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A greener and sustainable flow chemistry protocol for the synthesis of 3,3-disubstituted indolenines through interrupted Fischer indolisation reaction is described. First, two model aldehydes were reacted with phenylhydrazine in order to explore the reaction feasibility in a ‘greener’ fashion in batch mode. The best outcomes were then used as the starting point for the implementation of the reaction in continuous flow. A thorough exploration of key parameters allowed the identification of the most efficient reagent mixing mode, and the optimum temperature and residence time. The newly developed method allowed straightforward reaction channelling towards the formation of the indolenines, thus reducing the competitive formation of side products. We further broadened the scope of the conceived methodology by exploring the possibility of a heterogeneous in-line reduction of the indolenines to their indoline counterparts. This rapid approach nicely complements known batch chemistry and could facilitate synthesis and scale up of 3,3-disubstituted indolenines and indolines, offering a coupling point for additional and subsequent flow reactions for multistep syntheses for further derivatization.

Softwood kraft lignin SKL was fractionated using aprotic acetone and protic methanol to yield both previously reported “traditional” fractions and novel refined fractions thereof. Based on differences in mutual H-bonding affinities exerted by the solvents on one side and the OH group characteristics of the lignin oligo- and polymers on the other side, the 85% acetone-insoluble kraft lignin fraction AIKL, for example, could be further fractionated in 16% methanol-soluble and 67% methanol-insoluble parts. Sequential use of the two solvents practically resembled a refinement protocol that shed light on eventual “structural impurities” contained in the traditional fractions, which is not delineable easily in front of the background of a dominating spectroscopic image. Exploiting H-bonding characteristics offers a valid option for the facile generation of truly “structurally purified” lignin fractions. Acetone-insoluble, methanol-soluble MSAIKL, for example, exhibits 25% less aliphatic interunit bonding while being enriched in phenolic groups up to 25%, thus representing condensed, lower-molecular-weight structures with overall H-bond accepting character that was still contained in the overall larger-molecular-weight acetone-insoluble AIKL. More “polyphenylpropanoidic” parts practically free of condensed units, determining the overall structural picture of the unrefined lignins due to their overall abundance, are represented by the globally insoluble fractions like acetone-insoluble, methanol-insoluble MIAIKL. The original fractions and refined fractions generated based on targeting specific H-bonding affinities give a more homogeneous picture with respect to trends in glass transition temperatures, clearly indicating that Tg is dominated by actually both MW and nature of OH groups.

Phase change contrast agents for ultrasound (US) imaging consist of nanodroplets (NDs) with a perfluorocarbon (PFC) liquid core stabilized with a lipid or a polymer shell. Liquid ↔ gas transition, occurring in the core, can be triggered by US to produce acoustically active microbubbles (MBs) in a process named acoustic droplet vaporization (ADV). MB shells containing polymerized diacetylene moiety were considered as a good trade off between the lipid MBs, showing optimal attenuation, and the polymeric ones, displaying enhanced stability. This work reports on novel perfluoropentane and perfluorobutane NDs stabilized with a monolayer of an amphiphilic fatty acid, i.e. 10,12-pentacosadiynoic acid (PCDA), cured with ultraviolet (UV) irradiation. The photopolymerization of the diacetylene groups, evidenced by the appearance of a blue color due to the conjugation of ene-yne sequences, exhibits a chromatic transition from the nonfluorescent blue color to a fluorescent red color when the NDs are heated or the pH of the suspension is basic. An estimate of the molecular weights reached by the polymerized PCDA in the shell, poly(PCDA), has been obtained using gel permeation chromatography and MALDI-TOF mass spectrometry. The poly(PCDA)/PFC NDs show good biocompatibility with fibroblast cells. ADV efficiency and acoustic properties before and after the transition were tested using a 1 MHz probe, revealing a resonance frequency between 1 and 2 MHz similar to other lipidic MBs. The surface of PCDA shelled NDs can be easily modified without influencing the stability and the acoustic performances of droplets. As a proof of concept we report on the conjugation of cyclic RGD and PEG chains of the particles to support targeting ability toward endothelial cells. ispartof: LANGMUIR vol:35 issue:31 pages:10116-10127 ispartof: location:United States status: published

Stimuli-responsive behavior of lignin microcapsules (LMCs) has been investigated along with the detailed characterization of their stability profiles. The disassembly of LMCs was found to be salt species-dependent, indicating the specific relevance of inherent kosmotropic and chaotropic characteristics. For the first time, a connection between the Hofmeister series and the stability profile of lignin microscale materials is established. LMCs showed excellent stability in water and under high temperature and pressure (autoclaving conditions). Active release is efficiently triggered by pH changes and balancing chaotropic and kosmotropic effects via salinity tuning.

