Rapid Isomerization Research Articles

Silylium-Ion-Promoted (3 + 2) Annulation of Allenylsilanes with Internal Alkynes Involving a Pentadienyl-to-Allyl Cation Electrocyclization.

A (3 + 2) annulation of allenyl- and, after rapid isomerization, propargylsilanes with internal 1-aryl-1-alkynes to form 4-methylenecyclopentenes is reported. The reaction is initiated by a silylium ion, and the catalytic cycle is subsequently maintained by the self-regeneration of the silylium-ion promoter. Unlike the well-established Danheiser annulation, where the allenylsilane serves as a three-carbon synthon, the present transformation engages the allenylsilane as a two-carbon synthon. Experimental observations and DFT calculations unveil a reaction cascade involving various β-silicon-stabilized carbocations, where a pentadienyl-to-allyl cation electrocyclization is the key step.

Donor Strength Determination of Buchwald-Type Phosphines.

The electronic properties of eight Buchwald-type phosphines were determined and ranked by 13C NMR spectroscopy, highlighting the sensitivity of the Huynh electronic parameter (HEP). Furthermore, the 13Ccarbene NMR signals of 19 known and new P-donor complexes in CDCl3 (HEP values) are found to be highly correlated to those in C6D6. This enables the donor strength determination of P-donors even where the complexes are unstable or undergo rapid isomerization due to transphobia in CDCl3.

Isomerization, Protonation, and Hydrolysis Properties of Naphthalimide-Containing Spiropyran in Aqueous Media.

Synthesis of spiropyrans exhibiting ring-opening/closing isomerization driven by external stimuli is a challenging subject for the development of molecular sensors. A napthalimide-containing spiropyran (1) promotes rapid isomerization between the spirocyclic (SP) form and ring-opened merocyanine (MC) form by the change in solvent polarity even under the dark condition at room temperature. In this work, the effect of water on the isomerization behavior of 1 was studied. The addition of water caused an MC-water H-bonding interaction and shifted the resonance MC structure to the zwitterionic form with a lower ground-state energy. The stabilized MC form promoted spontaneous SP → MC isomerization and increased the equilibrium MC concentration. The effect of pH on the behavior of 1 was also studied. In acidic-neutral media, protonation/deprotonation of the naphthalimide moiety led to rapid and reversible changes in the absorption spectra. In contrast, strongly basic media (pH > 12) promoted irreversible base-catalyzed hydrolysis of the alkene moiety.

A Quantum‐Chemical Study of the Mechanism of 1‐Azadienes Formation from Aldimines and Arylacetylenes in the KOtBu/DMSO Superbasic Medium

AbstractThe mechanism of 1‐azadienes formation from aldimines and arylacetylenes in the KOtBu/DMSO superbasic medium has been explored theoretically using a DFT approach [B2PLYP(D2)/6‐311+G(d,p)//B3LYP/6‐31+G(d)] and a sophisticated model taking the potassium tert‐butoxide and the nearest solvation shell of the potassium cation explicitly into account. The main focus is placed on (i) a detailed analysis of the kinetic and thermodynamic characteristics of the isomerization of intermediate propargylamine into 1‐azadienes, and (ii) the factors determining the isomeric composition of 1‐azadienes with different substituents in the starting arylaldimines and arylacetylenes. Our findings clearly indicate that the mild experimental conditions of 1‐azadienes formation are accounted for by the relatively small activation barrier (ΔG≠≤16.2 kcal/mol) of the limiting step (ethynylation of aldimine with arylacetylenes). The rapid isomerization of propargylamine to 1‐azadiene is found to be due to low activation barriers (ΔG≠≤9.1 kcal/mol) related to the protons transfer. The isomeric composition in the case of various substituted 1‐azadienes is determined solely by the difference in the thermodynamic stability of the 1E,2E‐ and 1Z,2E‐forms.

