N-Heterocyclic carbene promoted enantioselective desymmetrization reaction of diarylalkane-bisphenols

Shoulei Li, Bin Liu, Li Chen, Xin Li* and Jin-Pei Cheng
State Key Laboratory of Elemento-Organic Chemistry, Collaborative Innovation Center of Chemical Science and Engineering, Nankai University, Tianjin 300071, China. E-mail: xin_li@nankai.edu.cn

Received 2nd December 2017 , Accepted 11th January 2018

First published on 12th January 2018


An enantioselective NHC-catalyzed desymmetrization reaction of diarylalkane-bisphenols with aldehydes was reported with linear free energy relationships (LFERs) to correlate the enantioselectivities with the steric and electronic parameters. The reaction scope is substantial, and a wide range of aromatic and aliphatic aldehydes, as well as diarylalkane-bisphenols were tolerated under the reaction conditions. As a result, a series of diarylalkane-bisphenol products were generated with excellent reactivities and enantioselectivities (up to 99% yield and up to 98.7[thin space (1/6-em)]:[thin space (1/6-em)]1.3 er). Furthermore, the reaction can be scaled up, and the desired product can be easily converted to valuable structures.


Introduction

In the past three decades, asymmetric catalysis, which is used to construct chiral structures, has boomed in both academic and industrial fields and has become the most economical and effective method for synthesizing chiral materials.1 Although intensive research efforts have been devoted to developing new chiral catalysts and novel asymmetric catalytic systems, their exploration remains mainly empirical and it is still a formidable challenge to design them rationally. The linear free energy relationship (LFER) analysis method2 is an essential and powerful tool that turns qualitative analysis into quantitative analysis in the study of the reaction mechanism and the influence of structural changes on the outcomes,3 which makes the rational design of the development of new catalysts and catalytic systems possible.4 In this regard, Sigman and co-workers have contributed a lot to this research area. Various parameters of molecules, such as Charton values,5 Hammett constants,6 and pKa values,7 have been used to correlate the reaction results to valuable rules of the relationship between the catalysts’ properties and the outcome of the reaction, which can be used in the design of substrates, catalysts and ligands.4,8 Furthermore, the emergence of a number of recent examples by other groups also reflects the powerful role of the LEFR analysis method in organic synthesis.9 Despite the wide application of the linear free energy relationship analysis, the examples of the corresponding strategy in asymmetric catalysis, especially for organocatalysis, are still very limited.8c,9f,10

Asymmetric desymmetrization has been developed as a significant methodology, in which enantiomerically pure products are generated by breaking the molecular symmetry from achiral or meso compounds.11 Particularly, the desymmetrization of diol type substrates has been frequently investigated. As a result, increasing attention has been paid to the development of efficient catalytic strategies, such as enantioselective acylation, sulfonylation, phosphorylation and silylation (Scheme 1a).12 Nevertheless, only a few cases of the desymmetrization of bisphenols, whose scaffolds are privileged in many biological and pharmaceutical compounds (Scheme 1b),13 were reported because of the great challenge in differentiating remote enantiotopic sites.14 Very recently, Zhao's group14a disclosed the first acylation desymmetrization reaction of bisphenols by NHC-catalysis.15 However, albeit with the diversity of substituents on bisphenols, there was only one aliphatic aldehyde to be used as an acylation reagent and the reaction did not occur with an aromatic aldehyde without an oxidant. Therefore, the development of novel methodologies for the preparation of bisphenol derivatives is still very meaningful and highly desirable.


image file: c7qo01083d-s1.tif
Scheme 1 (a) The catalytic strategies for the desymmetrization of diols; (b) bisphenol derivatives with biological properties; (c) overview of this work.

We herein reported a remote asymmetric desymmetrization reaction of diarylalkane-bisphenols with various aldehydes (aromatic and aliphatic aldehydes) catalyzed by a simple N-heterocyclic carbene (NHC) catalyst, wherein the prediction of substrates and the research on the mechanism were conducted by linear free energy relationship analysis (Scheme 1c).10a–c

