Mechanism and kinetics of Horner–Wadsworth–Emmons reaction in liquid–liquid phase-transfer catalytic system

• Phosphonate was totally deprotonated by excess naked OH− without the help of catalyst.
• Interfacial mechanism was changed into the extraction mechanism with increasing the catalyst usage.
• Kinetics model for each mechanism was deduced.
• Rate-determining step was ion-exchange reaction or distribution of the ion-pair for each mechanism.

Liquid–liquid phase-transfer catalytic (LL-PTC) system suitable for symmetric distyryl compounds could not be used to synthesize asymmetric derivates. Relatively low value, the time-dependence of the interfacial tension with only phosphonate in the organic phase and high background reaction rate demonstrated that “weakly acidic” phosphonate was adsorbed on the interfacial layer and deprotonated by excess “naked” OH− without the help of the catalyst. Phase-transfer catalyst participated in the transfer process of the carbanion and accelerated the bond-forming reaction. However, the distributing region for the carbanion ion-pair in this PTC system was dependent on the usage of the catalyst and the mechanism would change from the interfacial mechanism to the extraction mechanism. Accordingly, the mechanism including adsorption, deprotonation, ion-exchange reaction, distribution and bond-forming reaction was proposed, and the kinetic model was performed. The activation energy and the dependence of catalytic activity on lipophilicity and accessibility parameter also supported the proposed mechanism. The rate-determining step was not the bond-forming reaction for each mechanism and the apparent reaction rate was relevant to the generation rate of the carbanion and the ion-exchange reaction rate.

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