Letter pubs.acs.org/OrgLett
Rh/Wudaphos-Catalyzed Asymmetric Hydrogenation of Sodium α‑Arylethenylsulfonates: A Method To Access Chiral α‑Arylethylsulfonic Acids Xuguang Yin,† Caiyou Chen,† Xiu-Qin Dong,*,† and Xumu Zhang*,‡,† †
College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, Hubei 430072, P. R. China Department of Chemistry, South University of Science and Technology of China, Shenzhen, Guangdong 518055, P. R. China
‡
S Supporting Information *
ABSTRACT: A highly enantioselective hydrogenation of various sodium α-arylethenylsulfonates catalyzed by Rh/chiral ferrocenyl bisphosphorus ligand (Wudaphos) was successfully developed to construct a series of chiral α-arylethylsulfonic acids in the presence of CF3SO3H with full conversion and good to excellent enantioselectivity (>99% conversion, up to 96% ee) under mild reaction conditions for the first time. Moreover, the control experiment results showed that the noncovalent ion pair interaction between the α-arylethenylsulfonic acid and the Wudaphos ligand plays an important role in this asymmetric hydrogenation system.
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from the corresponding racemates by the resolution process with chiral amines.7 Only a few enantiomeric α-substituted sulfonic acids can be obtained by “Dutch resolution”,8 and the highest theoretical yield is just 50%. In 1992, Corey and coworkers found that chiral (S)-(−)-1-phenylethanesulfonic acid [(−)-PES] can be obtained from the transformation of (R)-1phenylethanol, which was obtained by the catalytic asymmetric reduction of methyl phenyl ketone.9 In 2005, Enders and coworkers reported the first auxiliary-controlled synthesis of various α-substituted sulfonic acid derivatives, in which enantiopure alcohols are used as chiral auxiliaries.10 In 2011, Adamo and co-workers successfully developed catalytic enantioselective addition of sodium bisulfite to α,β-unsaturated alkenes using a bifunctional amine−thiourea catalyst.11 Subsequently, Zhao and co-workers reported a chiral Ir- and Pd-complex-catalyzed asymmetric allylation reaction of allylic carbonates with sodium sulfite to afford allylic sulfonic acids in excellent yields and enantioselectivities.12 Despite the great progress that has been achieved, it is still necessary to develop efficient catalytic asymmetric methodologies for the preparation of chiral sulfonic acids. Nowadays, catalytic asymmetric hydrogenation of functionalized olefins is regarded as one of the most powerful and environmentally friendly methods to synthesize chiral compounds.13 To the best of our knowledge, catalytic asymmetric hydrogenation of unsaturated sulfonic acids has not been reported until now. Most recently, we developed a new chiral ferrocenyl bisphosphorus ligand (Wudaphos), which was successfully applied in Rh-catalyzed asymmetric hydrogenation of α-substituted acrylic acids with
s one of the most important classes of acids, chiral sulfonic acids have shown remarkable significance and owned wide application in the areas of biochemistry and natural products.1 For example, 6-gingesulfonic acid, which was isolated from Zingiberis rhizome, displays potent antiulcer activity;2 the semisynthetic penicillin α-sulfobenzylpenicillin shows excellent antibacterial activity against Pseudomonas aeruginosa;3 αphosphono sulfonates are squalene synthase inhibitors;4 and echinosulfonic acids, which were isolated from southern Australian marine sponges Echinodictyum, have good antibactarial activity (Figure 1).5 In addition, chiral α-arylethylsulfonic acids are excellent candidates as chiral resolving agents for numerous amino acids.6 Because of their great importance, much effort has been devoted to the development of efficient synthetic methodologies to approach chiral sulfonic acids. During the past decades, chiral α-substituted sulfonic acids have been obtained
Figure 1. Several examples of pharmaceuticals and natural products featuring a chiral sulfonic acid motif. © XXXX American Chemical Society
Received: April 4, 2017
