Origin of Stereoselectivity of the Photoinduced ... - ACS Publications

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Origin of Stereoselectivity of the Photoinduced Asymmetric PhaseTransfer-Catalyzed Perfluoroalkylation of β‑Ketoesters Chen Yang,†,‡ Wei Zhang,† Yi-He Li,‡ Xiao-Song Xue,† Xin Li,*,† and Jin-Pei Cheng†,‡ †

State Key Laboratory of Elemento-organic Chemistry, College of Chemistry, Nankai University, Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300071, P.R. China ‡ Department of Chemistry and Biology, College of Science, National University of Defense Technology, Changsha 410073, China S Supporting Information *

ABSTRACT: The sources of asymmetric induction in the photoinduced phase-transfer-catalytic perfluoroalkylation of βketoesters were interpreted by density functional theory calculations. The calculations indicated that multiple hydrogen-bonding interactions mode rather than π−π stacking interactions model better described the transition structure of the reaction. The quantitative estimation of these noncovalent interactions within the key transition states identified that the O−H···O and specific C−H···O hydrogen bonds play key roles for the stereocontrol.

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INTRODUCTION Though it is in infancy, the asymmetric catalysis driven by visible light1 has exhibited thriving vitality past a decade.2 Among the known enantioselective photocatalysis, the key chiral intermediates are generated mostly through covalent catalysis with photogenerated active species, such as chiral enamines,3 iminium ions,4 Breslow intermediates,5 metal complexes,6 and enolates.7 Recently, more challenging noncovalent strategy to control stereoselectivity in visible-light photochemistry has emerged. 8−16 As a result, several asymmetric transformations were achieved, such as αhydroxylation,8 aza-Pinacol cyclization,9 intramolecular11 and intermolecular12 cyclizations, oxidative C−H functionalization,13 redox neutral C−C bond forming reaction,14 and antiMarkovnikov hydroetherification.15 Besides the excellent examples mentioned above, recently, Melchiorre and co-workers exploited an attractive externalphotocatalyst-free strategy to achieve the phase-transfercatalyzed (PTC) enantioselective perfluoroalkylation of βketoesters,16 which relied on the formation of photoactive electron donor−acceptor (EDA) complexes17 between the chiral enolates and electron-deficient perfluoroalkyl iodides (Scheme 1). On basis of conducted diagnostic experiments on mechanism, they proposed a radical chain propagation pathway to interpret the asymmetric photochemical perfluoroalkylation. The transformation was triggered by irradiation of the chiral EDA complex, leading to the formation of the perfluoroalkyl radical RF· via the reductive cleavage of the perfluoroalkyl iodide RfI. The selective attack of the chiral enolate by the electrophilic RF· was believed to be the stereocontrol step. The resulting ketyl intermediate reacted with another RfI, yielding the product, the PTC catalyst and another RF·. The PTC catalyst and RF· entered into cycles to continuously provide products. © 2017 American Chemical Society

Although the PTC approaches, especially the cinchonaderived PTC catalysts, have found lots of applications on asymmetric transformations,18 we’re astonished that a full picture of the origin of stereoinduction remains obscure by screening literatures. It is well-known that the theoretical chemistry can offer alternative opportunity to study the mechanism,19 especially the untouchable transition states, besides experimental procedures. Recently, overcoming the large and flexible disadvantage of cinchona-derived20 PTC catalysts, some calculations were performed to throw new insights on PTC reactions.21−27 Lipkowitz and O’Donnell conducted a precedent theoretical study on the interaction between the N-benzylcinchoninium cation and Schiff basederived enolates with molecular dynamics and semiempirical quantum theory in 1991.21 They found the Z-enolates selective binding on the front side of the N-benzylcinchoninium may account for the observed enantioselectivity. The Palomo group proposed a dual activation model to fit the observed enantioselectivity in the N-benzylquininium ion catalyzed azaHenry reaction, where the catalyst 9-OH group interacts with the NO2 group of the nitro-compound and the catalyst +N− CH site interacts with the imine.22 Andrus and Ess disclosed the oxy-anion-ammonium interactions in transition states rather than π-face interactions in the ground states are responsible for the asymmetric allylation of Schiff base catalyzed by a 9-allyated cinchona derived PTC catalyst.23 Combination of density functional theory calculations and kinetic model, Pliego et al. pointed out both the hydrogen bond and electrostatic interaction involving the leaving chloride anion in the favored transition state are the key factors for the enantioselective inducement in the cinchoninium ion catalyzed alkylation of Received: May 9, 2017 Published: August 22, 2017 9321

