Cu-Catalyzed Enantioselective Atropisomer Synthesis via Thiolative

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Cu-Catalyzed Enantioselective Atropisomer Synthesis via Thiolative Ring Opening of Five-Membered Cyclic Diaryliodoniums Mengqing Hou, Ruixian Deng, and Zhenhua Gu* Department of Chemistry, Center for Excellence in Molecular Synthesis, and Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230026, P.R. China

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S Supporting Information *

ABSTRACT: A Cu-catalyzed asymmetric thiolative ring opening reaction of five-membered diaryliodonium salts and potassium thioates for the synthesis of atropisomeric 2′-iodo-[1,1′-biphenyl]-2-yl thioates was realized. The optimal catalytic system, Cu(CH3CN)4PF6/(Ph)-bis(oxazoline), showed the best performance with respect to both yields and stereocontrol. Finally, the utility of these products was briefly demonstrated by the synthesis of an axially chiral P,S-ligand.

B

iaryl atropisomers are a class of compounds in which their rotation around the aryl−aryl single bond was inhibited due to steric hindrance. These structures are widespread in a number of natural products, biologically active compounds, or pharmaceuticals.1 Owing to the high substituent-loading capability of the four ortho-positions, biaryl atropisomeric ligands and catalysts showed high substituent diversity and have demonstrated wide applications in asymmetric catalysis. Since it is highly sterically hindered around the axis, the construction of these aryl−aryl bonds is a particular challenge in terms of yields and selectivity.2 Thus, much effort has been made in this area, and several elegant ligands and catalysts have been developed to realize highly enantioselective aryl−aryl cross-coupling.3,4 Moreover, alternative methods, namely de novo aryl ring synthesis,5 asymmetric aromatic substitution,6 desymmetrization or (dynamic) kinetic resolution,7,8 and chirality transfer,9 have been established, and the diversity of these axially chiral biaryl analogues has significantly improved. Recently, organocatalysis has emerged as a powerful method for the preparation of axially chiral biaryl compounds.10,11 Through the lactone strategy, Bringmann and co-workers realized efficient syntheses for a series of axially chiral biaryls, including many atropisomeric natural products.12 Hayashi and co-workers reported a Ni-catalyzed ring-opening/cross-coupling reaction of dinaphthothiophene for the asymmetric synthesis atropisomeric (1,1′-binaphthalene)-2-thiol analogues.13 An impressive number of catalytically asymmetric reactions by the use of sulfur-containing ligands were documented, where axially chiral biaryl thioethers represent an important class of privileged ligands (Figure 1).14 However, these ligands usually have a finite number of skeletons, such as a binaphthalene structure, which are usually prepared from the optically active BINOL. Thus, structural diversity of these © XXXX American Chemical Society

Figure 1. Axially chiral ligands containing thioether.

ligands is very restricted. The diaryliodonium salt involved arylation reaction is a powerful method for the synthesis of substituted aromatic compounds by varying different nucleophiles. The transition-metal-catalyzed ring-opening/cross-coupling of five-membered diaryliodonium salts is a powerful method that enables access to functionalized 2-iodo-1,1′biphenyl derivatives (Scheme 1a,b).15−17 We reason that by the use of proper sulfur nucleophiles, optically active 2-thio-2′iodo-1,1′-biphenyl derivatives can be prepared through this ring-opening strategy (Scheme 1c). With this idea in mind, our investigation commenced with the reaction of iodonium salt 1a and potassium ethanethioate 2a (Table 1). With CuI as the catalyst and toluene as the solvent, the bis(oxazolinyl)pyridine (PyBox) ligands (L1−L3), which showed excellent performance in Cu-catalyzed amination reaction of these cyclic iodoniums,15b did not give satisfactory enantioselectivity (entries 1−3). Bisoxazolines L4− L6 showed better stereocontrol than PyBox, and the enantioselectivity reached 51% when L6 was used as the ligand (entries 4−6). Further investigation identified that the copper sources significantly affected this transformation. For example, catalyst Cu(MeCN)4PF6 improved the ee value to 68% with 99% yield (entry 7). The reaction delivered 3a with higher enantioselectivity in CH2Cl2 than that in toluene (entry Received: August 3, 2018

