Regioselective Gold-Catalyzed Hydration of CF3- and SF5-alkynes

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

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Regioselective Gold-Catalyzed Hydration of CF3- and SF5‑alkynes Mélissa Cloutier, Majdouline Roudias, and Jean-François Paquin* CGCC, PROTEO, Département de chimie, Université Laval, 1045 Avenue de la Médecine, Québec, Québec, G1V 0A6, Canada

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S Supporting Information *

ABSTRACT: The regioselective gold-catalyzed hydration of CF3- and SF5-alkynes is described. The corresponding trifluoromethylated and pentasulfanylated ketones are obtained in up to 91% yield as single regioisomers showcasing the use of CF3 and SF5 as highly efficient directing groups in this reaction. Notably, this transformation represents the first use of CF3- and SF5-alkynes in gold catalysis. Scheme 1. Synthesis of α-CF3 and α-SF5 Ketones

T

he trifluoromethyl group is vastly found in pharmaceuticals,1 agrochemicals,2 and fluorinated materials.3 Hence, various synthetic approaches for its incorporation into organic molecules have been developed.4 In that regard, α-trifluoromethyl ketones (cf. Scheme 1, eqs 1, 2, and 4) are useful building blocks for the preparation of a wide range of fluorinated compounds, including CF3-containing ones. Practically, almost all the methods for the synthesis of α-CF3 ketones rely on the introduction, as the key step, of the CF3 fragment in nucleophilic5 or electrophilic mode (Scheme 1, eq 1).6−8 Surprisingly, aside from a single report on the hydration of aromatic CF3-alkynes under strongly acidic media (Scheme 1, eq 2),9 readily accessible CF3-alkynes have not been investigated as starting substrates for the preparation of α-CF3 ketones. Considered as a “super CF3”, the pentafluorosulfanyl group (SF5)10 has also recently attracted its share of attention for its unique and interesting properties including, among others, a large dipole moment, a high lipophilicity, and a strong electronwithdrawing capacity.11 Not surprisingly, this substituent has started to find use in various fields.10,12 Contrary to the trifluoromethyl variant, access to α-SF5 ketone compounds, despite their potential utility as SF5-containing building blocks, is far less developed, and only five single examples have been reported up to now (Scheme 1, eq 3).13 Given our recent reports on the Au-catalyzed formal hydration14 or intramolecular hydroalkoxylation15 of propargylic gem-difluorides, we wanted to establish if CF3 and SF5 groups efficiently directed the addition of water to a neighboring alkyne. Herein, we report the regioselective gold-catalyzed hydration of CF3- and SF5-alkynes.16 The corresponding trifluoromethylated (Scheme 1, eq 4) and pentasulfanylated ketones (Scheme 1, eq 5) were obtained as single regioisomers. © XXXX American Chemical Society

Notably, this transformation represents the first use of CF3- and SF5-alkynes in gold catalysis.17 Received: April 19, 2019

A

DOI: 10.1021/acs.orglett.9b01379 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters

role of the in situ generated triflic acid (Table 1, entry 13).20 No conversion was observed when using 2 mol % of TfOH instead of (JohnPhos)AuCl/AgOTf, thus discarding a Brønsted acid catalyzed pathway. Overall, as running the reaction at 70 °C generally provided better conversions for a wider range of substrates (vide infra), the conditions reported in entry 7 were used as the optimal ones. With the optimized conditions in hand, we investigated the scope of this transformation using various CF3-alkynes (1), and results are reported in Scheme 2. A number of aliphatic CF3-

We began our investigation using CF3-alkyne 1a, which was prepared in a one-step procedure from the corresponding terminal alkyne through a copper-mediated reaction with the Ruppert−Prakash reagent.18,19 Selected optimization results are reported in Table 1. We initially used the conditions we had Table 1. Survey of Reaction Conditions for 1aa

Scheme 2. Gold-Catalyzed Hydration of CF3-alkynesa,b entry

solvent (ratio)

1c,d 2d 3

THF/MeOH (9:1) THF/MeOH (9:1) THF/MeOH/H2O (8:1:1) THF/MeOH/H2O (8:1:1) THF/H2O (9:1) THF/H2O (9:1) THF/H2O (9:1) THF/H2O (19:1) THF/H2O (4:1) THF/H2O (1:1) THF/H2O (9:1) THF/H2O (9:1) THF/H2O (9:1)

4 5 6 7 8 9 10 11e 12g 13h

Au/Ag cat. (mol %)

temp (°C)

yield (%)b

5 1 1

21 21 21

82 89 43

1

40

72

1 2 2 2 2 2 2 2 2

40 40 70 40 40 40 40 40 40

78 89 88 89 84 42 0f 0f 0f

a

See the Supporting Information for detailed experimental procedures. bIsolated yield of 2a after column chromatography. c (Ph3P)AuCl was used instead of (JohnPhos)AuCl. dAn acidic treatment (TFA/CH2Cl2 (1:3), 21 °C, 3 h) was performed after the reaction. eReaction was run without (JohnPhos)AuCl. fNo conversion was observed by NMR analysis of the crude mixture. gReaction was run without AgOTf. hTfOH (2 mol %) was used instead of (JohnPhos)AuCl/AgOTf.