Nanoencapsulation of active substances with controlled release in harmless matrices has been the subject of numerous scientific efforts mainly due to the significant biomedical potential of such endeavors. Lignin, the environmentally sustainable byproduct of the pulp and paper industry, contains a multitude of phenolic hydroxyl groups, some of which, are known to readily and strongly chelate with iron ions. In this effort we demonstrated that the concerted use of chelation chemistry, oil in water emulsion principles, and low energy sonication, offers a facile, one-pot strategy to assemble lignin nanocapsules (LNCs) of a controlled architecture. Under these conditions capsules are shown to rapidly assemble utilizing two driving forces, the p-stacking propensity of lignin and its metal chelating ability at alkaline pH. Detailed size exclusion chromatographic evidence validates that the formation of capsules is driven mainly by the enumerated physical interactions with no significant chemical modification of the lignin. The developed process was systematically optimized so as to create the foundations for the morphology and the yield of the capsules being modulated as a function of sonication time, power, and surface contact area. Both pure LNCs and Fe-LNCs were synthesized in high yields with size distributions varying from 0.3 to 6 mm and their release efficiencies were evaluated in detail. As anticipated, the complexation effects of the phenolic OH groups offered to the Fe-LNCs, increased stability, reduced shell thickness (allowing for greater loading efficiencies), and lower release kinetics, compared to LNCs.

The mechanism of lignin nanoprecipitation and subsequent self-assembly was elucidated by studying generation of lignin nanoparticles (LNPs) from aqueous ethanol. LNP formation was found to follow a kinetically controlled nucleation–growth mechanism in which large lignin molecules formed the initial critical nuclei. Using this information, we demonstrate entrapment of budesonide in LNPs and subsequent pH-triggered and surfactant-responsive release of this synthetic anti-inflammatory corticosteroid. Overall, our results not only provide a promising intestinal delivery system for budesonide but also deliver fundamental mechanistic understanding for the entrapment of actives in LNPs with controlled size and release properties.

The biotechnological conversion of lignocellulosic biomass is a pivotal aspect in the development of sustainable and environmentally friendly biorefinery and valorization processes for nonfossil biomass. Lipoxygenase, a noncellulolytic oxidizing enzyme normally operating on linoleic acid metabolism, is demonstrated to catalyze the oxidative functionalization of lignin. Lipoxygenase was used for structurally altering the most important types of technical lignins. Noteworthy oxidation is achieved by activation via a radical mechanism leading to both oxygen- and carbon-centered radicals. Since this activation also occurs on the lignin backbone, the ketyl and benzyl radicals generated can be used to functionalize lignins via formation of carbon-carbon bonds.

A simple and facile strategy for the creation of ferric tannin microcapsules around a liquid, non-sacrificial core is described. The assembly of the capsules occurs rapidly once ferric tannin complexes are subjected to ultrasonic treatment. The driving forces for the rapid capsule assembly reside in the strategy of adding ferric ions into the initial emulsion, which promotes shell formation and stability through well-known complexation effects. This is the first time that microcapsule assemblies of monomeric tannins like epigallocatechin-3-O-gallate has been demonstrated, which are reportedly unable to form dispersing microcapsules in the absence of a templating metal. The efficacy of the approach is demonstrated by the complete release of a hydrophobic molecule that is active against M. tuberculosis by using Acacia tannin capsules. The release kinetics of the active molecule were of zeroth order over a 12 h time frame.

