Cross-Coupling of Chloro(hetero)arenes with Thiolates Employing a

Letter pubs.acs.org/OrgLett

Cite This: Org. Lett. 2019, 21, 50−55

Cross-Coupling of Chloro(hetero)arenes with Thiolates Employing a Ni(0)-Precatalyst Paul H. Gehrtz, Valentin Geiger,‡ Tanno Schmidt,‡ Laura Sršan, and Ivana Fleischer* Institute of Organic Chemistry, Faculty of Science and Mathematics, Eberhard-Karls University Tübingen, Auf der Morgenstelle 18, 72076 Tübingen, Germany

Org. Lett. 2019.21:50-55. Downloaded from pubs.acs.org by EASTERN KENTUCKY UNIV on 01/11/19. For personal use only.

S Supporting Information *

ABSTRACT: A general and efficient Ni-catalyzed coupling of challenging aryl chlorides and in situ generated aliphatic and aromatic thiolates is described. The employed oncycle, air-stable defined Ni precatalysts allow for transformation of a broad scope of substrates. A variety of functional groups and heterocyclic motifs as well as structurally varied thiols are tolerated at unprecedented moderate catalyst loadings and reaction temperatures. Depending on reaction conditions, aryl thiols can selectively undergo C−S or C−C couplings.

T

Scheme 1. Methods for Aryl Thioether Synthesis and Our NiCatalyzed Migita Reactiona

he Migita reaction (MR) is the transition-metal catalyzed cross-coupling of haloarenes and thiols under basic conditions to yield aryl thioethers.1 This structural motif belongs to the classical series of bivalent isosteres (−CH2−, −NH−, −O−, −S−) and is thus valuable when modification of a lead structure in an agrochemical, pharmaceutical, or material chemistry context is necessary.2 It can also be accessed in a noncatalytic manner by nucleophilic (aromatic) substitution, charge transfer-mediated radical−radical coupling,3 or numerous conceptually different catalytic approaches (Scheme 1a), including (metallo)photoredox catalysis,4 cross-electrophile or -nucleophile couplings,5 decarbonylation,6 single-bond metathesis,7 and (photocatalytic) radical−nucleophilic aromatic substitutions.8 However, the Pd-catalyzed Migita reaction remains prevalent due to its generality and modularity. For example, it was successfully employed on multi-kilogram scale for the production of pharmaceuticals.9 Besides the typical Pd0/II mechanism, dinuclear PdI catalysts have been employed in a MR of thiolates with high selectivity toward the activation of bromoarenes.10 A large interest in the activation of aryl chlorides exists since these electrophiles are generally of low cost and high stability compared to other aryl halides. Second, such methods allow sequential cross-coupling sequences based on the bond dissociation energy order (PhCl, 408.4 kJ mol−1; PhBr, 345.6 kJ mol−1; PhI, 281.2 kJ mol−1) of aryl halides,11 which generally correlates with reactivity as explained by the distortion interaction model.12 Some methods for the thiolation of chloroarenes have been developed employing Pd-based systems, which required air sensitive ferrocenylphosphine ligands.13 Since oxidative addition of Ni0 complexes into electrophiles is more facile than with © 2018 American Chemical Society

a

PRC, photoredox catalyst; [TM], transition metal; xant, xantphos; X, (pseudo)halide.

