Iodine-Catalyzed Nazarov Cyclizations

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 cis–trans isomerization of the employed olefins
seem not to be an important side reaction here.