Two constitutional structural schemes are proposed attempting to unify and rationalize a series of focused NMR and chromatographic determinations aimed at providing an integrated picture for the structure of softwood kraft lignin. The complexity of native softwood lignin when coupled with the complexity of the kraft pulping process is known to lead to a rather heterogeneous material that has eluted us to date. The present work embarks at applying state-of-the-art quantitative 1D and 2D NMR methods on carefully isolated softwood kraft lignin samples and fractions. The accumulated data, when coupled with size exclusion chromatography, mass spectrometric analyses and literature accounts that pertain to the chemistry of kraft pulping, provide the following picture for softwood kraft lignin. Softwood kraft lignin is composed of two distinct fractions that can be separated by using anhydrous acetone. The acetone insoluble fraction is a somewhat branched polymeric material that still contains a variety of native wood lignin bonding patterns, albeit in significantly reduced abundance, as well as new structures induced during the process. The acetone soluble fraction is a significantly more branched and less polymeric material with an abundance of chemical structures that may be created when oligomeric phenols react under kraft pulping conditions. To account for the presence of the various moieties in these two fractions, kraft pulping fragmentation and repolymerization chemistries are extensively invoked, including radical processes initiated by sulfur.

Microcapsules (MCs) and nanoemulsions (NEs) were generated from both condensed and hydrolysable tannins, respectively, for controllable, stimuli-responsive active release. A rapid, simple and robust one step method based solely on the use of tannins, i.e., without the use of any additional cross-linking reagents, was developed via ultrasound treatment. The condensed tannins Schinopsis balansae wood extract (Sb) and Acacia mearnsii bark extract (Am) were studied for their potential use in microcapsule assembly, and the hydrolysable tannins tannic acid (TA) and epigallocatechin-3-O-gallate (EGCG) were studied for their potential use in forming nanoemulsions upon ultrasonic irradiation. Tannin microcapsules (TMCs) and tannin nanoemulsions (TNEs) were thoroughly characterized via cryo-SEM, as well as fluorescence and optical microscopy, to reveal their structural and morphological characteristics. GPC analyses revealed that microcapsules and nanoemulsions were formed due to efficient electronic interactions between the aromatic moieties present in the polyphenolic backbone of high molecular weight fractions of tannins, and apparently without any significant chemical modification. Both TMCs and TNEs were formed in high yields and their release efficiencies showed the higher stability of the MCs compared to the NEs. Active release from and the pH stability of both the TMCs and TNEs were evaluated. Tannins – and more specifically the polymeric structures of tannins – proved to be ideal scaffolds for ultrasound assisted capsule assembly with potential for the pH triggered delivery of hydrophobic active molecules.

A new fractionation protocol for wheat straw organosolv lignin was developed on the basis of the dominating H-bonding orientations of its components. Acetone as H-bond accepting aprotic polar solvent and methanol as H-bond donating and accepting protic polar solvent were used in sequence. Obtained fractions were structurally and thermally analysed. The protocol allowed for the generation of purified lignin fractions and the isolation of a novel, yet unobserved lignin carbohydrate complex (LCC) fraction. This LCC fraction was found to contain exclusively phenyl glycosides and &gamma, esters as LCC motifs.

Isolation of lignins from hardwood and softwood biomass samples, containing 26.1% and 28.1% lignin, respectively, has been performed with the use of alkaline and organosolv pretreatment methods. The effect of catalyst loading, ethanol content, particle size, and pretreatment time on the yields and properties of the isolated lignins were investigated. Alkaline lignins had higher carbohydrate content - up to 30% - and exhibited higher molecular weights in the range of 3000 Da, with a maximum phenolic hydroxyl content of 1 mmol g−1 for birch and 2 mmol g−1 for spruce. Organosolv lignins, on the other hand, showed high purity - 93% or higher - despite the more extensive biomass dissolution into the pretreatment medium; they also exhibited a lower range of molecular weights between 600 and 1600 Da depending on the source and pretreatment conditions. Due to the lower molecular weight, phenolic hydroxyl content was also increased, reaching as high as 4 mmol g−1 with a simultaneous decrease in aliphatic hydroxyl content as low as 0.6 mmol g−1. Efficient lignin dissolution of 62% for spruce and 69% for birch, achieved at optimal pretreatment conditions, was combined with extensive hemicellulose removal.

