10.1021/acs.joc.9b01083.s001 Jonas J. Koenig Jonas J. Koenig Thiemo Arndt Thiemo Arndt Nora Gildemeister Nora Gildemeister Jörg-M. Neudörfl Jörg-M. Neudörfl Martin Breugst Martin Breugst Iodine-Catalyzed Nazarov Cyclizations American Chemical Society 2019 metal-free reaction conditions apolar solvents halogen bond iodine-catalyzed reaction side reaction iodonium ion catalysis pericyclic reaction divinyl ketones Iodine-Catalyzed Nazarov Cyclizations Br ønsted acid carbonyl group Nazarov cyclization catalyst Nazarov system 2019-06-06 00:00:00 Journal contribution https://acs.figshare.com/articles/journal_contribution/Iodine-Catalyzed_Nazarov_Cyclizations/8264174 The Nazarov cyclization is an important pericyclic reaction that allows the synthesis of substituted cyclopentenones. We now demonstrate that this reaction can be performed under very mild, metal-free reaction conditions using molecular iodine as the catalyst. A variety of different divinyl ketones including aromatic systems undergo the iodine-catalyzed reaction with moderate to very good yields in both polar and apolar solvents. Our mechanistic studies indicate that the Nazarov system is activated through a halogen bond between the carbonyl group and the catalyst, and other modes of action like Brønsted acid or iodonium ion catalysis are unlikely. Furthermore, addition of iodine to the double bond or a putative iodine-catalyzed <i>cis</i>–<i>trans</i> isomerization of the employed olefins seem not to be an important side reaction here.