(Received: April 15, 2003; Accepted for publication: August 4, 2003; Published on Web: October 29, 2003)
The mechanism of the formose reaction catalyzed by thiazolium salt was investigated by MOPAC-PM3 semi-empirical molecular orbital (MO) method in order to elucidate the factor(s) stabilizing the reaction intermediates. We assumed that one factor was the formation of an ion pair with the ammonium ion in the reaction medium. Based on this assumption, possible intermediates were examined, including their structures and the heats of formation were calculated. It was suggested that the formation of an ion pair between the zwitterion intermediate (II*) and an ammonium ion and the subsequent formation of the intermediate, 2-hydroxymethyl-3-ethyl-5-(2-hydroxyethyl)-4-methylthiazolium (II**), were crucial for the reaction. A more plausible mechanism for the initial stage of the formose reaction is proposed based on calculation results.
Keywords: MOPAC-PM3, Formose reaction, Thiazolium salt, Reaction mechanism, Ion pair formation
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