DOI: 10.1021/acs.joc.7b01130 J. Org. Chem. 2017, 82, 9321−9327

Article

The Journal of Organic Chemistry Scheme 1. Photoinduced Enantioselectvie Pefluoroalkylation of β-Ketoesters

indanone.24a,b González and co-workers believed the hydrogenbond interaction dominated the favorable transition state but the π−π stacking interaction between substrate and the quinoline moiety of the catalyst destabilized the unfavorable transition state for it constrained the relative orientations of the substrates and catalyst, which led to the enantiopure epoxidation products of nitroalkenes.25 Capobianco and Palombi stressed the strength difference of hydrogen bond network established between 9-OH and 6′-OH of the quininium ion and the intermediate facilitated the desired product in an asymmetric tandem hemiaminal-heterocyclization-aza-Mannich reaction of 2-formylbenzonitriles.26 These theoretically enriched the understandings of the asymmetric induction mechanism of cinchona-derived phasetransfer catalysts, however, to our knowledge, phase-transfer catalytic examples involved in photochemical radical progress have not been reported by far. The researches on mechanisms and origins of stereoselectivity28,29 will undoubtedly accelerate the rational development of photochemistry. Herein, we reported the mechanism calculations on the photo-organocatalytic perfluoroalkylation of β-ketoesters.

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Their relative free energies were calculated at M06-2X-D3/ def2TZVPP-SMD(PhCl)// M06-2X/6-31G(d)-SMD(PhCl) level.

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RESULTS AND DISCUSSION Since the Melchiorre group proposed a radical chain propagation of the perfluoroalkylation reaction initiated by EDA complexes,16 we first evaluated the existence of the EDA complex by calculations. Simulating the catalyst with N-methyl quinuclidinium ion, eight possible ground EDA complexes were located (Figures 1 and S1). Through a TD-DFT calculation40 at M06-2X/6-31G(d)+Lanl2dz(I) level, we found that the EDA complexes through a C−I bond paralleled the plane of the enolate interaction (EDA1−2 and EDAS1−2) rather than halogen-bonding interaction EDA3−4 and EDAS3−4, between the CF3I and the enolate, are able to absorb visible light (wavelength in green). These results agreed well with the observed optical absorption spectra.16 The maximum absorption wavelength calculated at TD-CAM-B3LYP/6-311++G(d,p)+SDD(I) also gave the similar results (wavelength in blue). The calculated structures of EDA1−2 are very similar to the EDA complex suggested by Melchiorre and Maseras in their previous computational study.28d Meanwhile, the halogen-bonding complex seemed to be irradiated just by ultraviolet light, which is in line with the Chen’s recent study.41 The orbital analysis of EDA2 complex indicated the charge-transfer transition from HOMO (in red and green) to LUMO (in purple and yellow) should be responsible for the visible light absorption (λ = 434.7 nm). We also tried to obtain the structure of the singlet state of EDA complex, however, it failed because of the difficulty to keep the excited state in the same state during the optimization. Next, we came to consider the transition states (TSs) in the stereocontrolling step. Balancing the computational cost and precision, we adopted N-3,4,5-trifluorobenzyl cinchoninium ion as the catalyst and the reaction of methyl 1-oxo-2,3-dihydro1H-indene-2-carboxylate and CF3I as the model (Scheme 2). The existence of torsional freedom enabled cinchoninium cation to possess many conformations. Recently, the study from Pliego et al. pointed out that the N-4-trifluoromethylphenyl cinchoninium cation adopted the anti-open conformation in the alkylation reaction of indanone after they examined many

COMPUTATIONAL METHODS

All calculations were performed with Gaussian09 packages at T = 298.15 K and p = 1 atm.30 Unless noted, all energetics are reported in kcal/mol, and the bond lengths are reported in angstroms (Å). Structures were generated using CYLview,31 Multiwfn,32 and VMD.33 Geometry optimizations were conducted using M06-2X functional34 with 6-31G(d) basis set for C, H, O, N, and F atoms, and Lanl2dz35 for I atom at gas phase. The frequency calculations were conducted at the same level of theory to confirm the nature of stationary points and obtain the thermal corrections. The vertical excitations of EDA complexes were calculated at TD-M06-2X/6-31G(d)+Lanl2dz(I) and TD-CAM-B3LYP/6-311++G(d,p)+SDD(I) level without vibronic corrections.36 Single-point energies of the transition states were calculated at the M06-2X-D3/6-311++G(d,p) and M06-2X-D3/ def2TZVPP level, respectively.37,38 The solvent effect of chlorobenzene was evaluated with the SMD method.39 The thermal corrections derived from the vibrational frequency calculations were then added to the electronic energies calculated at M06-2X-D3/6-311++G(d,p) or M06-2X-D3/def2TZVPP level to obtain Gibbs free energies. Several transition states were also reoptimized in solution phase at M06-2X/6-31G(d) level based on the structures obtained in gas phase. 9322


