Unveiling Mechanism of a Quinine-Squaramide Catalyzed

Article pubs.acs.org/joc

Unveiling Mechanism of a Quinine-Squaramide Catalyzed Enantioselective Aza-Friedel−Crafts Reaction between Cyclic Trifluoromethyl Ketimine and Naphthol: A DFT Study Ping Wang, Yun Gao, Yang Zhao, Wei Liu,* and Yong Wang* College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, PR China S Supporting Information *

ABSTRACT: A mechanism study of quinine-squaramide catalyzed enantioselective aza-Friedel−Crafts (aza-F−C) reaction is described using density functional theory (DFT). The most favorable pathway is obtained through the discussions of four possible modes of hydrogen bond interactions, in which the nucleophile is activated by the squaramide N−H groups (N−Ha and N−Hb) and the electrophile binds to the protonated amine by hydrogen bonding. Meanwhile, we have also studied the energy barrier of the stereocontrolling transition states that might play a role of stereoselectivity. In addition, noncovalent interaction (NCI) analyses show a series of favorable cooperative noncovalent interactions, including N−H···O and C−H···F hydrogen-bonding, and π···π interactions. The strong interactions and lower barrier were found for TS3R, indicating the preference for the R-configuration adduct, which is in good agreement with the experimental observations.

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INTRODUCTION Over the years, dual hydrogen bond catalyzes, such as chiral thiourea and squaramide, have been widely applied as asymmetric catalysts in organic synthesis.1 In 2005, thiourea as an asymmetric organocatalyst was discovered by Chen,2 Soós,3 Connon,4 and Dixon.5 As a consequence, thiourea and its derivatives were applied in a wide range of asymmetric reactions including Michael addition,6 Mannich rection,7 Strecker,8 Henry,9 among others.10 Subsequently, in 2008, the group of Rawal11 reported a chiral squaramide-based organocatalyst catalyzed Michael addition of 1,3-dicarbonyl compounds to nitro olefins, which performed a highly chemical efficiency and stereocontrol. Since then, squaramide and its derivatives have also been widely used and developed in a broad range of asymmetric reactions, such as Mannich reaction,12 Friedel−Crafts reaction,13 Michael addition,14 among others.15 In 2008, Rawal determined that the distance between the two N−H groups in the squaramide is about 0.6 Å greater than that in thiourea, as determined by crystallographic structure and computational data.11 As well as the structure feature, the distinctly different pKa values show that squaramide is more acidic. Therefore, squaramide easily engages in stronger hydrogen bonds than the corresponding thiourea.16 Recently, the groups of Pihko and Pápai17 reported the foldamer catalyst which contain urea, thiourea, and tertiary amine groups catalyzed Mannich reactions with aliphatic imine and malonate esters. Of note, the folded structure of catalyst generates an active site which allows additional stabilizing interaction with the substrates, promoting highly enantioselectivity. First in 2017, Rawal proposed that thiosquaramides catalyzed the © 2017 American Chemical Society

conjugate addition reaction of the barbituric acid pharmacaphore to nitroalkenes with a high yield and excellent enantioselectivity, in which thiosquaramides are more acidic and more soluble in nonpolar solvents than in the case of thioureas and squaramides.18 With the increasing use of a bifunctional organocatalyst in organic asymmetric reactions, the catalytic mechanism is attracting more and more attention experimentally and theoretically. Takemoto et al. suggested a dual activation mechanism, where the quinuclidine amine of catalyst deprotonates an acidic proton of the nucleophile, and sequentially the thiourea activates the electrophile.19 Whereas, Pápai et al. proposed a different activation mechanism, in which the NH groups of the thiourea moiety simultaneously activate nucleophile, the electrophile binds to the N−H of the protonated amine.20 After that, a number of mechanistic studies have been carried out to explore the bifunctional organocatalyst catalyzed asymmetric reactions.21 The group of Wang reported a Michael reaction between α,β-unsaturated γbutyrolactam and chalcone catalyzed by cinchona alkaloid thiourea,22 in which the one N−H of thiourea moiety and the N−H of the protonated amine simultaneously activate α,βunsaturated γ-butyrolactam, then the other N−H of thiourea moiety activates chalcone though hydrogen bond. In 2017, Wong23 revealed that enantioselective sulfa-Michael addition reaction catalyzed by a cinchona alkaloid-squaramide proceeds through a mode of bifunctional activation, in which the NH Received: August 28, 2017 Published: November 11, 2017 13109