Characterizing the Oligomers Distribution along the Aggregation Pathway of Amyloid Aβ1-40 by NMR.

This study delves into the early aggregation process of the Aβ1-40 amyloid peptide, elucidating the associated oligomers distribution. Motivated by the acknowledged role of small oligomers in the neurotoxic damage linked to Alzheimer's disease, we present an experimental protocol for preparing 26-O-acyl isoAβ1-40, a modified Aβ1-40 peptide facilitating rapid isomerization to the native amide form at neutral pH. This ensures seed-free solutions, minimizing experimental variability. Additionally, we demonstrate the efficacy of coupling NMR diffusion ordered spectroscopy (DOSY) with the Inverse Laplace Transform (ILT) reconstruction method, for effective characterization of early aggregation processes. This innovative approach efficiently maps oligomers distributions across a wide spectrum of initial peptide concentrations offering unique insights into the evolution of oligomers relative populations. As a proof of concept, we demonstrate the efficacy of our approach assessing the impact of Epigallocathechin gallate, a known remodeling agent of amyloid fibrils, on the oligomeric distributions of aggregated Aβ1-40. The DOSY-ILT proposed approach stands as a robust and discriminating asset, providing a powerful strategy for rapidly gaining insight into potential inhibitors' impact on the aggregation process.

A DASA displaying highly efficient and rapid reversible isomerization within sustainable nano/micro capsules: one step closer to sustainability.

Donor-acceptor Stenhouse adducts (DASAs), derived from bio-based furfural, demonstrate reversible isomerization when exposed to light and heat, positioning them as attractive candidates for sustainable smart materials. However, achieving efficient and rapid isomerization in high bio-content solid-state matrices, especially under mild conditions, remains a significant hurdle due to restricted molecular mobility and limited matrix options. To address this, we developed a novel solid matrix in the form of sustainable nano/micro capsules, which boast the highest bio-content reported to date (57%). Composed of polymethylmethacrylate (PMMA) and a lauric-stearic acid eutectic mixture (L-SEM), these capsules facilitate highly efficient and rapid reversible isomerization of a third-generation DASA (DASA-1). Remarkably, the system achieves 84% forward and 90% reverse isomerization under mild temperatures, significantly enhancing the material's photo-switching capabilities. This advancement not only addresses the critical challenge of isomerization within high bio-content solid matrices but also opens broader possibilities for the application of bio-based DASAs in environmentally friendly technologies, such as color-rich rewritable papers. By innovating in the design of sustainable smart materials, this work has the potential to extend the utility of DASAs across various scientific fields, contributing to the global shift towards a low-carbon, environmentally sustainable society.

Heterobimetallic Ta–Nb MOF offering moderate Lewis/Brønsted acidity expedites glucose isomerization to fructose under microwave conditions

The heterobimetallic MOF with anchored sulfonic acid groups (functioning as a bridge between Ta and Nb metal nodes) enables rapid and augmented glucose isomerization to fructose for bioenergy development.

Investigation of ultrafast photoisomerization dynamics of azobenzene derivative (E)-1-phenyl-2-((triisopropylsilyl)ethynyl)diazene

When exposed to light at a specific wavelength, azobenzene and its derivatives experience a transformation from trans form to cis form through isomerization. Due to its ability to change color upon illumination, azobenzene finds extensive use in various molecular devices and functional materials. However, despite significant researches focused on practical applications, there are still ongoing investigations into the underlying mechanisms governing azobenzene’s photochemical reactions and isomerization. In this study, we employ femtosecond stimulated Raman spectroscopy (FSRS), and transient absorption spectroscopy, in conjunction with quantum chemical calculations, to elucidate the ultrafast isomerization dynamics of an azobenzene derivative known as trans-AZOTIPS ((E)-1-phenyl-2-((triisopropylsilyl)ethynyl)diazene). The results demonstrate that upon photoexcitation, rapid isomerization occurs along the C−N=N bonds via the singlet excited state S1 to hot ground state (S0*) state transition. Additionally, we explore the impact of solvent viscosity on the isomerization process and find that the duration of isomerization remains unaffected by variations in solvent viscosity. These results suggest that the isomerization pathway involves a volume-conserving motion known as “hula twist”. After that, the vibrational cooling process is obtained in S0 state.