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DOI: 10.1021/acs.orglett.7b01021 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters excellent results through the non-covalent ion pair interaction between the unsaturated acid and Wudaphos.14 Since this secondary interaction activation strategy played an important role to obtain high reactivity and excellent enantioselectivity, we believe that the asymmetric hydrogenation of α-arylethenylsulfonic acids should provide excellent results with our Wudaphos ligand. In the present work, this novel synergistic activation strategy was proved to be very efficient in Rh-catalyzed asymmetric hydrogenation of various α-arylethenylsulfonic acids formed in situ from sodium α-arylethylsulfonates using CF3SO3H (>99% conversion, up to 96% ee). The sodium α-arylethenylsulfonates were efficiently prepared from the arylmethanesulfonyl chlorides as the starting materials in three steps (Scheme 1).15 First, the methyl arylmethanesul-
Table 1. Optimization of the Reaction Conditions for the Asymmetric Hydrogenation of Sodium αPhenylethenylsulfonate (1a)a
entry
ligand
solvent
equiv of CF3SO3H
conv. (%)b
ee (%)c
1 2 3 4 5 6 7 8 9 10 11 12 13
(S)-Binap (S)-Segphos Josiphos Taniaphos Wudaphos Wudaphos Wudaphos Wudaphos Wudaphos Wudaphos Wudaphos Wudaphos Wudaphos
MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH EtOH i PrOH TFE THF
0 0 0 0 0 0.5 1.0 1.5 2.0 1.0 1.0 1.0 1.0
52 >99 >99 54 >99 >99 >99 80 20 >99 >99 7 >99
13 0 15 0 17 25 90 82 81 90 91 86 92
Scheme 1. Synthesis of Sodium α-Arylethenylsulfonates 1
a
All of the reactions were carried out with a substrate/catalyst ratio of 100:1 at 25 °C under 30 atm H2 for 12 h. bDetermined by 1H NMR spectroscopy. cDetermined by HPLC on a chiral phase.
fonates were easily synthesized through a simple nucleophilic substitution reaction of the arylmethanesulfonyl chlorides and sodium methoxide. Then the methyl 1-arylethene-1-sulfonates were obtained by the reaction between the methyl arylmethanesulfonates and paraformaldehyde in the presence of K2CO3 and tetrabutylammonium iodide (TBAI). Finally, the sodium 1-arylethene-1-sulfonates 1 were prepared through the replacement of the methyl 1-arylethene-1-sulfonates with NaI. We began our study by using sodium 1-phenylethene-1sulfonate (1a) as a model substrate in Rh-catalyzed asymmetric hydrogenation (substrate/catalyst (S/C) = 100, 30 atm H2, 25 °C). As shown in Table 1, several chiral phosphine ligands, such as (S)-Binap, (S)-Segphos, Josiphos, Taniaphos, and Wudaphos (Figure 2) displayed poor results (52−99% conversion, 0−17% ee; Table 1, entries 1−5). Compared with other chiral phosphine ligands, Wudaphos provided the best result (>99% conversion, 17% ee; Table 1, entry 5). To our delight, the enantioselectivity was gradually improved substantially in the Rh/Wudaphos catalytic system when the amount of CF3SO3H was increased from 0.5 to 1.0 equiv (>99% conversion, 25−90% ee; Table 1, entries 6 and 7). In addition, when the amount of CF3SO3H was increased to 1.5 or 2.0 equiv, both the conversion and the enantioselectivity became worse (20−80% conversion, 81−82% ee; Table 1, entries 8 and 9). It is possible that the extra CF3SO3H interacts with the dimethylamine motif of Wudaphos, which may reduce the reactivity and chiral induction ability in our catalytic system. Then, we inspected the solvent effects of this transformation in the presence of 1.0 equiv of CF3SO3H. Substrate 1a is soluble in a few kinds of solvents, such as MeOH, EtOH, iPrOH, trifluoroethanol (TFE), and tetrahydrofuran (THF). We found that the asymmetric hydrogenation proceeded well in MeOH, EtOH, iPrOH, and THF with full conversion and excellent enantioselectivity (>99% conversion, 90−92% ee; Table 1, entries 7, 10, 11, and 13). TFE provided poor conversion, albeit with good enantioselectivity (7% conversion, 86% ee; Table 1, entry 12). THF was selected as the best solvent in terms of both reactivity and enantioselectivity (>99% conversion, 92% ee; Table 1, entry 13).
