Asymmetric Photocatalysis by Intramolecular Hydrogen‐Atom Transfer in Photoexcited Catalyst–Substrate Complex

Abstract

3-(2-Formylphenyl)-1-pyrazol-1-yl-propenones undergo an asymmetric photorearrangement to benzo- [d]cyclopropa [b]pyranones with up to > 99 %e e, which is catalyzedb yabis-cyclometalated rhodium catalyst in the presence of visible light. Mechanistic experiments and DFT calculations support am echanism in which ap hotoexcited catalyst/substrate complex triggers an intramolecular hydrogen-atom transfer followed by ahighly stereocontrolled hetero-Diels-Alder reaction. In this reaction scheme,t he rhodium catalyst fulfills multiple functions by 1) enabling visible-light p!p*e xcitation of the catalyst-bound enone substrate, 2) facilitating the hydrogen-atom transfer,a nd 3) providing the asymmetric induction for the hetero-Diels-Alder reaction.Photochemistry provides new synthetic and mechanistic opportunities for asymmetric catalysis. [1] Forexample,visible light has been used as aconvenient and mild source of energy to generate free radicals,w hich are then interfaced with an asymmetric catalysis cycle. [2] In such systems,t he photochemistry is not part of the catalytic cycle and often serves as an inducer of at hermal chain process.T his role is distinct from mechanistic scenarios in which the photoexcited state is directly embedded within an asymmetric catalysis cycle, thereby opening untapped opportunities for exploiting the special reactivity of photoexcited states in the context of asymmetric catalysis. [3] Our group recently demonstrated that bis-cyclometalated iridium(III) and especially rhodium(III) complexes are exquisite catalysts for realizing visible-light-activated asymmetric catalysis. [4,5] Upon binding to as ubstrate and undergoing selective visible-light activation of the formed catalyst/ substrate complexes,s uch photoexcited assemblies are capa-ble of undergoing stereocontrolled chemical transformations, either through an initial outersphere electron transfer or by engaging in direct stereocontrolled bond-forming reactions (Figure 1). [5] Ther ealization of new reaction modes of photoexcited catalyst/substrate complexes is highly desirable for developing visible-light-activated asymmetric conversions that are otherwise elusive.Here we report au nique example of av isible-lightactivated catalyst/substrate complex engaging in ahydrogenatom transfer (HAT) reaction to generate ac atalyst-bound reactive intermediate,w hich is then primed to undergo ah ighly stereocontrolled intramolecular hetero-Diels-Alder reaction.We previously found that upon binding to bis-cyclometalated rhodium catalysts, a,b-unsaturated N-acyl pyrazoles can undergo p!p*e xcitation under visible light. We further demonstrated that these ligand-centered excited states can perform stereocontrolled cycloaddition reactions. [3d,f,6] At the onset of this study we were speculating that such photoexcited catalyst-bound enones might instead be capable of engaging in HATreactions to generate catalyst-bound reactive intermediates followed by stereocontrolled transformations.W e were inspired by areaction reported by Xia and co-workers in ...

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