Mechanistic Basis for Red Light Switching of Azonium Ions.

Azonium ions formed by the protonation of tetra-ortho-methoxy-substituted aminoazobenzenes photoisomerize with red light under physiological conditions. This property makes them attractive as molecular tools for the photocontrol of physiological processes, for example, in photopharmacology. However, a mechanistic understanding of the photoisomerization process and subsequent thermal relaxation is necessary for the rational application of these compounds as well as for guiding the design of derivatives with improved properties. Using a combination of sub-ps/ns transient absorption measurements and quantum chemical calculations, we show that the absorption of a photon by the protonated E-H+ form of the photoswitch causes rapid (ps) isomerization to the protonated Z-H+ form, which can also absorb red light. Proton transfer to solvent then occurs on a microsecond time scale, leading to an equilibrium between Z and Z-H+ species, the position of which depends on the solution pH. Whereas thermal isomerization of the neutral Z form to the neutral E form is slow (∼0.001 s-1), thermal isomerization of Z-H+ to E-H+ is rapid (∼100 s-1), so the solution pH also governs the rate at which E/E-H+ concentrations are restored after a light pulse. This analysis provides the first complete mechanistic picture that explains the observed intricate photoswitching behavior of azonium ions at a range of pH values. It further suggests features of azonium ions that could be targeted for improvement to enhance the applicability of these compounds for the photocontrol of biomolecules.

Evaluating pesticide degradation in artificial wetlands with compound-specific isotope analysis: A case study with the fungicide dimethomorph

Pesticide degradation in wetland systems intercepting agricultural runoff is often overlooked and mixed with other dissipation processes when assessing pesticide concentrations alone. This study focused on the potential of compound-specific isotope analysis (CSIA) to estimate pesticide degradation in a stormwater wetland receiving pesticide runoff from a vineyard catchment. The fungicide dimethomorph (DIM), with diastereoisomers E and Z, was the prevalent pesticide in the runoff entering the wetland from June to September 2020. DIM Z, the most commonly detected isomer, exhibited a significant change (Δ(13C) > 3 ‰) in its carbon isotopic composition in the wetland water compared to the runoff and commercial formulation, which indicated degradation. Laboratory DIM degradation assays, including photodegradation and biodegradation in oxic wetland water with and without aquatic plants and in anoxic sediments, indicated that DIM degradation mainly occurred in the wetland sediments. The rapid degradation of both DIM isomers (E:t1/2 = 1.2 ± 0.6, Z: t1/2 = 1.5 ± 0.8 days) in the wetland sediment led to significant carbon isotopic fractionation (εDIM-E = −3.0 ± 0.6 ‰, εDIM-Z = −2.0 ± 0.2 ‰). In contrast, no significant isotope fractionation occurred during DIM photodegradation, despite the rapid isomerization of the E isomer to the Z isomer and a half-life of 15.3 ± 2.2 days for both isomers. DIM degradation was slow (E: t1/2 = 56–62 days, Z: t1/2 = 82–103 days) in oxic water with plants, while DIM persisted (120 days) in water without plants. DIM CSIA was thus used to evaluate the in situ biodegradation of DIM Z in the wetland. The DIM Z degradation estimates based on a classical concentration mass balance (86–94 %) were slightly higher than estimates based on the isotopic mass balance (61–68 %). Altogether, this study shows the potential of CSIA to conservatively evaluate pesticide degradation in wetland systems, offering a reliable alternative to classical labor-intensive mass balance approaches.)

