Selective Ortho Thiolation Enabled by Tuning the Ancillary Ligand in

Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX

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Selective Ortho Thiolation Enabled by Tuning the Ancillary Ligand in Palladium/Norbornene Catalysis Wenqiang Cai 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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ABSTRACT: Site-selective introduction of a sulfur group into aromatic compounds is essential and useful in organic, material, and pharmaceutical chemistry. A palladium/ norbornene-catalyzed chemoselective ortho thiolation of aryl halides was reported. The selectivity of reductive elimination for C(Ar)−SR bond formation was well controlled by tuning the ancillary ligand in the aryl-NBE palladacycle Pd(IV) intermediate. The reaction showcased good substrate scope: both S-alkyl and S-aryl thiosulfonates were compatible. Scheme 1. Types of Ortho Functionalization Reagents

T

he development of new and practical tools for the siteselective synthesis of polysubstituted aromatic compounds is a hot research field in synthetic organic chemistry. Palladium/norbornene (NBE) cooperative catalysis is a unique and robust method for the construction of these polysubstituted aromatics. Taking advantage of this protocol, both the electrophiles and nucleophiles are introduced into one aryl ring at the ortho and ipso positions in one reaction, respectively. In 1997, Catellani and co-workers reported the first Pd/ norbornene-catalyzed reaction where ortho alkylation was realized by the use of alkyl halides.1 Subsequently, ortho arylation was reported by Catellani, and later by the Lautens groups.2 In the following one and a half decades after the discovery of this transformation, these two groups as well as others developed powerful transformations for the synthesis of highly functionalized aromatic compounds. These studies focused on ortho-arylation or alkylation and the development of versatile “nucleophilic” reagents, including bifunctional reagents, to functionalize the ipso position.3,4 In recent years, azirines,5 aziridines,6 and epoxides7 were further developed as alkylation reagents by the Lautens, Liang, Dong, and Zhou groups (Scheme 1a). The advances were also applied in complex natural product synthesis in a highly efficient and unique way.8 The recently developed bridgedhead-substituted norbornene by the Dong group enabled the successful monofunctionalization of ortho-unsubstituted aryl halides.9 In 2015, the Liang, Dong, and Gu groups independently reported ortho acylation with the acyl chlorides or acid (mixed) anhydrides (Scheme 1b).10 Further studies by the Dong group established an α-alkoxylcarbonylation method for selective cleavage of the less steric hindered C(O)−O bond of mixed carbonate anhydrides.11 Later, efforts by the Jiao12 and Gu13 groups successfully used carbamic chlorides and selenoates for the acylation reagents. Gu and co-workers developed a Pd/Cu/norbornene three-component-catalyzed strategy for the cleavage of the C(O)−SR bond of thioates.14 Surprisingly, no heteroatom was introduced into the ortho position of aryl halides until the seminal work of ortho © XXXX American Chemical Society

amination was reported by Dong and Dong in 2013, where Obenzoyl hydroxylamine served as the electrophilic reagent (Scheme 1c).15 In a nontypical Pd(II)/norbornene-catalyzed reaction, an N-directed meta−C-H/functionalization chlorination reaction was reported by Yu and co-workers in 2016.16 Received: March 14, 2019

A

DOI: 10.1021/acs.orglett.9b00923 Org. Lett. XXXX, XXX, XXX−XXX

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reductive elimination to form a new Ar−C(O)R2 bond (Scheme 2b). Interestingly, A did not undergo C−S bond reductive elimination, which was possibly attributed to the better leaving ability of the RS group than the acyl group. We anticipated that a group having better leaving ability than RS would enforce the reductive elimination between Ar and RS groups to form the C(Ar)−SR bond. Thiosulfonate was chosen as the potential thiolation reagent. It was reasoned that the oxidative addition of thiosulfonate would deliver D, which would tautomerize to palladium sulfite species (E).18 The sulfite anion is regarded as a better leaving group than RS and would act as an ancillary ligand. As a result, it would force the reductive elimination between the C(Ar) and RS group. To test this hypothesis, 1-iodonaphthalene (1a), thiosulfonate (2a), and ethyl acrylate were chosen as model substrates for this ortho thiolation reaction (Table 1). In the presence of

Heteroatom-attached aromatic compounds are extremely important in both synthetic chemistry and pharmaceutical chemistry. Impressive progress for ipso-functionalization has been made during the past two decades; however, so far, nitrogen is the only heteroatom that has been introduced into the ortho position in classic Catellanic-type domino reaction. This was possibly caused by the foreseeable challenges in this area: the oxidative addition of Pd(0) has to distinguish the aryl halides and electrophile E−X. (a) The reaction requires Pd(0) to react with aryl halides rather than the electrophile E−X. (b) The aryl-NBE palladacycle (ANP) is required to favor the oxidative addition with the electrophile E−X rather than aryl halides (Scheme 1d). We are interested in the development of ortho thiolation because of the extreme importance of sulfurcontaining compounds in organic synthesis and pharmaceuticals (Figure 1).17

Table 1. Optimization of Reaction Conditionsa

Figure 1. Ligands and pharmaceutical molecules containing sulfide.

