Kinetics of Asymmetric Transfer Hydrogenation, Catalyst Deactivation, and Inhibition with Noyori Complexes As Revealed by Real-Time High-Resolution FlowNMR Spectroscopy

DFT-Assisted Microkinetic Study of Transfer Hydrogenation over Homogeneous and Immobilized Cp*Ir Complexes

DFT calculations were done to investigate the kinetic mechanism of benzaldehyde transfer hydrogenation using [Cp*IrCl2]2 complexes in isopropyl alcohol in the presence of potassium tert-butoxide. Predicted energy barriers provide evidence that the inner-sphere (IS) mechanism (effective barrier 53.0 kJ/mol) is favored over the outer-sphere (OS) and Meerwein-Pondorf-Verley (MPV) mechanisms. Reaction kinetics was studied using both homogeneous and immobilized Cp*Ir complexes as catalysts. A mathematical model was developed to simulate the transfer hydrogenation of benzaldehyde on these catalysts, accounting for possible mass transfer limitations for the immobilized catalyst. A microkinetic model was constructed using both our density functional theory calculations and fitting of the kinetic parameters of catalyst activation and deactivation reactions. The simulation results predict that only about a quarter of Ir immobilized complexes are involved in the reaction, and this is the main reason for the observed higher activity of the homogeneous catalyst. The activity of the immobilized catalyst was found to be related to the hydride species concentration, which is a function of the base concentration. The results suggest that the amount of base has a drastic effect on the immobilized catalyst activity.

Pure Shift NMR in Continuous Flow

Flow NMR is an expanding analytical approach with applications that include in-line analysis for process control and optimisation, and real-time reaction monitoring. The samples monitored by flow NMR are typically mixtures that yield complex 1D 1H spectra. "Pure shift" NMR is a powerful approach to simplifying 1H NMR spectra, but its standard implementation is not compatible with continuous flow because of interference between sample motion and the position-dependent spin manipulations that are required in pure shift NMR. Here we show that pure shift NMR spectra can be successfully collected for continuously flowing samples, thanks to an adapted acquisition scheme, robust solvent suppression, and a velocity-compensation strategy. The resulting method is used to collect ultrahigh resolution reaction monitoring data. Pure shift NMR spectra are expected to benefit many applications of flow NMR.

Enantioselective Transfer Hydrogenation of α-Methoxyimino-β-keto Esters

α-Methoxyimino-β-keto esters are reported to undergo highly enantioselective catalytic transfer hydrogenation using the Noyori-Ikariya complex RuCl(p-cymene)[(S,S)-Ts-DPEN] in a mixture of formic acid-triethylamine and dimethylformamide at 25 °C. The experimental study performed on over 25 substrates combined with computational analysis revealed that a Z-configured methoxyimino group positioned alpha to a ketone carbonyl leads to higher reactivity and mostly excellent enantioselectivity within this substrate class. Density functional theory calculations of competing transition states were used in rationalizing the origins of enantioselectivity and the possible role of the methoxyimino group in the reaction outcome.

In-cell Catalysis by Tethered Organo-Osmium Complexes Generates Selectivity for Breast Cancer Cells

Anticancer agents that exhibit catalytic mechanisms of action offer a unique multi-targeting strategy to overcome drug resistance. Nonetheless, many in-cell catalysts in development are hindered by deactivation by endogenous nucleophiles. We have synthesised a highly potent, stable Os-based 16-electron half-sandwich ('piano stool') catalyst by introducing a permanent covalent tether between the arene and chelated diamine ligand. This catalyst exhibits antiproliferative activity comparable to the clinical drug cisplatin towards triple-negative breast cancer cells and can overcome tamoxifen resistance. Speciation experiments revealed Os to be almost exclusively albumin-bound in the extracellular medium, while cellular accumulation studies identified an energy-dependent, protein-mediated Os accumulation pathway, consistent with albumin-mediated uptake. Importantly, the tethered Os complex was active for in-cell transfer hydrogenation catalysis, initiated by co-administration of a non-toxic dose of sodium formate as a source of hydride, indicating that the Os catalyst is delivered to the cytosol of cancer cells intact. The mechanism of action involves the generation of reactive oxygen species (ROS), thus exploiting the inherent redox vulnerability of cancer cells, accompanied by selectivity for cancerous cells over non-tumorigenic cells.