Mechanistic Insights into Molecular Crystalline Organometallic Heterogeneous Catalysis through Parahydrogen-Based Nuclear Magnetic Resonance Studies.

The heterogeneous solid-gas reactions of crystals of [Rh(L2)(propene)][BArF4] (1, L2 = tBu2PCH2CH2PtBu2) with H2 and propene, 1-butene, propyne, or 1-butyne are explored by gas-phase nuclear magnetic resonance (NMR) spectroscopy under batch conditions at 25 °C. The temporal evolution of the resulting parahydrogen-induced polarization (PHIP) effects measures catalytic flux and thus interrogates the efficiency of catalytic pairwise para-H2 transfer, speciation changes in the crystalline catalyst at the molecular level, and allows for high-quality single-scan 1H, 13C NMR gas-phase spectra for the products to be obtained, as well as 2D-measurements. Complex 1 reacts with H2 to form dimeric [Rh(L2)(H)(μ-H)]2[BArF4]2 (4), as probed using EXAFS; meanwhile, a single-crystal of 1 equilibrates NMR silent para-H2 with its NMR active ortho isomer, contemporaneously converting into 4, and 1 and 4 each convert para-H2 into ortho-H2 at different rates. Hydrogenation of propene using 1 and para-H2 results in very high initial polarization levels in propane (>85%). Strong PHIP was also detected in the hydrogenation products of 1-butene, propyne, and 1-butyne. With propyne, a competing cyclotrimerization deactivation process occurs to afford [Rh(tBu2PCH2CH2PtBu2)(1,3,4-Me3C6H3)][BArF4], while with 1-butyne, rapid isomerization of 1-butyne occurs to give a butadiene complex, which then reacts with H2 more slowly to form catalytically active 4. Surprisingly, the high PHIP hydrogenation efficiencies allow hyperpolarization effects to be seen when H2 is taken directly from a regular cylinder at 25 °C. Finally, changing the chelating phosphine to Cy2PCH2CH2PCy2 results in initial high polarization efficiencies for propene hydrogenation, but rapid quenching of the catalyst competes to form the zwitterion [Rh(Cy2PCH2CH2PCy2){η6-(CF3)2(C6H3)}BArF3].

Hydrazone‐Based AIEgens with Photofluorochromic Ability for Rewritable, Intensity‐Variable, and High‐Resolution Photopattern

AbstractPhotoswitches with multiple fluorescence states known as photofluorochromism upon photo‐induced isomerization show practical applications in information storage, anticounterfeiting, and sensors. However, it is still challenging to realize rapid isomerization, efficient fluorescence, and gradient signal output simultaneously. Herein, by incorporating luminogenic units with aggregation‐induced emission (AIE) features into photo‐responsive hydrazone, a series of novel photofluorochromic AIE compounds are developed. These newly designed compounds exhibit quantum yields of up to 38.4% and could undergo reversible and swift photo‐isomerization upon 450/365 nm irradiation. Accordingly, the fabricated photopatterns based on them are utilized as quantitatively described images for information storage, which show excellent rewritability and intensity‐variable fluorescence with a high resolution of 10 µm. This work provides a new strategy to develop intelligent photopattern systems with continuous gradient fluorescence for information storage and cybernetics.

Ring-Opening Metathesis Polymerization and Related Olefin Metathesis Reactions in Benzotrifluoride as an Environmentally Advantageous Medium

