On the mechanism of a modified Perkin condensation leading to alpha-phenylcinnamic acid stereoisomers - experiments and molecular modelling

Pálinkó, István and Kukovecz, Ákos and Torok, B. and Körtvélyesi, Tamás (2000) On the mechanism of a modified Perkin condensation leading to alpha-phenylcinnamic acid stereoisomers - experiments and molecular modelling. MONATSHEFTE FÜR CHEMIE, 131 (10). pp. 1097-1104. ISSN 0026-9247

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Abstract

A modified Perkin condensation leading to alpha -phenylcinnamic acid stereoisomers affords predominantely or exclusively the (E)-isomer. Reaction duration, temperature, and polarity of the solvent affect the isomeric distribution only to a minor extent. However, geometry optimization of the stereoisomers by semiempirical quantum chemical methods revealed that their thermodynamic stabilities in the absolute minimum are the same. (E)-(Z)-isomerization under the conditions of the Perkin condensation resulted in an equilibrium mixture of nearly 1:1 composition, thus corroborating the results of the computations. Comparing the detailed potential energy maps of the isomers obtained at the semiempirical level of theory may explain the controversy. The (Z)-isomer has a structured potential energy surface with few but well-defined minima, whereas that of the (E)-isomer shows an extended flat plateau enabling this isomer to approach a minimum (which is close to the global one) right after its formation.

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