Results and discussion

We initiated our investigation by using triarylmethane-bisphenol 1a and benzaldehyde 2a as starting materials, in which after preliminary condition optimizations, only a moderate yield and low enantioselectivity (73% yield and 60.3[thin space (1/6-em)]:[thin space (1/6-em)]39.7 er. For details see the ESI, Table S1.) were achieved under the conditions of chiral azolium salt 3a as the pre-catalyst, DABCO as the base, and quinone as the oxidant in the solvent of DME at 0 °C under Ar (Table 1, entry 1). Inspired by Miller and Sigman's work,16 in which they found a strong steric effect on the enantioselective desymmetrization of symmetrical bisphenols in a peptide catalyzed acylation reaction, a series of bisphenols (Table 1, entries 2–5) with different size substituents at the pre-stereogenic center were synthesized to afford better results. To our delight, a well-behaved trend associated with the steric effect was observed so that the products of more bulky substituents (1d, 1e) were obtained with higher er values. To further study this trend in a quantitative way, we turned to the linear free energy relationship (LFER) analysis.17 As shown in Fig. 1a, a strong correlation between enantioselectivities and Charton values was exhibited with a slope of 2.31 and R2 = 0.94. The large positive slope indicated that better enantioselectivity should be obtained with a substrate bearing a large size substituent at the pre-stereogenic center. As expected, when the more sterically hindered substrates 1f and 1g were used, excellent enantioselectivities were achieved (Table 1, entries 6 and 7). And a better correlation with a slope of 2.49 and R2 = 0.99 was obtained with all the substrates 1a–1g (Fig. 1b). With a comprehensive consideration of the yield and the enantioselectivity, substrate 1g was selected as the substrate for further optimization of the conditions.
image file: c7qo01083d-f1.tif
Fig. 1 LFER analysis of the steric effect.
Table 1 Evaluation of steric parametersa

image file: c7qo01083d-u1.tif

Entry R Yieldb (%) erc Charton value ΔΔG(er)d (kcal mol−1)
a The reactions were conducted with 1 (0.1 mmol), 2a (0.1 mmol), 3a (5 mol%), DABCO (0.1 mmol), and oxidant (0.1 mmol) in DME (1.0 mL) at 0 °C for 17 h under Ar.b Yield of the isolated product as the average of two runs.c Determined by HPLC analysis, averaged over two runs.d ΔΔG = RT[thin space (1/6-em)]ln(er), R = 0.001986 kcal K−1 mol−1, T = 298.15 K.e 1-Ad = 1-adamantyl. DABCO = 1,4-diazobicyclo(2.2.2)octane, DME = 1,2-dimethoxyethane.
1 1a: Ph 4a: 73 60.3[thin space (1/6-em)]:[thin space (1/6-em)]39.7 0.57 0.25
2 1b: Et 4b: 78 62.5[thin space (1/6-em)]:[thin space (1/6-em)]37.5 0.56 0.30
3 1c: Me 4c: 70 58.5[thin space (1/6-em)]:[thin space (1/6-em)]41.5 0.52 0.20
4 1d: c-C6H11 4d: 70 82.9[thin space (1/6-em)]:[thin space (1/6-em)]17.1 0.87 0.94
5 1e: i-Pr 4e: 96 81.4[thin space (1/6-em)]:[thin space (1/6-em)]18.6 0.76 0.87
6 1f: 1-Ade 4f: 84 97.3[thin space (1/6-em)]:[thin space (1/6-em)]2.7 1.33 2.13
7 1g: t-Bu 4g: 98 96.9[thin space (1/6-em)]:[thin space (1/6-em)]3.1 1.24 2.04


As shown in Table 2, three other chiral NHC catalysts were examined, in which the initially used 3a was identified as the optimal one (Table 2, entries 1–4). Further screening of bases showed that the kind of base had almost no effect on the reaction outcome (Table 2, entries 1, 5 and 6). Although the solvent DCM gave the best 98.0[thin space (1/6-em)]:[thin space (1/6-em)]2.0 er, the yield is relatively low (Table 2, entries 1 and 7–9). Lowering the amounts of catalyst 3a resulted in a decrease in both reactivity and enantioselectivity (Table 2, entry 10). Collectively, the best result with respect to the yield and the er value was obtained by conducting the reaction with 5 mol% of catalyst 3a and DABCO as a base in DME at 0 °C under Ar. Under the optimized conditions, the product 4g was obtained with 98% yield and 97.2[thin space (1/6-em)]:[thin space (1/6-em)]2.8 er (Table 2, entry 1).

Table 2 Optimization of the reaction conditionsa

image file: c7qo01083d-u2.tif

Entry Cat. Base Solvent Yieldb (%) erc
a The reactions were conducted with 1g (0.1 mmol), 2a (0.1 mmol), 3 (5 mol%), base (0.1 mmol), and oxidant (0.1 mmol) in 1.0 mL of the solvent at 0 °C under Ar.b Yield of the isolated products.c Determined by HPLC analysis.d The catalyst loading was 2 mol%. DABCO = 1,4-diazobicyclo(2.2.2)octane, DME = 1,2-dimethoxyethane, DIPEA = ethyldiisopropylamine, DCM = dichloromethane.
1 3a DABCO DME 98 97.2[thin space (1/6-em)]:[thin space (1/6-em)]2.8
2 3b DABCO DME 96 9.5[thin space (1/6-em)]:[thin space (1/6-em)]90.5
3 3c DABCO DME 98 8.0[thin space (1/6-em)]:[thin space (1/6-em)]92.0
4 3d DABCO DME 72 39.5[thin space (1/6-em)]:[thin space (1/6-em)]60.5
5 3a DIPEA DME 98 96.9[thin space (1/6-em)]:[thin space (1/6-em)]3.1
6 3a Et3N DME 93 97.0[thin space (1/6-em)]:[thin space (1/6-em)]3.0
7 3a DABCO DCM 85 98.0[thin space (1/6-em)]:[thin space (1/6-em)]2.0
8 3a DABCO Et2O 93 97.3[thin space (1/6-em)]:[thin space (1/6-em)]2.7
9 3a DABCO Toluene 93 97.0[thin space (1/6-em)]:[thin space (1/6-em)]3.0
10d 3a DABCO DME 90 96.0[thin space (1/6-em)]:[thin space (1/6-em)]4.0


