A Rh-Catalyzed Air and Moisture Tolerable Aldehyde (Ketone

Letter Cite This: Org. Lett. 2018, 20, 4699−4703

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A Rh-Catalyzed Air and Moisture Tolerable Aldehyde (Ketone)Directed Fluorosulfonylvinylation of Aryl C(sp2)−H Bonds Chen Li, Shi-Meng Wang, and Hua-Li Qin* State Key Laboratory of Silicate Materials for Architectures; and School of Chemistry, Chemical Engineering and Life Science, Wuhan University of Technology, 205 Luoshi Road, Wuhan, 430070, China

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S Supporting Information *

ABSTRACT: The first Rh-catalyzed activation of ortho sp2 C−H bonds of aldehydes (ketones) for monoselective coupling with ethenesulfonyl fluoride was accomplished without covalent or transient preinstallation of imines. The 42 examples revealed that the developed method has the advantage of a wide scope and functional-group tolerability. Application of this method for complicated natural product modification was also accomplished.

T

Scheme 1. Competition Reactions of Rh-Catalyzed Aldehyde Directed Couplings of Olefins and Aryl C(sp2)−H Bonds

ransition-metal-catalyzed C−H bond functionalization has emerged as a powerful tool in a step- and atomeconomical fashion for organic synthesis, medicinal chemistry, and material sciences.1 To realize reactivity and selectivity, heteroatom-containing directing groups, such as ketones, amides, esters, nitriles, and alcohols, are typically required by coordinating with metal catalysts to form stable metallacyclic intermediates.2 Among those metal-coordinating moieties, aldehydes as a class of ubiquitously available and readily interconvertible motifs have rarely been successfully employed as directing groups for C−H activation,3 despite their outstanding usefulness in versatile pharmaceuticals, fine chemicals, vitamins, fragrances, and materials.4 And Rhcatalyzed aldehyde directed couplings of olefins and aryl C(sp2)−H bonds have also experienced limited exploration.5 To achieve Rh-catalyzed directed ortho-alkenylation of aryl C(sp2)−H bonds of aryl aldehydes, several major competition reactions need to be avoided or significantly minimized (Scheme 1).6 First, the desired transformation typically requires a stoichiometric amount of oxidants through an oxidative transition metal insertion progress, while those oxidants such as silver(I) or copper(II), even at catalytic amounts, will easily oxidize the aldehydes to their corresponding acids in the presence of air and moisture.7 Second, it has been well established that aryl aldehydes undergo decarbonylation with the promotion of Rh catalysts through oxidative addition of the aldehydic C(O)−H bond to generate a rhodium-acyl complex and subsequent migratory extrusion of carbon monoxide to generate arenes.8 Third, the possible hydroacylation of C(O)−H bonds with alkenes through an © 2018 American Chemical Society

acyl-Rh(III)-hydride transition is another possible challenge to the desired aryl C(sp2)−H olefination.9 Lastly, even if the aldehyde moiety remains untouched during the Rh-catalyzed process, ortho-aryl C(sp2)−H addition to alkenes generating the corresponding alkanes with a mono- or bis-selectivity concern is also a big challenge to the desired orthoalkenylation.10 To accomplish ortho-alkenylation of aryl C(sp2)−H of aryl aldehydes with minimal effects of the above competitions and challenges, either covalent transformation of aldehydes to imines with subsequent removal of imines to regenerate the original aldehydes upon completion of the desired transformation or transient imines formation is required (Scheme 2, Received: June 29, 2018 Published: July 18, 2018 4699

DOI: 10.1021/acs.orglett.8b02037 Org. Lett. 2018, 20, 4699−4703

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Organic Letters Scheme 2. Transition-Metal-Catalyzed Olefination of orthoAryl C(sp2)−H Bonds of Arylaldehydes

