Cinchona Alkaloid-Squaramide Catalyzed Sulfa-Michael Addition

Article pubs.acs.org/joc

Cinchona Alkaloid-Squaramide Catalyzed Sulfa-Michael Addition Reaction: Mode of Bifunctional Activation and Origin of Stereoinduction Jinlong Guo† and Ming Wah Wong*,†,‡ †

NUS Graduate School for Integrative Sciences and Engineering (NGS), National University of Singapore, 28 Medical Drive, Singapore 117456 ‡ Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543 S Supporting Information *

ABSTRACT: The mechanism of the enantioselective sulfaMichael addition reaction catalyzed by a cinchona alkaloidsquaramide bifunctional organocatalyst was studied using density functional theory (DFT). Four possible modes of dual activation mechanism via hydrogen bonds were considered. Our study showed that Houk’s bifunctional Brønsted acid−hydrogen bonding model, which works for cinchonidine or cinchona alkaloid-urea catalyzed sulfa-Michael addition reactions, also applies to the catalytic system under investigation. In addition, we examined the origin of the stereoselectivity by identifying stereocontrolling transition states. Distortion−interaction analysis revealed that attractive interaction between the substrates and catalyst in the C−S bond forming transition state is the key reason for stereoinduction in this catalytic reaction. Noncovalent interaction (NCI) analysis showed that a series of more favorable cooperative noncovalent interactions, namely, hydrogen bond, π-stacking, and C− H···π interaction and C−H···F interactions, in the major R-inducing transition state. The predicted enantiometric excess is in good accord with the observed value.

1. INTRODUCTION A bifunctional acid−base organocatalyst, which activates nucleophiles and electrophiles simultaneously, is most commonly used in organocatalysis.1 For sulfa-Michael addition of aromatic thiols to cycloalkenones, the discovery of natural cinchona alkaloids to achieve good enantioselectivity by Wynberg et al. in 1977 represents a major milestone in the field of hydrogen bonding organocatalysis.2 Subsequently, numerous cinchona alkaloid derivatives have been developed extensively to promote a wide range of asymmetric reactions.3 Wynberg postulated a transition state (TS) model, viz. an ion pair−hydrogen bonding model (Scheme 1), to explain the stereoselectivity of the catalytic thiol conjugate addition based on the results of NOESY NMR.2b In this TS model, a thiolatequinuclidinium tight ion pair is formed upon the deprotonation of the aromatic thiol, and the enone electrophile, activated by a hydrogen bond from the catalyst’s hydroxyl group, is attacked by the thiolate to afford the product (Scheme 1). However, in a recent computational investigation on Wynberg’s reaction,4 Houk et al. suggested that the reaction proceeds via a different mode of bifunctional activation. These authors proposed a model which involves a bifunctional Brønsted acid−hydrogen bonding activation mode, in which the enone is activated by the alkylammonium ion, while the thiolate electrophile is oriented toward the catalyst’s hydroxyl group (Scheme 1). This TS © 2017 American Chemical Society

Scheme 1. Wynberg and Houk Models of the Transition State

model is further supported by another theoretical investigation5 on sulfa-Michael addition promoted by cinchona alkaloid-urea.6 The preference for this mode of bifunctional activation is attributed to the better stabilization to developing alkoxide provided by the proton transfer from the quinuclidinium ion in this model than by the hydrogen bond interactions with urea or the hydroxyl group in Wynberg’s model.4,5 Received: February 18, 2017 Published: March 16, 2017 4362

DOI: 10.1021/acs.joc.7b00388 J. Org. Chem. 2017, 82, 4362−4368

Article

The Journal of Organic Chemistry

continuum solvation method in both optimization and single-point energy calculation. Intrinsic reaction coordinate (IRC)15 calculations were employed to confirm the corresponding species connecting to the TS’s. An ultrafine grid was employed for all DFT calculations. Unless otherwise stated, relative Gibbs energies (ΔG) reported in the text correspond to the M06-2X/def2-TZVPP//M06-2X/6-31G(d) level at 298.13 K in toluene solvent. All DFT calculations were carried out using the Gaussian 09 programs.16 Visualization of noncovalent interactions in transition states was carried out using the NCI plot.17 The NCI isosurfaces were visualized with the VMD program18 using data produced by the Multiwfn program.19 The strength of noncovalent interaction is indicated by the color of the isosurface: green represents weakly attractive, while blue denotes strongly attractive.

