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Article

Comparison of Cinchona Catalysts Containing Ethyl or Vinyl or Ethynyl Group at Their Quinuclidine Ring

1
Department of Organic Chemistry & Technology, Budapest University of Technology & Economics, Szent Gellért tér 4, H-1111 Budapest, Hungary
2
Chemical Department, Chemical Works of Gedeon Richter Plc., P.O. Box 27, H-1103 Budapest, Hungary
3
Department of Inorganic & Analytical Chemistry, Budapest University of Technology & Economics, Szent Gellért tér 4, H-1111 Budapest, Hungary
4
Furukawa Electric Institute of Technology, Késmárk utca 28/A, H-1158 Budapest, Hungary
5
Department of Chemical & Environmental Process Engineering, Budapest University of Technology and Economics, Műegyetem rkp. 3, H-1111 Budapest, Hungary
*
Authors to whom correspondence should be addressed.
Materials 2019, 12(18), 3034; https://doi.org/10.3390/ma12183034
Received: 22 August 2019 / Revised: 11 September 2019 / Accepted: 12 September 2019 / Published: 18 September 2019
(This article belongs to the Special Issue Advances in Organocatalysts: Synthesis and Applications)

Abstract

Numerous cinchona organocatalysts with different substituents at their quinuclidine unit have been described and tested, but the effect of those saturation has not been examined before. This work presents the synthesis of four widely used cinchona-based organocatalyst classes (hydroxy, amino, squaramide, and thiourea) with different saturation on the quinuclidine unit (ethyl, vinyl, ethynyl) started from quinine, the most easily available cinchona derivative. Big differences were found in basicity of the quinuclidine unit by measuring the pKa values of twelve catalysts in six solvents. The effect of differences was examined by testing the catalysts in Michael addition reaction of pentane-2,4-dione to trans-β-nitrostyrene. The 1.6–1.7 pKa deviation in basicity of the quinuclidine unit did not result in significant differences in yields and enantiomeric excesses. Quantum chemical calculations confirmed that the ethyl, ethynyl, and vinyl substituents affect the acid-base properties of the cinchona-thiourea catalysts only slightly, and the most active neutral thione forms are the most stable tautomers in all cases. Due to the fact that cinchonas with differently saturated quinuclidine substituents have similar catalytic activity in asymmetric Michael addition application of quinine-based catalysts is recommended. Its vinyl group allows further modifications, for instance, recycling the catalyst by immobilization.
Keywords: organocatalysis; cinchona; squaramide; thiourea; pKa; Michael addition organocatalysis; cinchona; squaramide; thiourea; pKa; Michael addition

1. Introduction

Asymmetric syntheses went through explosive growth in the last decades [1,2,3,4,5]. After transition metal- and enzyme-based catalysis, the application of organocatalysis has gained ground in the field of asymmetric syntheses. The aim of enantioselective organocatalytic synthesis is to produce enantiopure compounds from achiral substrates facilitated by asymmetric organocatalysts. The importance of the synthesis and application of enantiopure products lies in the fact that many industries are required to produce the appropriate enantiomer, for instance as drug or pesticide intermediates.
Within organocatalysis, bifunctional catalysts have become widespread. This bifunctionality means the capability to activate two components of a reaction simultaneously [6]. Cinchona moiety is one of the privileged chiral skeletons in asymmetric organocatalysis [7]. Its structure consists of two rigid rings, the aromatic quinoline, and the alicyclic quinuclidine rings. The tertiary amino group in the quinuclidine ring gives basic character for the molecule, hence this can activate/fix nucleophile or electrophile, and possessing a chiral skeleton, cinchona unit is also responsible for chiral induction.
There are many successful applications of cinchona-based organocatalysts in asymmetric reactions with high yields and enantiomeric excesses [8]. Such reactions include, but are not limited to, Morita–Baylis–Hillman reaction, Henry reaction, Diels–Alder reaction, and the most often examined, Michael addition. These reactions are used for the synthesis of drug intermediates, such as oseltamivir, baclofen, or prostaglandin E1 [9,10,11].
When bifunctional H-bond organocatalysts, such as cinchona-thioureas or squaramides were applied in Michael additions of 1,3-dioxo compounds to nitroalkenes, it was found [12,13,14,15,16] that the reaction can proceed by two mechanisms. In one of these mechanisms, the acidic NH groups (of thiourea or squaramide moieties) activate the enolate, while the protonated quinuclidine fixes the electrophile (Figure 1a). According to the other mechanism, the activation of substrates occurs on the opposite sites of catalysts (Figure 1b). Nevertheless, both mechanisms start with the deprotonation of the nucleophile by the basic N atom of the quinuclidine ring.
Based on these mechanisms, the basic N atom of the quinuclidine unit plays an important role in these reactions, which is related to its pKa value. These cinchona-based organocatalysts are usually used with saturated or unsaturated substituent (ethyl, vinyl, ethynyl) on the quinuclidine ring, although the effect of that saturation has not been examined before. Vinyl or ethynyl group is often the target for further transformations (Figure 2) such as polymerization or immobilization [17,18,19,20,21,22]; however, several methods for modification are described in the literature in other positions of the cinchonas as well (C9 position, or modification after demethylation of quinoline) [23,24].
Herein we report the study of substituent influence on the basicity of cinchonas. We synthesized the appropriate hydroxyl, amino, thiourea, and squaramide cinchona derivatives with different saturation of the quinuclidine substituent. We measured the pKa values of these derivatives in eight solvents and examined the catalytic activity of these twelve catalysts in Michael addition of pentane-2,4-dione to trans-β-nitrostyrene in seven solvents.

2. Results and Discussion

Since the strength of hydrogen bonding interactions plays an important role in H-bond organocatalysis [16,25,26,27], determination of pKa values could help understanding the mechanisms and catalytic activity, therefore, it can contribute the design of more efficient catalytic systems. In cases when deprotonation and/or hydrogen bonding are able to promote reactions or could increase enantioselectivity, knowing the pKa values of the corresponding groups is essential.

2.1. The pKa Values of Conjugate Acid Forms of Quinuclidine

To get further insight into the H-bond organocatalytic behavior of the aforementioned twelve catalysts, the pKa values of all four compound classes were measured in six different solvents by UV-spectrophotometric titrations (Pion Inc., Forest Row, UK) (Table 1 and Tables S1–S4). For better clarity, only those pKa values are shown in Table 1 which were measured in water for the conjugate acid forms of basic quinuclidine.

2.2. The Effect of the Substituents on Quinuclidine Basicity

Within the four different compound classes (cinchona hydroxyl, amino, thiourea, and squaramide derivatives) the effect of the substituents of quinuclidine on the basicity was examined. As expected, the saturation of the substituent leads to increased basicity due to electronic reasons. The difference among the pKa values of the tertiary amino nitrogen in quinuclidine is in a range of 1.6–1.7 in each compound class, depending on the substituents.

