Iodine(III) Derivatives as Halogen Bonding Organocatalysts

Hypervalent iodine(III) derivatives are known as versatile reagents in organic synthesis, but there is only one previous report on their use as Lewis acidic organocatalysts. Herein, we present first strong indications for the crucial role of halogen bonding in this kind of catalyses. To this end, the solvolysis of benzhydryl chloride and the Diels–Alder reaction of cyclopentadiene with methyl vinyl ketone served as benchmark reactions for halide ion and the activation of neutral compounds. Iodolium compounds (cyclic diaryl iodonium species) were used as activators or catalysts, and we were able to markedly reduce or completely switch off their activity by sterically blocking one or two of their electrophilic axes. Compared with previously established bidentate cationic halogen bond donors, the monodentate organoiodine derivatives used herein are at least similarly active (in the Diels–Alder reaction) or even decidedly more active (in benzhydryl chloride solvolysis). Activation with iodine: Hypervalent iodine(III) compounds were used as activators or organocatalysts in benchmark reactions. The crucial role of halogen bonding in these processes was demonstrated for the first time by a series of comparative experiments, including with sterically blocked derivatives. The activities of these monodentate Lewis acids match or exceed those of previously used bidentate cationic halogen bond donors.