Flow-NMR as a Process-Monitoring Tool for mRNA IVT Reaction

Messenger RNA (mRNA) based vaccines were instrumental in accelerating the end of the SARS-CoV-2 pandemic and are being aggressively developed as prophylaxes for a range of viral diseases. The swift adoption of mRNA-based therapeutics has also left open vast areas of opportunity for improving the development of mRNA-based drugs. One such area with immense potential focuses on the mRNA drug substance production, where mRNA is generated by a cell-free reaction called in vitro transcription (IVT). Process analytical technologies (PAT) are integral to the pharmaceutical industry and are necessary to facilitate agile process optimization and enhance process quality, control, and understanding. Due to the complexity and novelty inherent to the IVT reaction, there is a need for effective PAT that would provide in-depth, real-time insight into the reaction process to allow delivery of novel mRNA vaccines to patients faster in a more cost-effective way. Herein, we showcase the development of flow-nuclear magnetic resonance (flow-NMR) as a highly effective process-analytical tool for monitoring mRNA IVT reactions to support process development, optimization, and production.

Broadband ultrafast 2D NMR spectroscopy for online monitoring in continuous flow

Flow NMR is a powerful tool to monitor chemical reactions under realistic conditions. Here, we describe ultrafast (UF) 2D NMR schemes that make it possible to acquire broadband homonuclear 2D NMR spectra in 90 seconds or less for a continuously flowing sample. An interleaved acquisition strategy is used to address the spectral width limitation of UF 2D NMR. We show how, for a flowing sample, the use of a transverse axis for spatial encoding makes it possible to achieve the very high scan-to-scan stability required for interleaved acquisition. We also describe an optimised solvent suppression strategy that is effective for interleaved acquisition in continuous flow. These developments open the way to online monitoring with flow 2D NMR at high time resolution, as we illustrate with the monitoring of an organocatalysed condensation reaction.

Paramagnetic Relaxation Agents for Enhancing Temporal Resolution and Sensitivity in Multinuclear FlowNMR Spectroscopy

Sensitivity in FlowNMR spectroscopy for reaction monitoring often suffers from low levels of pre-magnetisation due to limited residence times of the sample in the magnetic field. While this in-flow effect is tolerable for high sensitivity nuclei such as 1 H and 19 F, it significantly reduces the signal-to-noise ratio in 31 P and 13 C spectra, making FlowNMR impractical for low sensititvity nuclei at low concentrations. Paramagnetic relaxation agents (PRAs), which enhance polarisation and spin-lattice relaxation, could eliminate the adverse in-flow effect and improve the signal-to-noise ratio. Herein, [Co(acac)3 ], [Mn(acac)3 ], [Fe(acac)3 ], [Cr(acac)3 ], [Ni(acac)2 ]3, [Gd(tmhd)3 ] and [Cr(tmhd)3 ] are investigated for their effectiveness in improving signal intensity per unit time in FlowNMR applications under the additional constraint of chemical inertness towards catalytically active transition metal complexes. High-spin Cr(III) acetylacetonates emerged as the most effective compounds, successfully reducing 31 P T1 values four- to five-fold at PRA concentrations as low as 10 mM without causing adverse line broadening. Whereas [Cr(acac)3 ] showed signs of chemical reactivity with a mixture of triphenylphosphine, triphenylphosphine oxide and triphenylphosphate over the course of several hours at 80° C, the bulkier [Cr(tmhd)3 ] was stable and equally effective as a PRA under these conditions. Compatibility with a range of representative transition metal complexes often used in homogeneous catalysis has been investigated, and application of [Cr(tmhd)3 ] in significantly improving 1 H and 31 P{1 H} FlowNMR data quality in a Rh-catalysed hydroformylation reaction has been demonstrated. With the PRA added, 13 C relaxation times were reduced more than six-fold, allowing quantitative reaction monitoring of substrate consumption and product formation by 13 C{1 H} FlowNMR spectroscopy at natural abundance.

Computer vision for non-contact monitoring of catalyst degradation and product formation kinetics

We report a computer vision strategy for the extraction and colorimetric analysis of catalyst degradation and product-formation kinetics from video footage. The degradation of palladium(ii) pre-catalyst systems to form 'Pd black' is investigated as a widely relevant case study for catalysis and materials chemistries. Beyond the study of catalysts in isolation, investigation of Pd-catalyzed Miyaura borylation reactions revealed informative correlations between colour parameters (most notably ΔE, a colour-agnostic measure of contrast change) and the concentration of product measured by off-line analysis (NMR and LC-MS). The breakdown of such correlations helped inform conditions under which reaction vessels were compromised by air ingress. These findings present opportunities to expand the toolbox of non-invasive analytical techniques, operationally cheaper and simpler to implement than common spectroscopic methods. The approach introduces the capability of analyzing the macroscopic 'bulk' for the study of reaction kinetics in complex mixtures, in complement to the more common study of microscopic and molecular specifics.