In this study, the application of different chemical and enzymatic treatment methods for the fractionation of the birch outer bark components was evaluated. More specifically, untreated and steam exploded, hydrothermally and organosolv treated bark samples were incubated with enzyme mixtures that consisted of cellulases, hemicellulases and esterases, and the effect of enzymes was analyzed with 31P NMR and {13C-1H} HSQC. The biocatalysts performed the cleavage of ester bonds resulting in reduction of methoxy and aliphatic groups in the remaining solid fraction, whereas the aromatic fraction remained intact. Moreover, the suberin and lignin fraction were isolated chemically and their properties were characterized by gas chromatography (GC-MS), 31P NMR, {13C-1H} HSQC and gel permeation chromatography (GPC). It was demonstrated that the lignin fraction was enriched in guaiacyl phenolics but still contained some associated aliphatic acids and carbohydrates, whereas the suberin fraction presented a polymodal pattern of structures with different molecular weight distributions. This work will help in getting a deeper fundamental knowledge of the bark structure, the intermolecular connection between lignin and suberin fractions, as well as the potential use of enzymes in order to degrade the recalcitrant bark structure toward its valorization. Validerad; 2016; Nivå 2; 2016-11-02 (andbra)

A pH- and light-responsive polymer has been synthesized from softwood kraft lignin by a two-step strategy, which aimed at incorporating diazobenzene groups. Initially, styrene oxide was reacted with the phenolic hydroxyl groups in lignin, to offer the attachment of benzene rings, thus creating unhindered reactive sites for further modifications. The use of advanced spectroscopic techniques (1H and 31P NMR, UV and FTIR) demonstrated that the reaction was quantitative and selective towards the phenolic hydroxyl groups. In a second step, the newly incorporated benzene rings were reacted with a diazonium cation to form the target diazobenzene motif, whose formation was again thoroughly verified. As anticipated, the diazobenzene-containing kraft lignin derivatives showed a pH-dependent color change in solution and light-responsive properties resulting from the cis-trans photoisomerization of the diazobenzene group.

The 31P NMR-spectroscopy-based spin trap technique involving 5-diisopropoxy-phosphoryl-5-methyl-1-pyrroline-N-oxide (DIPPMPO) was used to achieve quantitative analyses of the radical species that are generated in different bleaching solutions. These solutions comprised a mixture of an ammonium salt and hydrogen peroxide. This spin trap-based approach was also applied, with modifications, to the study of bleaching systems of human hair. The obtained results clearly revealed the subtle differences in both the nature and the amounts of the radical species generated in different bleaching solutions, and when allowed to react on hair samples. Generally, the main species involved in the oxidation processes were superoxide and amino radicals. Their amounts, however, showed a significant variation upon the kind of bleaching system and nature of the hair, i.e., virgin or dyed hair.

Tannase (EC 3.1.1.20) was quantitatively immobilised onto Eupergit® C 250 L and coated by layers of alternatively charged polystyrene sulfonate and polyallylamine hydrochloride, respectively. The layer-by-layer (LbL)-coated immobilised tannase, i.e., LbL-tannase retained its original activity and showed significant resistance to deactivation and maintained 50% of its activity after seven consecutive cycles of hydrolysis reactions, each run for 24 h. The LbL coating did not hinder the substrate access to the active site of the enzyme. Both gallo- and ellagitannins were efficiently hydrolysed upon treatment with LbL-tannase. The formation of gallic acid and the reaction patterns were followed by HPLC and the recently developed 31P NMR analytical method for the characterisation of tannins.

Two distinct functionalization schemes for Kraft lignin (KL) were developed to selectively incorporate furan and/or maleimide motifs as chain ends. The incorporation of furan functionalities was carried out by the selective and quantitative reaction of the lignin’s phenolic OH groups with furfuryl glycidyl ether (FGE). Maleimide groups were introduced by esterifying the lignin’s aliphatic and phenolic OH groups with 6 maleimidohexanoic acid (6-MHA), offering a high loading despite a somewhat incomplete conversion. Furan- and maleimide-functionalized lignins were subsequently combined to generate crosslinking via the Diels–Alder (DA) [4 + 2] cycloaddition reaction. The formation of the DA adduct was confirmed by 1H NMR. Under appropriate conditions, the formation of a gel was apparent, which turned back into the liquid state after performing the corresponding retro-DA reaction upon heating to 120 °C. This study reveals the significant versatility and potential of the developed strategy for the utilization of lignin-based recyclable networks.

