Enantioselective Lewis Acid Catalyzed <i>ortho</i> Photocycloaddition of Olefins to Phenanthrene‐9‐carboxaldehydes

Stegbauer, Simone; Jandl, Christian; Bach, Thorsten

doi:10.1002/anie.201808919

Cited by 83 publications

(48 citation statements)

References 47 publications

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“…2018 , 57, 14593–14596 . 3 The study revealed that catalyst loadings in Lewis acid-mediated photocycloaddition reactions can be significantly lowered when securing that Lewis acid coordination leads to an extensive bathochromic shift. Aldehydes were for the first time employed in a photochemical reaction catalyzed by a chiral 1,3,2-oxazaborolidine Lewis acid.…”

Section: Key Referencesmentioning

confidence: 99%

“…Along these lines, Stegbauer et al found that the bathochromic shift Δλ of phenanthrene-9-carboxaldehydes in the presence of EtAlCl 2 was larger than 70 nm, reaching beyond the nπ*-absorption of the uncomplexed substrate ( Figure 6 ). 3 …”

Section: Beyond Conventional Oxazaborolidines: New Catalysts and Reacmentioning

confidence: 99%

“… UV/vis spectra of phenanthrene-9-carboxaldehyde ( 25a , Scheme 9 ) in the absence and presence of EtAlCl 2 as Lewis acid. Adapted with permission from ref ( 3 ). Copyright 2018 John Wiley and Sons.…”

Section: Beyond Conventional Oxazaborolidines: New Catalysts and Reacmentioning

confidence: 99%

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Chiral 1,3,2-Oxazaborolidine Catalysts for Enantioselective Photochemical Reactions

Schwinger

Bach

2020

Acc. Chem. Res.

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Conspectus Asymmetric synthesis has posed a significant challenge to organic chemists for over a century. Several strategies have been developed to synthesize enantiomerically enriched compounds, which are ubiquitous in the pharmaceutical and agrochemical industries. While many organometallic and organic catalysts have been found to mediate thermal enantioselective reactions, the field of photochemistry lacks similar depth. Recently, chiral 1,3,2-oxazaborolidines have made the transition from Lewis acids that were exclusively applied to thermal reactions to catalysts for enantioselective photochemical reactions. Due to their modular structure, various 1,3,2-oxazaborolidines are readily available and can be easily fitted to a given chemical transformation. Their use holds great promise for future developments in photochemistry. This Account gives an overview of the substrate classes that are known to undergo enantioselective photochemical transformations in the presence of chiral 1,3,2-oxazaborolidines and touches on the catalytic mode of action, on the proposed enantiodifferentiation mechanism, as well as on recent computational studies. Based on the discovery that the presence of Lewis acids enhances the efficiency of coumarin [2 + 2] photocycloadditions, chiral 1,3,2-oxazaborolidines were applied in 2010 for the first time to prepare enantiomerically enriched photoproducts. These Lewis acids were then successfully used in intramolecular [2 + 2] photocycloaddition reactions of 1-alkenoyl-5,6-dihydro-4-pyridones and 3-alkenyloxy-2-cycloalkenones. In the course of this work, it became evident that the chiral 1,3,2-oxazaborolidine must be tailored to the specific reaction; it was shown that both inter- and intramolecular [2 + 2] photocycloadditions of cyclic enones can be conducted enantioselectively, but the aryl rings of the chiral Lewis acids require different substitution patterns. In all [2 + 2] photocycloaddition reactions in which chiral 1,3,2-oxazaborolidines were used as catalysts, the catalyst loading could not be decreased below 50 mol % without sacrificing enantioselectivity due to competitive racemic background reactions. To overcome this constraint, substrates that reacted exclusively when bound to an oxazaborolidine were tested, notably phenanthrene-9-carboxaldehydes and cyclohexa-2,4-dienones. The former substrate class underwent an ortho photocycloaddition, the latter an oxadi-π-methane rearrangement. Several new 1,3,2-oxazaborolidines were designed, and the products were obtained in high enantioselectivity with only 10 mol % of catalyst. Recently, an iridium-based triplet sensitizer was employed to facilitate enantioselective [2 + 2] photocycloadditions of cinnamates with 25 mol % of chiral 1,3,2-oxazaborolidine. In this case, the relatively low catalyst loading was possible because the oxazaborolidine–substrate complex exhibits a lower triplet energy and an improved electronic coupling compared to the uncomplexed substrate, allowing for a sele...

