FlowNMR spectroscopy is an excellent technique for non-invasive real-time reaction monitoring under relevant conditions that avoids many of the limitations that bedevil other reaction monitoring techniques.

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Catalytic hydrogen transfer from basic isopropyl alcohol to aryl ketones mediated by [(arene)­(TsDPEN)­RuCl] complexes has been investigated by operando 1H NMR spectroscopy using a recirculating flow setup. Selective excitation pulse sequences allowed fast and quantitative monitoring of the key [(mesitylene)­(TsDPEN)­RuH] intermediate during catalysis, which is shown to interact with both substrates by polarization transfer experiments. Comparison of reaction profiles with catalyst speciation traces in conjunction with reaction progress kinetic analysis using variable time normalization and kinetic modeling showed the existence of two independent catalyst deactivation/inhibition pathways: whereas excess base exerted a competitive inhibition effect on the unsaturated catalyst intermediate, the active hydride suffered from an inherent first-order decay that is not evident in early stages of the reaction where turnover is fast. Isotopic labeling revealed arene loss to be the entry point into deactivation pathways to Ru nanoparticles via hydride-bridged intermediates.

We demonstrate how FlowNMR spectroscopy can readily be applied to investigate photochemical reactions that require sustained input of light and air to yield mechanistic insight under realistic conditions. The Eosin Y mediated photo-oxidation of N-allylbenzylamine is shown to produce imines as primary reaction products from which undesired aldehydes form after longer reaction times. Facile variation of reaction conditions during the reaction in flow allows for probe experiments that give information about the mode of action of the photocatalyst.

Noyori–Ikariya type [(arene)RuCl(TsDPEN)] (TsDPEN, sulfonated diphenyl ethylenediamine) complexes are widely used C=O and C=N reduction catalysts that produce chiral alcohols and amines via a key ruthenium–hydride intermediate that determines the stereochemistry of the product. Whereas many details about the interactions of the pro-chiral substrate with the hydride complex and the nature of the hydrogen transfer from the latter to the former have been investigated over the past 25 years, the role of the stereochemical configuration at the stereogenic ruthenium center in the catalysis has not been elucidated so far. Using operando FlowNMR spectroscopy and nuclear Overhauser effect spectroscopy, we show the existence of two diastereomeric hydride complexes under reaction conditions, assign their absolute configurations in solution, and monitor their interconversion during transfer hydrogenation catalysis. Configurational analysis and multifunctional density functional theory (DFT) calculations show the λ-( R , R ) S Ru configured [(mesitylene)RuH(TsDPEN)] complex to be both thermodynamically and kinetically favored over its λ-( R , R ) R Ru isomer with the opposite configuration at the metal. Computational analysis of both diastereomeric catalytic manifolds show the major λ-( R , R ) S Ru configured [(mesitylene)RuH(TsDPEN)] complex to dominate asymmetric ketone reduction catalysis with the minor λ-( R , R ) R Ru [(mesitylene)RuH(TsDPEN)] stereoisomer being both less active and less enantioselective. These findings also hold true for a tethered catalyst derivative with a propyl linker between the arene and TsDPEN ligands and thus show enantioselective transfer hydrogenation catalysis with Noyori–Ikariya complexes to proceed via a lock-and-key mechanism.

In‐situ NMR spectroscopic analysis of homogeneous reactions is an essential tool for mechanistic analysis in organic and organometallic chemistry. However, rapid non‐equilibrium reactions, that are initiated by mixing, require specialized approaches. We report herein on a study of the factors that ensure quantitative results in a recently‐developed technique for stopped‐flow NMR spectroscopy. The influence of some of the key parameters on quantitation is studied by 19F NMR spectroscopic analysis of the kinetics and activation parameters for the base‐catalyzed protodeboronation of highly‐reactive polyfluorinated arylboronic acids, with half‐lives as low as 0.1 seconds. The effects of spin relaxation, pre‐magnetization, heat‐transfer versus reaction enthalpy, and mixing‐efficiency are analyzed in detail. We also compare and contrast choice of pulse angle, interscan delay, and use of pseudo real‐time by interleaving, as means to achieve an optimal balance between temporal resolution and sensitivity.

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Hyperpolarization techniques can enormously enhance the NMR signal thus allowing the exploitation of hyperpolarized substrates for in-vivo MRI applications. The short lifetime of hyperpolarized spin order poses significant limitations in such applications. Spin order storage can be prolonged through the use of long-lived spin states. Additionally, the storage of spin polarization–either in the form of longitudinal or singlet order–can be prolonged in low viscosity solutions. Here, we report the use of low viscosity liquid-CO2 solutions to store nuclear spin polarization in the form of longitudinal and singlet order for extended periods. Our results demonstrate that this storage time can be considerably sustained in liquid-CO2 solutions in comparison to other low viscosity solvents, opening up the possibility of new, exciting storage experiments in the future.