DOI: 10.1021/acs.joc.7b02168 J. Org. Chem. 2017, 82, 13109−13114

Article

The Journal of Organic Chemistry

nucleophilicity index36,37c N taking the HOMO energy of tetracyanoethylene (TCE) as a reference was defined as N = EHOMO(R) − EHOMO(TCE) by Domingo and co-workers,37 which was calculated basing on the HOMO energies of reactants. The HOMO and LUMO orbital energy, electronic chemical potential (μ), chemical hardness (η), electrophilicity (ω), and nucleophilicity (N) indices for reactants (R1, R2) are listed in Table 1, respectively. Obviously, the electrophilicity of R1 is

groups of the aquaramide simultaneously bind the nucleophile, while the N−H of the protonated amine activates the electrophile. Recently, we reported the reaction mechanism of an asymmetric conjugate addition of dimethyl phosphites to isatylidene malononitriles catalyzed by bifunctional tertiary amine−thiourea.24 As a continuation of our interests in investigating the mechanism of organocatalytic asymmetric reactions, herein we report the mechanisms of the quininesquaramide catalyzed aza-Friedel−Crafts (aza-F−C) reaction between cyclic trifluoromethyl ketimine and naphthol (Scheme 1).25

Table 1. HOMO (EHOMO) and LUMO (ELUMO) Energies, Electronic Chemical Potential (μ), Chemical Hardness (η), Electrophilicity (ω), and Nucleophilicity (N) Indices for R1 and R2

Scheme 1. Asymmetric Friedel−Crafts Reaction of Cyclic Trifluoromethyl Ketimine and Naphthol Catalyzed by Quinine-Squaramide

R1 R2

EHOMO(eV)

ELUMO(eV)

μ (eV)

η (eV)

ω (eV)

N (eV)

−6.160 −5.423

−2.570 −0.765

−4.365 −3.094

3.590 4.658

2.654 1.028

4.311 5.048

stronger than that of R2. Correspondingly, the nucleophilicity of R2 is stronger. Therefore, it can be expected that R1 could be susceptible to nucleophilic attack. Accordingly, the R2 could be susceptible to electrophilic attack in the reaction. Overall Catalytic Cycle. For the Friedel−Crafts reaction, the whole catalytic process involves three major steps through calculations: (1) The deprotonation step of naphthol (Nu) by quinuclidine amine of Cat, forming the Cat-Nu complex. (2) The C−C bond formation process. (3) Generation of the final product and recovery of Cat. All pathways for the reaction and the relative Gibbs free energy profiles are displayed in the following sections, in which the Gibbs free energy of Cat+Nu +EI was set as 0.0 kcal/mol. Deprotonation of Naphthol (Nu). Through DFT calculations, we successfully located some relatively stable catalystsubstrate intermediates. First, the Cat can easily interact with naphthol to form a Cat-Nu complex (Int1). Meanwhile, three corresponding unstable Cat-EI complexes Int2′, Int3′, and Int4′ with different hydrogen bonds between Cat and cyclic trifluoromethyl ketimine (EI) are also located (Figure S1). The binding energies of Cat-EI complexes Int2′, Int3′, and Int4′ are 0.5, 1.0, and 1.2 kcal/mol, respectively. In contrast, the binding energy of Cat-Nu complex is −2.5 kcal/mol, indicating a favorable pathway through Cat-Nu to Int1. Therefore, not surprisingly, the catalytic process starts with the deprotonation process of Nu. After the formation of the Cat-Nu, the quinuclidine amine of Cat obtains a proton from the Nu via transition state TS1, forming intermediate Int2. Transition state TS1 is reached by overcoming a relatively low activation barrier of 4.8 kcal/mol. The negligible activation barrier indicates that the protonation stage can easily occur. C−C Bond Formation. According to the two distinct activation mechanisms proposed by Takemoto19 and Pápai et al.,20 four possible activation pathways with different hydrogen bond interactions were discussed between Cat-Nu and EI (Figure 1). After the stable Cat-Nu intermediate (Int2), the coming of EI with Cat-Nu will possibly generate four different intermediates (Int3, Int4, Int5, and Int6). In path1, protons Ha and Hb of the squaramide NH groups activate the O, N of EI (length of 1.98 and 2.08 Å), respectively, while an oxygen atom of Nu coordinates to the ammonium NH function of the quinuclidine (1.50 Å). The intermediate Int3 is generated with a positive binding energy of 1.4 kcal/mol. Afterward, the deprotonated Nu attacks the EI via a transition state TS2 giving Int7, with a relatively higher