Figure 2. Several phosphine ligands for asymmetric hydrogenation of sodium 1-phenylethene-1-sulfonate (1a).
Encouraged by these excellent results, we turned our attention to an investigation of the generality of our catalytic system in the asymmetric hydrogenation of various sodium αarylethenylsulfonates under the optimized reaction conditions. These results are summarized in Scheme 2. A series of sodium α-arylethenylsulfonates bearing different substituent groups on the phenyl ring were hydrogenated well to afford the corresponding chiral α-arylethylsulfonic acids with full conversion and good to excellent enantioselectivity (87−96% ee). The electric property and position of the substituent groups on the phenyl ring of the substrates had little influence on both the reactivity and enantioselectivity. The substrates containing electron-donating groups (1b−d) or electron-withdrawing groups (1e−j) on the phenyl ring reacted efficiently with good to excellent results (full conversion, 87−99% ee). Moreover, our catalytic system is very efficient in this asymmetric hydrogenation. Substrate 1a was hydrogenated well on a gram scale under the optimized reaction conditions, and B
DOI: 10.1021/acs.orglett.7b01021 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Scheme 2. Scope of the Asymmetric Hydrogenation of Sodium α-Arylethenylsulfonates 1 Catalyzed by Rh/ Wudaphosa
Scheme 4. Control Experiments for the Investigation of the Interaction between the Substrate and Ligand
for the first time. A series of chiral α-arylethylsulfonic acids were obtained with full conversion and good to excellent enantioselectivity (87−96% ee). Additionally, this asymmetric hydrogenation was performed well on gram-scale without loss of enantioselectivity. Furthermore, control experiment results demonstrated that the non-covalent ion pair interaction between the α-arylethenylsulfonic acid and the Wudaphos ligand plays a critical role in obtaining high reactivity and excellent enantioselectivity in this asymmetric hydrogenation.
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a
All of the reactions were performed with 0.1 mmol of substrate and substrate/[Rh(NBD)2]BF4/Wudaphos = 1/0.01/0.011 at 25 °C under 30 atm H2 with 1.0 equiv of CF3SO3H in 1.0 mL of THF for 12 h. Conversion was determined by 1H NMR spectroscopy. The ee values were determined by HPLC on a chiral column. The absolute configuration was determined according to the literature.9
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b01021. Experimental details and characterization data (PDF)
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the desired product 2a was obtained with 99% conversion and 92% ee (Scheme 3).
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected].
Scheme 3. Gram-Scale Asymmetric Hydrogenation of 1a
ORCID
Xumu Zhang: 0000-0001-5700-0608 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Wuhan University (203273463, 203410100064), the “111” Project of the Ministry of Education of China, the National Natural Science Foundation of China (21372179, 21432007, 21502145), and the Natural Science Foundation of Hubei Province (2016CFB449) for financial support.
In order to explore the importance of the secondary interaction between the substrate and ligand, some control experiments were conducted. The asymmetric hydrogenation of 1a was carried out under the optimized conditions without CF3SO3H, and the corresponding product 2a was obtained with only 17% ee (Scheme 4a). Furthermore, no reaction was observed when methyl 1-phenylethene-1-sulfonate (3a) was used as the substrate (Scheme 4b). These results demonstrated that the α-arylethenylsulfonic acids were formed in situ by adding CF3SO3H of our catalytic system, providing the corresponding hydrogenation products with high enantioselectivities. In addition, a non-covalent ion pair interaction between the α-arylethenylsulfonic acid and the Wudaphos ligand also exists and plays a critical role in obtaining high activity and excellent enantioselectivity in this asymmetric hydrogenation. In summary, a novel synthetic method to prepare various chiral α-arylethylsulfonic acids through Rh/Wudaphos-catalyzed asymmetric hydrogenation of sodium α-arylethenylsulfonates in the presence of CF3SO3H was successfully developed
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REFERENCES
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DOI: 10.1021/acs.orglett.7b01021 Org. Lett. XXXX, XXX, XXX−XXX
