2D NOESY Research Articles

Using Poly(amidoamine) PAMAM-βCD Dendrimer for Controlled and Prolonged Delivery of Doxorubicin as Alternative System for Cancer Treatment

Background/Objectives: Doxorubicin (Dox) is an anticancer drug used in the treatment of a wide range of solid tumors; however, Dox causes systemic toxicity and irreversible cardiotoxicity. The design of a new nanosystem that allows for the control of Dox loading and delivery results is a powerful tool to control Dox release only in cancer cells. For this reason, supramolecular self-assembly was performed between a poly(amidoamine) (PAMAM) dendrimer decorated with four β-cyclodextrin (βCD) units (PAMAM-βCD) and an adamantane–hydrazone–doxorubicin (Ad-h-Dox) prodrug. Methods: The formation of inclusion complexes (ICs) between the prodrug and all the βCD cavities present on the surface of the PAMAM-βCD dendrimer was followed by 1H-NMR titration and corroborated by 2D NOESY experiments. A full characterization of the supramolecular assembly was performed in the solid state by thermal analysis (DSC/TGA) and scanning electron microscopy (SEM) and in solution by the DOSY NMR technique in D2O. Furthermore, the Dox release profiles from the PAMAM-βCD/Ad-h-Dox assembly at different pH values was studied by comparing the efficiency against a native βCD/Ad-h-Dox IC. Additionally, in vitro cytotoxic activity assays were performed for the nanocarrier alone and the two supramolecular assemblies in different carcinogenic cell lines. Results: The PAMAM-βCD/Ad-h-Dox assembly was adequately characterized, and the cytotoxic activity results demonstrate that the nanocarrier alone and its hydrolysis product are innocuous compared to the PAMAM-βCD/Ad-h-Dox nanocarrier that showed cytotoxicity equivalent to free Dox in the tested cancer cell lines. The in vitro drug release assays for the PAMAM-βCD/Ad-h-Dox system showed an acidic pH-dependent behavior and a prolonged profile of up to more than 72 h. Conclusions: The design of PAMAM-βCD/Ad-h-Dox consists of a new controlled and prolonged Dox release system for potential use in cancer treatment.

Modulating the Supramolecular Assembly of α-Cyclodextrin and Anderson-type Polyoxometalate through Covalent Modifications.

A series of unprecedented supramolecular complexes of covalently modified Anderson-type polyoxometalates (POMs) and α-cyclodextrins (α-CDs) have been obtained and characterized in solid state by single-crystal X-ray diffraction, and in aqueous solution using various techniques including 1H DOSY, 2D NOESY 1H NMR, isothermal titration calorimetry (ITC), and electrospray ionization time-of-flight mass spectroscopy (ESI-TOF-MS). It has been demonstrated that the supramolecular assembly process can be modulated by different covalent modification modes of the Anderson POMs, giving rise to a new type of POM/α-CD complexes featuring organic-inorganic pseudo-rotaxane structures, which are in good contrast to those of POM/γ-CD with poly-rotaxane type structures. Moreover, it is delighted to find that these pseudo-rotaxanes of POM/α-CD exhibit stable chirality in aqueous solution, which has not been accomplished in previously reported POM/γ-CD assemblies.

Ionic strength regulates IFT in microemulsion for extraction of botanical natural products

Microemulsion, an emerging environmentally friendly medium, has demonstrated significant potential in botanical natural products (BNP) extraction. We found suitable ionic strength could enhance the BNP extraction capability of microemulsion. However, this mechanism remains unclear. Elucidating this mechanism will help to develop extraction strategies to increase BNP yields. Therefore, this mechanism was explored through scanning electron microscope, 2D NOESY, and molecular dynamics simulation. Ionic strength was quantified by Na+ concentration. BNP was represented by acteoside, hyperoside, luteolin and apigenin in Rehmannia glutinosa Libosch. Leaves. The results revealed the position of coordination ring shift forward when ionic strength regulated IFT. This reduced the electrostatic repulsion among surfactants’ head groups. Head groups are thus closely arranged at interface. This improves the pool capacity, hydrogen bonding interaction between head group and cellulose, as well as the interaction between head group and BNP. These provide guarantees for effective microemulsion-mediated cell wall disruption and BNP extraction.

