent addition sequence and through the removal of gaseous byproducts. This Ritter-like coupling reaction proceeds in good yields with secondary cyclic alcohols under mild conditions. The stereochemical outcome likely due to fast nucleophilic capture of a non-planar carbocations (hyperconjomers) stabilized by ring hyperconjugation. / Includes bibliography. / Dissertation (Ph.D.)--Florida Atlantic University, 2014. / FAU Electronic Theses and Dissertations Collection

Abstract not available

Replacement of thymidine in DNA by halopyrimidines, such as bromodeoxyuridine (BrdU) and iododeoxyuridine (IdU), has long been known to enhance DNA damage and cell death induced by ionizing/UV radiation, but the mechanism of action of halopyrimidines at the molecular level is poorly understood. We have applied advanced time-resolved femtosecond laser spectroscopy to this molecular system of biological, chemical and medical significance. We obtained the first real-time observations of the transition states of the ultrafast electron transfer (UET) reactions of halopyrimidines with the ultrashort-lived precursor to the hydrated electron, which is a general product in ionizing/UV radiation. Our results provide a mechanistic understanding of these photo-/radiosensitizing drugs at the molecular level. We found that the UET reaction of BrdU is completed within 0.2 picosecond (ps) after the electronic exciataion, leading to the formation of the transition state BrdU* with a lifetime of ~1.5 ps that then dissociates into Br and a high reactive radical dU•. We have also demonstrated that the reaction efficiency for the formation of the reactive radical dU• to cause DNA damage and cell death is in the order of IdU>>BrdU>CldU>>FdU. This is due to the availability of two precursor states of ~0.2 ps and ~ 0.54 ps lifetimes for dissociative electron attachment (DEA) to IdU, of one precursor state of ~0.2 ps lifetime for DEAs to BrdU and CldU, and no precursors for DEA to FdU. This explains why BrdU and IdU were found to be effective radio-/photosensitizers and indicates that IdU should be explored as the most effective radiosensitizer among halopyrimidines. Moreover, as a by-product of this project, these halopyrimidines have been employed as quantum-state-specific molecular probes to resolve a long-standing controversy about the nature and lifetimes of prehydrated electrons. These findings also have a broader significance as they indicated that nonequilibrium precursor electrons may play an important role in electron-initiated reactions in many biological, chemical and environmental systems. We have also demonstrated UET reactions of nucleotides with the precursor to the hydrated electrons. Our results indicate that among DNA bases, adenine is the most efficient electron trapper and an effective electron transfer promoter, while guanine is the most effective in dissociative electron attachment. These results not only primarily explain the sequence selectivity of duplex DNA containing BrdU/IdU, but imply that the DEA of guanine is an important mechanism for radiation-induced DNA damage in ionizing radiation and radiotherapy of cancer.

Replacement of thymidine in DNA by halopyrimidines, such as bromodeoxyuridine (BrdU) and iododeoxyuridine (IdU), has long been known to enhance DNA damage and cell death induced by ionizing/UV radiation, but the mechanism of action of halopyrimidines at the molecular level is poorly understood. We have applied advanced time-resolved femtosecond laser spectroscopy to this molecular system of biological, chemical and medical significance. We obtained the first real-time observations of the transition states of the ultrafast electron transfer (UET) reactions of halopyrimidines with the ultrashort-lived precursor to the hydrated electron, which is a general product in ionizing/UV radiation. Our results provide a mechanistic understanding of these photo-/radiosensitizing drugs at the molecular level. We found that the UET reaction of BrdU is completed within 0.2 picosecond (ps) after the electronic exciataion, leading to the formation of the transition state BrdU* with a lifetime of ~1.5 ps that then dissociates into Br and a high reactive radical dU•. We have also demonstrated that the reaction efficiency for the formation of the reactive radical dU• to cause DNA damage and cell death is in the order of IdU>>BrdU>CldU>>FdU. This is due to the availability of two precursor states of ~0.2 ps and ~ 0.54 ps lifetimes for dissociative electron attachment (DEA) to IdU, of one precursor state of ~0.2 ps lifetime for DEAs to BrdU and CldU, and no precursors for DEA to FdU. This explains why BrdU and IdU were found to be effective radio-/photosensitizers and indicates that IdU should be explored as the most effective radiosensitizer among halopyrimidines. Moreover, as a by-product of this project, these halopyrimidines have been employed as quantum-state-specific molecular probes to resolve a long-standing controversy about the nature and lifetimes of prehydrated electrons. These findings also have a broader significance as they indicated that nonequilibrium precursor electrons may play an important role in electron-initiated reactions in many biological, chemical and environmental systems. We have also demonstrated UET reactions of nucleotides with the precursor to the hydrated electrons. Our results indicate that among DNA bases, adenine is the most efficient electron trapper and an effective electron transfer promoter, while guanine is the most effective in dissociative electron attachment. These results not only primarily explain the sequence selectivity of duplex DNA containing BrdU/IdU, but imply that the DEA of guanine is an important mechanism for radiation-induced DNA damage in ionizing radiation and radiotherapy of cancer.

Several workers have reduced acidic polysaccharides for structural studies, for sorption studies, and for studies on chemical reactivity. All these investigators have used reduction procedures which have not been extensively studied and do not completely reduce the acidic groups. In addition, it is not known to what extent the other functional groups such as esters and hemiacetal are reduced. The goal of the present study is to obtain a further understanding of the reduction with both a Lewis acid, diborane, and a Lewis base, lithium borohydride, by comparison of the percent reduction of the functional groups on an acidic polysaccharide. Alginic acid from the stipes of the brown algae Laminaria hyperborea was selected as the acidic polysaccharide for study.

No description available.

The present study was undertaken to elucidate the electrochemical reduction mechanisms of benzil derivatives using the techniques of polarography and controlled potential electrolysis. Another objective of this study was to determine a quantitative relationship between reactivity and structure. The electrochemical experiments have been conducted in a 50% ethanol-water solvent over the pH range of 1.5 to 13.5. Potentiometric titrations and ultraviolet spectra were obtained to help understand the ionization of hydroxy-substituted benzil derivatives and how the ionization affects the polarographic reductions. Benzoin and two benzoin derivatives were studied to help determine the reduction products.

No description available.

Nanocrystalline beta-silicon carbide (ß-SiC) was synthesized at relatively low temperature (<1300C) by carbothermal reduction (CTR) reaction in fine scale carbon/silica mixtures. The fine scale mixing of the reactants (i.e., carbon and silica) was achieved by solution-based processing and subsequent heat treatment. The mechanism of the CTR reaction in the current system was investigated from different aspects. The condensates of the volatile species generated during the CTR reaction was collected and analyzed. The results supported previous investigations which suggested that the CTR reaction is a multi-step process that involves silicon monoxide (SiO) vapor as a reaction intermediate. The kinetics of the CTR reaction was investigated by isothermal weight loss study and by the study which determined the amount of SiC formed via quantitative X- ray diffraction (QXRD) analysis. The results of kinetic study were consistent with the "shrinking-core" model, in which the reaction between SiO vapor and carbon at the carbon surface to produce SiC is the rate-controlling step. In addition, several techniques, including XRD, gas adsorption analysis, laser diffraction particle size analysis, SEM, TEM, etc., had been used to study the structural evolutions of the reaction product of CTR. It was demonstrated that the evolutions of product structure characteristics such as crystallite size, specific surface area, specific pore volume, pore size distribution, particle size distribution, and powder morphology, etc. were consistent with each other and provided support to the reaction mechanism proposed.