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calculations. Other conformations of the PTC catalyst, including different conformations of the catalyst’s 3,4,5trifluorobenzyl and vinyl group, however, were not considered in the following calculations. As described in Scheme 2, three possible transition-state models were proposed with respect to the stereocontrolling step. In model a, the TSs were stabilized by hydrogen bond interactions established between 9-OH and +N−CH of the quininium ion and the enolate, and π−π stacking interaction between enolate and the phenyl group of the catalyst. In the model b, the TSs were stabilized by hydrogen bond interactions established between 9-OH and +N−CH of the quininium ion and the enolate, and π−π stacking interaction between enolate and the quinoline moiety of the catalyst. In model c, the TSs were stabilized only by hydrogen bond network between the two oxygens of the β-ketoester and the catalyst. According to these TS models, we calculated corresponding transition states (Figure 2). The relative free energies were obtained at the M06-2X-D3/6-311++G(d,p)-SMD(PhCl)// M06-2X/6-31G(d) level (values in green). The ts1−2 belonged to model a, the ts3−4 belonged to model b, and the ts5−8 belonged to model c. Given the product is S enantiomer, ts1−4 failed to predict the right isomer. The proposed π−π stacking interaction8a,b seemed to have little influence on the enantioselectivity induction in the current reaction.24 Among the TSs corresponded to model c, the ts7-S and ts8-R structures are more stable than ts5-S and ts6-R. Because keto carbonyl contains more negative charge than ester carbonyl, the hydrogen-bonding interactions between OH group of catalyst and keto carbonyl of enolate is stronger than that between OH group of catalyst and ester carbonyl of enolate. The ts7-S, leading to the S product, is the most stable transition state among all TSs. The ts8-R is more unfavorable than ts7-S by 1.3 kcal/mol. The 1.3 kcal/mol difference in energy indicated an 81% ee value of S enantiomer, which is in good agreement with the observed 86% ee.42 Even if we analyze all calculated TSs with the Boltzmann equation, the predicted ee would be 71%, which is not in disagreement with the experimental result. Thus, the origin of the enantioselectivity can be attributed to the difference between ts7-S and ts8-R. Additional calculations were performed to confirm the importance of model c in the enantioselective inducement. In order to attenuate the basis set superposition error when energy calculations were conducted with 6-311++G(d,p), the triple-ζ

Figure 1. EDA complexes and maximum absorption wavelength.

structures for an anion-catalyst complex.24a,b The Houk group also found that the most stable comformer of a bis-quaternized cinchoninium cation adopted the anti-open conformation.24c Besides, the X-ray structure of the optimal cinchoninium PTC catalyst with a 2′-position modification of the quinoline ring reported in Melchiorre’s experiments is anti-open conformation, which can serve as a direct reference.16 Considering its resemblance to the N-4-trifluoromethylphenyl cinchoninium cation and bis-quaternized cinchoninium cation and the experimental X-ray data, the most stable structure of N-3,4,5trifluorobenzyl cinchoninium cation should be the anti-open conformation and was used as the initial structure in our Scheme 2. Model Reaction and Putative Transition-State Models

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experiments (see the Supporting Information of ref 16).16 If the proposed model c was indeed in charge of the stereoselectivity and different cinchona-derived PTC catalysts shared almost the same transition states, the observed enantioselectivites with different PTC catalysts could be reproduced via transition state calculations. For example, the 3,5-ditertbutylbenzyl cinchonidine-derived PTC catalyst provided product with only 1% ee in their original reports. We posited that the pseudoenantiomeric 3,5-dimethylbenzyl cinchonine-derived PTC catalyst should also lead to a low level of stereocontrol (Scheme 3). With the Scheme 3. Further Examination of the Hydrogen-Bonding Network Model c with a Cinchonine-Derived PTC Catalyst

established method (M06-2X-D3/6-311++G(d,p)-SMD(CH2Cl2)//M06-2X/6-31G(d)), ts9-S and ts10-R were located (Figure 3). Here again, it should be noted that C6F13I was