A tremendous number of solvents, either as liquids or vapors, contaminate the environment on a daily basis worldwide. Olefin metathesis, which has been widely used as high-yielding protocols for ring-opening metathesis polymerization (ROMP), ring-closing metathesis (RCM), and isomerization reactions, is typically performed in toxic and volatile solvents such as dichloromethane. In this study, the results of our systematic experiments with the Grubbs G1, G2, and Hoveyda-Grubbs HG2 catalysts proved that benzotrifluoride (BTF) can replace dichloromethane (DCM) in these reactions, providing high yields and similar or even higher reaction rates in certain cases. The ROMP of norbornene resulted not only in high yields but also in polynorbornenes with a high molecular weight at low catalyst loadings. Ring-closing metathesis (RCM) experiments proved that, with the exception of the G1 catalyst, RCM occurs with similar high efficiencies in BTF as in DCM. It was found that isomerization of (Z)-but-2-ene-1,4-diyl diacetate with the G2 and HG2 catalysts proceeds at significantly higher initial rates in BTF than in DCM, leading to rapid isomerization with high yields in a short time. Overall, BTF is a suitable solvent for olefin metathesis, such as polymer syntheses by ROMP and the ring-closing and isomerization reactions.

Mechanisms and Kinetics of the Dehydrogenation of C6–C8 Cycloalkanes, Cycloalkenes, and Cyclodienes to Aromatics in H-MFI Zeolite Framework

This work employs periodic density functional theory to elucidate dehydrogenation mechanisms of C6–C8 cycloalkanes, cycloalkenes, and cyclodienes into aromatics during methanol-to-olefin (MTO) chemistries in H-MFI zeolites. Aromatic compounds act as co-catalysts that predominantly form ethylene over propylene products and lead to site-blocking polyaromatic compounds; thus, understanding the formation of aromatic compounds during MTO is critical to understanding its selectivity and catalyst stability. Ring dehydrogenation reactions occur via sequential hydride transfer followed by deprotonation. The rate-controlling hydride transfer reactions were investigated with surface-bound protons (Z–H) and alkyls (Z–CnH2n+1) as hydride-accepting species. Hydride transfers via proton-mediated routes occur with intrinsic free energy barriers (623 K) of 215 kJ mol–1 for C6H12, 164 kJ mol–1 for C6H10, and 141 kJ mol–1 for C6H8, whereas methyl-mediated counterparts occur with intrinsic free barriers of 101, 95, and 81 kJ mol–1, respectively. Transition states for hydride transfer reactions prefer channel intersections within MFI networks, and their activation barriers suggest that methyl-mediated routes are favored over proton-mediated counterparts during MTO. Methyl substituents on hydrocarbon rings generally increase activation barriers for hydride transfers that occur proximal to the −CH3, partly because of steric hindrances between the hydride acceptor and ring-bound −CH3. Rapid double-bond isomerization within cyclohexenes and cyclohexadienes allows hydride transfer reactions to occur away from sterically hindering methyl substituents in methylated and dimethylated C6 ring hydrocarbons. The impact of carbocation substitution in hydride-accepting species was explored by contrasting barriers of methyl (Z–CH3), ethyl (Z–C2H5), 2-propyl (Z–C3H7), and tert-butyl (Z–C4H9) surface-bound alkyls. Intrinsic activation barriers decrease with increasing substitution of the alkyl hydride acceptor, consistent with those alkyls forming more stable carbocations. However, when accounting for steric hindrances associated with the co-adsorption of hydrocarbon rings near surface-bound alkyls, apparent free barriers are larger for more-substituted alkyls. Taking these apparent barriers into account, we predict that methyl-mediated hydride transfer reactions are responsible for the aromatization of hydrocarbon rings during MTO, leading to the CH4 formed during MTO reactions as the aromatic pool is enriched.

Synthesis and characterization of photo-responsive azobenzene side-chain liquid crystalline polystyrenes with a mesogenic phase of wide temperature range

