Synthesis of Enantioenriched Bromohydrins via Divergent Reactions

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Cite This: J. Am. Chem. Soc. 2018, 140, 10677−10681

Synthesis of Enantioenriched Bromohydrins via Divergent Reactions of Racemic Intermediates from Anchimeric Oxygen Borrowing Yi-Ming Cao, Dieter Lentz, and Mathias Christmann* Institute of Chemistry and Biochemistry, Freie Universität Berlin, Takustraße 3, 14195 Berlin, Germany

J. Am. Chem. Soc. 2018.140:10677-10681. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 08/29/18. For personal use only.

S Supporting Information *

ABSTRACT: We report a chiral phosphoric acid catalyzed bromocyclization/regiodivergent reaction of racemic intermediates sequence, which is enabled by anchimeric oxygen borrowing. Different types of alkenes are applicable, and both enantiomers of the bromohydrin products were obtained in generally excellent yields and enantioselectivities. In addition, an example of enantioconvergent synthesis from the two isomeric products is presented.

T

he catalytic asymmetric halofunctionalization of unactivated alkenes is a powerful synthetic transformation for the construction of chiral halogenated organic compounds, which are of great synthetic utility.1 Recently, important advances have been achieved not only in the discovery of new catalytic reactions but also regarding the reaction mechanism.2 Olefin-to-olefin transfer processes and β-halocarbenium ions mediated pathways are the main obstacles in methodology development that impede stereocontrol.3 Therefore, most work is focused on intramolecular asymmetric halocyclizations,4 which generally benefit from their favorable entropy of activation. Recent work from Borhan et al. offered a higher resolution of such reaction’s mechanistic picture, in which the “nucleophile-assisted alkene activation” plays a key role.5 In sharp contrast, less successful examples have been reported on its intermolecular counterpart.2m,6 By employing “activable” nucleophiles (for instance, carboxylic acids), which can be easily directed and/or activated by the catalyst, a couple of transformations have been realized.7 For halofunctionalizations with “inert” nucleophiles lacking coordination to the catalyst, the task becomes even more formidable (Figure 1a). While excellent work with conventional catalytic paradigms has been reported lately,8,9c,i,10a,b the development of alternative strategies is highly demanded. In this context, Burns et al. have recently reported a unique catalytic system9d,f,h for the asymmetric dihalogenation.9 In their strategy, a titanium halide is employed as the halogen anion source. The formation of a transient titanium complex including the substrate, ligand, and halogen cation source, allows the delivery of the halogen in an intramolecular fashion. Catalytic asymmetric halohydroxylation of unactivated alkenes using water as the nucleophile is the most efficient way to access valuable chiral halohydrins.11,1 However, because water is a rather inert nucleophile and difficult to activate, few examples have been reported.10,6e,9e Inspired by the aforementioned work of Burns, we sought for a design, in which a © 2018 American Chemical Society

Figure 1. Work proposal and initial investigation.

temporarily covalent interaction between an “inert” nucleophile and an assisting group (AG) in the substrate can render the reaction intramolecular. Thus, we envisioned an anchimeric heteroatom borrowing (AHAB) strategy for the targeted halohydroxylation. In the proposal, an ester12 serves as the nucleopile which attack the haliranium ion; water is incorporated by the addition to the forming oxocarbenium ion. It was anticipated that a cyclic hemiorthoester intermediate would be formed which subsequently collapses to give the desired product (Figure 1a). In a preliminary experiment, substrate 1t was tested by using NBS as the Br cation source, and only 2 equiv amount of water was added to the reaction. Using 10 mol % of chiral phosphoric acid (CPA) C1 (see Table 1) as catalyst, we were able to obtain a mixture of isomers 2t and 3t in a good yield. Interestingly, treatment of 3t with Et3N resulted in an intramolecular acyl transfer to afford ent-2t (Figure 1b). The relationship of yields and ee’s is indicative of a regiodivergent reaction of a racemic mixture (RRM) process. CPAs have been applied in many asymmetric reactions,13,14 including catalytic kinetic resolutions; divergent RRM is a powerful type of resolution,15 and such catalyst has scarcely been involved.16 Here, we disclose a CPA catalyzed bromocyclization/ regiodivergent RRM for the enantioselective synthesis of bromohydrins. In order to identify a suitable ester assisting group, we used C1 as the catalyst and the phenyl group as the starting point Received: June 19, 2018 Published: August 13, 2018 10677