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Section: Key Referencesmentioning

confidence: 99%

Section: Beyond Conventional Oxazaborolidines: New Catalysts and Reacmentioning

confidence: 99%

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Chiral 1,3,2-Oxazaborolidine Catalysts for Enantioselective Photochemical Reactions

Schwinger

Bach

2020

Acc. Chem. Res.

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“…[9] However,i ts applications in asymmetric dearomatization reactions are rare. [10] At the beginning, we noticed that highly reactive nitrosocarbonyl intermediates could be generated catalytically from bench-stable N-hydroxycarbamates in the presence of ap hotocatalyst under air. [11,12] Based on these results,o ur original design was to explore the natural nucleophilicity of indoles for chiral phosphoric acid catalyzed [13] asymmetric dearomatization with transient electrophilic nitrosocarbonyl species using visible-light as am ild driving energy force.…”

Section: Resultsmentioning

confidence: 99%

Asymmetric Dearomatization of Indole Derivatives with N‐Hydroxycarbamates Enabled by Photoredox Catalysis

et al. 2019

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Dearomatization of indoles provides efficient synthetic routes for substituted indolines.I nm ost cases,i ndoles serve as nucleophiles.R eported here is an asymmetric dearomatization reaction of indole derivatives that function as electrophiles.T he combination of ap hotocatalyst and chiral phosphoric acid open to air unlocksthe umpolung reactivity of indoles,enabling their dearomatization with N-hydroxycarbamates as nucleophiles.Avariety of fused indolines bearing intriguing oxy-amines were constructed in excellent yields with moderate to high enantioselectivities.Mechanistic studies show that the realization of two sequential single-electron transfer oxidations of the indole derivatives is key,g enerating the configurationally biased carbocation species while providing the source of stereochemical induction. These results not only providea ne fficient synthesis of enantioenriched indoline derivatives,but also offer anovel strategy for further designing asymmetric dearomatization reactions. Scheme 1. Catalytic asymmetric dearomatization of indole derivatives. Supportinginformation and the ORCID identification number(s) for the author(s) of this article can be found under: https://doi.Scheme 2. Mechanistic studies. a) Stern-Volmer luminescence quenchinge xperiments. b) Cyclic voltammetry experiments (1a and 2a). c) Cyclic voltammetry experimentso f1a with varying amounts of PA (diphenylphosphoric acid). Boc = tert-butylcarbonyl. Scheme 3. Proposed mechanism.

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“…The first intermolecular example using this catalyst system used cyclic ketones 346 with alkenes 347 to synthesise bicyclic compounds 348 ( Scheme 58 ) [ 135 ]. A similar reaction was also later developed for the cycloaddition of phenanthrene-derived aldehydes 349 with alkenes 350 , using 457 nm excitation, which was possible due to the increased conjugation of the substrate, and a lower catalyst loading ( Scheme 58 ) [ 136 ].…”

Section: Reviewmentioning

confidence: 99%

Recent developments in enantioselective photocatalysis

Prentice¹,

Morrisson²,

Smith³

et al. 2020

Beilstein J. Org. Chem.

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Enantioselective photocatalysis has rapidly grown into a powerful tool for synthetic chemists. This review describes the various strategies for creating enantioenriched products through merging enantioselective catalysis and photocatalysis, with a focus on the most recent developments and a particular interest in the proposed mechanisms for each. With the aim of understanding the scope of each strategy, to help guide and inspire further innovation in this field.

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Enantioselective Lewis Acid Catalyzed ortho Photocycloaddition of Olefins to Phenanthrene‐9‐carboxaldehydes

Cited by 83 publications

References 47 publications

Chiral 1,3,2-Oxazaborolidine Catalysts for Enantioselective Photochemical Reactions

Chiral 1,3,2-Oxazaborolidine Catalysts for Enantioselective Photochemical Reactions

Asymmetric Dearomatization of Indole Derivatives with N‐Hydroxycarbamates Enabled by Photoredox Catalysis

Recent developments in enantioselective photocatalysis

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