Letter pubs.acs.org/OrgLett
P(O)R2‑Directed Enantioselective C−H Olefination toward Chiral Atropoisomeric Phosphine−Olefin Compounds Shi-Xia Li,† Yan-Na Ma,† and Shang-Dong Yang*,†,‡ †
State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, P. R. China State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, P. R. China
‡
S Supporting Information *
ABSTRACT: An effective synthesis of chiral atropoisomeric biaryl phosphine−olefin compounds via palladium-catalyzed enantioselective C−H olefination has been developed for the first time. The reactions are operationally simple, tolerate wide functional groups, and have a good ee value. Notably, P(O)R2 not only acts as the directing group to direct C−H activation in order to make a useful ligand but also serves to facilitate composition of the product in a useful manner in this transformation.
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ligands which induced enantioselective C−H olefinations, the Colobert group succeeded in developing Pd(II)-catalyzed atroposelective C−H olefination of biphenyls by employing enantiopure p-tolyl sulfoxide as the directing group (b, Figure 1).18 Our group has also achieved asymmetric chiral P(O)R1R2directed palladium(II)-catalyzed C−H alkenylations of biaryl derivatives (b, Figure 1).19 Herein, we report a palladiumcatalyzed enantioselective C−H olefination of racemic 2diphenylphosphino-2′-methylbiphenyl for the synthesis of chiral atropoisomeric biaryl phosphine−olefin compounds (Figure 1c). Notably, P(O)R2 not only acts as the directing group to direct C−H activation and makes for a useful ligand but also serves to facilitate composition of the product in a manner conducive to this transformation. In recent years, Yu and co-workers achieved an elegant and highly successful strategy when they discovered that chiral monoprotected amino acids serve as efficient ligands to enable enantioselective C−H activation of a series of prochiral substrates.20 Subsequently, the You group has also successfully applied this strategy for the enantioselective synthesis of planar chiral ferrocenes by Pd-catalyzed direct functionalization of ferrocene with boronic acids.21 Inspired by the elegance of these achievements, we selected the potential axially chiral biaryls of 2diphenylphosphino-2′-methylbiphenyl (1a) and ethyl acrylate as model substrates, and chiral monoprotected amino acids as ligands in order to identify suitable reaction conditions. We first selected 5 mol % Pd(OAc)2 as the catalyst and AgOAc as the oxidant to screen different natural chiral amino acids. Results demonstrated that the unprotected amino acids such as L-Val-
hiral biaryl phosphine−olefin compounds as one novel type of atropoisomeric compounds have attracted much attention from researchers due to their high reactivities and selectivities in asymmetric catalysis.1 Biaryl axially chiral phosphine−olefins demonstrate a unique advantage, as they are significantly different from stereogenic atoms with four different substituents. In general, the preparation of enantiopure chiral biaryl phosphine−olefins relies on direct phosphinylation and olefination of optically pure binaphthyl skeletons of BINOL. Representative contributions hail from the Widhalm,2 Carreira,3 Du,4 and Xu5 groups. Moreover, the Gu group performs a palladium-catalyzed asymmetric synthesis of axially chiral 1vinylnaphthalen-2-yl phosphine oxides from aryl bromides and hydrazones.6 Despite these achievements, the binaphthyl skeleton has qualified their diversity in orientation frequently. As a result, the development of an accessible and concise method of synthesizing chiral phosphine−olefin compounds in a manner that tolerates a wide range of substrates represents an important and challenging task. As a practical alternative strategy, the emergence of transitionmetal-catalyzed direct enantioselective C−H activation provides a potential alternative strategy in large part due to their step and atom economy.7−14 Therefore, the advancements in the enantioselective C−H olefination have occurred by way of different strategies (Figure 1a). For example, Yu and co-workers demonstrated a Pd(II)-catalyzed enantioselective C−H olefination of carboxylic acids through desymmetrization.15 In 2016, they further developed a Pd(II)-catalyzed enantioselective C−H olefination of α-hydroxy and α-amino phenylacetic acids through kinetic resolution.16 In 2014, the You group reported an asymmetric Rh(III)-catalyzed direct alkenylation of biaryl derivatives with olefins.17 Besides the aforementioned chiral © XXXX American Chemical Society
Received: February 28, 2017
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DOI: 10.1021/acs.orglett.7b00608 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters
Scheme 1. Chiral ligands of L-Amino Acids Screeninga,b,c
a
Reaction conditions: 1a (0.2 mmol), ethyle acrylate (0.6 mmol), [Pd] (5 mol %), ligand (10 mol %), [Ag] (3.0 equiv), solvent (2.0 mL), temperature, air, 48 h. bYield of isolated product. cEnantioselectivities were determined by high performance liquid chromatography (HPLC) analysis employing a chiral column.
Figure 1. Different strategies for asymmetric C−H olefinations.