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COMPUTATIONAL DETAILS All geometry optimizations and frequency computations of the catalyst (Cat), cyclic trifluoromethyl ketimine (EI), naphthol (Nu), intermediates (Int), and transition states (TS) are performed using the hybrid density functional method B3LYP26 with the moderate 6-31G(d)27,28 basis set as implemented in the Gaussian 09 suite of program.29 The harmonic vibrational frequency computations justify that the nature of all transition-state structures possesses only one imaginary frequency and no imaginary frequency characterize the intermediates. In addition, we follow the intrinsic reactant coordinates (IRC)30 pathways to verify whether the correct configurations of reactant and product are connected by transition state structures on the potential energy surface. The single-point electronic energy of all optimized structures in the solvent phase (CH2Cl2) is further calculated by using m062x with much higher basis set 6-311G(d,p). Visualization of noncovalent interaction states is carried out using the NCI plot.31 The data of NCI isosurfaces are produced toward the Multiwfn program32 and are shown by the VMD program.33 The strength of noncovalent interactions is indicated by the color of the isosurface. The blue, green, and red represent strong, weak, and repulsive interaction, respectively. Optimized structures are generated using the CYL view program34 (as shown in Supporting Information).

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RESULTS AND DISCUSSION Reactivity of Reactants. Based on the optimized structures of reactants R1 and R2, the electronic chemical potential μ and chemical hardness η of reactants35,37c are obtained using the highest occupied molecular orbital (HOMO) energy (EHOMO) and the lowest unoccupied molecular orbital (LUMO) energy (ELUMO), where is μ = (EHOMO + ELUMO)/2 and η = ELUMO − EHOMO. Accordingly, the global electrophilicity index35 ω is measured by ω = (μ 2 /2η). The empirical (relative) 13110


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Figure 1. Free-energy profile diagram for the deprotonation of naphthol and the C−C bond formation at the level of B3LYP/6-31G(d).

with simultaneous regeneration of catalyst. Starting from a transient intermediate Int8, two possible pathways were investigated through the hydrogen bond between the protonated amine of catalyst and N or O of trifluoromethyl ketimine. For the O-site pathway, as depicted in Figure 2, intermediate O-Int11 (ΔG = 5.1 kcal/mol) is located by the protonated amine combines with O of trifluoromethyl ketimine via N−H···O hydrogen bond undergoing transition state OTS6, whose negligible activation barrier is −2.1 kcal/mol. Then, the intermediate O-Int11 undergoes an endothermic isomerization to O-Int12 by increasing the energy of 16.0 kcal/mol. After that, O-Int13 is located after the proton transfer through transition state O-TS7 with an unfavorable higher barrier of 28.2 kcal/mol. For the N-site pathway, the protonated amine of catalyst interacts with N of trifluoromethyl ketimine forming the N−H···N hydrogen bond through transition state N-TS6 (6.0 kcal/mol) giving intermediate N-Int11 by increasing favorable energy of 2.5 kcal/mol. After that, we tried to find final product through keto−enol tautomerization. Unfortunately, according to the experiment hypothesis, a very higher energy barrier of 62.8 kcal/mol is calculated, indicating an unfavorable pathway for keto−enol tautomerization. As expected, the tertiary amine group in the chiral scaffold could interact with naphthol through calculations. As shown in Figure 2, intermediate N-Int11 goes through an isomerization to forming a more stable intermediate N-Int12 by lowering energy of 3.2 kcal/mol. After N-Int12, intermediate N-Int13 is generated via transition state N-TS7 with a relatively lower energy barrier of 10.8 kcal/mol than that of O-TS7 (28.2 kcal/

activation barrier of 13.1 kcal/mol. In path 2, Int4 is generated though the NH groups of squaramide simultaneously activate Nu (N−Ha: 1.82 Å; N−Hb: 1.79 Å), while the N−H group of the protonated amine activates EI (1.80 Å). After Int4, the new C−C bond formation between Nu and EI takes place via transition state TS3 with a relatively lower barrier of 9.5 kcal/ mol than that of TS2. After TS3, intermediate Int8 is located. In this step, the C···C distances are shortened to 3.23 Å (Int4), 1.99 Å (TS3), and 1.69 Å (Int8), respectively. In path 3, one N−H group of squaramide (N−Ha) and the N−H of protonated amine simultaneously interact with Nu via a bidentate hydrogen bond (Ha−O, 1.85 Å and H−O, 1.54 Å), Meanwhile, the other N−H group (N−Hb) activates EI via a monodentate hydrogen bond (Hb−O, 1.95 Å) giving intermediate Int5. After Int5, the new C−C bond is formed through transition state TS4, whose activation barrier is 16.2 kcal/mol. In path 4, transition state TS5 connects intermediates Int6 and Int10 with the highest Gibbs activation barrier of 24.1 kcal/mol. Therefore, as depicted in Figure 1, the activation barrier of the rate-determining steps takes the sequence of TS3 (path 2, 9.5 kcal/mol) < TS2 (path 1, 13.1 kcal/mol) < TS4 (path 3, 16.2 kcal/mol) < TS5 (path 4, 24.1 kcal/mol). In other words, path 2 is kinetically most favorable. In addition, because of the negative charge transfer from Nu to EI in the C−C formation process, the EI is bound to the protonated amine, which could stabilize the negative charge developing on the unit upon C−C bond formation.22,38 Generation of the Final Product and Recovery of Cat. During this step, proton transfer furnishes the final product 13111


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Figure 2. Gibbs free energy profile (ΔG) of O-site and N-site pathways.

