The overall theme of my graduate research is to understand forces involved in supramolecular, hydrophobically-driven interactions, primarily in cyclodextrin systems and to use those interactions in applications ranging from fluorescence-based sensing to supramolecular catalysis. This research has included a highly interdisciplinary research project exploring the effects of cation-π interactions on surfactant/lipid bilayer vesicles for delivery applications. Cyclodextrins, which are commercially available, torus-shaped cyclic oligoamyloses, have been selected as the supramolecular hosts in these studies because of their well-defined hydrophobic interior cavity.
Cyclodextrin-based catalytic systems have been envisioned for mild, environmentally friendly transformations in high-impact organic reactions. The basis of this research stems from the ability of cyclodextrins to form hydrophobic complexes with small molecules, thereby lowering the entropic barrier for the formation of a transition state in selected organic reactions. Moreover, the hydrophobic complexes of cyclodextrin with small molecules have also been shown to be more reactive from the perspective of many organic transformations.
The first manuscript describes Diels Alder reactions of a model polycyclic aromatic hydrocarbon (PAH), 9-anthracenemethanol with N-substituted maleimides under mild reaction conditions in the presence of commercially available cyclodextrins. In this system, hydrophobic complexation of the N-substituent in turn modifies the electronics of the alkene double bond, resulting in its enhanced reactivity. We found that cyclodextrin complexation of the N-substituent on the maleimide was the key factor in determining the rate of the reaction and the overall conversion to product. Optimal results were found using N-cyclohexylmaleimide with a methyl-γ-cyclodextrin host, with 94% conversion obtained in 24 hours. A proposed model of the complexation with methyl-γ-cyclodextrin has been proposed, with cyclodextrin encapsulation perturbing the electronics of the dienophile double bond and enhancing its reactivity.
The second manuscript discusses the cyclodextrin-promoted oxidation of benzyl alcohols to benzaldehydes using an inexpensive, commercially available reagent, 1,3-dibromo-5,5-dimethylhydantoin (DBDMH). Catalyst reusability up to three consecutive runs has been observed without substantial loss of product yield and selectivity, which further enhances the atom economy of this method.
Non-covalent energy transfer has been used as a highly sensitive investigative tool in a wide variety of supramolecular systems. Owing to its exquisite sensitivity and dependence on a host of factors, this strategy has also been employed to study dynamic conformations of biomolecules such as nucleic acids and peptides. Our group has developed highly efficient energy transfer systems using γ-cyclodextrin as a supramolecular host for promoting non-covalent energy transfer from small molecule aromatic toxicants to high quantum yield fluorophores. Although γ-cyclodextrin has a cavity size that is well-known to be able to accommodate two small molecule guests simultaneously, limitations of γ-cyclodextrin include its limited specificity and ill-defined host: guest stoichiometry, as a result of its larger cavity size.
Additionally, the synthesis of a series of cyclodextrin-incorporated higher order architectures has also been described in the fourth manuscript. These architectures have been designed to exhibit higher binding affinity towards larger hydrophobic analytes like stilbene, tamoxifen and biphenyls based on hydrophobic binding of the guest from two or more distinct ends of the molecule. Two cyclodextrins were tethered by aromatic/alkyl amide linkages, and binding properties of these novel receptors were investigated for high quantum yield fluorophores.
To develop sensitive and selective sensors, efforts have even been extended to synthetic macrocycles for the efficient binding of PAHs and other analytes. The fifth manuscript entails the synthesis and application of a fluorenone integrated triazolophane for the efficient binding of PAHs and fluoride anions. UV-vis and 1H-NMR spectroscopy results showed that the macrocycle has high sensitivity for selected PAHs and binds fluoride anions in a 1:2 stoichiometry. The bilateral symmetry of the macrocycle creates two binding pockets for the relatively small fluoride anion. This conclusion is well supported by the binding curve fitting of 1H-NMR titrations and Job’s plot analysis of the chemical shift of the triazole proton. A high association constant value of 104 M-2 is observed for the binding of fluoride anion in DMSO.
The final chapter of the thesis describes the application of basic supramolecular science in an industrial setting. In cosmetic industries, hydrated surfactant vesicles are used to deliver encapsulated perfume ingredients and counter skin dryness. However, addition of small concentrations of perfume-raw-materials (PRMs) has a drastic effect on vesicular suspensions, perturbing their microstructures and altering their rheological properties. In the sixth manuscript, two model perfume-raw-material (PRM) compounds, linalyl acetate and eugenol, have been identified to have very different impacts on a multilamellar vesicular suspension made of diethylester dimethylammonium chloride (DEEDMAC) surfactant. While the former has negligible effect, the latter triggers a change from multilamellar to unilamellar vesicles, resulting in a sharp rise in the suspension viscosity. (Abstract shortened by ProQuest.)
|Commitee:||Bose, Arijit, DeBoef, Brenton, Deng, Ruitang, Kiesewetter, Matthew|
|School:||University of Rhode Island|
|School Location:||United States -- Rhode Island|
|Source:||DAI-B 79/03(E), Dissertation Abstracts International|
|Keywords:||Catalysis, Cyclodextrin, Derivatized, Sensing|
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