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A Case Study of Thiourea‐Assisted Iminium Formation by Hydroxyl Anion Binding: Kinetic, Spectroscopic and Computational Evidences

The experimental and computational study of the mechanism of the iminium-organocatalyzed formation of N-hydroxypyrrolidines from nitrones, as a case study, revealed a hitherto unknown mode of action for the Schreiner’s thiourea used as co-catalyst. The experimental data suggest that thiourea promotes the formation of the iminium ion through hydroxyl anion recognition forming a stable ion pair. Such an ion pair facilitates enolization of the nitrone through a H-bond network thus promoting the reaction with iminium ion. The computational model supports the mechanism and the catalytic cycle. This mechanistic rationale is supported by the lack of reactivity of preformed iminium ion with the nitrone in the absence of thiourea-hydroxyl complex and the observed reactivity when a complex thiourea-tetrabutylammonium hydroxide is added