The Thermal <i>Cis</i>-to-<i>trans</i> Isomerization of <i>N</i>,<i>N</i>′-Diacylindigos. Kinetic Pressure, Solvent, and Substituent Effects

Abstract

Abstract The kinetics of the thermal cis-to-trans isomerization of N,N′-diacylindigos(DAI) in various solvents and under high pressures has been studied. N,N′-Dibenzoylindigos(DBI) isomerize about 10 times slower than molecules not having a benzene ring in the acyl groups. The values of the activation volumes were about 8.0 cm3 mol−1 for the former and about 3.0 cm3 mol−1 for the latter. It was suggested that the thermal isomerization proceeds via the biradical mechanism. The positive values of the activation volume have been interpreted in terms of the increase in the intrinsic volume upon activation. The differences in the isomerization rates and the activation parameters between DBI and the others have been ascribed to an additional interaction between the two phenyl rings in the acyl groups in the cis-ground state.