DOI: 10.1021/jacs.8b06432 J. Am. Chem. Soc. 2018, 140, 10677−10681

Communication

Journal of the American Chemical Society Table 1. Reaction Developmenta

Table 2. Reaction Scopea

2 entry

R

1 2 3 4 5 6 7 8 9b 10b 11 12

C6H5 4-F-C6H4 4-Cl-C6H4 4-Br-C6H4 4-OMs-C6H4 4-Me-C6H4 3-Br-C6H4 3-MeO-C6H4 2-Br-C6H4 2-Me-C6H4 1-naphthyl 2-naphthyl

2a 2b 2c 2d 2e 2f 2g 2h 2i 2j 2k 2l

3

yield (%)

ee (%)

48 49 48 48 48 45 42 49 44 48 40 44

>99 99 >99 96 98 98 >99 96 96 98 97 98

3a 3b 3c 3d 3e 3f 3g 3h 3i 3j 3k 3l

yield (%)

ee (%)

time (h)

51 51 52 49 51 49 45 50 52 52 46 46

91 93 93 93 93 91 94 90 81 84 85 92

18 16 18 36 36 18 24 60 24 16 40 18

a

Unless otherwise noted, reactions were performed on a 0.025 mmol scale in 0.5 mL 1,2-DCE in the dark; >95% conversion was achieved after the indicated time; ee’s were determined by HPLC. b Ph > iPr > OMe), with the di-tButyl substitution giving the best ee’s (entry 13). After the readily accessible 3,5-di-tBu-4-MeO-benzoyl ester was selected as assisting group, catalysts and solvents were screened (see the Supporting Information (SI)). As a result, CPA C517a in dichloromethane as the solvent showed the best performance to give 2 and 3 in excellent yield and enantioselectivity (entry 14).18 With this system in hand, the substrate scope of the reaction was investigated (Table 2). Various substituted cinnamyl esters with either electron-withdrawing and electron-donating groups in the para or meta positions on the phenyl ring resulted in smooth conversions and gave the two corresponding isomers in excellent yields and high enantioselectivities (entries 1−8). It is noteworthy that electronic property of the substitution at the para position affected the reaction rate, similar to the substituent effect in the assisting group. This observation implies that both the alkene halogenation and hemiorthoester formation are kinetically relevant. Sterically hindered orthosubstituted phenyl and 1-naphthyl derivatives were well tolerated, giving the corresponding 2 isomers in excellent outcomes, and 3s in similar yields and good enantioselectivities (entries 9−11). Encouraged by these results, we further expanded the substrate scope to homoallylic esters (entries

a Unless otherwise noted, reactions were performed on a 0.1 mmol scale in 2 mL CH2Cl2 at rt in the dark for the indicated time; yields are of isolated products; ee’s were determined by HPLC. bYields and ee’s are determined after 4-nitrobenzoylation. cC2 was used as the catalyst. dC8 was used as the catalyst. eReaction was performed at 5 °C. Ar = 3,5-(tBu)2-4-MeO-C6H2.

13−18). First, alkyl substituted internal alkenes were tested (entries 13−16). The reactions proceeded smoothly and with high regioselectivity. In all examples, the hydroxyl group is installed in the C3 position. Isomers 2m−o were obtained in excellent ee while 3m−o obtained with lower enantioselectivities. It also worth noting that the reaction is diastereospecific, i.e., the E-isomer afforded the anti-bromohydrin while the Zisomer gave the syn-product (entries 13, 14). When symmetrical 1n was subjected to the reaction, the desymmetrization of the meso-bromonium ion is neglectable,14b,g suggesting that the CPA may not be directly involved in the ester activation (entry 15). Both aryl substituted and unsubstituted terminal alkenes are amenable as well and corresponding products were obtained in excellent yield and good to excellent enantioselectivity (entries 17−18). Configurations of 3l and 2p were determined by X-ray crystallography and optical rotation comparison respectively (see the SI).19 10678

DOI: 10.1021/jacs.8b06432 J. Am. Chem. Soc. 2018, 140, 10677−10681

Communication

Journal of the American Chemical Society

On the basis of the accumulated observations, we propose the following rationale for the reaction (Figure 2). The process