With this goal in mind, we reanalyzed our previous work on Pd/NBE-catalyzed ortho-acylation and ipso-thiolation via the cleavage of the C(O)−SR bond of thioates. A new question was raised for the purpose of ortho-thiolation: is the selectivity (C−S forming vs C−X forming) of the reductive elimination of Pd(IV) intermediate controllable (Scheme 2a)? In our previous work, the oxidative addition of ANP with thioates would give Pd(IV) intermediate A, which facilely underwent

entry

2

NBEs

solvent

base

time (h)

conv (%)

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

2a 2a 2a 2a 2a 2a 2a 2a 2a 2b 2c 2d

N1 N1 N1 N1 N1 N1 N2 N3 N4 N2 N2 N2

MeCN toluene THF dioxane DCE dioxane THF THF THF THF THF THF

K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 Cs2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3

12 24 24 24 24 12 24 24 24 24 24 24

trace 22 74 61 28 complex 84 (84)b 64 NR 44 43 23

Scheme 2. Rationale for Ipso and Ortho Thiolation

a

Unless stated otherwise, the reaction was conducted with 1a (0.20 mmol), 2a (0.30 mmol), ethyl acrylate (0.40 mmol), NBEs (3.0 equiv), Pd2(dba)3 (5.0 mol %), TFP (22 mol %) in the indicated solvent (2.0 mL) at 110 °C. Yields were determined by 1H NMR analysis using CH2Br2 as internal standard. bThe number in parentheses is the isolated yield.

Pd2(dba)3/trifurylphosphine (TFP), norbornene, and K2CO3, some typical solvents were examined and THF was identified as the optimal medium (Table 1, entries 1−5). The use of a stronger base Cs2CO3 resulted in the quick decomposition of 2a (entry 6). Substituted norbornenes (N2−N4) have been tested: N2 bearing an aldehyde (a mixture of exo and endo) functionality improved the yield of 3a, ketone derivative N3 gave similar efficacy as N1, while the carboxylic salt N4 showed no activity for this transformation at all (entries 7−9).19 It was reasoned that the low concentration of N4 in the THF solution made the reaction difficult to complete, since we did observe that the carboxylic salt N4 barely dissolved in THF. Further optimization of the reaction was attempted by tuning the electronic and steric properties of the sulfonyl part. Unfortunately, p-fluoro- (2b), p-methoxyl- (2c), and 2,4,6trimethylphenyl-derived (2d) thiosulfonates were not superior B

DOI: 10.1021/acs.orglett.9b00923 Org. Lett. XXXX, XXX, XXX−XXX

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other acrylate derivatives successfully participated in the reaction, although the corresponding amide was obtained in a relatively lower yield (3u−x). No desired product 3z was obtained when electron-deficient 1-iodo-2-(trifluoromethyl)benzene was used as the substrate. Some anomalies of this reaction were observed (Scheme 3). Under identical conditions, the reaction of 1-iodo-2-methox-

to 2a (entries 10−12). Finally, the reaction did not afford the thiolated product 3c by utilizing bis(4-methoxylphenyl) disulfide instead of 2a. With the optimal conditions in hand, the substrate scope was investigated (Figure 2). First, the effect of the substituents

Scheme 3. Anomalies of This Reaction

ybenzene did not afford the desired product 3aa. However, double-thiolation product 3bb was obtained in 15% yield, which might come from the desulfonylation of 2a. Disappointedly, the use of aryl iodide with two ortho C−H bonds, i.e., 1-iodo-4-methoxybenzene, did not result the formation of 3cc; instead, 3bb was isolated in 13%. The reaction with 1-iodo-4-methylbenzene merely increased the yield of 3bb. It should be noted that these reactions need to be conducted in the dark (covered by aluminum foil); otherwise, Z/E isomers of the CC double bond ranging from 10:1 to 5:1 were observed (Scheme 4a). Furthermore, the long-time storage of these pure products should be in the dark. It was found that irradiation of product 3a with a light lamp would increase the ratio of the Z isomer (see the Supporting Information).20 In sharp contrast, compound 4e, the

Figure 2. Substrate scope. The reactions were conducted with 1 (0.20 mmol), 2 (0.30 mmol), alkenes (0.40 mmol), N2 (0.6 mmol), Pd2(dba)3 (5.0 mol %), and TFP (22 mol %) in the indicated solvent (2.0 mL) at 110 °C for 24 h. (a) PdCl2 (10 mol %) was used.

on the arylthiol moiety was studied. The p-fluoro substituent on the phenylthiol moiety slightly decreased the yield of the product (3b), while p-methoxy and tert-butyl had marginal effect on the yields (3c and 3d). The o-methyl or o-chloro substituents on phenylthiol moiety are compatible for this transformation (3e and 3f). The yield of 3f decreased to 44% when the reaction was conducted on a 1.0 mmol scale. Moreover, sterically hindered S-(2,6-dimethylphenyl) 4-methylbenzenethiosulfonate smoothly reacted with 1a to deliver the product 3g in 79% yield. Second, series of substituted 1iodonaphthalenes, such as 7-methoxyl, 4-methyl, 4-NMe(Ms), and 6-OTs, were tested, and moderate to good yields were achieved (3h−k). The structure of 3k was unambiguously confirmed by single-crystal X-ray diffraction analysis. Third, the scope respected to the thiolation reagents is not limited to aryl thiosulfonates. The primary and secondary aliphatic thioethers were uneventfully introduced to the ortho position via this Pd/NBE-catalyzed C−H functionalization process (3l−o). Last, the reaction of iodobenzene derivatives did work, but with less efficacy than 1-iodonaphthalene derivatives. The yields ranged from decent to moderate (3p−t). Undoubtedly,