2.3. The Effect of the C9 Group of Cinchona on Quinuclidine Basicity

Not only the aliphatic substituent of quinuclidine unit has an effect on the basicity, but the groups of C9 position of the cinchona as well, since these groups and the aliphatic substituents are practically equidistant from the quinuclidine nitrogen. The decreasing order in basicity is the following: amino, hydroxyl, thiourea, and squaramide. This may be caused mainly by electronic effects. Thiourea derivatives have higher basicity than that of the corresponding squaramides. This may be explained by the squaramide moiety having aromatic feature, therefore the electron-withdrawing effect of the bis(trifluoromethyl)phenyl group can have a stronger effect than in the case of thiourea.

2.4. Relationship Between pKa Values and H-Bonding Abilities

Based on our published results [28,29,30] and the relevant literature [31,32], in asymmetric Michael additions of 1,3-dioxo compounds to β-nitroalkenes-cinchona, squaramides or thioureas are recommended to be used as enantioselective organocatalysts. Owing to their H-bonding ability (quantified by their pKa values) [33,34,35,36,37,38] and chiral skeleton, they not only give high yields but excellent enantioselectivities as well. Despite the fact that the N atom of the quinuclidine ring in thioureas is more basic, the observed enantiomeric excesses are similar than those given by squaramides. This observation can be explained by the different strength of H-bonds formed by thioureas and squaramides.
According to the mechanisms [12,13,14], cinchona thiourea or squaramide can fix substrates by three H-bonds, while hydroxyl or amino derivatives are able to interact different ways, for instance with less H-bonds. This causes the notable differences among the enantioselectivities. The strong H-bonds can be explained by the relatively high acidity of these units. The pKa values of thiourea and squaramide units measured in water can be seen in Table 2 (in other solvents see Tables S4 and S5).
Based on pKa values, squaramides are more acidic than thioureas, hence stronger H-bonds can be formed between this moiety and the corresponding substrate. Taking into account the bifunctionality of cinchona thioureas and squaramides, they possess similar catalytic activity: while the quinuclidine nitrogen of cinchona thioureas are more basic, squaramides acidic NH moiety can form stronger H-bonds with the substrate.
Reflecting to further importance of the acidity of the H-bond donor units, an analogue of squaramides, the thiosquaramides having more acidic NH groups should be mentioned [30,38,39,40,41,42]. Thiosquaramides perform better results (yield and ee) in asymmetric Michael addition than the corresponding squaramides. Thiosquaramides could act as Brønsted acids, too. Therefore, they can also catalyze aza-Diels–Alder reaction [30,42], in which neither thioureas nor squaramides can give the desired product.

2.5. Synthesis of the Catalysts

We synthesized the cinchona catalysts starting from the commercially available quinine (Q), which was transformed to hydroquinine (HQ) by catalytic hydrogenation (Scheme 1). Didehydroquinine (DQ) was prepared in a two-step reaction from quinine: a bromine addition was followed by HBr elimination [43].
Cinchona amino derivatives (HQ-N, Q-N, DQ-N) were prepared starting from the relevant hydroxy derivatives (HQ, Q, DQ) based on previously reported procedures [44]. In the next step, thiourea derivatives (HQ-TU, Q-TU, DQ-TU) were obtained by the addition of the appropriate amine derivatives to bis(trifluoromethyl)phenyl isothiocyanate (SCN) [45,46]. Finally, addition of the abovementioned amines to half-squaramide methyl ester (HSQ) resulted in the corresponding squaramides (HQ-SQ, Q-SQ, DQ-SQ) [46,47]. During the synthetic procedures, all compounds were characterized by well-established methods including MS, IR, 1H, and 13C NMR spectroscopies.

2.6. Application of the Catalysts in Asymmetric Michael Addition

We used hydroxy, amine, thiourea, and squaramide type catalysts in Michael addition (Scheme 2) of pentane-2,4-dione (2) to trans-β-nitrostyrene (1). Conventional and green solvents including dichloromethane (DCM), methyl tert-butyl ether (MTBE), toluene, acetonitrile (MeCN), ethyl acetate (EtOAc), methanol, and tetrahydrofuran (THF) were used (Table 3 and Table S5) under the same conditions. In methanol, which is a protic solvent, we got only lower yields (up to 82% vs. up to 100%) and enantioselectivities (up to 51% ee vs. up to 93% ee). This observation is in agreement with similar cinchona-based organocatalysis [48]. In methanol, hydrogen bonds could form between the solvent and the catalysts/reactants. Accordingly, to apply a hydrogen bonding organocatalyst in protic polar solvents, could result in low yields and enantiomeric excesses.
Michael adducts were obtained in high yields (up to 99%) when hydroxyl or amino derivatives of cinchona were applied, but these reactions gave only 34% ee (see Table S5). Concerning selectivity, no significant difference was found between the differently saturated cinchonas; however, among hydroxy derivatives in most of the solvents, HQ gave higher enantiomeric excesses. It can be explained by its higher basicity, but this tendency cannot be observed in cases of other derivatives.
The utilization of squaramides or thioureas served both high yields (up to 100%, see Table 3) and enantioselectivities (up to 93% ee, see Table 3 and Figure 3). The different pKa values of these derivatives did not lead to significant changes in yield or enantiomeric excess. Squaramides and thioureas gave good results in asymmetric Michael addition, without showing any considerable difference in the catalytic activity.

2.7. Theoretical Calculations

Due to the importance of hydrogen bonds and acidity in the catalytic mechanisms of cinchona-thiourea compounds [49,50], one of the main questions is how the ethyl, ethynyl, and vinyl substituents can affect their acid-base properties. Therefore, we investigated systematically the different tautomers of the synthesized HQ-TU, Q-TU, and DQ-TU catalysts and their protonated and deprotonated forms using quantum chemical calculations.
In agreement with their similar reactivities observed in the experiments, the geometric structures of the neutral cinchona and thiourea frameworks (see Figures S22 and S23 in the Supplementary Materials) and also the energetics of the different tautomers, as well as the protonation and deprotonation Gibbs free energies (Figure 4 and Figures S22 and S23, Table 4 and Table S6 in the Supplementary Materials) depend only slightly on the ethyl, ethynyl, or vinyl substituents. In all cases, the most stable tautomer is the thione form, containing two NH groups (i on Figure 4), while the quinuclidine nitrogen is protonated in the higher-lying tautomers ii and iii. Interestingly, in the higher-lying isomers the proton detaches from the bis(trifluoromethyl)phenyl side of the thiosquaramide. We expect that this is due to the interaction with the nitrogen lone pair with the π system, leading to an increased acidity at this site.
The thiol form iv is lying even higher in free energy. Thus, overall the calculations show that among the possible tautomers the catalytically most active [49,50] i is the most stable form and present in considerably higher amount than the others.
Electrostatic potential provides important insights into the intermolecular interactions of similar compounds [51,52]. The electrostatic potentials (see the Supplementary Materials, Figure S24) clearly show that both the sulfur and the nitrogen atoms are possible protonation sites. The calculations showed that in line with the above discussion, the quinuclidine nitrogen is the preferred site in all cases, with proton affinity of ~−44 kJ·mol−1 (Figure 4). The protonated thiol form of the cinchona-thiourea lies about 62 kJ·mol−1 higher in free energy, while the proton affinity decreases to −14 kJ·mol−1. Thus, the computations show that the quinuclidine is the main protonation site in acidic conditions.
Interestingly, in contrary to the neutral tautomers, there is only 9–12 kJ·mol−1 difference between the two possible deprotonated forms, indicating that both of the possible tautomers present in acidic conditions.

