Reversal of reaction type selectivity by Lewis acid coordination: the <i>ortho</i> photocycloaddition of 1- and 2-naphthaldehyde

Stegbauer, Simone; Jeremias, Noah; Jandl, Christian; Bach, Thorsten

doi:10.1039/c9sc03315g

Cited by 12 publications

(6 citation statements)

References 64 publications

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“… Under the chosen reaction conditions with oxazaborolidine catalyst 8d , no side reactions from carbonyl photoreactivity, e.g. a Paternò–Büchi reaction, were observed.…”

Section: Beyond Conventional Oxazaborolidines: New Catalysts and Reac...mentioning

confidence: 99%

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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“… Under the chosen reaction conditions with oxazaborolidine catalyst 8d , no side reactions from carbonyl photoreactivity, e.g. a Paternò–Büchi reaction, were observed.…”

Section: Beyond Conventional Oxazaborolidines: New Catalysts and Reac...mentioning

confidence: 99%

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

Schwinger

Bach

2020

Acc. Chem. Res.

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“…Additionally, we reported that tuning the substituents enables the construction of complex pyridine-fused 2D/3D ring systems via a cascade involving an ortho -cycloaddition with a subsequent rearrangement initiated by a second EnT . Despite the synthetic appeal of such cascade reactions, the photolability of the first addition products containing strained (four-membered) rings adjacent to an excitable alkene consequently hampers their synthetic access. , Thus, it is not surprising that intermolecular ortho -photocycloadditions of benzenoids that suppress a consecutive rearrangement are still underrepresented in the literature . We questioned whether a strain-release approach could serve as a thermodynamical pitfall to conserve the original ortho -adduct by forming cycloaddition products with a reduced tendency to undergo downstream rearrangements or cycloreversion (Scheme C) .…”

Section: Introductionmentioning

confidence: 99%

ortho-Selective Dearomative [2π + 2σ] Photocycloadditions of Bicyclic Aza-Arenes

Kleinmans,

Dutta,

Ozols

et al. 2023

J. Am. Chem. Soc.

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Dearomative photocycloadditions are valuable chemical transformations, serving as an efficient platform to create three-dimensional molecular complexity. However, the photolability of the original addition product especially within the context of ortho cycloadditions often causes undesired consecutive rearrangements, rendering these ortho cycloadducts elusive. Herein, we report an ortho-selective intermolecular photocycloaddition of bicyclic aza-arenes including (iso)quinolines, quinazolines, and quinoxalines by utilizing a strain-release approach. With bicyclo[1.1.0]butanes as coupling partners, this dearomative [2π + 2σ] cycloaddition enables the straightforward construction of C(sp3)-rich bicyclo[2.1.1]hexanes directly connected to N-heteroarenes. Photophysical experiments and DFT calculations revealed the origin of the [2π + 2σ] selectivity and indicate that, in addition to the originally proposed energy transfer or direct excitation pathways, a chain reaction mechanism is operative depending on the reaction conditions.

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“…16−29 We were interested in utilizing the Lewis acid complexation to alter the outcome of the reaction by changing the product being formed in the photochemical transformations as we envisioned altering the type of intermediates participating in the reaction pathways. 30 In this report, we disclose our findings with maleimides 31−34 in which the observed photoproduct is dictated by the mode of photocatalysis, viz., energy transfer or Lewis acid photocatalysis (Figure 1).…”

mentioning

confidence: 68%

Synergistic Excited State Photocatalysis: Divergent Energy Transfer vs Lewis Acid Mediated Phototransformations

Parthiban,

Garg,

Ahuja

et al. 2024

ACS Catal.

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Energy transfer vs Lewis acid photocatalysis dependent product selectivity is reported for the photochemical reaction of maleimides with alkenes. The photoproduct selectivity depends on the mode of photocatalysis that was deciphered using photochemical and photophysical studies. Under energy transfer conditions, hydroxy maleimides undergo a [3+2]-photocycloaddition, while in the presence of Lewis acids [2+2]-photocycloaddition was observed. Synergistic interactions between the Lewis acid, triplet sensitizer, and the substrate dictate both the outcome and efficiency of [2+2]-photocycloaddition.

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Reversal of reaction type selectivity by Lewis acid coordination: the ortho photocycloaddition of 1- and 2-naphthaldehyde

Abstract: Lewis acids, such as AlBr3, completely alter the photochemical behaviour of naphthaldehydes. Instead of typical carbonyl photochemistry, the aldehydes undergo cycloaddition reactions at the arene core upon visible-light irradiation.

Cited by 12 publications

References 64 publications

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

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

ortho-Selective Dearomative [2π + 2σ] Photocycloadditions of Bicyclic Aza-Arenes

Synergistic Excited State Photocatalysis: Divergent Energy Transfer vs Lewis Acid Mediated Phototransformations

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