Not antiaromaticity gain, but increased asynchronicity enhances the Diels-Alder reactivity of tropone

Tropone is an unreactive diene in normal electron demand Diels-Alder reactions, but it can be activated via carbonyl umpolung by using hydrazone ion analogs. Recently, the higher reactivity of hydrazone ion analogs was ascribed to a raised HOMO energy induced by antiaromaticity (L. J. Karas, A. T. Campbell, I. V. Alabugin and J. I. Wu, Org. Lett., 2020, 22, 7083). We show that this is incorrect, and that the activation barrier is lowered by increased asynchronicity.

Automated summary

Tropone is unreactive in normal electron-demand Diels-Alder reactions, but can be activated by using hydrazone ion analogs for carbonyl umpolung. Previous research attributed the higher reactivity of hydrazone ion analogs to raised HOMO energy induced by antiaromaticity. However, this study proves that the activation barrier is actually lowered by increased asynchronicity.