Figure 2. Stereodetermining transition state structures and their relative free energies (units: kcal/mol).

basis set def2-TZVPP was first applied.43 The resulting relative free energies were presented with blue color in Figure 2. This more precise method gave similar relative energies. The predicted ee value is 83% with the Boltzmann equation, also indicating model c is the preferred activation model. Performing the geometry optimization in solution phase with implicit solvent model did not change the geometry of transistion states much as well (Figure S2). At the M06-2X-D3/def2TZVPPSMD(PhCl)//M06-2X/6-31G(d)-SMD(PhCl) level, the energy difference between ts7-S1 and ts8-R1 reached up to 2.1 kcal/mol. And the energies of TSs belonging to model a and b are higher at least by 2.5 kcal/mol than that of ts7-S1, meaning the model c is prominent in control of enantioselectivity. Besides, another two model-c-type TSs (syn-ts7 and syn-ts8) with syn-open catalyst conformation were calculated (Figure S3). It turned out energies of transitions states catalyzed by synopen catalyst are much higher. For example, syn-ts7 is 6.9 kcal/ mol above ts7-S in energy. In the end, we noticed the substitution pattern at the benzyl group of the PTC catalyst had a profound influence on the enantioselectivity in Melchiorre’s

Figure 3. Transition state structures of the cinchonine-derived PTC catalyst and their relative free energies (units: kcal/mol).

replaced by CF3I in our calculations. Indeed, the difference of the two TSs in energy is only 0.02 kcal/mol, indicating the enantioselectivity of product is very low (3% ee) when the reaction was conducted in CH2Cl2. This result lent much support to the hydrogen-bonding network model in control of enantioselectivity. The influence of electronic modification of the benzyl group of the PTC catalyst on the enantioselectivity could be explained as follows. The introduction of withdrawing groups into the aromatic ring of the catalyst would strengthen the related hydrogen-bonding interaction between the enolate and the catalyst. For example, the distance of hydrogen bond (C22−H42···O77) in ts7-S is shorter than that (C22−H42··· O74) in ts9-S (2.38 Å versus 2.61 Å), meaning it is stronger in ts7-S. The importance of such a hydrogen bond has been proposed in some cinchona alkaloid-derived PTC catalyzed transformations24a,27b and also in bifunctional thiourea9324


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The Journal of Organic Chemistry catalyzed reactions.44 Meanwhile, the introduction of withdrawing groups can increase the strength of electrostatic interaction between the positive nitrogen center of the catalyst and the negative enolate, which contributed to the stabilization of the transition states.23,24a The distance (DN45−O77) in ts7-S catalyzed by 3,4,5-trifluorobenzyl cinchoninium is 3.50 Å, while it is 3.53 Å (D N45−O74 ) in ts9-S catalyzed by 3,5ditertbutylbenzyl cinchoninium. Forming a tight complex for the introduction of withdrawing group to the aromatic ring of catalyst would ensure discrimination of one of the enantiotopic faces in the TSs, indicating a better performance of stereocontrolling. In the following, we tried to figure out which hydrogen bond is dominant. Treating ts7-S and ts8-R with Yang’s noncovalent analytic method,45 we can clearly see the existence of hydrogen bond network in these two transition states (Figure 4). Taking

estimated the energies of the hydrogen bonds within key transition states using Espinosa equation. As we show in Table 1, the O−H···O hydrogen bond contributed most to stabilize Table 1. Estimated Strength of Hydrogen Bonds in the Transition States TS

type

ts7-S

O47−H44···O77 C16−H40···O77 C17−H41···O77 C22−H42···O77 C17−H41···O74 C19−H39···O74 C7−H35···O74 O47′-H44′···O77′ C16′-H40′···O77′ C17′-H41′···O77′ C22′-H42′···O77′ C17′-H41′···O74′ C19′-H39′···O74′ C7′-H35′···O74′

ts8-R

length 1.69 2.27 2.07 2.38 2.21 2.15 2.34 1.74 2.37 2.04 2.31 2.19 2.28 2.38

Å Å Å Å Å Å Å Å Å Å Å Å Å Å

energy (kcal/mol) −14.0 −2.8 −5.0 −3.8 −2.2 −3.7 −2.2 −12.1 −2.3 −5.3 −4.1 −2.5 −2.7 −2.1