3. Conclusions

In conclusion, we can approach bifunctional H-bond organocatalysts in two ways: focusing on their H-bond acceptor or donor abilities (Figure 5).
As H-bond acceptors: within each compound class (hydroxyl, amino, squaramide, thiourea) the bifunctional cinchona organocatalysts with differently saturated substituents on basic quinuclidine nitrogen have 1.6–1.7 difference in their pKa values. The strongest bases are the compounds substituted with ethyl, followed by vinyl and then ethynyl groups. No significant difference was found in yields and enantioselectivities in any compound classes applying them in Michael addition. The basicity in each compound class increases in the following order: amino, hydroxyl, squaramides, thiourea.
As H-bond donors: the catalysts, which are able to form dual H-bonds gave the highest enantiomeric excesses (thioureas and squaramides). The squaramides have by 1.3–1.6 lower pKa values than the corresponding thioureas, resulting in stronger H-bonds with the substrate.
By summarizing the H-bond abilities, cinchona thioureas are better acceptors, but as donors, squaramides show stronger interactions. As organocatalysts in Michael addition, there is no considerable difference among them, both the yields and enantiomeric excesses were similar in each solvent and, within these compounds, the saturation had no effect on yield and ee as well.
Based on our results, saturation of the substituent at the quinuclidine unit has no significant effect on the catalysis thus the most easily available quinine derivatives (vinyl substituent at quinuclidine) are worth to apply as cinchona-based organocatalysts as themselves or as starting materials of their C9 derivatives. Considering immobilization, vinyl is a suitable group for further modifications, for instance in polymerization, or forming a reactive group in the molecule for this purpose.
Our results were corroborated by computational studies. Accordingly, the saturation of the substituent on quinuclidine has a slight effect on basicity of the H-bond acceptor nitrogen atom. Moreover, we accomplished calculations on stability of different tautomer forms of thiourea catalysts. These calculations approved that the most stable tautomers in all cases are the thione forms with the proton residing on the nitrogen atom of the quinuclidine.

4. Experimental

The UV/pH titrations were performed using D-PAS technique (Dip-Probe Absorbance Spectrophotometry, a quartz fiber dip probe measuring absorbance using a flow-cell) attached to a SiriusT3 instrument (Pion Inc., Forest Row, UK) [53,54]. The pKa values were calculated by SiriusT3Refine™ software (Pion Inc., Forest Row, UK). All measurements were performed in solutions of 0.15M KCl under nitrogen atmosphere, at t = 25.0 ± 0.1 °C. The cosolvent dissociation constants (psKa values) of the compounds were also determined in various MeOH–water mixtures between 15 and 70 wt% and MeCN–water, dioxane–water, THF–water, DMSO–water mixtures between 15 and 50 wt%. Each sample was measured at least in minimum six different cosolvent–water ratios. To obtain the 98.0% and 99.9% cosolvent psKa values from psKa data, linear and Yasuda–Shedlovsky extrapolation methods have been used. The Yasuda–Shedlovsky method establishes a correlation with the dielectric constant and uses the following equation: psKa + log[H2O] = a ε + b, where log[H2O] is the molar water concentration of the given solvent mixture. This method is the most widely used procedure in cosolvent techniques [55,56].
Infrared spectra were recorded on a Bruker Alpha-T FT-IR spectrometer (Bruker, Ettlingen, Germany). Optical rotations were measured on a Perkin-Elmer 241 polarimeter (Perkin-Elmer, Waltham, MA, USA) that was calibrated by measuring the optical rotations of both enantiomers of menthol. NMR spectra were recorded at Directorate of Drug Substance Development, Egis Pharmaceuticals Plc., on a Bruker Avance III HD (Bruker, Ettlingen, Germany) (at 600 MHz for 1H and at 150 MHz for 13C spectra). Mass spectra were recorded on CAMAG LCMS Interface (CAMAG, Muttenz, Switzerland) (HPLC pump: Shimadzu LC-20AD Prominence SQ MS: Shimadzu LCMS-2020 MS settings: detector voltage: 1.10 kV, m/z: 105–1000, scan speed: 1075 u·s−1, DL temperature: 250 °C, nebulizing gas flow: 1.5 L·min−1, drying gas flow: 15 L·min−1, eluent: acetonitrile: 0.1% (v/v) formic acid 95:5, 1.5 mL·min−1). The exact mass measurements were performed using Q-TOF Premier mass spectrometer (Waters Corporation, 34 Maple St, Milford, MA, USA) in positive electrospray ionization mode. The enantiomeric ratios of the samples were determined by chiral high-performance liquid chromatography (HPLC) measurements using reversed-phase mode (Thermo Finnigan Surveyor LC System, Thermo Fisher Scientific, Waltham, MA, USA). Elemental analyses were performed in the Microanalytical Laboratory of the Department of Organic Chemistry, Institute for Chemistry, Eötvös Loránd University, Budapest, Hungary. Melting points were taken on a Boetius micro-melting point apparatus (VEB Dresden Analytik, Dresden, Germany) and they were uncorrected. Starting materials were purchased from Aldrich Chemical Company (St. Louis, MO, USA) unless otherwise noted. Silica gel 60 F254 (Merck, Darmstadt, Germany) plates and aluminium oxide 60 F254 (Merck) were used for TLC. The spots of materials on TLC plates were visualized by UV light at 254 nm. Silica gel 60 (70–230 mesh, Merck) was used for column chromatography. Ratios of solvents for the eluents are given in milliliters.
The lowest energy tautomers were obtained using the MMFF94 force, as it is implemented in the Avogadro program [57]. Subsequently, the geometry optimization of the neutral structures as well as of their protonated and deprotonated forms were carried out using Density Functional Theory applying the ωB97X-D functional [58] and 6-31G* basis set [59], as it is implemented in the Q-Chem 5.2 quantum chemical software package [60], while the final energy computations were performed using the 6-311++G** basis set. The applied density functional includes long range and dispersion corrections and the accuracy of this method has been tested for similar systems in our previous studies [28,29,30]. The (75,302) integration grid was applied in all cases. The geometries of the catalysts were optimized both in the gas phase and in THF solvent using SM8 [61] and the SS(V)PE [62]. Harmonic vibrational frequencies were computed in the gas phase for the most stable tautomers. These show no imaginary vibrational frequencies, confirming that the computed conformers are minima on the potential energy surface. Single-point calculations were carried out using the SM12 solvent model [63]. As it is expected, the different solvent models yield similar results for the relative energies of the different structures (the maximum absolute deviation is 14 kJ mol−1, see Table S6 in the Supplementary Materials) and similar accuracy is obtained for protonation free energies (the maximum absolute error is 13 kJ mol−1), however much larger differences are obtained for anions (maximum error in deprotonation free energies is 47 kJ mol−1). Nevertheless, both solvent models yield the same tendencies. During the computation of the relative Gibbs free energies, the vibrational contribution is neglected and only the solvation contribution is considered. For the protonation and deprotonation computation, the solvation Gibbs free energy of the proton in THF is necessary, however, its computation is a tedious task. We approximated this quantity using a supermolecule model consisting of a proton and two THF molecules, embedded in a continuum solvent [64]. The molecules were visualized using the PyMol program [65].