Received: October 31, 2018 Published: December 17, 2018 50

DOI: 10.1021/acs.orglett.8b03476 Org. Lett. 2019, 21, 50−55

Letter

Organic Letters Pd0 complexes,14 chloroarene activation should be favored for Ni catalysts. In addition, earth-abundant Ni catalysts could emerge as replacements for expensive Pd-complexes if the drawback of the relatively higher metal loadings (often 5−20 mol %) could be overcome.15 Only few reports on Ni-catalyzed C−S cross couplings with chloroarenes appeared. In 2015, Schönebeck and co-workers conducted a detailed study on the trifluoromethylthiolation of chloro(hetero)arenes under Ni catalysis (10 mol %) using (Me4N)SCF3.16 In 2018, Stewart employed a high-throughput ligand screening method, which allowed the identification of several bidentate phosphines for C−S cross couplings of chloro(hetero)arenes (10 mol % Ni metal) with arylthiols under reducing conditions (Zn), whereas alkylthiols were less competent coupling partners (although aryl bromides were reported to be working, only one reported example with chlorides was disclosed with a yield of 20%).17 In addition, NiSchiff base complexes catalyzed (5 mol %) reactions of aryl thiols and chlorides.18 During our study on the Ni-catalyzed Fukuyama coupling, we observed a MR of chloroarenes resulting from the liberated zinc thiolate.19 Herein, this Ni-catalyzed side reaction was studied in more detail, which resulted in the discovery of an efficient catalytic system for the MR of chloroarenes (Scheme 1b). It operates at exceptionally low catalyst loading and mild conditions compared to competitive Ni-based catalysts. For the first time, alkylthiols can be efficiently coupled with aryl chlorides under Ni catalysis. Moreover, a divergent behavior was observed for aryl thiols, which can undergo C−S or C−C coupling depending on reaction conditions. The reaction of chlorobenzene (1) with equimolar amounts of heptane-1-thiol (2, HeptSH) and the established precatalyst C1 was selected as a model system (Table 1, entry 1).20 PhMgBr·LiCl (3) was employed as a stoichiometric base to generate a reactive Mg thiolate.21 With a loading of 0.5 mol % C1, the reaction was completed within 15 min, providing the product 4 in an excellent yield. A shorter reaction time of 5 min was less satisfactory. Various control experiments confirmed the necessity of inert atmosphere, phosphine ligation, and a molecular precatalyst (entries 2−5). A strong ligand effect was suspected, and thus other literature-known Ni complexes of the type L2Ni(oTol)Cl were evaluated, featuring a small bite-angle bidentate phosphine in C2, a monodentate phosphine in C3, and a similar large bite-angle bidentate phosphine in C4. In all cases, no significant product formation occurred (entries 6−8). The use of a Grignard reagent as an organometallic base requires temperature control and dropwise addition to avoid a competing Kumada coupling. To circumvent this problem, 3 was replaced by the corresponding zinc reagent 5, which again resulted in an excellent yield of the product after 15 min with no cooling required (entry 9). Base 5 was prepared from a fresh solution of 3 by transmetalation with ZnCl2 in THF. In contrast, the use of a preformed HeptSZnCl·LiCl solution led to a disappointing result (entry 10). This indicates that a stoichiometric amount of organometallic reagent is required for an efficient transformation. It fulfils a dual role of base and reductant. In addition, commercially available iPrMgCl·LiCl failed to initiate efficient catalysis (entry 11). Following these preliminary investigations, the scope of coupling partners was evaluated by reaction of monosubstituted chloro(hetero)arenes with 2 (Scheme 2). Various electrophilic (e.g., ketone in 7, nitrile in 11) and Lewis basic (e.g., tertiary amine in 6, amide in 10) functional groups on chloroarenes are

Table 1. MR of Chlorobenzene with Heptanethiol under Ni Catalysis as a Model Reactiona

entry

deviation from standard conditions

conv. 1 (%)b

4 (%)b

1 2 3 4 5 6 7 8 9 10 11

5 min reaction conducted in air catalytic NiCl2c no metal, no ligandc catalytic NiB nanoparticlesc,d catalytic C2 catalytic C3c catalytic C4 PhZnCl·LiCl (5, 2.0 equiv), 15 min, rt HeptSZnCl·LiCl (1.3 equiv), 10 min, rte i PrMgCl·LiCl (1.1 equiv), 60 min

