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

Red-Light-Active N,C,N-Pincer Bismuthinidene: Excited State Dynamics and Mechanism of Oxidative Addition into Aryl Iodides

Despite the progress made in the field of synthetic organic photocatalysis over the past decade, the use of higher wavelengths, especially those in the deep-red portion of the electromagnetic spectrum, remains comparatively rare. We have previously disclosed that a well-defined N,C,N-pincer bismuthinidene (1a) can undergo formal oxidative addition into a wide range of aryl electrophiles upon absorption of low-energy red light. In this study, we map out the photophysical dynamics of 1a and glean insights into the nature of the excited state responsible for the activation of aryl electrophiles. Transient absorption and emission techniques reveal that, upon irradiation with red light, the complex undergoes a direct S0 → S1 metal-to-ligand charge transfer (MLCT) transition, followed by rapid intersystem crossing (ISC) to a highly reducing emissive triplet state (-2.61 V vs Fc+/0 in MeCN). The low dissipative losses incurred during ISC (∼6% of the incident light energy) help rationalize the ability of the bismuthinidene to convert low-energy light into useful chemical energy. Spectroelectrochemical and computational data support a charge-separated excited-state structure with radical-anion character on the ligand and radical-cation character on bismuth. Kinetic studies and competition experiments afford insights into the mechanism of oxidative addition into aryl iodides; concerted and inner-sphere processes from the triplet excited state are ruled out, with the data strongly supporting a pathway that proceeds via outer-sphere dissociative electron transfer.

Cross-Coupling Reactions with Nickel, Visible Light, and tert-Butylamine as a Bifunctional Additive

Transition metal catalysis is crucial for the synthesis of complex molecules, with ligands and bases playing a pivotal role in optimizing cross-coupling reactions. Despite advancements in ligand design and base selection, achieving effective synergy between these components remains challenging. We present here a general approach to nickel-catalyzed photoredox reactions employing tert-butylamine as a cost-effective bifunctional additive, acting as the base and ligand. This method proves effective for C-O and C-N bond-forming reactions with a diverse array of nucleophiles, including phenols, aliphatic alcohols, anilines, sulfonamides, sulfoximines, and imines. Notably, the protocol demonstrates significant applicability in biomolecule derivatization and facilitates sequential one-pot functionalizations. Spectroscopic investigations revealed the robustness of the dynamic catalytic system, while elucidation of structure-reactivity relationships demonstrated how computed molecular properties of both the nucleophile and electrophile correlated to reaction performance, providing a foundation for effective reaction outcome prediction.

Designing a visible light-mediated double photoswitch: a combination of biradical and azobenzene structural motifs that can be switched independently

A new molecular switch is presented that combines both biradical and azobenzene motifs to perform visible light-induced constitutional and stereo-isomerisation within the same molecule. The insertion of isonitrile-functionalised azobenzenes into the four-membered biradical [˙P(μ-NTer)2P˙] (1), yielding a phosphorus-centred cyclopentane-1,3-diyl (E-4B and E-5B), represents a straightforward method to generate the desired double switches (E-4B and E-5B) in excellent yields (>90%). The switching properties are demonstrated for the fluorinated species E-5B and, interestingly, can occur either stepwise or simultaneously, depending on the order in which the sample is irradiated with red and/or green light. All possible isomerisation reactions, i.e., housane formation in the phosphorus-centred cyclopentane-1,3-diyl fragment and E/Z isomerisation at the azobenzene, can be switched by irradiation and the reaction products E-5H, Z-5H and Z-5B (when performing the thermal reverse reaction in the dark) are identified using 19F{1H} and 31P{1H} NMR spectroscopy. Results from quantum chemical calculations contribute to the understanding and visualisation of the different isomers of each of the observed compounds (E-5B, E-5H, Z-5H, and Z-5B) caused by the unique structure of the double switches.

Protodefluorinated Selectfluor® Aggregatively Activates Selectfluor® for Efficient Radical C(sp3)-H Fluorination Reactions

Efficient fluorination reactions are key in the late-stage functionalization of complex molecules in medicinal chemistry, in upgrading chemical feedstocks, and in materials science. Radical C(sp3)-H fluorinations using Selectfluor® - one of the most popular fluorination agents - allow to directly engage unactivated precursors under mild photochemical or thermal catalytic conditions. However, H-TEDA(BF4)2 to date is overlooked and discarded as waste, despite comprising 95% of the molecular weight of Selectfluor®. We demonstrate that the addition of H-TEDA(BF4)2 at the start of fluorination reactions markedly promotes their rates and accesses higher overall yields of fluorinated products (~3.3 × higher on average across the cases studied) than unpromoted reactions. Several case studies showcase generality of the promotor, for photochemical, photocatalytic and thermal radical fluorination reactions. Detailed mechanistic investigations reveal the key importance of aggregation changes in Selectfluor® and H-TEDA(BF4)2 to fill gaps of understanding in how radical C(sp3)-H fluorination reactions work. This study exemplifies an overlooked reaction waste product being upcycled for a useful application.