OH and L-Pro-OH restrained the reaction, but the monoprotected amino acids such as Ac-L-Val-OH, Ac-L-Ala-OH, Ac-LLeu-OH, and others had the effect of promoting the reaction. The best ee value (84%) was obtained with Ac-L-Phe-OH as the chiral ligand (Scheme 1, L7). Based on these results, other monoprotected amino acids with different protective groups have also been investigated. As expected, the best result was observed by employing Boc-L-Val-OH as the ligand: chiral biaryl phosphine−olefin hybrid ligands of 2a afforded a 90% yield with 87% ee (Scheme 1, L9). Although the benzyl and acyl group protected amino acids also prompted the reaction to work smoothly, only either a lower ee value or racemic index was observed. Next, with Boc-L-Val-OH as the chiral ligand, solvents and other palladium catalysts were again evaluated. We found that the former result was still the best choice. Finally, the reaction temperature evaluation indicated that 60 °C is still the best choice (for details, please see Supporting Information). Thus, optimal reaction conditions were obtained by using Pd(OAc)2 (5 mol %) as the catalyst, Boc-L-Val-OH (10 mol %) as the ligand, and 3.0 equiv of AgOAc as the oxidant in 2.0 mL of combined solvent CF3CH2OH/DME (1:1) at 60 °C under air. In order to demonstrate the generality of this enantioselective direct C−H olefinative reaction, various acrylates and substituted diphenylphosphine oxide derivates were investigated under optimized conditions (Scheme 2). We first surveyed various acrylates with 2-diphenylphosphino-2′-methylbiphenyl (1a) (Scheme 2, entries 2a−2g). We were pleased to find that electron-deficient olefins such as butyl, benzyl, phosphite, and sulfonyl acrylates were compatible with this enantioselective direct C−H olefinative reaction and that the corresponding products were afforded in good yields with 84%−92% ee values. The absolute configuration of the product (S)-2f was confirmed by single-crystal X-ray crystallography, which also explains why the transition state of the (S)-Pd complex is more favorable than the (R)-Pd complex in the C−H activation step. The CH3 at the
2′-position was far from the phenyl group of diphenylphosphine oxide, so the steric repulsion was much smaller as a result. In addition, use of bulky P(O)(i-Pr)2 and P(O)(t-Bu)2 groups as directing groups was advantageous to obtain better selectivity (91% ee and 96% ee) (Scheme 2, (S)-2r−2s). These results were consistent with the formation of the products, which were faster with the S configuration. Furthermore, different aryl olefins have also worked very well in the reaction and afforded corresponding products of (S)-2h, 2i, 2t in moderate-to-good yields with 86− 93% ee values. Notably, the electronic effect in the enantioselective direct C−H olefination was very notable; the electron-donating substituent groups, such as methoxyl, and electron-withdrawing substituent groups, such as F− and Cl− at the 2′-position, were also well tolerated in this C−H olefination, but lower yields and a higher ee of products were observed (Scheme 2, (S)-2j−2l). Furthermore, some naphthyl diphenylphosphine oxide derivatives could also undergo enantioselective direct C−H olefination in order to obtain products in good yields with the best ee values (Scheme 2, (S)-2q and (R)-2w). To our delight, the 1,4-dihydronaphthalene also tolerated the reaction and the chiral product of (S)-2z was obtained in 30% yield with 79% ee. We also extended this method to modify estrone and afforded chiral phosphine−olefinstrone (S)-2ac in 83% yield and 88% ee. Moreover, we further examined different substituents including −CHO, −COMe, −CO2Me, −iPr, −Ph, and OEt at the 2′-position. All of these substituents could been tolerated in the reaction and afforded corresponding products in good yields and ee value. Finally, we investigated two heterocyclic substrates of pyrrolidine and thiophene, which were suited to the reaction and products of (S)-2aa and (S)-2ab were obtained in good yields and moderate ee values. B
DOI: 10.1021/acs.orglett.7b00608 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Scheme 2. Investigation of Various Acrylates and Substituted Diphenylphosphine Oxide Derivatesa,b,c
Scheme 3. Further Applications
selected product (R)-2w to react with 4-chlorophenylboronic acid through Michael addition in order to yield the chiral monophosphate ligand precursor (R,S)-3w in 83% yield with a 96:4 dr value, thus demonstrating the utility of our chemistry (eq 3, Scheme 3). In summary, an effective synthesis of chiral atropoisomeric biaryl phosphine−olefin compounds via palladium-catalyzed enantioselective C−H olefination through dynamic kinetic resolution (DKR) has been developed for the first time. The reactions are operationally simple, tolerate a wide array of functional groups, and have a good ee value. This research opens a new door to the useful synthesis of new axis chiral biaryl phosphine−olefin hybrid ligands.
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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.orglett.7b00608. Experimental details and characterization data for all new compounds (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Shang-Dong Yang: 0000-0002-4486-800X a
Reaction conditions: 1a (0.2 mmol), ethyle acrylate (0.6 mmol), [Pd] (5 mol %), ligand (10 mol %), [Ag] (3.0 equiv), solvent (2.0 mL), temperature, air, 48 h. bYield of isolated product. cEnantioselectivities were determined by high performance liquid chromatography (HPLC) analysis employing a chiral column.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful for the NSFC (Nos. 21472076 and 21532001) and PCSIRT (IRT_15R28 and lzujbky-2016-ct02) financial support.
The chiral binaphthyl skeleton phosphine−olefin hybrid ligand is important to many metal-catalyzed organic transformations, including many chiral reactions.22 Thus, we have also used the racemic binaphthyl diphenyl oxide (1ad) as a substrate in order to examine this enantioselective C−H olefination reaction and obtained the desired products (S)-2ad in 62% yield with 88% ee, which could be improved to 99% ee by simple recrystallization (eq 1, Scheme 3). Furthermore, (S)-2ad has also been reduced to the phosphine−olefin hybrid ligand (S)-3ad by using HSiCl3 (eq 2, Scheme 3). Chiral monophosphate ligands are also important in asymmetric catalysis. Therefore, we
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DOI: 10.1021/acs.orglett.7b00608 Org. Lett. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.orglett.7b00608 Org. Lett. XXXX, XXX, XXX−XXX