With the optimized conditions in hand, the substrate scope for both bisphenols and aldehydes was examined. As for the substituents on the bisphenols (4g–4m), all the desired products were obtained in excellent yields (81–99%) with excellent enantioselectivities (up to 98.7[thin space (1/6-em)]:[thin space (1/6-em)]1.3 er). Then different substituted aldehydes were tested.

As shown in Table 3 (4n–4ae), the position of the substituent had little effect on the enantioselectivity, no matter if the substituents were at the o-position, m-position or p-position, and all the target products containing electron-neutral and electron-rich aryls exhibited extremely good er values with moderate to good yields. In contrast, electron-deficient substrates gave lower enantioselectivities, in which a well-behaved trend was found that the stronger the electron-withdrawing substituent was, the lower was the er value obtained (4x–4ae). These results indicated that the electronic effect had a great impact on enantioselectivity. We next examined some double substituted and naphthyl-substituted aldehydes, which delivered the corresponding products with moderate yields and very good er values (4af–4al). In addition, heterocyclic aldehydes and cinnamaldehyde were also well-tolerated in this enantioselective desymmetrization reaction (4am–4ao). Moreover, aliphatic aldehydes could also be used to prepare the desired chiral products, giving an excellent outcome under the optimal conditions (4ap–4aq).

Table 3 Substrate scopea,b,c
a The reactions were conducted with 1 (0.1 mmol), 2 (0.1 mmol), 3a (5 mol%), DABCO (0.1 mmol), and oxidant (0.1 mmol) in 1.0 mL of DME at 0 °C under Ar.b Yield of the isolated products.c The er values were determined by HPLC analysis.d The reaction was conducted at 25 °C. DABCO = 1,4-diazobicyclo(2.2.2)octane, DME = 1,2-dimethoxyethane.
image file: c7qo01083d-u3.tif


Besides, we also conducted the reactions of bisphenols bearing different size substituents with different aldehydes (Table 4). These results again showed that both the steric effect and the electronic effect had a great impact on enantioselectivity. We tried to improve some of the poor results. Take 4d for example, when 4-methylbenzaldehyde was used, only a slight improvement was observed (4ar). And the results of compounds 4d, 4ar, 4f, 4as, and 4at further verified the finding that more electron-rich aldehydes favored the stereoselective control.

Table 4 Substrate scopea,b,c
a The reactions were conducted with 1 (0.1 mmol), 2 (0.1 mmol), 3a (5 mol%), DABCO (0.1 mmol), and oxidant (0.1 mmol) in 1.0 mL of DME at 0 °C under Ar.b Yield of the isolated products.c The er values were determined by HPLC analysis.d The reaction was conducted at 25 °C. DABCO = 1,4-diazobicyclo(2.2.2)octane, DME = 1,2-dimethoxyethane.
image file: c7qo01083d-u4.tif


The absolute configuration of product 4p was determined by X-ray analysis.18 The configurations of other products were tentatively assigned by referring to that of 4p.

In order to study the observed electronic effect of the substrate scope in a quantitative way, the LFER analysis was used again. And the Hammett constants (σ) were selected as electronic parameters to correlate enantioselectivities (Table 5). As shown in Fig. 2, there was a strong correlation between enantioselectivity and Hammett constants (σ) with R2 = 0.97 (Fig. 2), indicating that the electronic effect was another key factor for the enantio-control as well as the steric effect. From the outcome of the electronic effect, we speculated that the hydrogen bond played an important role in reactions. When an aldehyde with an electron-donating group was used, a tighter hydrogen bond was created because of the enhancement of carbonyl's Lewis basicity. Thereby, a tighter transition state could be achieved to amplify the steric effect and then lead to a higher enantioselectivity.


