Thermal and photochemical cis/trans isomerizations of some alpha,beta-unsaturated iminium salts

Abstract

<p>This thesis presents some investigations of the ground and excited state properties and reactions of some α,β-unsaturated iminium salts. These studies were instigated primarily by the vast number of reports on the photoreceptor pigments, rhodopsin and bacteriorhodopsin. Both of these natural systems are believed to contain iminium salt chromophores which undergo efficient cis/trans isomerizations in their ground and/or excited states. A series of aliphatic and aromatic substituted α,β-unsaturated iminium salts were prepared and characterized by ¹H nmr spectroscopy. The ground state structure and charge distribution were investigated by MIND0/3 calculations, an analysis of solution and solid state ¹³C nmr chemical shift data and x-ray crystal structure determinations. It was concluded that the positive charge of these systems resides principally in the iminium moiety and that the s-trans conformation is predominant in solution. The photochemical reactions of the iminium salts in strong acid media (FSO₃H, H₂SO₄ and TFA) were examined. All of the systems formed a photostationary state mixture among some or all of the possible geometric isomers. The unambiguous identification of a photon-induced cis/trans isomerization about the C=N⁺ bond was particularly significant as this was the first report of such an occurrence. The quantum yields for some of these isomerization processes were determined in the above solvents. The substituent and viscosity dependence of these yields were used in an attempt to elucidate the mechanism of isomerization and the charge distribution of the excited state. It was concluded that on going from the ground to the excited state there was a shift of the positive charge from the iminium moiety onto the carbon framework. This was completely in accord with the sudden polarization effect proposed by Salem and Bruckmann. A mechanism involving the rotation about one of the multiple bonds per quantum absorbed was consistent with these data. A quantitative investigation of the ground state cis to trans isomerizations about the C=C bonds in a series of para-substituted aromatic iminium salts was undertaken. The results were indicative of a change in mechanisms with a variation in the electron demand of the substituent. Salts with electron-withdrawing substituents were concluded to isomerize by the Michael addition of a nucleophile whereas, a pathway involving protonation at the iminium nitrogen was proposed for the salts with electron-donating substituents.</p>