Asymmetric Photocatalysis by Hydrogen Atom Transfer with Chiral-at-Rhodium Complexes
Hydrogen atom transfer provides straightforward methods to generate open-shell radical intermediates from C-H bonds and offers unique opportunities for green and sustainable synthesis. Visible-light-induced C-H functionalization enabled by HAT is an emerging strategy for substrate activation in pho...
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|Hydrogen atom transfer provides straightforward methods to generate open-shell radical intermediates from C-H bonds and offers unique opportunities for green and sustainable synthesis. Visible-light-induced C-H functionalization enabled by HAT is an emerging strategy for substrate activation in photocatalyzed organic synthesis. In recent years, photoinduced HAT reactions have seen substantial development of their versatility, efficiency, and selectivity. However, asymmetric photocatalysis via hydrogen atom transfer has rarely been reported.
In the first section, a visible-light-induced asymmetric rearrangement of 3-(2-formylphenyl)-1-pyrazol-1-yl-propenones to benzo-[d]cyclopropa[b]pyranones with up to > 99% ee is introduced, which is catalyzed by a bis-cyclometalated chiral-at-metal rhodium complex (RhS). Mechanistic experiments and DFT calculations support a mechanism whereby the photoexcited catalyst/substrate complex generates triplet excited-state species through photoiduced intersystem crossing, which triggers an intramolecular hydrogen atom transfer subsequent highly stereocontrolled hetero-Diels–Alder reaction. In this reaction scheme, the rhodium catalyst fulfills multiple functions by 1) enabling visible-light π→π* excitation of the catalyst-bound enone substrate, 2) facilitating the hydrogen atom transfer, and 3) providing the asymmetric induction for the hetero-Diels–Alder reaction.
In the second section, a visible light driven deracemization of ketones as a new type of deprotonation and asymmetric protonation is demonstrated for the first time, which leads to the formation of chiral carbonyl compounds in an efficient way with high yield (up to 97%) and high enantioselectivity (up to 97%). This new photoinduced deprotonation process is achieved by single electron transfer and subsequent hydrogen atom transfer. A bis-cyclometalated chiral-at-metal rhodium complex (RhInd) is used as the photocatalyst to induce the redox process and is responsible for the asymmetric induction, while the amine acts as the single electron reductant, HAT reagent and proton source. This conceptually simple light-driven strategy of coupling a photoredox deprotonation with a stereocontrolled protonation serves as a blueprint for other deracemizations of ubiquitous carbonyl compounds.