image file: c7qo01083d-f2.tif
Fig. 2 LFER analysis of the electronic effect.
Table 5 Evaluation of the electronic effecta
Entry Group Yieldb (%) erc Hammett constants (σ) ΔΔG(er)d (kcal mol−1)
a The reactions were conducted with 1 (0.1 mmol), 2 (0.1 mmol), 3a (5 mol%), DABCO (0.1 mmol), and oxidant (0.1 mmol) in DME (1.0 mL) under Ar.b Yield of the isolated product.c Determined by HPLC analysis.d ΔΔG = RT[thin space (1/6-em)]ln(er), R = 0.001986 kcal K−1 mol−1, T = 298.15 K.
1 4g: H 98 97.2[thin space (1/6-em)]:[thin space (1/6-em)]2.8 0.00 2.10
2 4x: NO2 77 81.9[thin space (1/6-em)]:[thin space (1/6-em)]18.1 0.78 0.89
3 4y: F 99 97.1[thin space (1/6-em)]:[thin space (1/6-em)]2.9 0.06 2.08
4 4z: CH3 75 98.0[thin space (1/6-em)]:[thin space (1/6-em)]2.0 −0.17 2.30
5 4aa: OMe 99 98.1[thin space (1/6-em)]:[thin space (1/6-em)]1.9 −0.27 2.34
6 4ab: CF3 61 91.0[thin space (1/6-em)]:[thin space (1/6-em)]9.0 0.54 1.37
7 4ac: OCF3 74 95.2[thin space (1/6-em)]:[thin space (1/6-em)]4.8 0.35 1.77
8 4ad: CN 66 86.2[thin space (1/6-em)]:[thin space (1/6-em)]13.8 0.66 1.09
9 4ae: Ph 78 97.3[thin space (1/6-em)]:[thin space (1/6-em)]2.7 −0.01 2.12


The reaction of bisphenol 5 with 2a was then conducted under the optimal conditions to further explore the reaction mechanism (Scheme 2). Only a 52.4[thin space (1/6-em)]:[thin space (1/6-em)]47.6 er value was obtained. In this reaction, the reactive site was far away from the pre-stereogenic center, which weakened the steric hindrance between the intermediate generated from the catalyst and aldehyde and t-Bu group of bisphenol. And due to the long distance of the two hydroxyl groups of 5, it was very difficult to generate a hydrogen bond in the transition state. All these made the difference in transition state energy smaller, thus resulting in the almost racemic outcome, proving the importance of steric hindrance and the hydrogen bond. Furthermore, from the substrate scope, we knew that the products of the three kinds of aldehydes with different steric hindrances (benzaldehyde, propanal and isobutyraldehyde) were obtained with almost the same er values (Table 3, 4g, 4ap and 4aq), indicating that it was not the steric hindrance interaction between the t-Bu group and aldehyde section of the generated intermediates that played a key role in enantioselectivity but the catalyst section might have contributed to the steric effect. According to the above-mentioned results, a possible transition state was proposed (Fig. 3), in which the steric hindrance between the aromatic ring of catalyst 3a and the t-Bu group at the pre-stereogenic carbon center of the bisphenol substrate is likely to be crucial to induce enantioselectivity.


image file: c7qo01083d-s2.tif
Scheme 2 Control reaction for the mechanism study.

image file: c7qo01083d-f3.tif
Fig. 3 The proposed transition state.

To investigate the synthetic potential of the present desymmetrization strategy, a gram scale reaction of 1g and 2a was conducted under the optimal conditions (Scheme 3a). The product 4g was generated in a good yield with a slight loss of enantioselectivity. More importantly, product 4g could be converted to chiral phosphorus compound 9 which has the potential ability to act as a catalyst14b through only three steps (Scheme 3b). Firstly, 4g was methylated to yield 7 in 50% yield with little erosion of the er value. Then, the Grignard reagent (CH3MgBr) was used to afford product 8. Finally, 9 was obtained by an esterification reaction of 8 with 2-(diphenylphosphino)benzoic acid.


image file: c7qo01083d-s3.tif
Scheme 3 Synthetic transformations.

Conclusions

In summary, we have presented a highly enantioselective NHC-catalyzed desymmetrization reaction of diarylalkane-bisphenols with various aldehydes. The optimal substrate structure was screened by a LFER analysis of the steric effect. As a result, a series of chiral bisphenol type products were generated with excellent reactivities and enantioselectivities (up to 99% yield and 98.7[thin space (1/6-em)]:[thin space (1/6-em)]1.3 er value). According to the results of the LFER analyses of both steric and electronic effects, and other controlled experiments, a possible transition state was proposed. The synthetic transformations demonstrated the effectiveness of our developed method.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We are grateful to the National Natural Science Foundation (Grant No. 21390400 and 21421062) for financial support.

Notes and references

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  18. CCDC 1574456 contains the supplementary crystallographic data for compound 4p. For details, see the ESI..

Footnote

Electronic supplementary information (ESI) available. CCDC 1574456. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7qo01083d

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