In 2010, Chen and co-workers demonstrated that a cinchona alkaloid-squaramide catalyst (Cat) (Scheme 2) promoted Scheme 2. Examples of Common Acid−Base Bifunctional Organocatalysts

3. RESULTS AND DISCUSSION 3.1. Four Possible Modes of Bifunctional Activation. Since there are three N−H hydrogen bond donors in the protonated cinchona alkaloid-squaramide catalyst, we envisage four possible modes of hydrogen bond interactions (A−D, Scheme 4) in the dual activation mechanism of bifunctional

efficient and highly enantioselective sulfa-Michael addition of thiols to trans-chalcones under mild reaction conditions (Scheme 3).7 Since squaramide is more acidic than a hydroxyl

Scheme 4. Four Possible Modes of Bifunctional Activation of the C−S Bond Forming Stepa

Scheme 3. Enantioselective Sulfa-Michael Addition Reaction between Benzyl Thiol and trans-Chalcone Catalyzed by Cinchona Alkaloid-Squaramide Catalyst (Cat)

or (thio)urea group,8 it is expected to provide better stabilization to the developing alkoxide through hydrogen bonding interactions in Wynberg’s model of activation. It is intriguing to ask whether Houk’s model is still applicable to the cinchona alkaoid-squaramide catalytic sysytem. Although the cinchona alkaloid and their derivatives have received great attention in organocatalysis,3 knowledge of their catalytic behaviors4,5,9 is still somewhat limited. This theoretical study investigates the mode of bifunctional activation for the sulfaMichael addition reported by Chen et al. and the origin of stereoselectivity. An in-depth knowledge of the mode of dual activation mechanism and key factors that control stereoselectivity in cinchonda alkaloid-derived organocatalysts should serve as a useful guide for the rational development of more efficient asymmetric bifunctional catalytic systems based on hydrogen-bonding activation.

a

El and Nu represent electrophile and nucleophile, respectively.

acid−base organocatalysis. The electrophile (El) and nucleophile (Nu) can interact with the catalyst via a monodentate or bidentate hydrogen bond. In Mode A,20,21 the deprotonated nucleophile that is directed by the alkylammonium attacks the electrophile, which is activated by the Brønsted acid in the ratedetermining step. This bifunctional mode of activation was proposed by Takemoto et al. in their study on asymmetric Michael reaction catalyzed by a bifunctional thiourea catalyst 1 (Scheme 2).22 However, a subsequent theoretical investigation23 conducted by Pápai et al. on catalyst 1 showed that another mode of bifunctional activation (Mode B, Scheme 4)5,24,25 is preferred. In Mode B, the Brønsted acid orients the deprotonated nucleophile, while the electrophile is activated by the alkylammonium of the protonated catalyst. Another activation mode, Mode C (Scheme 4), was postulated in a joint experimental/theoretical study on a bifunctional cinchona alkaloid-thiourea catalyst 2 by Wang and co-workers.26 In this bifunctional mode of activation, the distal N−H of the Brønsted acid activates the electrophile, and the deprotonated nucleophile is oriented by both the alkylammonium and the other N−H group of the Brønsted acid. Wang et al. also argued that Mode C may work for squaramide-based bifunctional organocatalysts. However, a computational study25 by Pápai showed that Mode B is favored for a squaramide catalyst 3 (Scheme 2), which behaves in a manner similar to that of

2. COMPUTATIONAL DETAILS Geometry optimizations of relevant equilibrium and transition state (TS) structures were performed using the M06-2X10 density functional with the standard 6-31G(d) basis set. The M06-2X functional was chosen as it is better suited to handling kinetics, thermodynamics, and noncovalent interactions for organic molecular systems10,11 and the sulfa-Michael addition reaction.12 It is worth noting that Houk et al. have shown that inclusion of diffusion functions in the basis set for geometry optimizations is not necessary for sulfa-Michael addition reactions.4 Harmonic frequency analysis was performed on the optimized structure to identify whether it is a true transition state (with only one imaginary frequency) or a local energy minimum (with no imaginary frequency). To obtain more accurate energetics, single-point energy calculation with a larger def2-TZVPP13 basis set was performed for each optimized structure. Solvation effect (in toluene solvent) was taken into account using Truhlar’s SMD14 4363