Photocatalytic applications of covalent organic frameworks: synthesis, characterization, and utility

Photocatalysis has emerged as an energy efficient and safe method to perform organic transformations, and many semiconductors have been studied for use as photocatalysts. Covalent organic frameworks (COFs) are an established class of crystalline, porous materials constructed from organic units that are easily tunable. COFs importantly display semiconductor properties and respectable photoelectric behaviour, making them a strong prospect as photocatalysts. In this review, we summarize the design, synthetic methods, and characterization techniques for COFs. Strategies to boost photocatalytic performance are also discussed. Then the applications of COFs as photocatalysts in a variety of reactions are detailed. Finally, a summary, challenges, and future opportunities for the development of COFs as efficient photocatalysts are entailed.

Light, Switch, Action! The Influence of Geometrical Photoisomerization in an Adaptive Self-Assembled System

The ubiquitous ability of natural dynamic nanostructures to adapt to environmental changes is a highly desirable property for chemical systems, particularly in the development of complex matter, molecular machines, and life-like materials. Designing such systems is challenging due to the generation of complex mixtures with responses that are difficult to predict, characterize, and diversify. Here, we navigate between self-assembled architectures using light by operating an intrinsic photoswitchable building block that governs the state of the system. When complementary units are present, the photoswitch determines the predominant architecture, reversibly adapting between the cage and macrocycles, including (otherwise inaccessible) higher-energy assemblies. Our study showcases this concept with seven different transformations, offering an unprecedented degree of control, diversification, and adaptation by self-selecting complementary units. These findings could enable applications of on-demand dissipative macrocycles based on dynamic bonds. We also envision different transient nanostructures, e.g., reticular and polymeric materials, being explored by fine-tuning the nature of the complementary unit.

In situ Monitoring of Photocatalysis on Polymeric Carbon Nitride Thin Films

Polymeric carbon nitride has attracted significant interest in heterogeneous photocatalysis due to its activity under visible-light irradiation. Herein, we report on using carbon nitride-coated NMR tubes for in situ studies of photocatalytic reaction mechanisms. In a first step, we exploited carbon nitride-coated crimp vials as batch photoreactors for visible photocatalytic fluorinations of unactivated C(sp3)-H bonds, with moderate to excellent yields and reusability over multiple cycles. Eventually, carbon nitride-coated NMR tubes were used as a photoreactor by coupling them with optical fiber irradiation directly inside the spectrometer. This enabled us to follow the reaction with in situ NMR spectroscopy identifying reactive intermediates otherwise elusive in conventional analyses. The method provides advantages for the study of photocatalytic mechanisms of complex reactions and substantially reduces the need of comparative tests for depicting reaction intermediates and conversion pathways.

Photoinduced Group Transposition via Iridium-Nitrenoid Leading to Amidative Inner-Sphere Aryl Migration

We herein report a fundamental mechanistic investigation into photochemical metal-nitrenoid generation and inner-sphere transposition reactivity using organometallic photoprecursors. By designing Cp*Ir(hydroxamate)(Ar) complexes, we induced photo-initiated ligand activation, allowing us to explore the amidative σ(Ir-aryl) migration reactivity. A combination of experimental mechanistic studies, femtosecond transient absorption spectroscopy, and density functional theory (DFT) calculations revealed that the metal-to-ligand charge transfer enables the σ(N-O) cleavage, followed by Ir-acylnitrenoid generation. The final inner-sphere σ(Ir-aryl) group migration results in a net amidative group transposition.

Cryogen-free 400-MHz nuclear magnetic resonance spectrometer as a versatile tool for pharmaceutical process analytical technology

The discovery of new ceramic materials containing Ba-La-Cu oxides in 1986 that exhibited superconducting properties at high temperatures in the range of 35 K or higher, recognized with the Nobel Prize in Physics in 1987, opened a new world of opportunities for nuclear magnetic resonance (NMRs) and magnetic resonance imaging (MRIs) to move away from liquid cryogens. This discovery expands the application of high temperature superconducting (HTS) materials to fields beyond the chemical and medical industries, including electrical power grids, energy, and aerospace. The prototype 400-MHz cryofree HTS NMR spectrometer installed at Amgen's chemistry laboratory has been vital for a variety of applications such as structure analysis, reaction monitoring, and CASE-3D studies with RDCs. The spectrometer has been integrated with Amgen's chemistry and analytical workflows, providing pipeline project support in tandem with other Kinetic Analysis Platform technologies. The 400-MHz cryofree HTS NMR spectrometer, as the name implies, does not require liquid cryogens refills and has smaller footprint that facilitates installation into a chemistry laboratory fume hood, sharing the hood with a process chemistry reactor. Our evaluation of its performance for structural analysis with CASE-3D protocol and for reaction monitoring of Amgen's pipeline chemistry was successful. We envision that the HTS magnets would become part of the standard NMR and MRI spectrometers in the future. We believe that while the technology is being developed, there is room for all magnet options, including HTS, low temperature superconducting (LTS) magnets, and low field benchtop NMRs with permanent magnets, where utilization will be dependent on application type and costs.

Reversibly Tuning the Viscosity of Peptide-Based Solutions Using Visible Light

Light can be used to design stimuli-responsive systems. We induce transient changes in the assembly of a low molecular weight gelator solution using a merocyanine photoacid. Through our approach, reversible viscosity changes can be achieved via irradiation, delivering systems where flow can be controlled non-invasively on demand.