reported for propargylic gem-difluorides (Table 1, entry 1),14 and we were pleased to isolate the desired ketone 2a in 82% yield. Switching from (Ph3P)AuCl to (JohnPhos)AuCl while reducing the catalyst loading gave 89% yield (Table 1, entry 2). Though these results were interesting, we wondered if we could get away from this two-step procedure by using water as the nucleophile instead of methanol, which could not be achieved with the propargylic gem-difluorides.14 To our delight, using a solvent system containing water (THF/MeOH/H2O in a 8:1:1 ratio) gave 2a in a promising 43% yield with an incomplete conversion (Table 1, entry 3). Increasing the temperature to 40 °C provided a better conversion, which resulted in an improved yield of 72% (Table 1, entry 4). The presence of methanol was not mandatory as a slightly improved yield of 78% was obtained when the reaction was performed in a THF/H2O (9:1) mixture (Table 1, entry 5). The yield could be increased further to 89% with a higher catalyst loading of 2 mol % (Table 1, entry 6). Running the reaction at 70 °C gave a similar result (Table 1, entry 7). In terms of water content, while running the reaction with less water provided a similar result (Table 1, entry 8), increasing the water content was found to be detrimental to the reaction (Table 1, entries 9−10). At this point, some control experiments were performed. Removing either (JohnPhos)AuCl (Table 1, entry 11) or AgOTf (Table 1, entry 12) resulted in the absence of conversion demonstrating that both components are essential to the reaction. Finally, we also evaluated the potential

a See the Supporting Information for detailed experimental procedures. bIsolated yield of 2 after column chromatography. c Reaction was run at 40 °C. dReaction was run on a 1 mmol scale. e Isolated yield of the corresponding ketone with the free alcohol (R = H). f1,4-Dioxane was used instead of THF. gFive mol % of (JohnPhos)AuCl and of AgOTf were used.

alkynes (1a−i) were subjected to the reaction conditions and provided, in all cases, the α-CF3 ketones (2a−i) in low to excellent yields (10−91%). Notably, substrates bearing a protected alcohol in the form of a benzoate (1c−d) or a benzyl (1e−f) worked well. However, a TBS protecting group was not tolerated and its deprotection was observed, leading to a low yield of ketone 2g. 21 However, in this reaction, the corresponding α-CF3-ketone with the free alcohol (2g with R = H instead of TBS) was isolated in 71% yield. Aromatic CF3alkynes (1j−t) bearing various substituents performed well and provided the corresponding α-CF3 ketones (2j−t) in moderate B


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Organic Letters

Scheme 4. Gold-Catalyzed Hydration of SF5-alkynesa,b

to excellent yields (62−90%).22 Finally, the bis(CF3-alkyne) compound 1u gave the corresponding bis(α-CF3 ketones) 2u in 68% yield. It is important to note that in all cases, a single regioisomer of the product was observed and isolated. Finally, we turned out attention to SF5-alkynes with the latter being readily accessible through a two-step from alkynes via a radical addition of SF5Cl using Dolbier’s protocol followed by a base-promoted elimination (Scheme 3).23 Scheme 3. Synthesis of SF5-alkynes

The results of the optimization are shown in Table 2. At 40 °C, no conversion was observed (Table 2, entry 1). At 70 °C, the Table 2. Survey of Reaction Conditions for 3aa

entry

cat. loading (mol %)

cosolvent

temp (°C)

yield (%)b

1 2 3 4 5 6 7 8 9

2 2 2 2 2 2 3 4 5

THF THF 1,4-dioxane 1,4-dioxane toluene 2-MeTHF 1,4-dioxane 1,4-dioxane 1,4-dioxane

40 70 70 100 70 70 70 70 70

0 37 62 62 0c 44 77 78 75

a

See the Supporting Information for detailed experimental procedures. bIsolated yield of 4 after column chromatography. c Contaminated with residual JohnPhos (ca. 6%). dFull conversion was observed, but the desired product could not be detected by NMR analysis of the crude mixture.

and material sciences, are obtained in up to 90% yield as single regioisomer. Not only does this transformation showcase using CF3 and SF5 as highly efficient directing groups but it also represents the first use of CF3- and SF5-alkynes in gold catalysis.

a See the Supporting Information for detailed experimental procedures. bIsolated yield of 4a after column chromatography. cNo conversion was observed.

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ASSOCIATED CONTENT

* Supporting Information

desired α-SF5 ketone 4a was isolated in a promising 37% yield (Table 2, entry 2). Switching to 1,4-dioxane resulted in a significant increase in yield (Table 2, entry 3). Heating the reaction to 100 °C had no effect on the isolated yield (Table 2, entry 4). Other solvents such as toluene and 2-MeTHF did not provide better results at 70 °C (Table 2, entries 5−6). Finally, increasing the catalyst loading to 3 mol % proved beneficial (Table 2, entry 7), while further increase had virtually no effect (Table 2, entries 7−8). Hence, conditions found in entry 7 were considered the optimal ones. The scope of this transformation using various SF5-alkynes (3) is reported in Scheme 4. Overall, both aliphatic and aromatic SF5-alkynes could be used and the corresponding α-SF5 ketones (4a−l) could be isolated, as a single regioisomer, in moderate to good yields (60−77%). The only exception noted was for an aromatic alkyne bearing an ortho-chlorine substituent (3m) for which the desired product 4m was not observed. It is interesting to note that hydration of 3a under strongly acidic conditions, as reported for aromatic CF3-alkynes,9b resulted in no conversion. This result illustrates the relevance of this gold-catalyzed transformation. In summary, we have described the regioselective goldcatalyzed hydration of CF3- and SF5-alkynes. The corresponding trifluoromethylated and pentasulfanylated ketones, potentially useful fluorinated building blocks for medicinal, agrochemistry,

S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.9b01379. Detailed experimental procedures and full spectroscopic data for all new compounds (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jean-François Paquin: 0000-0003-2412-3083 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Sciences and Engineering Research Council of Canada, the Fonds de recherche du Quebec−Nature et technologies, OmegaChem, and the Université Laval.

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REFERENCES

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Regioselective Gold-Catalyzed Hydration of CF3- and SF5-alkynes

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