This study has shown that ultrafiltration allows the selective extraction from industrial black liquors of lignin fraction with specific thermo-mechanical properties, which can be matched to the intended end uses. Ultrafiltration resulted in the efficient fractionation of kraft lignin according to its molecular weight, with an accumulation of sulfur-containing compounds in the low-molecular weight fractions. The obtained lignin samples had a varying quantities of functional groups, which correlated with their molecular weight with decreased molecular size, the lignin fractions had a higher amount of phenolic hydroxyl groups and fewer aliphatic hydroxyl groups. Depending on the molecular weight, glass-transition temperatures (Tg) between 70 and 170C were obtained for lignin samples isolated from the same batch of black liquor, a tendency confirmed by two independent methods, DSC, and dynamic rheology (DMA). The Fox-Flory equation adequately described the relationship between the number average molecular masses (Mn) and T g's-irrespective of the method applied. DMA showed that low-molecular-weight lignin exhibits a good flow behavior as well as high-temperature crosslinking capability. Unfractionated and high molecular weight lignin (Mw >5 kDa), on the other hand, do not soften sufficiently and may require additional modifications for use in thermal processings where melt-flow is required as the first step.

Over the past decade, the integration of synthetic chemistry with flow processing has resulted in a powerful platform for molecular assembly that is making an impact throughout the chemical community. Herein, we demonstrate the extension of these tools to encompass complex natural product synthesis. We have developed a number of novel flow-through processes for reactions commonly encountered in natural product synthesis programs to achieve the first total synthesis of spirodienal A and the preparation of spirangien A methyl ester. Highlights of the synthetic route include an iridium-catalyzed hydrogenation, iterative Roush crotylations, gold-catalyzed spiroketalization and a late-stage cis-selective reduction.

Lignin is the second-most abundant polymer after cellulose within the biomass of our planet. Structurally, it displays random oligomeric units without fixed repetition schemes beyond the stage of dimers. Quantitative 1H-3C HSQC measurements have recently greatly facilitated lignin analyses. In some cases, however, long acquisition times needed for obtaining quantitative HSQCs are not compatible with the chemical integrity of (a potentially functionalised) lignin sample. We thus compared different methods that were developed for more time-efficient quantitative HSQC measurements with respect to their usefulness in lignin analyses: reliable and reproducible results were obtained using both the QQ-HSQC and the HSQC0 method. © 2013 Sette et al. Licensee: Computational and Structural Biotechnology Journal. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are properly cited.

The mechanism (or mechanisms) of enthalpy− entropy (H/S) compensation in protein−ligand binding remains controversial, and there are still no predictive models (theoretical or experimental) in which hypotheses of ligand binding can be readily tested. Here we describe a particularly well-defined system of protein and ligands - human carbonic anhydrase (HCA) and a series of benzothiazole sulfonamide ligands with different patterns of fluorination - that we use to define enthalpy/entropy (H/S) compensation in this system thermodynamically and structurally. The binding affinities of these ligands (with the exception of one ligand, in which the deviation is understood) to HCA are, despite differences in fluorination pattern, indistinguishable; they nonetheless reflect significant and compensating changes in enthalpy and entropy of binding. Analysis reveals that differences in the structure and thermodynamic properties of the waters surrounding the bound ligands are an important contributor to the observed H/S compensation. These results support the hypothesis that the molecules of water filling the active site of a protein, and surrounding the ligand, are as important as the contact interactions between the protein and the ligand for biomolecular recognition, and in determining the thermodynamics of binding.