Evolution of a Combined UV/Vis and NMR Setup with Fixed Pathlengths for Mass-limited Samples

The investigation of reaction mechanisms is a complex task that usually requires the use of several techniques. To obtain as much information as possible on the reaction and any intermediates - possibly invisible to one technique - the combination of techniques is a solution. In this work we present a new setup for combined UV/Vis and NMR spectroscopy and compare it to an established alternative. The presented approach allows a versatile usage of different commercially-available components like mirrors and fiber bundles as well as different fixed pathlengths according to double transmission or single transmission measurements. While a previous approach is based on a dip-probe setup for conventional NMR probes, the new one is based on a micro-Helmholtz coil array (LiquidVoxel™). This makes the use of rectangular cuvettes possible, which ensure well-defined pathlengths allowing for quantification of species. Additionally, very low quantities of compound can be analyzed due to the microfabrication and small cuvette size used. As proof-of-principle this new setup for combined UV/Vis and NMR spectroscopy is used to examine a well-studied photochromic system of the dithienylethene compound class. A thorough comparison of the pros and cons of the two setups for combined UV/Vis and NMR measurements is performed.

Azotriptycenes: Photoswitchable Molecular Brakes

The control of molecular motions is a central topic of molecular machine research. Molecular brakes are fundamental building blocks towards such goal as they allow deliberately decelerating specific motions after an outside stimulus is applied. Here we present azotriptycenes as structural framework for light-controlled molecular brakes. The intrinsic kinetics and their changes upon azotriptycene isomerization are scrutinized comprehensively by a mixed theoretical and variable temperature NMR approach. With azotriptycenes C-N bond rotation rates can be decelerated or accelerated reversibly by up to five orders of magnitude. Rate change effects are highly localized and are strongest for the C-N bond connecting a triptycene rotor fragment to the central diazo group. The detailed mechanistic insights provide a solid basis for further conscious design and applications in the future.

Detection of sub-nmol amounts of the antiviral drug favipiravir in 19F MRI using photo-chemically induced dynamic nuclear polarization

In biological tissues, 19F magnetic resonance (MR) enables the non-invasive, background-free detection of 19F-containing biomarkers. However, the signal-to-noise ratio (SNR) is usually low because biomarkers are typically present at low concentrations. Measurements at low magnetic fields further reduce the SNR. In a proof-of-principal study we applied LED-based photo-chemically induced dynamic nuclear polarization (photo-CIDNP) to amplify the 19F signal at 0.6 T. For the first time, 19F MR imaging (MRI) and spectroscopy (MRS) of a fully biocompatible model system containing the antiviral drug favipiravir has been successfully performed. This fluorinated drug has been used to treat Ebola and COVID-19. Since the partially cyclic reaction scheme for photo-CIDNP allows for multiple data acquisitions, averaging further improved the SNR. The mean signal gain factor for 19F has been estimated to be in the order of 103. An in-plane resolution of 0.39 × 0.39 mm2 enabled the analysis of spatially varying degrees of hyperpolarization. The minimal detectable amount of favipiravir per voxel was estimated to about 500 pmol. The results show that 19F photo-CIDNP is a promising method for the non-invasive detection of suitable 19F-containing drugs and other compounds with very low levels of the substance.

Exo-cage catalysis and initiation derived from photo-activating host-guest encapsulation

Coordination cage catalysis has commonly relied on the endogenous binding of substrates, exploiting the cavity microenvironment and spatial constraints to engender increased reactivity or interesting selectivity. Nonetheless, there are issues with this approach, such as the frequent occurrence of product inhibition or the limited applicability to a wide range of substrates and reactions. Here we describe a strategy in which the cage acts as an exogenous catalyst, wherein reactants, intermediates and products remain unbound throughout the course of the catalytic cycle. Instead, the cage is used to alter the properties of a cofactor guest, which then transfers reactivity to the bulk-phase. We have exemplified this approach using photocatalysis, showing that a photoactivated host-guest complex can mediate [4 + 2] cycloadditions and the aza-Henry reaction. Detailed in situ photolysis experiments show that the cage can both act as a photo-initiator and as an on-cycle catalyst where the quantum yield is less than unity.

MIDA- and TIDA-Boronates Stabilize α-Radicals Through B-N Hyperconjugation

Multifunctional organoboron compounds increasingly enable the simple generation of complex, Csp3 -rich small molecules. The ability of boron-containing functional groups to modify the reactivity of α-radicals has also enabled a myriad of chemical reactions. Boronic esters with vacant p-orbitals have a significant stabilizing effect on α-radicals due to delocalization of spin density into the empty orbital. The effect of coordinatively saturated derivatives, such as N-methyliminodiacetic acid (MIDA) boronates and counterparts, remains less clear. Herein, we demonstrate that coordinatively saturated MIDA and TIDA boronates stabilize secondary alkyl α-radicals via σB-N hyperconjugation in a manner that allows site-selective C-H bromination. DFT calculated radical stabilization energies and spin density maps as well as LED NMR kinetic analysis of photochemical bromination rates of different boronic esters further these findings. This work clarifies that the α-radical stabilizing effect of boronic esters does not only proceed via delocalization of radical character into vacant boron p-orbitals, but that hyperconjugation of tetrahedral boron-containing functional groups and their ligand electron delocalizing ability also play a critical role. These findings establish boron ligands as a useful dial for tuning reactivity at the α-carbon.