Recognition of carboxylic acids and phosphonic acids using a 1,8-diphenylanthracene-based diguanidine in solution and solid states

A 1,8-diphenylanthracene-based diguanidine 1 has been designed and synthesized for the recognition of carboxylic acids and phosphonic acid derivatives. The diguanidine 1a forms 1:1 complexes with the dicarboxylic acids and the diphosphonic acid derivatives in a DMSO solution, and the formation of these complexes was confirmed by a DOSY NMR analysis. The binding mode towards the guanidyl group (Type I, Type II, two existing types of binding modes toward three nitrogen atoms) with a diphosphonic acid derivative was successfully demonstrated by the 2D NOESY analysis. The diguanidine 1a does not show any fluorescence in a DMSO solution, but after the addition of carboxylic acids, the formation of a stable complex or the formation of the protonated guanidinium (1a+2H+) were distinguishable by the observation of a light-blue fluorescence around 430 nm. These fluorescence observations can be determined the stability of the complexes of the diguanidine 1a with carboxylic acids. On the other hand, the DMSO solution of diguanidine 1a showed a light-blue fluorescence after the addition of phosphonic acids. Interestingly, diguanidine 1a recognized methylphosphonic acid 9a in the solid state by grinding and showed a green fluorescence, while the other phosphonic acids 9 and dicarboxylic acids (4d: pimelic acid, 4f: azelaic acid) showed a light-blue fluorescence in the solid state, and a selectivity was observed. These fluorescence characteristics of the diguanidine 1 are applicable for the detection of carboxylic acids and phosphonic acids, especially for methylphosphonic acid 9a in the solid state.

Photoisomerization of two 2-hydroxy-5-arylazobenzaldehydes in solvents of different polarities.

Two azo dyes 2-hydroxy-5-(4-nitrophenylazo)benzaldehyde and 2-hydroxy-5-(4-chlorophenylazo)benzaldehyde dissolved in carbon tetrachloride, hexane, acetone and acetonitrile were irradiated with 365nm UV light, and processes, occurring in them, were studied by NMR and UV-vis spectroscopy. It was established that reversible trans/cis photoisomerization of the molecules occurs in the non-polar solvents and is not observed in the polar solvents. 2D NOESY NMR spectroscopy was used to identify isomers of the azo compounds. Based on the chemical shifts of the signals, it was established that these compounds are in the trans-form before UV irradiation. Spectra of the azo dyes before and after UV irradiation allowed assignment of the chemical shifts of the cis-isomers. In polar solvents these compounds undergo a hypochromic effect under heating or irradiation with UV light. Both compounds exhibit solvatochromism. The shifts in NMR signals caused by photoisomerization of the molecules were compared with the shifts in the NMR signals of other azo compounds such as Disperse Orange 3, Disperse Red 1 and azobenzene.

Synthesis of base-modified fluorescent furo[3,2-c]coumarin nucleosides and their photophysical studies

