Zur Umwandlung von Triazatrimethincyaninen in Azamethincyanine (Kinetik und Chemismus)

Abstract

Benzothiazole-triazatrimethinecyanines 1, 3, 5 are almost stable at 200 °C in dry solvents as o-Dichlorobenzene and Benzonitrile, but in solvents containing water change to Azamethinecyanines 2, 4, 6 by eliminating nitrogen. The transformation of Bis[3-ethyl-benzothiazole(2)]-triazatrimethinecyanine-perchlorate 11 to Bis[3-ethyl-benzothiazole(2)]-azamethinecyanine-perchlorate 12 in Dimethylsulfoxide/water-mixture, 80 °C, buffered by Ethyl-diisopropylamine/Ethyl-diisopropylammonium-perchlorate (μ = 0,1 M) has been measured.
The reaction follows the kinetic system: 

A + OH^⊖ → O + J + N₂ (+ H^⊕)   (k₁, k'₁)

A + J → M + J + N₂ (k₂)

JH^⊕ ⇄ J + H^⊕ (K)

A: Triazacyanine 11
O: 2-Oxo-3-ethyl-benzothiazoline 9
J: 2-Imino-3-ethyl-benzothiazoline 10
M: Azacyanine 12 

The appearent rate constants could be estimated:

a = k₁∙[OH^⊖] + k'₁∙[H₂O] = (6.3 ± 0.2)∙10^-3 min^-1

b = k₂∙K / (K + [H^⊕]) = 8.1 ± 0.4 M^-1∙min^-1

As a probable mechanism of the transformation of 11 to 12 we postulate a competing hydrolysis (by OH^⊖, H₂O) and autocatalytic iminolysis (by 10).