Scheme 4. E/Z Isomerization

C

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corresponding sulfoxide of 3e, did not undergo isomerization with irradiation, indicating the importance of thioether moiety for isomerization (Scheme 4b). Nevertheless, the property of light sensitive of these thioethers provided an opportunity for the synthesis of their Z-isomers. For instance, the irradiation of compounds 3a and 3m gave Z-3a (73% isolated yield) and Z3m (68% isolated yield) as the major products (Scheme 4c). According to previous reports on the mechanistic studies in this area, a plausible catalytic cycle is briefly proposed in Scheme 5. The oxidative addition of Pd(0) with aryl iodide

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 NSFC (21622206, 21871241), the “973” project from the MOST of China (2015CB856600), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB20000000), and the Fundamental Research Funds for the Central Universities (WK2060190086).

Scheme 5. Plausible Catalytic Cycle

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REFERENCES

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followed by norbornene insertion and palladation gave palladacycle ANP. The selective oxidative addition of Pd(II) (ANP) with thiosulfonate delivered the Pd(IV) intermediate D, which underwent tautomerization to afford palladium sulfite E. Subsequent reductive elimination formed a new C−S bond to bring F. Finally, the standard norbornene extrusion and termination delivered the final desired products 3. In summary, we have successfully realized a palladium/ norbornene-catalyzed ortho-thiolation reaction via rational design of S-containing electrophiles.The reaction features a tautomerization of sulfonyl to sulfinyl ligand for palladium(IV) species, which enables the change of ancillary ligands for palladium from RS to sulfite. Thus, the reaction successfully undergoes C(Ar)−SR reductive elimination to give orthothiolation products.

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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.9b00923. Experimental procedures, characterization of products, and spectroscopic data (PDF) Accession Codes

CCDC 1889729 contains 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. D

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DOI: 10.1021/acs.orglett.9b00923 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters 4)(CO)(PPH3)2] (M = Ru, Os), and the role of the adduct [OsCl(C6H4Me-4)(CO)(PPh3)2(SO2)]. J. Organomet. Chem. 1988, 353, 243−249. (b) Jackson, W. G.; Rahman, A. F. M. M.; Craig, D. C. The First Oxygen-Bonded Sulfenate Ion: Crystal and Molecular Structures of Bis(ethylenediamine)(2-pyridinesulfenato-O)cobalt(III) and Bis(ethylenediamine)(2-pyridinesulfinato-O)cobalt(III). Inorg. Chem. 2003, 42, 383−388. (19) Currently, we could not rule out or confirm if there is any coordination of the carbonyl groups in N2 and N4 to the palladium center. However, in both cases of N1 and N2, the thiosulfonate was consumed after 24 h, and the distinction in reaction rate between N1 and N2 was not detected apparently. However, an inseparable byproduct was formed when N1 was used, which resulted in a reduced efficiency of N1. (20) (a) An, X.-D.; Zhang, H.; Xu, Q.; Yu, L.; Yu, S. Stereodivergent Synthesis of α-Aminomethyl Cinnamyl Ethers via PhotoredoxCatalyzed Radical Relay Reaction. Chin. J. Chem. 2018, 36, 1147− 1150. (b) Singh, A.; Fennell, C. J.; Weaver, J. D. Photocatalyst size controls electron and energy transfer: selectable E/Z isomer synthesis via C−F alkenylation. Chem. Sci. 2016, 7, 6796. (c) Singh, K.; Staig, S. J.; Weaver, J. D. Facile Synthesis of Z-Alkenes via Uphill Catalysis. J. Am. Chem. Soc. 2014, 136, 5275−5278. (d) Metternich, J. B.; Artiukhin, D. G.; Holland, M. C.; von Bremen-Kuhne, M.; Neugebauer, J.; Gilmour, R. Photocatalytic E → Z Isomerization of Polarized Alkenes Inspired by the Visual Cycle: Mechanistic Dichotomy and Origin of Selectivity. J. Org. Chem. 2017, 82, 9955−9977. (e) Molloy, J. J.; Metternich, J. B.; Daniliuc, C. G.; Watson, A. J. B.; Gilmour, R. Contra-Thermodynamic, Photocatalytic E→Z Isomerization of Styrenyl Boron Species: Vectors to Facilitate Exploration of Two-Dimensional Chemical Space. Angew. Chem., Int. Ed. 2018, 57, 3168−3172.

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DOI: 10.1021/acs.orglett.9b00923 Org. Lett. XXXX, XXX, XXX−XXX