A small library of fluorescent furo[3,2-c]coumarin nucleoside analogues has been successfully synthesized via a one-pot condensation reaction of 3′,5′-di-O-acetyl-5-formyl-2′-deoxyuridine, alkyl isocyanides, and differently substituted 4-hydroxycoumarins in acetonitrile at 80 °C. Further, the synthesized compounds have been characterized by IR, 1H NMR, 13C NMR, 1H–1H COSY, 1H–13C HETCOR, 2D NOESY NMR experiments, HRMS measurements and single crystal X-ray structure analysis of compound 5-(2′'-N-(cyclohexyl)-amino-4′'H-furo[3,2-c]chromen-4-one-3′'-yl)-2′-deoxyuridine. The electronic structure of these modified nucleosides has been examined by Density Functional Theory (DFT). The synthesized nucleoside analogues exhibited a prominent emission spectrum, featuring a band around 500 nm (with excitation at 380 nm). Photophysical investigations revealed a noteworthy fluorescence intensity, along with excellent Stokes shift values (54–194 nm) and higher quantum yields (0.010–0.515) in comparison to other pyrimidine-based fluorescent nucleosides. The solvatochromic studies of 5-(2′'-N-(cyclohexyl)-amino-8′'‑chloro-4′'H-furo[3,2-c]chromen-4-one-3′'-yl)-2′-deoxyuridine determined the quantum yield (ΦF) to be 0.515 in DMSO (λabs = 388 nm). This emphasizes the potential utility of these modified nucleosides in probing the local structure and dynamics of nucleic acids. Moreover, the scalability of the synthesis protocol was effectively demonstrated by producing one of the compounds on a gram scale, exhibiting its practical viability for large-scale production.

Basic accuracy of a1D NOESY with presaturation method using standard solutions of amino and maleic acids.

1D NOESY with presaturation (NOESY-presat) is the most popular water suppression method. When D2O solutions of L-phenylalanine or L-valine were measured using NOESY, the absolute concentration biases increased with longer mixing and evolution times, reaching a maximum of 54% with respect to the preparation values. At mixing and evolution times of 0ms and 0µs, respectively, the absolute concentration biases were reduced to less than 3%. The remaining biases were caused by the off-resonance effect, which was prevented by setting the frequency offset to an intermediate value between the analyte and internal standard 3-(trimethylsilyl)-1-propanesulfonic acid-d6 (DSS-d6) signals. Nevertheless, NOESY-presat gave maximum absolute biases of 26% and 11% for glycine and maleic acid concentrations, respectively, in three H2O/D2O (90/10 vol%) solutions. The proposed NOESY-dual-presat method reduced the absolute biases to below 4%. However, water suppression was insufficient but was improved by setting the frequency offset to the same as the presaturation offset with the H2O signal, although the absolute biases rose to 5 to 13%. Quantitative analyses using NOESY-presat and NOESY-dual-presat require careful consideration of the off-resonance effect.

Supramolecular approach to obtain folate-functionalized nanoassociates

Nanoassociates containing folic acid (FA) were obtained through solubilization by colloid solutions of three betaine calixresorcinarenes (sulfobetaine tetrapentylcalixresorcinarene С5-SB, hydroxysulfobetaine tetrapentyl- С5-hSB and tetraundecylcalixresorcinarene C11-hSB). FA was solubilized in its anionic form and fraction of bound molecules of FA was of from 89 % to 99 % (FT-PGSE NMR method data). The driving forces of FA encapsulation in the nanoassociates has been considered with the help of 1Н NMR, 2D NOESY NMR, and FT-PGSE NMR data. The macrocycle-FA nanoassociates have average hydrodynamic diameters of 3–5 nm (PDI of 0.1840.238) and a zeta potential close to zero; however, they were stable for 4–8 weeks. The enhancement of the internalization of the macrocycle-FA nanoassociates into M-HeLa cells compared to unloaded macrocycles self-associates was shown by the flow cytometry and fluorescence microscopy methods. Additionally, the growth of cytotoxicity of Rhodamine B by 19 % in the presence of С5-SB+FA nanoassociates compared to С5-SB self-associates in the in vitro PDT experiment on M-HeLa cells was found. Overall, the potential of a supramolecular approach to obtaining folate-functionalized nanocontainers has been demonstrated.