the corresponding transition states among all the hydrogen bonds. The energy of this type hydrogen bond is −14.0 kcal/ mol in ts7-S and −12.1 kcal/mol in ts8-R, respectively, which is in line with the distance (O47−H44···O77 1.69 Å vs O47′H44′···O77′ 1.74 Å). And the difference of O−H···O hydrogen bond in ts7-S and ts8-R reached 1.9 kcal/mol, reflecting the importance of O−H···O hydrogen bond in control of enantioselectivity. The C19−H39···O74 hydrogen bond in ts7-S (2.15 Å) is 1.0 kcal/mol stronger than that in ts8-R (2.28 Å). The C16−H40···O77 hydrogen bond in ts7-S (2.27 Å) is 0.5 kcal/mol stronger than that in ts8-R (2.37 Å). These two interactions are other factors in control of enantioselectivity. The energy difference of remaining hydrogen bonds is 0.1−0.3 kcal/mol as the tiny difference of distance. Taking all the energy difference of hydrogen bonds together, these interactions make ts7-S more stable than ts8-R by 2.6 kcal/ mol. Though this value (2.6 kcal/mol) derived from hydrogenbonding interactions is somewhat bigger than the difference (1.3 kcal/mol) between ts7-S and ts8-R, the quantitative estimation of hydrogen-bonding strength is a very powerful tool to interpret the origin of enantioselectivity. Compared to the Re face attack, the attack of chiral enolate by trifluoromethyl radical from the Si face is more favorable resulting in multiple hydrogen-bonding interactions, leading to the S product through ts7-S.

Figure 4. Noncovalent interaction analysis of transition states ts7-S and ts8-R.

ts7-S as an example, there are O47−H44···O77, C16−H40··· O77, C17−H41···O77, C22−H42···O77, C17−H41···O74, C19−H39···O74, and C7−H35···O74 hydrogen bonds. The existence of multiple hydrogen bonds makes it still not easy to pick up which hydrogen bond contributes more for the enantioselectivity with the noncovalent analysis, while the quantitative description of the strength of the hydrogen bond will lend much help to give a clear picture of the selectivity. In 1998, Espinosa et al. disclosed the energy of a given hydrogen bond (Eint) is proportionate to the local electron potential energy density V(rcp) of the bond critical point with a coefficient of 0.5.46 With this relationship (Eint = 0.5* V(rcp)), Yu and Cheng have analyzed the strength of hydrogen bonds in the transition states of the asymmetric olefin isomerization47 catalyzed by a cinchona alkaloid derivative in 2013 and successfully revealed the importance of multiple C− H···O hydrogen bonds in the selectivity-control.48 Here we

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CONCLUSIONS The stereoselective origin of the photoinduced, asymmetric phase-transfer catalytic perfluoroalkylation of β-ketoesters was interpreted by density functional theory calculations. By means of TD-DFT calculations at M06-2X/6-31G(d)+Lanl2dz(I) level, EDA complexes through a C−I bond paralleled the plane of the enolate interaction rather than halogen-bonding interaction between the CF3I and the enolate are proposed to be visible light photoactive. The examination of three putative transition state models suggests that the simple multiple hydrogen-bonding mode rather than the other two mixed π−π stacking interaction and hydrogen-bonding modes better describes the transition structures of the perfluoroalkylation reaction. The reason why introduction of withdrawing groups 9325


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into the aromatic ring of the PTC catalyst would gain better enantioselectivity was attributed to a better molecular recognization as the strength of the related hydrogen-bonding and electrostatic interactions between the enolate and the PTC catalyst increased. In the case of N-3,4,5-trifluorobenzyl cinchoninium cation, we believed that multiple hydrogenbonding interactions between the 9-OH moiety and +N−CH site of the cinchoninium ion and enolate are the key factors to obtain high enantioselectivity. These noncovalent interactions within the key transition states were estimated quantitatively and clarified that the O−H···O and specific C−H···O interactions governed the stereocontrolling step. These findings enriched the understanding of Cinchona alkaloids based phasetransfer catalysis in photochemistry and may lend help to design novel phase-transfer catalysts and related reactions.

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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.7b01130. EDA complexes with TD-DFT descriptions, Gibbs free energies of calculated transition states, stereo-determining transition state structures, transition state structures, and Cartesian coordinates of all the optimized structures (PDF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected]. ORCID

Xiao-Song Xue: 0000-0003-4541-8702 Xin Li: 0000-0001-6020-9170 Jin-Pei Cheng: 0000-0001-8822-1577 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the National Natural Science Foundation (Grant Nos. 21390400, 21421062) and the State Key Laboratory on Elemento-organic Chemistry for financial support.

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REFERENCES

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