Figure 3. Transition states TS3R and TS3S for C−C bond formation. The relative Gibbs energies are given in kcal/mol and intermolecular distances are in Å.

state TS3S. As shown in Figure 3, the barrier of TS3R (9.5 kcal/ mol) is lower than that of TS3S (19.8 kcal/mol) by 10.3 kcal/ mol. Therefore, as expected, the R-configuration adduct is dominant in the final products, which is in good agreement with the experimental observations. Since the process from Int4 to Int8 through TS3 of C−C bond formation is the rate-determining step, the relative stability of TS3R and TS3S is crucial. The noncovalent interactions (NCI) are employed to identify the intermolecular interaction within TS3R and TS3S, and the relative NCI plots are depicted in Figure 4. For the graphical depiction of NCI, the color is representative of the nature and strength. The blue

mol). In the following step, intermediate N-Int14 is easily located through N-Int13 deprotonation via the transition state N-TS8 by negligible barrier of −0.5 kcal/mol. Finally, the formation of the final R-configuration product and the recovery of Cat could occur. As discussed above, the N-site pathway is more favorable than the O-site pathway. Analysis of Stereoselectivity through NCI. As mentioned before, the rate-determining step of the total reaction is C−C bond formation. After the Nu attacks EI from Re-face, Rconfiguration of adduct could be obtained through transition state TS3R. In contrast, S-configuration of the product is obtained after the Si-face of EI attacked by Nu via transition 13112


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Figure 4. NCI analyses of transition states TS3R and TS3S. The blue, green, and red surfaces indicate strong attraction, weak interaction, and steric effects, respectively. The isosurface value is 0.6 au, and the color scale is from −0.04 to 0.04 au.

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regions represent strong attractive interactions, green regions show dispersion or weak noncovalent interactions and red regions express repulsive interaction. As shown in Figure 4, the dark blue surfaces between the squaramide and the substrates in TS3R mean strong interactions, indicating the stronger N− H···O interactions in TS3R than those in TS3S. Clearly, the activation energy barrier takes the sequence of TS3S (19.8 kcal/ mol) > TS3R (9.5 kcal/mol). Moreover, in TS3R, the larger green clouds between the Nu and EI contribute to stronger π···π interactions. Meanwhile, the stronger C−H···F interactions are also observed between the squaramide and EI in TS3R. In addition, the stronger steric effect is found in TS3S. Therefore, not surprisingly, the differential noncovalent interactions, namely N−H···O interactions, π···π interactions, C−H···F interactions, presenting in transition states could be the most important factors leading to the barrier difference. Consequently, the attack of the Re-face of EI by Nu results in the R-configuration of adducts corresponding to the major product, which is in good agreement with the experimental observations.

*E-mail: [email protected] *E-mail: [email protected] ORCID

Yong Wang: 0000-0002-1481-5118 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors appreciate the financial support from Starting-up Foundation (Q410900111 and Q410900211) and Scientific Research Foundation of Soochow University (SDY2012A07), and Natural Science Foundation of China (21201127). This project was funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and the project of scientific and technologic infrastructure of Suzhou (SZS201708).

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CONCLUSION In summary, we have investigated the mechanisms of organocatalyzed cyclic trifluoromethyl ketimine and naphthol. It was found that R-configuration adduct is energetically and kinetically favorable, which the squaramide NH groups (N−Ha and N−Hb) activate Nu, and the N−H group of the protonated amine activates EI, simultaneously. Through NCI analyses, more favorable noncovalent interaction, such as hydrogen bond, π···π interactions, and C−H···F interactions, between the catalyst and the substrates in transition state TS3R result in the excellent stereoselectivity. Hopefully, our investigations could provide further insights into the understanding of hydrogen-bond-mediated catalysis in enantioselective aza-Friedel−Crafts (aza-F−C) reactions.

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AUTHOR INFORMATION

Corresponding Authors

REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.joc.7b02168. Detailed reaction figures and optimized Cartesian coordinates for all stationary points (PDF) 13113


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