A “Hand‐In‐Hand” Thermal‐Responsive Cyclodextrin Material Designed for Liquid Smart Windows

AbstractThermochromic smart windows have emerged as a focal point in energy research. In this study, a novel approach is introduced for developing thermoresponsive materials for smart windows, utilizing microscopic agglomeration of molecules to regulate incident light. The groundbreaking material, β‐CD‐Val‐IBAm4, is constructed by grafting four isobutylated valines onto natural β‐cyclodextrins. This material exhibits a unique “hand‐in‐hand” effect, resulting from the interaction of isobutylated valine side chains, which leads to molecular aggregation and reduced light transmission at elevated temperatures. The hydration–dehydration process is related to molecular aggregation rather than crosslinking, ensuring the structural integrity and uniformity of the smart windows. Molecular dynamics simulations and 2D NOESY confirm the thermal response mechanism of β‐CD‐Val‐IBAm4. When dissolve in a glycerol‐water binary solvent system, the material forms a thermochromic liquid with outstanding optical performance (ΔTsol of 1 mm liquid can reach 69.8%) and long‐term stability. This innovative approach opens new avenues for the advancement of next‐generation smart window technologies.

Systematic exploration of structure-property relationships in colorless polyimides derived from stereoisomeric dianhydrides and various diamines

This study explores the structure-property relationships in colorless polyimides (CPIs) synthesized from stereoisomeric (exo- and endo-form) alicyclic dianhydrides, combined with various diamines. The structural configurations of 5,5’-(1,4-phenylene)bis[hexahydro-(3aR,4R,5R,7S,7aS)-rel-4,7-Methanoisobenzofuran-1,3-dione] (BzDA-exo) and 5,5-(1,4-phenylene)bis(hexahydro-4,7-methanoisobenzofuran-1,3-dione) (BzDA-endo) were confirmed as exo- and endo-forms, respectively, through 1H NMR and 2D NOESY NMR analyses. Wide-angle X-ray scattering (WAXS) data revealed that stereoisomeric structures influence polymer chain packing in synthesized CPIs. The investigation extended to the chemical and physical structure-dependent properties such as physical, thermal and optical properties of the PIs. Notably, structure-dependent physical and thermal properties were evident, with a particularly strong structure-property relationship manifesting in the coloration and yellow index (Y.I.) of PIs. These systematic characterizations offer profound insights into the design and optimization of chemical and physical structures in CPIs, highlighting the versatility of stereoisomeric monomers for various applications.

Solution-state methyl NMR spectroscopy of large non-deuterated proteins enabled by deep neural networks

Methyl-TROSY nuclear magnetic resonance (NMR) spectroscopy is a powerful technique for characterising large biomolecules in solution. However, preparing samples for these experiments is demanding and entails deuteration, limiting its use. Here we demonstrate that NMR spectra recorded on protonated, uniformly 13C labelled samples can be processed using deep neural networks to yield spectra that are of similar quality to typical deuterated methyl-TROSY spectra, potentially providing information for proteins that cannot be produced in bacterial systems. We validate the methodology experimentally on three proteins with molecular weights in the range 42–360 kDa. We further demonstrate the applicability of our methodology to 3D NOESY spectra of Escherichia coli Malate Synthase G (81 kDa), where observed NOE cross-peaks are in good agreement with the available structure. The method represents an advance in the field of using deep learning to analyse complex magnetic resonance data and could have an impact on the study of large biomolecules in years to come.

Benzoselenadiazole-Functionalized H-Bonded Arylamide Foldamers: Solvent-Responsive Properties and Helix Self-Assembly Directed by Chalcogen Bonding in Solid State.

In this study, a series of H-bonded arylamide foldamers bearing benzoselenadiazole ends with solvent-responsive properties have been synthesized. In dichloromethane or dimethyl sulfoxide solvents, the molecules exhibit meniscus or linear structures, respectively, which can be attributed to the unique intramolecular hydrogen bonding behavior evidenced by 1D 1H NMR and 2D NOESY spectra. UV-vis spectroscopy experiments show that the absorption wavelength of H-bonded arylamide foldamers are significantly red-shifted due to the presence of benzoselenadiazole group. In addition, the crystal structures reveal that effective intermolecular dual Se ⋅ ⋅ ⋅ N interactions between benzoselenadiazole groups induce further assembly of the monomers. Remarkably, supramolecular linear and double helices structures are constructed under the synergistic induction of intramolecular hydrogen bonding and intermolecular chalcogen bonding. Additionally, 2D DOSY diffusion spectra and theoretical modelling based on density functional theory (DFT) are performed to explore the persistence of intermolecular Se ⋅ ⋅ ⋅ N interactions beyond the crystalline state.