36 13 35 6 31 2 10 8 96 18 3

33 13 0 1 0 0 0 7 96 18 2

Standard reaction conditions: PhCl (50 μL, 500 μmol), heptane-1thiol (80 μL, 500 μmol), PhZnCl·LiCl (1.2 mmol based on titer [typically 0.5 M], in THF), C1 (2.0 mg, 2.5 μmol, 0.5 mol %), dry THF (500 μL), 0 °C to rt, 15 min. bDetermined by quant. GC-FID analysis. cFormation of biphenyl. dPrepared as a THF slurry by a literature procedure (SI). eHeptSZnCl·LiCl was generated from thiol and PhZnCl·LiCl, which was prepared from PhMgBr·LiCl. a

tolerated in the para- or meta-position generally resulting in good to excellent yields after 0.5−4 h either at room temperature or 60 °C with C1 loadings from 0.5 to 2 mol %. An outlier is the moderate performance of electron-poor 4-chloro-pentafluorosulfanylbenzene to give 12. Electron-rich chloroarenes are tolerated, but generally require harsher reaction conditions and higher loadings of C1. However, thiolations of ortho-substituted chloroarenes did not proceed when catalysis with C1 was attempted. The use of analogue catalyst C5 introduced by Stradiotto showed again excellent activity for these substrates (leading to compounds 13 and 14).22 Some chloroarenes with Lewis basic substituents were unreactive, possibly due to coordination-induced catalyst poisoning. The addition of MeCN, which was demonstrated by Schönebeck to have a positive effect on couplings of challenging ArCl, was unfruitful in our case.16 However, we found that NMP had a positive effect as cosolvent. For example, in THF alone, no product formation of 10 and 17 was observed under standard conditions. In contrast, by using NMP as a cosolvent in conjunction with a catalyst loading of 2 mol % at 60 °C, the products were obtained in good to excellent yield. With these more robust conditions in hand, we were able to convert a range of diverse chloroheteroarenes to the corresponding sulfides (15−24), including pyrimidine and purine motifs, in moderate to excellent yields. A limitation is the presence of a Michael acceptor as in the structure leading to product 22 (the full set of limitations is disclosed in the SI). After oxidation, the sulfide 21 is an entry point for desulfonylative olefinations or cross-coupling reac51


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Organic Letters Scheme 2. Scope of the Ni-Catalyzed MR of Chloroarenes with Alkylthiolsa

a

Isolated yields. Conditions: chloroarene (1.0 mmol), thiol (1.0 mmol), PhZnCl·LiCl or PhMgBr·LiCl (1.2 mmol of a titrated solution in THF), dry NMP or THF (400 μL), C1 or C5 (0.5−2.0 mol %), 0.5 to 4.0 h, RT−60 °C (for RZnX) or 0 °C−RT (for RMgX, removal of ice−water bath after Grignard addition). b0.5 mmol scale. cContains unknown impurities.

tions.23 Notably, the catalytic system also operates on more complex chlorinated compounds, such as 8-chlorocaffeine to give 25 in moderate yield or indomethacin ethyl ester leading to 26 in good yield. The reaction of electron-poor chloroarenes leading to 7, 11, and 17 was performed without the catalyst. No products were obtained, which suggests that the SNAr mechanism can be ruled out. Next, the reactivity of structurally varied thiols was investigated. It was found that primary and secondary alkylthiols

performed well (sulfides 27 to 31), while tertiary thiols gave lower yields (33 and 34). The synthesis of 31 was repeated on a 10 mmol scale with isolated yield of 80%. A bifunctional thiol could also be employed, giving the aryl sulfide 29. Electronically less activated thiols performed poorly in catalysis (e.g., leading to 32). Notably, the keto-sulfide 36 was obtained in moderate yield from an atom-economical tandem intramolecular Fukuyama− 52


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Organic Letters

base at 110 °C with 1 h reaction time. However, biaryls and symmetrical sulfides were also produced. The formation of symmetrical sulfides was independently explained by Percec and Kentaro as a result of reinsertion of LnNi0 into the product sulfide, followed by thiolate scrambling.26 Yang and Wang reported a transition-metal-free cross coupling of aryl zinc halides with thiophenols under thermal conditions.27 Thus, n BuZnCl·LiCl was used instead, leading to an improved selectivity profile (Scheme 5).