Cobalt Homeostatic Catalysis for Coupling of Enaminones and Oxadiazolones to Quinazolinones

Transition metal catalysis has revolutionized modern synthetic chemistry for its diverse modes of coordination reactivity. However, this versatility in reactivity is also the predominant cause of catalyst deactivation, a persisting issue that can significantly compromise its synthetic value. Homeostatic catalysis, a catalytic process that can sustain its productive catalytic cycle even when chemically disturbed, is proposed herein as an effective tactic to address the challenge. In particular, a cobalt homeostatic catalysis process has been developed for the water-tolerant coupling of enaminones and oxadiazolones to quinazolinones. Dynamic covalent bonding serves as a mechanistic handle for the preferred buffering of water onto enaminone and reverse exchange by a released secondary amine, thus securing reversible entry into cobalt's dormant and active states for productive catalysis. Through this homeostatic catalysis mode, a broad structural scope has been achieved for quinazolinones, enabling further elaboration into distinct pharmaceutically active agents.

Performance of homogeneous catalysts viewed in dynamics

Effective assessment of catalytic performance is the foundation for the rational design and development of new catalysts with superior performance. The ubiquitous screening/optimization studies use reaction yields as the sole performance metric in an approach that often neglects the complexity of the catalytic system and intrinsic reactivities of the catalysts. Using an example of hydrogenation catalysis, we examine the transient behavior of catalysts that are often encountered in activation, deactivation and catalytic turnover processes. Each of these processes and the reaction environment in which they take place are gradually shown to determine the real-time catalyst speciation and the resulting kinetics of the overall catalytic reaction. As a result, the catalyst performance becomes a complex and time-dependent metric defined by multiple descriptors apart from the reaction yield. This behaviour is not limited to hydrogenation catalysis and affects various catalytic transformations. In this feature article, we discuss these catalytically relevant descriptors in an attempt to arrive at a comprehensive depiction of catalytic performance.

NMR methods for the analysis of mixtures

NMR spectroscopy is a powerful approach for the analysis of mixtures. Its usefulness arises in large part from the vast landscape of methods, and corresponding pulse sequences, that have been and are being designed to tackle the specific properties of mixtures of small molecules. This feature article describes a selection of methods that aim to address the complexity, the low concentrations, and the changing nature that mixtures can display. These notably include pure-shift and diffusion NMR methods, hyperpolarisation methods, and fast 2D NMR methods such as ultrafast 2D NMR and non-uniform sampling. Examples or applications are also described, in fields such as reaction monitoring and metabolomics, to illustrate the relevance and limitations of different methods.

Heterocycle-containing Noyori-Ikariya catalysts for asymmetric transfer hydrogenation of ketones

The synthesis of a range of N-(heterocyclesulfonyl)-functionalised Noyori-Ikariya catalysts is described. The complexes were prepared through a short sequence from C2-symmetric 1,2-diphenylethylene-1,2-diamine (DPEN) and were characterised by a range of methods including X-ray crystallography. The complexes were active catalysts for the asymmetric transfer hydrogenation (ATH) of a range of acetophenone derivatives, giving products of high ee in most cases, with notably good results for ortho-substituted acetophenones.

Basic Promotors Impact Thermodynamics and Catalyst Speciation in Homogeneous Carbonyl Hydrogenation

Homogeneously catalyzed reactions often make use of additives and promotors that affect reactivity patterns and improve catalytic performance. While the role of reaction promotors is often discussed in view of their chemical reactivity, we demonstrate that they can be involved in catalysis indirectly. In particular, we demonstrate that promotors can adjust the thermodynamics of key transformations in homogeneous hydrogenation catalysis and enable reactions that would be unfavorable otherwise. We identified this phenomenon in a set of well-established and new Mn pincer catalysts that suffer from persistent product inhibition in ester hydrogenation. Although alkoxide base additives do not directly participate in inhibitory transformations, they can affect the equilibrium constants of these processes. Experimentally, we confirm that by varying the base promotor concentration one can control catalyst speciation and inflict substantial changes to the standard free energies of the key steps in the catalytic cycle. Despite the fact that the latter are universally assumed to be constant, we demonstrate that reaction thermodynamics and catalyst state are subject to external control. These results suggest that reaction promotors can be viewed as an integral component of the reaction medium, on its own capable of improving the catalytic performance and reshaping the seemingly rigid thermodynamic landscape of the catalytic transformation.

Ru-Catalyzed Enantioselective Hydrogenation of 2-Pyridyl-Substituted Alkenes and Substrate-Mediated H/D Exchange

A highly efficient and enantioselective asymmetric hydrogenation catalyzed by Ru-DTBM-segphos is reported for a broad range of pyridine-pyrroline tri-substituted alkenes. Kinetic, spectroscopic, and computational studies suggest that addition of H2 is rate-determining and that alkene insertion is the enantio-determining step. These studies also reveal an intriguing Ru-catalyzed H/D exchange process that is facilitated by the substrate at room temperature and low pressure where hydrogenation activity is suppressed. These studies lead to a mechanistic proposal that further defines the roles of hydrogen gas, Ru-H species, and protic solvents in this catalytic system.

