Enantiodiscrimination of carboxylic acids using single enantiomer thioureas as chiral solvating agents

Enantiodifferentiation of Thiohydantoin Derivatives by NMR in the Presence of Chiral Solvating Agent: (1R,2S)-Ephedrine and Chiral Symmetrical and Unsymmetrical Thioureas

2-Thiohydantoin derivatives, including different substitutions at N-1 and C-5 (5-methyl-, 5-isopropyl-, 1-acetyl-5-methyl-, and 1-acetyl-5-isopropyl-) (1-4, respectively), were synthesized by the known literature methods. In these synthetic pathways, it was reported that enantiomerically pure 2-thiohydantions were obtained in the absence of any solvent via the reaction of L-amino acids with thiourea (1&2) and via the reaction of L-amino acids with NH4SCN and acetic anhydride (3&4). However, in this study, in contrary to the previous literature studies, racemic mixtures of 2-thiohydantoins were obtained although the same synthetic methods were used. The racemic nature of 2-thiohydantoin derivatives (1-4) was proved by using 1H NMR analysis in the presence of (1R,2S)-(-)-ephedrine as a chiral auxiliary. In addition, the enantiomers of 3&4 were also resolved on chiral stationary phases by HPLC analyses. Furthermore, newly synthesized unsymmetrical chiral thioureas (S-1&S-2) and previously synthesized symmetrical ones (SS-3&SS-4) were used as chiral solvating agent (CSA) for the enantiodiscrimination of the thiohydantoins (5&6), previously reported. Optimal CSA/substrate ratios were determined for the best enantiodiscrimination.

Revisiting the Mechanism of Asymmetric Ni-Catalyzed Reductive Carbo-Carboxylation with CO2: The Additives Affect the Product Selectivity

The mechanistic details of the asymmetric Ni-catalyzed reductive cyclization/carboxylation of alkenes with CO2 have been revisited using DFT methods. Emphasis was put on the enantioselectivity and the mechanistic role of Lewis acid additives and in situ formed salts. Our results show that oxidative addition of the substrate is rate-limiting, with the formed Ni(II)-aryl intermediate preferring a triplet spin state. After reduction to Ni(I), enantioselective cyclization of the substrate occurs, followed by inner sphere carboxylation. Our proposed mechanism reproduces the experimentally observed enantiomeric excess and identifies critical C-H/O and C-H/N interactions that affect the selectivity. Further, our results highlight the beneficial effect of Lewis acids on CO2 insertion and suggest that in situ formed salts influence if the 5-exo or 6-endo product will be formed.

A Squaramide-Based Organocatalyst as a Novel Versatile Chiral Solvating Agent for Carboxylic Acids

A squaramide-based organocatalyst for asymmetric Michael reactions has been tested as a chiral solvating agent (CSA) for 26 carboxylic acids and camphorsulfonic acid, encompassing amino acid derivatives, mandelic acid, as well as some of its analogs, propionic acids like profens (ketoprofen and ibuprofen), butanoic acids and others. In many cases remarkably high enantiodifferentiations at 1H, 13C and 19F nuclei were observed. The interaction likely involves a proton transfer from the acidic substrates to the tertiary amine sites of the organocatalyst, thus allowing for pre-solubilization of the organocatalyst (when a chloroform solution of the substrate is employed) or the simultaneous solubilization of both the catalyst and the substrate. DOSY experiments were employed to evaluate whether the catalyst-substrate ionic adduct was a tight one or not. ROESY experiments were employed to investigate the role of the squaramide unit in the adduct formation. A mechanism of interaction was proposed in accordance with the literature data.

Enantiospecific Analysis of Carboxylic Acids Using Cinchona Alkaloid Dimers as Chiral Solvating Agents

Cinchona alkaloid derivatives as Brønsted base catalysts have attracted considerable attention in the field of asymmetric catalysis. However, their potential application as chiral solvating agents has not been described. In this research, we investigated the use of the Cinchona alkaloid dimer, namely, (DHQ)2PHAL, as a chiral solvating agent for discerning various mandelic acid derivatives through 1H NMR spectroscopy. The addition of catalytic amounts of DMAP facilitated this process. Our experimental results demonstrate that dimeric (DHQ)2PHAL exhibits remarkable chiral discrimination properties regarding the diagnostic split protons of 1H NMR signals (including 24 examples, up to 0.321 ppm). Furthermore, it serves as an excellent chiral discriminating agent and provides good resolution for racemic chiral phosphoric acid as determined by 31P NMR spectroscopy. The quality of enantiodifferentiation has also been evaluated by means of the parameter "resolution (Rs)". Significantly, this class of CSAs based on (alkaloid)2linker systems with an azaaromatic linker can be directly employed, which is commercially available in an enantiopure form at very low cost and exhibits promising potential in determining the enantiopurity of α-hydroxy acids by chemoselective and biocatalytic reactions.