4.1. Hydroquinine

A solution of quinine (2.00 g, 6.17 mmol) in methanol (20 mL) was added to a suspension of Pd/C in methanol (20 mL) under argon atmosphere. The quinine was hydrogenated at 25 °C under atmospheric pressure. When the reaction was completed (1 h), the reaction mixture was filtered through a pad of Celite. The solvent was evaporated, giving pure hydroquinine (HQ, Figure 6) as a white solid (2.01 g, 99%). Spectral data are fully consistent with those reported in the literature [66].

4.2. (S)-((1S,2S,4S,5S)-5-Ethynylquinuclidin-2-yl)(6-methoxyquinolin-4-yl)methanamine

Triphenylphosphine (976 mg, 3.72 mmol) was added to a solution of DQ (1.00 g, 3.10 mmol) in DCM (6 mL) under argon atmosphere and stirred for 1 h at room temperature. The reaction mixture was cooled down to 0 °C and a solution of DIAD (740 µL, 753 mg, 3.72 mmol) in DCM (1 mL) was added to it dropwise. After addition, the reaction mixture was stirred at room temperature overnight. Then the reaction mixture was cooled down to 0 °C, and water (1.35 mL, 75 mmol) was added to it in one portion. After this step, a solution of triphenylphosphine (1.46 g, 5.58 mmol) in DCM (5 mL) was added dropwise to the reaction mixture and it was stirred overnight at 25 °C. After the reaction was completed, the mixture was diluted to 50 mL with DCM, and it was shaken with an aqueous hydrochloric acid solution (2 × 50 mL, 10 w%). The combined inorganic phase was cooled down to 0 °C, and its pH was adjusted to 10 with aqueous sodium hydroxide solution and extracted with DCM (2 × 50 mL). The combined organic phase was dried over MgSO4, and the solvent was evaporated under reduced pressure. Further purification was not necessary. The product is a brownish-yellow, viscous oil (DQ-N, Figure 7, 673 mg, 69%). TLC (SiO2 TLC; DCM:MeOH:25% NH4OH = 10:1:0.01, Rf = 0.52;   a 20 D + 86.7 (c 1.00, CHCl3); IR νmax 3424, 3294, 2930, 2862, 1621, 1589, 1508, 1474, 1454, 1431, 1356, 1620, 1260, 1229, 1028 cm−1; 1H NMR (600 MHz, DMSO-d6, 25 °C): δ (ppm) 0.76 (1 H, m), 1.35 (1 H, m), 1.41 (1 H, m), 1.49 (1 H, m), 1.67 (1 H, bm), 2.51 (1 H, m), 2.62 (1 H, m), 2.79 (1 H, m), 2.83 (1 H, m), 3.07 (1 H, bm), 3.22 (1 H, bm), 3.32 (1 H, m), 3.94 (3 H, s), 4.64 (1 H, bm), 7.42 (1 H, dd, J1,H,H 2.8 Hz, J2,H,H 9.2 Hz), 7.59 (1 H, bm), 7.80 (1 H, bm), 7.95 (1 H, d, JH,H 9.2 Hz), 8.72 (1 H, d, JH,H 4.5 Hz); 13C NMR (150 MHz, DMSO-d6, 25 °C): δ (ppm) 25.9, 26.0, 26.7, 26.9, 55.7, 57.2, 61.5, 71.2, 79.4, 88.5, 102.9, 120.5, 121.4, 128.7, 129.0, 131.4, 144.3, 147.9 (2 C), 157.1; HRMS-ESI+ m/z [M + H+] calcd. for C20H24N3O: 322.1919, found: 322.1923.

4.3. 3-((3,5-Bis(trifluoromethyl)phenyl)amino)-4-(((S)-((1S,2S,4S,5R)-5-ethynylquinuclidin-2-yl)(6-methoxyquinolin-4-yl)methyl)amino)cyclobut-3-ene-1,2-dione

A solution of DQ-N (Figure 7) (200 mg, 0.67 mmol) in chloroform (0.7 mL) was added to a solution of half squaramide (HSQ: 322 mg, 0.74 mmol) in chloroform (1.5 mL). This mixture was stirred for 12 h at room temperature. The solvent was removed under reduced pressure, then the crude product was purified by column chromatography on silica gel using DCM: methanol:25% NH4OH (10:1:0.01) mixture as an eluent to obtain the pure product as white crystals (DQ-SQ, Figure 8, 387 mg, 99%). M.p. 240 °C (decomposes). TLC (SiO2 TLC; DCM:MeOH:25% NH4OH = 10:1:0.01, Rf = 0.52;   a 20 D −104.8 (c 1.00, CHCl3); IR νmax 3441, 3315, 3213, 2934, 1796, 1669, 1623, 1573, 1512, 1455, 1380, 1279, 1186, 1129, 1042 cm−1; 1H NMR (600 MHz, DMSO-d6, 25 °C): δ (ppm) 0.76 (1 H, bm), 1.45 (1 H, bm), 1.55 (1 H, bm), 1.65 (1 H, bm), 1.77 (1 H, bm), 2.60 (1 H, m), 2.88 (1 H, m), 2.96 (1 H, bm), 3.28 (1 H, m), 3.34 (1 H, m), 3.57 (1 H, bm), 3.97 (3 H, s), 6.11 (1 H, bm), 7.48 (1 H, dd, J1,H,H 2.3 Hz, J2,H,H 9.1 Hz), 7.62 (1 H, d, JH,H 4.0 Hz), 7.67 (1 H, bm), 7.76 (1 H, bm), 7.97 (2 H, m), 8.01 (1 H, d, JH,H 9.1 Hz), 8.39 (1 H, bm), 8.86 (1 H, d, JH,H 4.0 Hz), 10.15 (1 H, bs); 13C NMR (150 MHz, DMSO-d6, 25 °C): δ (ppm) 25.7, 26.3, 26.7 (2 C), 55.1, 55.9, 57.0, 59.0, 71.5, 88.3, 101.6, 115.2, 118.6, 119.3, 122.2, 123.3 (q, JC,F 273 Hz), 127.7, 128.9, 129.0, 131.4 (JC,F 33 Hz), 131.8, 141.0, 143.2, 144.5, 148.0, 158.0, 162.9, 168.8, 180.4, 185.1; HRMS-ESI+ m/z [M+H+] calcd. for C32H27N4O3F6: 629.1987, found: 629.1990.