Mechanistic snapshots of rhodium-catalyzed acylnitrene transfer reactions

Rhodium acylnitrene complexes are widely implicated in catalytic C-H amidation reactions but have eluded isolation and structural characterization. To overcome this challenge, we designed a chromophoric octahedral rhodium complex with a bidentate dioxazolone ligand, in which photoinduced metal-to-ligand charge transfer initiates catalytic C-H amidation. X-ray photocrystallographic analysis of the Rh-dioxazolone complex allowed structural elucidation of the targeted Rh-acylnitrenoid and provided firm evidence that the singlet nitrenoid species is primarily responsible for acylamino transfer reactions. We also monitored in crystallo reaction of a nucleophile with the in situ generated Rh-acylnitrenoid, providing a crystallographically traceable reaction system to capture mechanistic snapshots of nitrenoid transfer.

Amber Light Control of Peptide Secondary Structure by a Perfluoroaromatic Azobenzene Photoswitch

The incorporation of photoswitches into the molecular structure of peptides and proteins enables their dynamic photocontrol in complex biological systems. Here, a perfluorinated azobenzene derivative triggered by amber light was site-specifically conjugated to cysteines in a helical peptide by perfluoroarylation chemistry. In response to the photoisomerization (trans→cis) of the conjugated azobenzene with amber light, the secondary structure of the peptide was modulated from a disorganized into an amphiphilic helical structure.

Spin Hyperpolarization in Modern Magnetic Resonance

Magnetic resonance techniques are successfully utilized in a broad range of scientific disciplines and in various practical applications, with medical magnetic resonance imaging being the most widely known example. Currently, both fundamental and applied magnetic resonance are enjoying a major boost owing to the rapidly developing field of spin hyperpolarization. Hyperpolarization techniques are able to enhance signal intensities in magnetic resonance by several orders of magnitude, and thus to largely overcome its major disadvantage of relatively low sensitivity. This provides new impetus for existing applications of magnetic resonance and opens the gates to exciting new possibilities. In this review, we provide a unified picture of the many methods and techniques that fall under the umbrella term "hyperpolarization" but are currently seldom perceived as integral parts of the same field. Specifically, before delving into the individual techniques, we provide a detailed analysis of the underlying principles of spin hyperpolarization. We attempt to uncover and classify the origins of hyperpolarization, to establish its sources and the specific mechanisms that enable the flow of polarization from a source to the target spins. We then give a more detailed analysis of individual hyperpolarization techniques: the mechanisms by which they work, fundamental and technical requirements, characteristic applications, unresolved issues, and possible future directions. We are seeing a continuous growth of activity in the field of spin hyperpolarization, and we expect the field to flourish as new and improved hyperpolarization techniques are implemented. Some key areas for development are in prolonging polarization lifetimes, making hyperpolarization techniques more generally applicable to chemical/biological systems, reducing the technical and equipment requirements, and creating more efficient excitation and detection schemes. We hope this review will facilitate the sharing of knowledge between subfields within the broad topic of hyperpolarization, to help overcome existing challenges in magnetic resonance and enable novel applications.

Light-Controlled Lyotropic Liquid Crystallinity of Polyaspartates Exploited as Photo-Switchable Alignment Medium

Two polyaspartates bearing ortho-fluorinated azobenzenes (pFAB) as photo-responsive groups in the side chain were synthesized: PpFABLA (1) and co-polyaspartate PpFABLA-co-PBLA [11, 75%(n/n) PpFABLA content]. As a consequence of the E/Z-isomerization of the side chain, PpFABLA (1) undergoes a visible-light-induced reversible coil-helix transition in solution: Green light (525 nm) affords the coil, and violet light (400 nm) affords the helix. pFAB significantly increases the thermal stability of the Z-isomer at 20 °C (t_1/2 = 66 d for the Z-isomer) and effectively counters the favored back formation of the helix. At 20%(w/w) polymer concentration, the helical polymer forms a lyotropic liquid crystal (LLC) that further orients unidirectionally inside a magnetic field, while the coil polymer results in an isotropic solution. The high viscosity of the polymer solution stabilizes the coexistence of liquid crystalline and isotropic domains, which were obtained with spatial control by partial light irradiation. When used as an alignment medium, PpFABLA (1) enables (i) the measurement of dipolar couplings without the need for a separate isotropic reference and (ii) the differentiation of enantiomers. PpFABLA-co-PBLA (11) preserves the helical structure, by intention, independently of the E/Z-isomerization of the side chain: Both photo-isomers of PpFABLA-co-PBLA (11) form a helix that─at a concentration of 16%(w/w)─form an LLC. Despite the absence of a change in the secondary structure, the E/Z-isomerization of the side chain changes the morphology of the liquid crystal and leads to different sets of dipolar coupling for the same probe molecule.