Polystyrenes (PSs) that provide photochemical stability are commonly used as the chromophore macromolecular carriers. Herein, PS-based alternating copolymers, i.e., alt-P(D-SiH/Ip), alt-P(D-2SiH/Ip) and alt-P(D-SiH/St), are synthesized through anionically copolymerized Si–H functionalized 1,1-diphenylethylene (DPE-SiH/2SiH) with isoprene (Ip)/styrene (St). Anionically polymerized (4-vinylphenyl)-dimethyl-silane is used as the uniform hydrosilylated PS, i.e., Uni.-P(St-SiH). A series of photo-responsive azobenzene side-chain liquid crystalline PSs combine the above PS-based main chains and the mesogenic moiety composed of azobenzene (Azo) and cholesterol (Chol) groups, i.e., Azo-Chol (M), via the hydrosilylation. The polarized optical microscopic (POM) morphologies and thermal properties of Azo-Chol (M) and Azo-Chol-PSs are studied. As desired, they produce the cholesteric phase of wide temperature range due to the PSs and Chol-groups. Moreover, the photo-responsive behavior irradiated under UV-light of different wavelengths are quantitatively evaluated by time-dependent UV–Vis spectroscopy. Azo-Chol-PSs undergo rapid and reversible trans-cis isomerization under the alternating irradiation of UV and visible light. The trans-to-cis isomerization efficiency under 365 nm UV-light is over that under 290–320 nm UV-light, while over 80% of cis-to-trans efficiency can be realized under 400–500 nm Vis-light irradiation This work would provide guidelines for the effective design of photo-responsive anisotropic materials.

Investigation of relative stabilities of CPAM zirconium cycloalkylmethyl vs isomeric ω-alkenyl complexes: Crystal structures of the cyclobutylmethyl and cyclopentylmethyl complexes

Investigations of the syntheses, structural characterization, and relative stabilities of the cyclopentadienyl, amidinate Zr cycloalkylmethyl complexes, (CPAM)Zr(X)[CH2(cyclo-CmH2m-1)] (CPAM = (η5-C5Me5)[(N,N′)-κ2-N(Et)-C(Me)N(t-Bu)]) for X = Cl and m = 3 (1a), 4 (1b) and 5 (1c) and for X = Me, and m = 4 (2b), and of the corresponding isomeric ω-alkenyl complexes, (CPAM)Zr(Cl)[CH2(CH2)nCH2CH = CH2] for n = 0 (3a), 1 (3b) and 2 (3c), have been conducted. Hydrozirconation of methylenecyclopropane by (CPAM)Zr(X)Cl (X = H or D) (8), which is generated in situ through hydrogenolysis of the Zr-Si bond of (CPAM)Zr(SiMe2Ph)Cl (7), provided a high yield of 3a due to rapid isomerization of transiently generated 1a. In contrast, crystalline 1b could be isolated in high yield via hydrozirconation of methylenecyclobutane by 8, and it has been structurally characterized by X-ray crystallography. Thermolysis of 1b in solution provided a quantitative yield of 3b, while quantitative production of 1c was achieved through intramolecular cyclization of 3c by reversible chloride abstraction that was catalyzed by addition of the ion pair, {(CPAM)Zr(Me)}{B(C6F5)4} 10. The cyclobutylmethyl complex 2b was also prepared from 1b through traditional methylation using MeLi, and it too was structurally characterized by X-ray crystallography. In solution, 2b thermally decomposes at 25 °C to an intractable mixture. Finally, reaction of 2b with a stoichiometric amount of the borate B1 provided the corresponding ion pair complex, {(CPAM)Zr[CH2CH2CH2CH = CH2]}{B(C6F5)4} (9) in which the terminal alkenyl group is intramolecularly coordinated to the transition metal center as determined by 1H NMR spectroscopy. The results of a DFT (B3LYP / LANL2DZ (Zr) / 6-31G**) computational investigation for the 1b to 3b isomerization and for the structure of 9 are also included.