A modular library for fast prototyping of solution-state nuclear magnetic resonance experiments

Abstract. We present a framework library (Modular Elements, ME) for the development of pulse sequences for Bruker spectrometers. It implements a two-level abstraction approach – the lower level comprises basic functional elements of pulse sequences and the higher one often-reused blocks comprising several evolution periods. The low-level abstractions reduce code duplication between variants of experiments such as hard-pulse and selective variants of individual NMR experiments. The high-level modules enable further reuse of pulse program code and aid in the construction of complex experiments. We show the library's functionality by presenting pulse programs that can be switched between standard and TROSY variants as well as hard and shaped pulses and that can seamlessly incorporate real-time homodecoupling. Adaptability is further demonstrated in a configurable 4D NOESY program.

NMR characterization and ligand binding site of the stem-loop 2 motif from the Delta variant of SARS-CoV-2.

The stem-loop 2 motif (s2m) in SARS-CoV-2 (SCoV-2) is located in the 3'-UTR. Although s2m has been reported to display characteristics of a mobile genomic element that might lead to an evolutionary advantage, its function has remained unknown. The secondary structure of the original SCoV-2 RNA sequence (Wuhan-Hu-1) was determined by NMR in late 2020, delineating the base-pairing pattern and revealing substantial differences in secondary structure compared to SARS-CoV-1 (SCoV-1). The existence of a single G29742-A29756 mismatch in the upper stem of s2m leads to its destabilization and impedes a complete NMR analysis. With Delta, a variant of concern has evolved with one mutation compared to the original sequence that replaces G29742 by U29742. We show here that this mutation results in a more defined structure at ambient temperature accompanied by a rise in melting temperature. Consequently, we were able to identify >90% of the relevant NMR resonances using a combination of selective RNA labeling and filtered 2D NOESY as well as 4D NMR experiments. We present a comprehensive NMR analysis of the secondary structure, (sub)nanosecond dynamics, and ribose conformation of s2m Delta based on heteronuclear 13C NOE and T 1 measurements and ribose carbon chemical shift-derived canonical coordinates. We further show that the G29742U mutation in Delta has no influence on the druggability of s2m compared to the Wuhan-Hu-1 sequence. With the assignment at hand, we identify the flexible regions of s2m as the primary site for small molecule binding.

Aggregation-induced emission enhancement (AIEE) active bispyrene-based fluorescent probe: “turn-off” fluorescence for the detection of nitroaromatics

The detection of nitroaromatic explosives in real samples is essential for environmental monitoring because of their strongly powerful nature and wide applications in industries. Aggregation-induced emission enhancement (AIEE) active fluorescent probe has been widely employed to detect nitroaromatic explosives. Hereby, a simple V-shaped bispyrene-based fluorescent probe (called py-o) with AIEE properties was designed and synthesized, which was fully charactered by 1D NMR, ESI, FTIR, and 2D NOESY spectra. The py-o displayed bright blue-green fluorescence excimer emission at 480 nm in DMF/H2O (v/v 1:1). It is observed that the fluorescence excimer emission of py-o at 480 nm was quenched by PA in solution with a quenching constant of 5.45 × 104 M–1, and the limit of detection was approximately 0.139 μM. The details of the sensing mechanism were explained using 1H NMR titrations, Job’s plot and Bensi-Hildebrand methods, which revealed a 1:1 binding ratio via the π-π interactions between PA and py-o. Meanwhile, it exhibited outstanding anti-interference ability in the detection of PA when interfering analytes were added under the same conditions. Furthermore, low-cost thin-layer chromatography (TLC) plates coated with py-o were developed as fluorescent tools for naked-eye detection of PA in the solid state. Therefore, this work provides a new method for constructing an AIEE fluorescent probe for the detection of nitroaromatic explosives to utilize in environmental monitoring.

