Abstract
Chiral cis-cyclopropanes are strained rigid analogues of alkyl chains, whose study and application are limited by their difficult synthesis. A modular approach from olefin materials is enabled by the discovery of the electron donor–acceptor (EDA) interaction between 2-substituted benzothiazolines and N-hydroxyphthalimide esters. These complexes are activated by visible light without photocatalysts, and the benzothiazoline reagent plays a triple role as a photoreductant, a stereoselective hydrogen-atom donor, and a Brønsted acid. Beyond the enantioselective synthesis of cis-cyclopropanes, these results introduce benzothiazolines as accessible and easily tunable self-sensitized photoreductants.
Highlights
Background and Synthetic ApplicationsACS Catal. 2016, 6 (3), 1389−1407. (d) Mori, T.; Inoue, Y
The photoreductions using Hantzsch ester[18] or N-butyl dihydronicotinamide (5b)[19] recently developed by Shang et al.18a and our group[19] were promising, but further attempts to increase the yield or diastereoselectivity by tuning the structure of the dihydropyridines proved unsuccessful. On account of these results, we explored the possibility of employing a reductant with a more sterically hindered hydrogen atom to impose a higher kinetic barrier in the hydrogen atom transfer (HAT) toward the undesired diasteroisomer trans-4a. 2-Substituted benzothiazolines (BTA, 6) have been used as alternative hydride sources to Hantzsch esters in transfer hydrogenation reactions.[20]
We explored several benzothiazolines in this context, finding promising results
Summary
1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 solvent footnote e CHCl3 DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO. The photoreductions using Hantzsch ester[18] or N-butyl dihydronicotinamide (5b)[19] recently developed by Shang et al.18a and our group[19] were promising (entries 3 and 4), but further attempts to increase the yield or diastereoselectivity by tuning the structure of the dihydropyridines proved unsuccessful (see the Supporting Information for details) On account of these results, we explored the possibility of employing a reductant with a more sterically hindered hydrogen atom to impose a higher kinetic barrier in the HAT toward the undesired diasteroisomer trans-4a. This protocol allows for quick and modular access to ring-strained and conformationally strained compounds from available olefin materials, facilitating the synthesis of interesting bioactive molecules These advances are bestowed by a new, efficient, and stereoselective photodecarboxylation driven by a novel EDA complex between redox-active esters and benzothiazoline reagents.
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