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solution.29a The squaramide moiety is found to be coplanar to the 3,5-bis(trifluoromethyl)phenyl group to which it is connected. This coplanar orientation is stabilized by an attractive C−H···O interaction between one carbonyl oxygen of the squaramide moiety and the acidic ortho-proton of the 3,5-bis(trifluoromethyl)phenyl group (see Figure 1). The 6′methoxy group of the quinoline moiety is also found to be coplanar to the quinoline ring. These structural features are further supported by a previous computational study28 and crystal structures of several cinchona alkaloids.30 3.3. Transition States of Different Activation Modes. The transition states for various bifunctional activation modes, Modes A, B (Figure 2), and C (Figure S3, SI), were successfully located for the C−S bond forming step with the anti-open conformation of the protonated catalyst. The lowest-energy TS is TS-B-R, via Mode B. It yields the major R-product, in excellent agreement with experiments. Both TS’s of Mode A (TS-A-R and TS-A-S) are less stable by more than 13 kJ/mol. TS’s in which the catalyst adopts the syn-open arrangement (Figure S2, SI) are significantly less stable. All TS’s related to Mode C (Figure S3, SI) are significantly higher in energy by at least 26 kJ/mol; thus, they will not be discussed further in the main text (see Table S1 and Figure S4, SI). Our various attempts to locate TS’s for Mode D were not successful. In most cases, the optimizations led to TS’s of Mode B. This is likely attributed to the steric repulsion between the bulky chalcone and the catalyst. TS-B-S and TS-A-S, leading to the minor product via Mode B and Mode A, respectively, are considerably disfavored given their higher relative energies compared to that of TS-B-R. A theoretical enantiometric excess (ee) of 99% was obtained based on the Boltzmann distribution of various low-lying TS’s located. This predicted value is in close agreement with the observed ee of 96%.7 The absolute activation free energies of the lowest R- and S-inducing TS’s are 85.5 and 101.1 kJ/mol, respectively (with respect to the free reactants and catalyst). These moderate activation barriers are consistent with the experimental condition.7 3.4. Origin of Stereoinduction. What is the origin of stereoselectivity? To shed light on the source of the energy difference between two transition states, distortion−interaction analysis31 was carried out. The energy decomposition method, first adopted by Wheeler et al.,31 is similar in spirit but different in aim compared to Houk’s distortion−interaction model and Bickelhaupt’s activation−strain model.32 The distortion− interaction model treats the activation barrier of a transition state as two quantities, distortion energy (ΔE⧧dist) and interaction energy (ΔE⧧int). In our catalytic system, the distortion term has two parts: substrates and catalysts. We calculated the gas-phase energy difference (ΔΔE⧧) between two TS’s at their solution-phase geometries. Then, the electronic energy difference ΔΔE⧧ was decomposed into three contributions: distortion energy difference of the substrates (ΔΔE⧧sub), distortion energy difference of the catalyst (ΔΔE⧧cat), and difference in interaction energies of the substrates with the catalyst (ΔΔE⧧int). As evidenced in Table 1, the net effect of the solvation and entropy reduces the energy gap between two TS’s but without changing the selectivity. As a result, ΔΔE⧧int is mainly responsible for the energy difference between two TS’s. Our distortion−interaction analysis reveals that the favorable interaction in TS-B-R is considerably stronger than that in TS-A-R and TS-A-S (entries 2 and 3 in Table 1), by 39 and 49 kJ/mol, respectively. This stronger interaction leads to the preference for Mode B despite

thiourea catalyst 1. Finally, in activation Mode D, the electrophile interacts with the alkylammonium and one of the N−H groups of the Brønsted acid, while the nucleophile interacts with the distal N−H of the acid (Scheme 4). All these four plausible modes of dual activation will be investigated in detail. 3.2. Conformations of the Protonated Catalyst. It is important to first establish the possible active conformations of the catalyst (Cat). Both the initial protonation and final deprotonation of the tertiary amine in the catalytic cycle are facile in the dual activation mechanism of bifunctional catalysis.23,26 It has been shown convincingly that the C−S bond forming step is rate-determining in the base-catalyzed sulfa-Michael addition reaction.12 Hence, our focus of the present study is on the C−S bond forming step, in which the catalyst is protonated (CatH+). We explored mainly the conformational space of the protonated catalyst to identify various possible catalytically active conformations. The two N− H functionalities of squaramide should be aligned syn to each other in the catalytically active conformations. 25,27 A comprehensive conformational analysis on epiquinine has shown that the vinyl group on the quinuclidine moiety is anti with respect to C2−C3 bond.28 Thus, the syn alignment of the two N−H functionalities of the squaramide and the trans conformation of the vinyl group to C2−C3 were adopted in our conformational analysis to simplify the calculations. The rotations along C8−C9 and C4′-C9 bonds lead to six key conformations,28 namely, anti-open, syn-open, anti-closed, synclosed, anti-hindered, and syn-hindered (see definitions in Scheme 5). The hindered conformations are not catalytically Scheme 5. Definitions of Torsional Angles of the Cinchona Alkaloid-Squaramide Catalyst (Cat) and Nomenclatures of Different Conformations