Migita reaction (Scheme 3), which formally corresponds to insertion of a 1,4-phenylene spacer into the thioester C−S bond. Scheme 3. Example of a Tandem Fukuyama−Migita Reactiona

Scheme 5. Ni-Catalyzed MR of Chloroarenes with Thiophenols Driven by Alkylzinc Basesa

a

Conditions: 35 (0.5 mmol), 4-Cl-C6H4ZnCl·LiCl (3.6 mL of a 0.25 M THF solution), C1 (2 mol %), THF (0.5 mL), 1.5 h, rt.

Unfortunately, arylthiols gave no conversion using PhZnCl· LiCl as the base under mild conditions. It was investigated if the choice of organometallic base affects the reaction outcome. Using Grignard reagents as the base and thiophenols as potential nucleophiles, a desulfenylative Kumada-type cross-coupling24 was observed even in the presence of chloroarenes, together with formation of the Migita product and symmetrical sulfides. The use of sterically encumbered Grignard bases to disfavor C−C coupling did not lead to a clearer reaction outcome. Thus, the scope of the so far unexplored reaction of aryl thiols and Grignard reagents using C1 was studied (Scheme 4). Several biaryls (37−45) were obtained in fair to good yields. The combination of electron-rich nucleophile and electron-poor electrophile appears to provide the best results, as expected for a cross-coupling reaction.25 During further screening of reaction conditions, it was found that toluene was the optimal cosolvent using PhZnCl·LiCl as the Scheme 4. Ni-Catalyzed Desulfenylative Kumada Coupling of Thiophenols with Grignard Reagentsa

a

Conditions: chloroarene (1.0 mmol), arenethiol (1.0 mmol), BuZnCl·LiCl (1.2 mmol of a THF solution [typically 0.5 M]), C1 (2 mol %), toluene (usually 2.4 mL, equal to THF volume), 1 h, 110 °C.

While the method is excellent for challenging electron-rich aryl chlorides (providing 46, 49, 50, and 53), electron-poor reactants still lead to the formation of symmetrical sulfides, which decreases overall yield (e.g., 48). Thus, our method is complementary to the visible-light driven chloroarene thiophenolation,3 which works best with the latter class of electrophiles. In conclusion, an efficient Ni-catalyzed coupling of in situ generated zinc thiolates with aryl chlorides was developed. The method is characterized by short reaction times and low catalyst loadings for unfunctionalized coupling partners (TOF of 800 h−1). Even under robust conditions for challenging heterocyclic and Lewis basic coupling partners, the catalytic system surpasses in many parameters the known methods involving the activation of chloroarenes and should be of general interest to the practicing synthetic chemist. While the MR of thiophenols requires thermally harsher conditions, short reaction times (1 h) can be achieved with challenging, electron-rich aryl chlorides. In the absence of other electrophiles, catalyst C1 can be also employed for a desulfenylative Kumada-type reaction of thiophenols with Grignard reagents.

a

Conditions: arenethiol (1.0 mmol), ArMgBr·LiCl (2.2 mmol of a THF solution [typically 1 M]), C1 (2 mol %), THF (0.5−1.0 mL), 4.0 h, 60 °C. The formation of sulfide was proven by lead acetate paper test. bThe product could not be completely purified (NMR yields are given). 53


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Organic Letters

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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.8b03476.

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Experimental procedures, analytical data, and mechanistic discussion (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: ivana.fl[email protected]. ORCID

Ivana Fleischer: 0000-0002-2609-6536 Author Contributions ‡

These authors contributed equally. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank B. Ciszek (University of Tübingen) for helpful discussions. Financial support from Boehringer Ingelheim Stiftung (Exploration Grant) and the University of Tübingen (Institutional Strategy: Deutsche Forschungsgemeinschaft ZUK 63) is gratefully acknowledged.

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REFERENCES

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Cross-Coupling of Chloro(hetero)arenes with Thiolates Employing a

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