LED based NMR illumination device for mechanistic studies on photochemical reactions – Versatile and simple, yet surprisingly powerful

Feldmeier, Christian; Bartling, Hanna; Riedle, Eberhard; Gschwind, Ruth M.

doi:10.1016/j.jmr.2013.04.011

Cited by 137 publications

(159 citation statements)

References 30 publications

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“…Our new optical fiber‐based UVNMR‐illumination setup combines a UV/Vis‐reflection dip probe (Avantes) with our in situ LED‐illumination device14 inside an NMR spectrometer (Figure 2 A). The optical fiber for illumination (with a sandblasted tip) and the reflection dip probe for UV/Vis measurements are placed together inside a coaxial quartz glass insert within an amberized NMR tube.…”

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confidence: 99%

Combined In Situ Illumination‐NMR‐UV/Vis Spectroscopy: A New Mechanistic Tool in Photochemistry

2018

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Synthetic applications in photochemistry are booming. Despite great progress in the development of new reactions, mechanistic investigations are still challenging. Therefore, we present a fully automated in situ combination of NMR spectroscopy, UV/Vis spectroscopy, and illumination to allow simultaneous and time‐resolved detection of paramagnetic and diamagnetic species. This optical fiber‐based setup enables the first acquisition of combined UV/Vis and NMR spectra in photocatalysis, as demonstrated on a conPET process. Furthermore, the broad applicability of combined UVNMR spectroscopy for light‐induced processes is demonstrated on a structural and quantitative analysis of a photoswitch, including rate modulation and stabilization of transient species by temperature variation. Owing to the flexibility regarding the NMR hardware, temperature, and light sources, we expect wide‐ranging applications of this setup in various research fields.

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confidence: 99%

Combined In Situ Illumination‐NMR‐UV/Vis Spectroscopy: A New Mechanistic Tool in Photochemistry

2018

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“…800. [9] This LED device can be used in a pulsed and in a continuouswave mode and facilitates time-resolved photo-CIDNP as well as the detection of entire reaction pathways. Therefore, with this LED setup now the full potential of NMR spectroscopy can be applied to study the mechanisms of photocatalytic reactions.…”

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confidence: 99%

LED‐Illuminated NMR Studies of Flavin‐Catalyzed Photooxidations Reveal Solvent Control of the Electron‐Transfer Mechanism

et al. 2014

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Mechanistic insights into chemical photocatalysis are mainly the domain of UV/Vis spectroscopy, because NMR spectroscopy has been limited by the type of illumination so far. An improved LED-based illumination device can be used to obtain NMR reaction profiles of photocatalytic reactions under synthetic conditions and perform both photo-CIDNP and intermediate studies. Flavin-catalyzed photooxidations of alcohols show the potential of this setup. After identical initial photoreaction steps the stabilization of a downstream intermediate is the key to the further reaction mechanism and the reactivity. As a chemical photocatalyst flavin can act either as a one- or a two-electron mediator when the stability of the zwitterionic radical pair is moldulated in different solvents. This demonstrates the importance of downstream intermediates and NMR-accessible complementary information in photocatalytic reactions and suggests the control of photoorganic reactions by solvent effects.

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“…Works on this subject were described by Fisher long time ago employing low temperature NMR measurement and external illumination with a mercury lamp. 27 Applying the LED based NMR illumination device developed in our group, 33 we explored the feasibility of ketimines 2a – i isomerization by in situ irradiation of the NMR sample by using a glass fiber coupled with a 365 nm LED. It was possible, in this way, to follow the isomerization by recording 1 H NMR spectra with regular intervals of time while the sample was subjected to a continuous illumination at 180 K. Low temperature measurements were necessary in order to prevent thermal back reaction and allow for Z -imine characterization.…”

Section: Resultsmentioning

confidence: 99%

Decrypting Transition States by Light: Photoisomerization as a Mechanistic Tool in Brønsted Acid Catalysis

2017

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Despite the wide applicability of enantioselective Brønsted acid catalysis, experimental insight into transition states is very rare, and most of the mechanistic knowledge is gained by theoretical calculations. Here, we present an alternative approach (decrypting transition state by light = DTS-hν), which enables the decryption of the transition states involved in chiral phosphoric acids catalyzed addition of nucleophiles to imines. Photoisomerization of double bonds is employed as a mechanistic tool. For this class of reactions four pathways (Type I Z, Type I E, Type II Z, Type II E) are possible, leading to different enantiomers depending on the imine configuration (E- or Z-imine) and on the nucleophilic attack site (top or bottom). We demonstrated that the imine double bond can be isomerized by light (365 nm LED) during the reaction leading to a characteristic fingerprint pattern of changes in reaction rate and enantioselectivity. This characteristic fingerprint pattern is directly correlated to the transition states involved in the transformation. Type I Z and Type II Z are demonstrated to be the competing pathways for the asymmetric transfer hydrogenation of ketimines, while in the nucleophilic addition of acetylacetone to N-Boc protected aldimines Type I E and Type II E are active. Accelerations on reaction rate up to 177% were observed for ketimines reduction. Our experimental findings are supported by quantum chemical calculations and noncovalent interaction analysis.

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LED based NMR illumination device for mechanistic studies on photochemical reactions – Versatile and simple, yet surprisingly powerful

Cited by 137 publications

References 30 publications

Combined In Situ Illumination‐NMR‐UV/Vis Spectroscopy: A New Mechanistic Tool in Photochemistry

Combined In Situ Illumination‐NMR‐UV/Vis Spectroscopy: A New Mechanistic Tool in Photochemistry

LED‐Illuminated NMR Studies of Flavin‐Catalyzed Photooxidations Reveal Solvent Control of the Electron‐Transfer Mechanism

Decrypting Transition States by Light: Photoisomerization as a Mechanistic Tool in Brønsted Acid Catalysis

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