Correlations between structure and phase-properties of phosphonium salts ionic liquid crystals and media for stereoselective brominations and solutue structural studies by NMR spectroscopy
Thesis (Ph.D.)--Georgetown University, 2009.; Includes bibliographical references. The structural factors influencing the formation, organization, and temperature ranges of the solid and smectic phases of a structurally-diverse family of phosphonium salts in the absence or presence of a small amount of an organic liquid (solute) have been examined. The salts consist of one short group and three (equivalent) long n-alkyl chains attached to a positively charged phosphorus atom and either a free or covalently-attached counterion, the latter resulting in zwitterionic salts. Most of these salts pack in lamellae within their solid phases. Based on the single-crystal X-ray structures of an amidomethyl- and a carboxymethyl-tri-tetradecylphosphonium bromide and proposed packing models based on powder X-ray diffraction data, the ordered phases consist almost exclusively of stacks of bilayers in which the phosphonium cationic centers and anions are arranged alternately to form ionic planes that separate lipophilic layers.; The tri-alkylmethylphosphonium salts with small anions do not form liquid-crystalline phases as a consequence of strong alternating intra- and inter-molecular electrostatic interactions within their ionic planes. Decreasing the order within their ionic planes has been accomplished by increasing the size of anions, by attaching covalently a functional group (hydroxymethylene, carboxy, or amido) to the methyl group attached to phosphorus, or by adding a small amount of an organic liquid. The order within the lipophilic layers was increased by extending the length of the three long n-alkyl chains of salts. Correlations among all of these molecular structure factors and the properties of the phases are presented in order to discern the optimal structures leading to liquid crystallinity over broad temperature ranges that commence at low temperatures.; The order parameters of selectively deuterated phosphonium salts in their liquid-crystalline phases and their solutes, as low as 10-2, have been measured by NMR spectroscopy. These order parameters are much lower than the values found when the host is a commonly employed thermotropic liquid crystal. The dependence of the order parameters on temperature, the type and concentration of the solutes, the nature of the anions, and the lengths of long n-alkyl chains of salts has been assessed; the factors leading to the lowest (non-zero) order parameters have been determined. The results, in toto, indicate that several of the phosphonium salts are very promising as hosts to determine solute structures, as demonstrated by 13C-enriched acetonitrile as a model molecule.; A new reagent, tri-decylmethylphosphonium tribromide, and new procedures for bromination reactions of unsaturated substrates (including one that allows several substrates to be reacted in sequence) are described. The procedures exploit the diffusion of components, and the densities and immiscibilities of layers, including a fluorous 'spacer' layer, within a reaction vessel. The stereoselectivities achieved in the reactions are superior in some cases to those found with other brominating reagents.
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