Photo enhancement reveals (E,Z) and (Z,Z) configurations as additional intermediates in iminium ion catalysis

Imidazolidinone-based α,β-unsaturated iminium ions are the reactive species within countless synthetic protocols in asymmetric organocatalysis. However, (E,Z) and (Z,Z) imidazolidinone iminium ions, i.e. (Z)-CC configurations, have been elusive so far. Herein we describe how in situ photoisomerization enables the observation and assignment of high energetic (Z)-configured intermediates below the detection limit of NMR spectroscopy for (E,Z) and (Z,Z) iminium perchlorate complexes derived from MacMillan's 1st generation catalyst and cinnamaldehyde. Traces of (E,Z) could even be detected under synthetic conditions at 25 °C in MeCN. Using back isomerization studies and diffusion ordered spectroscopy, conditions were found to stabilize the (E,Z) and (Z,Z) isomers for several hours via ion pair aggregation. Thus, at least (E,Z) should be considered for future investigations in asymmetric iminium ion catalysis.

Benzoates as photosensitization catalysts and auxiliaries in efficient, practical, light-powered direct C(sp3)-H fluorinations

Of the methods for direct fluorination of unactivated C(sp3)-H bonds, photosensitization of SelectFluor is a promising approach. Although many substrates can be activated with photosensitizing catalysts, issues remain that hamper fluorination of complex molecules. Alcohol- or amine-containing functional groups are not tolerated, fluorination regioselectivity follows factors endogenous to the substrate and cannot be influenced by the catalyst, and reactions are highly air-sensitive. We report that benzoyl groups serve as highly efficient photosensitizers which, in combination with SelectFluor, enable visible light-powered direct fluorination of unactivated C(sp3)-H bonds. Compared to previous photosensitizer architectures, the benzoyls have versatility to function both (i) as a photosensitizing catalyst for simple substrate fluorinations and (ii) as photosensitizing auxiliaries for complex molecule fluorinations that are easily installed and removed without compromising yield. Our auxiliary approach (i) substantially decreases the reaction's induction period, (ii) enables C(sp3)-H fluorination of many substrates that fail under catalytic conditions, (iii) increases kinetic reproducibility, and (iv) promotes reactions to higher yields, in shorter times, on multigram scales, and even under air. Observations and mechanistic studies suggest an intimate 'assembly' of auxiliary and SelectFluor prior/after photoexcitation. The auxiliary allows other EnT photochemistry under air. Examples show how auxiliary placement proximally directs regioselectivity, where previous methods are substrate-directed.

Facile Detection of Light-Controlled Radical Scavengers from Natural Products Using In Situ UV-LED NMR Spectroscopy

With the recent development of chemical analysis technology, attention has been placed on natural light-sensitive compounds that exhibit photoreactivity to expand the structural diversity of natural product chemistry. Photochemical reactions that proceed via a free radical mechanism could be used to modulate the radical-scavenging ability of natural products as well as involve structural change. As the health benefits of radicals are also presented, there is a need for a controllable radical scavenging method for topical and selective application. In this study, we developed a novel acquisition and processing method to identify light-controlled radical scavengers in plant extracts and evaluate their antioxidant activity under light irradiation based on in situ UV-LED NMR spectroscopy. Using the developed method, licochalcones A and B, in which the trans and cis isomers undergo reversible photoisomerization, were selectively identified from licorice root extract, and their light-induced free radical scavenging activity was confirmed.

Noncovalent Stabilization of Radical Intermediates in the Enantioselective Hydroamination of Alkenes with Sulfonamides

Noncovalent interactions (NCIs) are critical elements of molecular recognition in a wide variety of chemical contexts. While NCIs have been studied extensively for closed-shell molecules and ions, very little is understood about the structures and properties of NCIs involving free radical intermediates. In this report, we describe a detailed mechanistic study of the enantioselective radical hydroamination of alkenes with sulfonamides and present evidence suggesting that the basis for asymmetric induction in this process arises from attractive NCIs between a neutral sulfonamidyl radical intermediate and a chiral phosphoric acid (CPA). We describe experimental, computational, and data science-based evidence that identifies the specific radical NCIs that form the basis for the enantioselectivity. Kinetic studies support that C-N bond formation determines the enantioselectivity. Density functional theory investigations revealed the importance of both strong H-bonding between the CPA and the N-centered radical and a network of aryl-based NCIs that serve to stabilize the favored diastereomeric transition state. The contributions of these specific aryl-based NCIs to the selectivity were further confirmed through multivariate linear regression analysis by comparing the measured enantioselectivity to computed descriptors. These results highlight the power of NCIs to enable high levels of enantioselectivity in reactions involving uncharged open-shell intermediates and expand our understanding of radical-molecule interactions.

Kinetics of a Ni/Ir-Photocatalyzed Coupling of ArBr with RBr: Intermediacy of ArNiII(L)Br and Rate/Selectivity Factors

The Ni/Ir-photocatalyzed coupling of an aryl bromide (ArBr) with an alkyl bromide (RBr) has been analyzed using in situ LED-¹⁹F NMR spectroscopy. Four components (light, [ArBr], [Ni], [Ir]) are found to control the rate of ArBr consumption, but not the product selectivity, while two components ([(TMS)₃SiH], [RBr]) independently control the product selectivity, but not the rate. A major resting state of nickel has been identified as ArNi^II(L)Br, and ¹³C-isotopic entrainment is used to show that the complex undergoes Ir-photocatalyzed conversion to products (Ar-R, Ar-H, Ar-solvent) in competition with the release of ArBr. A range of competing absorption and quenching effects lead to complex correlations between the Ir and Ni catalyst loadings and the reaction rate. Differences in the Ir/Ni Beer–Lambert absorption profiles allow the rate to be increased by the use of a shorter-wavelength light source without compromising the selectivity. A minimal kinetic model for the process allows simulation of the reaction and provides insights for optimization of these processes in the laboratory.