Lignin is the second most abundant natural polymer. Its use and targeted functionalisation within biomass refinery processes, however, still needs to be further explored and developed. The oxidative functionalisation, and thus valorisation of lignin, is a very promising way to go, since it holds the possibilities to yield highly functionalised, monomeric or oligomeric products that can serve as starting materials for other valorisation processes in the chemical and pharmaceutical industries. Gaining a profound knowledge about the structure of lignin, being able to analyse structural features, and understanding the mechanisms that guide the reactions leading to the oxidative derivatisation, depolymerisation and functionalisation of lignin samples from different renewable sources are key requirements for developing successful valorisation protocols for lignin. In this review, we wish to revisit, and set into context, some important achievements in the field of oxidatively upgrading lignin. We will focus on organometal catalyses (MTO, salen complexes, POMs), biomimetic catalyses (porphyrins), and enzymatic catalyses (laccase, peroxidase) for upgrading lignin and lignin model compounds. Details of mechanistic implications and means of potential manipulations of reaction outcomes are discussed.

Tannins and flavonoids are secondary metabolites of plants that display a wide array of biological activities. This peculiarity is related to the inhibition of extracellular enzymes that occurs through the complexation of peptides by tannins. Not only the nature of these interactions, but more fundamentally also the structure of these heterogeneous polyphenolic molecules are not completely clear. This first paper describes the development of a new analytical method for the structural characterization of tannins on the basis of tannin model compounds employing an in situ labeling of all labile H groups (aliphatic OH, phenolic OH, and carboxylic acids) with a phosphorus reagent. The 31P NMR analysis of 31P-labeled samples allowed the unprecedented quantitative and qualitative structural characterization of hydrolyzable tannins, proanthocyanidins, and catechin tannin model compounds, forming the foundations for the quantitative structural elucidation of a variety of actual tannin samples described in part 2 of this series.

Having demonstrated in the preceding publication the flow synthesis of aryl azides, we describe here a general protocol for the in-line purification of these versatile intermediates. As part of this investigation, we evaluated the use of ReactIR 45m as a tool for real-time detection of hazardous azide contaminants. This azide synthesis and purification process was then incorporated into a multistep flow sequence to generate a small collection of 5-amino-4-cyano-1,2,3-triazoles directly from aniline starting materials in a fully automated fashion.

Herein we describe the application of a monolithic triphenylphosphine reagent to the Appel reaction in flow-chemistry processing, to generate various brominated products with high purity and in excellent yields, and with no requirement for further off-line purification.

We report a rapid microwave-assisted Sonogashira cross-coupling of aryl iodides and bromides with terminal alkynes using Pd-EnCatTM TPP30. Both electron-rich and electron-deficient aryl halides reacted smoothly with a broad variety of terminal alkynes in MeCN at 100–120 °C. The coupling products were obtained in good to excellent yields and in high purity. This reaction can be performed under copper- and DMF-free conditions and does not require an inert atmosphere. Furthermore, the encapsulated catalyst can be recovered and recycled by a simple filtration of the reaction mixture. It can be reused in further reactions with only minor decrease in activity. Additionally, we were able to produce a variety of enyne derivatives under modified conditions employing the same Pd-EnCatTM source. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Enantioenriched a-stannylated benzyl carbamates were used in highly stereospecific coupling reactions employing Cu(I) as catalyzing transition metal. Acid chlorides and allyl bromide derivatives were used as electrophilic coupling partners. The reaction was applied in the synthesis of two highly enantioenriched indanoles and one enantioenriched tetraline via intramolecular cyclization reactions.

The title compounds were generated by deprotonation of different benzyl-type carbamates with sec-butyllithium in the presence of chiral diamines (-)-sparteine or diisopropyl and di-tert-butyl bis(oxazoline)s. These lithiated species exhibit configurational lability at -78 degC. In the case of the chiral di-tert-butyl bis(oxazoline), the equilibrium of the epimeric complexes can be used synthetically to obtain highly enantioenriched secondary benzyl carbamates. The enantiodetermining step was proven to be a dynamic thermodynamic resolution. The absolute configurations of the products were determined, and the stereochemical pathways of selected substitution reactions were thus elucidated. High level quantum chemical investigations were performed to gain insight into the experimentally investigated system. To obtain an accuracy for the energy difference (DDH) between two epimeric complexes of about 0.5 kcalmol-1 as well as the correct sign, a theoretical procedure was established. It included geometry optimization at the dispersion-corrected DFT level, computation of zero-point vibrational energies, and single-point SCS-MP2 energy calculations with large atomic-orbital basis sets.