active due to the lack of cooperativity between the three N−H functionalities of the protonated catalyst. Previous computational studies also concluded that the hindered conformations of cinchona alkaloids are much higher in energy than the open and closed forms.4,5,28 In fact, only the open and closed conformations could be observed in experiments.29 Hence, only the open and closed structures were considered in our conformational exploration. The protonated catalyst CatH+ is found to favor strongly the open conformations over the closed ones, by at least 16 kJ/mol (see Figure 1). Our finding here is consistent with previous computational studies on epiquinine28 and cinchona urea.5 Thus, the closed conformations were not examined in subsequent transition states search. The anti-open form is the lowest-energy conformation with the syn-open conformer less stable by 7.0 kJ/mol. Our result agrees with Sharpless’ finding that only an anti-open conformer exists in the epiquinine 4364


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Figure 1. Open and closed conformers of the protonated catalyst (CatH+). Relative Gibbs energies (in kJ/mol) are given in parentheses. The C− H···O distances are in Å.

and substrates in TS-B-R are significantly more distorted than their counterparts in TS-B-S. To probe further the origin of the interaction energy difference between these two TS’s, ΔΔE⧧int was decomposed into contributions from different noncovalent interactions between the substrates and the three components of the catalyst, namely, the skeleton, squaramide, and N-aryl moiety (i.e., 3,5-bis(trifluoromethyl)phenyl group) (see SI for details). The hydrogen bond interactions with the substrates and squaramide moiety are similar in strength in both TS-B-R and TS-B-S (Figure S6, SI). On the other hand, the interaction energy with the skeleton in TS-B-R is significantly larger, by 55.3 kJ/mol, than that in TS-B-S (Figure S6, SI). This difference in hydrogen bond strength is further substantiated by the hydrogen bond parameters (N−H···O distance = 1.675 and 1.796 Å, N−H···O angle = 143° and 132° in TS-B-R and TS-BS, respectively; Figure 2). The noncovalent interactions within TS-B-R and TS-B-S are illustrated with the NCI plots (Figure 3). The noncovalent interactions (NCI) index developed by Yang et al. enables the visualization of the noncovalent interaction.17 The result of NCI analysis transforms noncovalent interaction from the reduced density gradient into the surface, in which color is representative of the nature (repulsive or attractive) and strength of the NCI. In Figure 3, the dark blue surface between the skeleton and the developing alkoxide in the NCI plots demonstrates that the developing negative charge is stabilized substantially by the quinuclidinium ion via N−H···O hydrogen bond. The interaction between the N-aryl moiety and the substrates is stronger by 15.5 kJ/mol in TS-B-R compared to TS-B-S (Figure S6, SI). This is in agreement with a larger surface in the NCI plot of TS-B-R (Figure 3), which reflects that the presence of π-stacking exists between the N-aryl moiety and substrates. An additional C−H···π interaction between the thiolate and N-aryl moiety in TS-B-R is also revealed by the NCI plots. Interestingly, these NCI plots show that stronger

the fact that the total distortion in TS-B-R is significantly greater than that in TS-A-R or TS-A-S by more than 15 kJ/mol. Jacobsen argued that more effective organocatalysts should work in a way close to enzymes,33 i.e., the preferred pathway results from more favorable noncovalent interactions rather than steric repulsion. Thus, our catalytic system examined here provides another good example of an ideal organocatalyst according to Jacobsen’s argument. In our ongoing computational studies in organocatalysis, we showed that weak noncovalent interactions, such as N−H···π interaction,11b C− H···O oxyanion hole,11d donor−acceptor interactions,11c,e,f halogen bonding,34 and aryl−aryl interaction,35 are the key factors influencing stereoselectivity of asymmetric reactions. The preference for Mode B over Mode A can readily be explained in terms of the better stabilization to the developing alkoxide offered by the proton transfer from the quinuclidinium ion in Mode B than by the hydrogen bonding interactions with the squaramide moiety in Mode A.4,5 The strength of these stabilizations can be estimated by the interactions between the substrates and catalyst fragments. The interaction energy of the substrates with the skeleton of Cat in TS-B-R are greater than those of the substrates with the N-aryl squaramide in TS-A-R or TS-A-S by more than 200 kJ/mol (Figure S5, SI). This is consistent with Houk’s explanation on the preference for Mode B over Mode A.4,5 Mode C is disfavored over Mode B for the same reason as Mode A (see SI for further details). The large stabilization provided by the protonated catalyst to the developing alkoxide in Mode B also supports the postulate that the protonated bifunctional squaramide catalyst can make a good oxyanion hole to stabilize the developing negative charge on alkoxide.25 Distortion−interaction analysis between TS-B-R and TS-B-S (entry 1 in Table 1) reveals that the very strong noncovalent interactions within TS-B-R can account for its lower energy (by 45 kJ/mol relative to TS-B-S), even though both the catalyst 4365


