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Conformational analysis of 2-aryl-3-methylpropyl derivatives by nuclear magnetic resonance spectroscopy: The photochemistry of some unsaturated ketones
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|Title:||Conformational analysis of 2-aryl-3-methylpropyl derivatives by nuclear magnetic resonance spectroscopy: The photochemistry of some unsaturated ketones|
|Authors:||Carlson, David Arthur|
Nuclear magnetic resonance
|Abstract:||Conformer populations were estimated by nmr in a series of compounds consisting of a 3-carbon chain containing an asymmetric carbon and different l- and 3-substituents. The energy differences between conformers were estimated from chemical shift differences between geminal methylene protons and vicinal coupling constants. Some mercury adducts of methallylarenes were prepared: they were 3-aryl-2-methyl-2-methoxypropylmercuric halides. Chemical shift differences (Δʋ) of up to 50 Hz were observed, which indicated considerable nonequivalence between methylene groups and suggested some intramolecular interaction. A derivative, 2-methoxy-2-methyl-3phenylbromopropane, showed no nonequivalence. Nonorganic compounds prepared for nmr spectra included the sodium salt of 3-(anisyl)-2-methylpropanol, and 2-methyl-3-phenylpropyllithium. A Grignard reagent, 2-methyl-3-phenylpropylmagnesium bromide, was formed in dimethyl ether and converted to the corresponding alkylborane, alkylcadmium chloride, alkyldichlorophosphine and alkylmercuric chloride. The nmr spectra of these compounds were recorded and some usable spectra were obtained. Increases in Δʋ were seen only for the alkylmercuric chloride. Organic compounds prepared included 2-methyl-3-phenylpropyl bromide and the corresponding iodide, nitrile, acid, amide, amine and nitro compounds. The nitro compound and the amide showed nonequivalence at the methylene a to the entire group. The α-methylene and benzylic groups of 2-methyl-3-phenylpropyl amine showed considerable nonequivalence. In addition, a series of four primary 2-methyl-3-ary1propano1s was prepared and their nmr spectra examined at different temperatures, concentrations and solvents from 100° to -80°. Fairly large effects on chemical shift differences and coupling constants were observed over the temperature range studied on the gemina1 benzylic and amethylene protons, while a derivative, 2-methyl-3-pheny1-1-propy1 methyl ether, showed smaller changes over the same temperature range. Four methods were used to estimate Free Energy Differences (-ΔE) between the most populous conformers of the organic compounds. Two calculations based on Karp1us-type relationships involving vicinal coupling constants gave reasonable and consistent answers to problems of conformer populations. The third was a graphical method which plotted coupling constants against estimated -ΔE. The fourth, another graphical method, plotted chemical shift differences between the benzylic hydrogens against -ΔE. All four methods gave similar results. The values obtained were small (0 to 300 ca1/mo1e) but reasonable and self-consistent. In addition, a Van't Hoff relationship was used to determine enthalpy interactions.|
Thesis (Ph. D.)--University of Hawaii, 1970.
2 pt. in 1 v illus., tables
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|Appears in Collections:||Ph.D. - Chemistry|
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