The Mechanism of Syn-Anti Isomerization of Azomethine Dyes

Abstract

In previous work it has been demonstrated that irradiation of azomethine dyes brings about syn-anti isomerization, a change in the configuration about the carbon-nitrogen double bond. We have now systematically measured substituent and steric effects on the rates of thermal isomerization following photochemical excitation of benzoylacetanilide and pyrazolone azomethine dyes. For all dyes, bulk added in the form of methyl groups adjacent to the azomethine bond increases the rate of thermal isomerization. With substituents in the para position of the aromatic ring attached to the nitrogen atom of the azomethine bond, both electron-donating and electron with drawing substituents provide rate enhancement; for each class of dye, plots of the logarithm of the rate constant vs. the substituent σconstant are V-shaped with the minima near equal to zero. We believe that our results can be interpreted in terms of different mechanisms of isomerization; with electron-donating substituents, the isomerization proceeds by torsion about the central azomethine bond and with electron attractors, inversion about the azomethine nitrogen is the preferred path. © 1973, American Chemical Society. All rights reserved.