Iodine-Catalyzed Diels-Alder Reactions

The Diels-Alder cycloaddition is the most popular pericyclic reaction with numerous applications in synthesis and catalysis. We now demonstrate that we can perform this reaction under mild and metal-free conditions relying on molecular iodine as the catalyst. Cycloadditions with cyclohexadiene, cyclopentadiene, or isoprene with various dienophiles can be performed typically within minutes in moderate to good yields and high endo selectivity. The mechanistic studies including kinetic and DFT investigations clearly indicate a halogen-bond activation and rule out other modes of activation. Furthermore, iodine performs equally well as typical metallic Lewis acids like AlCl3, SnCl4, or TiCl4.

Automated summary

This study demonstrates that the Diels-Alder cycloaddition reaction can be efficiently conducted under mild and metal-free conditions using molecular iodine as the catalyst, leading to high endo selectivity in a short amount of time. Mechanistic studies revealed a halogen-bond activation mechanism and showed that iodine performs as well as typical metallic Lewis acids in this reaction.