Next, a few control experiments were carried out to gain insight into the reaction mechanism. Replacing the benzoyl substrates with a benzyl protected cinnamyl alcohol under standard conditions (SC) (see Table 2) did not result in any observable bromohydrin formation despite the presence of stoichiometric amount of water. This result emphasizes the enabling role of the ester group in the reaction (Scheme 1a). Scheme 1. Preliminary Mechanism Studiesa

Figure 2. Proposed mechanism.

can be divided into two catalytic stages. In the first stage (diastereoselective bromocyclization), CPA catalyzes the halocyclization by activation of NBS, and the water nucleophilic attacks the forming oxocarbenium ion. The corresponding cyclic hemiorthoesters (INT and ent-INT) are formed with excellent diastereoselectivity. In the second stage (regiodivergent RRM), catalyzed by the chiral Brønsted acid, the hemiorthoester enantiomers can collapse to different constitutional isomers. We rationalize that the selectivity results from the activation of the different oxygen atoms for each enantiomer. The pathway B is slightly disfavored over path A due to the R group pointing toward the catalyst. As a side reaction, some of INT may collapse through pathway C, yielding ent-3 in small amounts. This process is postulated to contribute to the ee erosion of the 3 isomer. The possibility of an enantioconvergent synthesis22 starting from the two constitutional isomers was also demonstrated (Scheme 2). Isomer anti-3m was first transformed to epoxide 4m. Treatment of 4m with CPA catalyst C217c in CH2Cl2 with 4 equiv of water gave diol 5m. In this process, it is believed that the hemiorthoester was again involved in the regioselective epoxide opening. Exhaustive mesylation of both 5m and anti2m in one-pot followed by deprotection and ring closure afforded chiral tetrahydrofuran 6m in good yield and enantioselectivity. In summary, an anchimeric oxygen borrowing strategy was developed to realize a catalytic bromocyclization/regiodivergent RRM sequence. This method employs commercially available CPAs as catalyst, readily synthesized alkenes, NBS, and notably, only stoichiometric amount of water as the reactant. Both enantiomers of the bromohydrin can be obtained in generally excellent yields and enantioselectivities. Preliminary mechanistic studies hint at the formation of enantiomeric cyclic hemiorthoester intermediates and a subsequent regiodivergent resolution. In addition, an example of an enantioconvergent synthesis from the two products was

a

Ar = 3,5-(tBu)2-4-MeO-C6H2.

When racemic constitutional isomers 2c or 3c were subjected to the SC, no resolution was observed. This result indicates that the resolution does not take place via isomerization between the two products (Scheme 1b). Subjecting E-1m and Z-1m to same conditions, the Z-isomer reacted significantly slower due to steric repulsion (Scheme 1c). In the SC, substrate 1c afforded the products 3c and 2c in an equimolar ratio (1.07:1). In contrast, using the less bulky catalyst C0 changed the ratio to 0.48:1 (Scheme 1d). These observations are in good agreement with the model depicted in Scheme 1d. In line with previous literature reports,20,21 we assume the intermediacy of cyclic hemiorthoesters and furthermore we suggest the involvement of CPA in their collapse to the two constitutional isomers. During the reaction, 1f provided a decreasing ratio of 3f/2f approaching toward about 1.05:1 (Scheme 1e). Such profile is similar to an independent RRM,15g,t suggesting the accumulation of hemiorthoester intermediates during the course. In the 18O labeling experiment (Scheme 1f), the reaction of 1b with H218O gave both isomers with the 18O atom exclusively incorporated into the carbonyl group (see the SI). This result indicates that for both isomers the addition of water proceeds via the proposed cyclic hemiorthoester pathway. The mechanisms concerning direct water addition or acyclic tetrahedral intermediate nucleophilic attack are excluded. 10679