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Figure 2. C−S bond forming transition states for modes A and B. Relative Gibbs energies (in kJ/mol) are given in parentheses. Intermolecular distances are given in Ångstroms. Nonessential hydrogen atoms are omitted for clarity.

4. CONCLUSION In summary, our DFT computations provide mechanistic insights into the sulfa-Michael addition reaction promoted by the cinchona alkaloid-squaramide catalyst. We find that bifunctional activation Mode B, i.e., the Houk’s Brønsted acid−hydrogen bonding model, is still applicable when the squaramide is used as hydrogen bond donors in the catalyst for the sulfa-Michael addition reaction. The preference for Mode B is attributed to the developing alkoxide stabilized by a greater extent by the quinuclidinium ion than hydrogen bond interactions in the other modes. On the basis of NCI analysis, more favorable cooperative noncovalent interactions, including hydrogen bond, π-stacking, C−H···π interaction, and C−H···F interactions, between the catalyst and the substrates in the major transition state led to the excellent stereoselectivity of Mode B. The NCI plots clearly demonstrate that the commonly used 3,5-bis(trifluoromethyl)phenyl moiety in bifunctional acid−base organocatalysts is capable of forming multiple attractive interactions with the substrates to differentiate the stereo-controlling transition states. The catalytic system studied here provides another good example to Jacobsen’s argument that organocatalysts can work in an enzyme-like manner.

Table 1. Distortion−Interaction Analysis of Selected Transition Statesa entry

TS’s

1 2 3

TS-B-R vs TS-B-S TS-B-R vs TS-A-R TS-B-R vs TS-A-S

ΔΔG⧧ ΔΔE⧧ 34.4 13.2 15.6

45.5 24.2 32.8

ΔΔE⧧sub ΔΔE⧧cat ΔΔE⧧int −13.4 6.3 5.5

−26.7 −21.6 −22.1

85.5 39.6 49.4

a

At M06-2X/def2-TZVPP level of theory. Energy difference is in kJ/ mol. TS-B-R as reference. Corresponding solution-phase Gibbs energy differences (ΔΔG⧧, kJ/mol) are also included.

C−H···F interactions are present between the substrates and Naryl moiety in TS-B-R. In summary, a series of stronger cooperative noncovalent interactions, namely, hydrogen bonds, π-stacking, C−H···π interaction, and C−H···F interactions, in the major transition state TS-B-R is the key reason for the excellent stereoselectivity of Mode B, as pointed out by the distortion−interaction analysis above. Furthermore, this study provides supporting evidence that the 3,5-bis(trifluoromethyl)phenyl group, which is more than an electron-withdrawing group that enhances the acidity of the hydrogen bond donors,36 is a privileged motif as part of the catalyst to form noncovalent interactions with reactants37 to differentiate different transition states to achieve stereoselectivity. 4366


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Figure 3. NCI plots of transition states TS-B-R and TS-B-S showing the key noncovalent interactions between the substrates and catalyst. The isosurfaces were generated for s = 0.4 au, and the color scale is −0.04 < ρ < 0.04 au. The blue, green, and red regions represent strong attractive, weak attractive, and repulsive interactions, respectively.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.joc.7b00388. Optimized transition states for Mode C, transition states in which the catalyst adopts the syn-open conformation, energetic stabilization of developing alkoxide in various transition states, estimation of interaction energies between substrates and catalyst fragments, Cartesian coordinates of all reported structures, and their number of imaginary frequencies and computed total energies (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ming Wah Wong: 0000-0003-2162-1220 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by National University of Singapore (Grant No. R-143-000-555-112). REFERENCES

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