elevated significantly to 69% and 85%, respectively (Table 1, entries 5 and 6). Interestingly, further increasing the temperature to 120 °C led to an obvious decrease of desired product (from 85% to 47%) (Table 1, entry 7). In contrast, when the temperature was lowered to 80 °C, the isolated yield of the product was increased to 92% (Table 1, entry 8). However, further decreasing the temperature to 60 °C provided a significantly lower yield of the product (Table 1, entry 9). Interestingly, under an argon atmosphere the yield of the coupling product was slightly lower than when the reaction occurred under air (Table 1, entry 10 compared to entry 8). In the absence of either AgSbF6 or Cu(OAc)2·H2O, the desired transformation did not effectively proceed (Table 1, entries 11 and 12). Therefore, the conditions of Table 1, entry 8 were chosen as the standard procedure for functional group tolerance and substrate scope examinations. With these optimized conditions in hand, we subsequently explored the substrate scope and limitations of the developed fluorosulfonylvinylation. As shown in Scheme 3, most

eq 1).11 These available indirect methodologies are sometimes incompatible with many functional groups for complicated molecules synthesis and advanced synthesis; therefore, a direct and one single step process of Rh-catalyzed ortho-alkenylation of aryl C(sp2)−H bonds of aryl aldehydes is highly desirable. Herein, we report the first example of a Rh-catalyzed air and moisture tolerable direct fluorosulfonylvinylation of aryl C(sp2)−H bonds of aryl aldehydes for accessing a class of novel 2-aryl ethenesulfonyl fluorides for SuFEx click chemistry.12 We initiated our studies by testing the feasibility of coupling para-chloro-benzaldehyde 1a with ESF 2 for the desired direct fluorosulfonylvinylation of aryl C(sp2)−H bonds of aryl aldehydes. After screening a large variety of conditions (see the Supporting Information (SI) for details), we were pleased to find that the desired coupling product was generated (in 28%−42% yields) when [Cp*RhCl2]2 (5 mol %)/AgSbF6 (50 mol %) was used as a cocatalyst and Cu(OAc)2, Cu(OAc)2· H2O, or AgNO2 served as an oxidant in PhCF3 at 100 °C under an air atmosphere for 20 h without any additive (Table 1, entries 1−3). Switching the solvent from PhCF3 to DCE slightly increased the yield to 55% (Table 1, entry 4). Remarkably, when a stoichiometric amount of AcOH or Ac2O was added to this reaction, the yields of the product were

Scheme 3. Scope of Oxidative Couplings of Arylaldehydes and ESF 2a,b

Table 1. Optimizations of the Oxidative Coupling of Arylaldehydes with ESFa

entry

oxidant

additive

solvent

temp (°C)

3a, yield (%)b

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

Cu(OAc)2 Cu(OAc)2·H2O AgNO2 Cu(OAc)2·H2O Cu(OAc)2·H2O Cu(OAc)2·H2O Cu(OAc)2·H2O Cu(OAc)2·H2O Cu(OAc)2·H2O Cu(OAc)2·H2O Cu(OAc)2·H2O −

− − − − AcOH Ac2O Ac2O Ac2O Ac2O Ac2O Ac2O Ac2O

PhCF3 PhCF3 PhCF3 DCE DCE DCE DCE DCE DCE DCE DCE DCE

100 100 100 100 100 100 120 80 60 80 80 80

35 42 28 55 69 85 47 92 56 81 trace trace

a

Reaction conditions: 1 (1.0 mmol, 1.0 equiv), 2 (5.0 mmol, 5.0 equiv), [Cp*RhCl2]2 (5 mol %), AgSbF6 (50 mol %), Cu(OAc)2·H2O (1.0 equiv), and Ac2O (1.0 equiv) with DCE (5 mL) for 20 h at 80 °C in a screw-capped vial. bIsolated yields. cAgNO2 (1.0 equiv) was used for the reaction instead of Cu(OAc)2·H2O (1.0 equiv). d [Cp*RhCl2]2 (10 mol %), AgSbF6 (1.0 equiv), and AgNO2 (1.0 equiv) were used for the reaction.

substrates with both electron-withdrawing groups, such as halogen groups and the carbonyl group, and electron-donating groups, such as methyl, methoxyl, and hydroxyl, at the ortho- or para-positions of the benzene ring were well tolerated. For instance, aldehydes possessing halogen groups at the parapositions of the benzene ring (1a, 1b, 1c, and 1d) underwent fluorosulfonylvinylation with ESF 2 efficiently and afforded 3a, 3b, 3c, and 3d in good to high yields. Benzaldehyde 1e

a Reaction conditions: A mixture of 1a (0.2 mmol, 1.0 equiv), 2 (1.0 mmol, 5.0 equiv), [Cp*RhCl2]2 (5.0 mol %), AgSbF6 (50 mol %), oxidant (1.0 equiv), additive (1.0 equiv), and solvent (2.0 mL) was reacted under an air atmosphere at 80−120 °C for 20 h. bIsolated yield. cUnder argon atmosphere. dWithout AgSbF6.

