CHEMISTRY OF PYRROLE PIGMENTS .28. THERMAL-STABILITY OF THE GEOMETRICAL-ISOMERS OF BILATRIENES-ABC

In the equilibrium between the two diastereomeric (Z, Z, Z) and (E, Z, Z) forms of aetiobiliverdine-IV-γ the former is preferred by about 20kJ/mol. For the thermal isomerization a barrier of 108kJ/mol has to be surmounted-as measured for the latter isomer. A value of 12.4 for the logarithm of the preexponential factor leads to a polar singlet mechanism, which was deduced previously for pyrromethenones and arylmethylenepyrrolinones, but a much more positive activation entropy for the verdinoid system indicates a pronounced delocalisation of charge over the entire conjugated system in the corresponding transition state. © 1979 Springer-Verlag.