A

DOI: 10.1021/acs.orglett.8b02477 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters

materials, L4 was chosen as the optimal ligand for further studies. Under the standard conditions, a 2.0 mmol scale reaction of 1b and 2a worked with equal efficacy to give 3a in identical enantioselectivity (see the Supporting Information). It is interesting to note that in the presence of K2CO3 the reaction with thioacetic acid 4 as the substrate gave inferior results (Scheme 2a). For example, stirring the mixture of CuI

Scheme 1. Atropisomer Synthesis via Asymmetric Ring Opening of Cyclic Diaryliodoniums

Scheme 2. Substrate Effect

Table 1. Optimization of Reaction Conditionsa

entry

X−

Cu/L

solv/temp (°C)

yield (%)

ee (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14

TfO− TfO− TfO− TfO− TfO− TfO− TfO− TfO− TfO− TfO− TfO− PF6− PF6− PF6−

CuI/L1 CuI/L2 CuI/L3 CuI/L4 Cu/L5 CuI/L6 Cu(MeCN)4PF6/L6 Cu(MeCN)4PF6/L6 Cu(MeCN)4PF6/L6 Cu(MeCN)4PF6/L6 Cu(MeCN)4PF6/L4b Cu(MeCN)4PF6/L4b,c Cu(MeCN)4PF6/L7b,c Cu(MeCN)4PF6/L8b,c

toluene/40 toluene/40 toluene/40 toluene/40 toluene/40 toluene/40 toluene/40 CH2Cl2/40 CH2Cl2/0 CH2Cl2/−20 CH2Cl2/−20 CH2Cl2/−20 CH2Cl2/−20 CH2Cl2/−20

90 99 92 92 90 45 99 99 99 99 93 92 81 97

9 6 8 44 17 51 68 83 92 94 95 98 99 97

and L6 in toluene for 1 h, followed by the addition of K2CO3, 1a, and 4, resulted no stereoinduction. The acidity of ethanethioic acid might cause the decomposition of bis(oxazoline) and resulted no enantioselectivity. However, the slow addition of thioacetic acid 4 to a mixture of 1a, K2CO3, and CuI/L6 gave comparable enantioselectivity, while the yield of 3a decreased. With the optimal conditions in hand, we tested the substrate scope of this Cu-catalyzed enantioselective ring-opening/ thiolation reaction (Scheme 3). The reactions with aliphatic thioates worked smoothly, albeit the one with potassium cyclohexylthioate gave a slightly decreased enantioselectivity (3b and 3c). Potassium benzothioates, bearing either electrondonating (3e−f,k−l) or withdrawing groups (3g−j,3m) on the phenyl rings are compatible substrates, and good to excellent yields with 96−99% ees were achieved. Notably, sterically bulky potassium thioates reacted uneventfully with 1b, though potassium 2,6-dimethoxybenzothioate delivered the corresponding product in a relatively lower yield (3i−k). Additionally, the reaction of potassium naphthalene-2-carbothioate proceeded steadily to give excellent stereoinduction (3n). The counteranion of the iodonium salt strongly affected the enantioselectivity. For comparison, enantiomeric excess of thioester 3i dropped from 97% to 87% when 1a was used as the substrate in lieu of 1b. A similar anion effect was described by Gaunt in Cu-catalyzed semipinacol rearrangement with diaryliodoniums.19 For our case, we reasoned that this phenomenon was caused by the weak coordination ability of triflate anion. Thus, the corresponding reaction of cyclic diaryliodonium with tetrafluoroborate anion gave excellent enantioselectivity (Scheme 2b). The absolute configuration was finally determined to be S by single-crystal X-ray diffraction analysis of 3i. The scope of substituted cyclic diaryliodoniums is summarized in Scheme 4. The reaction tolerated m- or pmethyl groups on the phenyl rings to the C−I bonds, and it afforded the corresponding products with exceptional enantioselectivity (3o−r). For those nonsymmetric cyclic diaryliodoniums, a group was introduced to the adjacent position of the C−I bond. With an adjacent methyl group or chloro atom, the reactions with potassium ethanethioate 2a

a Unless stated otherwise, the reaction was conducted with 1a (0.10 mmol), 2a (0.15 mmol), Cu salt (5.0 mol %), and ligand (10.0 mol %) in the indicated solvent (1.0 mL). b7.5 mol % of ligand was used. c 0.10 mmol of 2a was used.