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DOI: 10.1021/acs.orglett.8b02037 Org. Lett. 2018, 20, 4699−4703

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Organic Letters

decreasing the temperature to 80 °C, the yield of the product 5a was improved to 90% (Table 2, entry 6). However, further decreasing the temperature obviously hindered the efficiency of the coupling (Table 2, entry 7). The use of 50 mol % of AgSbF6 provided higher yields than when using a 20 mol % and 100 mol % AgSbF6 loading for this transformation (Table 2, entries 8 and 9). The scope of aryl ketones was extended under the optimized conditions. In most examples, good yields of fluorosulfonylvinylation products were obtained. The results are illustrated in Scheme 4. Para-substituted substrates with electron-donating

provided the corresponding product 3e in 76% yield. Benzaldehydes with para-electron-donating functionalities of methyl (1f), phenyl (1g), dimethylamino (1h), and hydroxyl (1i) on the benzene rings provided their corresponding products 3f, 3g, 3h, and 3i in moderate to excellent yields (40% to 87%). Notably, terephthalaldehyde 1j was transformed to 3j in 50% yield after coupling with ESF 2. Not surprisingly, ortho-substituted benzaldehyde with either electron-withdrawing functionalities of −F, −Cl, −Br (1k, 1l, 1m) or electron-donating functionalities of −CH3, −OCH3, −OH (1n, 1o, 1p) afforded their corresponding products in moderate to excellent yields (39% to 87%). However, the coupling of meta-substituted benzaldehydes with ESF 2 could not generate the desired products. The 2-naphthaldehyde (1q) was also smoothly converted to the desired product 3q in good yield (78%). It is worth noting that the couplings of ESF 2 with substituted thiophenecarboxaldehydes (1r, 1s, 1t, 1u) were proven to be more effective than those with benzaldehydes to provide the corresponding 2-thiophenylESF products in excellent yields (85% to 95%). In addition, 2furancarboxaldehyde 1v and 2-benzofurancarboxaldehyde 1w were also transformed to the corresponding ESF derived products 3v and 3w in 42% and 72% yield, respectively. The products with the presence of aldehyde S-heterocyclic and Oheterocyclic functionalities offered an opportunity for further manipulation. Inspired by the oxidative coupling reaction between aryl aldehydes 1 and ESF 2, we continued our studies by examining the coupling of acetyl benzene 4a and ESF 2 under the same conditions. Interestingly, using the identical conditions for coupling aldehyde with ESF resulted in only a trace amount of the desired product (Table 2, entries 1 and 2). It is pleasing to

Scheme 4. Scope of Oxidative Couplings of Arylketones and ESF 2a,b

Table 2. Optimization of the Oxidative of Arylketone and ESFa

a

entry

oxidant

additive

solvent

temp (°C)

5a, yield (%)b

1 2 3 4 5 6 7 8c 9d

Cu(OAc)2·H2O Cu(OAc)2·H2O AgNO2 AgNO2 AgNO2 AgNO2 AgNO2 AgNO2 AgNO2

Ac2O Ac2O − − AcOH AcOH AcOH AcOH AcOH

DCE DCE DCE PhCF3 PhCF3 PhCF3 PhCF3 PhCF3 PhCF3

80 100 120 120 120 80 60 80 80

trace trace 28 62 78 90 55 63 87

Reaction conditions: 4 (1.0 mmol, 1.0 equiv), 2 (5.0 mmol, 5.0 equiv), [Cp*RhCl2]2 (5 mol %), AgSbF6 (50 mol %), AgNO2 (1.0 equiv), and AcOH (1.0 equiv) with PhCF3 (5 mL) for 20 h at 80 °C in a screw-capped vial. bIsolated yields. c5 mL of DCE were used as solvent.