Process Analytical Technologies - Advances in bioprocess integration and future perspectives

Process Analytical Technology (PAT) instruments include analyzers capable of measuring physical and chemical process parameters and key attributes with the goal of optimizing process controls. PAT in the form of a probe or sensor is designed to integrate within the pharmaceutical manufacturing line and is coupled with computing equipment to perform chemometric modeling for result interpretation and multilayer statistical control of processes. PAT solutions are intended for understanding bioprocesses with a goal to control quality at all stages of product manufacturing and achieve quality by design (QbD). The goal of PAT implementation is to promote real-time release of products to decrease the cycle time and cost of production. This review focuses on the applications of PAT solutions at different stages of the manufacturing process for vaccine production, the advantages, challenges at present state, and the vision of the future development of biopharmaceutical industries.

Mapping Catalyst Activation, Turnover Speciation and Deactivation in Rh/PPh3-catalysed Olefin Hydroformylation

We report new insights into the fate of the precious metal during hydroformylation catalysis of 1-hexene with Rh/PPh3 complexes using multi-nuclear operando FlowNMR spectroscopy. By applying selectively excited 1H and...

Modern advancements in continuous-flow aided kinetic analysis

Although kinetic analysis has traditionally been conducted in a batch vessel, continuous-flow aided kinetic analysis continues to swell in popularity.

Fast and Accurate Diffusion NMR Acquisition in Continuous Flow

FlowNMR spectroscopy has become a popular and powerful technique for online reaction monitoring. DOSY NMR is an established technique for obtaining information about diffusion rates and molecular size on static...

Carboxamide carbonyl-ruthenium(ii) complexes: detailed structural and mechanistic studies in the transfer hydrogenation of ketones

The organo-carboxamide carbonyl-ruthenium(ii) complexes displayed moderate catalytic activities in the transfer hydrogenation of a broad spectrum of ketones.

Convenient and accurate insight into solution-phase equilibria from FlowNMR titrations

Solution phase titrations are made easy by multi-nuclear FlowNMR spectroscopy with automated, continuous titre addition to give accurate insights into Brønsted acid/base, hydrogen bonding, Lewis acid/base and metal/ligand binding equilibria under native conditions.

Does the Configuration at the Metal Matter in Noyori-Ikariya Type Asymmetric Transfer Hydrogenation Catalysts?

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.

Multi-nuclear, high-pressure, operando FlowNMR spectroscopic study of Rh/PPh3 - catalysed hydroformylation of 1-hexene

The hydroformylation of 1-hexene with 12 bar of 1 : 1 H2/CO in the presence of the catalytic system [Rh(acac)(CO)2]/PPh3 was successfully studied by real-time multinuclear high-resolution FlowNMR spectroscopy at 50 °C. Quantitative reaction progress curves that yield rates as well as chemo- and regioselectivities have been obtained with varying P/Rh loadings. Dissolved H2 can be monitored in solution to ensure true operando conditions without gas limitation. 31P{1H} and selective excitation 1H pulse sequences have been periodically interleaved with 1H FlowNMR measurements to detect Rh-phosphine intermediates during the catalysis. Stopped-flow experiments in combination with diffusion measurements and 2D heteronuclear correlation experiments showed the known tris-phosphine complex [RhH(CO)(PPh3)3] to generate rapidly exchanging isomers of the bis-phosphine complex [Rh(CO)2(PPh3)2] under CO pressure that directly enter the catalytic cycle. A new mono-phosphine acyl complex has been identified as an in-cycle reaction intermediate.

Chitosan as a chiral ligand and organocatalyst: preparation conditions–property–catalytic performance relationships

Properties of chitosan prepared by alkaline deacetylation of chitin under various conditions were correlated with their performance as ligands or organocatalysts in asymmetric catalytic reactions.

Mechanistic in situ investigation of heterogeneous hydrogenation over Rh/TiO2 catalysts: selectivity, pairwise route and catalyst nature

We report a catalyst with the highest selectivity toward pairwise hydrogen addition of 7% among supported metal catalysts, found as a result of variation of Rh/TiO₂ catalyst preparation procedures.

Online reaction monitoring by single-scan 2D NMR under flow conditions

Single-scan 2D NMR based on spatial encoding can be used to monitor chemical reactions with a flow unit in realistic reaction conditions.

Insight into catalyst speciation and hydrogen co-evolution during enantioselective formic acid-driven transfer hydrogenation with bifunctional ruthenium complexes from multi-technique operando reaction monitoring

Operando spectroscopy shows a transition from dehydrogenation to hydrogen transfer during the reaction, and allows measuring optimal conditions for maximum rate and efficiency.