Fluorescence Modulation of ortho-Green Fluorescent Protein Chromophores Following Ultrafast Proton Transfer in Solution

Photophysical and photochemical properties of the green fluorescent protein (GFP) chromophore and derivatives underlie their bioimaging applications. To date, ultrafast spectroscopic tools represent the key for unraveling fluorescence mechanisms toward rational design of this powerful biomimetic framework. To correlate the excited-state intramolecular proton transfer (ESIPT) with chromophore emission properties, we implement experimental and computational tool sets to elucidate real-time electronic and structural dynamics of two archetypal ortho-GFP chromophores (o-HBDI and o-LHBDI) possessing an intramolecular hydrogen bond to undergo efficient ESIPT, only differing in a bridge-bond constraint. Using excited-state femtosecond stimulated Raman spectroscopy (FSRS), a low-frequency phenolic (P)-ring-deformation mode (∼562 cm-1) was uncovered to accompany ESIPT. The tautomerized chromophore undergoes either rapid P-ring isomerization to reach the ground state with essentially no fluorescence for o-HBDI or enhanced (up to an impressive 180-fold in acetonitrile) and solvent-polarity-dependent fluorescence by P-ring locking in o-LHBDI. The significant dependence of the fluorescence enhancement ratio on solvent viscosity confirms P-ring isomerization as the dominant nonradiative decay pathway for o-HBDI. This work provides crucial insights into the dynamic solute-solvent electrostatic and steric interactions, enabling the application-specific improvement of ESIPT-capable molecules as versatile fluorescence-based sensors and imaging agents from large Stokes shift emission to brighter probes in physiological environments.

Front Cover: 2‐ and 6‐Purinylmagnesium Halides in Dichloromethane: Scope and New Insights into the Solvent Influence on the C−Mg Bond (Eur. J. Org. Chem. 7/2022)

The Front Cover shows shows the influence of reaction solvent on the reaction of purin-2-yl Grignards with carbonyl compounds. In the ether (THF), the reaction gives 8-carbinol products due to a rapid isomerisation to the purin-8-yl, whereas performing the reaction in dichloromethane stops the isomerisation and yields 2-carbinols in good yields. In contrast to aldehydes, reactions with ketones require prior complexation with LaCl3.2LiCl before addition to the Grignard reagent. More information can be found in the Full Paper by M. R. Gordon, S. Gazzola et al.

2‐ and 6‐Purinylmagnesium Halides in Dichloromethane: Scope and New Insights into the Solvent Influence on the C−Mg Bond

AbstractThe generation of positionally stable purin‐2‐ and 6‐yl magnesium halides is complicated by the often very rapid isomerization to give the 8‐yl Grignards. By conducting the reaction in dichloromethane (DCM), we demonstrated that the anion isomerization can be stopped and these stable purin‐2‐ and 6‐yl Grignards react directly with a broad scope of aldehydes in good yields. Furthermore, purine functionalization with ketones has been achieved for the first time in the presence of LaCl3 ⋅ 2LiCl. Density functional theory calculations offer a possible explanation of the special role played by solvent in this chemistry and show that in DCM the C−Mg bond has a less polar character, whereas in THF it is predominantly ionic and much more basic in nature.

Rapid and Continuous Astaxanthin Isomerization in Subcritical Ethanol

Recent studies have shown that Z-isomers of astaxanthin have greater bioavailability and biological activity than the all-E-isomer. Hence, it is important to develop efficient and practical Z-isomerization methods. This study attempted to establish a continuous-flow isomerization system for (all-E)-astaxanthin in subcritical ethanol. Specifically, the effects of temperature, pressure, time, and addition of antioxidants and a natural catalyst, allyl isothiocyanate, on the Z-isomerization and degradation were investigated. Continuous high-temperature processing (>140 °C) efficiently promoted astaxanthin Z-isomerization within a few minutes, but simultaneously accelerated its degradation. We found that the thermal degradation could be inhibited by adding specific antioxidants, especially ascorbic acid. Finally, using the continuous-flow system with ascorbic acid, 60.3% of total Z-isomer ratio of astaxanthin was achieved while inhibiting the degradation at 200 °C and 10 MPa in just 30 s. This efficient and environmentally friendly isomerization technology is an effective means for the production of Z-isomer-rich astaxanthin materials.