groups were compatible, giving the desired products 5a−5d in good to excellent yields. Ketones with the halogen groups at para-positions of the benzene ring worked smoothly, delivering the corresponding coupling products 5e−5h in 65%−97% yields. Notably, the 4-CF3 and 4-OCF3 substituted substrates also afforded the desired products 5i and 5j in good yields. It is interesting to note that aryl ketone 4k with a ketone and a para-ester moiety was regioselectively transformed to 5k, which indicated the ketone predominately directed the coupling process rather than the carboxylate. Aryl ketone 4l with a conjugated olefin motif was also smoothly converted to the desired product 5l in moderate yield. Fluorosulfate (−OSO2F, 5m) was tolerated well under the coupling conditions. The meta-methyl substituted substrate 4n was examined, which provided an obviously lower yield of 5n (59%) compared to para-methyl substituted 4b which generated 5b in 95% yield. 2-Acetonaphthone 4o and bissubstituted aryl ketone 4p were successfully transformed to the corresponding fluorosulfonylvinylation products 5o and 5p in

a Reaction conditions: A mixture of 4a (0.1 mmol, 1.0 equiv), 2 (0.5 mmol, 5.0 equiv), [Cp*RhCl2]2 (5.0 mol %), AgSbF6 (50 mol %), oxidant (1.0 equiv), additive (1.0 equiv), and solvent (2.0 mL) was reacted under an air atmosphere at 80−120 °C for 20 h. bIsolated yield. cAgSbF6 (20 mol %). dAgSbF6 (100 mol %).

find that when AgNO2 was used as the oxidant, the coupling of acetyl benzene 4a and ESF 2 generated the desired product in 28% yield (Table 2, entry 3). Gratifyingly, switching the solvent from DCE to PhCF3 provided a significantly improved yield of 62% (Table 2, entry 4). Addition of 1.0 equiv of AcOH elevated the yield of 5a to 78% (Table 2, entry 5). By 4701

DOI: 10.1021/acs.orglett.8b02037 Org. Lett. 2018, 20, 4699−4703

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65% and 47% yield, respectively. It is worth mentioning that Sheterocyclic 2-acetylthiophene 4q provided 5q in 96% yield while O-heterocyclic 2-acetylfuran 4r generated 5r in a significantly lower yield (62%) under the same conditions. The fact that many natural compounds and bioactive molecules possess aldehyde and ketone motifs has attracted significant interest in medicinal chemistry. We also applied this procedure to the structure modification of derivative of complicated natural product cholesterol (Scheme 5). Not surprisingly, the complicated aldehyde (1x) was smoothly transformed into its corresponding product (3x) under the developed conditions in moderate yield.

a

Reaction conditions: 1x (0.5 mmol, 1.0 equiv), 2 (2.5 mmol, 5.0 equiv), [Cp*RhCl2]2 (10 mol %), AgSbF6 (50 mol %), Cu(OAc)2· H2O (1.0 equiv), and Ac2O (1.0 equiv) with 5 mL of DCE for 20 h at 80 °C in a screw-capped vial. bIsolated yields.

In summary, we have developed a novel method for the synthesis of a new class of 2-(hetero) arylethenesulfonyl fluorides possessing ortho-aldehyde (ketone) moieties through Rh-catalyzed ortho-alkenylation of aryl C(sp2)−H of aryl aldehydes (ketones). In addition, we demonstrated that the ortho-fluorosulfonylvinylation process can be applied for latestage complicated natural product modification. Moreover, 42 structurally diverse vinyl sulfonyl fluorides were synthesized demonstrating the wide scope and functional-group tolerability of this practical method. Studies on the potential bioactivities of these compounds are currently underway in our laboratory.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b02037. General methods, synthetic procedures, and characterization (PDF)

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REFERENCES

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Scheme 5. Fluorosulfonylvinylation of Natural Product Cholesterol Derived Aldehyde 1xa,b

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Letter

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hua-Li Qin: 0000-0002-6609-0083 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Fundamental Research Funds for the Central Universities (2016-YB-012 to H.L.Q.). We are grateful to Professor K. B. Sharpless (the Scripps Research Institute) for helpful discussions. 4702

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