LED‐NMR Monitoring of an Enantioselective Catalytic [2+2] Photocycloaddition

Skubi, Kazimer L.; Swords, Wesley B.; Hofstetter, Heike

doi:10.1002/cptc.202000094

Cited by 26 publications

(23 citation statements)

References 61 publications

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“…This reaction is particularly useful in many natural product syntheses, where it provides a single-step route to challenging fused multicyclic and/or heterocyclic moieties, − and it was famously used as a key step in the landmark syntheses of caryophyllene and cubane . More recently, Jiang et al and Skubi et al reported new catalytic platforms for the [2 + 2] photocycloaddition with unparalleled regio- and/or stereoselectivity, demonstrating that even after more than a century the potential scope of this reaction has still not been fully explored.…”

Section: Introductionmentioning

confidence: 99%

Mechanisms of the Cu(I)-Catalyzed Intermolecular Photocycloaddition Reaction Revealed by Optical and X-ray Transient Absorption Spectroscopies

et al. 2021

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The [2 + 2] photocycloaddition provides a simple, single-step route to cyclobutane moieties that would otherwise be disfavored or impossible due to ring strain and/or steric interactions. We have used a combination of optical and X-ray transient absorption spectroscopies to elucidate the mechanism of the Cu(I)-catalyzed intermolecular photocycloaddition reaction using norbornene and cyclohexene as model substrates. We find that for norbornene the reaction proceeds through an initial metal-to-ligand charge transfer (MLCT) state that persists for 18 ns before the metal returns to the monovalent oxidation state. The Cu K-edge spectrum continues to evolve until ∼5 μs and then remains unchanged for the 50 μs duration of the measurement, reflecting product formation and ligand dissociation. We hypothesize that the MLCT transition and reverse electron transfer serve to sensitize the triplet excited state of one of the norbornene ligands, which then dimerizes with the other to give the product. For the case of cyclohexene, however, we do not observe a charge transfer state following photoexcitation and instead find evidence for an increase in the metal−ligand bond strength that persists for several ns before product formation occurs. This is consistent with a mechanism in which ligand photoisomerization is the initial step, which was first proposed by Salomon and Kochi in 1974 to explain the stereoselectivity of the reaction. Our investigation reveals how this photocatalytic reaction may be directed along strikingly disparate trajectories by only very minor changes to the structure of the substrate.

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Section: Introductionmentioning

confidence: 99%

Mechanisms of the Cu(I)-Catalyzed Intermolecular Photocycloaddition Reaction Revealed by Optical and X-ray Transient Absorption Spectroscopies

et al. 2021

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“…To demonstrate the effectiveness of this photocatalyst system, they tested the intramolecular photocycloaddition of quinolones 206 ( Scheme 32 ) [ 90 ]. The proposed mechanism again proceeds via a triplet sensitisation of the substrate within a hydrogen bonding complex 207 that provides the desired products 208 in excellent yields and good enantioselectivities (13 examples, up to 96:4 er) and a quantum yield of 0.31 [ 91 ]. As with Bach’s catalysts, the scope is limited to lactams.…”

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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“…To conclude the introduction, in the present work, we focus on the investigation of photoreactions in the mixture of anthracenes. To do this, we employ 1 H NMR spectroscopy, which is being more and more often employed in the field of photochemistry and reaction monitoring [43][44][45][46][47]. We demonstrate that a simple NMR approach can easily deliver data which enables us to obtain insights into the kinetics of the photoreactivity between varying anthracenes.…”

Section: Introductionmentioning

confidence: 99%

Photoreactivity of an Exemplary Anthracene Mixture Revealed by NMR Studies, including a Kinetic Approach

et al. 2021

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Anthracenes are an important class of acenes. They are being utilized more and more often in chemistry and materials sciences, due to their unique rigid molecular structure and photoreactivity. In particular, photodimerization can be harnessed for the fabrication of novel photoresponsive materials. Photodimerization between the same anthracenes have been investigated and utilized in various fields, while reactions between varying anthracenes have barely been investigated. Here, Nuclear Magnetic Resonance (NMR) spectroscopy is employed for the investigation of the photodimerization of two exemplary anthracenes: anthracene (A) and 9-bromoanthracene (B), in the solutions with only A or B, and in the mixture of A and B. Estimated k values, derived from the presented kinetic model, showed that the dimerization of A was 10 times faster in comparison with B when compounds were investigated in separate samples, and 2 times faster when compounds were prepared in the mixture. Notably, the photoreaction in the mixture, apart from AA and BB, additionally yielded a large amount of the AB mixdimer. Another important advantage of investigating a mixture with different anthracenes is the ability to estimate the relative reactivity for all the reactions under the same experimental conditions. This results in a better understanding of the photodimerization processes. Thus, the rational photofabrication of mix-anthracene-based materials can be facilitated, which is of crucial importance in the field of polymer and material sciences.

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LED‐NMR Monitoring of an Enantioselective Catalytic [2+2] Photocycloaddition

Cited by 26 publications

References 61 publications

Mechanisms of the Cu(I)-Catalyzed Intermolecular Photocycloaddition Reaction Revealed by Optical and X-ray Transient Absorption Spectroscopies

Mechanisms of the Cu(I)-Catalyzed Intermolecular Photocycloaddition Reaction Revealed by Optical and X-ray Transient Absorption Spectroscopies

Recent developments in enantioselective photocatalysis

Photoreactivity of an Exemplary Anthracene Mixture Revealed by NMR Studies, including a Kinetic Approach

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