Interaction of cationic 5,10,15,20-tetra(N-methylpyridyl)porphyrin and its Co(III) complex with premicellar SDS aggregates: Structure and properties

The study explores the interactions between 5,10,15,20-tetra(N-methylpyridyl)porphyrin (H2P) and sodium dodecyl sulfate (SDS) at concentrations below the critical micelle concentration (CMC) in aqueous solutions employing spectrophotometric and fluorescence methods. The composition and structure of the complexes involved in the association of H2P with premicellar SDS aggregates of varying compositions denoted as [H2P-5·SDS], [H2P-10·SDS] and [H2P-20·SDS] were characterized using mass spectrometry, DOSY, and 1D NOESY techniques. Possible reasons for the decrease in the intensity of absorption and emission of porphyrin in micellar aggregates of SDS compared to porphyrin in an aqueous environment are analyzed.The research has revealed that SDS can act as both a catalyst and an inhibitor, depending on its concentration in the reaction of complexation of H2P with metal cations (Co2+). The spectrophotometric approach examined interaction processes between 5,10,15,20-tetra(N-methylpyridyl)porphyrinate Co(III) and premicellar SDS aggregates, which proceed similarly to the processes with the porphyrin ligand. The study demonstrated the impact of premicellar aggregates on the binding capacity of Co(III) porphyrinate with electron-donating molecules using imidazole as an illustrative example.

Synthesis, Spectroscopic Evaluations and UV-Vis Titration Studies of New Symmetrical Amide Compounds Derived from N-6-[(4-pyridylmethylamino)carbonyl]-pyridine-2-carboxylic Acid Methyl Ester

In this study, 2 flexible and pre-organized tetraamide compounds derived from N-6-[(4-pyridylmethylamino)carbonyl]-pyridine-2-carboxylic acid methyl ester namely 1,2-bis[N,N’-6-(4-pyridylmethylamido)pyridyl-2-carboxyamido]pentane (L1) and 1,2-bis[N,N’-6-(4-pyridylmethylamido)pyridyl-2-carboxyamido]hexane (L2) have been successfully synthesized when reacted with diamines in 1:2 ratio. These new compounds were built from combination of 3 main components, as a trend requires for anion receptor which are (i) 2,6-pyridine dicarboxamide moieties as targeted anion binding host, (ii) amino methyl pyridine pendants arms as the flexible moieties and (iii) pentyl (-C5H10-) and hexyl (-C6H12-) unit as the spacer. Compounds L1-L2 were fully characterized by using elemental analyzer, Fourier transform infrared (FTIR) spectroscopy, gas chromatography-mass spectroscopy (GC-MS), 1H, 2D NOESY and 13C Nuclear Magnetic Resonance (NMR) spectroscopies, and Ultraviolet-visible (UV-Vis) spectroscopies. In this study, anion titration methods were used to identify the affinity towards selected anions. The results showed that L1 (having a pentyl spacer) had the highest affinity towards phosphate anions as compared to L2, where the red shift changes were observed in the UV-vis spectrum at the amide region. HIGHLIGHTS Two novel amide compounds with flexible and pre-organized structure are designed for use as potential anion receptors The flexible moieties at the amide and the linker allow hydrogen bonding interaction of the molecules with variety anions with different geometries Anion titration studies revealed good affinities towards phosphate anions as suggested in Hofmeister trend GRAPHICAL ABSTRACT

Discovery of unguisin J, a new cyclic peptide from Aspergillus heteromorphus CBS 117.55, and phylogeny-based bioinformatic analysis of UngA NRPS domains.

