MECHANISTIC ASPECTS OF THE THERMAL EQUILIBRATION OF RETINYLIDENE IMINES AND IMMONIUM SALTS

The reactions of isomeric retinylidene n-butylimines and protonated imines in the dark were examined by proton nuclear magnetic resonance and ultraviolet absorption spectroscopy and by high-performance liquid chromatography. The immonium hydrochlorides were shown to decompose by a complex system of pathways. However, 13-cis-retinylidene-n-butylamine was shown to undergo thermodynamic equilibration with the trans isomer by general base catalysis, whereas the 9-cis isomer was thermodynamically stable. The specificity of the reaction for the 13-cis isomer suggests that it occurs via the 13-methylene enamine intermediate. Protonated retinylidene imines have been shown to be important as chromophores in the visual cycle and in a light-driven proton pump in certain halophilic bacteria. The mechanisms of action for these systems are briefly contrasted in the light of the result reported here. © 1979, American Chemical Society. All rights reserved.