Bis-Thiourea Chiral Sensor for the NMR Enantiodiscrimination of N-Acetyl and N-Trifluoroacetyl Amino Acid Derivatives

A C2-symmetrical bis-thiourea chiral solvating agent (CSA), TFTDA, for NMR spectroscopy has been obtained by reacting (1R,2R)-1,2-bis(2-hydroxyphenyl)ethylenediamine and 3,5-bis(trifluoromethyl)phenyl isothiocyanate. TFTDA shows remarkable propensity to enantiodiscriminate N-trifluoroacetyl (N-TFA) and N-acetyl (N-Ac) derivatives of amino acids with free carboxyl functions, with the co-presence of 1,4-diazabicyclo[2.2.2]octane (DABCO) as the third achiral additive, which is needed for substrate solubilization. TFTDA shows enhanced enantiodiscriminating efficiency in comparison with the corresponding monomeric counterpart, TFTMA, pointing out cooperativity between its two symmetrical entities. A wide range of amino acid derivatives have been efficiently enantiodiscriminated in CDCl₃, with high enantioresolution quotients, which guarantee high quality in applications devoted to the quantification of enantiomers. High enantiodiscriminating efficiency is maintained also in diluted 5 mM conditions or in the presence of sub-stoichiometric amounts of CSA (0.3 equiv). The role of phenolic hydroxyls in the DABCO-mediated interaction mechanism between TFTDA and the two enantiomeric substrates has been pointed out by means of diffusion-ordered spectroscopy (DOSY) and rotating frame Overhauser effect spectroscopy (ROESY) experiments. A conformational model for both the CSA and its diastereomeric solvates formed with the two enantiomers of N-acetyl leucine has also been conceived on the basis of ROE data in order to give a chiral discrimination rationale.

Chiral Recognition of Hydantoin Derivatives Enabled by Tetraaza Macrocyclic Chiral Solvating Agents Using 1H NMR Spectroscopy

Enantiomers of a series of hydantoin derivatives were prepared from d- and l-amino acids with p-tolyl isocyanate and 3,5-bis(trifluoromethyl)phenyl isocyanate as guests for chiral recognition by ¹H NMR spectroscopy. Meanwhile, several tetraaza macrocyclic compounds were synthesized as chiral solvating agents from d-phenylalanine and (1S,2S)-(+)-1,2-diaminocyclohexane. An uncommon enantiomeric discrimination has been successfully established for hydantoin derivatives, representatives of five-membered N,N-heterocycles, in the presence of tetraaza macrocyclic chiral solvating agents (TAMCSAs) 1a-1c by means of ¹H NMR spectroscopy. Several unprecedented nonequivalent chemical shifts (up to 1.309 ppm) were observed in the split ¹H NMR spectra. To evaluate practical applications in the determination of enantiomeric excess (ee), the ee values of samples with different optical purities (up to 95% ee) were accurately calculated by the integration of relevant proton peaks. To better understand the chiral discriminating behavior, Job plots of (±)-G1 with TAMCSA 1a were investigated. Furthermore, in order to further explore any underlying intermolecular hydrogen bonding interactions, theoretical calculations of the enantiomers of (S)-G1 and (R)-G1 with TAMCSA 1a were performed by means of the hybrid density functional theory (B3LYP/6-31G*) of the Gaussian 16 program.

A Dimeric Thiourea CSA for the Enantiodiscrimination of Amino Acid Derivatives by NMR Spectroscopy

The reaction of benzoyl isothiocyanate with (1R,2R)-1,2-bis(2-hydroxyphenyl)ethylenediamine afforded a new thiourea chiral solvating agent (CSA) with a very high ability to differentiate 1H and 13C NMR signals of simple amino acid derivatives, even at low concentrations. The enantiodiscrimination efficiency was higher with respect to that of the parent monomer, a thiourea derivative of 2-((1R)-1-aminoethyl)phenol, thus putting into light the relevance of the cooperativity between the two molecular portions of the dimer in a cleft conformation stabilized by interchain hydrogen bond interactions. An achiral base additive (DABCO or DMAP) played an active role in the chiral discrimination processes, mediating the interaction between the CSA and the enantiomeric mixtures. The chiral discrimination mechanism was investigated by NMR spectroscopy through the determination of complexation stoichiometries, association constants, and the stereochemistry of the diastereomeric solvates.

Azaheterocyclic diphenylmethanol chiral solvating agents for the NMR chiral discrimination of alpha-substituted carboxylic acids

A series of small-membered heterocycle probes, so-called azaheterocycle-containing diphenylmethanol chiral solvating agents (CSAs), have been developed for NMR enantiodiscrimination. These chiral sensors were readily synthesized were inexpensive and efficiently used for the chiral analysis of alpha-substituted carboxylic acids. The sensing method was operationally simple and the processing was straightforward. Notably, we propose (S)-aziridinyl diphenylmethanol as a promising CSA, which has excellent chiral discriminating properties and offers multiple detectable possibilities pertaining to the 1H NMR signals of diagnostic split protons (including 25 examples, up to 0.194 ppm, 77.6 Hz). Its ability to detect the molecular recognition of fluorinated carboxylic acids were further investigated, with a good level of discrimination via the 19F NMR spectroscopic analysis. In addition, an accurate enantiomeric excess (ee) analysis of the p-methoxyl-mandelic acid with different optical compositions have been calculated based on the integration of well-separated proton signals.