Substitution reactions that employ primary-carbamoyl-protected arylmethanethiols are described. The enantiodetermining step was found to occur in the post-deprotonation step as a dynamic thermodynamic resolution with a chiral bis(oxazoline) ligand. The configurationally labile lithium complexes were trapped with various electrophiles to yield different substitution products in good to excellent yields and enantiomeric excesses. The absolute configurations of the substitution products were determined, and the stereochemical pathway of the substitution reaction was elucidated for different classes of electrophiles. The temperature-dependent epimerization process was monitored by 1H and 6Li NMR spectroscopy. Die asymmetrische Deprotonierung prochiraler S-Benzylthiocarbamate wird beschrieben und untersucht. Die Enantiomerenanreicherung entsteht im Post-Deprotonierungsschritt mittels dynamisch-thermodynamischer Resolution unter Verwendung eines chiralen Bis(oxazolin)-Liganden. Die bei −30 °C konfigurationslabilen Diaminkomplexe equilibrieren und konnen mit verschiedenen Elektrophilen abgefangen werden. So werden in sehr guten Ausbeuten hoch enantiomerenangereicherte α-substituierte S-Benzylthiocarbamate erhalten. Die Absolutkonfiguration und der stereochemische Verlauf der Substitutionsreaktionen werden aufgeklart. Zudem wird die Temperatur-abhangige Epimerisierung mittels 1H- und 6Li-NMR-Spektroskopie verfolgt und visualisiert.

The title compounds were generated by deprotonation of different benzyl-type carbamates with sec-butyllithium in the presence of chiral diamines (−)-sparteine or diisopropyl and di-tert-butyl bis(oxazoline)s. These lithiated species exhibit configurational lability at −78 °C. In the case of the chiral di-tert-butyl bis(oxazoline), the equilibrium of the epimeric complexes can be used synthetically to obtain highly enantioenriched secondary benzyl carbamates. The enantiodetermining step was proven to be a dynamic thermodynamic resolution. The absolute configurations of the products were determined, and the stereochemical pathways of selected substitution reactions were thus elucidated. High-level quantum chemical investigations were performed to gain insight into the experimentally investigated system. To obtain an accuracy for the energy difference (ΔΔH) between two epimeric complexes of about 0.5 kcal mol−1 as well as the correct sign, a theoretical procedure was established. It included geometry optimization at the dispersion-corrected DFT level, computation of zero-point vibrational energies, and single-point SCS-MP2 energy calculations with large atomic-orbital basis sets. Prochirale Benzylcarbamate werden mittels sec-Butyllithium in der Gegenwart eines chiralen Diamins ((−)-Spartein oder Bisoxazoline) deprotoniert. Die resultierenden lithiierten Spezies sind bei −78 °C konfigurativ labil. Im Falle des chiralen di-tert-Butyl-Bisoxazolin-Liganden setzen sich die epimeren Komplexe in ein Gleichgewicht, in dem ein Lithiumkomplex im grosen Uberschuss vorliegt (dynamisch-thermodynamische Resolution). Umsetzungen mit verschiedenen Elektrophilen liefern hoch enantiomerenangereicherte sekundare Benzylcarbamate. Der stereochemische Verlauf der Substitutionen wird anhand der Absolutkonfiguration der Produkte aufgeklart. Zudem werden die experimentell untersuchten Epimerengleichgewichte mit quantenchemischen Methoden erfasst. Um bei der Bestimmung der Energiedifferenz ΔΔH zwischen zwei epimeren Komplexen eine Genauigkeit von ungefahr 0.5 kcal mol−1 und ein korrektes Vorzeichen zu erhalten, wurde eine theoretische Vorgehensweise entwickelt. Diese beinhaltet Geometrieoptimierungen auf Dispersions-korrigiertem DFT-Level, Berechnung der Nullpunktschwingungsenergien und SCS-MP2-Energie Einzelpunktberechnungen unter Verwendung groser AO-Basissatze.