DOI: 10.1021/jacs.8b06432 J. Am. Chem. Soc. 2018, 140, 10677−10681

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Z.; Yeung, Y. Y. Org. Biomol. Chem. 2014, 12, 2333. (k) Tan, C. K.; Yu, W. Z.; Yeung, Y. Y. Chirality 2014, 26, 328. (l) Zheng, S.; Schienebeck, C. M.; Zhang, W.; Wang, H.-Y.; Tang, W. Asian J. Org. Chem. 2014, 3, 366. (m) Zhou, L.; Chen, J. Synthesis 2014, 46, 586. (3) (a) Neverov, A. A.; Brown, R. S. J. Org. Chem. 1996, 61, 962. (b) Denmark, S. C.; Burk, M. T.; Hoover, A. J. J. Am. Chem. Soc. 2010, 132, 1232. (c) Olah, G. A.; Bollinger, J. M. J. Am. Chem. Soc. 1967, 89, 2993. (d) Yousefi, R.; Ashtekar, K. D.; Whitehead, D. C.; Jackson, J. E.; Borhan, B. J. Am. Chem. Soc. 2013, 135, 14524. (4) For selected recent examples, see: (a) Jaganathan, A.; Staples, R. J.; Borhan, B. J. Am. Chem. Soc. 2013, 135, 14806. (b) Ashtekar, K. D.; Marzijarani, N. S.; Jaganathan, A.; Holmes, D.; Jackson, J. E.; Borhan, B. J. Am. Chem. Soc. 2014, 136, 13355. (c) Ke, Z.; Tan, C. K.; Chen, F.; Yeung, Y. Y. J. Am. Chem. Soc. 2014, 136, 5627. (d) Nakatsuji, H.; Sawamura, Y.; Sakakura, A.; Ishihara, K. Angew. Chem., Int. Ed. 2014, 53, 6974. (e) Tay, D. W.; Leung, G. Y.; Yeung, Y. Y. Angew. Chem., Int. Ed. 2014, 53, 5161. (f) Toda, Y.; Pink, M.; Johnston, J. N. J. Am. Chem. Soc. 2014, 136, 14734. (g) Arai, T.; Watanabe, O.; Yabe, S.; Yamanaka, M. Angew. Chem., Int. Ed. 2015, 54, 12767. (h) Shen, Z.; Pan, X.; Lai, Y.; Hu, J.; Wan, X.; Li, X.; Zhang, H.; Xie, W. Chem. Sci. 2015, 6, 6986. (i) Vara, B. A.; Struble, T. J.; Wang, W.; Dobish, M. C.; Johnston, J. N. J. Am. Chem. Soc. 2015, 137, 7302. (j) Woerly, E.; Banik, S. M.; Jacobsen, E. N. J. Am. Chem. Soc. 2016, 138, 13858. (k) Griffin, J. D.; Cavanaugh, C. L.; Nicewicz, D. A. Angew. Chem., Int. Ed. 2017, 56, 2097. (l) Jiang, H. J.; Liu, K.; Yu, J.; Zhang, L.; Gong, L. Z. Angew. Chem., Int. Ed. 2017, 56, 11931. (m) Liu, Y.; Tse, Y.-L. S.; Kwong, F. Y.; Yeung, Y.-Y. ACS Catal. 2017, 7, 4435. (n) Samanta, R. C.; Yamamoto, H. J. Am. Chem. Soc. 2017, 139, 1460. (o) Chan, Y. C.; Yeung, Y. Y. Angew. Chem., Int. Ed. 2018, 57, 3483. (p) Knowe, M. T.; Danneman, M. W.; Sun, S.; Pink, M.; Johnston, J. N. J. Am. Chem. Soc. 2018, 140, 1998. (q) Mennie, K. M.; Banik, S. M.; Reichert, E. C.; Jacobsen, E. N. J. Am. Chem. Soc. 2018, 140, 4797. (r) Salehi Marzijarani, N.; Yousefi, R.; Jaganathan, A.; Ashtekar, K. D.; Jackson, J. E.; Borhan, B. Chem. Sci. 2018, 9, 2898. (s) Lu, Y.; Nakatsuji, H.; Okumura, Y.; Yao, L.; Ishihara, K. J. Am. Chem. Soc. 2018, 140, 6039. (5) Ashtekar, K. D.; Vetticatt, M.; Yousefi, R.; Jackson, J. E.; Borhan, B. J. Am. Chem. Soc. 2016, 138, 8114. (6) For examples using activated alkenes, see: (a) Cai, Y.; Liu, X.; Hui, Y.; Jiang, J.; Wang, W.; Chen, W.; Lin, L.; Feng, X. Angew. Chem., Int. Ed. 2010, 49, 6160. (b) Cai, Y.; Liu, X.; Jiang, J.; Chen, W.; Lin, L.; Feng, X. J. Am. Chem. Soc. 2011, 133, 5636. (c) Cai, Y.; Liu, X.; Li, J.; Chen, W.; Wang, W.; Lin, L.; Feng, X. Chem. - Eur. J. 2011, 17, 14916. (d) Alix, A.; Lalli, C.; Retailleau, P.; Masson, G. J. Am. Chem. Soc. 2012, 134, 10389. (e) Honjo, T.; Phipps, R. J.; Rauniyar, V.; Toste, F. D. Angew. Chem., Int. Ed. 2012, 51, 9684. (f) Cai, Y.; Liu, X.; Zhou, P.; Kuang, Y.; Lin, L.; Feng, X. Chem. Commun. 