Variable Temperature LED-NMR: Rapid Insights into a Photocatalytic Mechanism from Reaction Progress Kinetic Analysis

A multitude of techniques are available to obtain a useful understanding of photocatalytic mechanisms. The combination of LED illumination with nuclear magnetic resonance spectroscopy (LED-NMR) provides a rapid, convenient means to directly monitor a photocatalytic reaction in situ. Herein, we describe a study of the mechanism of an enantioselective intermolecular [2 + 2] photocycloaddition catalyzed by a chiral Ir photocatalyst using LED-NMR. The data-rich output of this experiment is suitable for same-excess and variable time normalization analyses (VTNA). Together, these identified an unexpected change in mechanism between reactions conducted at ambient and cryogenic temperatures. At -78 °C, the kinetic data are consistent with the triplet rebound mechanism we previously proposed for this reaction, involving sensitization of maleimide and rapid reaction with a hydrogen-bound quinoline within the solvent cage. At room temperature, the cycloaddition instead proceeds through intracomplex energy transfer to the hydrogen-bound quinolone. These results highlight the potential sensitivity of photocatalytic reaction mechanisms to the precise reaction conditions and the further utility of LED-NMR as a fast, data-rich tool for their interrogation that compares favorably to conventional ex situ kinetic analyses.

Sample illumination device facilitates in situ light-coupled NMR spectroscopy without fibre optics

In situ illumination of liquid-state nuclear magnetic resonance (NMR) samples makes it possible for a wide range of light-dependent chemical and biological phenomena to be studied by the powerful analytical technique. However, the position of an NMR sample deep within the bore of the spectrometer magnet renders such illumination challenging. Here, we demonstrate the working principles of a sample illumination device (NMRtorch) where a lighthead containing an LED array is positioned directly at the top of an NMRtorch tube which is inserted into the NMR spectrometer. The wall of the tube itself acts as a light guide, illuminating the sample from the outside. We explore how this new setup performs in a number of photo-NMR applications, including photoisomerisation and photo-chemically induced dynamic nuclear polarisation (photo-CIDNP), and demonstrate the potential for ultraviolet (UV) degradation studies with continuous online NMR assessment. This setup enables users of any typical liquid-state spectrometer to easily perform in situ photo-NMR experiments, using a wide range of wavelengths.

Photoisomerization of Arylidene Heterocycles: Toward the Formation of Fused Heterocyclic Quinolines

We report herein the photoinduced isomerization of a series of arylidene heterocycles 1. The photoreaction mechanism was investigated by a combined UV-vis/photo-NMR spectroscopic study, and we showed that Ar-TZDs exhibit a positive P-type photochromism, which limits their isomerization efficiency. By exploring the solvatochromism in a series of solvents, the conditions favoring the conversion toward one or the other stereoisomer have been studied, in particular by choosing the appropriate wavelengths. Finally, the extension of this photoisomerization study was proposed with a convenient preparation of various fused heterocyclic quinolines in good overall yields.

Discovery and Photoisomerization of New Pyrrolosesquiterpenoids Glaciapyrroles D and E, from Deep-Sea Sediment Streptomyces sp

Two new pyrrolosesquiterpenes, glaciapyrroles D (1) and E (2) were discovered along with the previously reported glaciapyrrole A (3) from Streptomyces sp. GGS53 strain isolated from deep-sea sediment. This study elucidated the planar structures of 1 and 2 using nuclear magnetic resonance (NMR), mass spectrometry (MS), ultraviolet (UV), and infrared (IR) spectroscopic data. The absolute configurations of the glaciapyrroles were determined by Mosher’s method, circular dichroism spectroscopy, and X-ray crystallography. Under 366 nm UV irradiation, the glaciapyrroles were systematically converted to the corresponding photoglaciapyrroles (4–6) via photoisomerization, resulting in the diversification of the glaciapyrrole family compounds. The transformation of the glaciapyrrole Z to E isomers occurred in a 1:1 ratio, based on virtual validation of the photoisomerization of these olefinic compounds by ¹H-NMR spectroscopy and liquid chromatography/mass spectrometry (LC/MS) analysis. Finally, when encapsulated in poly(lactic-co-glycolic acid) nanoparticles, glaciapyrrole E and photoglaciapyrrole E displayed significant inhibitory activity against influenza A virus. This is the first report of antiviral effects from glaciapyrrole family compounds, whose biological functions have only been subjected to limited studies so far.

Mechanistic analysis by NMR spectroscopy: A users guide

A 'principles and practice' tutorial-style review of the application of solution-phase NMR in the analysis of the mechanisms of homogeneous organic and organometallic reactions and processes. This review of 345 references summarises why solution-phase NMR spectroscopy is uniquely effective in such studies, allowing non-destructive, quantitative analysis of a wide range of nuclei common to organic and organometallic reactions, providing exquisite structural detail, and using instrumentation that is routinely available in most chemistry research facilities. The review is in two parts. The first comprises an introduction to general techniques and equipment, and guidelines for their selection and application. Topics include practical aspects of the reaction itself, reaction monitoring techniques, NMR data acquisition and processing, analysis of temporal concentration data, NMR titrations, DOSY, and the use of isotopes. The second part comprises a series of 15 Case Studies, each selected to illustrate specific techniques and approaches discussed in the first part, including in situ NMR (^1/2H, ^10/11B, ¹³C, ¹⁵N, ¹⁹F, ²⁹Si, ³¹P), kinetic and equilibrium isotope effects, isotope entrainment, isotope shifts, isotopes at natural abundance, scalar coupling, kinetic analysis (VTNA, RPKA, simulation, steady-state), stopped-flow NMR, flow NMR, rapid injection NMR, pure shift NMR, dynamic nuclear polarisation, ¹H/¹⁹F DOSY NMR, and in situ illumination NMR.