8), and the ee value was further advanced to 94% by lowering the temperature to −20 °C (entries 9 and 10). There was no negative effect when the loading of the ligand decreased to 7.5 mol % [Cu/bis(oxazoline) = 1:1.5] (entry 11). The counterion effect of iodonium salts was also investigated. The reaction of 1b, with a hexafluorophosphate anion, gave slightly better stereoselectivity (entry 12). Ligands L7 and L8, where cyclopentyl or cyclopropyl was introduced in lieu of the dimethyl moiety, worked at a very similar efficacy as L4 (entries 13 and 14). Notably, no O-arylation product was observed in the reactions.18 Considering the availability of the B

DOI: 10.1021/acs.orglett.8b02477 Org. Lett. XXXX, XXX, XXX−XXX

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acceptable, ee values (3t, 3v, and 3x). In the case of the diaryliodonium salts containing p-tosyloxyl groups adjacent to the chiral axis, the stereoselectivity was also perfect, albeit the yields slightly decreased (3y and 3z). This ring-opening reaction of diaryliodoniums with binaphthyl structure worked equally efficient to give 3aa in 97% ee. A plausible catalytic cycle for this Cu(I)-catalyzed asymmetric ring-opening/thiolation reaction was proposed in Scheme 5. The interaction of Cu(I)/bisoxazoline complex A Scheme 5. Plausible Catalytic Cycle

with 1b has two possible models: B and C. In model C, the steric repulsion between the phenyl ring in bisoxazoline and the methyl group resulted this disfavored conformation. Cleavage of the C−I bond in B would give Cu(III) complex D, which bonded with potassium thioate, followed by reductive elimination, to give the final product 3.20 To further demonstrate the utility of these axially chiral thioesters, a synthesis of P,S-ligand from 3a was performed (Scheme 6). Hydrolysis with NaOH in methanol worked

a

The reactions were conducted with 1a (0.10 mmol), 2 (0.10 mmol), Cu(CH3CN)4PF6 (5.0 mol %), and L4 (7.5 mol %) in CH2Cl2 (1.0 mL) at −20 °C.

Scheme 4. Substrate Scope with Iodonium Saltsa

Scheme 6. Synthesis of P,S-ligand

uneventfully to deliver the thiophenol, followed by methylation to yield the thioether 5 in 80% overall yields without decreasing the optical activity. Palladium/dppb-catalyzed phosphorylation, followed by reduction, gave 6 in 98% ee. In conclusion, we have developed an asymmetric ringopening/thiolation reaction of five-membered cyclic diaryliodoniums for the synthesis of atropisomeric 2′-iodo[1,1′biphenyl]-2-yl thioates. With commercially available Phbis(oxazoline) L4, this thiolative ring-opening reaction usually gave high enantioselectivity. Finally, a typical P,S-ligand has been synthesized from axially chiral product 3a. Further studies regarding the utilities of these chiral thioesters and P,S-ligand are underway in our laboratory.

a

The reactions were conducted with 1 (0.10 mmol), 2 (0.10 mmol), Cu(CH3CN)4PF6 (5.0 mol %), L4 (7.5 mol %) in CH2Cl2 (1.0 mL) at −20 °C.

gave equally high enantioselectivity (3s, 3u, and 3w), while the one with potassium benzothioate afforded decreased, yet C

DOI: 10.1021/acs.orglett.8b02477 Org. Lett. XXXX, XXX, XXX−XXX

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b02477. Experimental procedures and spectroscopic data (PDF) Accession Codes

CCDC 1852000 and 1854391 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Zhenhua Gu: 0000-0001-8168-2012 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for financial support from the NSFC (21622206, 21871241), the “973” Project from the MOST of China (2015CB856600), and the Fundamental Research Funds for the Central Universities (WK2060190086).

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DOI: 10.1021/acs.orglett.8b02477 Org. Lett. XXXX, XXX, XXX−XXX