2013, 49, 8054. (g) Zhou, P.; Cai, Y.; Zhong, X.; Luo, W.; Kang, T.; Li, J.; Liu, X.; Lin, L.; Feng, X. ACS Catal. 2016, 6, 7778. (7) (a) Li, G.-x.; Fu, Q.-q.; Zhang, X.-m.; Jiang, J.; Tang, Z. Tetrahedron: Asymmetry 2012, 23, 245. (b) Zhang, W.; Liu, N.; Schienebeck, C. M.; Zhou, X.; Izhar, I. I.; Guzei, I. A.; Tang, W. Chem. Sci. 2013, 4, 2652. (c) Li, L.; Su, C.; Liu, X.; Tian, H.; Shi, Y. Org. Lett. 2014, 16, 3728. (d) Qi, J.; Fan, G. T.; Chen, J.; Sun, M. H.; Dong, Y. T.; Zhou, L. Chem. Commun. 2014, 50, 13841. (8) Soltanzadeh, B.; Jaganathan, A.; Staples, R. J.; Borhan, B. Angew. Chem., Int. Ed. 2015, 54, 9517. (9) (a) Cresswell, A. J.; Eey, S. T.; Denmark, S. E. Angew. Chem., Int. Ed. 2015, 54, 15642. (b) Snyder, S. A.; Tang, Z.-Y.; Gupta, R. J. Am. Chem. Soc. 2009, 131, 5744. (c) Nicolaou, K. C.; Simmons, N. L.; Ying, Y.; Heretsch, P. M.; Chen, J. S. J. Am. Chem. Soc. 2011, 133, 8134. (d) Hu, D. X.; Shibuya, G. M.; Burns, N. Z. J. Am. Chem. Soc. 2013, 135, 12960. (e) Snyder, S.; Brucks, A.; Treitler, D.; Liu, S.-A. Synthesis 2013, 45, 1886. (f) Hu, D. X.; Seidl, F. J.; Bucher, C.; Burns, N. Z. J. Am. Chem. Soc. 2015, 137, 3795. (g) Huang, W. S.; Chen, L.; Zheng, Z. J.; Yang, K. F.; Xu, Z.; Cui, Y. M.; Xu, L. W. Org. Biomol. Chem. 2016, 14, 7927. (h) Landry, M. L.; Hu, D. X.; McKenna, G. M.; Burns, N. Z. J. Am. Chem. Soc. 2016, 138, 5150. (i) Soltanzadeh, B.; Jaganathan, A.; Yi, Y.; Yi, H.; Staples, R. J.; Borhan, B. J. Am. Chem.

Scheme 2. Enantioconvergent Synthesis

accomplished. Future work will include reaction optimization, substrates scope expansion and applications in target-oriented synthesis. Further studies on the mechanism, as well as application of this strategy to other reaction types, are currently underway.

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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/jacs.8b06432. Data for the configuration determination of 3l (CIF) Experimental procedures and compound characterization (PDF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Dieter Lentz: 0000-0002-0583-7024 Mathias Christmann: 0000-0001-9313-2392 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Alexander von Humboldt Foundation for postdoctoral fellowships (Yi-Ming Cao). We thank Guoli He and Thomas Siemon (both FU Berlin) for MS measurement, Christiane Groneberg (FU Berlin) for analytical support.

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DOI: 10.1021/jacs.8b06432 J. Am. Chem. Soc. 2018, 140, 10677−10681

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DOI: 10.1021/jacs.8b06432 J. Am. Chem. Soc. 2018, 140, 10677−10681