4.4. General Procedure for Michael Addition of Pentane-2,4-dione (2) to Trans-β-nitrostyrene (1).

To a solution of trans-β-nitrostyrene (1) (23.4 mg, 0.16 mmol) in the corresponding solvent (1 mL), organocatalyst (5 mol%) was added. Then, pentane-2,4-dione (2) (41.5 µL, 40.7 mg, 0.41 mmol) was added to this solution and the resulting reaction mixture was stirred at room temperature. After 24 h, the volatile components were removed under reduced pressure. The crude product was purified by preparative thin-layer chromatography on silica gel using hexane:ethyl acetate 2:1 mixture (Rf = 0.36) as eluent to obtain Michael adduct (S)-3 as white crystals. Yields and enantiomeric excess (ee) values can be seen in in the Table 3 and Table S5. Spectral data are fully consistent with those reported in the literature (the absolute configuration was determined by the optical rotation of the products) [28]. HPLC: Phenomonex Lux Cellulose-3 column (3 mm, 250 × 4.6 mm), eluent CH3CN/20 mM NH4OAc in H2O = 40/60, isocratic mode; 0.6 mL min−1; UV detector 222 nm, 5 µL or 10 µL injection, 20 °C. Retention time for (S)-3: 11.94 min, for (R)-3: 14.20 min.

Supplementary Materials

Supplementary data to this article can be found online at https://www.mdpi.com/1996-1944/12/18/3034/s1. These data include solvent screening of asymmetric Michael additions, FT-IR spectra, characterization of 1H, 13C and 2D NMR spectra and MS spectra of new products, furthermore, the chiral HPLC profiles, the pKa measurements, and theoretical calculations.

Author Contributions

Methodology, S.N., P.K., and J.K.; synthesis of compounds, S.N. and Z.F.; performing pKa measurements, data analysis, Gergő Dargó and G.T.B.; performing quantum-chemical studies, conducting discussion of the results, J.B. and T.H.; performing NMR experiments, data analysis, Z.G.; determination of ee values by chiral HPLC measurements, B.M.; writing―original draft preparation, S.N. and J.B.; writing―review and editing, J.K., T.H., P.H., G.T.B., P.K., Z.F., and Gyula Dargó; project administration, J.K.; funding acquisition, P.H. and J.K.; resources, P.H. and J.K.; supervision, J.K.

Funding

This research was funded by the New National Excellence Program of the Ministry of Human Capacities, grant numbers ÚNKP-18-4-BME-270 and ÚNKP-18-2-II-BME-243, and the János Bolyai Research Scholarship of the Hungarian Academy of Science. It was also supported by the National Research, Development, and Innovation Office (grant number K128473), the Servier–Beregi PhD Research Fellowship, and the Gedeon Richter’s Talentum Foundation.

