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.
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.
Iodine can be considered as the simplest halogenbond donor. Previous investigations have revealed its remarkable catalytic effect in various reactions. The catalytic activity of iodine can often even compete with that of traditional Lewis acids. So far, iodine was typically used to activate carbonyl derivatives like Michael acceptors. We now demonstrate that iodine can also be used to activate allyl aryl ethers in Claisen rearrangements. The formed ortho-allylic phenols rapidly undergo iodocyclizations to afford dihydrobenzofurans, which are important building blocks for medicinal applications. A comparison with different catalysts further highlights the potential of iodine catalysis for this reaction. Computational and mechanistic investigations provide deeper insights into the underlying non-covalent interactions and their role for the catalysis.
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