Several under-explored Aspergillus sp. produce intriguing heptapeptides containing a γ-aminobutyric acid (GABA) residue with as yet unknown biological functions. In this study, a new GABA-containing heptapeptide - unguisin J (1) - along with known unguisin B (2) were isolated from a solid culture of Aspergillus heteromorphus CBS 117.55. The structure of compound 1 was elucidated by extensive 1D and 2D NMR spectroscopic analysis including HSQC, HMBC, COSY, and 2D NOESY as well as HRESIMS. The stereochemistry of 1 and 2 was determined by Marfey's method. A biosynthetic gene cluster (BGC) encoding unguisins B and J was compared to characterized BGCs in other Aspergillus sp. Since the unguisin family of heptapetides incorporate different amino acid residues at different positions of the peptide, the A and C domains of the UngA NRPS were analyzed in an attempt to understand the lack of substrate specificity observed.

Fructooligosaccharides from Cynoglossum tubiflorus: Effect of the molecular size on their antidiabetic activity in high-fat diet and alloxan induced diabetic rats

The use of acetylation followed by silica gel column purification allowed the isolation of eight fructooligosaccharides (FOS) from the ethanol extract of Cynoglossum tubiflorus roots. Each FOS was identified by analyzing its FT-IR, HRMS/MS and NMR data, including 1H, 13C and 2D NMR HH COSY, HMBC and NOESY. In diabetic rats treated with a series of FOS from Glc-(Fru)3 to Glc-(Fru)7, a significant inhibition of intestinal α-amylase was observed. This activity increases proportionally with the FOS molecular size. It was found that they delay the absorption of total cholesterol (TC), ldl-cholesterol (LDL-C) and increase HDL-cholesterol (HDL-C) in a molecular size-dependent manner.This inhibitory effect on the activity of the digestive enzyme causes a significant (p < 0.05) reduction in the level of glucose in the blood as an anti-diabetic action. The ethanolic extract (E.E) exerts a significant effect against α-amylase as well as antihyperglycemic and antihyperlipidemic actions, while its acetylation suppresses these effects. Therefore, this study demonstrates for the first time that pure FOS act as an efficient agent in preventing hyperglycemia and hyperlipidemia and that this action evolves in the same manner with their molecular size.

Speciation and kinetics of fluoride transfer from tetra-n-butylammonium difluorotriphenylsilicate ('TBAT').

Tetra-n-butylammonium difluorotriphenylsilicate (TBAT) is a conveniently handled anhydrous fluoride source, commonly used as a surrogate for tetra-n-butylammonium fluoride (TBAF). While prior studies indicate that TBAT reacts rapidly with fluoride acceptors, little is known about the mechanism(s) of fluoride transfer. We report on the interrogation of the kinetics of three processes in which fluoride is transferred from TBAT, in THF and in MeCN, using a variety of NMR methods, including chemical exchange saturation transfer, magnetisation transfer, diffusion analysis, and 1D NOESY. These studies reveal ion-pairing between the tetra-n-butylammonium and difluorotriphenylsilicate moieties, and a very low but detectable degree of fluoride dissociation, which then undergoes further equilibria and/or induces decomposition, depending on the conditions. Degenerate exchange between TBAT and fluorotriphenylsilane (FTPS) is very rapid in THF, inherently increases in rate over time, and is profoundly sensitive to the presence of water. Addition of 2,6-di-tert-butylpyridine and 3 Å molecular sieves stabilises the exchange rate, and both dissociative and direct fluoride transfer are shown to proceed in parallel under these conditions. Degenerate exchange between TBAT and 2-naphthalenyl fluorosulfate (ARSF) is not detected at the NMR timescale in THF, and is slow in MeCN. For the latter, the exchange is near-fully inhibited by exogenous FTPS, indicating a predominantly dissociative character to this exchange process. Fluorination of benzyl bromide (BzBr) with TBAT in MeCN-d3 exhibits moderate progressive autoinhibition, and the initial rate of the reaction is supressed by the presence of exogenous FTPS. Overall, TBAT can act as a genuine surrogate for TBAF, as well as a reservoir for rapidly-reversible release of traces of it, with the relative contribution of the pathways depending, inter alia, on the identity of the fluoride acceptor, the solvent, and the concentration of endogenous or exogenous FTPS.