Acknowledgments

The authors are grateful to Joanna Kacperek and Magdalena Potępa from Lodz University of Technology, International Faculty of Engineering, for their assistance with the synthetic work, to László Drahos from MS Proteomics Research Group, Research Centre for Natural Sciences, Hungarian Academy of Sciences, for his assistance with the HRMS measurements, and to András Dancsó from Egis Pharmaceuticals Plc., Directorate of Drug Substance Development, for his assistance with the NMR measurements.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Nag, A.; Gergelitsová, I.; Veselý, J.; Adly, F.G.; Ghanem, A.; Moriuchi, T.; Ohmura, S.D.; Hirao, T.; McCluskey, A.; Simone, M.I.; et al. Asymmetric Synthesis of Drugs and Natural Products, 1st ed.; CRC Press: Boca Raton, FL, USA, 2019; pp. 1–485. [Google Scholar]
  2. Houk, K.N.; List, B. Asymmetric organocatalysis. Acc. Chem. Res. 2004, 37, 487. [Google Scholar] [CrossRef]
  3. MacMillan, D.W.C. The advent and development of organocatalysis. Nature 2008, 455, 304–308. [Google Scholar] [CrossRef] [PubMed]
  4. Zhan, G.; Du, W.; Chen, Y.C. Switchable divergent asymmetric synthesis via organocatalysis. Chem. Soc. Rev. 2017, 46, 1675–1692. [Google Scholar] [CrossRef] [PubMed]
  5. Hughes, D.L. Biocatalysis in Drug Development-Highlights of the Recent Patent Literature. Org. Proc. Res. Dev. 2018, 22, 1063–1080. [Google Scholar] [CrossRef]
  6. Connon, S.J. Asymmetric catalysis with bifunctional cinchona alkaloid-based urea and thiourea organocatalysts. Chem. Commun. (Camb.) 2008, 22, 2499–2510. [Google Scholar] [CrossRef] [PubMed]
  7. Yoon, T.P.; Jacobsen, E.N. Privileged chiral catalysts. Science 2003, 299, 1691–1693. [Google Scholar] [CrossRef] [PubMed]
  8. Yeboah, E.M.O.; Yeboah, S.O.; Singh, G.S. Recent applications of Cinchona alkaloids and their derivatives as catalysts in metal-free asymmetric synthesis. Tetrahedron 2011, 67, 1725–1762. [Google Scholar] [CrossRef]
  9. Kato, K.; Zhang, M.R.; Suzuki, K. Synthesis of (R,S)-[4-C-11]baclofen via Michael addition of nitromethane labeled with short-lived C-11. Bioorg. Med. Chem. Lett. 2009, 19, 6222–6224. [Google Scholar] [CrossRef]
  10. Hajzer, V.; Fisera, R.; Latika, A.; Durmis, J.; Kollar, J.; Frecer, V.; Tucekova, Z.; Miertus, S.; Kostolansky, F.; Vareckova, E.; et al. Stereoisomers of oseltamivir—Synthesis, in silico prediction and biological evaluation. Org. Biomol. Chem. 2017, 15, 1828–1841. [Google Scholar] [CrossRef]
  11. Jung, J.C.; Park, O.S. Efficient asymmetric synthesis of prostaglandin E-1. Z Naturforsch B 2007, 62, 556–560. [Google Scholar] [CrossRef]
  12. Okino, T.; Hoashi, Y.; Furukawa, T.; Xu, X.N.; Takemoto, Y. Enantio- and diastereoselective Michael reaction of 1,3-dicarbonyl compounds to nitroolefins catalyzed by a bifunctional thiourea. J. Am. Chem. Soc. 2005, 127, 119–125. [Google Scholar] [CrossRef] [PubMed]
  13. Hamza, A.; Schubert, G.; Soos, T.; Papai, I. Theoretical studies on the bifunctionality of chiral thiourea-based organocatalysts: Competing routes to C-C bond formation. J. Am. Chem. Soc. 2006, 128, 13151–13160. [Google Scholar] [CrossRef] [PubMed]
  14. Kotai, B.; Kardos, G.; Hamza, A.; Farkas, V.; Papai, I.; Soos, T. On the Mechanism of Bifunctional Squaramide- Catalyzed Organocatalytic Michael Addition: A Protonated Catalyst as an Oxyanion Hole. Chem. Eur. J. 2014, 20, 5631–5639. [Google Scholar] [CrossRef] [PubMed]
  15. Varga, E.; Mika, L.T.; Csampai, A.; Holczbauer, T.; Kardos, G.; Soos, T. Mechanistic investigations of a bifunctional squaramide organocatalyst in asymmetric Michael reaction and observation of stereoselective retro-Michael reaction. RSC Adv. 2015, 5, 95079–95086. [Google Scholar] [CrossRef]
  16. Grayson, M.N. Mechanism and Origins of Stereoselectivity in the Cinchona Thiourea- and Squaramide-Catalyzed Asymmetric Michael Addition of Nitroalkanes to Enones. J. Org. Chem. 2017, 82, 4396–4401. [Google Scholar] [CrossRef] [PubMed]
  17. Porta, R.; Benaglia, M.; Coccia, F.; Cozzi, F.; Puglisi, A. Solid Supported 9-Amino-9-deoxy-epi-quinine as Efficient Organocatalyst for Stereoselective Reactions in Batch and Under Continuous Flow Conditions. Adv. Synth. Catal. 2015, 357, 377–383. [Google Scholar] [CrossRef]
  18. Kacprzak, K.M.; Maier, N.M.; Lindner, W. Highly efficient immobilization of Cinchona alkaloid derivatives to silica gel via click chemistry. Tetrahedron Lett. 2006, 47, 8721–8726. [Google Scholar] [CrossRef]
  19. Lee, J.W.; Mayer-Gall, T.; Opwis, K.; Song, C.E.; Gutmann, J.S.; List, B. Organotextile Catalysis. Science 2013, 341, 1225–1229. [Google Scholar] [CrossRef]
  20. Parvez, M.M.; Haraguchi, N.; Itsuno, S. Synthesis of Cinchona Alkaloid-Derived Chiral Polymers by Mizoroki-Heck Polymerization and Their Application to Asymmetric Catalysis. Macromolecules 2014, 47, 1922–1928. [Google Scholar] [CrossRef]
  21. Kohout, M.; Wernisch, S.; Tuma, J.; Hettegger, H.; Picha, J.; Lindner, W. Effect of different immobilization strategies on chiral recognition properties of Cinchona-based anion exchangers. J. Sep. Sci. 2018, 41, 1355–1364. [Google Scholar] [CrossRef]
  22. Fredriksen, K.A.; Kristensen, T.E.; Hansen, T. Combined bead polymerization and Cinchona organocatalyst immobilization by thiol-ene addition. Beilstein J. Org. Chem. 2012, 8, 1126–1133. [Google Scholar] [CrossRef] [PubMed]
  23. Song, C.E.; Blaser, H.-U.; Ager, D.J.; Deshmukh, R.R.; Ryu, D.H.; Shin, U.S.; Lee, J.E.; Yang, J.W.; Park, H.-G.; Jeong, B.-S.; et al. Cinchona Alkaloids in Synthesis and Catalysis, 1st ed.; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2009; pp. 1–506. [Google Scholar]
  24. Kumpuga, B.T.; Itsuno, S. Synthesis of Crosslinked Chiral Polysiloxanes of Cinchona Alkaloid Derivatives and their Applications in Asymmetric Catalysis. Asian J. Org. Chem. 2019, 8, 251–256. [Google Scholar] [CrossRef]
  25. Etzenbach-Effers, K.; Berkessel, A. Noncovalent Organocatalysis Based on Hydrogen Bonding: Elucidation of Reaction Paths by Computational Methods. Top. Curr. Chem. 2009, 291, 1–27. [Google Scholar]
  26. Knowles, R.R.; Jacobsen, E.N. Attractive noncovalent interactions in asymmetric catalysis: Links between enzymes and small molecule catalysts. Proc. Natl. Acad. Sci. USA 2010, 107, 20678–20685. [Google Scholar] [CrossRef] [PubMed]
  27. Doyle, A.G.; Jacobsen, E.N. Small-molecule H-bond donors in asymmetric catalysis. Chem. Rev. 2007, 107, 5713–5743. [Google Scholar] [CrossRef] [PubMed]
  28. Didaskalou, C.; Kupai, J.; Cseri, L.; Barabas, J.; Vass, E.; Holtzl, T.; Szekely, G. Membrane-Grafted Asymmetric Organocatalyst for an Integrated Synthesis-Separation Platform. ACS Catal. 2018, 8, 7430–7438. [Google Scholar] [CrossRef]
