Photocatalyzed Diastereoselective Isomerization of Cinnamyl Chlorides to Cyclopropanes
(2020) Journal of the American chemical society — Vol. 142, n° 13, p. 6206-6215 (2020)
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- Authors
Tambar, Uttam K. 
- Abstract
- 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.
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Citations
Xu, B., Troian-Gautier, L., Dykstra, R., Martin, R. T., Gutierrez, O., & Tambar, U. K. (2020). Photocatalyzed Diastereoselective Isomerization of Cinnamyl Chlorides to Cyclopropanes. Journal of the American chemical society, 142(13), 6206-6215. https://doi.org/10.1021/jacs.0c00147 (Original work published 2020)