Photochemical and Thermal E/Z Isomerizations of Some α,β-Unsaturated Iminium Salts

Abstract

<p>Geometric isomerizations about the C=C and C=N bonds of a series of diaryl α,β-unsaturated iminium salts are investigated in this thesis. These isomerizations are important in several unsaturated iminium ions found in natural systems. The vision process relies on light absorption by the protein rhodopsin followed by Z/E isomerization. A similar reaction is used by certain bacteria to convert light energy into energy the organisms can use for cell functions.</p> <p>A series of nine iminium salts, N-methyl, N-aryl-3-aryl-2-propenylidene iminium perchlorates, with various electron-withdrawing or electron-donating substituents on the aryl rings were synthesized and characterized. Three methods of isomerization of these molecules were examined--photochemical, electron transfer initiated, and thermal isomerization.</p> <p>The electronic absorption and emission properties of these iminium salts, and the effect of the substituents on the regioselectivity of photoisomerization, led to some conclusions about the excited states involved in the isomerization. Although the molecules reach an initial excited state where the positive charge of the iminium group has migrated process into the carbon framework of the molecule, the isomerization process is not governed by such an intermediate. It is suggested that the state that governs photoisomerization has biradical character.</p> <p>E/Z isomerization about the C=N bond of the unsubstituted iminium salt was accomplished by photoinitiated electron transfer from the donor, tris (2,2'-bipyridine)-ruthenium(II)dichloride, the first observation of this reaction.</p> <p>The iminium salts crystallize as the E,E isomers, but undergo thermal isomerization in solution to produce a mixture of E and Z isomers about the C=N bond. In strong acid media, two mechanisms of isomerization were found. Electron-withdrawing substituted iminium salts isomerize by a nucleophile-catalyzed mechanism, and electron-donating substituents cause isomerization by protonation of the iminium salt.</p>