  29. Kisszekelyi, P.; Alammar, A.; Kupai, J.; Huszthy, P.; Barabas, J.; Holtzl, T.; Szente, L.; Bawn, C.; Adams, R.; Szekely, G. Asymmetric synthesis with cinchona-decorated cyclodextrin in a continuous-flow membrane reactor. J. Catal. 2019, 371, 255–261. [Google Scholar] [CrossRef]
  30. Nagy, S.; Dargo, G.; Kisszekelyi, P.; Feher, Z.; Simon, A.; Barabas, J.; Holtzl, T.; Matravolgyi, B.; Karpati, L.; Drahos, L.; et al. New enantiopure binaphthyl-cinchona thiosquaramides: Synthesis and application for enantioselective organocatalysis. New J. Chem. 2019, 43, 5948–5959. [Google Scholar] [CrossRef]
  31. Vakulya, B.; Varga, S.; Csampai, A.; Soos, T. Highly enantioselective conjugate addition of nitromethane to chalcones using bifunctional cinchona organocatalysts. Org. Lett. 2005, 7, 1967–1969. [Google Scholar] [CrossRef]
  32. Malerich, J.P.; Hagihara, K.; Rawal, V.H. Chiral Squaramide Derivatives are Excellent Hydrogen Bond Donor Catalysts. J. Am. Chem. Soc. 2008, 130, 14416–14417. [Google Scholar] [CrossRef]
  33. Jakab, G.; Tancon, C.; Zhang, Z.G.; Lippert, K.M.; Schreiner, P.R. (Thio)urea Organocatalyst Equilibrium Acidities in DMSO. Org. Lett. 2012, 14, 1724–1727. [Google Scholar] [CrossRef] [PubMed]
  34. Ni, X.; Li, X.; Cheng, J.P. Equilibrium acidities of cinchona alkaloid organocatalysts bearing 6′-hydrogen bonding donors in DMSO. Org. Chem. Front. 2016, 3, 170–176. [Google Scholar] [CrossRef]
  35. Ni, X.; Li, X.; Wang, Z.; Cheng, J.P. Squaramide Equilibrium Acidities in DMSO. Org. Lett. 2014, 16, 1786–1789. [Google Scholar] [CrossRef] [PubMed]
  36. Xue, X.S.; Yang, C.; Li, X.; Cheng, J.P. Computational Study on the pK(a) Shifts in Proline Induced by Hydrogen-Bond-Donating Cocatalysts. J. Org. Chem. 2014, 79, 1166–1173. [Google Scholar] [CrossRef] [PubMed]
  37. Zwicker, V.E.; Yuen, K.K.Y.; Smith, D.G.; Ho, J.M.; Qin, L.; Turner, P.; Jolliffe, K.A. Deltamides and Croconamides: Expanding the Range of Dual H-bond Donors for Selective Anion Recognition. Chem. Eur. J. 2018, 24, 1140–1150. [Google Scholar] [CrossRef]
  38. Ho, J.M.; Zwicker, V.E.; Yuen, K.K.Y.; Jolliffe, K.A. Quantum Chemical Prediction of Equilibrium Acidities of Ureas, Deltamides, Squaramides, and Croconamides. J. Org. Chem. 2017, 82, 10732–10736. [Google Scholar] [CrossRef] [PubMed]
  39. Busschaert, N.; Elmes, R.B.P.; Czech, D.D.; Wu, X.; Kirby, I.L.; Peck, E.M.; Hendzel, K.D.; Shaw, S.K.; Chan, B.; Smith, B.D.; et al. Thiosquaramides: pH switchable anion transporters. Chem. Sci. 2014, 5, 3617–3626. [Google Scholar] [CrossRef]
  40. Lu, T.X.; Wheeler, S.E. Origin of the Superior Performance of (Thio)Squaramides over (Thio)Ureas in Organocatalysis. Chem. Eur. J. 2013, 19, 15141–15147. [Google Scholar] [CrossRef]
  41. Rombola, M.; Sumaria, C.S.; Montgomery, T.D.; Rawal, V.H. Development of Chiral, Bifunctional Thiosquaramides: Enantioselective Michael Additions of Barbituric Acids to Nitroalkenes. J. Am. Chem. Soc. 2017, 139, 5297–5300. [Google Scholar] [CrossRef]
  42. Rombola, M.; Rawal, V.H. Dicyclopentyl Dithiosquarate as an Intermediate for the Synthesis of Thiosquaramides. Org. Lett. 2018, 20, 514–517. [Google Scholar] [CrossRef]
  43. Braje, W.M.; Frackenpohl, J.; Schrake, O.; Wartchow, R.; Beil, W.; Hoffmann, H.M.R. Synthesis of 10,11-didehydro Cinchona alkaloids and key derivatives. Helv. Chim. Acta 2000, 83, 777–792. [Google Scholar] [CrossRef]
  44. McCooey, S.H.; Connon, S.J. Readily accessible 9-epi-amino cinchona alkaloid derivatives promote efficient, highly enantioselective additions of aldehydes and ketones to nitroolefins. Org. Lett. 2007, 9, 599–602. [Google Scholar] [CrossRef] [PubMed]
  45. Vakulya, B.; Varga, S.; Soos, T. Epi-cinchona based thiourea organocatalyst family as an efficient asymmetric Michael addition promoter: Enantioselective conjugate addition of nitroalkanes to chalcones and alpha,beta-unsaturated N-acylpyrroles. J. Org. Chem. 2008, 73, 3475–3480. [Google Scholar] [CrossRef] [PubMed]
  46. del Pozo, S.; Vera, S.; Oiarbide, M.; Palomo, C. Catalytic Asymmetric Synthesis of Quaternary Barbituric Acids. J. Am. Chem. Soc. 2017, 139, 15308–15311. [Google Scholar] [CrossRef] [PubMed]
  47. Bae, H.Y.; Some, S.; Lee, J.H.; Kim, J.Y.; Song, M.J.; Lee, S.; Zhang, Y.J.; Song, C.E. Organocatalytic Enantioselective Michael-Addition of Malonic Acid Half-Thioesters to beta-Nitroolefins: From Mimicry of Polyketide Synthases to Scalable Synthesis of gamma-Amino Acids. Adv. Synth. Catal. 2011, 353, 3196–3202. [Google Scholar] [CrossRef]
  48. Grosseheilmann, J.; Kragl, U. Simple and Effective Catalyst Separation by New CO2-Induced Switchable Organocatalysts. ChemSusChem 2017, 10, 2685–2691. [Google Scholar] [CrossRef]
  49. Madarasz, A.; Dosa, Z.; Varga, S.; Soos, T.; Csampai, A.; Papai, I. Thiourea Derivatives as Bronsted Acid Organocatalysts. ACS Catal. 2016, 6, 4379–4387. [Google Scholar] [CrossRef]
  50. Hammar, P.; Marcelli, T.; Hiemstra, H.; Himo, F. Density functional theory study of the Cinchona thiourea-catalyzed henry reaction: Mechanism and enantioselectivity. Adv. Synth. Catal. 2007, 349, 2537–2548. [Google Scholar] [CrossRef]
  51. Quinonero, D.; Frontera, A.; Suner, G.A.; Morey, J.; Costa, A.; Ballester, P.; Deya, P.M. Squaramide as a binding unit in molecular recognition. Chem. Phys. Lett. 2000, 326, 247–254. [Google Scholar] [CrossRef]
  52. Quinonero, D.; Prohens, R.; Garau, C.; Frontera, A.; Ballester, P.; Costa, A.; Deya, P.M. A theoretical study of aromaticity in squaramide complexes with anions. Chem. Phys. Lett. 2002, 351, 115–120. [Google Scholar] [CrossRef]
  53. Allen, R.I.; Box, K.J.; Comer, J.E.; Peake, C.; Tam, K.Y. Multiwavelength spectrophotometric determination of acid dissociation constants of ionizable drugs. J. Pharm. Biomed. Anal. 1998, 17, 699–712. [Google Scholar] [CrossRef]
  54. Tam, K.Y.; Takacs-Novak, K. Multi-wavelength spectrophotometric determination of acid dissociation constants: A validation study. Anal. Chim. Acta 2001, 434, 157–167. [Google Scholar] [CrossRef]
  55. Avdeef, A.; Comer, J.E.A.; Thomson, S.J. Ph-Metric Log. 3. Glass-Electrode Calibration in Methanol Water, Applied to Pka Determination of Water-Insoluble Substances. Anal. Chem. 1993, 65, 42–49. [Google Scholar] [CrossRef]
  56. TakacsNovak, K.; Box, K.J.; Avdeef, A. Potentiometric pK(a) determination of water-insoluble compounds: Validation study in methanol/water mixtures. Int. J. Pharm. 1997, 151, 235–248. [Google Scholar] [CrossRef]
