Photocatalyzed Diastereoselective Isomerization of Cinnamyl Chlorides to Cyclopropanes

Endergonic
isomerizations are thermodynamically unfavored processes
that are difficult to realize under thermal conditions. We report
a photocatalytic and diastereoselective isomerization of acyclic cinnamyl
chlorides to strained cyclopropanes. Quantum mechanical calculations
(uM06-2X and DLPNO), including TD-DFT calculations, and experimental
studies provide evidence for the energy transfer from an iridium photocatalyst
to the allylic chloride substrate followed by C–Cl homolytic
cleavage. Subsequent Cl• radical migration forms
a localized triplet 1,3-diradical intermediate that, after intersystem
crossing, undergoes ring-closing to form the desired product. The
mild reaction conditions are compatible with a broad range of functional
groups to generate chlorocyclopropanes in high yields and diastereoselectivities.
A more efficient process is developed by addition of a catalytic amount
of a nickel complex, and we propose a novel role for this cocatalyst
to recycle an allyl chloride byproduct generated in the course of
the reaction. The reaction is also shown to be stereoconvergent, as
an E/Z mixture of cinnamyl chlorides
furnish the anti-chlorocyclopropane product in high
diastereoselectivity. We anticipate that the use of a visible light
activated photocatalyst to transform substrates in combination with
a transition metal catalyst to recycle byproducts back into the catalytic
cycle will provide unique opportunities for the discovery of new reactivity.