Rapid Online Analysis of Photopolymerization Kinetics and Molecular Weight Using Diffusion NMR

Online, high-throughput molecular weight analysis of polymerizations is rare, with most studies relying on tedious sampling techniques and batchwise postanalysis. The ability to track both monomer conversion and molecular weight evolution in real time could underpin precision polymer development and facilitate study of rapid polymerization reactions. Here, we use a single time-resolved diffusion nuclear magnetic resonance (NMR) experiment to simultaneously study the kinetics and molecular weight evolution during a photopolymerization, with in situ irradiation inside the NMR instrument. As a model system, we used a photoinduced electron transfer reversible addition-fragmentation chain transfer (PET-RAFT) polymerization. The data allow diffusion coefficients and intensities to be calculated every 14 s from which the polymer size and monomer conversion can be extracted. Key to this approach is (1) the use of shuffled gradient amplitudes in the diffusion NMR experiment to access reactions of any rate, (2) the addition of a relaxation agent to increase achievable time resolution and, (3) a sliding correction that accounts for viscosity changes during polymerization. Diffusion NMR offers a uniquely simple, translatable handle for online monitoring of polymerization reactions.

Stereoselective, Ruthenium-Photocatalyzed Synthesis of 1,2-Diaminotruxinic Bis-amino Acids from 4-Arylidene-5(4H)-oxazolones

The irradiation of (Z)-2-phenyl-4-aryliden-5(4H)-oxazolones 1 in deoxygenated CH₂Cl₂ at 25 °C with blue light (465 nm) in the presence of [Ru(bpy)₃](BF₄)₂ (5% mole ratio) as a triplet photocatalyst promotes the [2+2] photocycloaddition of the C=C bonds of the 4-arylidene moiety, thus allowing the completely regio- and stereoselective formation of cyclobutane-bis(oxazolone)s 2 as single stereoisomers. Cyclobutanes 2 have been unambiguously characterized as the μ-isomers and contain two E-oxazolones coupled in an anti-head-to-head form. The use of continuous-flow techniques in microreactors allows the synthesis of cyclobutanes 2 in only 60 min, compared with the 24–48 h required in batch mode. Ring opening of the oxazolone heterocycle in 2 with a base affords the corresponding 1,2-diaminotruxinic bis-amino esters 3, which are also obtained selectively as μ-isomers. The ruthenium complex behaves as a triplet photocatalyst, generating the reactive excited state of the oxazolone via an energy-transfer process. This reactive excited state has been characterized as a triplet diradical ³(E/Z)-1* by laser flash photolysis (transient absorption spectroscopy). This technique also shows that this excited state is the same when starting from either (Z)- or (E)-oxazolones. Density functional theory calculations show that the first step of the [2+2] cycloaddition between ³(E/Z)-1* and (Z)-1 is formation of the C(H)–C(H) bond and that (Z) to (E) isomerization takes place at the 1,4-diradical thus formed.

Photons or Electrons? A Critical Comparison of Electrochemistry and Photoredox Catalysis for Organic Synthesis

Redox processes are at the heart of synthetic methods that rely on either electrochemistry or photoredox catalysis, but how do electrochemistry and photoredox catalysis compare? Both approaches provide access to high energy intermediates (e.g., radicals) that enable bond formations not constrained by the rules of ionic or 2 electron (e) mechanisms. Instead, they enable 1e mechanisms capable of bypassing electronic or steric limitations and protecting group requirements, thus enabling synthetic chemists to disconnect molecules in new and different ways. However, while providing access to similar intermediates, electrochemistry and photoredox catalysis differ in several physical chemistry principles. Understanding those differences can be key to designing new transformations and forging new bond disconnections. This review aims to highlight these differences and similarities between electrochemistry and photoredox catalysis by comparing their underlying physical chemistry principles and describing their impact on electrochemical and photochemical methods.

Controlled release and characterisation of photocaged molecules using in situ LED illumination in solution NMR spectroscopy

We demonstrate that photo-uncaging reactions triggered by LED illumination can be conveniently monitored in situ by solution NMR, offering new ways to characterise and optimise photocages.

Photochemical transformation of chlorobenzenes and white phosphorus into arylphosphines and phosphonium salts

Chlorobenzenes are important starting materials for the preparation of commercially valuable triarylphosphines and tetraarylphosphonium salts, but their use for the direct arylation of elemental phosphorus has been elusive. Here we describe a simple photochemical route toward such products. UV-LED irradiation (365 nm) of chlorobenzenes, white phosphorus (P₄) and the organic superphotoreductant tetrakis(dimethylamino)ethylene (TDAE) affords the desired arylphosphorus compounds in a single reaction step.