  57. Hanwell, M.D.; Curtis, D.E.; Lonie, D.C.; Vandermeersch, T.; Zurek, E.; Hutchison, G.R. Avogadro: An advanced semantic chemical editor, visualization, and analysis platform. J. Cheminform. 2012, 4, 17. [Google Scholar] [CrossRef]
  58. Chai, J.D.; Head-Gordon, M. Long-range corrected hybrid density functionals with damped atom-atom dispersion corrections. Phys. Chem. Chem. Phys. 2008, 10, 6615–6620. [Google Scholar] [CrossRef] [PubMed]
  59. Hariharan, P.C.; Pople, J.A. The influence of polarization functions on molecular orbital hydrogenation energies. Theor. Chim. Acta 1973, 28, 213–222. [Google Scholar] [CrossRef]
  60. Shao, Y.H.; Gan, Z.T.; Epifanovsky, E.; Gilbert, A.T.B.; Wormit, M.; Kussmann, J.; Lange, A.W.; Behn, A.; Deng, J.; Feng, X.T.; et al. Advances in molecular quantum chemistry contained in the Q-Chem 4 program package. Mol. Phys. 2015, 113, 184–215. [Google Scholar] [CrossRef]
  61. Cramer, C.J.; Truhlar, D.G. A universal approach to solvation modeling. Acc. Chem. Res. 2008, 41, 760–768. [Google Scholar] [CrossRef]
  62. Chipman, D.M.; Dupuis, M. Implementation of solvent reaction fields for electronic structure. Theor. Chem. Acc. 2002, 107, 90–102. [Google Scholar] [CrossRef]
  63. Marenich, A.V.; Cramer, C.J.; Truhlar, D.G. Generalized Born Solvation Model SM12. J. Chem. Theory Comput. 2013, 9, 609–620. [Google Scholar] [CrossRef] [PubMed]
  64. Zhan, C.G.; Dixon, D.A. Absolute hydration free energy of the proton from first-principles electronic structure calculations. J. Phys. Chem. A 2001, 105, 11534–11540. [Google Scholar] [CrossRef]
  65. The PyMOL Molecular Graphics System, V.S.; LLC Schrodinger: New York, NY, USA, 2010.
  66. Palacio, C.; Connon, S.J. A New Class of Urea-Substituted Cinchona Alkaloids Promote Highly Enantioselective Nitroaldol reactions of Trifluoromethylketones. Org. Lett. 2011, 13, 1298–1301. [Google Scholar] [CrossRef] [PubMed]
Figure 1. Two possible mechanisms (a and b) for Michael addition in the presence of bifunctional H-bond organocatalysts.
Figure 1. Two possible mechanisms (a and b) for Michael addition in the presence of bifunctional H-bond organocatalysts.
Materials 12 03034 g001
Figure 2. The possible direct modifications on the vinyl or ethynyl group of quinuclidine unit.
Figure 2. The possible direct modifications on the vinyl or ethynyl group of quinuclidine unit.
Materials 12 03034 g002
Scheme 1. The synthesis of cinchona-based organocatalysts.
Scheme 1. The synthesis of cinchona-based organocatalysts.
Materials 12 03034 sch001
Scheme 2. Michael Addition of pentane-2,4-dione (2) to trans-β-nitrostyrene (1).
Scheme 2. Michael Addition of pentane-2,4-dione (2) to trans-β-nitrostyrene (1).
Materials 12 03034 sch002
Figure 3. Enantioselectivities given in different solvents when squaramides (a) or thioureas (b) were applied in Michael addition.
Figure 3. Enantioselectivities given in different solvents when squaramides (a) or thioureas (b) were applied in Michael addition.
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a Reaction conditions: pentane-2,4-dione (2) (0.41 mmol) was added to the solution of trans-β-nitrostyrene (1) (0.16 mmol) and 5 mol% catalysts in 1 mL of solvent, then the resulting mixture was stirred at room temperature for 24 h; b Isolated yield; c Determined by chiral HPLC (S enantiomer).
Figure 4. The significant neutral, deprotonated, and protonated tautomer forms of thioureas.
Figure 4. The significant neutral, deprotonated, and protonated tautomer forms of thioureas.
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Figure 5. The bifunctional sites of the represented cinchona organocatalysts.
Figure 5. The bifunctional sites of the represented cinchona organocatalysts.
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Figure 6. Structure of HQ.
Figure 6. Structure of HQ.
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Figure 7. Structure of DQ-N.
Figure 7. Structure of DQ-N.
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Figure 8. Structure of DQ-SQ.
Figure 8. Structure of DQ-SQ.
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Table 1. The pKa values of the conjugate acids of basic quinuclidine tertiary amine part in cinchona catalysts a.
Table 1. The pKa values of the conjugate acids of basic quinuclidine tertiary amine part in cinchona catalysts a.
EntryCatalystbpKaEntryCatalystbpKa
1HQ9.107HQ-SQ7.86
2Q8.528Q-SQ6.95
3DQ7.409DQ-SQ6.13
4HQ-N9.7110HQ-TU8.61
5Q-N9.1011Q-TU8.06
6DQ-N8.1012DQ-TU7.02
a Conditions: water, 25 °C; b The structures are shown in Scheme 1.
Table 2. The pKa values of acidic squaramide or thiourea units measured in water.
Table 2. The pKa values of acidic squaramide or thiourea units measured in water.
EntryCatalystpKa, NH1pKa, NH2
1HQ-SQ9.5711.73
2Q-SQ9.29>12#
3DQ-SQ9.20>12#
4HQ-TU10.87 12 #
5Q-TU10.84 12 #
6DQ-TU10.78 12 #
# nonmeasurable by UV-spectrophotometric titration method.
Table 3. Catalysts applied in Michael addition using trans-β-nitrostyrene (1) and pentane-2,4-dione (2) a.
Table 3. Catalysts applied in Michael addition using trans-β-nitrostyrene (1) and pentane-2,4-dione (2) a.
EntryCatalystSolventYield [%]bee [%]c
1HQ-SQDCM9985
2HQ-SQMTBE10075
3HQ-SQtoluene10082
4HQ-SQMeCN9784
5HQ-SQEtOAc9981
6HQ-SQMeOH7745
7HQ-SQTHF9985
8Q-SQDCM9887
9Q-SQMTBE9990
10Q-SQtoluene9989
11Q-SQMeCN9986
12Q-SQEtOAc10089
13Q-SQMeOH7651
14Q-SQTHF9981
15DQ-SQDCM9989
16DQ-SQMTBE7682
17DQ-SQtoluene8790
18DQ-SQMeCN8880
19DQ-SQEtOAc8983
20DQ-SQMeOH7638
21DQ-SQTHF9570
22HQ-TUDCM9387
23HQ-TUMTBE9991
24HQ- TUtoluene9590
25HQ- TUMeCN9484
26HQ- TUEtOAc9986
27HQ- TUMeOH7625
28HQ- TUTHF9778
29Q- TUDCM10085
30Q- TUMTBE9989
31Q- TUtoluene10091
32Q- TUMeCN9978
33Q- TUEtOAc10085
34Q- TUMeOH8225
35Q- TUTHF9982
36DQ- TUDCM9891
37DQ- TUMTBE9766
38DQ- TUtoluene9993
39DQ- TUMeCN8573
40DQ- TUEtOAc8086
41DQ- TUMeOH7617
42DQ- TUTHF9890
Table 4. Relative Gibbs free energies (kJ·mol−1) of the energetically lowest-lying tautomers of thioureas (HQ-TU, Q-TU, DQ-TU) and their deprotonated anion and protonated cation forms in THF.
Table 4. Relative Gibbs free energies (kJ·mol−1) of the energetically lowest-lying tautomers of thioureas (HQ-TU, Q-TU, DQ-TU) and their deprotonated anion and protonated cation forms in THF.
CompoundNeutralDeprotonated (anion)Protonated (cation)
iiiiiiivvviviiviii
HQ-TU03259640 (150)9 (159)0 (−44)62 (−14)
Q-TU03263650 (148)10 (157)0 (−46)69 (−9)
DQ-TU04066700 (144)12 (157)0 (−40)64 (−9)
Protonation and deprotonation Gibbs free energies of the corresponding neutral tautomer are indicated in parentheses. (ωB97X-D/6-311++G**+SM12// ωB97X-D/6-31G*+SM8 method; see Figure 4 for notation).
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