Photocatalytic Arylation of P4 and PH3 : Reaction Development Through Mechanistic Insight

Detailed 31 P{1 H} NMR spectroscopic investigations provide deeper insight into the complex, multi-step mechanisms involved in the recently reported photocatalytic arylation of white phosphorus (P4 ). Specifically, these studies have identified a number of previously unrecognized side products, which arise from an unexpected non-innocent behavior of the commonly employed terminal reductant Et3 N. The different rate of formation of these products explains discrepancies in the performance of the two most effective catalysts, [Ir(dtbbpy)(ppy)2 ][PF6 ] (dtbbpy=4,4'-di-tert-butyl-2,2'-bipyridine) and 3DPAFIPN. Inspired by the observation of PH3 as a minor intermediate, we have developed the first catalytic procedure for the arylation of this key industrial compound. Similar to P4 arylation, this method affords valuable triarylphosphines or tetraarylphosphonium salts depending on the steric profile of the aryl substituents.

Photocatalysis in the Life Science Industry

In the pursuit of new pharmaceuticals and agrochemicals, chemists in the life science industry require access to mild and robust synthetic methodologies to systematically modify chemical structures, explore novel chemical space, and enable efficient synthesis. In this context, photocatalysis has emerged as a powerful technology for the synthesis of complex and often highly functionalized molecules. This Review aims to summarize the published contributions to the field from the life science industry, including research from industrial-academic partnerships. An overview of the synthetic methodologies developed and strategic applications in chemical synthesis, including peptide functionalization, isotope labeling, and both DNA-encoded and traditional library synthesis, is provided, along with a summary of the state-of-the-art in photoreactor technology and the effective upscaling of photocatalytic reactions.

1,3-Alkyl Transposition in Allylic Alcohols Enabled by Proton-Coupled Electron Transfer

A method is described for the isomerization of acyclic allylic alcohols into β-functionalized ketones via 1,3-alkyl transposition. This reaction proceeds via light-driven proton-coupled electron transfer (PCET) activation of the O-H bond in the allylic alcohol substrate, followed by C-C β-scission of the resulting alkoxy radical. The transient alkyl radical and enone acceptor generated in the scission event subsequently recombine via radical conjugate addition to deliver β-functionalized ketone products. A variety of allylic alcohol substrates bearing alkyl and acyl migratory groups were successfully accommodated. Insights from mechanistic studies led to a modified reaction protocol that improves reaction performance for challenging substrates.

Artificial Supramolecular Pumps Powered by Light

The development of artificial nanoscale motors that can use energy from a source to perform tasks requires systems capable of performing directionally controlled molecular movements and operating away from chemical equilibrium. Here, the design, synthesis and properties of pseudorotaxanes are described, in which a photon input triggers the unidirectional motion of a macrocyclic ring with respect to a non-symmetric molecular axle. The photoinduced energy ratcheting at the basis of the pumping mechanism is validated by measuring the relevant thermodynamic and kinetic parameters. Owing to the photochemical behavior of the azobenzene moiety embedded in the axle, the pump can repeat its operation cycle autonomously under continuous illumination. NMR spectroscopy was used to observe the dissipative non-equilibrium state generated in situ by light irradiation. We also show that fine changes in the axle structure lead to an improvement in the performance of the motor. Such results highlight the modularity and versatility of this minimalist pump design, which provides facile access to dynamic systems that operate under photoinduced non-equilibrium regimes.

Second-Generation Light-Fueled Supramolecular Pump

We describe the modular design of a pseudorotaxane-based supramolecular pump and its photochemically driven autonomous nonequilibrium operation in a dissipative regime. These properties derive from careful engineering of the energy maxima and minima along the threading coordinate and their light-triggered modulation. Unlike its precursor, this second-generation system is amenable to functionalization for integration into more complex devices.

Insights Into the Micelle-Induced β-Hairpin-to-α-Helix Transition of a LytA-Derived Peptide by Photo-CIDNP Spectroscopy

A choline-binding module from pneumococcal LytA autolysin, LytA_239–252, was reported to have a highly stable nativelike β-hairpin in aqueous solution, which turns into a stable amphipathic α-helix in the presence of micelles. Here, we aim to obtain insights into this DPC-micelle triggered β-hairpin-to-α-helix conformational transition using photo-CIDNP NMR experiments. Our results illustrate the dependency between photo-CIDNP phenomena and the light intensity in the sample volume, showing that the use of smaller-diameter (2.5 mm) NMR tubes instead of the conventional 5 mm ones enables more efficient illumination for our laser-diode light setup. Photo-CIDNP experiments reveal different solvent accessibility for the two tyrosine residues, Y249 and Y250, the latter being less accessible to the solvent. The cross-polarization effects of these two tyrosine residues of LytA_239–252 allow for deeper insights and evidence their different behavior, showing that the Y250 aromatic side chain is involved in a stronger interaction with DPC micelles than Y249 is. These results can be interpreted in terms of the DPC micelle disrupting the aromatic stacking between W241 and Y250 present in the nativelike β-hairpin, hence initiating conversion towards the α-helix structure. Our photo-CIDNP methodology represents a powerful tool for observing residue-level information in switch peptides that is difficult to obtain by other spectroscopic techniques.